US20050193863A1

US20050193863A1 - Selective reduction of cupriferous calcine

Info

Publication number: US20050193863A1
Application number: US10/794,304
Authority: US
Inventors: Mika Muinonen; Samel Marcuson; Ahmed Vahed; Anthony Warner
Original assignee: Vale Canada Ltd
Current assignee: Vale Canada Ltd
Priority date: 2004-03-05
Filing date: 2004-03-05
Publication date: 2005-09-08
Also published as: KR20060122942A; JP4885119B2; NO20064066L; AU2004316863A1; WO2005085484A1; CA2557849A1; AU2004316863B2; CA2557849C; JP2007526397A

Abstract

A process for selectively reducing calcine from a copper/nickel containing sulfidic material includes subjecting the calcine to a mildly reducing atmosphere at relative high temperatures. CuO is selectively converted to Cu₂O which can easily be separated by leaching from other metals which may be present in the material.

Description

FIELD OF THE INVENTION

The present disclosure relates to recovery of copper from sulfide materials which contain metals intimately mixed with the copper. It is particularly useful for the treatment of cupriferous mattes or fractions of such mattes.

BACKGROUND OF THE INVENTION

Copper and nickel are close relatives in terms of dissolution thermodynamics and it has been difficult to efficiently separate nickel from copper during processing of ore and matte. U.S. Pat. No. 4,168,217 (the “'217 patent”) describes a process for recovering copper from sulfidic material, e.g., a cupriferous matte or matte fraction (such as bessemer matte), containing at least one of the metals iron, nickel and cobalt, by dead-roasting the material at a temperature of at least about 750° C. to provide a calcine which is essentially sulfur-free, cooling the calcine and thereafter leaching it in an aqueous sulfuric acid solution at a temperature of at least about 50° C., and separating pregnant leach liquor in which is dissolved most of the copper initially present in the material from leach residue containing most of any iron, nickel and cobalt initially present in the material. In the '217 patent process, the resulting calcine contains a significant amount of CuO in solid solution with NiO which is not easily leached.
U.S. Pat. No. 4,135,918 describes a process where 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° 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° 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. As above, the calcine contains a significant amount of CuO in solid solution with NiO or with ferrites which are not easily leached.
A process for recovering copper from a sulfidic material is also described in U.S. Pat. No. 4,120,697 which includes roasting the sulfidic material to produce a calcine, mixing the calcine with a particulate, carbonaceous reductant and with at least one halide salt, which is heat transformable to a gaseous halogen or to a hydrogen halide at a segregation roasting temperature, in small amounts to halogenate copper values contained in the calcine, heating the mixture to a segregation roasting temperature between about 650° C. and about 700° C. at which temperature copper values in the calcine react to form a copper halide which is transported to the particulate carbonaceous reductant where metallic copper is precipitated on the carbonaceous reductant from the copper halide, and the precipitated metallic copper is recovered. This process involves formation of salts after the calcine to reduce the copper to metallic form which must be further processed and separated by flotation or some other physical separation.
Thus, it has been difficult to achieve the desired degree of selectivity between copper and other metals, such as nickel, cobalt and iron, which are present in mixtures without various further processing steps which increase the time and cost of manufacture. There is a continuing need to provide more efficient methods of purifying copper and other metals from mixtures.

