Getting Gold
by
J. C. F. Johnson
Part 2 out of 3
breaking.
"(5) Consists of fragments of crystallised carbonate of lime from
Tarrawingee, in which the gold is deposited in spots, in appearance
like ferrous oxide, until submitted to the magnifying glass.
"The whole subject is worthy of much more time than I can possibly
give it. The importance lies in this: That having found how the much
desired metal may have been deposited in its matrix, the knowledge
should help to suggest how it may be economically extracted
therefrom."
A very remarkable nugget weighing 16 3/4 oz. was sluiced from near the
surface in one of my own mining properties at Woodside, South
Australia, some years ago, which illustrated the nuclear theory very
beautifully. This nugget is very irregular in shape, fretted and
chased as though with a jeweller's graving tool, showing plainly the
shape of the pyritous crystals on which it was formed while the
interstices were filled with red hematite iron just as found in
artificially formed nuggets on a sulphide of iron base. The author has
a nugget from the same locality weighing about 1 1/2 oz. which
exhibits in a marked degree the same characteristics, as indeed does
most of the alluvial gold found in the Mount Lofty Ranges; also a
nugget from near the centre of Australia weighing four ounces, in
which the original crystals of pyrites are reproduced in gold just as
an iron horse-shoe, placed in a launder through which cupriferously
impregnated water flows, will in time be changed to nearly pure copper
and yet retain its shape.
Now with regard to the four points I have put as to the apparent
anomalies of occurrence of alluvial gold. The reason why alluvial gold
is of finer quality as a rule than reef is probably because while gold
and silver, which have a considerable affinity for each other, were
presumably dissolved from their salts and held in solution in the same
mineral water, they would in many cases not be deposited together, for
the reason that silver is most readily deposited in the presence of
alkalies, which would be found in excess in mineral waters coming
direct from the basic rocks, while gold is induced to precipitate more
quickly in acid solutions, which would be the character of the waters
after they had been exposed to atmospheric action and to contact with
organic matters.
This, then, may explain not only the comparatively greater purity of
the alluvial gold, but also why big nuggets are found so far from
auriferous reefs, and also why heavy masses of gold have been
frequently unearthed from among the roots even of living trees, but
more particularly in drifts containing organic matter, such as ancient
timber.
All, then, that has been adduced goes to establish the belief that the
birthplace of our gold is in certain of the earlier rocks comprising
the earth's crust, and that its appearance as the metal we value so
highly is the result of electro-chemical action, such as we can
demonstrate in the laboratory.
CHAPTER VI
GOLD EXTRACTION
We now come to a highly important part of our subject, the practical
treatment of ores and matrixes for the extraction of the metals
contained. The methods employed are multitudinous, but may be divided
into four classes, namely, washing, amalgamating with mercury,
chlorinating, cyaniding and other leaching processes, and smelting.
The first is used in alluvial gold and tin workings and in preparing
some silver, copper, and other ores for smelting, and consists merely
in separating the heavier metals and minerals from their gangues by
their greater specific gravity in water. The second includes the
trituration of the gangue and the extraction of its gold or silver by
means of mercury. Chlorinating and leaching generally is a process
whereby metals are first changed by chemical action into their mineral
salts, as chloride of gold, nitrate of silver, sulphate of copper, and
being dissolved in water are afterwards redeposited in the metallic
form by means of well-known re-agents.
In really successful mining it is in the last degree important that
the mode of extraction of metals in the most scientific manner should
be thoroughly understood, but as a general rule the science of
metallurgy is but very superficially grasped even by those whose
special business it is to treat ore bodies in order to extract their
metalliferous contents, and whether in quartz crushing mill,
lixiviating, or smelting works there is much left to be desired in the
method of treating our ores.
My attention was recently attracted to an article written by Mr. F. A.
H. Rauft, M.E., from which I make the following extract:
He says, speaking of the German treatment of ores and the mode of
procedure in Australia, "It is high time that Government stepped in
and endeavoured by prompt and decisive action to bring the mining
industry upon a sound and legitimate basis. Though our ranges abound
in all kinds of minerals that might give employment to hundreds of
thousands of people, mining is carried on in a desultory, haphazard
fashion. There is no system, and the treatment of ores is of necessity
handed over to the tender mercies of men who have not even an idea of
what an intricate science metallurgy has become in older countries.
During many years of practical experience I have never known a single
instance where a lode, on being worked, gave a return according to
assay, and I have never known any mine where some of the precious
metals could not be found in the tailings or slag. The Germans employ
hundreds of men in working for zinc which produces some two or three
per cent to the ton; here the same percentage of tin could hardly be
made payable, and this, mark you, is owing not to cheaper labour
alone, but chiefly to the labour-saving appliances and the results of
the researches of such gigantic intellects as Professor Kerl and many
others, of whom we in this country never even hear. Go into any of the
great mining works of central Germany, and you may see acres covered
by machinery ingeniously constructed to clean, break, and sort, and
ultimately deliver the ores into trucks or direct into the furnace,
and the whole under the supervision of a youngster or two. When a
parcel of ore arrives at any of the works, say Freiberg or Clausthal,
it is carefully assayed by three or four different persons and then
handed over to practical experts, who are expected to produce the full
amount of previous metal according to assay; and if by any chance they
do not, a fixed percentage of the loss is deducted from their salary;
or, if the result is in excess of this assay which is more frequently
the case, a small bonus is added to their pay. Compare this system
with our own wasteful, reckless method of dealing with our precious
metals, and we may hide our heads in very shame."
All really practical men will, I think, endorse Mr. Rauft's opinion.
Well organised and conducted schools of mines will gradually
ameliorate this unsatisfactory state of things, and I hope before long
that we shall have none but qualified certificated men in our mines.
In the meantime a few practical hints, particularly on that very
difficult branch of the subject, the saving of gold, will, it is
hoped, be found of service.
The extraction of gold from the soil is an industry so old that its
first introduction is lost in the mist of ages. As before stated, gold
is one of the most widely disseminated of the metals, and man, so soon
as he had risen from the lowest forms of savagery, began to be
attracted by the kingly metal, which he found to be easily fashioned
into articles of ornament and use, and to be practically non-
corrodable.
What we now term the dish or pan, then, doubtless generally a wooden
bowl, was the appliance first used; but they had also an arrangement,
somewhat like our modern blanket tables, over which the auriferous
sand was passed by means of a stream of water. The sands of some of
the rivers from which portions of the gold supply of the old world was
derived are still washed over year after year in exactly the same
manner as was employed, probably, thousands of years ago, the labour,
very arduous, being often carried on by women, who, standing knee deep
in water, pan off the sand in wooden bowls much as the digger in
modern alluvial fields does with his tin dish. The resulting gold
often consists of but a grain or two of fine dust-gold, which is
carefully collected in quills, and so exported or traded for goods.
The digger of to-day having discovered payable alluvial dirt at such a
depth as to permit of its being profitably worked by small parties of
men with limited or no capital, procures first a half hogshead for a
puddling tub, a "cradle," or "long tom," and tin dish. The "wash
dirt," as the auriferous drift is usually termed, contains a
considerable admixture of clay of a more or less tenacious character,
and the bulk of this has to be puddled and so disintegrated before the
actual separation of the gold is attempted in the cradle or dish. This
is done in the tub by constantly stirring with a shovel, and changing
the water as it becomes charged with the floating argillaceous, or
clayey, particles. The gravel is then placed in the hopper of the
cradle which separates the larger stones and pebbles, the remainder
passing down over inclined ledges as the cradle is slowly rocked and
supplied with water. At the bottom of each ledge is a riffle to arrest
the particles of gold. Sometimes, when the gold is very fine,
amalgamated copper plates are introduced and the lower ledges are
covered with green baize to act as blanket tables and catch gold which
might otherwise be lost.
A long tom is a trough some 12 feet in length by 20 inches in width at
the upper end, widening to 30 inches at the lower end; it is about 9
inches deep and has a fall of 1 inch to a foot. An iron screen is
placed at the lower end where large stones are caught, and below this
screen is the riffle box, 12 feet long, 3 feet wide, and having the
same inclination as the upper trough. It is fitted with several
riffles in which mercury is sometimes placed.
Much more work can be done with this appliance than with the cradle,
which it superseded. Of course, the gold must be coarse and water
plentiful.
When, however, the claim is paying, and the diggings show signs of
some permanency, a puddling machine is constructed. This is described
in the chapter called "Rules of Thumb."
Hydraulicing and ground sluicing is a very cheap and effective method
of treating large quantities of auriferous drift, and, given
favourable circumstances, such as a plentiful supply of water with
good fall and extensive loose auriferous deposits, a very few grains
to the ton or load can be made to give payable returns. The water is
conveyed in flumes, or pipes to a point near where it is required,
thence in wrought iron pipes gradually reduced in size and ending in a
great nozzle somewhat like that of a fireman's hose. The "Monitor," as
it is sometimes called, is generally fixed on a movable stand, so
arranged that the strong jet of water can be directed to any point by
a simple adjustment. A "face" is formed in the drift, and the water
played against the lower portion of the ledge, which is quickly
undermined, and falls only to be washed away in the stream of water,
which is conducted through sluices with riffles, and sometimes over
considerable lengths of amalgamated copper plates. This class of
mining has been most extensively carried out in California and New
Zealand, and some districts of Victoria, but the truly enormous drifts
of the Shoalhaven district in New South Wales must in the near future
add largely to the world's gold supply. These drifts which are
auriferous from grass roots to bed rock extend for nearly fifty miles,
and are in places over 200 feet deep. Want of capital and want of
knowledge has hitherto prevented their being profitably worked on a
large scale.
The extraction of reef gold from its matrix is a much more complicated
process, and the problem how most effectively to obtain that great
desideratum--a complete separating and saving operation--is one which
taxes the skill and evokes the ingenuity of scientific men all over
the world. The difficulty is that as scarcely any two gangues, or
matrixes, are exactly alike, the treatment which is found most
effective on one mine will often not answer in another. Much also
depends on the proportion of gold to the ton of rock under treatment,
as the most scientific and perfect processes of lixiviation hitherto
adopted will not pay, even when all other conditions are favourable,
if the amount of gold is much under half an ounce to the ton and even
then will leave but a very small profit. If, however, the gold is
"free," and the lode large, a very few pennyweights (or "dollars," as
the Americans say) to the ton will pay handsomely. The mode of
extraction longest in vogue, and after all the cheapest and most
effective, for free milling ores where the gold is not too fine, is
amalgamation with mercury, which metal has a strong affinity for gold,
silver, and copper.
As to crushing appliances, I shall not say much. "Their name is legion
for they are many," and the same may be said of concentrators. It may
be old-fashioned, but I admit my predilection is still in favour of
the stamper-battery, for the reason that though it may be slower in
proportion to the power employed, it is simple and not liable to get
out of order, a great advantage when one has so often to depend on men
who bring to their work a supply principally of main strength and
stupidity. For the same reason I prefer the old draw and lift, and
plunger pumps to newer but more complicated water-lifters.
On both these points, however, I am constrained to admit that my
opinion has recently been somewhat shaken.
