Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invention relates to a process for separating a noble
metal, especially silver, from an alloy, especially coin metal, of the noble
metal, copper, and one or more other non-ferrous metals, such as zinc.
There is an increasing quantity of silver alloys in scrap metal.
One important object is silver coins, the silver of which must be recovered
in pure form. A known method is described in British Pat~N~71~ ~B2. The
proces9 is based on adding copper to the silver alloy~ whereby the silver
content of the alloy decreases to 5-25 percent. This material is dissolved
in a sulphate-based electrolyte, anodically potential-controlled so that the
potential of the anode does not rise to the dissolving potential of silver.
The residue is a porous "rusk" with a silver content of over 90%. The copper
of the solution is ultimately recovered electrolytically and the nickel sul-
phate and zinc sulphate by crystallization by evaporation. The "rusk" is
Jmelted and cast into anodes, which are treated by silver electrolysis in a
known manner to recover the silver.
The above process has, among others, the following disadvantages:
- on a technological scale it is very difficult to economically cast
the approx. 5 mm thick anodes required at the first stage.
- at the first stage it is necessary to electrolyze a quantity of copper
almost 4.5-fold in proportion to the produced quantity
- when several electrolytic cells are coupled in a series, there is a
risk of the silver melting in some cell
In the procecs suggested by Heinz Janus (published in Demag Nach-
richten Heft 166 pp. 21-22 Verfahlen 2) and in the related process according
to German Pat. 819 949, a silver alloy is dissolved electrolytically in a
nitrate based solution, whereby a copper-silver alloy is precipitated as a
fine-grained powder. This powder is roasted, whereby the copper oxidizes
into copper oxide. The roasted product is dissolved in a dilute sulphuric
acid, whereby the copper dissolves in the sulphate. The dissolved copper
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is recovered electrolytically or by crystalli~ation into copper sulphate.
The residue is smelted and cast into anodes, which are treated electrolytical-
ly by the method according to Pat. No. 829,949.
The process has, among others, the following disadvantages:
- when powder is prepared electrolytically from the described
alloy, it is in practice difficult to prevent the formation of poisonous
nitrogen oxides and simultaneously to dissolve the anode
- the roasting of substances containing noble metals is known to
be difficult and involves special and inconvenient arrangements for the
recovery of dusts
- furthermore, nitrogen oxides are released in the roasting process.
The object of the present invention is to provide a process for
separating a noble metal from an alloy of the noble metal, copper, and some
other non-ferrous metal, especially from a coin metal, a process in which
the above disadvantages and problems have been eliminated.
According to the present invention, there is provided a process for
separating a noble metal from an alloy of the noble metal, copper, and one
or more other non-ferrous metals, said process comprising smelting said alloy,
granulating said alloy into fine-grained granules which have a specific
surface area of at least 1200 cm2/cm3 and contain a large number of Cu2O
segregations for promoting dissolving, and dissolving the obtained granules
in sulphuric acid having a concentration of at least 10 g/l at an elevated
temperature of above 40C but below the boiling point of the solution to
recover the copper and the noble metal.
In a preferred embodiment of the process according to the invention,
a molten metal alloy is granulated in water into granules with a very large
surface area. Cu20 segregations are simultaneously created in the granules,
which facilitates the dissolving. The copper and the other non-ferrous
metals are dissolved, with the help of air or oxygen, in a dilute sulphuric
acid in a suspension-type reactor at an elevated temperature. It has been
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observed in practice that the dissolving can be regulated, in one stage, so
that copper is dissolved but the silver can be fully recovered by using
fresh granules countercurrently, whereby the possibly dissolved silver is
re-cemented. It has been shown that the large surface area and Cu20 segre-
gations of the granules have a substantial effect on the dissolving. From
the dissolving stage the solution is fed into copper electrolysis tanks which
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operate with insoluble anodes. The copper dissolved at the dissolving stage
is precipitated onto the cathode, and the solution, in which free acid in a
quantity corresponding to the copper has been produced, is retu~ned to the
dissolving stage. Thus the circulation process is utilized in the dissolv-
ing and only acid losses must be compensated for. The dissolution residue
is screened and the coarse fraction is returned to the dissolving stage.
The fine fraction is cast into anodes and treated in silver electrolysis in
a known manner. The electrolytic silver is smelted and cast into bullions
or granules.
The invention is described below in more detail with reference to
the enclosed drawings~ in which Figure 1 shows schematically a device desig-
nated for the application of a process according to the invention and Figure
2 shows a cross section of a detail of it on a larger scale.
The raw material used in the embodiment described above is a coin
metal with the following composition: 35% Ag, 57% Cu, and 8% Zn. For example,
a batch of 1000 kg of the coin metal is smelted in an induction furnace 1
at the temperature t200 C, and the metal is simultaneously slightly oxidized
with air. The molten metal is poured along duct 2 into granulation duct 3,
along which water flows at the rate of~ for example, 100 Nm3/h at the speed
of 20 m/s. The water carries the formed granules further into a settler 4.
Screen analysis of the granules:
+ 1.50 mm 21%
+ 0.75 25%
+ 0.42 28%
+ 0.25 24%
-_ 0.25 2%
The specific surface is 1200 cm /cm , which corresponds to a diameter
of approx. 0.05 mm if the granules are assumed to be spherical.
The composition of the granules is: 35.3% Ag~ 57.4% Cu, and 7.3% Zn.
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Owing to the slight oxidation, Cu20 segregations are created both on the
surface and in the inner parts.
The granules are dissolved in a dissolution reactor 5 using as the
initial batch a solution obtained from the copper electrolysis stage, with
the composition: Cu 40g/l, H2S04 100 g/l, Zn 0-80 g/l.
Approx. 600 kg of granules a are batched into the reactor and the
solution is heated by a direct steam blast b to approx. 90C, and oxygen is
blown through a bottom nozzle c into the reactor at a rate of approx. 20
Nm /h. When the copper content of the solution has risen to 70-80 g/l~ a
dilution is performed with the solution d from the copper electrolysis, where-
by the excess solution is removed through an overflow pipe onto a filter 6.
When the dissolving process has progressed so far that even silver has dis-
solved for approx. an hour~ the feeding of oxygen is discontinued and the
dissolved silver is allowed to be cemented by the still undissolved copper.
The reactor is emptied through the bottom valve and the liquid is
separated from the dissolution residue with the filter 6. The filtrate~
which contains 70-80 g/l Cu~ i9 fed to the copper electrolysis ~tage.
The dissolution residue is screened with a 12-mesh screen 8. The
unpassed fraction, which comprises 11% of the material, contains approx. 20%
copper and is returned to the dissolving stage. The passed fraction is Ag
94.2%, Cu 3.9%, and Zn 1.9%, whereby 97.~% of the copper and 90.3% of the
zinc have been dissolved.
Side by side with the granule dissolution in the closed circuit there
is a copper electrolysis 10, which works according to the electrowinning prin-
ciple. There are eight tanks and the current density used is 100 A/m .
The analysis of the electrolyte is: Cu 35-40 g/l, H2S04 approx. 100 g/l,
and Zn 0-80 g/l. This solution is used in the reactor for a new dissolving.
When the Zn content of the solution rises to 80 g/l, a complete electrolytic
removal of copper is performed and the copper-free solution is either used
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for the production of zinc or zinc sulphate is crystallized from it.
The fraction which has passed the screen 8 and which has been obtained
as the product of the dissolving is smelted and cast into anodes 9 for an
electrolytic production of fine silver.