SUMMARY OF THE INVENTION

A process for recovering copper from sulfide-containing material which contains, in addition to copper, at least one other metal selected from the group consisting of iron, nickel and cobalt, is provided which includes roasting said sulfide-containing material for sufficient time to provide a substantially sulfur-free calcine, subjecting the calcine to sufficient heat and a reducing atmosphere to selectively reduce CuO in the calcine to Cu₂O and form a reduced calcine, subjecting the reduced calcine to an oxidizing leach and recovering the copper. The reduction step separates copper from nickel oxide matrix due to the insolubility of Cu₂O in NiO.
A process for selectively reducing CuO to Cu₂O in a calcine which contains, in addition to copper, at least one other metal selected from the group consisting of iron, nickel and cobalt, is provided which includes roasting said material for sufficient time to provide a substantially sulfur-free calcine, subjecting the calcine to sufficient heat and a reducing atmosphere to selectively reduce CuO in the calcine to Cu₂O and form a reduced calcine.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a copper and nickel predominance diagram.
FIG. 2 is a graph depicting curves showing oxygen pressures (atm) as a function of temperature and CO₂/CO ratios at a total pressure of 1 atmosphere. The shaded area below the dashed cross curve represents metastable conditions.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention incorporates selective reduction of CuO (tenorite) to Cu₂O (cuprite) in calcine under a suitable reducing atmosphere and sufficiently high temperature prior to leaching to provide surprisingly effective separation of copper from mixtures of nickel, iron, cobalt and/or other metals which may be present. In addition to making the copper oxide available for leaching, the reduction step also causes the NiO to be refractory. For example, in one exemplary experiment, subjecting bulk roasted bessemer matte having a Cu/Ni ratio of 1.0 to techniques described herein yielded about 97.8% copper extraction while extracting only about 0.3% nickel.
The process disclosed herein is applicable to a wide range of copper-containing materials, and is even useful for the treatment of materials wherein the amounts of metals other than copper are relatively low. For example, it is suitable for materials wherein the copper content exceeds, and is much greater than, the sum of the contents of the other metals. This will not usually be the case for ores or concentrates which can also be treated according to the techniques described herein, 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.
In one aspect, it is contemplated that any substantially sulfur-free calcine material containing copper and at least one other metal can be utilized herein. Those skilled in the art are familiar with various techniques for obtaining such calcine. Indeed, any technique may be utilized to treat a sulfide containing material to remove most of the sulfur. For example, the temperature at which the roasting is carried out is usually considered important for achieving desired good leaching properties of the resulting calcine. In one well-known technique, (see, e.g., U.S. Pat. No. 4,168,217) a minimum temperature of about 750° C. is recommended, about 800° C. preferred, and about 950° C. not to be exceeded. According to the present invention, the roasting temperature can be higher than previous practice. However, too high a roasting temperature may lead to detrimental melting of the calcine and therefore the temperature usually should not exceed about 1050° C. Thus, according to the present invention, roasting can proceed at a faster rate, when temperatures are high, but below the calcine melting point. Maximizing the temperature increases the rate of reaction. In a preferred embodiment, roasting of the sulfide material takes place in an atmosphere containing excess oxygen to ensure that nearly 100% of the sulfur is removed. At such a temperature a retention time of between about 0.5 hour and about 3 hours has been found satisfactory to lower the sulfur content of the feed to about 0.5% or less. Unless otherwise specified, all percentages quoted herein are percentages by weight. “Substantially” as used herein is intended to mean nearly or precisely.
In the case of mattes or matte fractions, in contrast to prior techniques, the composition of the matte or matte fraction treated is less relevant to the copper extraction and also to the selectivity which can be achieved. Indeed, the relative concentration of each component is not critical since the present process is effective at nearly any range of copper, iron and nickel ratio.
Roasting can be effected in any of the various known forms of apparatus and can proceed autogenously. Preferably a fluid-bed roaster is used because of the higher throughput possible in such a roaster, as well as because of the relatively high sulfur dioxide concentration in its off-gases. For the purpose of fluid-bed roasting the feed should preferably be in the form of particles of between about 100 and about 600 micron diameter. Where the initial feed is a fine powder pelletization may be needed. However where the feed material is derived from a previously molten matte the desired particles can be produced by water granulation of the hot matte. The latter approach is preferred in that it leads to less dusting during the fluid-bed roasting operation.
