CA2273067A1 - Process for recovering value metals from iron-containing alloys - Google Patents

Abstract

A process for selectively extracting a value metal from a ferrous solid, the value metal being selected from the group consisting of cobalt, nickel and copper, and the ferrous solid being selected from mattes and alloys containing iron and the value metal in metallic form. In a first leach step, an aqueous solution is provided in contact with an oxygen-containing atmosphere, the first aqueous solution having a pH of less than about 2.0 and containing sufficient sulfate ion to form a soluble sulfate with the value metal contained in the ferrous solid.
The ferrous solid is added to the first aqueous solution over a period of at least one hour to increase the pH to the range of about 4 to 6, while maintaining the temperature of the first aqueous solution at less than about 100°C during addition of at least the last fifty percent of the solid. At the end of the first leach, a solid/liquid separation is performed to separate; a first liquid fraction containing soluble sulfates of the value metal from a solid iron-containing residue. The solid residue is preferably treated in a second leach step to recover copper and unleached cobalt. The second leach step comprises treating the solid residue with an acidic aqueous solution at elevated pressure and a temperature of about 120 to 220°C, to thereby oxidize copper sulfides present in the residue and cause copper to enter the liquid phase as soluble copper salts.

Description

PROCESS FOR RECOVERING VALUE METALS FROM
IRON-CONTAINING ALLOYS
FIELD OF THE INVENTION
The present invention relates to a process for recovering value metals such as cobalt and/or nickel from alloys and mattes containing substantial amounts of iron.
BACKGROUND OF THE INVENTION
Substantial reserves are known to exist of ores., both of the oxidic and sulfidic types, as well as slags, which contain relatively small amounts of value metals such as cobalt, copper and/or nickel and relatively large amounts of iron. The first step in recovering value metals from such ores and/or stags is a pyrometallurgical reduction process which converts oxides of metals in the ore or slag to metals having a zero oxidation state. This reduction process is conducted at high temperature in a reduction furnace, and the material produced by the reduction is typically referred to as a "matte" or "alloy".
Alloys typically contain about 35 to 70 percent iron, and less than about 10 to 60 percent of value metals such as cobalt, copper and/or nickel. Mattes are similar in composition to alloys, but have a relatively high sulfur content, typically exceeding about 10 percent. The sulfur is typically associated with copper, which can comprise up to about 30 percent of the matte or alloy.
Metals are recovered from mattes and alloys by a hydrometallurgical process involving an acidic leach process conducted under oxidative conditions, in which all the metals present in the matte or alloy are oxidized and dissolved in the form of soluble metal salts.
After separation of the liquid fraction from the solid residue, iron is separated from the remaining metals in the liquid fraction by precipitation.
One disadvantage with conventional processes is that the hydrometallurgical leaching step involves dissolving all the metals to produce a liquid fraction in which the value metals account for only a small fraction of the total dissolved metals. Clearly, it would be desirable from an economic standpoint to reduce the volumes of liquid used in the leaching step and thereby improve its efficiency.
SUMMARY OF THE INVENTION
The present invention overcomes the above-mentioned problems of prior art processes for recovering value metals from a matte or alloy by providing a process in which the iron in the matte or alloy remains in a substantially insoluble state during the leaching process, thereby significantly reducing the volume of liquid which is required to dissolve the metals in the matte or alloy.
The process of the present invention is particularly adapted for recovery of value metals such as cobalt, copper and/or nickel from mattes and alloys in which the value metals are entrained with relatively large amounts of iron. The process of the present invention involves a first hydrometallurgical leaching step in which a quantity of matte or alloy is contacted with an aqueous, acidic solution containing sulfate ion to form a soluble sulfate of the value metal contained in the matte or alloy. This first leach is conducted under conditions in which the iron contained in the matte or alloy is oxidized and transiently forms soluble sulfates which are immediately converted to insoluble compounds which precipitate out of the solution. A
solid/liquid separation then separates the value-metal containing liquid fraction from the iron-containing solid residue, and the liquid fraction is purified, where necessary, and further processed to recover the value metals therefrom.
Therefore, in one aspect, the present invention provides a process for selectively extracting a value metal from a ferrous solid, said value metal being selected from one or more members of the group consisting of cobalt and nickel and said ferrous solid being selected from mattes and alloys containing iron and said value metal in metallic form, wherein said process comprises: (a) providing a first aqueous solution in contact with an oxygen-containing atmosphere, said first aqueous solution having an initial pH of less than about 2.0 and containing sufficient sulfate ion to form a soluble sulfate with said value metal contained in a predetermined quantity of said ferrous solid; (b) adding said predetermined quantity of said ferrous solid to said first aqueous solution over a period of at least one hour to increase the pH
of the first aqueous solution to the range of from about pH 4 to about pH 6, wherein a temperature of the first aqueous solution during addition of at least a final 50 percent of said ferrous solid is maintained at less than about 100°C; (c) conducting a solid/liquid separation to separate a first liquid fraction containing soluble sulfates of said value metal from a first solid residue containing substantially all of the iron in said predetermined quantity of said ferrous solid; and (d) reducing and recovering said value metal from said first liquid fraction.
