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Page 35
[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before being withdrawn from the chlorinator.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.
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