Cupriferous calcine produced, e.g., by dead roasting, may contain a significant amount of CuO in solid solution with Ni. The calcine is then subjected to selective reduction using relatively high temperatures and a suitably mild reducing atmosphere which is believed to form Cu₂O outside of NiO, effectively removing CuO from the nickel oxide matrix. In a preferred embodiment, a reducing gas mixture is chosen that is not too high in reductant to ensure that reduction is selective and that formation of nickel metal is avoided. The range of pO₂(partial pressure of oxygen) and temperatures required to achieve successful selective reduction is exemplified in phase diagrams and is determinable using well-known techniques. FIG. 1 is a copper and nickel predominance diagram in which the gray region exemplifies where reduction of CuO to Cu₂O can be affected without forming nickel metal that could harm downstream leaching efficiency. The minimum pO₂is exemplified in the region where nickel metal is stable (dashed line). The arrows indicate that increasing temperature and operating at low pO₂achieve maximum kinetics and recovery. The black diamond indicates potential target conditions for reduction. The maximum pO₂is where CuO is reduced to Cu₂O. Preferably, conversion to Cu₂O is affected at the lowest possible pO₂without forming nickel metal and at as high a temperature as possible. Even if copper metal forms, it can be successfully leached by oxidative leaching.
The pO₂can be controlled according to well known techniques by mixing suitable gases in appropriate amounts. For example, CO, CO₂, H₂and H₂O mixtures are well known for creating reducing atmospheres. FIG. 2 illustrates that pO₂can be controlled by mixing CO and CO₂gas in the appropriate ratio. In particular, FIG. 2 depicts curves showing oxygen pressures (atm) as a function of temperature and CO₂/CO ratios at a total pressure of 1 atmosphere. The shaded area below the dashed curve represents metastable conditions. Similar tables exist for H₂/H₂O mixtures. H₂and CO₂can be mixed to form mixtures of CO, CO₂, H₂and H₂O. pO₂below 10⁻⁶may be utilized, with about 10⁻¹⁰being preferred. An example of a mixture is a gas mix of H₂and CO₂containing about 3-5% H₂(volume %) and balance CO₂to affect a pO₂of ˜10⁻¹⁰-10⁻¹²at about 1000° C. Another mixture could encompass a CO₂/CO of ˜1000 (pO₂of ˜10⁻¹¹) at about 1000° C. In one aspect, pO₂can be controlled by the partial combustion of fuel as is commonly done by those skilled in the art. By measuring the concentration of CO and CO₂in the gas, oxygen concentration (pO₂) can be indirectly measured, and the fuel/air ratio is varied to maintain the proper CO₂/CO ratio, i.e., pO₂. CO₂/CO gas generation can also be obtained from solid carbon via the Boudouard reaction.
Relatively high temperatures utilizable herein are those that are suitable to augment the reduction conditions. Those skilled in the art can readily determine appropriate temperatures to maximize the kinetics of reduction. See, e.g., FIGS. 1 and 2. As a practical matter, copper oxides begin to melt at about 1050° C., which effectively limits the upper range. Therefore, a preferred temperature range runs from about 750° C. to about 1050° C., and is even more preferably about 1000° C. Although any suitable oven or furnace may be utilized as a vessel for reduction as described herein, a highly mixed reactor such as a dynamic fluid bed apparatus is preferred to achieve high kinetics.
The reduced calcine produced by reduction as described herein, after cooling, and if necessary, grinding, may be subjected to an oxidative leach, e.g., slurried with an aqueous sulfuric acid solution which can conveniently comprise the spent electrolyte from a copper electrowinning operation to selectively dissolve the Cu₂O. The lixiviant may contain some dissolved copper in addition to the free sulfuric acid. While a leach temperature of at least about 50° C. is typically needed for practicable leach rates to be achieved, temperatures of about 80° C. or more may have the undesirable effect of increasing nickel and iron dissolution. In a preferred embodiment, a leach temperature of about 60°-70° C. along with a retention time of about 2-3 hours has been found to give good results. After solid-liquid separation, the liquor can be used to electrowin a high purity copper product, while the residue is treated to recover any nickel and cobalt, as well as any precious metals which may have been present in the feed.
The following examples are included for illustrating certain aspects of the present invention. The examples should therefore not be construed as limiting any aspect of the invention. Some examples of the process of the invention will now be described with reference to the accompanying drawings.

