Roast-reductive leach process for copper recovery

A sulfide containing copper and one or more of iron, nickel and cobalt, is dead-roasted at a temperature of 750.degree. C or above, then subjected to a pressure leach under reducing conditions to produce elemental copper and an aqueous solution of the other metals.

The present invention relates to the recovery of copper from 
sulfide-containing materials, and more particularly from materials which 
contain iron and nickel in addition to the copper. 
Various hydrometallurgical processes have been suggested for the treatment 
of cupriferous materials to recover copper. Where the materials are 
sulfide concentrates which contain copper, iron, nickel and possibly 
cobalt and precious metals it has been suggested to leach the concentrate 
under pressure in ammoniacal or in acid solutions. However such leaches 
are not found to be sufficiently selective between the copper, iron and 
nickel. It has also been suggested to subject such sulfidic materials to a 
roasting treatment, so that copper and other sulfides are oxidized to 
their respective sulfate and oxides, and thereafter leaching the roasted 
material. Such techniques also exhibit poor selectivity with respect to 
copper, so that separate electrolysis has to be relied on to separate the 
copper from other metals dissolved in the lixiviant. Moreover another 
difficulty encountered when leaching roasted sulfide concentrates lies in 
the poor metal recoveries which can be accounted for by the known tendency 
for ferrites to form during a roast. Copper tied up in ferrites has proved 
particularly difficult to solubilize during a subsequent leach. 
It is an object of the present invention to provide an improved process for 
efficiently recovering copper from such sulfidic 
copper-iron-nickel-containing materials. 
According to the invention copper is recovered from a particulate 
sulfide-containing material which contains in addition to copper at least 
one other metal from the group: iron, nickel and cobalt, by roasting the 
material at a temperature of at least about 750.degree. C. for a period of 
sufficient duration to provide a substantially sulfur-free calcine, 
forming a slurry of the calcine with water or an aqueous solution 
containing at least a sufficient amount of sulfuric acid to supplement any 
sulfuric acid formed in situ and satisfy the stoichiometry of formation of 
sulfates of the other metal(s), heating the slurry to at least 110.degree. 
C. under pressure and in the presence of a reducing gas to sulfate the 
other metal(s) and reduce the copper to elemental form, and separating the 
product of the pressure-heat treatment into a liquor containing the other 
metal(s) and a solids residue containing the elemental copper. 
The process of the invention is applicable to a wide range of 
copper-containing materials, but it is particularly useful for the 
treatment of materials wherein the amounts of metals other than copper are 
relatively low. For example it is particularly suitable for materials 
wherein the copper content exceeds, and preferably is much greater than, 
the sum of the contents of the other metals. This will not usually be the 
case for ores or concentrates, but might be true of mattes as well as 
various metallurgical residues obtained for example from an acid leaching 
process or a nickel carbonylation process. Thus the practical 
implementation of the process of the invention might consist either in the 
direct treatment of suitable mattes, or given a starting material too high 
in nickel, cobalt and iron, the material could be subjected to leaching or 
carbonylation and the residue treated in accordance with the invention. 
Matte to be treated by the described process could advantageously be 
prepared by oxygen flash smelting. The high strength SO.sub.2 steam 
produced by the flash furnace is then conveniently used for the reductive 
leaching. The smelting operation could also reject part of the iron 
content of the concentrate so that less ferrous sulfate is produced in the 
reductive leaching operation. 
The dead-roasting must be carried out at a temperature no lower than about 
750.degree. C. in order to ensure that the copper is present as oxide 
rather than as sulfate in the calcine. While it is possible to resort to 
higher temperatures for the sake of rapidity of the roast, temperatures 
higher than about 950.degree. C. are neither necessary nor indeed 
desirable. We prefer to roast the feed material at a temperature in the 
range 750.degree.-800.degree. C. The roasting period is of course 
dependent on the initial sulfur content of the material and typically a 
period of the order of 3-6 hours is used. Advantageously the roasting is 
carried out in a fluid bed roaster apparatus whereby off-gases are 
obtained which are concentrated with respect to sulfur dioxide. 
When the process is carried out some ferrites are inevitably formed during 
the `dead-roasting` which is intended to drive off all the sulfur from the 
material and leave the copper and other metals essentially in the form of 
oxides. However we have found that the reductive leach process used after 
the roasting enables even the ferrites to be leached readily so that a 
high proportion of the iron, nickel and cobalt are solubilized while a 
high proportion of the copper reports in elemental form in the solids 
residue. 
The calcine is slurried with an aqueous sulfuric acid solution and treated 
in an autoclave. For this purpose a slurry consistency of up to 40% solids 
can be used. The amount of sulfuric acid in the solution must be 
sufficient to ensure that when taken in combination with any sulfuric acid 
generated in situ it will balance the stoichiometric requirement for the 
formation of sulfates of all the nickel, cobalt and iron in the calcine. 
