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 what has since become a very prominent Australian 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, &c.,
which, under the action of the crushing mill become finely divided and
float off in the 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 to secure the heavy base metals which hold
a percentage of gold. There are many, and some do excellent work, but do
not act equally well in all circumstances. The first and most primitive
is the blanket table (p. 79); 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 removed when 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 with a lateral motion, and this,
whether with amalgamated plates, 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
and consistently fine. It is an adaptation of the old simple apparatus
used in China and India for washing gold dust from river sand. 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.
[Illustration: FIG. 24. FRUE VANNER.]
Chinamen were working these forerunners of the Frue vanner fifty years
ago in Australia, and getting fair returns.
The Frue vanner (Fig. 24) is an endless india-rubber band drawn over an
inclined table, to which a revolving and side motion is given by
ingenious 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 (Fig. 25) 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.
[Illustration: FIG. 25. WATSON AND DENNY PAN.]
There is much misconception, even among men with some practical
knowledge, as to the proper function of these saving appliances; and
sometimes good machines are condemned because they fail to perform work
they were not intended for.
It cannot be too clearly realised 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 twenty-five years since I also 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
falling therefrom 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 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, clean, easy flowing 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 £4. 17_s._ 10-2/3_d._ a ton,
have been put through for a saving of £1 9_s._ 1-2/3_d._ 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. Twenty-five 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 that 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 gas-tight 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 mine in the eighties, 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 Krumm 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 the estimated cost per ton was not
more than 30_s._, 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 other 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 earths. 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 acidity 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 8_s._ 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.”