In addition to containing large amounts of iron;, mattes and alloys may also contain up to about 30 percent copper. Copper is typically contained in a matte or alloy in the form of insoluble sulfides and passes through the first leach substantially unleached, being present in the solid residue recovered from the first leach. In circumstances where it is desired to also recover copper from the matte or alloy, the process of the present invention includes optional steps for treating the solid residue of the first leach under conditions in which the copper sulfides in the solid residue are oxidized to soluble copper sulfates. After a second solid/liquid separation, the liquid fraction containing copper sulfates is purified and treated to recover copper therefrom.
Accordingly, in a second aspect, the present invention provides the process as described above, wherein said ferrous solid additionally contains up to about 10 percent by weight sulfur in the form of copper sulfides, said copper sulfides being substantially unoxidized during said steps (a) and (b) and being contained in said first solid residue, said copper being extracted from the first solid residue by a process comprising: (a) oxidizing said copper sulfides in said first solid residue to produce soluble copper sulfates by contacting said first solid residue with a second aqueous solution containing sulfate ion and having an initial pH of less than about 3.0 in the presence of a pressurized oxygen-containing atmosphere and at a temperature of from about 120 to about 220°C; (b) conducting a liquid/solid separation to produce a second liquid fraction containing said soluble copper sulfates and a second solid residue; (c) reducing and recovering said copper from said second liquid fraction.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described, by way of example only, with reference to the accompanying drawings, in which:
Figure 1 is a flow diagram showing a preferred two-stage leaching process according to the present invention; and Figure 2 is a flow diagram showing an alternate two-stage leaching process according to the present invention.
DETAIL DESCRIPTION OF PREFERRED EMBODIMENTS
A preferred process for extracting value metals from a matte or alloy will now be described below with reference to the flow diagrams shown in Figures 1 and 2.
As discussed above, the present invention is useful for extracting value metals from mattes and alloys which are produced by reduction of iron-rich ores. These mattes or alloys typically contain for example about 3 5 to 70 percent by weight iron, less than about 10 up to about 60 percent cobalt and/or nickel and up to about 30 percent copper. Alloys typically contain little or no sulfur, whereas mattes contain greater than about 10 percent sulfur. The method of the present invention is also useful for extracting value metals from materials having a sulfur content of up to about 10%, which are neither mattes nor alloys. These materials are sometimes referred to as "malloys". However, the terms "matte" and "alloy" as used herein are intended to include materials containing from 0 to 10 percent sulfur.
In a particularly preferred embodiment, the process of the present invention is utilized to recover cobalt and copper from a material which contains about 65 percent iron, 6 to 7 percent cobalt (this amount includes nickel which has similar chemistry), about 13 to 27 percent (average 19 percent) copper, relatively small amounts of zinc, selenium, tellurium, manganese, chromium, cadmium, arsenic and lead, which may or may not be recovered according to the process of the invention. The material also contains about 5 percent sulfur, and therefore, strictly speaking, it can neither be categorized as a matte nor an alloy, but is nevertheless referred to herein as a matte or alloy.
Most of the metals in the matte or alloy, including iron and value metals, are present in their zero oxidation state, having being reduced in the previous ore processing step. However, at least some of the copper in the matte or alloy is present in the form of the copper sulfide Cu2S
and, if the sulfur content is relatively high, will also be present as CuS.
The first step in the preferred process of the present invention comprises leaching cobalt from the matte or alloy by an acidic leaching step conducted under oxidative conditions. In a preferred embodiment, the matte or alloy is charged over a period of time into a reactor containing an acidic solution of sulfate ion under an oxygen-containing atmosphere. The amount of sulfate ion contained in the leaching liquid is preferably stoichiometrically matched to the value metal content ofthe matte or alloy. The adjustment of'sulfate content is discussed more fully below.
Under the conditions employed in the preferred leaching step, value metals such as cobalt and nickel are oxidized and form soluble sulfates, namely CoS04 and NiS04. The metallic iron present in the matte or alloy is oxidized and reacts with sulfuric acid in the leaching solution to form the soluble iron sulfates FeS04 and Fe2(S04)3. However, these iron sulfates immediately undergo disproportionation, primarily forming iron hydroxide Fe0(OH) and iron hydroxysulfate Fe(OH)504, both of which are insoluble and precipitate from the solution.
Sulfuric acid is regenerated in the production of these insoluble iron compounds. Therefore, iron is transiently dissolved during the first leaching step, but is immediately precipitated as insoluble compounds.
The transient dissolution of iron does not need to be accounted for in the design of the leach step, in terms of reactor volume, liquid volume or sulfixric acid concentration, and therefore iron may effectively be regarded as being insoluble during the first leach.
Under the conditions used in the first leaching step, copper which is present in the matte or alloy in the form of copper sulfides, is substantially unoxidized and remains in the solid phase.
Therefore, the first leach effectively separates value metals such as cobalt and/or nickel from iron and copper. The preferred conditions for the first leach are now discussed below.

