"METHOD FOR PRODUCTION OF METALLIC COBALT FROM THE NICKEL SOLVENT EXTRACTION RAFFINATE".

The present report relates to a method for production of metallic cobalt from the nickel solvent extraction raffinate , and more specifically, from the nickel solvent extraction raffinate from refineries where mixed hydroxides of nickel and cobalt are subjected to ammonia leaching.

As is known by the person skilled in the art, there have been developed techniques for cobalt recovery by direct precipitation of cobalt carbonate in a column for total removal (stripping) of ammonia from the solution. Said techniques, however, remain technically and economically unfeasible.

One of the disadvantages aforementioned is the fact that, without addition of reagent to the raffinate prior to ammonia stripping, only a small amount of cobalt precipitates from the solution, which renders the recovery technique low profitable. A counterpart disadvantage arises from increasing the consumption of reagent for said refining process, which consequently increases cobalt recovery by said technique. The high consumption of reagent renders the process economically unfeasible.

Therefore, one of the objectives of the present invention is to provide a method for production of metallic cobalt from the nickel solvent extraction raffinate so that significant cobalt recovery efficiency is achieved without significant increase in costs.

One other objective of the present invention is to provide a method for production of metallic cobalt from the nickel solvent extraction raffinate comprising an operational sequence that is both technically and economically feasible.

As is known, according to conventional technique, there are four types of processes or routes for nickel-ore treatment, namely, nickel matte production (pyrometallurgical), ferronickel production (pyrometallurgical),

ammoniacal reduction-leaching (pyrometallurgical/hydrometallurgical), and pressure acid leaching (hydro-metallurgical).

With regard to the routes aforementioned, the first is applicable to ores bearing nickel associated with sulfur, and in this process, use is made of the caloric power of the minerals present in the smelting stage.

The other three processes are used where the nickel metal is associated with oxygen (oxidized or lateritic ore), the choice of which process to use being dependent on the composition of the ore to be treated.

The ferronickel production process is used for high-grade magnesium ores with a Mg/Si ratio such that the gangue produced by the process has sufficient fluidity to flow out of the electric furnace and is not corrosive to the refractory lining materials of the electric furnace. Typically, the ores treated by this process contain iron grades lower than those of ores treated by the two processes hereinabove. On the other hand, nickel grades are higher. As for the ammoniacal reduction-leaching process, a combination of pyrometallurgical and hydrometallurgical techniques is used to separate nickel and cobalt from the iron contained in the ores normally treated by this process. Notwithstanding its high selectivity in the ammoniacal leaching stage, this process requires higher energy consumption while concurrently achieving lower nickel and cobalt recoveries as compared with pressure acid leaching.

In the pressure acid leaching process, as in the case of ammoniacal reduction-leaching, the ores used have lower nickel contents and higher iron contents. In this process, practically all the minerals present are dissolved in acid solution, and consequently, nickel and cobalt recoveries are high. Selectivity to iron is also high, and occurs predominantly during the leaching stage.

Therefore, with respect to the pressure acid leaching process, a technique is proposed for obtaining the leached cobalt metal in its metallic form.

After the leachate from the pressure acid leaching stage has been treated for removal of impurities that had been leached with the nickel and cobalt, these metals are precipitated from the solution as hydroxides, and this precipitate is leached with either ammonia or ammonium carbonate; the nickel and cobalt return to the liquid form, forming nickel and cobalt complexes with ammonia in solution.

This process has high selectivity over iron, manganese, and magnesium. Nickel and cobalt separation occurs in a solvent extraction stage, after cobalt has been oxidized to the trivalent state (Co 3+) so as to not be coextracted with nickel. In the extraction step of the solvent extraction stage, nickel is extracted from the stripped ammoniacal leach liquor with an organic extractant, and subsequently recovered in the metallic form by electrowinning, while cobalt remains in said liquor (raffinate).

The objective of the present invention is to recover the cobalt metal contained in the ammoniacal liquor - i.e. in the nickel solvent extraction raffinate - in its metallic form.

