AU2003244581B8

AU2003244581B8 - Process for producing cobalt compound and process for producing solution of cobalt sulfate

Info

Publication number: AU2003244581B8
Application number: AU2003244581A
Authority: AU
Inventors: Masaki Imamura; Nobuhiro Matsumoto; Satoshi Matsumoto; Kazuyuki Takaishi; Naoyuki Tsuchida
Original assignee: Sumitomo Metal Mining Co Ltd
Current assignee: Sumitomo Metal Mining Co Ltd
Priority date: 1998-09-21
Filing date: 2003-09-05
Publication date: 2006-05-18
Anticipated expiration: 2018-10-09
Also published as: AU2003244581A1; AU2003244581B2

Description

Our Ref:78267533 P/00/011 Regulation 3:2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Applicant(s): Address for Service: Invention Title: Sumitomo Metal Mining Company Limited 11-3, Shinbashi Minato-ku Tokyo 105-8716 Japan DAVIES COLLISON CAVE Patent Trade Mark Attorneys Level 10, 10 Barrack Street SYDNEY NSW 2000 Process for producing cobalt compound and process for producing solution of cobalt sulfate The following statement is a full description of this invention, including the best method of performing it known to me:- 5020 PROCESS FOR PRODUCING COBALT COMPOUND AND PROCESS FOR PRODUCING SOLUTION OF COBALT SULFATE BACKGROUND OF THE INVENTION 1. Field of the Invention: The present invention relates to a process for producing a high-purity cobalt compound from a raw cobalt compound containing ammonia and nitrate as impurities by removal of nitrogen compounds such as ammonia and nitrate. The present invention relates also to a process for producing a solution of high-purity cobalt sulfate economically by efficient dissolution of a high-purity cobalt compound obtained by the process mentioned above.

2. Description of the Related Art: Cobalt finds general use as a metal for special alloys (such as heat resistant alloy and cemented carbide) and also as a magnetic material (such as permanent magnet and magnetic tape). In general, cobalt for magnetic materials is used in the form of cobalt sulfate.

Cobalt sulfate (in crystal form) is conventionally produced by dissolving metallic cobalt or a cobalt-containing compound (such as cobalt hydroxide and cobalt oxide) in sulfuric acid and then concentrating the solution for crystallization.

Cobalt ores naturally occur mostly in the form of oxide and sulfide. Cobalt is also obtained as aby-product of nickel smelting. In this case, it is essential to isolate cobalt from nickel and other impurities.

It has recently become possible to completely separate and remove impurity metals (such as nickel, copper, and zinc) in the form of ion by the aid of solvent extraction or ion exchange resin. Now, there is available a high-purity cobalt compound containing said metal impurities in an extremely small amount.

In the meantime, among common processes for producing nickel and cobalt is ammonia leaching. This process consists of roasting (for reduction) of raw ores (usually in the form of oxide), leaching with ammonia, solvent extraction, and recovery of a raw cobalt compound. Solvent extraction gives rise to a raw cobalt compound in the form of cobalt carbonate or cobalt hydroxide with a small amount of metal impurities.

However, ammonia leaching inevitably contaminates the raw cobalt compound with ammonia (NH 3 at the time of recovery from the ammoniac solution. Moreover, ammonia partly oxidizes into nitric acid, thereby increasing the concentration of nitrate or nitrite (NO 3

NO

2 in the raw cobalt compound.

Removal of ammonia, nitrate or nitite from the raw cobalt compound has been accomplished in the following manner.

A first one is repeated water washing. (Ammonia and nitrate are readily soluble in water.) A second one is heating.

(Ammonia is volatile and nitrate is decomposed at a high temperature.) Unfortunately, removing ammonia and nitrate completely from a raw cobalt compound by water washing needs copious water and repeated washing. In addition, water washing is not satisfactory in the case where ammonia and nitrate are present in the form of slightly soluble compound, and water washing needs close attention to waste water disposal. Therefore, it is not an effective means.