_g_ The solution which forms the liquid fraction of the first leach is acidic, preferably containing sulfuric acid and having an initial pH no greater than about 2Ø
More preferably, the leaching solution is a recycled electrolyte from a cobalt and/or copper electrowinning process, discussed in greater detail below. Where the liquid fraction is a recycled electrolyte from copper or cobalt electrowinning, it will contain sulfate ion in the form of sulfi~ric acid, and in the form of soluble sulfates of metals such as copper. Sulfate ion in these forms is referred to herein as "exchangeable sulfate", since it is available for formation of soluble sulfates of value metals such as cobalt and/or nickel.
In order to provide the oxidative conditions necessary to oxidize the value metals and the iron in the matte or alloy, the acidic leaching solution is preferably in contact with an oxygen-containing atmosphere, such as air (20 percent oxygen) or an atmosphere containing higher amounts of oxygen, for example 93 percent oxygen from a vacuum swing absorption plant or 99.5 percent from a cryogenic plant. The oxidative atmosphere preferably has a pressure at or above atmospheric pressure. More preferably, the oxygen-containing atmosphere is pressurized, thereby increasing the speed of the leaching reaction b;y increasing the amount of oxygen in solution. Most preferably, the oxygen-containing atmosphere has a pressure of up to about 1,000 kPa, and even more preferably from about 300 to about 700 kPa.
The temperature of the acidic leaching solution is preferably maintained at an average temperature of less than about 100°C during the leach in order to avoid oxidizing copper sulfides to soluble copper sulfates. More preferably, the temperature is maintained in a range of from about 65 to about 100 ° C, and even more preferably from about 75 to 85 ° C.
In the process of the present invention, the solid matte or alloy is added to the acidic leaching liquid gradually. The gradual addition of the matte or alloy results in a gradual rise in the pH level of the liquid from an initial pH of less than about 2.0, typically about 1.6, to a pH in the range of from about 4 to 6, and preferably from about 4.5 to about 5.5. This rise in pH reflects the consumption of HZS04 in the oxidation of value metals such as cobalt and/or nickel to produce soluble metal sulfates.
The addition of the matte or alloy may either be continuous or step-wise in portions. The size and number of portions is variable, with each portion preferably comprising from about 2 to about 50 percent of the total mass of the solid added. In general, continuous addition or addition of a relatively large number of small portions produces better results than the addition of relatively few large portions.
The matte or alloy is preferably added to the leaching liquid over a period of time of from about 1 to about 6 hours, and more preferably from about 2 to about 3 hours.
The matte or alloy may be added at either a constant or variable rate over this period of time.
The reaction between the metals in the matte or alloy with the acidic leaching liquid is highly exothermic. In prior art leaching processes, all of the matte or alloy is typically added to the leaching liquid in one portion at the beginning of the leaching process, resulting in a rapid increase in temperature to at least 130°C. Allowing the temperature to increase into this range causes the oxidation of copper sulfides present in the matte or alloy, producing soluble copper sulfates, some of which react further to produce additional sulfizric acid and hydrolyzed copper compounds. This prevents the pH from increasing to the above-mentioned range, resulting in copper contamination of cobalt rich liquors, and a more difficult separation of the liquid and solid fractions. In contrast, the step-wise addition of matte or alloy to the acidic leaching liquid in the process of the present invention removes heat from leach slurry residues and reduces the extent of sulfur oxidation, thereby improving the purity of the cobalt rich liquor.
Preferably, the matte or alloy is added to the liquid such that the temperature is kept below 100 ° C. However, it is to be noted that some excursions in temperature above 100 ° C may be tolerated at the start of the leach. Since the matte or alloy is added gradually, there is typically only a small proportion of the total matte or alloy present in the leaching liquid at the beginning of the leach. The oxidation of the copper sulfides in this portion of the matte or alloy will typically not have a significant detrimental effect on the extraction of the value metals or the purity of the cobalt rich liquor.
For example, the addition of the initial portions of the matte or alloy to the liquid may result in an exotherm to the range of from 100 to 150°C. After this, the temperature may be reduced, for example by flashing off steam, to the preferred range of less than about 100°C.
Preferably, the temperature then remains at less than about 100 ° C for the remainder of the leach.
In the preferred embodiment of the present invention, the temperature is brought down to the preferred range of less than 100°C for the addition of at least the final 50 percent of the matte or alloy. More preferably, the temperature remains at less than 100 ° C
for substantially the entire addition of the matte or alloy to the acidic leaching liquid.
The inventors have found that performing a first leaching step according to the preferred method described above results in substantially all the copper and iron remaining in the solid phase with from about 70 percent to about 95 to 100 percent of the value metals such as cobalt and/or nickel being extracted into the liquid phase. After completion of the leach, a solid/liquid separation is conducted to separate a value metal-containing stream from the solid residue. This separation may preferably be accomplished by filtration or by counter current decantation (CCD).
In leaches conducted with a "malloy" having the composition set out above, the following ratios of cobalt to other metals are typically observed in the value metal stream:
Co/Fe > 10,000 Co/Cu > 100 Co/As > 10,000 Co/Pb > 10,000.
Therefore, it can be seen that the first leach in the preferred process of the present invention provides a liquid stream which is predominantly comprised of soluble salts, primarily sulfates, of value metals such as cobalt and/or nickel. As shown in the flow diagram of Figure 1, this value metal stream may preferably be subjected to fizrther purification by conventional means to remove impurities such as nickel or zinc from the solution. After purification, cobalt is recovered from the liquid stream, preferably by electrowinning in which cobalt ions are reduced and precipitated as cobalt metal in its zero oxidation state. The spent electrolyte from the electrowinning step contains sulfizric acid and residual amounts of metal salts. Preferably, at least a portion of this electrolyte is re-circulated for incorporation into the acidic leaching liquid in the first leach, with optional bleeding of a portion of the spent electrolyte or neutralization of the acid in the electrolyte to adjust the amount of sulfate ion contained therein.
Although a specific purification circuit is shown in Figure 1, it is to be appreciated that numerous other methods exist for purifying the value metal-containing stream obtained from the solid/liquid separation conducted at the end of the first leach. In another preferred embodiment of the present invention, illustrated in Figure 2, cobalt is precipitated from the value metal-containing stream in the form of cobalt carbonate by addition of sodium carbonate and sodium hydroxide to the value metal stream. The cobalt carbonate then undergoes a series of additional steps by which it is purified, culminating in a cobalt electrowinning step to produce cobalt metal.
Where the matte or alloy contains significant amounts of copper, and where it is desired to recover the copper contained in the matte or alloy, the solid residue obtained from the solid/liquid separation in the first leach is subjected to fiarther processing in a second leaching step. The object of the second leach is to extract copper and the remaining cobalt unextracted in the first leach into the aqueous phase while iron remains in the solid phase. As in the first leach, the solid phase is contacted with an acidic leaching liquid which contains sulfuric acid, the leaching liquid preferably comprising a spent electrolyte from either a cobalt or copper electrowinning step which, as discussed above, may contain residual amounts of soluble metal salts. The initial pH of the leaching liquid is typically less than about 2.0, more typically about 1.4, reflecting the sulfizric acid content of the leaching liquid. After addition of the solid residue to the leaching liquid, the pH may increase to about 2. S to 5.0, but decreases during the second leach to about 1.4 to 1.6, reflecting the liberation of HZS04 during the formation of insoluble iron compounds.