These objectives, as well as other objectives and advantages of the present invention, are attained with a method for production of metallic cobalt from the nickel solvent extraction raffinate , which comprises the following stages: (a) obtaining the nickel solvent extraction raffinate for production of cobalt; (B) adding a sulfide precipitation agent to said raffinate, for cobalt sulfide and zinc sulfide precipitation; (C) removing all ammonia from the sulfidized pulp (solids and liquid); (D) subjecting the filtered solid - cobalt sulfide (and impurities) - to atmospHeric leaching; (E) reducing to a minimum the zinc concentration in the cobalt round, by means of solvent extraction with D ₂ EPHA extractant diluted in Escaid 110 solvent or equivalent solvent, in any solvent extraction system comprising the required number of extraction, scrubbing, and stripping stages, with resident time of not less than 1 minute at each of the stages used; (F) performing nickel removal by ion exchange, for nickel purification; (G) adding

sodium carbonate to the solution, for cobalt carbonate precipitation; (H) leaching the produced cobalt carbonate in a system that utilizes an acid, preferably sulfuric acid, and more preferably, the anolyte from cobalt electrolysis; (I) performing cobalt electrowinning, so as to recover the cobalt from the solution in its metallic form; and (J) allowing the metallic cobalt to settle on insoluble stainless steel plates for the time required for the production of cobalt cathodes.

The present invention is described hereinunder in relation to the drawing annexed hereto, in which a single item represents a block diagram of the method for production of metallic cobalt from the nickel solvent extraction raffinate .

According to said drawing, the raffinate from the nickel solvent extraction stage (A) is utilized for the production of cobalt. A sulfide precipitation agent, preferably sodium hydrosulfide (NaHS), is added to this raffinate (B), using a stoichiometric amount of base for the cobalt and zinc contained in the solution and most preferably an excess of 1.5 times the stoichiometric dosage for the cobalt and zinc in the solution, for the precipitation of cobalt sulfide and zinc sulfide. The precipitation temperature must be maintained within the range 4O ⁰ C to 55°C and most preferably at 45 ⁰ C. After the precipitation of cobalt sulfide (and impurities), all the ammonia present in the sulfidized pulp (solids and liquid) is removed (C), preferably by vapor injection so as to increase pulp temperature to preferably between 95 ⁰ C and 110 ⁰ C and most preferably to 100 ⁰ C. The equipment for ammonia removal may be a column or any other system designed for such purpose. After ammonia removal, solid-liquid separation of the pulp (C1) is carried out in either filters or thickeners and more preferably in thickeners. The solids are then filtered (C2).

Alternatively to this proposed method, the solids in the pulp may be filtered in a filtration system, without the removal of ammonia by the technique described above. In this case, a flocculating agent is used to assist in the decantation of the solids in equipment suitable for such purpose, such as a

thickener, a clarifier, or other.

The use of this technique, however, may lead to higher consumption of reagent, and more specifically an acid, in the subsequent stage of the process. On the other hand, when ammonia is removed from the pulp containing the sulfidized solids by the technique described above, the filtering performance of the filtering agent chosen is enhanced.

When ammonia is removed from the pulp, some redissolution of cobalt may occur during the removal process. In this case, a sulfide precipitation agent can be added to the ammonia removal system used, during this process.