I have lately seen two appliances which appear to mark a new era in
the scientific progress of mining. One is the "Griffin Mill," the
other the "Lemichel Siphon Elevateur."
The first is in some respects on the principle of the Huntingdon Mill.
The latter, if the inventor may be believed and the results seem to
show he can be, will be a wonderful factor in developing not only
mining properties where a preponderance of water is the trouble, but
also in providing an automatic, and therefore extremely cheap, mode of
water-raising and supply, which in simplicity is thus far unexampled.
Atmospheric pressure alone is relied on. The well-known process of the
syphon is the basis, but with this essential difference, that a large
proportion of the water drawn up to the apex of the syphon is super-
elevated to heights regulated by the fall obtained in the outlet leg.
This elevation can be repeated almost indefinitely by returning the
waste water to the reservoirs.
The Lemichel Syphon is a wonderful, yet most simple application of
natural force. The inlet leg of the syphon is larger in diameter than
the outlet leg, and is provided at the bottom with a valve or "clack."
The outlet leg has a tap at its base. At the apex are two chambers,
with an intermediary valve, regulated by a counterpoise weighted
lever. The first chamber has also a vertical valve and pipe.
When the tap of the outlet leg is turned, the water flows as in an
ordinary syphon, but owing to the rapid automatic opening and shutting
of the valve in the first chamber about 45 per cent of the water is
diverted, and may be raised to a height of many feet above the top of
the syphon.
It need not be impressed on practical men that if this invention will
perform anything like what is claimed for it, its value can hardly be
calculated. After a careful inspection of the appliance in operation,
I believe it will do all that is stated.
Another invention is combined with this which, by a very small
expenditure of fuel, will enable the first point of atmospheric
pressure to be attained. In this way the unwatering of mines may be
very inexpensively effected, or water for irrigation purposes may be
raised from an almost level stream.
The Griffin Mill is a centrifugal motion crusher with one roller only,
which, by an ingenious application of motive force, revolves in an
opposite direction to its initial momentum, and which evolves a force
of 6000 lb. against the tire, which is only 30 inches in diameter. For
hard quartz the size should be increased by at least 6 inches. It is
claimed for this mill that it will pulverise to a gauge of 900 holes
to the square inch from 1 1/2 to 2 1/2 tons per hour, or, say roughly,
150 tons per week.
The Huntingdon mill is a good crusher and amalgamator where the
material to be operated on is comparatively soft, but does not do such
good work when the stone is of a hard flinty nature.
A No. 4 Dodge stone-breaker working about 8 hours will keep a five-
foot Huntingdon mill going 24 hours, and an automatic feeder is
essential. For that matter both are almost essential for an ordinary
stamper battery, and will certainly increase the crushing capacity and
do better work from the greater regularity of the feed.
A 10 h.-p. (nominal) engine of good type is sufficient for Huntingdon
mill, rock breaker, self-feeder and steam pump. A five-foot mill under
favourable circumstances will crush about as much as eight head of
medium weight stamps.
The Grusonwek Ball Mills, made by Krupp of Germany, also that made by
the Austral Otis Company, Melbourne, are fast and excellent crushing
triturating appliances for either wet or dry working, but are
specially suited only for ores when the gold is fine and evenly
distributed in the stone. The trituration is effected by revolving the
stone in a large cylinder together with a number of steel balls of
various sizes, the attrition of which with the rock quickly grinds it
to powder of any required degree of fineness.
More mines have been ruined by bad mill management probably than by
bad mining, though every experienced man must have seen in his time
many most flagrant instances of bungling in the latter respect. Shafts
are often sunk on the wrong side of the lode or too near or too far
away therefrom, while instances have not been wanting where the (mis)
manager has, after sinking his shaft, driven in the opposite direction
to that where the lode should be found.
A common error is that of erecting machinery before there is
sufficient ore in sight to make it certain that enough can be provided
to keep the plant going. In mines at a distance from the centre of
direction it is almost impossible to check mistakes of this
description, caused by the ignorance or over sanguineness of the mine
superintendent, and they are often as disastrous as they are
indefensible. Another fertile source of failure is the craze for
experimenting with untried inventions, alleged to be improvements on
well-known methods.
A rule in the most scientific of card games, whist, is "when in doubt
lead trumps." It might be paraphrased for mining thus: "When in doubt
about machinery use that which has been proved." Let some one else do
the experimenting.
The success of a quartz mine depends as much on favourable working
conditions as on its richness in gold. Thus it may be that a mine
carrying 5 or 6 oz. of gold to the ton but badly circumstanced as to
distance, mountainous roads, lack of wood and water, in some cases a
plethora of the latter, or irregularly faulted country, may be less
profitable than another showing only 5 or 6 dwt., but favourably
situated.
It is usually desirable to choose for the battery site, when possible,
the slope of a hill which consists of rock that will give a good
foundation for your battery.
The economical working depends greatly on the situation, which is
generally fixed more or less, in the proximity of the water. The
advantages of having ample water for battery purposes, or of using
water as a motive power, are so great that it is very often desirable
to construct a tramway of considerable length, when, by so doing, that
power can be utilised; hence most quartz mills are placed near
streams, or in valleys where catchment dams can be effectively
constructed, except, of course, in districts where much water has to
be pumped from the mine.
If water-power can be used, the water-motor will necessarily be placed
as low as possible, in order to obtain the fullest available power.
One point is essential. Special care must be taken to keep the
appliances above the flood-level. If the water in the stream is not
sufficient to carry off the tailings, the battery should be placed at
such a height as to leave sufficient slope for tailings' dumps. This
is more important when treating ore of such value that the tailings
are worth saving for secondary treatment. In this case provision
should be made for tailings, dams, or slime pits.
Whether the battery is worked by water, steam, or gas power, an ample
supply of water is absolutely necessary, at least until some
thoroughly effective mode of dry treatment is established. If it can
be possibly arranged the water should be brought in by gravitation,
and first cost is often least cost; but where this is impossible,
pumps of sufficient capacity, not only to provide the absolute
quantity used, but to meet any emergency, should be erected.
The purer the water the better it will be for amalgamating purposes,
and in cold climates it is desirable to make provision for heating the
water supplied to the battery. This can be done by means of steam from
the boiler led through the feed tanks; but where the boiler power is
not more than required, waste steam from the engine may be employed,
but care must be taken that no greasy matter comes in contact with the
plates. The exhaust steam from the engine may be utilised by carrying
it through tubes fitted in an ordinary 400 gallon tank.
Reducing appliances have often to be placed in districts where the
water supply is insufficient for the battery. When this is so every
available means must be adopted for saving the precious liquid, such
as condensing the exhaust steam from the engine. This may be done by
conducting it through a considerable length of ordinary zinc piping,
such as is used for carrying the water from house roofs. Also tailings
pits should be made, in which the tailings and slimes are allowed to
settle, and the cleared water is pumped back to be again used. These
pits should, where practicable, be cemented. It is usual, also, to
have one or two tailings dams at different levels; the tailings are
run into the upper dam, and are allowed to settle; the slimes overflow
from it into the lower dam, and are there deposited, while the cleared
water is pumped back to the battery. Arrangements are made by which
all these reservoirs can be sluiced out when they are filled with
accumulated tailings. It is well not to leave the sluicing for too
long a period, as when the slimes and tailings are set hard they are
difficult to remove.
Where a permanent reducing plant is to be erected, whatever form of
mill may be adopted, it is better for many reasons to use automatic
ore feeders. Of these the best two I have met are the "Tulloch" and
"Challenge" either of which can be adapted to any mill and both do
good work.
By their use the reducing capacity of the mill is increased, and the
feeding being regular the wear and tear is decreased, while by the
regulated feeding of the "pulp" in the battery box or mortar can be
maintained at any degree of consistency which may be found desirable,
and thus the process of amalgamation will be greatly facilitated. The
only objection which can be urged against the automatic feeder is that
the steel points of picks, gads, drills, and other tools may be
allowed to pass into the mortar or mill, and thus cause considerable
wear and tear. This, I think, can be avoided by the adoption of the
magnet device, described in "Rules of Thumb."
There are many mines where 3 to 4 dwt. of gold cover all the cost, the
excess being clear profit. In fact there are mines which with a yield
of 1 1/2 to 2 dwt. a ton, and crushing with water power, have actually
yielded large profits. On the other hand, mines which have given
extraordinary trial crushings have not paid working expenses.
Everything depends on favourable local conditions and proper
management.
Having decided what class of crushing machinery you will adopt, the
first point is to fix on the best possible site for its erection. This
requires much judgment, as success or failure may largely depend on
the position of your machinery. One good rule is to get your crusher
as reasonably high as possible, as it is cheaper to pump your feed
water a few feet higher so as to get a good clear run for your
tailings, and also to give you room to erect secondary treatment
appliances, such as concentrators and amalgamators below your copper
plates and blanket strakes.
Next, and this is most important, see that your foundations are solid
and strong. A very large number of the failures of quartz milling
plants is due to neglect of this rule.
I once knew a genius who erected a 10-Lead mill in a new district, and
who adopted the novel idea of placing a "bed log" laterally beneath
his stampers. The log was laid in a little cement bed which, when the
battery started, was not quite dry. The effect was comical to every
one but the unfortunate owners. It was certainly the liveliest, but at
the same time one of the most ineffective batteries I have seen.
In a stamp mill the foundations are usually made of hard wood logs
about 5 to 6 feet long, set on end, the bottom end resting on rock and
set round with cement concrete. These are bolted together, and the
"box" or mortar is bolted to them. The horizontal logs to carry the
"horses" or supports for the battery frame should also be of good
size, and solidly and securely bolted. The same applies to your
engine-bed, but whether it be of timber, or mason work, above all
things provide that the whole of your work is set out square and true
to save after-wear and friction.
Considerable difference of opinion exists as to the most effective
weight for stamps. My experience has been that this largely depends on
the nature of your rock, as does also the height for the drop. I have
usually found that with medium stamps, say 7 to 7 1/2 cwt. with fair
drop and lively action, about 80 falls per minute, the best results
were obtained, but the tendency of modern mill men is towards the
heavier stamps, 9 cwt. and even heavier.
To find the horse-power required to drive a battery, multiply the
weight of one stamp by the number of stamps in the battery; the height
of lift in feet by the number of lifts per minute; add one-third of
the product for friction, and the result will be the number of feet-
lbs. per minute; divide this by 33,000 which is the number of feet-
lbs. per minute equal to 1 h.-p. and the result will be the h.-p.
required. Thus if a stamp weighs 800 lb. and you have five in the box,
and each stamp has a lift of 9 in. = 0.75 ft. and strikes 80 blows per
minute, then 800 x 5 x 0.75 x 80 = 240,000; one-third of 240,000 =
80,000 which added to 240,000 = 320,000; and 320,000 divided by 33,000
= 9.7 h.-p. or 1.9 h.-p. each stamp.
The total weight of a battery, including stamper box, stampers, etc.,
may be roughly estimated at about 1 ton per stamp. Medium weight
stampers, including shank cam, disc, head, and shoe, weigh from 600 to
700 lb., and need about 3/4 h.-p. to work them.