Now, in the case of a raw cobalt compound containing ammonia alone, it is possible to decompose and volatilize ammonia completely by heating at about 200* C in the air.

However, decomposition and volatilization of nitrate need heating at an extremely high temperature. For example, heating at 500*C is necessary to decrease the content of nitrate below 0.01 wt% in a raw cobalt compound. In other words, nitrate remains almost intact at a low temperature of 200-300°C, which is high enough for ammonia to decompose and volatilize.

On the other hand, a raw cobalt compound containing ammonia alone also poses a problem. When it is heated (or roasted) in the air (or oxygen-containing atmosphere), ammonia is decomposed and volatilized once but oxidized into nitrate in the air. This nitrate enters the raw cobalt compound, and its complete removal needs heating at 5000 C or up as mentioned above.

The conventional method of removing ammonia and nitrate from a raw cobalt compound by heating for their decomposition and volatilization needs a high temperature above 500 C. This is economically disadvantageous from the standpoint of energy consumption.

Moreover, the above-mentioned process for converting a raw cobalt compound into a cobalt compound and then giving crystalline cobalt sulfate requires that the solution for concentration and crystallization should be kept at pH 1-2.

The reason for this is that the final cobalt sulfate crystallized from this solution should have pH 3.5-5.5. (With a pH lower than 3.5, the cobalt sulfate has to be neutralized with an expensive high-purity neutralizer when it is dissolved again with a pH higher than 5.5, the purity of CoSO 4 becomes lower because of Co(OH) 2 contamination.) On the other hand, trivalent cobalt compounds in the form of hydroxide and oxide are slightly soluble in sulfuric acid under normal conditions. For their rapid dissolution (to save processing time), it has been necessary to add an excess amount of sulfuric acid, thereby greatly lowering the pH of the solution, and to raise the temperature of the solution.

The result is that the cobalt solution greatly decreases in pH and hence the resulting crystalline cobalt sulfate decreases in pH. The process in this way needs acid- and heat-resistant facilities made of special materials.

SUMMARY OF THE INVENTION The present invention was completed to address the 00 00 5 above-mentioned problem. The present invention seeks to provide a process for removing ammonia, nitrate and nitrite Cc from a raw cobalt compound containing them efficiently in a simple manner with less energy consumption, thereby decreasing their concentration. The present invention seeks to provide a process for producing cobalt sulfate efficiently, from the resulting cobalt compound by reduction and dissolution.

The present inventors carried out a series of researches on the process for efficient production of cobalt sulfate from a cobalt compound, which led to the finding that if a raw cobalt compound containing ammonia and nitrate is roasted at 300-500 0 C, preferably 350-450 0 C, in an inert gas stream or together with a reducing agent, it is possible to remove ammonia and nitrate from the raw cobalt compound, thereby giving a cobalt compound containing ammonia, nitrate and nitrite in an amount less than 0.01 wt% each, and that by dissolving the resulting cobalt compound together with a reducing agent in sulfuric acid, it is possible to produce cobalt sulfate efficiently. The present invention is based on this finding.

The above-mentioned object is achieved in the present invention by the first embodiment which covers a process for producing a cobalt compound. This process comprises roasting a raw cobalt compound at a temperature equal to or higher than 3000C and lower than 5000C in an inert gas stream, thereby removing ammonia and nitrate from the raw cobalt compound which is not yet roasted. This roasting should preferably be carried out at a temperature not lower than 3500C and not higher than 4500C in a nitrogen gas atmosphere.

The above-mentioned object is achieved in the present invention by the second embodiment which covers a process for producing a cobalt compound. This process comprises roasting a raw cobalt compound together with a reducing agent at a temperature equal to or higher than 350 C and lower than 500 C, thereby removing ammonia, nitrate and nitrite from the raw cobalt compound which is not yet roasted. The reducing agent should be at least one member selected from carbon, activated carbon, hydrogen gas, and carbon monoxide. The amount of carbon or activated carbon should be no less than 0.05 wt% and no more than 5 wt% of the raw cobalt compound which is not yet roasted.

