Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF INVENTION
It has long been known (see Habashi F. and Du~dale R.,
Metall, 28, 129 (1974) - Reference 1) that a variety of
copper sulphites, including cupric-cuprous sulphite
(Chevreul's salt), can be precipitated from solutions of
cupric sulphate, using sulphur dioxide sulphurous acid
or soluble salts of sulphurous acid as the source of sul-
phite ion. Copper sulphites have a variety of stoichio-
metries and some contain both copper (II) and copper (I),
so it is difficult to define them other than as those
salts which are precipitated from copper salt solutions
in water by addition of soluble salts of sulphurous acid,
which contains sulphite ions. But all are potentially
useful intermediates in copper processing. This observation
has proved difficult to utilize in the extractive metallurgy
of copper, because effective reduction of copper sulphites
to metal has required elevated pressures and temperatures
well above 100C (see reference 1 above and Arbiter ~.,
Milligan D., and Mc Clincy, R., I, Chem E. Symposium
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.
- :. ' :
~ . : . :.. ::: -
'' ' ~ -' .~ :
10 74S30
Series No. 42, 1.1 (1975) - Reference 2~. Reduction is
often incomplete. Recent work has shown that cuprous
ammonium sulphites can be precipitated from copper ammine
salt solutions obtained by the oxidative ammonia leaching
of chalcopyrite (see Reference 2 above). These were
converted to copper metal by heating in an autoclave at
150C and 150 psig. It has also been shown that solutions
of cuprous sulphate in water containing per mole of cuprous
ion at least 3 moles of certain organic nitriles, notably
acetonitrile and 2-hydroxycyanoethane, can be disproportion-
ated either thermally (if acetonitrile or other volatile
nitriles) to give particul~e copper and cupric sulphate
solutions or electrochemically (if acetonitrile or 2-
hydroxycyanoethane) to give copper cathodes and cupric
sulphate solutions, i.e. Cu2S04 ~ CuS04 + Cu. Solutions
of cuprous sulphate in water containing organic nitriles
have considerable value as a source of copper (see Parker
A.J., Search, 4, 426 (1973) - Reference 3). Thus a method
of converting cupric sulphate and slightly soluble copper
sulphites to solutions of cuprous sulphate in water con-
taining organic nitriles, has useful applications.
SUMMARY OF INVENTION
-
In one form the invention resides in a method of pre-
paring cuprous sulphate solutions suitable for thermal
or electrochemical disproportionation which comprises
11~7~53Q
dissolving copper sul.phites such as Chevreul's salt or
copper ammonium sulphites in acetonitrile-water or 2-
hydroxycyanoethane-water mixtures, the amount of acet-
onitrile or 2-hydroxycyanoethane being at least sufficient
to stabilize the resulting cuprous sulphate solution.
Preferably the amount of organic nitrile is greater than
3 moles per mole of Cu+.
In another form, the invention resides in a method of
preparing cuprous sulphate solutions suitable for thermal.
or electrochemical disproportionation which comprises
dissolving copper sulphites such as Chevreul's salt in
acetonitrile-water or 2-hydroxycyanoethane-water mixtures
in the presence of cupric sulphate, the amount of acet-
onitrile or 2-hydroxycyanoethane being at least sufficient
to stabilize the resulting cuprous sulphate solution.
In another form the invention resides in a method of
preparing cuprous sulphate solutions suitable for thermal
or electrochemical disproportionation which comprises
treating a copper sulphate solution with soluble salts
of sulphurous acid or with sulphurous acid to precipitate
copper sulphites such as Chevreul's salt or cuprous amm-
onium sulphite by known processes, separating the salt
from the supernatant liquors and dissolving it in an
acetonitrile-water or 2-hydroxycyanoethane-water mixture
containing cupric sulphate, the amount of acetonitrile or
1~7453~ I
2-hydroxycyanoethane being sufficient to stabilize the
cuprous ion with respect to its disproportionation. It
should be noted that 2-hydroxycyanoethane is suitable
only in the case where the solution is electrochemically
disproportionated, because it is a high boiling nitrile
and cannot be distilled from aqueous solution.
In yet another form the invention resides in a method of
recovering copper from copper sulphides including chal-
copyrite, which comprises roasting the copper sulphide
to produce a calcine containing acid soluble copper salts,
such as CuS04 and CuO, leaching the calcine by known
methods to produce a solution of cupric sulphate, treating
the cupric sulphate solution with a soluble salt of sul-
phurous acid or with sulphurous acid by known methods to
produce copper sulphites such as Chevreul's salt or cuprous
ammonium sulphite, separating the copper sulphite from the
supernatant liquor and dissolving it in an acetronitrile-
water or 2-hydroxycyanoethane-water mixture containing
cupric sulphate to produce a cuprous sulphate solution and
disproportionating the cuprous sulphate solution to
produce copper.