The filtered solid - cobalt sulfide (and impurities) - is subjected to oxidizing atmospHeric leaching (D), which is carried out in an appropriate system of the type comprising a tank made of or lined with a material resistant to the corrosion that may occur during the process, or else, in a system comprising the use of agitators also made of or lined with corrosion-resistant material. Thus, the solid is leached with sulfuric acid at atmospHeric pressure with the temperature maintained between 8O ⁰ C and 90°C, and most preferably at 85 ⁰ C, for such a time as to allow recovery of a minimum of 90% of the mass of cobalt present in the solids. (D1) Oxygen is added to the system throughout the leaching period at a flow rate of between 5 and 25 L/hr, and more preferably between 10 and 20 L/hr, and most preferably 15 L/hr. The oxygen source may be either industrial oxygen of any purity or air. The pulp from this leaching stage is then filtered (D2) in a filtration system. The unleached residual solids collected in the filtration system may be recycled for recovery (D3) of residual cobalt, or may be either disposed of or used in some other part of the process. The filtrate (i.e. the solution from the filtration system) containing cobalt (and impurities) is cooled (D4) in a heat exchanger or other equipment used for such purpose, prior to removal of zinc from the solution. Owing to heating of the pulp, liquid evaporation may occur during the leaching process, which may lead to an increase in its

viscosity. In such case, dispersion of the oxygen added to the pulp may be hampered, and as a consequence there may be a decrease in cobalt recovery in this process.

Thus, during this process the pulp being leached is monitored for its characteristics and, if necessary, a correction of its viscosity is applied by adding a liquid (D5), which may be water, to the leaching system. In this way, oxygen dispersion in the pulp will not be affected.

Zinc removal from the solution is required, to reduce to a minimum the zinc concentration in the cobalt round, since in the final product the concentration of this element in solution is estimated to increase 25 times.

Separation of the zinc and cobalt contained in the solution, after atmospHeric leaching and filtration, is carried out by solvent extraction (E), using D ₂ EPHA extractant diluted in Escaid 110 solvent or other equivalent solvent, in any solvent extraction system comprising the required number of extraction, scrubbing, and stripping stages, with resident time of not less than 1 minute at each of the applied stages.

In the extraction stage, the pH is controlled so as to be in the range between 1.4 and 1.8 and most preferably at 1.6, by the addition of a base (E1 ), more preferably sodium hydroxide. Zinc is transferred from the aqueous solution to the extractor. The process occurs in a continuous aqueous medium with an organic/aqueous ratio of between 1.00 and 2.11 and more preferably of 2.00. The aqueous continuity (organic/aqueous ratio of approximately 1 :1 ) is achieved by means of internal recycles (E2) within each of the employed extraction stages. The temperature of this process shall be maintained between

40°C and 6O ⁰ C and more preferably at 50°C, by any system appropriate for such purpose. It should be noted that for an organic/aqueous ratio of 1.00 the concentration of cobalt in the organic will be minimum.

On the other hand, zinc contamination in the raffinate from the

extraction process will be maximum. At the other extreme, such condition is reversed: owing to the lower zinc load in the organic, the loss of cobalt into the organic will be higher, and zinc contamination in the raffinate will be minimum.

If necessary, the raffinate (i.e. the zinc-free solution) from the extraction stage is purified of its impregnated organic content in any system appropriate for such purpose, such as coal columns.

Depending on the nickel concentration in the cobalt solution, nickel purification can be carried out in a system appropriate for such purpose, such as, for example, ion exchange columns for nickel removal (F) containing a resin capable of performing this process, and especially Dowex M 4195 resin. Nickel concentration in solution shall not exceed 70 mg/L.

After the purification processes described hereinabove, the cobalt solution will have been purified with respect to most impurities, but may contain a high concentration of sodium, owing to the pH adjustment in the zinc solvent extraction stage (E), where sodium hydroxide (NaOH) is used for said adjustment. On the other hand, the concentration of cobalt in this solution is still too low for effective electrowinning.

With the purpose of removing the sodium from the solution and increasing the cobalt concentration, sodium carbonate is added to the solution for the precipitation of cobalt carbonate (G). The precipitation temperature is maintained between 28°C and 45° and more preferably at 35°. The sodium carbonate is dosed so as to elevate the solution pH to between 7.0 and 9.0, more preferably to 8.0. The recovery of cobalt mass from the solution is not less than 90%. The cobalt carbonate produced by the technique described above is then filtered (G1) in an appropriate filtration system, preferably one that includes a washing stage, so as to ensure that any and all excess sodium carbonate is washed from the produced solid.