The quantity of water required for the effective treatment of gold-
bearing rock in a stamper battery varies according to the composition
of the material to be operated upon, but generally it is more than the
inexperienced believe. For instance, "mullocky" lode stuff, containing
much clayey matter or material carrying a large percentage of heavy
metal, such as titanic iron or metallic sulphides, will need a larger
quantity of water per stamp than clean quartz. A fair average quantity
would be 750 to 1000 gallons per hour for each box of five stamps. In
general practice I have seldom found 1000 gallons per hour more than
sufficient.
As to the most effective mesh for the screen or grating no definite
rule can be given, as that depends so largely on the size of the gold
particles contained in the gangue. The finer the particles the closer
must be the mesh, and nothing but careful experiment will enable the
battery manager to decide this most important point. The American
slotted screens are best; they wear better than the punched gratings
and can be used of finer gauge. Woven steel wire gauze is employed
with good effect in some mills where especially fine trituration is
required. This class of screen requires special care as it is somewhat
fragile, but with intelligent treatment does good work.
The fall or inclination of the tables, both copper and blanket
strakes, is also regulated by the class of ore. If it should be heavy
then the fall must be steeper. A fair average drop is 3/4 inch to the
foot. Be careful that your copper tables are thoroughly water-tight,
for remember you are dealing with a very volatile metal, quicksilver;
and where water will percolate mercury will penetrate.
The blanket tables are simply a continuation of the mercury tables,
but covered with strips of coarse blanket, green baize, or other
flocculent material, intended to arrest the heavier metallic particles
which, owing to their refractory nature, have not been amalgamated.
The blanket table is, however, a very unsatisfactory concentrator at
best, and is giving place to mechanical concentrators of various
descriptions.
An ancient Egyptian gold washing table was used by the Egyptians in
treating the gold ores of Lower Egypt. The ore was first ground, it is
likely by means of some description of stone arrasts and then passed
over the sloping table with water, the gold being retained in the
riffles. In these the material would probably be mechanically
agitated. Although for its era ingenious it will be plain to practical
men that if the gold were fine the process would be very ineffective.
Possibly, but of this I have no evidence, mercury was used to retain
the gold on the riffles, as previously stated. This method of saving
the precious metal was known to the ancients.
At a mine of which I was managing director the lode was almost
entirely composed of sulphide of iron, carbonate of lime or calcspar,
with a little silica. In this case it has been found best to crush
without mercury, then run the pulp into pans, where it is
concentrated. The concentrates are calcined in a common reverberatory
furnace, and afterwards amalgamated with mercury in a special pan, the
results as to the proportion of gold extracted being very
satisfactory; but it does not therefore follow that this process would
be the most suitable in another mine where the lode stuff, though in
some respects similar, yet had points of difference.
I was lately consulted with respect to the treatment of a pyritic ore
in a very promising mine, but could not recommend the above treatment,
because though the pyrites in the gangue was similar, the bulk of the
lode consisted of silica, consequently there would be a great waste of
power in triturating the whole of the stuff to what, with regard to
much of it, would be an unnecessary degree of fineness. I am of
opinion that in cases such as this, where it is not intended to adopt
the chlorination or cyanogen process, it will be found most economical
to crush to a coarse gauge, concentrate, calcine the concentrates, and
finally amalgamate in some suitable amalgamator.
Probably for this mode of treatment Krom rolls would be found more
effective reducing agents than stampers, as with them the bulk of the
ore can be broken to any required gauge and there would consequently
be less loss in "slimes."
The great art in effective battery work is to crush your stuff to the
required fineness only, and then to provide that each particle is
brought into contact with the mercury either in box, trough, plate, or
pan. To do this the flow of water must be carefully regulated; neither
so much must be used as to carry the stuff off too quickly nor so
little as to cause the troughs and plates to choke. In cold weather
the water may be warmed by passing the feed-pipe through a tank into
which the steam from the engine exhausts, and this will be found to
keep the mercury bright and lively. But be careful no engine oil or
grease mingles with the water, as grease on the copper tables will
absolutely prevent amalgamation.
The first point, then, is to crush the gangue effectively, the degree
of fineness being regulated by the fineness of the gold itself. This
being done, then comes the question of saving the gold. If the quartz
be clean, and the gold unmixed with base metal, the difficulty is
small. All that is required is to ensure that each particle of the
Royal metal shall be brought into contact with the mercury. The main
object is to arrest the gold at the earliest possible stage;
therefore, if you are treating clean stone containing free gold,
either coarse or fine, I advise the use of mercury in the boxes, for
the reason that a considerable proportion of the gold will be caught
thereby, and settling to the bottom, or adhering to amalgamated plates
in the boxes, where such are used, will not be afterwards affected by
the crushing action, which might otherwise break up, or "flour," the
mercury. On the whole, I rather favour the use of mercury in the box
at any time, unless the ore is very refractory--that is, contains too
great a proportion of base metals, particularly sulphides of iron,
arsenic, etc., when the result will not be satisfactory, but may
entail great loss by the escape of floured mercury carrying with it
particles of gold. Here only educated intelligence, with experience,
will assist the battery manager to adopt the right system.
The crushed stuff--generally termed the "pulp"--passes from the boxes
through the "screens" or "gratings," and so on to the "tables"--i.e.,
sheets of copper amalgamated on the upper surface with mercury, and
sometimes electroplated with silver and afterwards treated with
mercury. Unless the quartz is very clean, and, consequently light, I
am opposed to the form of stamper box with mercury troughs cast in the
"lip," nor do I think that a trough under the lip is a good
arrangement, as it usually gets so choked and covered with the heavy
clinging base metals as to make it almost impossible for the gold to
come in contact with the mercury. It will be found better where the
gold is fine, or the gangue contains much base metal, to run the pulp
from the lip of the battery into a "distributor."
The distributor is a wooden box the full width of the "mortar," having
a perforated iron bottom set some three to four inches above the first
copper plate, which should come up under the lip. The effect of this
arrangement is that the pulp is dashed on the plate by the falling
water, and the gold at once coming in contact with the mercury begins
to accumulate and attract that which follows, till the amalgam becomes
piled in little crater-shaped mounds, and thus 75 per cent of the gold
is saved on the top plate.
I have tried a further adaptation of this process when treating ores
containing a large percentage of iron oxide, where the bulk of the
gold is impalpably fine, and contained in the "gossan." At the end of
the blanket table, or at any point where the crushed stuff last passes
before going to the "tailings heap," or "sludge pit," a "saver" is
placed. The saver is a strong box about 15 in. square by 3 ft. high,
one side of which is removable, but must fit tight. Nine slots are cut
inside at 4 in. apart, and into these are fitted nine square
perforated copper plates, having about eighty to a hundred 1/4 in.
holes in each; the perforations should not come opposite each other.
These plates are to be amalgamated on both sides with mercury, in
which a very little sodium has been placed (if acid ores are being
treated, zinc should be employed in place of sodium, and to prevent
the plates becoming bare, if the stuff is very poor, thick zinc
amalgam may be used with good effect; but in that case discontinue the
sodium, and occasionally, if required, say once or twice in the day,
mix an ounce of sulphuric acid in a quart of water and slowly pour it
into the launder above the saver). Underneath the "saver" you require
a few riffles, or troughs, to catch any waste mercury, but if not
overfed there should be no waste. This simple appliance, which is
automatic and requires little attention, will sometimes arrest a
considerable quantity of gold.
We now come to the subsidiary processes of battery work, the
"cleaning" of plates, and "scaling" same when it is desired to get all
the gold off them, the cleaning and retorting of amalgam, and of the
mercury, smelting gold, etc.
Plates should be tenderly treated, kept as smooth as possible, and
when cleaning up after crushing, in your own battery, the amalgam--
except, say, at half-yearly intervals--should be removed with a rubber
only; the rubber is simply a square of black indiarubber or soft pine
wood.
When crushing rich ore, and you want to get nearly all the gold off
your plates, the scraper may be resorted to. This is usually made by
the mine blacksmith from an old flat file which is cut in half, the
top turned over, beaten out to a sharp blade, and kept sharp by
touching it up on the grinding-stone. This, if carefully used, will
remove the bulk of the amalgam without injury to the plate.
Various methods of "scaling" plates will be found among "Rules of
Thumb."
Where base metals are present in the lode stuff frequent retortings of
the mercury, say not less than once a month, will be found to have a
good effect in keeping it pure and active. For this purpose, and in
order to prevent stoppage of the machinery, a double quantity is
necessary, so that half may be used alternately. Less care is required
in retorting the mercury than in treating the amalgam, as the object
in the one case is more to cleanse the metal of impurities than to
save gold, which will for the most part have been extracted by
squeezing through the chamois leather or calico. A good strong heat
may therefore at once be applied to the retort and continued, the
effect being to oxidise the arsenic, antimony, lead, etc., which, in
the form of oxides, will not again amalgamate with the mercury, but
will either lie on its surface under the water, into which the nozzle
of the retort is inserted, or will float away on the surface of the
water. I have also found that covering the top of the mercury with a
few inches of broken charcoal when retorting has an excellent
purifying effect.
In retorting amalgam, much care and attention is required.
First, never fill the retort too full, give plenty of room for
expansion; for, when the heat is applied, the amalgam will rise like
dough in an oven, and may be forced into the discharge pipe, the
consequence being a loss of amalgam or the possible bursting of the
retort. Next, be careful in applying the heat, which should be done
gradually, commencing at the top. This is essential to prevent waste
and to turn out a good-looking cake of gold, which all battery
managers like to do, even if they purpose smelting into bars.
Sometimes special difficulties crop up in the process of separating
the gold from the amalgam. At the first "cleaning up" on the Frasers
Mine at Southern Cross, West Australia, great consternation was
excited by the appearance of the retorted gold, which, as an old miner
graphically put it, was "as black as the hind leg of a crow," and
utterly unfit for smelting, owing to the presence of base metals. Some
time after this I was largely interested in the Blackborne mine in the
same district when a similar trouble arose. This I succeeded in
surmounting, but a still more serious one was too much for me--i.e.,
the absence of payable gold in the stone. I give here an extract from
the /Australian Mining Standard/, of December 9th, 1893, with
reference to the mode of cleaning the amalgam which I adopted.
NEW METHOD OF SEPARATING GOLD FROM IMPURE AMALGAM.
I had submitted to me lately a sample of amalgam from a mine in West
Australia which amalgam had proved a complete puzzle to the manager
and amalgamator. The Mint returns showed a very large proportion of
impurity, even in the smelted gold. When retorted only, the Mint
authorities refused to take it after they had treated two cakes, one
of 119 oz., which yielded only 35 oz. 5 dwt. standard gold, and one of
140 oz., which gave 41 oz. 10 dwt. The gold smelted on the mine was
nearly as bad proportionately. Thus, 128 oz. smelted down at the Mint
to 87 oz. 8 dwt. and 109 oz. to 55 oz. 10 dwt. The impurity was
principally iron, a most unusual thing in my experience, and was due
to two causes revealed by assay of the ore and analysis of the mine
water, viz., an excess of arsenate of iron in the stone, and the
presence in large proportions of mineral salts, principally chloride
of Calcium CaCl., sodium NaCl, and magnesium MgCl2, in the mine water
used in the battery. The exact analysis of the water was as follows:--
Carbonate of Iron FeCO3 2.76 grains per gallon
Carbonate of Calcium CaCO3 7.61 grains per gallon
Sulphate of Calcium CaSO4 81.71 grains per gallon
Chloride of Calcium CaCl2 2797.84 grains per gallon
Chloride of Magnesium MgCl2 610.13 grains per gallon
Chloride of Sodium or
Common Salt NaCl 5072.65 grains per gallon
Total solid matter 8572.70 = 19.5 oz. to the gallon.