EXAMPLE 1

Bessemer matte (Cu (40.7%), Ni (39.7%), Co (0.54%), Fe (0.91%) and S (18%)) was roasted in a 12 inch (30.4 cm) fluid bed. The calcine was used as feed for the reduction tests.
A 500-gram sample of calcine was charged to a 2 inch (5.1 cm) diameter mini plant fluid bed and heated to 1000° C. The CO₂/CO ratio was controlled at 1000 to affect a pO₂of about 10⁻⁸(See FIG. 2). Fluidizing gas flow rates were 10 L/min N2, 25 L/min CO₂and 0.025 L/min CO. The conditions were maintained for 1 hour and the furnace heater was turned off. The material was allowed to cool rapidly with the gases on. A second reduction test was performed in order to determine the effect of a longer residence time during reduction. The test was conducted under the same conditions but for 2 hours.
Several bench-scale leaching tests were conducted on the two reduced samples. 25-gram samples of calcine in 250 mL of solution containing 180-g/L of sulfuric acid were utilized. In the first tests, H₂O₂at 30% strength was added to oxidize the copper to CuO to allow leaching. Test results are shown in Table 1 below. Additional tests used oxygen and air sparging to oxidize the copper. See Table 2 below. The solution was allowed to leach at 80° C. for 2 hours. The samples were filtered and solution and residue sent for analysis.
Other leach tests were conducted by grinding the calcine for 15-s in a Blueler mill with air sparging for oxidation. Tests were also conducted using spent copper electrolyte stream from a copper electrowinning plant in place of pure sulfuric acid solution. Most tests utilized a 500 mL beaker and magnetic stirrer. A final test utilized a hydrometallurgical leaching apparatus for improved particle agitation and increased gas dispersion. Results of phase identification microprobe analysis of test #1 are shown in Table 3 below.
The reduced calcine had a porous structure when observed under optical microsopy. It is likely that under reduction, the CuO in solid solution with NiO in the roasted Bessemer Matte was ejected as Cu₂O, thus forming the observed porous structure. Removal of CuO from the NiO matrix during reduction makes the copper available for leaching. Moreover, the NiO is rendered more refractory after the CuO is removed from solid solution. A baseline test was conducted on roasted bessemer matte resulting in 9.4% of the nickel reporting to solution and only 73% copper dissolution, thus demonstrating that the NiO is not refractory when in solid solution with CuO. A SEM/EDX (semiquantitative) analysis (Table 3) from the 1-hour reduction-leach test indicated that the CuO was reduced from ˜30% in solid solution with NiO, to 6.6% (±4.2). This correlates to the residue assay of 4.4% CuO. The improved leach results observed in the 2-hour reduction indicate that even more CuO was ejected (2.3% CuO in residue).
The coarse calcine was not ground prior to leaching for experiments conducted using H₂O₂for oxidation. The resulting leach residue was very fine, indicating that removal of the copper oxide resulted in liberation of the remaining NiO grains.
Leach tests conducted with air and no grinding resulted in only 65% copper recovery (Table 2). Grinding the reduced calcine prior to leaching provided results comparable to tests performed using peroxide. Copper recovery of 95% in test #7 (Table 2, ground calcine, air sparging) is comparable to Tests #1 and #2 (Table 1, no grind, peroxide), which also had copper recovery of 95%. Ni, Fe, and Co losses to solution were slightly increased with sample grinding.
Leach test #12 was conducted using increased agitation. The sample was not ground and achieved a 90% copper recovery compared to 65% copper recovery using air sparging and a magnetic spin bar (Test #6). This test demonstrates that increased agitation improves dissolution of copper.

Thus, by optimizing grinding, increasing agitation and/or utilizing oxygen to recover copper, results equivalent to use of hydrogen peroxide can be achieved.

TABLE 1


Analysis of Selected Tests Using Hydrogen Peroxide for Oxidation

Samples Were Not

Assays

Distribution

Ground	Ni	Cu	Co	Fe	Ni	Cu	Co	Fe

Test #

1	Residue %	74.2	3.5	1.1	2.0	99.8	4.7	99.8	99.3
(1-Hour	Liquor gpL	0.11	42.89	0.001	0.017	0.2	95.3	0.2	0.7
Reduction)
Test #2	Residue %	74.1	3.5	1.0	2.0	99.8	4.5	99.8	98.6
(1-Hour	Liquor gpL	0.092	38.1	0.001	0.008	0.2	95.5	0.2	1.4
Reduction)
Test #5 (2	Residue %	75.7	1.8	1.1	2.0	99.7	2.2	99.6	99.0
Hour	Liquor gpL	0.177	49.91	0.003	0.015	0.3	97.8	0.4	1.0
Reduction)