The sulphur dioxide produced during the oxidative roasting
of copper sulphides may be absorbed in a solution of a
water solubls base such as acqueous ammonia, sodium
hydroxide, or sodium carbonate to produce a water soluble
salt of sulphurous acid which may be used in the production
lV'~'4~3C~
of the copper sulphites such as Chevreul's salt or cuprous
ammonium sulphite, whilst the cupric sulphate and
acetonitrile or 2-hydroxycyanoethane from the disproportiion-
ation of the cuprous sulphate may be recycled.
DESCRIPTION
.
It has been found that the presence of acetonitrile or
2-hydroxycyanoethane strongly enhances the solubility of
Chevreul's salt and other cuprous salts, including
CuNH4S03 in water. The reaction is less useful but still
possible in the presence of ammonium ions (CuNH4S03) than
with Chevreul's salt because ammonium sulphate which is
a product of the dissolution decreases the solubility of
acetonitrile in water and sometimes two layers of solvent
form. It has been found that (equationl~ Chevreul's salt
under ambient conditions dissolves readily in water con-
taining at least 3 moles of acetonitrile per mole of copper
produced, to give a solution of cuprous sulphate and
cuprous sulphite. Naturally the rate and extent of
dissolution increases at higher temperatures.
2CU2So3.CUS03.2H20 ~ Cu2S04 + 2CuHS03 + Cu2S03 + 3H20) ..... (1)
Cuprous ammonium sulphite gives a solution of cuprous
sulphate, cuprous sulphite and ammonium sulphite in water
containing acetonitrile or 2-hydroxycyanoethane.
107~S3(J
An excess of Chevreul's salt gave a solution containing 20
g/litre cuprous ion at 25C and 35 g/litre Cu+ at 50C
when treated with water containing 5.8 M acetonitrile. On
distillation of this solution, some S02 was evolved and the
acidity increased to 0.04 molar H . Under otherwise
identical conditions, but in the absence of acetonitrile
less than 2 g/litre of copper could be dissolved from
Chevreul's salt.
Distillation of the acetonitrile from one litre of the above
acetonitrile-containing solution from reaction (1) at 50C
with the addition of sulphuric acid such as to maintain
a pH of 2 or less gave 17 g of particulate copper and blue
cupric sulphate solution. Some sulphur dioxide was evolved.
Another useful method of preparing a cuprous sulphate
solution is to react a copper sulphite such as Chevreul's
salt with various proportions of cupric sulphate in water
containing acetonitrile. The sulphite ion reduced cupric
sulphate in the presence of acetonitrile provided that the
pH is above 1. The proportion of acetonitrile should be at-
least 3 moles per mole of Cu+ produced and preferably
greater. The reaction proceeds at 25C but faster reactions
and higher concentrations of cuprous ion (up to 160 g/litre)
can be obtained if reaction is at 50-70C. The resulting
cuprous sulphate solution is acidic. Distillation of
acetonitrile gives copper if the pH is maintained in the
1074S3(~
vicinity of and preferably below 2 during distillation,
the cupric sulphate and acetonitrile are recycled if
necessary. The reaction is thought to be as follows:
3CuS04 + Cu2S03.CuS03.2H20 ~ 3Cu2S04 2 4 -(2)
The final acidity of the solution depends on the pro-
portion of Chevreul's salt to cupric sulphate, the pH
being lower the greater the proportion of cupric sulphate
according to equation 2. A desirable ratio is 3 moles of
CuS04 per mole of Chevreul's salt. The pH preferably
should not go below 1 during reaction, otherwise reaction
2 is incomplete, probably because of acid decomposition of
Chevreul's salt to give sulphurous acid. Reactions 1 and
2 produce various amounts of cuprous sulphate, according
not only to the proportions of cupric sulphate and Chevreul's
salt, but also to the concentration of acetonitrile, the
temperature, and the pH of the solution. Some relevant
data are in Table I.
Table I: Formation of cuprous sulphate from excess
Chevreul's salt and cupric sulphate at 25C in-
acetonitrile-water.