It will be seen, then, that this water is nearly four times more salt
that that of the sea. The effect of using a water of this character,
as I have previously found, is to cause the amalgamation of
considerable quantities of iron with the gold as in this case.
I received 10 oz. of amalgam, and having found what constituted its
impurities proceeded to experiment as to its treatment. When retorted
on the mine it was turned out in a black cake so impure as almost to
make it impossible to smelt properly. I found the same result on first
retorting, and after a number of experiments which need not be
recapitulated though some were fairly effective, I hit on the
following method, which was found to be most successful and will
probably be so found in other localities where similarly unfavourable
conditions prevail.
I took a small ball of amalgam, placed it in a double fold of new fine
grained calico, and after soaking in hot water put it under a powerful
press. The weight of the ball before pressing was 1583 gr. From this
383 gr. of mercury was expressed and five-eighths of a grain of gold
was retorted from this expressed mercury. The residue, in the form of
a dark, grey, and very friable cake, was powdered up between the
fingers and retorted, when it became a brown powder; it was afterwards
calcined on a flat sheet in the open air; result, 510 gr. of russet-
coloured powder. Smelted with borax, the iron oxide readily separated
with the slag; result, 311 gr. gold 871-1000 fine; a second smelting
brought this up to 914-1000 fine. Proportion of smelted gold to
amalgam, one-fifth.
The principal point about this mode of treatment is the squeezing out
of the mercury, whereby the amalgam goes into the retort in the form
of powder, thus preventing the slagging of the iron and enclosure of
the gold. The second point of importance is thorough calcining before
smelting.
Of course it would be practicable, if desired, to treat the powder
with hydrochloric acid, and thus remove all the iron, but in a large
way this would be too expensive, and my laboratory treatment, though
necessarily on a small scale, was intended to be on a practical basis.
The amalgam at this mine was in this way afterwards treated with great
success.
For the information of readers who do not understand the chemical
symbols it may be said that
FeCO3 is carbonate of iron;
CaCO3 is carbonate of calcium;
CaSO4 is sulphate of calcium;
CaCl2 is chloride of calcium;
MgCl2 is chloride of magnesium;
NaCl is chloride of sodium, or common salt.
CHAPTER VII
GOLD EXTRACTION--SECONDARY PROCESSES AND LIXIVIATION
Before any plan is adopted for treating the ore in a new mine the
management should very seriously and carefully consider the whole
circumstances of the case, taking into account the quantity and
quality of the lode stuff to be operated on, and ascertain by analysis
what are its component parts, for, as before stated, the treatment
which will yield most satisfactory results with a certain class of
gangue on one mine will sometimes, even when the material is
apparently similar, prove a disastrous failure in another. Some time
since I was glad to note that the manager of a prominent mine strongly
discountenanced the purchase of any extracting plant until he was
fully satisfied as to the character of the bulk of the ore he would
have to treat. It would be well for the pockets of shareholders and
the reputation of managers, if more of our mine superintendents
followed this prudent and sensible course.
Having treated on gold extraction with mercury by amalgamated plates
and their accessories, something must be said about secondary modes of
saving in connection with the amalgamation process. The operations
described hitherto have been the disintegration of the gold-bearing
material and the extraction therefrom of the coarser free gold. But it
must be understood that most auriferous lode stuff contains a
proportion of sulphides of various metals, wherein a part of the gold,
usually in a very finely divided state, is enclosed, and on this gold
the mercury has no influence. Also many lodes contain hard heavy
ferric ores, such as titanic iron, tungstate of iron, and hematite, in
which gold is held. In others, again, are found considerable
quantities of soft powdery iron oxide or "gossan," and compounds such
as limonite, aluminous clay, etc., which, under the action of the
crushing mill become finely divided and float off in water as
"slimes," carrying with them atoms of gold, often microscopically
small. To save the gold in such matrixes as these is an operation
which even the best of our mechanical appliances have not yet fully
accomplished.
Where there is not too great a proportion of base metals on which the
solvent will act, and when the material is rich enough in gold to pay
for the extra cost of treatment, chlorination or cyanisation are the
best modes of extraction yet practically adopted.
Presuming, however, that we are working by the amalgamation process,
and have crushed our stone and obtained the free gold, the next
requirement is an effective concentrator. Of these there are many
before the public, and some do excellent work, but do not act equally
well in all circumstances. The first and most primitive is the blanket
table, previously mentioned; but it can hardly be said to be very
effective, and requires constant attention and frequent changing and
washing of the strips of blanket.
Instead of blanket tables percussion tables are sometimes used, to
which a jerking motion is given against the flow of the water and
pulp, and by this means the heavier minerals are gathered towards the
upper part of the table, and are from thence removed from time to time
as they become concentrated.
I have seen this appliance doing fairly good work, but it is by no
means a perfect concentrator.
Another form of "shaking table" is one in which the motion is given
sideways, and this, whether amalgamated, or provided with small
riffles, or covered with blanket, keeps the pulp lively and encourages
the retention of the heavier particles, whether of gold or base metals
containing gold. There has also been devised a rocking table the
action of which is analogous to that of the ordinary miner's cradle.
This appliance, working somewhat slowly, swings on rockers from side
to side, and is usually employed in mills where, owing to the
complexity of the ore, difficulties have been met with in amalgamating
the gold. Riffles are provided and even very fine gold is sometimes
effectively recovered by their aid.
The Frue vanner will, as a rule, act well when the pulp is
sufficiently fine. It is really a adaptation of an old and simple
apparatus used in China and India for washing gold dust from the sands
of rivers. The original consisted of an endless band of strong cloth
or closely woven matting, run on two horizontal rollers placed about
seven feet apart, one being some inches lower than the other. The
upper is caused to revolve by means of a handle. The cloth is thus
dragged upwards against a small stream of water and sand fed to it by
a second man, the first man not only turning the handle but giving a
lateral motion to the band by means of a rope tied to one side.
Chinamen were working these forerunners of the Frue vanner forty years
ago in Australia, and getting fair returns.
The Frue vanner is an endless indiarubber band drawn over an inclined
table, to which a revolving and side motion is given by ingenious
automatic mechanism, the pulp being automatically fed from the upper
end, and the concentrates collected in a trough containing water in
which the band is immersed in its passage under the table; the lighter
particles wash over the lower end. The only faults with the vanner are
--first, it is rather slow; and secondly, though so ingenious it is
just a little complicated in construction for the average non-
scientific operative.
Of pan concentrators there is an enormous selection, the principle in
most being similar--i.e., a revolving muller, which triturates the
sand, so freeing the tiny golden particles and admitting of their
contact with the mercury. The mistake with respect to most of these
machines is the attempt to grind and amalgamate in one operation. Even
when the stone under treatment contains no deleterious compounds the
simple action of grinding the hard siliceous particles has a bad
effect on the quicksilver, causing it to separate into small globules,
which either oxidising or becoming coated with the impurities
contained in the ore will not reunite, but wash away in the slimes and
take with them a percentage of the gold. As a grinder and
concentrator, and in some cases as an amalgamator, when used
exclusively for either purpose, the Watson and Denny pan is effective;
but although successfully used at one mine I know, the mode there
adopted would, for reasons previously given, be very wasteful in many
other mines.
There is considerable misconception, even among men with some
practical knowledge, as to the proper function of these secondary
saving appliances; and sometimes good machines are condemned because
they will not perform work for which they were never intended. It
cannot be too clearly realized that the correct order of procedure for
extracting the gold held in combination with base metals is--first,
reduction of the particles to a uniform gauge and careful
concentration only; next, the dissipation, usually by simple
calcination, of substances in the concentrates inimical to the
thorough absorption of the gold by the mercury; and lastly, the
amalgamation of the gold and mercury.
For general purposes, where the gangue has not been crushed too fine,
I think the Duncan pan will usually be found effective in saving the
concentrates. In theory it is an enlargement of the alluvial miner's
tin dish, and the motion imparted to it is similar to the eccentric
motion of that simple separator.
The calcining may be effectively carried out in an ordinary
reverberatory furnace, the only skill required being to prevent over
roasting and so slagging the concentrates; or not sufficiently
calcining so as to remove all deleterious constituents; the subject,
however, is fully treated in Chapter VIII.
For amalgamating I prefer some form of settler to any further grinding
appliance, but I note also improvements in the rotary amalgamating
barrel, which, though slow, is, under favourable conditions, an
effective amalgamator. The introduction of steam under pressure into
an iron cylinder containing a charge of concentrates with mercury is
said to have produced good results, and I am quite prepared to believe
such would be the case, as we have long known that the application of
steam to ores in course of amalgamation facilitates the process
considerably.
Some seventeen years since I was engaged on the construction of a dry
amalgamator in which sublimated mercury was passed from a retort
through the descending gangue in a vertical cylinder, the material
thence falling through an aperture into a revolving settler, the
object being to save water on mines in dry country. The model, about
quarter size, was completed when my attention was called to an
American invention, in which the same result was stated to be attained
more effectively by blowing the mercury spray through the triturated
material by means of a steam jet. I had already encountered a
difficulty, since found so obstructive by experimentalists in the same
direction, that is, the getting of the mercury back into its liquid
metallic form. This difficulty I am now convinced can be largely
obviated by my own device of using a very weak solution of sulphuric
acid (it can hardly be too weak) and adding a small quantity of zinc
to the mercury. It is perfectly marvellous how some samples of mercury
"sickened" or "floured" by bad treatment, may be brought back to the
bright limpid metal by a judicious use of these inexpensive materials.
Thus it will probably be found practicable to crush dry and amalgamate
semi-dry by passing the material in the form of a thin pasty mass to a
settler, as in the old South American arrastra, and, by slowly
stirring, recover the mercury, and with it the bulk of the gold.
The following is from the /Australian Mining Standard/, and was headed
"Amalgamation Without Overflow":
"Recent experiments at the Ballarat School of Mines have proved that a
deliverance from difficulties is at hand from an unexpected quarter.
The despised Chilian mill and Wheeler pan, discarded at many mines,
will solve the problem, but the keynote of success is amalgamation
without overflow. Dispense with the overflow and the gold is saved.
"Two typical mines--the Great Mercury Proprietary Gold Mine, of
Kuaotunu, N.Z., the other, the Pambula, N.S.W.--have lately been
conducting a series of experiments with the object of saving their
fine gold in an economical manner. The last and best trials made by
these companies were at the Ballarat School of Mines, where
amalgamation without overflow was put to a crucial test, in each case
with the gratifying result that ninety-six per cent of the precious
metal was secured. What this means to the Great Mercury Mine, for
instance, can easily be imagined when it is understood that
notwithstanding all the latest gold-saving adjuncts during the last
six months 1260 tons of ore, worth 4l. 17s. 10 2/3d. a ton, have been
put through for a saving of 1l. 9s. 1 2/3d. only; or in other words
over two-thirds of the gold has gone to waste (for the time being) in
the tailings, and in the tailings at the present moment lie the
dividends that should have cheered shareholders' hearts.