TABLE 2


Analysis of Selected Tests using Air as Leach Oxidant

Tests done with 1-Hour

Assays

Distribution

Reduction	Ni	Cu	Co	Fe	Ni	Cu	Co	Fe

Test #6	Residue %	57.4	22.5	0.8	1.7	99.3	34.8	99.3	97.6
(No grind)	Liquor gpL	0.448	41.4	0.007	0.046	0.7	65.2	0.7	2.4
Test #7	Residue %	70.9	4.08	0.981	1.8	99.1	5.0	99.1	96.9
(Grind)	Liquor gpL	0.512	52.3	0.007	0.051	0.9	95.0	0.9	3.1
Test #12	Residue %	70.2	7.3	1.0	1.9	99.7	9.6	99.4	98.4
(No grind, Agitation)	Liquor gpL	0.132	33.5	0.03	0.017	0.3	90.4	0.6	1.6

TABLE 3


SEM/EDX Microprobe Analysis of Leach Residue

	Phase	Average	2 S.D.	Max	Min

FeO	2.34	1.65	4.39	6.47
CoO	1.77	1.17	3.7	0.71
NiO	89.34	4.8	94.91	85.15
CuO	6.55	4.16	10.82	3.91

EXAMPLE 2

The effect of iron in a Cu—Ni—Fe system was evaluated. Three samples were tested: 1) Flash Furnace Matte (Cu-23%, Ni-22.6%, Co-0.664%, Fe-25.2%, S-26.7%), 2) Synthetic Matte (Cu-14.9%, Ni-16%, Co-0.435%, Fe-39.2%, S-29.4%), and 3) Bulk Cu—Ni Concentrate (Cu-12.1%, Ni-9.25%, Co-0.278%, Fe-37.5%, S-32.3%) Good separation of copper was achieved in each case. Table 4 summarizes the test results. The following procedure was used with respect to each sample:

The sample was melted and water granulated. About 2 kg of granulated sample was charged into a refractory pan and heated to ˜900° C. for ˜24 hours in an ambient air atmosphere to provide a roasted material. Fluid bed reduction was effected by first charging 1 kg of the roasted material into a 2 inch (5.1 cm) diameter fluid bed and heating to 1000° C. (external electrical elements) using air for fluidization at ˜45 slpm. The material was allowed to “re-roast” for 10 minutes at 1000° C. and then the air atmosphere was replaced by a gas mixture (20 slpm N₂, 25 slpm CO₂, 0.025 slpm CO). The material was then allowed to reduce for 2 hours at 1000° C., at which point the furnace power was turned off (leaving the gas mixture on) and allowed to cool for ˜30 minutes. To provide an oxidizing leach, 25 grams sulfuric acid was mixed with 250 mL distilled water in a 500 mL beaker, heated and maintained at 80° with stirring using a hotplate and a stirring mechanism. 25 grams of material to be leached was added to the beaker. 30 mL of hydrogen peroxide was gradually added at time=0 hours and gradually added again at time=1 hour. The test was stopped at 2 hours and the sample was filtered. Respective weights and volumes were recorded.

TABLE 4


Result for Roast-Selective Reduction-Leach of High Iron Mattes

Samples Were Not

Assays

Distribution

Ground	Ni	Cu	Co	Fe	Ni	Cu	Co	Fe

Flash	Residue %	40.7	3.1	1.0	32.5	96.4	13.3	89.8	70.8
Furnace	Liquor	0.891	21.77	0.069	8.22	3.6	93.3	10.2	29.2
Matte	gpL
Synthetic	Residue %	25.3	3.0	0.6	44.8	95.2	13.3	93.6	78.0
High Iron	Liquor	0.836	12.41	0.031	9.35	4.8	86.7	6.4	22.0
Matte¹	gpL
Bulk Cu-Ni	Residue %	18.3	5.5	0.5	50.3	92.4	21.7	90.2	71.7
Concentrate	Liquor	0.925	10.863	0.036	13.108	7.6	78.3	9.8	28.3
	gpL

¹Synthetic High Iron Matte was prepared by mixing Flash Furnace matte with iron sulfide. The mixture was melted and water granulated.