The second leaching step is also conducted under oxidative conditions in order to facilitate oxidation of the copper sulfides present in the solid phase. As in the first leach, the oxygen-containing atmosphere may be comprised of air or oxygen in varying states of purity. In the second leach, the oxygen-containing atmosphere is pressurized, preferably to a pressure of from about 400 to about 2,500 kPa, and more preferably from about 700 to about 1,200 kPa. As in the first leach, pressurization of the oxygen-containing atmosphere ensures that sufficient oxygen enters the liquid phase to oxidize any remaining metal compounds, including sulfides, in the matte or alloy.
The temperature in the second leach is preferably higher than that of the first leach in order to provide more vigorous conditions for copper sulfide oxidation.
Preferably, the temperature in the second leach is from about 120 to 220 ° C, and more preferably from about 13 0 to 170 ° C. In general, the higher the pressure and temperature in the second leach, the higher will be the degree of copper extracted from the solid phase. However, it will be appreciated that the actual pressure and temperature conditions used in the second step are largely determined by economic considerations, with the increased value of extracted copper being weighed against the costs associated with raising the pressure and/or temperature of the second leach. For example, the use of a temperature of 130 to 170°C and a pressure of from 700 to 1,200 kPa will typically extract about 70 percent of the copper from the solid phase during the second leach. However, the application of more vigorous conditions can increase the level of copper extraction to about 95 to 100 percent.
Under the above conditions, copper is leached into the liquid phase as soluble salts while substantially all the iron remains in the solid phase, primarily in the form of Fe0(OH), with some iron (III) oxide (Fez03) being formed, both of these compounds being substantially insoluble in the liquid phase. The relative proportions of the iron compounds formed in the second leach is at least partially dependent on the leach temperature and pressure. For example, higher temperatures and pressures favour the formation of iron III oxide over Fe0(OH).
In the second leach, the solid residue from the first leach may preferably be added in one or more portions, the size and number of the portions being relatively unimportant because temperature control is relatively unimportant in the second leach, as long as it is sufficient to oxidize copper sulfides. Preferably, the second leach is continued for a period of from about 1 to 8 hours, most preferably about 3 to 5 hours.
In addition to extraction of copper from the solid phase, the second leach also is usefi~l for removing any residual value metals remaining in the solid residue after the first leach. These value metals may be entrained in the solid residue in the form of soluble sulfates, or in the form of insoluble compounds such as hydroxysulfates or metallics. These compounds are converted in the second leach to soluble sulfates, such as cobalt sulfate,. and are extracted into the liquid phase during the second leach. The amounts of value metals extracted during the second leach may be significant, depending on the amounts extracted during the first leach. For example, the second leach may extract up to about 30 percent of the cobalt originally contained in the matte or alloy.
A fizrther variation of the preferred process of the present invention involves the exothermic nature of the second leach. In many cases, the unleached sulfur in the form of sulfides present in the solid residue from the first leach is sufficient to provide the exothermic heat required to sustain the leach temperature of the slurry during the second leach. Where an excess of heat is generated, this excess may be recovered as steam in a flash-down process or other cooling method. Where the sulfide content is insufficient to support an autogenous second leach, then a fizrther variation of the preferred process comprises diversion of some of the matte/alloy from the first leach directly to the second leach, thereby producing an energy source for the second leach step.
After the completion of the second leach, a solid/liquid separation is conducted in order to separate the copper containing liquid stream, which may also contain some other value metals, from the iron-containing solid residue. As in the first leach, the separation is preferably accomplished by filtration or counter current decantation (CCD) methods. In order to remove soluble copper and cobalt compounds from the solid residue as completely as possible, the solid residue is preferably washed at this point by addition of fresh water, and the wash liquors are added to the copper containing liquid stream. In preferred embodiments of the invention in which spent electrolytes from cobalt and/or copper electrowinning are used as the leaching liquids, this stage is a convenient location for intake of fresh water into the process.
After washing the solid residue, the combined liquid phase is subjected to further purification in order to recover copper therefrom.
As a first purification step, shown in Figure 1, the copper containing liquid phase may be subjected to a conventional process in which selenium and/or tellurium are precipitated from the liquid phase. These metals are typically present in the :matte or alloy in small amounts. After recovery of selenium and/or tellurium, the purified liquid stream is then preferably subjected to copper electrowinning, during which copper ions are reduced to provide copper metal in its zero oxidation state.
The spent electrolyte from the copper electrowinning step primarily contains sulfizric acid, but typically also contains some amounts of soluble metal sulfates, such as copper sulfate, iron sulfates and cobalt sulfate. The amount of copper in the spent electrolyte can be significant, on the order of about 30 g/L. Similarly, as discussed above the spent electrolyte may contain up to about 30 percent of the cobalt originally present in the matte or alloy. In order to recover these residual metals, the spent electrolyte is preferably recycled for reuse as the leaching liquid in the first and/or second leach, and may optionally be combined with spent electrolyte from the cobalt electrowinning step discussed above. The recycling of spent electrolyte is also preferred because sulfuric acid generated during electrowinning reduces the need to input fresh sulfuric acid into the process.
In the preferred process of the invention in which spent electrolyte is recycled, the sulfur contained in the matte or alloy forms the primary source of sulfate ion in the leaching liquids for the first and second leaches. As discussed above, mattes and alloys contain varying amounts of sulfur. Where the sulfi~r content is relatively low, the amount of sulfate generated may be insufficient to completely leach value metals from the matte or alloy. Under these circumstances, the sulfate content of the liquid phase is preferably supplemented by addition of sulfizric acid or by recycling cobalt and copper spent electrolytes.
Conversely, in mattes and alloys having a relatively high sulfur content, it may be preferred to reduce the sulfate content of the spent electrolyte in order to balance the exchangeable sulfate demand over the first and second leaches. Sulfate content may preferably be reduced by bleeding some of the spent electrolyte and replacing it with fresh water (preferably during the washing step discussed above) or by neutralizing some of the acid in the spent electrolyte by addition of a base.
An alternate preferred method for reducing sulfate content in the liquid phase is to add a jarosite-forming salt to the second leach in order to form jarosite, a sulfate-containing iron mineral, having a formula such as NaFe3(S04)Z(OH)6, which is insoluble in the leaching liquid, thereby efi~ectively removing some of the sulfate from the liquid phase.
Preferred jarosite-forming salts include alkaline metal salts such as sodium or potassium sulfate.