The produced cobalt carbonate is leached (G2) in an

appropriate system of the type comprising a tank made of or lined with a corrosion-resistant material, or else, in a system comprising the use of agitators also made of or lined with corrosion-resistant material. Leaching is carried out using an acid, preferably sulfuric acid, and most preferably the anolyte from cobalt electrolysis. The resulting product is the strong electrolyte, concentrated in cobalt, whose concentration of this element in solution is not less than 40 g/L, preferably between 40 and 80 g/L, and most preferably 70 g/L of cobalt, for electrolysis feed.

Next, the product from the cobalt carbonate leaching stage is filtered (H) in any appropriate filtration system. The solids recovered in the filtration process are repulped with demineralized water and are returned to the cobalt carbonate leaching system.

One cobalt electrowinning stage (I) is carried out so as to recover cobalt in the metallic from the solution. Electrolysis is carried out in an appropriate electrowinning system in which the electrodes are insoluble plates made of lead-calcium-tin alloy. In the cells, the anodes are enclosed in membrane bags so as to prevent the migration of hydrogen ions to the cathode. In this way, the anolyte and catholyte solutions are processed separately. Where the anolyte is used for cobalt carbonate dissolution, the cobalt carbonate is filtered in an appropriate filtration system and directed to the cobalt carbonate leaching stage. After leaching, it is passed through a heat exchange system so as to control its temperature between 55 ⁰ C and 70 ⁰ C and more preferably at 65°C, and is returned to the electrowinning feed tank, which may be a tank loaded with cobalt (strong electrolyte).

The catholyte, which is processed separately from the anolyte, is returned to the electrowinning feed system, where it is mixed with the strong electrolyte from the cobalt carbonate acid leaching. After mixing, the cobalt concentration in the electrolyte is not less than 40 g/L. The electrolyte is dosed

with either a base or an acid, more preferably with caustic soda or sulfuric acid, for adjusting the pH within the range from 3.6 to 2.7. During the operation, the characteristics of the product being formed are observed, and pH adjustment is carried out accordingly. In this electrowinning feed system, barium hydroxide may be added to remove from the solution any lead that might have dissolved from the anode and contaminated the solution.

The metallic cobalt is deposited in the form of discs onto insoluble plates made of stainless steel for the time required for production of the cathodes, more preferably for 5 days. The stainless steel plate on which the cobalt is deposited may, or may not, have a mold made of specified material, such as a resin, that is resistant to the acid, so as to give the cobalt cathodes the form of discs (J).

Example of the invention

The following example illustrates the usefulness of the process in question. The nickel solvent extraction raffinate has the following composition

After the addition of NaHS (1.5 times the stoichiometric dosage), precipitation of the cobalt sulfide occurred, and the liquor presented the following composition (mg/L):

The produced precipitate was subjected to a removal (stripping) stage of all the ammonia present.

The chemical composition of the produced precipitate was as

follows:

The pulp containing the cobalt precipitate was filtered. The cobalt sulfide was subjected to a twelve-hour oxidizing atmospHeric leaching period, after which the following extraction (leaching) efficiencies were obtained:

The test for cobalt refining, described below, includes cobalt precipitation, acid releaching, zinc solvent extraction, nickel ion exchange, and cobalt electrowinning (electrolysis).

The feed to the cobalt circuit is an aqueous solution containing cobalt, referred to as nickel raffinate. Said solution is a byproduct from the downstream part of the process.

Cobalt sulfide precipitation

The precipitation was carried out in 250-L tanks that were fed with the nickel raffinate with composition given in table 1 , and with 1.5 times the stoichiometric amount of NaHS for Zn and Co. The operation was carried out at

5O ⁰ C for 30 minutes, and the final product obtained was a mixed precipitate of cobalt and zinc sulfides. Said residence time established for the tanks is sufficient for the precipitation of 99.7% of the cobalt, 99.4% of the zinc, and 99.9% of the nickel. Said values can be confirmed by analyzing the composition of the precipitation discharge liquor, shown in table 2

Table 1 :

Table 1 : Composition of the nickel raffinate:

Concentration , mg/L Liquor

Batch volume No.