The amount of acid used will therefore clearly depend on the composition 
of the calcine; furthermore it will depend on the reducing agent used 
since the latter may or may not result in generation of sulfuric acid as 
explained below. 
The pressure leach must be carried out in the presence of a reducing gas, 
and two gases we have found particularly effective for this task are 
hydrogen and sulfur dioxide. The leach should be carried out for a period 
of the order of 1/2 to 2 hours at a temperature of at least 110.degree. 
C., preferably 130.degree.-180.degree. C. and most preferably, 
150.degree.-160.degree. C., under a total pressure in the autoclave of the 
order of 0.75-3 megapascals (MPa). Where hydrogen is used as the reducing 
gas it should be present, at the leaching temperature, at a partial 
pressure of not less than about 1 MPa, while where sulfur dioxide is used 
the partial pressure thereof at the leaching temperature should be at 
least about 0.5 MPa. It is possible to achieve this by feeding the 
approximate amount of hydrogen or sulfur dioxide into the autoclave. 
However in the case of sulfur dioxide, we prefer to rely on the high 
solubility thereof in water at lower temperatures, and adopt the procedure 
of introducing sulfur dioxide into the slurry at a temperature below 
80.degree. C., e.g., at room temperature, to dissolve the necessary amount 
and thereafter heat the slurry in a closed autoclave. By forming a 
saturated solution at room temperature, at which temperature the partial 
pressure of sulfur dioxide is about 0.2 MPa, and heating this to 
150.degree. C. a total pressure of 2 MPa is achieved with a sulfur dioxide 
partial pressure of about 1.5 MPa. 
When hydrogen is used as the reducing gas, the reactions occurring during 
leaching are believed to be as follows: 
EQU CuO + H.sub.2 .fwdarw. Cu.degree. + H.sub.2 O 
EQU niO + H.sub.2 SO.sub.4 .fwdarw. NiSO.sub.4 + H.sub.2 O 
EQU fe.sub.2 O.sub.3 + 2H.sub.2 SO.sub.4 + H.sub.2 .fwdarw. 2FeSO.sub.4 + 
3H.sub.2 O 
EQU fe.sub.3 O.sub.4 + 3H.sub.2 SO.sub.4 + H.sub.2 .fwdarw. 3FeSO.sub.4 + 
4H.sub.2 O 
EQU co.sub.2 O.sub.3 + 2H.sub.2 SO.sub.4 + H.sub.2 .fwdarw. 2CoSO.sub.4 + 
3H.sub.2 O 
EQU co.sub.3 O.sub.4 + 3H.sub.2 SO.sub.4 + H.sub.2 .fwdarw. 3CoSO.sub.4 + 
4H.sub.2 O 
EQU fe.sub.2 NiO.sub.4 + 3H.sub.2 SO.sub.4 + H.sub.2 .fwdarw. 2FeSO.sub.4 + 
NiSO.sub.4 + 4H.sub.2 O 
however when sulfur dioxide is used, the following reactions are believed 
to take place: 
EQU CuO + SO.sub.2 + H.sub.2 O .fwdarw. Cu.degree. + H.sub.2 SO.sub.4 
EQU niO + H.sub.2 SO.sub.4 .fwdarw. NiSO.sub.4 + H.sub.2 O 
EQU fe.sub.2 O.sub.3 + H.sub.2 SO.sub.4 + SO.sub.2 .fwdarw. 2FeSO.sub.4 + 
H.sub.2 O 
EQU fe.sub.3 O.sub.4 + 2H.sub.2 SO.sub.4 + SO.sub.2 .fwdarw. 3FeSO.sub.4 + 
2H.sub.2 O 
EQU co.sub.2 O.sub.3 + H.sub.2 SO.sub.4 + SO.sub.2 .fwdarw. 2CoSO.sub.4 + 
H.sub.2 O 
EQU co.sub.3 O.sub.4 + 2H.sub.2 SO.sub.4 + SO.sub.2 .fwdarw. 3CoSO.sub.4 + 
2H.sub.2 O 
EQU fe.sub.2 NiO.sub.4 + 2H.sub.2 SO.sub.4 + SO.sub.2 .fwdarw. 2FeSO.sub.4 + 
NiSO.sub.4 + 2H.sub.2 O 
it will be clear from the above equations that when sulfur dioxide is used 
as reductant, the dissolution of iron, nickel and cobalt consumes sulfuric 
acid, whereas the reduction of copper generates the acid. Thus if the 
amount of copper is sufficiently high in relation to the iron, nickel and 
cobalt contents, little or no acid may be needed to balance the 
stoichiometry. In fact it is a particular advantage of the process that, 
with a calcine of suitable copper/other metal ratio, the sulfuric acid 
solution used for the leach can be spent electrolyte from an 
electrowinning operation. It will also be apparent that if the 
copper/other metal ratio is too high the result can be a high acid 
concentration at the end of the leach. This is undesirable because of the 
tendency for copper to be solubilized if the concentration of acid is too 
high. Generally the proportions should be controlled so that the final 
acid concentration is not greater than 50 grams per liter. 
