Gilpin observer., 1900-05-24
PARTING GOLD AND SILVER.
Description of Three Processrs That Are
In Use,

Description of Three Processrs That Are
in Use
There are three methods of parting
gold and silver: the nitric acid, the
sulphuric acid, and the electrolytic pro-
cess, respectively. The latter is well
adapted to large smelling works. The
nitric acid process is the oldest. 1t is
thoroughly satisfactory, but the corn
paratively high price of nitric acid and
the necessity of using either platinum
or porcelain vessels has led to its beng
superseded in many. modern retnerie,,
by the sulphuric acid parting process.
Diluted sulphuric acid has no action OH
silver, but the strong acid, when heat
ed, is decomposed by the metal. giving
OF sulphurous acid and forming silver
oxide, which passes into solution in
the excess of acid rs silver sulphate.
If gold Is present In percentage, not to
exceed One-third of the whole strong
sulphuric acid will extract the silver
and leave the gold as a granular brown
sediment Should the bullion contain
One-half gold, the latter metal will
protect the silver against acid attack,
and the result will be failure. 1t is
the business of the retiner to adjust for
the melting pot gold deposits. with
those of silver carrying small quanti-
ties of gold, or to add fine silver so as
to have from two to three parts of sill
yer for each part of gold. Such an ad-
mixture is melted in II large crucible.
thoroughly mixed, and then. with
dipping cup, is poured into cold water.
Most bullion contains varying guam
titles of copper, and this metal works
very badly in strong sulphuric acid.
While decompOsing the acid with for
nation of copper sulphate, the latter.
unlike the silver sulphate. is insoluble
In oil of yitrfol. hence. If present in any
considerable quantity, copper soon acts
as protector to the bullion and the
chemical action either ceases or be-
comes tediOuSly slow. Besides appor-
tioning the silver to gold, it is neces-
sary, therefore, to SO combine deposits
that the granulations do not contain
more than six to eight per cent. of cop
per- The success of this process really
depends upon the fact that while weak
sulphuric acid rapidly dissolves iron.
yet. If the acid he strong enough. ii
has no appreciabl action-just the re-
verse of its relations to silver. Cast
iron vessels may, therefore. he EN-
ployed. Large iron kettles are used.
weighing half Il ton, mounted over Il
furnace. Into this n charge of 300 to
400 pounds of granulated bullion is
placed, and covered with three or four
times Its weight OF acid-66 b. Heat
is applied, when a lively evolution of
sulphurous acid sets in. which. If too
violent, must be checked by the addi-
tion of little cold acid and slackening
of the fire. This boiling is continued
for several hours, during which time
the escaping and consumed acids are
gradually replaced. When the solution
Is complete the fire Is withdrawn and
the contents of the kettle allowed to
quietly settle. A little cold acid is
added to aid the precipitation of any
suspended gold. Hoods over the ket-
ties carry off the fumes to condensing
and suppression apparatus. The strong
ly acid solution, while still hot is
siphoned off into the reducing houses.
These are long, rectangular watts lined
with lead. and provided with sloping
covers. Slabs or ingOts of metallic cop
per are placed on the bottom and sides
of these watts, which are then partly
filled with cold water The hot solu-
tiOn Is then run in. with resultant
cracking and splutterlng. The result
is weak solution of silver sulphatc.
rendered milky by the separation of
fine crystals of that salt. the solubility
of which Is very slight in pure water.
The copper Immediately begins to pre-
cipltate the silver us crystalline
mass, easily detachable from the plates
when the operation is complete. This
requires about twentyfour hours.
Meanwhile the copper has taken the
place Of silver. leaving strong solu-
tion Of copper sulphate (blue vitrioD.
This blue solution is drawn off for
crystallizatiOn, and the spongy silver
carefully transferred to teaching tubs
for thorough sweetening with fresh
water. The silver, now in the form of
minute, haky crystals. Is cOmpacted
into cakes by means Of hydraulic pres
sure and these cakes dried in current
of warm water. after which n simple
fusion in n black lend crucible without
fiuxes gives n bullion O.99S or O999
fine. III silenium or tellurium,is pres.
ent In the bullion II will pass into So
luton with the silver and be deposited
with 1t by the copper plates. In this
case the spongy silver will be dark
colored and the melt will be brittle.
II must then be tiuxed with inter. The
sulphate of copper Is then carefully
prepared for the market. the proceeds
thus materially reducing the expense.
Mining and Scientific Press.