The present invention is further illustrated by the following examples.

This Example comprised a two-step process according to the preferred embodiment of the present invention. The conditions for the first leach are shown in Table 1, comprising a batch leach log sheet. In this Example, 1,700 g of an alloy containing 65 percent Fe, 6 to 7 percent Co, 19 percent Cu and 5 percent S was added step-wise in portions to 3,000 ml of a leaching liquid comprising the spent electrolyte from a copper electrowinning process. As shown in Table 1, most of the alloy was added to the leaching liquid during the first hour of the leach, during which time the pH was raised to 4.70. The leach was continued for over five hours, at the end of which time the pH had increased to 5.25. The leach was run at a canstant pressure of 500 lcPa and at a temperature of approximately 80°C.
Table 2 shows the extraction data for the first leach of Example 1. As shown in Table 2, the copper content in the liquid phase, initially at 40.7 g/1, was reduced to 0.078 g/1 after one hour. This reflects the conversion of soluble copper sulfate in the spent electrolyte to insoluble copper hydroxysulfate , 2 Cu(OH)Z.Cu 504. Also after one hour of leach time, the amount of cobalt in the liquid phase increased from 4.53 g/1 to 21.2 g/1, translating to an extraction of 74.4 percent of the cobalt in the solid alloy. Table 2 also shows the content in the liquid phase of a number of other metals. Most notably, iron is present in the liquid phase in an amount of less than 0.45 ppm.

