A number of years ago the author devised a method for refining silver
bullion by sulphuric acid, in which iron was substituted for copper as
precipitant of silver, the principal feature being the separation of
pure crystals of silver sulphate. A full description of this process
may be found in Percy's Metallurgy, "Silver and Gold," page 479. The
process has been extensively worked in San Francisco and in Germany in
refining bullion to the amount of more than a hundred million dollars'
worth of silver. Its more general application has been hampered,
however, by the circumstance that the patent had been secured by one
firm which limited itself to its utilization in its California works.
The patent having expired, the author lately introduced a modification
of the process by which the apparatus and manipulations are greatly
cheapened and simplified. In the following account is given a short
description of the process in its present shape.
_Preparing the Silver Sulphate._--The bullion, containing,
essentially, silver, copper and gold, is dissolved by boiling with
sulphuric acid in cast iron pots. The difference between the new
process and the usual practice consists in the use of a much larger
quantity of acid. Thus, in refining ordinary silver "dore," four parts
of acid are used to one part of bullion. Of this acid one part is
chemically and mechanically consumed in the dissolving process, and
the remaining three parts are fully recovered and at once ready for
reutilization, as will be described hereafter. In the usual
process--understanding thereby, here and in the following, the process
practiced at the United States mints, for instance--two parts of acid
are employed for one of bullion; all of this is lost, partly through
the dissolving and partly in being afterward mixed with water,
previous to the precipitation of the silver by copper. Economy in acid
being therefore imperative, the silver solution finally becomes much
concentrated, and it requires high heat and careful management to
finish the solution of the bullion. Bars containing more than about 10
per cent. of copper cannot be dissolved at all, owing to the
separation of copper sulphate insoluble in the small amount of free
acid finally remaining. The advantage gained by dissolving bullion
with abundance of free acid in the improved process is so evident that
it merely requires to be pointed out. For bullion containing 20 per
cent. of copper the author employs six parts of acid to one of
bullion; for baser metal still more acid, and so on, never losing more
than the stochiometrical percentage of acid and recovering the
remainder. In this description he, however, confines himself to the
treatment of ordinary silver ore with less than 10 per cent. of
copper.
In the diagram A A represent two refining pots, 4 ft. in diameter and
3 ft. in depth, each capable of dissolving at one operation as much as
400 pounds of bullion. The acid is stored in the cast iron reservoir,
B, which is placed on a level sufficiently high to charge into A by
gravitation, and is composed of fresh concentrated acid mixed with the
somewhat dilute acid regained from a previous operation. After the
bullion is fully dissolved all the acid still available is run from B
into A A. The temperature and strength are thereby reduced, the fuming
ceases, any still undissolved copper sulphate dissolves, and the gold
settles. In assuming that the settling of the gold takes place in A
itself, the author follows the practice of the United States mints. In
private refineries, where refining is carried on continuously, the
settling may take place in an intermediate vessel, and A A be at once
recharged. Owing to the large amount of free acid present, the
temperature must fall considerably before the separation of silver
sulphate commences, and sufficient time may be allowed for settling if
the intermediate vessel be judiciously arranged.
[Illustration]
_Separating the Silver Sulphate._--The clarified solution is siphoned
off the gold from A A into C, which is an open cast iron pan, say 8
ft. by 4 ft. and 1 ft. deep. It is supported by means of a flange in
another larger pan--not shown in the diagram--into which water may be
admitted for cooling. Steam is blown into the acid solution, still
very hot, as soon as C is filled. The steam is introduced about 1 in.
below the surface of the liquid, blowing perpendicularly downward from
a nozzle made of lead pipe through an aperture 1/8 in. in diameter.
Under these circumstances the absorption of the steam is nearly
perfect, and takes place without any splashing. The temperature rises
with the increasing dilution, and may be regulated by the less
experienced by manipulating the cooling tank. An actual boiling is not
desired, because it protracts unnecessarily the operation by the less
perfect condensation of the steam. No separation of silver sulphate
occurs during this operation (and, consequently, there is no clotting
of the steam nozzle), the large amount of free acid, combined with the
increase of temperature, compensating for the diminution of the
solubility of the sulphate by the dilution. The most important point
in this procedure is to know when to stop the admission of steam. To
determine this, the operator takes a drop or two of the solution upon
a cold iron plate by means of a glass rod and observes whether after
cooling the sample congeals partly or wholly into a white mass of
silver bisulphate, or whether the silver separates as a monosulphate
in detached yellow crystals, leaving a mother liquor behind. As soon
as the latter point has been reached, steam is shut off and the
solution is allowed to crystallize, cold water being admitted into the
outer pan. The operator may now be certain that the liquid will no
longer congeal into a soft mass of silver bisulphate, which on contact
with water will disintegrate into powder, obstinately retaining a
large amount of free acid; but the silver will separate as a
monosulphate in hard and large yellow crystals retaining no acid and
preserving their physical characteristics when thrown into water.