Ni Co Fe Mg Mn Cu Zn Ca Na Cr S(t) (L) 10

6 1758 0 26 0 2 734 10 34 0 4896 256

Table 2:

Table 2: Composition of the precipitation discharge liquor

The precipitate was then fed to a column counter-current to steam flow, for complete removal of free ammonia prior to the decantation and filtration steps, the temperature of the feed vapor being 100 ⁰ C. During this operation, approximately 2.3% of cobalt dissolution occurred, and the amounts of ammonia in the discharge were found to be less than 0.5 ppm. The values of feed, discharge, and percent of dissolution of metals during vapor separation are listed in table 3. The products from the slurries of NaHS precipitate and NaHS/NH ₃ separate were pumped to the decanters and maintained at approximately 5O ⁰ C. After flocculation, which was carried out with the addition of 350 g/t of Magnafloc 919, the solids were filtered, washed with demineralized water, and kept as feed to the cobalt sulfide leaching.

Table 3:

Table 3: Dissolution of metals during the complete steam separation of the NaHS reci itation roduct:

Cobalt sulfide leaching

The oxidizing leaching was carried out in a 60-L polypropylene vessel, with direct injection of steam to prevent corrosion. The temperature was maintained at 85 ⁰ C by means of a heated water jacket.

The optimal leaching conditions for 36 L of pulp volume at 85 ⁰ C were as follows: oxygen rate 10 L/min, percent of solids in the cobalt sulfide

pulp 10% (w/w), addition of 303 kg/t of 98% sulfuric acid, pH controlled at 1.5, and residence time 20 hours.

The leaching rate for the cobalt sulfide precipitate was low, requiring at least 12 hours for 99% of dissolution of the cobalt to be achieved. In contrast, zinc dissolution was faster (3 to 4 hours).

The leach product was filtered, then the pie was subjected to releaching with sulfuric acid for higher cobalt recovery. The relation between the composition of the CoS feed and that of the leaching residue is given in table 4.

Table 4:

Table 4: Compositions of the feed and of the residue from cobalt sulfide leaching:

Feed to CoS

28 0,04 0,01 0,01 12,36 leaching 33,18 No.3

Residue from

1 ,31 0,00 0,000 0,00 0,03 89,80 leaching No. 3

Zn and Co extractions are greater than 99%.

Table 5:

Table 5: Com osition of the leach li uor:

The solution containing cobalt was treated in a solvent extraction stage so as to separate cobalt from zinc.

The solvent extraction circuit comprised 4 extraction stages, 2 scrubbing stages, and 3 re-extraction (stripping) stages. The temperature in the 4 extraction stages was maintained at 5O ⁰ C.

Zinc solvent extraction

The zinc solvent extraction circuit (liquid-liquid separation) comprised 4 extraction stages, 2 scrubbing stages, and 3 stripping stages.

Extraction Zinc solvent extraction was carried out in 4 extraction steps, the feeds to this stage being the solution from the cobalt sulfide oxidizing leaching and the organic component. The temperature was maintained at 5O ⁰ C, and the organic/aqueous ratio in the separation circuit was maintained at approximately 2:1 so as to minimize zinc contamination in the extraction raffinate. The organic extractant utilized was D ₂ EHPA, diluted to 30% v/v using Escaid 110. The conditions and levels of Zn and Co extraction are listed in table 6, for the process conditions as defined for this stage.

Table 6: Zn and Co extraction rofiles

The average compositions of zinc in the aqueous pHase and cobalt in the organic pHase along the extraction steps are shown in tables 7 and 8 respectively.