Following filtration of the leach mixture, the solid residue is treated by a second leach under the conditions shown in Table 3. In this leach, the liquid phase comprised spent electrolyte from a copper electrowinning process (Reagent 1 ), supplemented with additional sulfuric acid (Reagent 2). The composition of the copper spent electrolyte was Cu = 19.7 g/1, Co = 3.31 g/1, HzS04 = 28.4 g/1. To this liquid phase was added 2259.8 g of the solid residue obtained from the first leach of Example 1, the residue being added in four portions. This leach was conducted at a pressure of 680 kPa and at a temperature of about 160°C. The pH of the leach liquid decreased from an initial pH of 4.69 to a final pH of 1.56 after a leach time of eight hours.
As shown in Table 4, most of the copper was removed from the solid residue and into the liquid phase, with the extraction of copper from the solid residue after a leach time of five hours being 92.0 percent. Furthermore, after a leach time of five hours, 97.1 percent of residual cobalt contained in the solid residue was extracted into the liquid phase. In contrast, as shown in Table 4, the content of iron in the liquid phase remains fairly low, being 996 ppm after a leach time of nine hours.

As shown in Table 5, 850 g of the alloy of Example 1 was added into a solution of spent electrolyte from a copper electrowinning process, the alloy being added over a period of one hour at a temperature of about 80°C and a pressure of SOO 1cI'a. The pH of the liquid phase increased gradually from 2.45 to 5.37 over a period of four hours. The extraction data for the first leach of Example 4 is shown in Table 6, and is generally consistent with that discussed above in Example The conditions for the second leach of Example 4 are shown in Table 7. In the second leach, 981.76 g of residue was added to a solution containing spent electrolyte and sulfuric acid at a pressure of 1,100 to 1,200 kPa and a temperature of 150°C, causing the pH to be reduced to 1.36 after a leach time of eight hours. No fresh sulfuric acid was added, only that recycled in the spent electrolyte (Reagent 1) having the following composition: Cu = 0.55 g/l, Co - 20.3 g/l, HzS04 = 69.6 g/1. The extraction data for the second leach is shown in Table 8, showing excellent extraction of cobalt and copper during the second leach.

This Example shows only conditions and extraction data for the first leach in the process.
As shown in Table 9, 1,400 g of the alloy of Example 1 was added in small portions to a spent electrolyte solution at a pressure of SO kPa and a temperature of 85 to 90°C, causing the pH to rise to 5.11 after about ten hours.
As shown in Table 10, the conditions used in the leach of Example 5 resulted in poor extraction of cobalt from the solid phase, whereas most of the copper in the liquid phase was transferred to the solid residue. Significant soluble iron remained in the leach liquor (8.2 g/1).

As shown in Table 11, 1,800 g of the alloy of Example 1 was added in small portions over a period of five to six hours to a solution containing spent electrolyte at a pressure of 700 kPa and a temperature of about 70 to 85 ° C, raising the pH to 5.23 after a leach time of seven hours.
The results of the extraction in the first leach are shown in Table 12, with the cobalt extraction reaching a maximum of 84.7 percent after five hours, and the copper content of the liquid phase being reduced to 0.238 g/1 at the same time.
The conditions for the second leach of Example 6 are shown in Table 13, in which the solid residue from the first leach was added to a leach liquid at a pressure of 1,200 kPa and a temperature of about 160°C, decreasing the pH to 1.66 after eight hours. These conditions resulted in high extraction of both cobalt and copper fi~om the residue, with cobalt extraction reaching a maximum of 97.7 after five hours, and copper extraction reaching a maximum of 95.4 after nine hours.

As shown in Table 15, 1,450 g of the alloy of 1?xample 1 was added in small portions over a period of six hours to a solution of spent electrolyte at a pressure of 300 kPa and a temperature of about 80 ° C, increasing the pH from 1.22 to 5.08 after five hours.
As shown in Table 16, the copper content of the liquid phase was reduced to 0.07 g/1 after five hours, whereas cobalt was extracted to 74 percent after the same time.
In the second leach of Example 7, the conditions of which are shown in Table 17, 2173.33 g of the solid residue from the first leach was added to a solution of spent electrolyte and sulfuric acid at a pressure of 700 kPa and a temperature of about 130°C, decreasing the pH to 1.40. As shown in Table 18, copper extraction reached a maximum of 91.2 percent after seven hours and cobalt extraction reached a maximum of 94.8 percent after seven hours.