(CH3CN) moles litre 1 o 1.95 3.90 5-85 8.ooC
(CuS04) moles litre~1 0.63 0.63 o.63 0.63 excessa
(Cu+~a g litre~1 0 33 55 77 l40C
Copper g 0 16 22.5 38.5 70
1~74S30
a) As sulphate b) Copper produced by thermal
disproportionation of one litre of solution.
c3 At 65C after 3 hours, total copper (Cu+ + Cu2+) is
158 g/l.
Table II shows the effect of varying the ratio of cupric
sulphate to Chevreul's salt in a solution containing 3.90M
acetonitrile and .05 M sulphur dioxide at 25C. The
greater the proportion of cupric sulphate to Chevreul's
salt, the greater the acidity of the solutions from
reaction (2). This variation of the proportion of CuS04
provides a means of controlling the acidity and this is
important because in a continuous process the acid gen-
erated can be recycled to leach any basic calcine. The
reaction has a half life of about 10 minutes and is usually
complete after one hour at 25C with stirring. However,
the final proportions of Cu+ and Cu2+ are dependent on the
temperature. More cuprous sulphate is produced as the
temperature is raised above 25 (Table I).
Table II: Effect of proportion of CuS04 to Chevreul~s
salt on reaction (2) at 25C.
(CH3CN) = 3.9 M: (S02) = 0.5 M: Reaction time 1 hour.
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~074S3~
(Clls04) ChevraeUl's (Cu +Cu ) (cu+) (H2S04)
Ma Ma g/litreb g/litreb Mb
o.oo 0.31 49 46 o-ol
0.16 0.26 58 51 0.02
0.32 0.21 58 51 o.o6
o.50 0.15 59 5 0.14
o .64 o . lo 38 23 0.13
o .80 o . oS 38 23 0.18
l.oO - 34 16 0.28
a) Initial concentrations b) Concentrations after 1 hour
of reaction.
We believe that the optimum conditions for producing
copper from Chevreul's salt via reaction ( 2) is to mix
cuso4 and Chevreul~s salt in the molar proportion of 2: 1-
3:1 at 50-65C in water containing 30-40% v/v acetonitrile
to give a solution containing 100-120 g/l Cu+ as Cu2S04,
15-30 g/l Cu as CUS04, 0.2 - 0.3 M H2S04 and about
0.05 M S02. Thermal disproportionation, gives approximately
50-60 g of pure copper powder per litre of solution if
the pH is below 2. Disproportionation at higher pH gives
some copper oxides and copper sulphites together with the
copper.
One method of producing copper sulphites is indicated below.
Ammonium sulphite solution in water was prepared by bubbling
1~74~30
S2 into 7 M ammonia to give a colourless~ odourless
solution of pH 6. This was mixed with various proportions
of 0.5 M cupric sulphate solutions in water at 60C to
precipitate dark red crystalline Chevreul's salt or
brick red (NH4)2S03Cu2S03.2H20)~depending on the molar
ratio of cupric ion to ammonium sulphate. The final pH
was 3.5 and some S02 was evolved.
With 1.5 moles (NH4)2S03 to one mole of cupric ion 96%
of the copper was precipitated mainly as Chevreul's sal~;
with 2 ~oles of (NH4)2S03 to one mole of Cu2+, 98% of
the copper was precipitated mainly as (NH4)2S03Cu2S03.2H20.
The reactions described above, when coupled with existing
technology, provide two promising and rapid methods for -
converting copper sulphides to pure copper.
(a) The concentrate is roasted by known methods (cf.
USBM Report of Investigation 7996 - Reference 4) at a
temperature of the order 500-800C preferably in a fluid
bed roaster and preferably with oxygen enriched air,
containing S02 and S03. The reaction is strongly exother-
mic, i.e. ~H973K is about 1400 kJ mole 1 of CuFeS2 (10).
The products are often CuO and some copper ferrite as well
as the CuS04 shown in equation (5)
S + 15 2 ~ 2CuSo4 + 2so2 2 3
according to the roasting conditions, but the ferrites can
be kept very low and more than 95% of acid-soluble copper
1(~7~30
can be produced in such a roast. The effluent gas normally
contains 5-12% S02, with some S03. The heat produced by
the roast may be used for the thermal disproportionation
step (Vide infra~.
(b) The calcine is leached of its copper by known methods
to give a solution of cupric sulphate.
(c) The S02 produced from the roast is converted to sodium
or ammonium sulphite solution by scrubbing the exhaust
gases from the roaster with solution of the appropriate
base (Na2C03, NaOH, NH3)-
(d) The solutions (b) and (c) are mixed so as to precipitate
copper sulphites, including CuNH4S03, by known methods.