The above-mentioned object is achieved in the present invention by the third embodiment which covers a process for producing a solution of cobalt sulfate. This process comprises adding to a solution containing a cobalt compound sulfuric acid together with at least one reducing agent selected from methanol, ethanol, sodium sulfite, and sulfur dioxide gas, thereby dissolving said cobalt compound at a temperature not lower than 500C. This process should be carried out by using sulfur dioxide gas as said reducing agent such that the solution containing a cobalt compound has an oxidation-reduction potential not lower than 300 mV and not higher than 700 mV.

In the third embodiment, the cobalt compound should be one which is obtained by roasting a raw cobalt compound at a temperature equal to or higher than 3000 C and lower than 500* C, preferably not lower than 350 C and not higher than 450* C, in an inert gas stream (such as nitrogen) and which is free from ammonia and nitrate contained in the raw cobalt compound which is not yet roasted.

In the third embodiment, the cobalt compound should be one which is obtained by roasting a raw cobalt compound together with a reducing agent at a temperature equal to or higher than 350 C and lower than 500* C and which is free from ammonia and nitrate contained in the raw cobalt compound which is not yet roasted, with said reducing agent being at least one member selected from carbon, activated carbon, hydrogen gas, and carbon monoxide. The amount of carbon or activated carbon should be no less than 0.05 wt% and no more than 5 wt% of the raw cobalt compound which is not yet roasted.

DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the present invention, the process for producing a solution of cobalt sulfate comprises adding to a solution containing a cobalt compound (obtained by removing ammonia and nitrate from a raw cobalt compound as mentioned below) sulfuric acid together with at least one reducing agent selected from methanol, ethanol, sodium sulfite, and sulfur dioxide gas, thereby dissolving said cobalt compound at a temperature not lower than 50°C. This process should be carried out by using sulfur dioxide gas as said reducing agent such that the solution containing a cobalt compound has an oxidation-reduction potential not lower than 300 mV and not higher than 700 mV.

Although ammonia is readily volatile at about 200 C as mentioned above, it is considered that nitrate and nitrite decompose and volatilize to be removed according to the equation below respectively. The reaction involved needs a high temperature above 500'C.

3 2NO 30 H 2 0 (1) 2HN2 2NO O H 2 0 (2) In view of this, the present invention employs the following two methods to remove nitrate and nitrite instead of the conventional method mentioned above.

That is, the first method for removing nitrate and nitrite, which is used in the first embodiment, utilizes the characteristic of a raw cobalt compound which contains both ammonia, nitrate and nitrite. It consists of roasting a raw cobalt compound in an inert gas stream, thereby causing ammonia and nitrate to react with each other. Thus ammonia and nitrate radical are removed at a temperature lower than 5000C.

In other words, this method employs ammonia in the raw cobalt compound as a reducing agent for removal of nitrate.

This method does not need high temperatures for removal of nitrate unlike the conventional one. Ordinary roasting carried out in the air causes nitrate to be removed from the raw cobalt compound by decomposition and volatilization. In fact, ammonia contained in the raw cobalt compound can be used as a reducing agent to remove nitrate at a low temperature.

This is true only when there exists ammonia sufficient for reaction with nitrate. Fortunately, the raw cobalt compound obtained from the ammonia leaching method only contains ammonia in principle, although it contains nitrate and nitrite formed secondarily by oxidation of ammonia.

Therefore, ammonia exists usually in large excess relative to nitrate and nitrite. It follows that both ammonial, nitrate and nitrite are removed by roasting in an inert gas atmosphere.

The amount of ammonia necessary for reduction of nitrate is 0.3 wt% or above.