This example indicates that for the roast-reduction-leach process, as the level of iron in the feed is reduced, the extraction of copper increases. Copper ferrites can be transformed to cuprite. The free cuprite can be readily leached. Another important observation is that the nickel extraction to solution is higher with increased iron in feed. Nickel extractions of 3.6-7.6% were observed as compared with low iron (1% Fe) bessemer matte calcine where only 0.3% of the nickel was extracted.
In accordance with the provisions of the statute, there is illustrated and described herein specific embodiments of the invention. Various modifications may be made to the examples and embodiments set forth herein without departing from the scope and spirit of the invention which is defined by the appended claims. Those skilled in the art will understand that changes may be made in the form of the invention covered by the claims and that certain features of the invention may sometimes be used to advantage without a corresponding use of the other features.

Claims

1. A process for recovering copper from sulfide-containing material which contains, in addition to copper, at least one other metal selected from the group consisting of iron, nickel and cobalt, comprising roasting said material for sufficient time to provide a substantially sulfur-free calcine, subjecting the calcine to sufficient heat and a reducing atmosphere to selectively reduce CuO in the calcine to Cu₂O and form a reduced calcine, subjecting the reduced calcine to an oxidizing leach and recovering the copper.

2. A process according to claim 1 wherein the reducing atmosphere is characterized by a partial pressure of oxygen sufficiently low to reduce CuO to Cu₂O, but no so low as to reduce NiO to Ni metal.

3. A process according to claim 1 wherein the reducing atmosphere is characterized by pO₂˜10⁻¹⁰.

4. A process according to claim 1 wherein the sufficient heat ranges from about 750° C. to about 1050° C.

5. A process according to claim 1 wherein the sulfide containing material is bessemer matte.

6. A process according to claim 1 wherein the oxidizing leach is in sulfuric acid.

7. A process according to claim 1 wherein the reducing atmosphere contains a mixture selected from the group consisting of CO/CO₂, H₂/H₂O and combinations thereof.

8. A process according to claim 1 wherein the calcine is reduced in a controlled atmosphere using a fluid bed apparatus.

9. A process according to claim 6 wherein leach liquor from the oxidizing leach is subjected to electrowinning to recover the copper therefrom, and spent electrolyte from the electrowinning operation is recycled to constitute the aqueous sulfuric acid solution for leaching a further supply of reduced calcine material.

10. A process according to claim 5 wherein the bessemer matte is dead roasted to remove all the sulfur.

11. A process according to claim 4 wherein the temperature is about 1000° C.

12. A process for selectively reducing CuO to Cu₂O in a calcine which contains, in addition to copper, at least one other metal selected from the group consisting of iron, nickel and cobalt, comprising roasting said material for sufficient time to provide a substantially sulfur-free calcine, subjecting the calcine to sufficient heat and a reducing atmosphere to selectively reduce CuO in the calcine to Cu₂O and form a reduced calcine.

13. A process for selectively reducing CuO to Cu₂O in a calcine according to claim 12 wherein the reducing atmosphere is characterized by a partial pressure of oxygen sufficiently low to reduce CuO to Cu₂O, but no so low as to reduce NiO to Ni metal.

14. A process for selectively reducing CuO to Cu₂O in a calcine according to claim 13 wherein the reducing atmosphere is characterized by pO₂˜10⁻¹⁰.

15. A process for selectively reducing CuO to Cu₂O in a calcine according to claim 12 wherein the sufficient heat ranges from about 750° C. to about 1050° C.

16. A process for selectively reducing CuO to Cu₂O in a calcine according to claim 12 wherein the temperature is about 1000° C.

17. A process for selectively reducing CuO to Cu₂O in a calcine according to claim 12 wherein the sulfide containing material is bessemer matte.

18. A process for selectively reducing CuO to Cu₂O in a calcine according to claim 17 wherein the Bessemer matte is dead roasted to remove all the sulfur.

19. A process for selectively reducing CuO to Cu₂O in a calcine according to claim 12 wherein the calcine is reduced in a controlled atmosphere using a fluid bed apparatus.

20. A process for selectively reducing CuO to Cu₂O in a calcine according to claim 12 further comprising subjecting the reduced calcine to an oxidizing leach and recovering copper.