The copper sulphite is separated from supernatant liquor.
(e) The copper sulphites are dissolved in a solution of
cupric sulphate containing water and approximately 40%
v/v acetonitrile so as to give a solution of cuprous
sulphate. This is filtered from any residual solids.
(f) The cuprous sulphate solution is thermally dispropor-
tionated to give copper powder. Cupric sulphate and the
acetonitrile-water distillate are recycled, with provision
for a bleed circuit as necessary, to purify the recycling
liquors.
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~1~'74530
COPPER PURITY
Thermal disproportionation of cuprous sulphate solutions
has given coarse copper powder of 99.9% purity. Some
relevant data is set out in Table III. They indicate that
a relatively coarse, high purity copper powder, suitable
for briquetting, should be a possible product of thermal
disproportionation. A considerable amount of control over
the type of powder produced by thermal disproportionation
is possible.
Table III: Purity (ppm) of copper powder produced from
cupric sulphate solutions via precipitation of Chevreul's
salt, dissolution as cuprous sulphate and thermal dispro-
portionation of the resulting cuprous sulphate solution.
.
Impurity Ni Fe Mg Zn
Solutiona 9000 9000 24000C 9000
Powder ~5 ~ 5 ~ 7 4
a) These elements were added as their sulphate to a cupric
sulphate solution (40 g/litre Cu2+) which was converted to
Chevreul's salt, then as described herein dissolved in
acetonitrile/water/CuS04 andthermally disproportionated
to give copper powder of the purity shown below. b) by
atomic absorption. c) This proportion of magnesium as
sulphate was in the cuprous sulphate solution, prior to
disproportionation.
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74530
It should be noted that the Parker method (Reference 3)
of refining particulate copper via solutions of cuprous
sulphate and disproportionation is applicable to particulate
copper from S02 reduction of copper ammine solutions via
thermal decomposition of copper sulphites (Reference 2).
~he following specific examples will serve to further
describe the invention.
Example I
500 g of chalcopyrite concentrate supplied by Mt. Isa Mines
Pty. Ltd. (25% Cu, 28% Fe, 32% S) was roasted wit;h air to
690C in a rotating kiln. When leached with dilute H2S04
at constant pH 2 for one hour, 95% of the copper and 2% of
the iron was extracted to give a solution containing 41
g/litre cupric ion. A 0.7 M magnesium bisulphite solution
was prepared by bubbling S02 from a cylinder through
magnesium carbonate. This was mixed at 70C with an equal
volume (500 ml) of the 0.7 M cupric sulphate solution
leached from the calcine to precipitate 36 g of Chevreul's
salt. A further 4 g of Chevreul's salt was obtained by
adding a further 250 ml of 0.7 M Mg (HS03)2.
The 40 g of Chevreul's salt was dissolved in 500 ml of
o.63 M cupric sulphate solution at 50C containing 80 g
acetonitrile. The solution contained 24 g/litre Mg as
MgS04. After 10 minutes a clear lime green solution was
1074S3~3
produced. Steam distillation precipitated 14 g of cry-
stalline copper powder containing 10 ppm Mg, and less than
5 ppm Fe, Ni or Zn. The experiment was repeated using
copper sulphate containing 9 g/litre Fe, Zn and Ni as
sulphates and 24 g/l Mg as sulphate. The copper powder
produced, contained 7 ppm Mg, 4 ppm Zn and <5 ppm Fe and
Ni by atomic absorption.
Example II
The above experiment was repeated to the stage of diss-
olution of the Chevreul's salt, prior to steam distillation
at 1/10 the quantities indicated, using twice the con-
centration of 2-hydroxycyanoethane (MW = 70) in place of
acetonitrile (MW = 41). A solution of 65 gl lCu+ as cuprous
sulphate was obtained. 56 g of cuprous ammonium sulphite
was dissolved in 500 ml of a solution containing 160 g
acetonitrile water and 140 g of CuS04.5H20 at 65C. A
clear solution containing about 100 gl 1 cuprous sulphate
was obtained and distillation of the acidified solution
gave particulate copper.
The various aspects of the invention are illustrated in
the self explanatory flow diagrams shown in the accompanying
drawings wherein:
Fig. 1 is a flow diagram showing the production of
copper from Chevreul's salt;
Fig. 2 is a flow diagram showing the production of
copper from copper ammonium sulphate; and
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1~74S30
Fig. 3 is a flow diagram showing the production of
copper from copper ore via the Chevreul's salt
procedure.
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