According to the present invention, the temperature for roasting in an inert gas stream is not the one necessary for decomposition of nitrate but the one necessary for reaction between nitrate and ammonia. Therefore, it is much lower than the temperature (above 5000C) necessary for roasting in the air. The reaction proceeds at a temperature down to 3000 C to remove both ammonia and nitrate almost completely.

Removal of nitrate approaches perfection as the roasting temperature increases. However, roasting at 5000C or above is not desirable because it needs as much energy as the conventional technology. Moreover, roasting at an excessively high temperature tends to prevent cobalt from being leached out in the subsequent leaching step. A preferred temperature range is 350-450* C, although removal of ammonia and nitrate is not substantially affected by temperatures exceeding 450'C.

Examples of the inert gas are nitrogen, argon, and helium which are in general use. They are not specifically restricted but nitrogen is preferable because of its low price and its easy handling. Pure nitrogen is essential because removal of nitrate and nitrite is susceptible to oxygen however small its amount may be. Incidentally, the inert gas is not specifically restricted in flow rate and other factors because it is not involved directly with reaction.

Incidentally, the process of the present invention is designed to perform roasting in an inert gas atmosphere at a temperature equal to or higher than 3000 C, so that excess ammonia which does not react with nitrate volatilizes to be removed efficiently without oxidation. Besides, the process of the present invention can be applied directly to the raw cobalt compound obtained by the ammonia leaching method which is in the form of fine powder. In the case where the raw cobalt compound is in the form of large granules, it is desirable to crush it into particles of adequate size prior to treatment.

The second method for removing nitrate and nitrite, which is used in the second embodiment, is characterized by the reduction of nitrate (which is less volatile than ammonia) into ammonia (which is readily volatile), followed by efficient removal of the volatilized ammonia without oxidation. As mentioned above, ammonia is readily volatile and it decompose and volatilize at about 200 C, whereas nitrate and nitrite decomposes and volatilizes through the conceivable reaction represented by the equation and (2) given above, and this reaction needs a high temperature.

However, the fact that ammonia can be removed more easily than nitrate and nitrite suggest that nitrate would be volatilized and removed easily if it is reduced into ammonia by the aid of an appropriate reducing agent. In other words, the second method in the present invention is designed to roast a raw cobalt compound containing nitrate in the presence of a reducing agent which is deliberately added. Therefore, it reduces hard-to-remove nitrate into ammonia, and at the same time, it efficiently decomposes and volatilizes reduced ammonia as well as originally contained ammonia. In this way it is possible to remove both nitrate and ammonia at a temperature equal to or higher than 350 C and lower than 500 C.

The reducing agent used in the process of the present invention includes carbon, activated carbon, hydrogen gas, carbon monoxide gas, and sulfur dioxide, which are in general use. Of these, solid ones such as carbon and activated carbon are easy to handle because they can be simply mixed with the raw cobalt compound prior to roasting. Excess carbon will undesirably remain unreacted in the resulting cobalt compound. The amount of carbon or activated carbon varies depending on the concentration of nitrate in the raw cobalt compound; it is usually two equivalents for nitrate. In the case where the raw cobalt compound is one which is obtained by the ammonia leaching method, the amount of carbon or activate carbon should preferably be 0.05-5 wt% of it, in view of the fact that it contains a very small amount of nitrate which originates secondarily from ammonia by oxidation. The reducing agent less than 0.05 wt% is not enough for reduction into ammonia, and the reducing agent in excess of 5 wt% will partly remain unreacted in the resulting cobalt compound.

The advantage of reducing hard-to-remove nitrate into readily volatile ammonia is that the removal of ammonia and nitrate can be accomplished at a low temperature equal to or higher than 3500C and lower than 5000C as mentioned above.

This advantage leads to energy saving and cost reduction.

Moreover, roasting in a reducing atmosphere permits volatile ammonia to be removed efficiently without oxidation into nitrate.