"And now for the /modus operandi/, which, it must be remembered, is
not hedged in by big royalties to any one, rights, patent or
otherwise. The ore to be treated is first calcined, then put through a
rock-breaker or stamper battery in a perfectly dry state. If the
battery is used, ordinary precautions, of course, must be taken to
prevent waste, or the dust becoming obnoxious to the workmen. The ore
is then transferred to the Chilian mill and made to the consistency of
porridge, the quicksilver being added. When the principal work of
amalgamation is done (experience soon teaching the amount of grinding
necessary), from the Chilian mill the paste (so to say) is passed to a
Wheeler or any other good pan of a similar type, when the gold-saving
operation is completed."
This being an experiment in the same direction as my own, I tried it
on a small scale. I calcined some very troublesome ore till it was
fairly "sweet," triturated it, and having reduced it with water to
about the consistency of invalid's gruel, put it into a little berdan
pan made from a "camp oven," which I had used for treating small
quantities of concentrates, and from time to time drove a spray of
mercury, wherein a small amount of zinc had been dissolved, into the
pasty mass by means of a steam jet, added about half an ounce of
sulphuric acid and kept the pan revolving for several hours. The
result was an unusually successful amalgamation and consequent
extraction--over ninety per cent.
Steam--or to use the scientific term, hydro-thermal action--has played
such an important part in the deposition of metals that I cannot but
think that under educated intelligence it will prove a powerful agent
in their extraction. About fourteen years ago I obtained some rather
remarkable results from simply boiling auriferous ferro-sulphides in
water. There is in this alone an interesting, useful, and profitable
field for investigation and experiment.
The most scientific and perfect mode of gold extraction (when the
conditions are favourable) is lixiviation by means of chlorine,
potassium cyanide, or other aurous solvent, for by this means as much
as 98 per cent of the gold contained in suitable ores can be converted
into its mineral salt, and being dissolved in water, re-deposited in
metallic form for smelting; but lode stuff containing much lime would
not be suitable for chlorination, or the presence of a considerable
proportion of such a metal as copper, particularly in metallic form,
would be fatal to success, while cyanide of potassium will also attack
metals other than gold, and hence discount the effect of this solvent.
The earlier practical applications of chlorine to gold extraction were
known as Mears' and Plattner's processes, and consisted in placing the
material to be operated on in vats with water, and introducing
chlorine gas at the bottom, the mixture being allowed to stand for a
number of hours, the minimum about twelve, the maximum forty-eight.
The chlorinated water was then drawn off containing the gold in
solution which was deposited as a brown powder by the addition of
sulphate of iron.
Great improvements on this slow and imperfect method have been made of
late years, among the earlier of which was that of Messrs. Newbery and
Vautin. They placed the pulp with water in a gaslight revolving
cylinder, into which the chlorine was introduced, and atmospheric air
to a pressure of 60 lb. to the square inch was pumped in. The cylinder
with its contents was revolved for two hours, then the charge was
withdrawn and drained nearly dry by suction, the resultant liquid
being slowly filtered through broken charcoal on which the chloride
crystals were deposited, in appearance much like the bromo-chlorides
of silver ore seen on some of the black manganic oxides of the Barrier
silver mines. The charcoal, with its adhering chlorides, was conveyed
to the smelting-house and the gold smelted into bars of extremely pure
metal. Messrs. Newbery and Vautin claimed for their process decreased
time for the operation with increased efficiency.
At Mount Morgan, when I visited that celebrated mine, they were using
what might be termed a composite adaptation process. Their
chlorination works, the largest in the world, were putting through
1500 tons per week. The ore as it came from the mine was fed
automatically into Krom roller mills, and after being crushed and
sifted to regulation gauge was delivered into trucks and conveyed to
the roasting furnaces, and thence to cooling floors, from which it was
conveyed to the chlorinating shed. Here were long rows of revolving
barrels, on the Newbery-Vautin principle, but with this marked
difference, that the pressure in the barrel was obtained from an
excess of the gas itself, generated from a charge of chloride of lime
and sulphuric acid. On leaving the barrels the pulp ran into settling
vats, somewhat on the Plattner plan, and the clear liquid having been
drained off was passed through a charcoal filter, as adopted by
Newbery and Vautin. The manager, Mr. Wesley Hall, stated that he
estimated cost per ton was not more than 30s., and he expected shortly
to reduce that when he began making his own sulphuric acid. As he was
obtaining over 4 oz. to the ton the process was paying very well, but
it will be seen that the price would be prohibitive for poor ores
unless they could be concentrated before calcination.
The Pollok process is a newer, and stated to be a cheaper mode of
lixiviation by chlorine. It is the invention of Mr. J. H. Pollok, of
Glasgow University, and a strong Company was formed to work it. With
him the gas is produced by the admixture of bisulphate of sodium
(instead of sulphuric acid, which is a very costly chemical to
transport) and chloride of lime. Water is then pumped into a strong
receptacle containing the material for treatment and powerful
hydraulic pressure is applied. The effect is stated to be the rapid
change of the metal into its salt, which is dissolved in the water and
afterwards treated with sulphate of iron, and so made to resume its
metallic form.
It appears, however, to me that there is no essential difference in
the pressure brought to bear for the quickening of the process. In
each case it is an air cushion, induced in the one process by the
pumping in of air to a cylinder partly filled with water, and in the
other by pumping in water to a cylinder partly filled with air.
The process of extracting gold from lode stuff and tailings by means
of cyanide of potassium is now largely used and may be thus briefly
described:--It is chiefly applied to tailings, that is, crushed ore
that has already passed over the amalgamating and blanket tables. The
tailings are placed in vats, and subjected to the action of solutions
of cyanide of potassium of varying strengths down to 0.2 per cent.
These dissolve the gold, which is leached from the tailings, passed
through boxes in which it is precipitated either by means of zinc
shavings, electricity, or to the precipitant. The solution is made up
to its former strength and passed again through fresh tailings. When
the tailings contain a quantity of decomposed pyrites, partly
oxidised, the acidity caused by the freed sulphuric acid requires to
be neutralised by an alkali, caustic soda being usually employed.
When "cleaning up," the cyanide solution in the zinc precipitating
boxes is replaced by clean water. After careful washing in the box, to
cause all pure gold and zinc to fall to the bottom, the zinc shavings
are taken out. The precipitates are then collected, and after
calcination in a special furnace for the purpose of oxidising the
zinc, are smelted in the usual manner.
The following description of an electrolytic method of gold deposition
from a cyanide solution was given by Mr. A. L. Eltonhead before the
Engineers' Club of Philadelphia.
A description of the process is as follows:--"The ore is crushed to a
certain fineness, depending on the character of the gangue. It is then
placed in leaching vats, with false bottoms for filtration, similar to
other leaching plants. A solution of cyanide of potassium and other
chemicals of known percentage is run over the pulp and left to stand a
certain number of hours, depending on the amount of metal to be
extracted. It is then drained off and another charge of the same
solution is used, but of less strength, which is also drained. The
pulp is now washed with clean water, which leaches all the gold and
silver out, and leaves the tailings ready for discharge, either in
cars or sluiced away by water, if it is plentiful.
"The chemical reaction of cyanide of potassium with gold is as
follows, according to Elsner:--
2Au + 4KCy + O + H2O = 2KAuCy2 + 2KHO.
That is, a double cyanide of gold and potassium is formed.
"All filtered solutions and washings from the leaching vats are saved
and passed through a precipitating 'box' of novel construction, which
may consist either of glass, iron or wood, and be made in any shape,
either oval, round, or rectangular--if the latter, it will be about 10
ft. long, 4 ft. wide and 1 ft. high--and is partitioned off lengthwise
into five compartments. Under each partition, on the inside or bottom
of the 'box,' grooves may be cut a quarter- to a half-inch deep,
extending parallel with the partitions to serve as a reservoir for the
amalgam, and give a rolling motion to the solution as it passes along
and through the four compartments. The centre compartment is used to
hold the lead or other suitable anode and electrolyte.
"The anode is supported on a movable frame or bracket, so it may be
moved either up or down as desired, it being worked by thumb-screws at
each end.
"The electrolyte may consist of saturated solutions of soluble
alkaline metals and earth. The sides or partitions of each compartment
dip into the mercury, which must cover the 'box' evenly on the bottom
to the depth of about a half-inch.
"Amalgamated copper strips or discs are placed in contact with the
mercury and extended above it, to allow the gold and silver solution
of cyanide to come in contact.
"The electrodes are connected with the dynamo; the anode of lead being
positive and the cathode of mercury being negative. The dynamo is
started, and a current of high amperage and low voltage is generated,
generally 100 to 125 amperes, and with sufficient pressure to
decompose the electrolyte between the anode and the cathode.
"As the gas is generated at the anode, a commotion is created in the
liquid, which brings a fresh and saturated solution of electrolyte
between the electrodes for electrolysis, and makes it continuous in
its action.
"The solution of double cyanide of gold, silver, and potassium, which
has been drained from the leaching vats, is passed over the mercury in
the precipitating 'box' when the decomposition of the electrolyte by
the electric current is being accomplished, the gold and silver are
set free and unite with the mercury, and are also deposited on the
plates or discs of copper, forming amalgam, which is collected and
made marketable by the well known and tried methods. The above
solution is regenerated with cyanide of potassium by the setting free
of the metals in the passage over the 'box.'
"In using this solution again for a fresh charge of pulp, it is
reinforced to the desired percentage, or strengthened with cyanide of
potassium and other chemicals, and is always in good condition for
continuing the operation of dissolving.
"The potassium acting on the water of the solution creates nascent
hydrogen and potassium hydrate; the nascent hydrogen sets free the
metals (gold and silver), which are precipitated into the mercury and
form amalgam, leaving hydrocyanic acid; this latter combines with the
potassium hydrate of the former reaction, thus forming cyanide of
potassium. There are other reactions for which I have not at present
the chemical formulas.
"As the solution passes over the mercury, the centre compartment of
the 'box' is moved slowly longitudinally, which spreads the mercury,
the solution is agitated and comes in perfect contact with the
mercury, as well as the amalgamated plates or discs of copper,
ensuring a perfect precipitation.
"It is not always necessary to precipitate all the gold and silver
from the solution, for it is used over and over again indefinitely;
but when it is required, it can be done perfectly and cheaply in a
very short time.
"No solution leached from the pulp, containing cyanide of potassium,
gold and silver, need be run to waste, which is in itself an enormous
saving over the use of zinc shavings when handling large quantities of
pulp and solution.