In the first leach of Example 8, the conditions of which are shown in Table 19, 1,250 g of the alloy of Example 1 was added in small portions over a period of six to seven hours at atmospheric pressure and a temperature of about 85°C, increasing the pH
to 4.46 after six and one half hours. As shown in Table 20, cobalt extraction was 92.9 percent after three hours, and the copper content of the liquid phase was reduced to a minimum of 1.2 ppm after five hours.
In the second leach, as shown in Table 21, 2,618.4 g of the solid residue from the first leach was added to a solution of sulfizric acid at a pressure of 1,000 kPa and a temperature of about 170°C, lowering the pH of the liquid phase to 1.40 after seven hours.
As shown in Table 22, copper extraction from the solid residue reached a maximum of 95.5 percent after eight hours, and cobalt extraction reached a maximum of 95.2 percent, also after eight hours. It is to be noted that since the leach liquid in the second leach comprised a solution of sulfuric acid, and not a spent electrolyte solution, the initial concentration of copper and cobalt in the liquid phase was zero.

In the first leach of Example 9, shown in Table 23, 1,800 g of the alloy of Example 1 was added in small portions over a period of about six hour's to a spent electrolyte solution at a pressure of 500 kPa, bringing the pH from 1.42 to 5.06 after six hours. It is to be noted that the temperature of the leach was allowed to rise to a high of 150°C over the first hour of the leach, and was subsequently reduced to about 80 ° C for the remainder of the leach time. About 28 percent (500 g) of the total mass of the alloy was added during the first hour of the leach.
As shown in Table 24, the copper content of th.e liquid phase was not reduced during the first hour of the leach, but was subsequently decreased to a minimum of 53 ppm after a leach time of seven hours. The cobalt extraction on the other hand reached a maximum of 94.1 percent after one hour. The leach obtained after five hours, at which time copper was reduced to 0.57 g/1 and zinc was reduced to 0.29 ppm, while about 75 percent of cobalt was extracted, represents highly preferred reaction conditions to obtain a pure cobalt-containing liquid stream. The lixiviant in the leach had a composition of Cu = 25.3 g/1, Co = 9.73 gfl, HZS04 = 17.6 g/1.

In the second leach of Example 9, shown in Table 25, a total of 2471.93 g of the residue from the first leach was added to a leaching liquid containing spent electrolyte and sulfuric acid at a pressure of 1,000 kPa and a temperature of 160°C, the pH being reduced from 2.50 to 1.40 after a leach time of eight hours. It can be seen from Table 26 that copper extraction in the second leach reached a maximum of 92 percent after a leach time of nine hours, and cobalt extraction reached a maximum of 96.8 percent after nine hours. However, it will also be seen that the copper and cobalt extractions were also high after a leach time of only three hours.

In the first leach of this Example, the conditions of which are shown in Table 27, 2,800 g of the alloy of Example 1 was added in small portions over a period of about three hours to a solution of spent electrolyte at a temperature of about 140 ° C and a pressure of 600 lcPa. In this Example, the pH rose from 1.03 to 3.84 after 195 minutes, reaching a maximum of 4.45 after 150 minutes.
In this Example, the temperature in the first leach is substantially higher than that in any of the other Examples and, as discussed above, oxidation of copper sulfides present in the alloy occurs, resulting in production of sulfuric acid and therefore the increase in the pH is less than that in the other Examples. The lixiviant in this leach had a composition of Cu = 24.5 g/1, Co =
8.69 g/1, HZS04 = 16.7 g/1.

The erect on the increased temperature on the copper and cobalt content of the liquid phase is significant. As shown in Table 28, the copper content of the liquid phase could not be reduced below 0.6 g/1, iron was not completely reduced and cobalt extractions were lower considering the feed alloy to lixiviant ratio. The lixiviant composition was Cu = 24.5 g/1, Co =
8.69 g/1, HZS04 = 16.7 g/1.
In the second leach, shown in Table 29, a total of about 120 g of the solid residue from the first leach was extracted with a mixture of spent electrolyte and sulfuric acid at a pressure of 1,000 kPa and a temperature of about 160°C, decreasing the pH to 1.41 after a leach time of five hours. However, Table 30 shows that the copper extraction reached a maximum of 94.7 after six hours and cobalt extraction reached a maximum of 88.3 percent after six hours.

As shown in Table 31, 3986 g of alloy was added in portions over a period of 135 minutes to a synthetic solution containing 16.9 g/1 copper, 10 g/1 cobalt and 40 g/1 free acid. The leach was conducted over a period of five hours at a temperature of about 85°C and a pressure of 500 kPa. The test extraction data for the first leach is shown in Table 32, with the copper being reduced from 16.9 g/1 to 0.35 g/1 after five hours, and cobalt extraction reaching 68.97 percent after the same time frame.
In the second leach, shown in Table 33,1708 g of the solid residue obtained from the first leach was added to a solution of sulfuric acid in portions over a period of ten hours, and a pressure of 1,000 to 1,200 kPa and a temperature of about 170°C. As shown in Table 34, copper extraction reached a maximum of 93.5 percent after five hours and cobalt extraction reached a maximum of 96.5 percent (also after five hours) of the unleached cobalt fraction.
Although the invention has been described in connection with certain preferred embodiments, it is to be understood that it is not limited thereto. Rather, the invention includes within its scope all embodiments which may fall within the scope of the following claims.