The above-mentioned methods pertaining to the first and second embodiments of the present invention give rise to a cobalt compound, with nitrate removed. This cobalt compound is a mixture of divalent and trivalent cobalt hydroxide (as major components) and cobalt oxide (as minor components) which result from cobalt hydroxide by roasting.

According to the present invention, a solution containing the cobalt compound (with ammonia and nitrate removed by roasting) is efficiently dissolved in sulfuric acid. This object is achieved by reducing trivalent cobalt in the cobalt compound. Dissolution in this manner saves sulfuric acid and prevents the cobalt sulfate solution from decreasing in pH (keeping it pH This permits the cobalt compound to be dissolved efficiently at 50 C and above for the production of cobalt sulfate.

It is considered that the following reaction (equation 3) takes place for dissolution when sulfuric acid is added to cobalt oxyhyroxide (CoOOH) as one of the cobalt compounds.

CoOOH H 2 S0 4 Co 2 S0 4 2 3 2

H

2 0 1/4 02 (3) On the other hand, it is also considered that the following reaction (equation 4) takes place for dissolution when sulfuric acid is added together with sulfur dioxide gas as the reducing agent.

CoOOH 1/2 H 2 S0 4 I/2 SO 2 Co 2 S042- H 2 0 (4) The reaction is slower than the reaction and the former needs a high temperature.

In short, the present invention is designed to produce cobalt sulfate by the efficient dissolution of a cobalt compound at 500 C or above, which is achieved by adding sulfuric acid together with a reducing agent to a solution containing a cobalt compound. The molar ratio of sulfuric acid (H 2 S0 4 to cobalt (Co) should preferably be 0.5-0.7. Thereducing agent may be a water-soluble one selected from methanol, ethanol, sodium sulfite, and sulfur dioxide gas, which are in general use. Of these reducing agents, sulfur dioxide gas is suitable for the production of a solution of high-purity cobalt sulfate, because it is free from carbon and sodium.

However, excess sulfur dioxide should be avoided because it remains in the cobalt solution, giving rise to sulfate ions, which enter the cobalt sulfate (as the product) to lower its pH. In this case it is necessary to decompose sulfate ions with an oxidizing agent (such as hydrogen peroxide) for post treatment.

Consequently, the amount of sulfur dioxide gas for reduction and dissolution should be such that the solution containing a cobalt compound has an oxidation-reduction potential of 300 mV or above. If sulfur dioxide gas is used in an excess amount (with an oxidation-reduction potential lower than 300 mV), post treatment with an oxidizing agent would be necessary for decomposition. To attain a practical rate of dissolution, it is necessary to add sulfur dioxide in an amount sufficient to produce an oxidation-reduction potential lower than 700 mV. Therefore, an adequate amount of sulfur dioxide gas is such that the resulting solution has an oxidation-reduction potential ranging from 300 mV to 700 mV.

Dissolution should be carried out at 50 0 C or above because the saturated concentration of cobalt sulfate is low at temperatures lower than 500 C. On the other hand, dissolution at temperatures in excess of 1000 C is not economical due to energy loss. Hence, the upper limit is about 1000 C.

The invention will be described in more detail with reference to the following examples and comparative examples.

Example 1 (Removal of impurities by heating in an inert gas stream) This example uses cobalt hydroxide containing 63.6 wt% cobalt as a raw cobalt compound. This cobalt hydroxide (10.0 g) was roasted at 300-500°C (as shown in Table 1 below) in an nitrogen gas stream. For comparison, roasting was carried out at the same temperature in the air. Incidentally, this cobalt hydroxide contains 0.84 wt% ammonia (NH 3 and 0.42 wt% nitrate

(NO

3 After roasting for 1 hour at the specified temperature, the samples were analyzed for NH 3 and NO 3 The results are shown in Table 1.