Thus if excessive acid formation is expected in view of calcine 
composition, the problem can be overcome by relying on hydrogen rather 
than sulfur dioxide for the reductive leach. Alternatively, it may be 
preferable to reduce the copper content of the material to be reductively 
leached. A particularly useful way of accomplishing this purpose involves 
resorting to the preferential copper leaching method described in 
co-pending U.S. patent application Ser. No. 894,900 filed Apr. 10, 1978, 
assigned in common with the present invention. The technique in question 
involves dead-roasting of a sulfide material at 750.degree. C. or above 
followed by a sulfuric acid leach, at 50.degree. C. or above such that 
copper is solubilized preferentially to nickel, iron and cobalt in the 
material. Thus, inasmuch as both this preferential leach technique and the 
copper recovery method of the present invention require an initial high 
temperature dead-roasting treatment, they can be combined conveniently in 
a process wherein the sulfide material which is high in copper is first 
dead-roasted, then leached in sulfuric acid to dissolve some of the copper 
preferentially, and the residue of this leach is separated from the 
copper-bearing solution, reslurried with water or an aqueous sulfuric acid 
solution and pressure-leached in the presence of a reducing gas. 
An important application of the process of the invention lies in the 
treatment of materials which contain, in addition to the copper, iron, 
etc., a significant amount of precious metals. When such materials are 
subjected to roasting followed by reductive leaching, the precious metals 
are separated from the iron, nickel and cobalt, and report together with 
elemental copper in the solids residue of the leach. As a result it is 
possible to recover iron and nickel from the liquor in a conventional 
fashion, while the copper and precious metals can be conveniently 
separated from one another by a pyrometallurgical route or a 
leach/electrowinning route in well known fashion. 
The invention will now be particularly described by way of examples, with 
reference to preferred embodiments thereof. All percentages referred to 
herein, unless otherwise specified, are percentages by weight.

EXAMPLE 1 
A copper-containing flash furnace matte was used which contained: 
______________________________________ 
copper 
: 43.0% 
nickel 
: 11.6% 
iron : 21.0% 
cobalt 
: 0.08% 
sulfur 
: 22.5% 
______________________________________ 
A sample of this matte was roasted at 800.degree. C. for a period of 6 
hours. The resulting calcine was slurried with a 130 grams per liter (g/l) 
sulfuric acid solution in the ratio of 150 grams of calcine to 1 liter of 
solution. The slurry was then heated in an autoclave to 130.degree. C. 
Hydrogen was fed into the autoclave such that the total pressure therein 
was about 2.6 MPa while the hydrogen partial pressure was 2.2 MPa. 
Leaching was carried out for one hour, during which the slurry temperature 
rose from 130.degree. to 180.degree. C. At the end of that time, the leach 
liquor was separated from the solid residue and each was analyzed to 
obtain the results shown in Tables 1 to 3 below. 
TABLE 1 
______________________________________ 
Weight Composition (%) 
Phase (g) Copper Nickel Iron Cobalt 
______________________________________ 
Calcine 150. 41.6 11.2 20.3 0.08 
Leach 
Residue 64.8 88 0.86 1.30 0 
______________________________________ 
TABLE 2 
______________________________________ 
Composition (g/l) 
Phase Copper Nickel Iron Cobalt H.sub.2 SO.sub.4 
______________________________________ 
Initial 
Solution -- -- -- -- 130 
Final 
Liquor 1.54 15.7 31.1 0.096 48 
______________________________________ 
TABLE 3 
______________________________________ 
Metal Distribution (%) 
Phase Copper Nickel Iron Cobalt* 
______________________________________ 
Final 
Liquor 2.5 96.6 97.1 80 
Leach 
Residue 97.5 3.4 2.9 20 
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*The low cobalt levels could not be accurately determined 
It will be seen from the above results that the roast-reductive leach 
process resulted in recovery of a very high proportion of the available 
copper in the form of an 88% pure product. 
EXAMPLE 2 
A further 150 gram sample of the calcine produced as described in Example 1 
by roasting of flash furnace matte was slurried with 1 liter of water. 