"Some of the advantages the electro-chemical process has over other
cyanide processes are: Its cleanliness, quickness of action,
cheapness, and large saving of cyanide of potassium by regeneration;
not wasting the solutions, larger recovery of the gold and silver from
the solutions; the cost of recovery less; the loss of gold, silver,
and cyanide of potassium reduced to a minimum; the use of caustic
alkali in such quantity as may be desired to keep the cyanide solution
from being destroyed by the solidity of the pulp, and also sometimes
to give warmth, as a warm cyanide solution will dissolve gold and
silver quicker than a cold one. These caustic alkalies do not
interfere with or prevent the perfect precipitation of the metals. The
bullion recovered in this process is very fine, while the zinc-
precipitated bullion is only about 700 fine.
"The gold and silver is dissolved, and then precipitated in one
operation, which we know cannot be done in the 'chlorination process';
besides, the cost of plant and treatment is much less in the above-
described process.
"The electro-chemical process, which I have hastily sketched will, I
think, be the future cheap method of recovering fine or flour gold
from our mines and waste tailings or ore dumps.
"Without going into details of cost of treatment, I will state that
with a plant of a capacity of handling 10,000 tons of pulp per month,
the cost should not exceed 8s. per ton, but that may be cheapened by
labour-saving devices. There being no expensive machinery, a plant
could be very cheaply erected wherever necessary."
CHAPTER VIII
CALCINATION OR ROASTING OF ORES
The object of calcining or roasting certain ores before treatment is
to dissipate the sulphur or sulphides of arsenic, antimony, lead,
etc., which are inimical to treatment, whether by ordinary mercuric
amalgamation or lixiviation. The effect of the roasting is first to
sublimate and drive off as fumes the sulphur and a proportion of the
objectionable metals. What is left is either iron oxide, "gossan," or
the oxides of the other metals. Even lead can thus be oxidised, but
requires more care as it melts nearly as readily as antimony and is
much less volatile. The oxides in the thoroughly roasted ore will not
amalgamate with mercury, and are not acted on by chlorine or cyanogen.
To effect the oxidation of sulphur, it is necessary not only to bring
every particle of sulphur into contact with the oxygen of the air, but
also to provide adequate heat to the particles sufficient to raise
them to the temperature that will induce oxidation. No appreciable
effect follows the mere contact of air with sulphur particles at
atmospheric temperature; but if the particles be raised to a
temperature of 500 degrees Fahr., the sulphur is oxidised to the
gaseous sulphur dioxide. The same action effects the elimination of
the arsenic and antimony associated with gold and silver ores, as when
heated to a certain constant temperature these metals readily oxidise.
The science of calcination consists of the method by which the
sulphide ores, having been crushed to a proper degree of fineness, are
raised to a sufficient temperature and brought into intimate contact
with atmospheric air.
It will be obvious then that the most effective method of roasting
will be one that enables the particles to be thoroughly oxidised at
the lowest cost in fuel and in the most rapid manner.
The roasting processes in practical use may be divided into three
categories:
/First or A Process./--Roasting on a horizontal and stationary hearth,
the flame passing over a mass of ore resting on such hearth. In order
to expose the upper surface of the ore to contact with air the
material is turned over by manual labour. This furnace of the
reverberatory type is provided with side openings by which the turning
over of the ore can be manually effected, and the new ore can be
charged and afterwards withdrawn.
/Second or B Process./--Roasting in a revolving hearth placed at a
slight incline angle from the horizontal. The furnace is of
cylindrical form and is internally lined with refractory material. It
has projections that cause the powdered ore to be lifted above the
flame, and, at a certain height, to fall through the flame and so be
rapidly raised to the temperature required to effect the oxidation of
the oxidisable minerals which it is desired to extract.
The rate, or speed, of revolution of this revolving furnace obviously
depends upon the character of the ore under treatment; it may vary
from two revolutions per minute down to one revolution in thirty
minutes. Any kind of fuel is available, but that of a gaseous
character is stated to be by far the most efficient.
Any ordinary cylinder of a length of 25 ft., and a diameter of 4 ft. 6
in., inclined 1 ft. 6 in. in its length, will calcine from 24 to 48
tons per diem.
Another form of rotating furnace is one in which the axis is
horizontal. It is much shorter than the inclined type, and the feeding
and removal of the ore is effected by the opening of a retort lid door
provided at the side of the furnace. Openings provided at each end of
the furnace permit the passage of the flame through it, and the
revolution of the furnace turns over the powdered ore and brings it
into more or less sustained contact with the oxidising flame. The
exposure of the ore to this action is continued sufficiently long to
ensure the more or less complete oxidation of the ore particles.
/Third or C Process./--In this process the powdered ore is allowed to
fall in a shower from a considerable height, through the centre of a
vertical shaft up which a flame ascends; the powdered ore in falling
through the flame is heated to an oxidising temperature, and the
sulphides are thus depleted of their sulphur and become oxides.
Another modification of this direct fall or shaft furnace is that in
which the fall of the ore is checked by cross-bars or inclined plates
placed across the shaft; this causes a longer oxidising exposure of
the ore particles.
When the sulphur contents of pyritous ores are sufficiently high, and
after the ore has been initially fired with auxiliary carbonaceous
fuel, it is unnecessary, in a properly designed roasting furnace, to
add fuel to the ore to enable the heat for oxidation to be obtained.
The oxidation or burning of the sulphur will provide all the heat
necessary to maintain the continuity of the process. The temperature
necessary for effecting the elimination of both sulphur and arsenic is
not higher than that equivalent to dull red heat; and provided that
there is a sufficient mass of ore maintained in the furnace, the
potential heat resulting from the oxidation of the sulphur will alone
be adequate to provide all that is necessary to effect the
calcination.
TYPES OF FURNACES OF THE DIFFERENT CLASSES
THAT ARE IN ACTUAL USE.
"A" OR REVERBERATORY CLASS.
The construction of this furnace has already been sufficiently
described. If the roasting is performed in a muffle chamber, the
arrangement employed by Messrs. Leach and Neal, Limited, of Derby, and
designed by Mr. B. H. Thwaite, C.E., can be advantageously employed in
this furnace, which is fired with gaseous fuel. The sensible heat of
the waste gases is utilised to heat the air employed for combustion;
and by a controllable arrangement of combustion, a flame of over 100
feet in length is obtained, with the result that the furnace from end
to end is maintained at a uniform temperature. By this system, and
with gaseous fuel firing, a very considerable economy in fuel and in
repairs to furnace, and a superior roasting effect, have been
obtained.
Where the ordinary reverberatory hearth is fired with solid coal from
an end grate, the temperature is at its maximum near the firing end,
and tails off at the extreme gas outlet end. The ores in this furnace
should therefore be fed in at the colder end of the hearth and be
gradually worked or "rabbled" forward to the firing end.
One disadvantage of the reverberatory furnace is the fact that it is
impossible to avoid the incursion of air during the manual rabbling
action, and this tends to cool the furnace.
The cost of roasting, to obtain the more or less complete oxidation,
or what is known in mining parlance as a "sweet roast" (because a
perfectly roasted ore is nearly odourless) varies considerably, the
variation depending of course upon the character of the ore and the
cost of labour and fuel.
There are several modifications of the reverberatory furnace in use,
designed mechanically to effect the rabbling. One of the most
successful is that known as the Horse-shoe furnace. In plan the hearth
of the furnace resembles a horse-shoe.
The stirring of the ore over the hearth is effected by means of
carriages fixed in the centre of the furnace and having laterally
projecting arms, carrying stirrers, that move along the hearth and
turn over the pulverised ore.
In operation, half the carriages are traversing the furnace, and half
are resting in the cooling space, so that a control over the
temperature of the stirrers is established.
This furnace is stated to be more economical in labour than other
mechanically stirred reverberatory furnaces, and there is also said to
be an economy in fuel.
Usually the mechanical stirring furnaces give trouble and should be
avoided, but the horse-shoe type possesses qualifications worthy of
consideration.
"B."--THE REVOLVING CYLINDER FURNACE.
Of these the best known to me are: The Howell-White, the Bruckner, the
Thwaite-Denny, and the Molesworth.
The Bruckner is a cylinder, turning on the horizontal axis and carried
by four rollers.
The batch of ore usually charged into the two charging hoppers weighs
about four tons. When the two charging doors are brought under the
hopper mouth, the contents of the hopper fall directly into the
cylinder.
The ends or throats of the furnace are reduced just sufficiently to
allow the flame evolved from a grated furnace to pass completely
through the cylinder.
A characteristic size for this Bruckner furnace is one having a length
of 12 feet and a diameter of 6 feet. A furnace of this capacity will
have an inclusive weight (iron and brickwork) of 15 tons.
The time of operation, with the Bruckner, will vary with the character
of the ore under treatment and the nature of the fuel employed. Four
hours is the minimum and twelve hours should be the maximum time of
operation.
By the addition of common salt with the batch of ore, such of its
constituents as are amenable to the action of chlorine are chlorinated
as well as freed from sulphur.
Where the ore contains any considerable quantity of silver which
should be saved, the addition of the salt is necessary as the silver
is very liable to become so oxidised in the process of roasting as to
render its after treatment almost impossible. I know a case in point
where an average of nearly five ounces of silver to the ton, at that
time worth 30s., was lost owing to ignorance on this subject. Had the
ore been calcined with salt, NaCl, the bulk of this silver would have
been amalgamated and thus saved. It was the extraordinary fineness of
the gold saved by amalgamation as against my tests of the ore by fire
assay that put me on the track of a most indefensible loss.
/The Howell-White Furnace./--This furnace consists of a cast iron
revolving cylinder, averaging 25 feet in length and 4 ft. 4 in. in
diameter, which revolves on four friction rollers, resting on truck
wheels, rotated by ordinary gearing.
The power required for effecting the revolution should not exceed four
indicated horse-power.
The cylinder is internally lined with firebrick, projecting pieces
causing the powdered ore to be raised over the flame through which it
showers, and is thereby subjected to the influence of heat and to
direct contact oxidation.
The inclination of the cylinder, which is variable, promotes the
gradual descension of the ore from the higher to the lower end. It is
fed into the upper end, by a special form of feed hopper, and is
discharged into a pit at the lower end, from which the ore can be
withdrawn at any time.
The gross weight of the furnace, which is, however, made in segments
to be afterwards bolted together, is some ninety to one hundred tons.
The furnace is fired with coal on a grated hearth, built at the lower
end; it is more economical both in fuel and in labour than an ordinary
reverberatory furnace.
/The Thwaite-Denny Revolving Furnace./--This new type of furnace,
which is fired with gaseous fuel, is stated to combine the advantages
of the Stetefeldt, the Howell-White, and the Bruckner.
It is constructed as follows:--Three short cylinders, conical in shape
and of graduated dimensions, are superimposed one over the other,
their ends terminating in two vertical shafts of brickwork, by which
the three cylinders are connected. The powdered ore is fed into the
uppermost cylinder and gravitates through the series. The highest
cylinder is the largest in diameter, the lowest the smallest.
The gas flame, burnt in a Bunsen arrangement, enters the smallest end
of the lowest cylinder and passes through it; then returns through the
series and the ore is reduced by the expulsion of its sulphur,
arsenic, etc., as it descends from the top to the bottom. The top
cylinder is made larger than the one below it and the middle cylinder
is made larger than the lowest one in proportion to the increased bulk
of gases and ore.