Table 1 Roasting Roasted in the air Roasted in nitrogen gas temperature NH 3

NO

3

NH

3

NO

3 (wt%) Before roasting 0.84 0.42 0.84 0.42 300 <0.005 1.1 0.008 0.012 350 <0.005 0.63 0.033 0.008 400 <0.005 0.27 <0.005 0.008 450 <0.005 0.077 <0.005 0.008 500 <0.005 0.008 <0.005 <0.005 It is noted from Table 1 that in the case of roasting in nitrogen gas, the content of ammonia and nitrate is reduced below about 0.01 wt% at a temperature as low as 300 0 C. By contrast, in the case of roasting in the air, ammonia is removed almost completely at 300* C but nitrate as much as 0.077 wt% remains even at 4500 C. Roasting at a temperature higher than 5000C is necessary for its complete removal.

Example 2 (Removal of impurities by heating in an inert gas stream) The same raw cobalt compound as used in Example 1 was roasted at 4000 C for 1 hour in a nitrogen gas stream containing oxygen in varied amounts as shown in Table 2 below. After roasting, the samples are analyzed for NH 3 and NO 3 The results are shown in Table 2.

Table 2 02 NH 3

NO

3 (wt%) Before roasting 0.84 0.42 0 <0.005 0.008 7 <0.005 0.19 <0.005 0.30 13 <0.005 0.27 <0.005 0.27 It is noted from Table 2 that pure nitrogen is desirable as the roasting atmosphere. The more the amount of oxygen, the more it is difficult to remove nitrate (NO 3 Example 3 (Removal of impurities by heating in the presence of a reducing agent) This example uses cobalt hydroxide containing 63.6 wt% cobalt as a raw cobalt compound. After thorough mixing with carbon (0.1 this cobalt hydroxide (30.0 g) was roasted at 300-500° C (as shown in Table 3 below) in the air. Likewise, after thorough mixing with activated carbon (0.5 this cobalt hydroxide (150.0 g) was roasted at 300-500 C (as shown in Table 3).

For comparison, roasting was carried out in the same manner as above except that neither carbon nor activated carbon was used.

Incidentally, this cobalt hydroxide contains 0.84 wt% ammonia (NH 3 and 0.42 wt% nitrate (NO 3 After roasting for 1 hour at the specified temperature, the samples were analyzed for NH 3 and NO 3 The results are shown in Table 3.

Table 3 Roasting None Carbon Activated carbon temperature NH 3

NO

3

NH

3

NO

3

NH

3

NO

3 (wt%) Before roasting 0.84 0.42 0.84 0.42 0.84 0.42 300 <0.01 1.1 <0.01 0.37 350 <0.01 0.63 <0.01 0.08 <0.01 0.06 400 <0.01 0.27 <0.01 0.03 <0.01 <0.01 450 <0.01 0.08 <0.01 <0.01 <0.01 <0.01 500 <0.01 0.01 <0.01 <0.01 <0.01 <0.01 It is noted from Table 3 that roasting at 400 C without carbon removes ammonia almost completely (with the concentration of residual ammonia being less than 0.01 wt%) but removes nitrate incompletely (with the concentration of residual nitrate being 0.27 By contrast, it is also noted that roasting with 0.1 wt% carbon under the same conditions removes ammonia as well as nitrate down to 0.01 wt% or less. This demonstrates the effect of the reducing agent.

Likewise, roasting with 0.5 wt% activated carbon under the same conditions removes ammonia as well as nitrate down to 0.01 wt% or less.

Example 4 The same raw cobalt compound as used in Example 3 was roasted at 450"C for 1 hour in the air, with the amount of carbon changed as shown in Table 4. The roasted samples were analyzed for NH 3 and NO 3 The results are shown in Table 4.

Table 4 Carbon NH 3

NO

3 (wt%) 0 <0.01 0.08 0.05 <0.01 0.01 0.1 <0.01 <0.01 0.2 <0.01 <0.01 <0.01 <0.01 2.3 <0.01 <0.01 It is noted from Table 4 that roasting with 0.05 wt% carbon removes ammonia as well as nitrate down to 0.01 wt%.