Sulfur dioxide at a pressure of 0.2 MPa was then introduced into the 
slurry at room temperature to the point of saturation. The saturated 
slurry was thereafter heated in an autoclave for one hour at 150.degree. 
to 165.degree. C. under a total pressure of about 2 MPa and a sulfur 
dioxide partial pressure of about 1.5 MPa. The slurry was then cooled and 
the liquor separated from the leach residue. The liquor was analyzed and 
found to contain: 
______________________________________ 
copper : 1.1 g/l 
nickel : 14.5 g/l 
iron : 32.0 g/l 
cobalt : 0.074 g/l 
sulfuric acid : 43.5 g/l. 
______________________________________ 
Thus it will be seen that the reduction of copper oxide by sulfur dioxide 
produced more than enough sulfuric acid to satisfy the stoichiometry of 
sulfation of the other metals. The leach residue, weighing 62 grams, was 
found to contain: 
______________________________________ 
copper 
: 91.8% 
nickel 
: 0.62% 
iron : 1.02% 
cobalt 
: &lt;0.02% 
______________________________________ 
so that the distribution of the metals in the final slurry was as shown in 
Table 4 below: 
TABLE 4 
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Metal Distribution (%) 
Phase Copper Nickel Iron Cobalt* 
______________________________________ 
Final Liquor 1.7 97.1 98.0 92 
Leach Residue 
98.3 2.9 2.0 8 
______________________________________ 
*Not accurately determined. 
EXAMPLE 3 
A test was carried out on a copper concentrate which analyzed: 
______________________________________ 
copper 
: 30.0% 
nickel 
: 2.0% 
iron : 31.5% 
cobalt 
: 0.08% 
sulfur 
: 30.0% 
______________________________________ 
The concentrate was roasted at 800.degree. C. for 6 hours, and 100 grams of 
the resulting calcine were slurried with one liter of water. The slurry 
was saturated at room temperature with sulfur dioxide, and heated in an 
autoclave in the manner described in Example 2. The results are shown in 
Tables 5 and 6 below: 
TABLE 5 
______________________________________ 
ASSAYS 
Phase Units Copper Nickel 
Iron Cobalt 
H.sub.2 SO.sub.4 
______________________________________ 
Calcine % 31.5 2.15 33.1 0.08 -- 
Leach Residue 
% 70.5 0.02 0.47 &lt;0.02 -- 
Final Liquor 
g/l 0.42 2.09 33.0 0.083 19 
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TABLE 6 
______________________________________ 
Metal Distribution (%) 
Phase Copper Nickel Iron Cobalt* 
______________________________________ 
Final Liquor 1.3 99.6 99.4 90 
Leach Residue 
98.7 0.4 0.6 10 
______________________________________ 
*Not accurately determined. 
EXAMPLE 4 
The feed material for this test was a residue from a carbonylation process, 
which residue contained: 
______________________________________ 
copper 
: 55.1% 
nickel 
: 7.4% 
iron : 8.0% 
cobalt 
: 3.7% 
sulfur 
: 17.4% 
______________________________________ 
The feed was roasted at 900.degree. C. for 3 hours to provide a calcine, 
100 grams of which were slurried with 1 liter of water. The slurry was 
saturated with sulfur dioxide at 25.degree. C. (partial pressure 0.2 MPa) 
then heated under pressure (total pressure: 2 MPa, partial pressure of 
sulfur dioxide about 1.5 MPa) for one hour at between 150.degree. and 
160.degree. C. Analysis of the final liquor separated from the solids 
residue (which weighed 56 grams) showed that the copper reduction had 
produced a more than adequate supply of acid. The tests results are shown 
in Tables 7 and 8 below: 
TABLE 7 
______________________________________ 
ASSAYS 
Phase Units Copper Nickel 
Iron Cobalt 
H.sub.2 SO.sub.4 
______________________________________ 
Calcine % 57.3 7.61 8.33 3.75 -- 
Leach Residue 
% 90.9 0.03 0.25 0.04 -- 
Final Liquor 
g/l 3.34 7.65 8.41 3.71 57 
______________________________________ 
TABLE 8 
______________________________________ 
Metal Distribution (%) 
Phase Copper Nickel Iron Cobalt 
______________________________________ 
Final Liquor 
5.8 99.8 98.3 99.4 
Leach Residue 
94.2 0.2 1.7 0.6 
______________________________________ 
Example 4 is a good illustration of the excellent separation which can be 
achieved by the roast-reductive leach process of the invention between 
copper on one hand and iron, nickel and cobalt on the other. As already 
stated precious metals contained in the calcine would report in the copper 
product. Of the metals which might be present in addition to those already 
referred to, selenium would be concentrated in the metallic copper 
residue, while zinc and arsenic would be solubilized in the same way as 
iron. 
Thus while the invention has been described with reference to preferred 
embodiments thereof, various modifications may be made to such embodiments 
without departing from the scope of the invention which is defined by the 
appended claims.