The powdered ore in descending through the cylinders is lifted up and
showers through the flame, falling in its descent a distance of over
1000 feet. By the time it reaches the bottom the ore is thoroughly
roasted.
Provision is made for the introduction of separate supplies of air and
gas into each cylinder; this enables the oxidising treatment to be
controlled exactly as desired so as to effect the best results with
all kinds of ore. Each cylinder is driven from its own independent
gearing, and the speed of each cylinder can be varied at will.
The output of this type of furnace, the operations of which appear to
be more controllable than those of similar appliances, depends, of
course, upon the nature of the ore, but may be considered to range
within the limits of twelve to fifty tons in twenty-four hours, and
the cost of roasting will vary from 2s. 6d. to 4s. per ton, depending
upon the quality of ore and of fuel.
The gaseous fuel generating system permits not only the absolute
control over the temperature in the furnace, but the use of the
commonest kinds of coal, and even charcoal is available.
The power required to drive the Thwaite-Denny furnace is four
indicated horse-power.
/The Molesworth Furnace/ also is a revolving cylindrical appliance,
which, to say the least of it, is in many respects novel and
ingenious. It consists of a slightly cone-shaped, cast-iron cylinder
about fourteen feet long, the outlet end being the larger to allow for
the expansion of the gases. Internal studs are so arranged as to keep
the ore agitated; and spiral flanges convey it to the outlet end
continually, shooting it across the cylinder. The cylinder is encased
in a brick furnace. The firing is provided from /outside/, the
inventor maintaining that the products of combustion are inimical to
rapid oxidisation, to specially promote which he introduces an excess
of oxygen produced in a small retort set in the roof of the furnace
and fed from time to time with small quantities of nitrate of soda and
sulphuric acid. Ores containing much sulphur virtually calcine
themselves. I have seen this appliance doing good work. The
difficulties appeared to be principally mechanical.
There are other furnaces which work with outside heat, but I have not
seen them in action.
"C."--SHAFT TYPE OF FURNACE
In one form of this furnace, instead of allowing the ore to descend in
a direct clear fall the descent is impeded by inclined planes placed
at different levels in the height of the shaft, the ore descending
from one plane to the other.
/The Stetefeldt Shaft Furnace./--Although very expensive in first
cost, has many advantages. No motive power is required and the
structure of the furnace is of a durable character. Its disadvantages
are:--Want of control, and the occasionally imperfect character of the
roasting originating therefrom.
Three sizes of Stetefeldt's furnaces are constructed:
The largest will roast from 40 to 80 tons per diem.
The intermediate will roast from 20 to 40 tons per diem.
The smallest will roast from 10 to 20 tons per diem.
A good furnace should bring down the sulphur contents even of
concentrates so as to be innocuous to mercuric amalgamation. The
sulphur left in the ore should never be allowed to exceed two per
cent.
A forty per cent pyritous or other sulphide ore should be roasted in a
revolving furnace in thirty to forty minutes, and without any
auxiliary fuel.
For ordinary purposes a 40-foot chimney is adequate for furnace work;
such a chimney four feet square inside at the base, tapering to 2' 6"
at the summit, will require 12,000 red bricks, and 1500 fire-bricks
for an internal lining to a height of 12 feet from the base of the
chimney shaft.
When second-hand Lancashire or Cornish boiler flues are available,
they make admirable and inexpensive chimneys. The advantage of
wrought-iron or steel chimneys lies in the convenience of removal and
erection. They should be made in sections of 20 feet long, three steel
wire guy-ropes attached to a ring, riveted to a ring two-thirds of the
height of the chimney, and attached to holdfasts driven into the
ground; tightening couplings should be provided for each wire.
Flue dust depositing chambers should be built in the line of the flues
between the furnace and the chimney; they consist simply of carefully
built brick chambers, with openings to enable workmen to enter and
rapidly clear away the deposited matters. The chambers, three or four
times the cross sectional area of the chimney flue, and ten to twenty
feet long, can be built of brickwork, set in cement; the walls are
provided with a cavity, filled with sand or Portland cement, so that
there will be no danger of the incursion of air. In all furnace work
the greatest possible precautions should be taken to prevent the least
cracking of either joints or bricks. It is surprising how much the
inadequate draft of a good chimney is due to cracks or orifices in the
flues; and therefore a competent furnace-man should see to it that his
flues are thoroughly sound, and free from openings through which the
air can enter.[*]
[*] For full details of the most recent improvements in the cyanide
process and in other methods of extraction, the reader is referred
to Dr. T. K. Rose's "Metallurgy of Gold," third edition.
CHAPTER IX
MOTOR POWER AND ITS TRANSMISSION
It is unnecessary to describe methods by which power for mining
purposes has been obtained--that is, up to within the last five years
--beyond a general statement, that when water power has been available
in the immediate locality of the mine, this cheap natural source of
power has been called upon to do duty. Steam has been the alternative
agent of power production applied in many different ways, but
labouring under as many disadvantages, chief of which are lack of
water, scarcity of fuel and cost of transit of machinery. Sometimes
condensing steam-engines have been employed. For the generation of
steam the semi-portable and semi-tubular have been the type of boiler
that has most usually been brought into service. Needless to say, when
highly mineralised mine water only is available the adoption of this
class of boiler is attended with anything but satisfactory results.
Recently, however, there is strong evidence that where steam is the
power agent to be employed the water-tube type of boiler is likely to
be employed, and to the exclusion of all other forms of apparatus for
the generation of steam. The advantages of this type, particularly the
tubulous form (or a small water tube), made as it is in sections,
offers unrivalled facilities for transport service. The heaviest parts
need not exceed 3 cwt. in weight, and require neither heavy nor yet
expensive brickwork foundations.
WATERLESS POWER.
The difficulties in finding water to drive a steam plant are often of
such a serious character as to involve the abandonment of many payable
mines; therefore, a motive power that does not require the aqueous
agent will be a welcome boon.
It will be a source of gratification to many a gold-claim holder to
know that practical science has enabled motive power to be produced
without the necessity of water, except a certain very small quantity,
which once supplied will not require to be renewed, unless to
compensate for the loss due to atmospheric evaporation.
Any carbonaceous fuel, such as, say, lignite, coal, or charcoal, can
be employed. The latter can be easily produced by the method described
in the Chapter on "Rules of Thumb," or by building a kiln by piling
together a number of trunks of trees, or fairly large-sized branches,
cut so that they can be built up in a compact form. The pile, after
being covered with earth, is then lighted from the base, and if there
are no inlets for the air except the limited proportion required for
the smouldering fire at the base, the whole of the timber will be
gradually carbonised to charcoal of good quality, which is available
for the waterless power plant.
The waterless power plant consists of two divisions: First, a gas
generating plant; secondly, an internal combustion or gas engine in
which the gas is burnt, producing by thermo-dynamic action the motive
power required. The system known as the Thwaite Power Gas System is
not only practically independent of the use of water, but its
efficiency in converting fuel heat into work is so high that no
existing steam plant will be able to compete with it.
The weight of raw timber, afterwards to be converted into charcoal,
that will be required to produce an effective horse-power for one hour
equals 7 lb.
If coal is the fuel 1 1/3 lb. per E.H.P. for one hour's run.
If lignite is the fuel 2 1/2 lb. per E.H.P. for one hour's run.
The plant is simple to work, and as no steam boiler is required the
danger of explosions is removed. No expensive chimney is necessary for
the waterless power plant.
Where petroleum oil can be cheaply obtained, say for twopence per
gallon, one of the Otto Cycle Oil Engines, for powers up to 20
indicated horse-power, can be advantageously employed.
These engines have the advantage of being a self-contained power,
requiring neither chimney nor steam boiler, and may be said to be a
waterless power. The objection is the necessity to rely upon oil as
fuel, and the dangers attending the storage of oil. A good oil engine
should not require to use more than a pint of refined petroleum per
indicated horse-power working for one hour.
Fortunately for the mining industry electricity, that magic and
mysterious agency, has come to its assistance, in permitting motive
power to be transmitted over distances of even as much as 100 miles
with comparatively little loss of the original power energy.
Given, that on a coal or lignite field, or at a waterfall, 100 horse-
power is developed by the combustion of fuel or by the fall of water
driving a turbine, this power can be electrically transmitted to a
mine or GROUP OF MINES, say 100 miles away, with only a loss of some
30 horse-power. For twenty miles the loss on transmission should not
exceed 15 horse-power so that 70 and 85 horse-power respectively are
available at the mines. No other system offers such remarkable
efficiencies of power transmission. The new Multiphase Alternating
Electric Generating and Power Transmission System is indeed so perfect
as to leave practically no margin for improvement.
The multiphase electric motor can be directly applied to the stamp
battery and ore-breaker driving-shaft and to the shaft of the
amalgamating pans.
APPROXIMATE POWER REQUIRED TO DRIVE THE MACHINERY OF A MINE.
Rock breaker 10 effective horse-power
Amalgamating pan 5 effective horse-power
Grinding pan 6 effective horse-power
Single stamp of 750 lb. dropping
90 times per minute 1.25 effective horse-power
Settlers 4 effective horse-power
Ordinary hoisting lift 20 effective horse-power
Allow 10 per cent in addition for overcoming friction.
Besides this electrical distribution power, which should not cost more
than three farthings per effective horse-power per hour, the
electrical energy can be employed for lighting the drives and the
shafts of the mine. The modern electrical mine lamps leave little to
be desired. Also it is anticipated that once the few existing
difficulties have been surmounted electric drilling will supplant all
other methods.
Electric power can be employed for pumping, for shot firing, for
hauling, and for innumerable purposes in a mine.
Electricity lends itself most advantageously to so many and varied
processes, even in accelerating the influence of cyanide solutions on
gold, and in effecting the magnetic influence on metallic particles in
separating processes; while applied to haulage purposes, either on
aerial lines or on tram or railroads, it is an immediate and striking
success.
It is anticipated that in the near future the mines on the Randt,
South Africa, will be electrically driven from a coalfield generating
station located on the coalfields some thirty miles from Johannesburg.
Such a plant made up of small multiples of highly efficient machines
will enable mine-owners to obtain a reliable power to any extent at
immediate command and at a reasonable charge in proportion to the
power used. This wholesale supply of power will be a godsend to a new
field, enabling the opening up to be greatly expedited; and no
climatic difficulties, such as dry seasons, or floods, need interfere
with the regular running of the machinery. The same system of power-
generation at a central station is to be applied to supply power to
the mines of Western Australia.
CHAPTER X
COMPANY FORMATION AND OPERATIONS
All the world over, the operation of winning from the soil and
rendering marketable the many valuable ores and mine products which
abound is daily becoming more and more a scientific business which
cannot be too carefully entered into or too skilfully conducted. The
days of the dolly and windlass, of the puddler, cradle, and tin dish,
are rapidly receding; and mining, either in lode or alluvial working,
is being more generally recognised as one of the exact sciences. In
the past, mining has been carried on in a very haphazard fashion, to
which much of its non-success may be attributed.
But the dawn of better days has arrived, and with the advent of
schools of mines and technical colleges there will in future be less
excuse for ignorance in this most important industry.
This chapter will be devoted to Company formation and working, in
which mistakes leading to very serious consequences daily occur.
