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 re-deposited in the metallic form by
means of well-known reagents.
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 precious
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 gold-dust, 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 (Fig. 8) 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 (cut off in the manner shown in diagram)
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.
[Illustration: FIG. 8. LONG TOM.]
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 and
illustrated in Chapter XII., 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.
[Illustration: FIG. 9. FLUME, GREAT SOUTHERN GOLD-FIELD,
SHOALHAVEN, N.S.W.]
[Illustration: FIG. 10. MONITOR AT WORK, GREAT SOUTHERN
GOLD-FIELD.]
The water is conveyed in flumes (Fig. 9), 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 (Fig. 10). A “face” is formed in
the drift, and the water played against the lower portion of the ledge,
which is quickly undermined, falls, and is soon 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 are
auriferous from grass roots to bed rock, extend for nearly fifty miles,
and are in places as much as 300 feet deep. Want of capital and want of
knowledge has hitherto prevented them 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 Élévateur.”
The first is in some respects on the principle of the Huntingdon Mill.
The latter, if the inventor is to be believed, and the results seem to
show he may, 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 (Fig. 11) 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 (Fig. 11).
[Illustration: FIG. 11. “LEMICHEL” SYPHON.]
[Illustration: FIG. 12. “GRIFFIN” MILL.]
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 1½ to 2½ tons per hour, or, say roughly, 150 tons
per week.
The Huntingdon mill (Fig. 13) 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.
[Illustration: FIG. 13. HUNTINGDON MILL.]
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 necessary.
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.
[Illustration: FIG. 14. STONE-BREAKER.]
[Illustration: FIG. 15. STONE-BREAKER (SECTIONAL VIEW).]
The Grusonwerk Ball Mills (Fig. 16), made by Krupp of Germany, as also
that made by the Austral Otis Company, Melbourne, are fast and excellent
triturating appliances for either wet or dry working, but are specially
suitable 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.
[Illustration: FIG. 16. GRUSONWERK BALL MILL.]
Probably more mines have been ruined by bad mill management 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 management 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.
[Illustration: FIG. 17. DOUBLE FAULTED LODE.]
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 (Fig. 17), 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 highest 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” (Fig. 18),
and “Challenge” (Fig. 19) 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. A remedy for this is suggested (p. 148).
[Illustration: PLATE IV.--TAIL RACE _To face p._ 74]
[Illustration: FIG. 18. TULLOCH AUTOMATIC ORE FEEDER.]
[Illustration: FIG. 19. CHALLENGE AUTOMATIC ORE FEEDER.]
If local conditions are favourable and the management good, some mines
have paid well with a yield of from 1½ to 4 dwts. of gold per ton of
ore; but others have not paid with much higher yields in test crushings.
Great judgment is required in selecting the best possible site for the
class of crushing machinery decided upon. The crusher should be as high
as can be managed so as to give a good clear run for the tailings and
room for the concentrators and amalgamators, &c., below the copper
plates and blanket strakes. Solid, strong foundations are essential;
many failures are due to a neglect of this point.
In a stamp mill (Fig. 20) 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. Fig. 21 represents a 10-head stamp mill.
My experience has been that the most effective weight for stamps and
height for drop largely depends on the nature of the rock foundation. I
have usually found that with medium stamps, say 7½ to 8 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, 10 cwt. and even heavier.
Great improvements have been made in stamp mills since the sixteenth
century, as is evident by comparing Fig. 21A with Figs. 20 and 21; even
in the writer’s time they have been considerable.
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 lbs. 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
× 5 × 0·75 × 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.
[Illustration: FIG. 20. STAMP MILL, SHOWING PART OF FOUNDATIONS.]
[Illustration: FIG. 21. 10-HEAD STAMP MILL.]
The total weight of a battery, including stamper box, stampers, &c, may
be roughly estimated at about 1 ton per stamp. Medium weight stampers,
including shank, cam, disc, head, and shoe, weigh from 600-700 lbs., and
need about 3/4 h.-p. to work them.
[Illustration: FIG. 21A. SIXTEENTH CENTURY STAMP MILL.]
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, that depends
largely on the size of the gold particles contained in the gangue. The
finer the particles the closer must be the mesh, so that 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 specially
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, 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 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.
I have a device at present unpatented which will do away with the
necessity for blanket tables or copper plates, as by the expenditure of
from 1/2 to 1 h.-p. every particle of free gold after leaving the boxes
must come within the embrace of a mercury amalgam. No cleaning of plates
is required.
An ancient Egyptian gold washing table is shown in Fig. 22. It is a
representation of an old stone table such as is referred to in p. 2,
which 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 arrastra, 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 in the riffles, for, as
previously stated, this method was known to the ancients.
[Illustration: FIG 22. WASHING TABLE OF STONE WITH RIFFLES.]
At a mine at 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 was found best to crush without mercury,
then run the pulp into pans, where it was concentrated. The concentrates
were calcined in a common reverberatory furnace, and afterwards
amalgamated with mercury in a special pan; the results were very
satisfactory; but it does not follow that this process would be the most
suitable for a slightly different lode stuff.
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; it should
not carry the stuff off too quickly nor allow the troughs and plates to
be choked. 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 effectively crush the gangue, 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 gold shall be
taken up by 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, &c., when the result will not be
satisfactory, but may entail great loss by the escape of floured mercury
carrying with it particles of gold. Intelligent 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 stamper box or
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 to 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, and the top plate “fed”
from time to time 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 half gallon 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 automatic appliance
sometimes arrests 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, &c.
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 india-rubber or soft pine wood.
[Illustration: FIG. 23. SCRAPER.]
When crushing rich ore, and you want to get nearly all the gold off your
plates, the scraper (Fig. 23) 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, &c., 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 the first 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_{2}, sodium NaCl, and magnesium MgCl_{2}, 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 ” ”
Sulphate of Calcium CaSO₄ 81·71 ” ”
Chloride of Calcium CaCl₂ 2797·84 ” ”
Chloride of Magnesium MgCl₂ 610·13 ” ”
Chloride of Sodium or
Common Salt NaCl 5072·65 ” ”
----------
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
than 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 proved 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 were 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