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Claims (21)

1. A process for selectively extracting a value metal from a ferrous solid, said value metal being selected from the group consisting of cobalt and nickel and said ferrous solid being selected from mattes and alloys containing iron and said value metal in metallic form, wherein said process comprises:
(a) providing a first aqueous solution in contact with an oxygen-containing atmosphere, said first aqueous solution having a pH of less than about 2.0 and containing sufficient sulfate ion to form a soluble sulfate with said value metal contained in a predetermined quantity of said ferrous solid;
(b) adding said predetermined quantity of said ferrous solid to said first aqueous solution over a period of at least one hour to increase the pH of the first aqueous solution to the range of from about pH 4 to about pH 6, wherein a temperature of the first aqueous solution during addition of at least a final 50 percent of said ferrous solid is maintained at less than about 100°C;
(c) conducting a solid/liquid separation to separate a first liquid fraction containing soluble sulfates of said value metal from a first solid residue containing substantially all of the iron in said predetermined quantity of said ferrous solid; and (d) reducing and recovering said value metal from said first liquid fraction.

2. The process of claim 1, wherein the temperature of the first aqueous solution is about 65 to 100°C.

3. The process of claim 1, wherein the temperature of the first aqueous solution is about 75 to 85°C.

4. The process of claim 1, wherein the temperature is maintained at less than about 100°C
during addition of substantially all of said ferrous solid.

5. The process of claim 1, wherein the ferrous solid contains about 35 to 70 percent by weight metallic iron and less than about 10 percent metallic cobalt.

6. The process of claim 1, wherein said step (d) comprises an electrowinning process.

7. The process of claim 6, wherein spent electrolyte from said electrowinning process is recycled for use in the first aqueous solution in said step (a).

8. The process of claim 1, wherein said ferrous solid is added over a time of from about 1 to 6 hours.

9. The process of claim 1, wherein said ferrous solid is added intermittently as a plurality of portions, each of said portions comprising from about 2 to about 50 percent of the predetermined quantity of said ferrous solid.

10. The process of claim 1, wherein a pressure of said oxygen-containing atmosphere is at a pressure ranging from atmospheric pressure to about 1,000 kPa.

11. The process of claim 10, wherein the pressure of said oxygen-containing atmosphere is from about 300 to about 700 kPa.

12. The process of claim 1, wherein said value metals include both nickel and cobalt and said ferrous solid additionally comprises zinc, and wherein said process further comprises a first purification step in which said nickel and zinc are removed from said first liquid fraction prior to said cobalt electrowinning step.

13. The process of claim 1, wherein said ferrous solid additionally contains copper in the form of copper sulfides, said copper comprising up to 30 percent by weight of said ferrous solid and being substantially unleached during said steps (a) and (b), said copper being extracted from the first solid residue by a process comprising:

(e) oxidizing said copper sulfides in said first solid residue to produce soluble copper sulfates by contacting said first solid residue with a second aqueous solution containing sulfate ion and having an initial pH of less than about 3.0 in the presence of a pressurized oxygen-containing atmosphere and at a temperature of from about 120 to about 220°C;
(f) conducting a liquid/solid separation to produce a second liquid fraction containing said soluble copper sulfates and a second solid residue;
(g) reducing and recovering said copper from said second liquid fraction.

14. The process of claim 13, wherein said temperature during said step (e) is maintained at about 130 to about 170°C.

15. The process of claim 13, wherein said pressurized oxygen-containing atmosphere is maintained at a pressure in the range of from about 400 to about 2,500 kPa during said step (e).

16. The process of claim 13, wherein said pressurized oxygen-containing atmosphere is maintained at a pressure in the range of from about 700 to about 1,200 kPa during said step (e).

17. The process of claim 13, wherein said step (g) comprises electrowinning said copper from said second liquid fraction.

18. The process of claim 17, wherein spent electrolyte from said copper electrowinning is recycled to at least one of steps (a) and (e), said spent electrolyte containing sulfuric acid and dissolved sulfates of copper, iron and said value metal.

19. The process of claim 13, wherein said second solid residue is washed with water and said water is added to said second liquid fraction.

20. The process of claim 13, wherein said ferrous solid additionally comprises at least one of selenium and tellurium, and wherein said second liquid fraction is purified before said step (g) by rejection of selenium and/or tellurium.

21. The process of claim 13, wherein an amount of said ferrous solid is added to said second aqueous solution during said step (e), said amount of said ferrous solid being sufficient to increase a temperature of said second aqueous solution from an ambient temperature to said temperature of from about 120 to about 220°C.