It is not necessary to go deeply into the question why, in the mining
industry more than any other, it should be deemed desirable as a
general rule to carry on operations by means of public Companies, but,
as a matter of fact, few names can be mentioned of men who mine
extensively single handed. Yet, risky as it is, mining can hardly be
said to be more subject to unpreventable vicissitudes than, say,
pastoral pursuits, in which private individuals risk, and often lose
or make, enormous sums of money.
However, it is with Mining Companies we are now dealing, and with the
errors made in the formation and after conduct of these Associations.
The initial mistake most often made is that sufficient working capital
is not called up or provided in the floating of the Company. Promoters
trust to get sufficient from the ground forthwith to ensure further
development; the consequence being that, as nearly 99 per cent of
mining properties require a very considerable expenditure of capital
before permanent profits can be relied on, the inexperienced
shareholders who started with inflated hopes of enormous returns and
immediate dividends become disheartened and forfeit their shares by
refusing to pay calls, and thus many good properties are sacrificed.
In England, the companies are often floated fully paid-up, but the
same initial error of providing too little money for the equipment and
effective working of the mine is usually fallen into.
Again, far too many Companies are floated on the report of some self-
styled mining expert, often a man, who, like the schoolmaster of the
last century, has qualified for the position by failing in every other
business he has attempted. These men acquire a few geological and
mining phrases, and by more or less skilfully interlarding these with
statements of large lodes and big returns they supply reports
seductive enough to float the most worthless properties and cause the
waste of thousands of pounds. But the trouble does not end here.
When the Company is to be formed, some lawyer, competent or otherwise,
is instructed to prepare articles of association, rules, etc.; which,
three times out of four, is accomplished by a liberal employment of
scissors and paste. Such rules may, or may not, be suited to the
requirements of the organisation. Generally no one troubles much about
the matter, though on these rules depends the future efficient working
of the Company, and sometimes its very existence.
Then Directors have to be appointed, and these are seldom selected
because of any special knowledge of mining they may possess, but as a
rule simply because they are large shareholders or prominent men whose
names look well in a prospectus. These gentlemen forthwith engage a
Secretary, usually on the grounds that he is the person who has
tendered lowest, to provide office accommodation and keep the
accounts; and not from any particular knowledge he has of the true
requirements of the position.
The way in which some Directors contrive to spend their shareholders'
money is humorously commented on by a Westralian paper which describes
a great machinery consignment lately landed in the neighbourhood of
the Boulder Kalgoorlie.
"It would seem as if the purchaser had been let loose blindfold in a
prehistoric material-founder's old iron yard, and having bought up the
whole stock, had shipped it off. The feature of the entire
antediluvian show is the liberal allowance of material devoted to
destruction. Massive kibbles, such as were used in coal mines half a
century ago, are arranged alongside a winding engine, built in the
middle of the century, and evidently designed for hauling the kibbles
from a depth of 1000 feet. Nothing less than horse-power will stir the
trucks for underground use, and their design is distinctly of the
antique type. The engine is built to correspond--of a kind that might
have served to raise into position the pillars of Baalbec, and the
mass of metal in it fairly raises a blush to the iron cheek of frailer
modern constructions. The one grand use to which this monster could be
put would be to employ it as a kedge for the Australian continent in
the event of it dragging its present anchors and drifting down south,
but as modern mining machinery the whole consignment is worth no more
than its value as scrap-iron, which in its present position is a
fraction or two less than nothing."
Next, a man to manage the mine has to be obtained, and some one is
placed in charge, of whose capabilities the Directors have no direct
knowledge. Being profoundly ignorant of practical mining they are
incompetent to examine him as to his qualifications, or to check his
mode of working, so as to ascertain whether he is acting rightly or
not. All they have to rely on are some certificates often too
carelessly given and too easily obtained. Finally, quite a large
proportion of the allottees of shares have merely applied for them
with the intention of selling out on the first opportunity at a
premium, hence they have no special interest in the actual working of
the mine.
Now let us look at the prospects of the Association thus formed. The
legal Manager or Secretary, often a young and inexperienced man, knows
little more than how to keep an ordinary set of books, and not always
that. He is quite ignorant of the actual requirements of the mine, or
what is a fair price to pay for labour, appliances, or material. He
cannot check the expenditure of the Mining Manager, who may be a rogue
or a fool or both, for we have had samples of all sorts to our sorrow.
The Directors are in like case. Even where the information is honestly
supplied, they cannot judge whether the work is being properly carried
out or is costing a fair price, and the Mining Manager is left to his
own devices, with no one to check him nor any with whom he can consult
in specially difficult cases. Thus matters drift to the almost certain
conclusion of voluntary or compulsory winding up; and so many a good
property is ruined, and promising mines, which have never had a
reasonable trial, are condemned as worthless. But let us ask, would
any other business, even such as are less subject to unforeseen
vicissitudes than mining, succeed under similar circumstances?
It is now very generally agreed that to the profitable development of
mining new countries, at all events, must look mainly for prosperity,
while other industries are growing. Therefore, we cannot too seriously
consider how we may soonest make our mines successful.
What is the remedy for the unsatisfactory state of affairs we have
experienced? The answer is a more practical system of working from the
inception. Although it may evoke some difference of opinion I consider
it both justifiable and desirable that the State should take some
oversight of mining matters, at all events in the case of public
Companies. It would be a salutary rule that the promoters of any
mining undertaking should, before they are allowed to place it on the
market, obtain and pay for the services of a competent Government
Mining Inspector, who need not necessarily be a Government officer,
but might, like licensed surveyors, be granted a certificate of
competency either by a School of Mines or by some qualified Board of
Examiners. The certificate of such Inspector that the property was as
represented, should be given before the prospectus was issued. It is
arguable whether even further oversight might not be properly be taken
by the State and the report of a qualified officer be compulsory that
the property was reasonably worth the value placed upon it in the
prospectus.
Probably it will be contended that such restrictions would be an undue
interference with private rights, and the old aphorism about a fool
and his folly will be quoted. There are doubtless fools so infatuated
that if they were brayed in a ten hundred-weight stamp-battery the
"foolishness that had not departed from them" would give a highly
payable percentage to the ton. Yet the State in other matters tries by
numerous laws to protect such from their folly. A man may not sell a
load of wood without the certificate from a licensed weighbridge or a
loaf of bread without, if required, having to prove its weight; and we
send those to gaol who practise on the credulity and cupidity of fools
by means of the "confidence trick." Why not, therefore, where
interests which may be said to be national are involved, endeavour to
ensure fair dealing?
Then with regard to the men who are to manage the mines, seeing that a
man may not become captain or mate of a river steamboat without some
certificate on competency, nor drive her engines before he has passed
an examination to prove his fitness, surely it is not too much to say
that the mine manager or engineer, to whose care are often confided
the lives of hundreds of men, and the expenditure of thousands of
pounds, should be required to obtain a recognised diploma to prove his
qualifications. The examinations might be made comparatively easy at
first, but afterwards, when by the establishment of Schools and Mines
the facilities have been afforded for men to thoroughly qualify, the
standard should be raised; and after a date to be fixed no man should
be permitted to assume the charge of a mine or become one of its
officers without a proper certificate of competency from some
recognised School of Mines or Technical College. The effect of such a
regulation would in a few years produce most beneficial results.
In New Zealand, whose "progressive" legislature I do not generally
commend, they have, in the matter of mine management, at all events,
taken a step in the right direction. There a mine manager, before he
obtains his certificate, must have served at least two years
underground, and has to pass through a severe examination, lasting for
days, in all subjects relating to mining and machinery connected with
mining. In addition, he must prove his capacity by making an
underground survey, and then plotting his work. The examination is a
stiff one, as may be judged from the fact that between 1886 and 1891,
only 27 candidates passed. Then the conditions were made easier, and
from that date to 1895, 19 passed. Of the 46 students who gained
first-class honours, 30 have left for South Africa or Australia, in
both of which countries New Zealand certificated men are held in high
estimation.
But returning to the formation of the Company, care should be taken in
appointing Directors that at least one member of the Board is selected
on account of his special technical knowledge of mining, and others
for their special business capacity. The ornamental men with high
sounding names should not be required in legitimate ventures. Also, it
is most important that the business Manager or Secretary should be a
specially qualified man, who by experience has learned what are the
requirements of a mine doing a certain amount of work, so that a
proper check may be kept on the expenses. The more Companies such a
Secretary has the better, as one qualified man can supervise a large
staff of clerks, who would themselves be qualifying for similar work,
and gaining a useful and varied experience of mining business. An
office of this description having charge of a large number of mines
is, in its way, a technical school, and lads trained therein would be
in demand as mine pursers, a very responsible and necessary officer in
a big mine.
With respect to the men to whom the actual mining and treatment of
ores and machinery is committed the greatest mistakes of the past have
been that too much has been required from one man, a combination not
to be found probably in one man in a thousand. Such Admirable
Crichtons are rare in any profession or business, and that of mining
is no exception. Men who profess too much are to be distrusted. Your
best men are they who concentrate their energies and intellects in
special directions. The Mining Manager should, if possible, be chosen
from men holding certificates of competency from some technical mining
school and, of course, should, in addition, have some practical
experience, not necessarily as Head Manager. He should understand
practical mine surveying and calculation of quantities, be able to
dial and plot out his workings, and prepare an intelligible plan
thereof for the use of the Directors, and should understand sufficient
of physics, particularly pneumatics and hydraulics, to ensure
thoroughly efficient pumping operations without loss of power from
unnecessarily heavy appliances. Any other scientific knowledge
applicable to his business which he may have acquired will tell in his
favour, but he must, above all things, be a thoroughly practical man.
Such men will in time be more readily procurable, as boys who have
passed through the various Schools of Mines will be sent to learn
their business practically at the mines just as we now, having given a
lad a course of naval instruction, send him to sea to learn the
practical part of his life's work.
But, of course, more is wanted on a mine than a man who can direct the
sinking of shafts, driving of levels, and stoping of the lode. Much
loss and disappointment have resulted in the past from unsuitable,
ineffective, or badly designed and erected machinery, whether for
working the mine or treating the ores. To obviate this defect a first-
class mining engineer is required.
Then, also, day by day we are more surely learning that mining in all
its branches is a science, and that the treatment of ores and
extraction of the metals is daily becoming more and more the work of
the laboratory rather than of the rule-of-thumb procedure of the past.
Every mine, whether it be of gold, silver, tin, copper, or other
metal, requires the supervision of a thoroughly qualified metallurgist
and chemist, and one who is conversant with the newest processes for
the extraction of the metals from their ores and matrices.
It has then been stated that to ensure effective working each mine
requires, in addition to competent directors, a business manager,
mining-manager, and assistants, engineer, chemist, and metallurgist,
with assistant assayers, etc., all highly qualified men. But it will
be asked, how are many struggling mines in sparsely populated
countries to obtain the services of all these eminent scientists? The
reply is by co-operation. One of the most ruinous mistakes of the past
has been that each little mining venture has started on an independent
course, with different management, separate machinery, etc. Can it
then be wondered at that our gold-mining is not always successful?
Under a co-operative system all that each individual mine would
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