The process of extracting gold from ores by absorption of the precious
metal in chlorine gas, from which it is reduced to a metallic state,
is not a very new discovery. It was first introduced by Plattner many
years ago, and at that time promised to revolutionize the processes
for gold extraction. By degrees it was found that only a very clever
chemist could work this process with practically perfect results, for
many reasons. Lime and magnesia might be contained in the quartz, and
would be attacked by the chlorine. These consume the reagents without
producing any results, earthy particles would settle and surround the
small gold and prevent chlorination, then lead and zinc or other
metals in combination with the gold would also be absorbed by the
chlorine; or, again, from some locally chemical peculiarity in the
water or the ore, gold held in solution by the water might be again
precipitated in the tailings before filtration was complete, and thus
be lost. Henderson, Clark, De Lacy, Mears, and Deacon, all introduced
improvements, or what were claimed to be improvements, on Plattner,
but these chiefly failed because they did not cover every particular
variety of case which gold extraction presented. Therefore, where
delicate chemical operations were necessary for success, practice
generally failed from want of knowledge on the part of the operator,
and many times extensive plants have been pronounced useless from this
cause alone. Hence it is not to be wondered that processes requiring
such care and uncommon knowledge are not greatly in favor.
Mr. Claude Vautin, a gentleman possessed of much practical experience
of gold mining and extraction in Queensland, together with Mr. J.
Cosmo Newbery, analytical chemist to the government of Victoria, have
developed a process which they claim to combine all the advantages of
the foregoing methods, and by the addition of certain improvements in
the machinery and mode of treatment to overcome the difficulties which
have hitherto prevented the general adoption of the chlorination
process.
By reference to the illustrations of the plant below, the system by
which the ore is treated can be readily understood. The materials for
treatment--crushed and roasted ore, or tailings, as the case may
be--are put into the hopper above the revolving barrel, or
chlorinator. This latter is made of iron, lined with wood and lead,
and sufficiently strong to bear a pressure of 100 lb. to the square
inch, its capacity being about 30 cwt of ore. The charge falls from
the hopper into the chlorinator. Water and chlorine-producing
chemicals are added--generally sulphuric acid and chloride of
lime--the manhole cover is replaced and screwed down so as to be gas
tight. On the opposite side of the barrel there is a valve connected
with an air pump, through which air to about the pressure of four
atmospheres is pumped in, to liquefy the chlorine gas that is
generated, after which the valve is screwed down. The barrel is then
set revolving at about ten revolutions a minute, the power being
transmitted by a friction wheel. According to the nature of the ore,
or the size of the grains of gold, this movement is continued from one
to four hours, during which time the gold, from combination with the
chlorine gas, has formed a soluble gold chloride, which has all been
taken up by the water in the barrel. The chlorinator is then stopped,
and the gas and compressed air allowed to escape from the valve
through a rubber hose into a vat of lime water. This is to prevent the
inhalation of any chlorine gas by the workmen. The manhole cover is
now removed and the barrel again set revolving, by which means the
contents are thrown automatically into the filter below. This filter
is an iron vat lined with lead. It has a false bottom, to which is
connected a pipe from a vacuum pump working intermittently. As soon as
all the ore has fallen from the chlorinator into the filter, the pump
is set going, a partial vacuum is produced in the chamber below the
false bottom in the filter, and very rapid filtration results. By this
means all the gold chlorides contained in the wet ore may be washed
out, a continual stream being passed through it while filtration is
going on. The solution running from the filter is continually tested,
and when found free from gold, the stream of water is stopped, as is
also the vacuum pump. The filter is then tipped up into a truck below,
and the tailings run out to the waste heap. The process of washing and
filtration occupies about an hour, during which time another charge
may be in process of treatment in the chlorinator above. The discharge
from the filter and the washings are run into a vat, and from this
they are allowed to pass slowly through a tap into a charcoal filter.
During the passage of the liquid through the charcoal filter, the
chloride of gold is decomposed and the gold is deposited on the
charcoal, which, when fully charged, is burnt, the ashes are fused
with borax in a crucible, and the gold is obtained.
[Illustration: THE NEWBERY-VAUTIN CHLORINATION PROCESS.]
We have specified above the objections to the old processes of
chlorination, so it may be fairly asked in what way the Newbery-Vautin
process avoids the various chemical actions which have hitherto proved
so difficult to contend with.
For any system of chlorination yet introduced it is necessary to free
the ore from sulphides. This is done by roasting according to any of
the well-known systems in vogue. It is a matter which requires great
care and considerable skill. The heat must be applied and increased
slowly and steadily. If, through any neglect on the part of the
roaster, the ore is allowed to fuse, in most cases it is best to throw
the charge away, as waste. This roasting applies equally to the Vautin
process as to any others. So on this head there is no alteration. One
of the most important advantages is not a chemical one, but is the
rapidity with which the charge can be treated. In the older styles of
treatment the time varied from thirty six to ninety hours. Now this is
accomplished in from three to six hours with a practically perfect
result. The older processes required a careful damping of the ore,
which, to get good results, must leave the ore neither too wet nor too
dry. Now "damping" is entirely done away with, and in its place water
is poured into the barrel. Pressure to the extent of four atmospheres
causes chlorine gas to leave its vaporous form. Thus the pressure
applied not only enables a strong solution of chlorine to be formed
with the water in the barrel, but forces this into contact with the
gold through every crevice in the ore. Chlorine gas also takes up any
silver which may exist in association with the gold. In the older
processes this is deposited as a film of chloride of silver around the
fine gold grains, and from its insolubility in water prevents the
absorption of the gold. The rotary motion of the barrel in the
Newbery-Vautin method counteracts this by continually rubbing the
particles together; this frees the particles from any accumulations,
so that they always present fresh surfaces for the action of the
solvent. Again, the short time the ore is in contact with the chlorine
does not allow of the formation of hydrochloric acid, which has a
tendency to precipitate the gold from its soluble form in the water
before being withdrawn from the chlorinator.