Table 7:

Table 7: Average composition of the zinc raffinate, mg/L

Table 8:

Table 8: Average cobalt concentration in the organic pHase, mg/L

Scrubbing

The organic load was passed through two scrubbing steps, with pH maintained at 1.4 and 1.3 respectively, and with addition of acid so as to prevent cobalt advance in the zinc stripping stage and also to recover cobalt, which can be recirculated to the cobalt sulfide leaching stage. The compositions of the aqueous and organic pHases in the two steps, in mg/L, are shown in table 9.

Table 9:

Table 9: Aqueous and organic compositions of the depurate, in

Zinc stripping

The circuit for removal of the zinc contained in the organic pHase comprises three steps. A feed of H ₂ SO ₄ of 200 g/L was utilized. Sulfuric acid consumption was equivalent to 1 ,246 kg/t of Zn. The compositions of the acid solution before and after the separation, as well as the compositions of the organic pHase after the separation, are shown in tables 10, 11 , and 12

respectively.

Table 10:

Table 10: Com osition of the separation feed, in m /L

Table 11 :

Table 11 : Composition of the separation product, mg/L

Table 12:

Table 12: Composition of the organic pHase of the separation,

After solvent extraction, the solution containing cobalt (i.e. the raffinate) was purified of nickel by means of ion exchange.

Removal of nickel by ion exchange

Ion exchange was used to remove any nickel impurity from the cobalt liquor prior to electrowinning. The circuit comprised five columns, four of which for extraction, and one for elution. The initial feed to the nickel ion exchange circuit was a liquor with a nickel content of 85 mg/L. During loading, the breakthrough occurred in column 1 within the first two hours, and after 24 hours of operation, in which the concentration of the discharge liquor from the fourth extraction column was of 0.5

mg/L of nickel, the effluent from column 1 contained approximately 40 mg/L of nickel.

Cobalt carbonate precipitation and leaching

The feeds for the precipitation of cobalt carbonate are either the discharge liquor from the nickel ion exchange (when utilized) or the zinc solvent extraction raffinate after it has passed through coal columns for removal of impregnated organic and anhydrous Na ₂ CO ₃ . Precipitation occurs at 35 ⁰ C at pH 8.0 for 30 minutes. The proportion of sodium carbonate added was 1.64 mols per cobalt mol, or 2.987 kg/kg of cobalt. Approximately 99% of the cobalt was precipitated. The composition of the cobalt precipitate CoCO ₃ (H ₂ SO ₄ leaching feed) is specified in table 13.

Table 13:

Table 13: Com osition of the cobalt reci itate COCO ₃ :

After being filtered, the precipitate is leached with sulfuric acid, at ambient temperature (25 ⁰ C) and pH 3.0, for 30 minutes, forming a solution with 70 g/L of Co. The composition of the leaching discharge liquor is given in table 14.

Table 14:

Table 14: Composition of the cobalt carbonate liquor from acid leaching, in mg/L

Cobalt electrowinning

The concentrated cobalt solution was fed to an electrolytic cell at a current density of 350 A/m ² . The feed rate was 2 mL/min. The pH was

adjusted to 3.0 with the addition of 2 M of sodium hydroxide solution, and the temperature was maintained at 6O ⁰ C. The residence time in this operation was five days, at the end of which a Grade 1 cobalt cathode with Co purity of up to 99.96% and 5 mg/kg of Ni was produced. A comparison between the composition of the cathode produced in the test described above and the specifications defined for GRADE 1 cobalt cathode is given in table 15. These values show that the impurities in the produced cathode are within the limits of GRADE 1 specification.

Table 15:

Table 15: Composition of the 5-day cobalt cathode ldentifi Composition

Mass cation ppm kg %Co

5-day Cu Ni Fe Zn Mn Mg Ca Al S As Na Cr Si cathod e 14 11

4 5 20 29 1 2 10 8 25 20 10 Grade 0.293 99.961 5 6 1 40 3500 100 55 10 10 50 40 10 25 specif

Although a preferred methodological sequence has been described and illustrated, it should be noted that modifications are possible and achievable without departing the scope of the present invention.