After cooling to, say, 80° F., the silver sulphate will have coated
the pan C about 1 in. thick. There will also be found a deposit of
copper sulphate when the mother acid, after having been used over and
over again, has been sufficiently saturated therewith. Lead sulphate
separates in a cloud, which, however, will hardly settle at this
stage.
The whole operation just described, which constitutes the most
essential feature of the author's improvement upon his old process
described in Dr. Percy's work, is a short one, as the acid requires by
no means great dilution. The steam has merely to furnish enough water
to dilute the free acid present to, say, 62° B. Areometrical
determination is, of course, not possible, on account of the dissolved
sulphates.
_Reducing the Silver Sulphate to Fine Silver._--The mother acid is
pumped from C to the reservoir, B, for this purpose an iron pipe
connecting the top of B with a recess in the bottom of C. The tank, B,
is cast as a closed vessel, with a manhole in the top, which is
ordinarily kept closed by an iron plate resting on a rubber packing.
The air is exhausted from B by a steam injector, and the acid rises
from C and enters B without coming in contact with any valves. The
volume of fresh commercial acid necessary for another dissolving
operation, say 800 pounds, more or less, for refining 800 pounds of
bullion in A A, is lifted from some other receptacle into B in the
same manner. The mixture of the two acids in B now represents the
volume of acid to be employed for dissolving and settling the next
charge of 800 pounds of bullion in A A. In this reservoir, B, the
cloud of lead sulphate mentioned above finds an opportunity for
settling.
The crystals of silver sulphate are detached from C by an iron shovel
and thrown into D. D is a lead lined tank about 4 ft. by 4 ft. and 3
ft. deep. It is divided into two compartments by means of a
horizontal, perforated false bottom made of wood. From the lower
compartment a lead pipe discharges into the lead lined reservoir, E.
Warm distilled water is allowed to percolate the crystals until the
usual ammonia test indicates that the copper sulphate has been
sufficiently dissolved. Then the outflow is closed, sheets of iron are
thrown on and into the crystals, the apparatus is filled with hot
distilled water, and steam is moderately admitted into the lower
compartment. Ferrous sulphate is formed, and in connection with the
iron rapidly reduces the silver sulphate to the metallic state, the
reduced silver retaining the heavy compact character of the crystals.
When the reaction is completed, as indicated by the chlorine test, the
liquid is discharged into E, the iron sheets are removed and the
silver is sweetened either in the same vessel, D, or in a special
filtering vessel which rests on wheels and may be run directly to the
hydraulic press.
The vat, E, is the great reservoir where all liquids holding silver
sulphate in solution are collected; for instance, that from sweetening
the gold and from washing the tools. Sheets of iron here precipitate
all silver and copper, and the resulting solution of ferrous sulphate
is, with the usual precautions, discharged into the sewer.
Occasionally when copper and silver have accumulated in E in
sufficient amount the mass is thrown into D, silver sulphate crystals
are added and sheet copper is thrown in, instead of sheet iron. There
results a hot, neutral, concentrated solution of copper sulphate,
which may be run at once into a crystallizing vat for the separation
of commercial crystals of copper sulphate. It will be readily
understood, of course, that if there should be any advantage in
manufacturing that commercial article, besides the amount prepared as
described, which represents merely the copper contained in the
bullion, copper sheets may be regularly employed for reducing the
silver sulphate in D. The author trusts that the practical refiner
will recognize that the manufacture of commercial copper sulphate is
thus effected in a more rational and economical manner than by the
present method of evaporating from 25° B. to 35° B., and of saturating
by oxidized copper, generally in a very incomplete manner, the large
amount of free acid left from the refining by the usual process.
However, the sale of copper sulphate is but rarely so profitable that
a refinery should not gladly dispense with that troublesome and bulky
manufacture, especially the government establishments, which, besides,
waste much valuable space with the crystallizing vats.
The great saving in sulphuric acid, amounting to about 50 per cent. of
the present consumption, has already been pointed out. Another
advantage the author merely mentions, namely, the easier condensation
of the sulphurous fumes in refineries situated in cities, because the
larger amount of acid available for dissolving greatly facilitates
working and makes the usual frequent admission of air into the
refining pot for the purpose of stirring and testing unnecessary.
The more air is excluded from the refining fumes the easier they can
be condensed.
Work may be carried on continuously, the vessels C and D being empty
by the time a new solution is finished in A A. Thus, the plant shown
in the diagram, covering 26 ft. by 16 ft., allows the refining of
40,000 ounces of fine silver in 24 hours; that is, four charges in A A
of 800 pounds each.--_F. Gutzkow, Eng. and Mining J._
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