Hitherto, when the ore was very fine or contained slimes, the
difficulty of filtration was increased, sometimes in extreme cases to
such an extent that chlorination became impracticable. By the
introduction of the vacuum pump this is greatly facilitated; then by
making the action intermittent a jigging motion is given to the
material in the filter which prevents any clogging except in cases of
extreme fineness.
The advantage of using charcoal as a decomposing agent for chloride of
gold was pointed out by Mr. Newbery some twenty years ago; four or
five years since the idea was patented in the United States, but as
this was given gratis to the world years before, the patent did not
hold good. The form of precipitation generally adopted was to add
sulphate of iron to the liquid drawn from the filter. This not only
threw down the gold it contained, but also the lime and magnesia. Then
very great care was necessary, and a tedious process had to be gone
through to divide the gold from these. Now, by filtration through
charcoal everything that is soluble in hydrochloric acid passes away
with the water; for instance, lime and magnesia, which before gave
such great trouble. In passing through the charcoal, the chloride of
gold is decomposed and all fine gold particles are taken up by the
charcoal, so that it is coated by what appears to be a purple film.
Should copper be associated with the gold, the water, after running
through the charcoal filter, is passed over scrap iron, upon which the
copper is precipitated by a natural chemical action. If silver is
contained in the ore, it is found among the tailings in the filter, in
a chloride which is insoluble in water. Should the quantity prove
sufficiently large, it may be leached out in the usual way by
hyposulphites.
One of the great advantages common to all systems of chlorination is
that ores may be crushed dry and treated, so that the loss from float
gold may be avoided. Of this loss, which is most serious, we shall
have something to say on another occasion. An advantage in
amalgamation with chlorine gas instead of amalgamation with
quicksilver in the wet way, is that the ore need not be crushed so
finely. Roasting takes the place of fine crushing, as the ore from the
roasting furnace is either found somewhat spongy in texture or the
grains of silica in which fine gold may be incased are split or flawed
by the fire. For quicksilver amalgamation very fine crushing is
necessary to bring all gold particles in contact with it. Quicksilver
being so thick in substance, it will not find its way readily in and
out of a microscopically fine spongy body or through very fine flaws
in grains of silica, whereas chlorine gas or a solution of liquefied
chlorine does this, and absorbs the gold far more readily.
There are cases when gold is contained in ores in what is known as a
perfectly "free" form--that is, there is an absence of all sulphides,
arsenides, etc.--when it is not practicable to extract it either with
the ordinary forms of quicksilver amalgamation of or any process of
chlorination, without first roasting. This is because the finer gold
is locked up inside fine grains of silica and hydrated oxide of iron.
No ordinary crushing will bring this fine enough, but when roasting is
resorted to by drawing it rapidly through a furnace heated to a cherry
red, these grains are split up so that chlorine gas is enabled to
penetrate to the gold.
It may be said that an equally clever chemist will be required to work
this improved process as compared with those that have, one by one,
fallen into disuse, mainly from want of knowledge among the operators.
To a certain extent this is so. The natural chemical actions are not
so delicate, but an ignorant operator would spoil this process, as he
does nearly every other. When a reef is discovered, practice shows
that its strongest characteristics are consistently carried throughout
it wherever it bears gold. Before Messrs. Newbery and Vautin leave a
purchaser to deal himself with their process, they get large samples
of his ore to their works and there experiment continually until a
practically perfect result is obtained; then any one with a moderate
amount of knowledge can work with the formula supplied. It has been
their experience that the ore from any two mines rarely presents the
same characteristics. Experiments are begun by treating very coarse
crushings. These, if not satisfactory, are gradually reduced until the
desired result is obtained.
To treat the whole body of ore from a mine, dry crushing is strongly
recommended. To accomplish this in the most efficient manner, a stone
breaker which will reduce to about ¼ in. cubes is necessary. For
subsequent crushing Kroms rolls have, up to the present time, proved
most satisfactory. They will crush with considerable evenness to a
thirty mesh, which is generally sufficient. The crushings are then
roasted in the ordinary way in a reverberatory furnace and the whole
of the roastings are passed through the machine we have just
described. By this it is claimed that over 90 per cent. of the gold
can be extracted at very much the same cost as the processes now in
general use in gold producing countries, which on the average barely
return 50 per cent. If so, the gentlemen who have brought forward
these improvements deserve all the success their process
promises.--_Engineering._
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