This demonstrates the effect of the reducing agent.

Example 6 (Dissolution with a reducing agent) This example uses the cobalt hydroxide containing <0.005 wt% ammonia, 0.008 wt% nitrate, and 63.6 wt% cobalt, which was obtained in Example 1 by roasting at 4000 C in nitrogen. This cobalt hydroxide was made into a slurry having a concentration of 100 g/L. Conc. sulfuric acid was added to the slurry in such an amount that the molar ratio of sulfuric acid to cobalt is 0.6. Further, sulfur dioxide gas (as a reducing agent) was added to effect dissolution at 70 0

C

such that the oxidation-reduction potential (ORP) was maintained at 300-700 mV. The results are shown in Table below.

Table Molar ratio of Reaction Final pH Final ORP Solubility of

H

2

SO

4 Co time (min) (mv) Co Cobalt hydroxide 0.6 360 1.5 440 99.7 It is noted from Table 5 that the reducing agent helps increase the solubility of cobalt (up to 99.7%) despite the less amount of sulfuric acid. Incidentally, the final pH was and the final oxidation-reduction potential was 440 mV.

Example 7 This example is intended to confirm that roasting in Examples 1 to 5 permits efficient dissolution even though the cobalt hydroxide (CoOOH) undergoes partial oxidation to give cobalt oxide (C030 4 Cobalt oxide (C030 4 containing 72 wt% cobalt was dissolved at 700 C in sulfuric acid (to which sulfur dioxide gas as a reducing agent had been added) in the same manner as in Example 6. The results are shown in Table 6.

Table 6 Molar ratio of Reaction Final pH Final ORP Solubility of

H

2 SO/Co time (min) (mV) Co Cobalt oxide 0.6 360 2.0 480 97.0 It is noted from Table 6 that the reducing agent helps increase the solubility of cobalt (up to 97.0%) despite the less amount of sulfuric acid. Incidentally, the final pH was and the final oxidation-reduction potential was 480 mV.

Comparative Example The same procedure as in Example 6 was repeated except that sulfur dioxide gas was not added and dissolution was carried out at 85-95°C. The results are shown in Table 7.

Table 7 Molar ratio of Reaction time Final pH Solubility of Co Final ORP (mV)

H

2 SO4Co (min) 0.8 360 0.6 87.0 900 360 0.4 98.6 700 1.8 360 0.5 97.0 600 2.4 180 <0 100 400 It is noted from Table 7 that excess sulfuric acid is necessary to increase the solubility of cobalt. As the results, the resulting solution decreases in pH (lower than pH 1).

[Effect of the invention] As shown above, the present invention provides a process for freeing a raw cobalt compound of ammonia and nitrate down to low concentrations in an easy, efficient manner with less energy consumption, and also provides a process for efficient production of cobalt sulfate from the thus obtained cobalt compound by reduction and dissolution.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

The reference to any prior art in this specification is not,: and should not be taken as, an acknowledgment or any form of suggestion that that prior art forms part of the common general knowledge in Australia.

Claims

2. A process for producing a cobalt compound as defined in Claim i, wherein the roasting is carried out at a temperature not lower than 350 C and not higher than 450'C in a nitrogen gas atmosphere.
3. A process for producing a cobalt compound which comprises roasting a raw cobalt compound together with a reducing agent at a temperature equal to or higher than 3500 C and lower than 5001C, thereby removing ammonia and nitrate impurities from the raw cobalt compound.
4. A process for producing a cobalt compound as defined in Claim 3, wherein the reducing agent is at least one member selected from carbon, activated carbon, hydrogen gas, and carbon monoxide. A process for producing a cobalt compound as defined in Claim 4, wherein roasting is carried out by using carbon or activated carbon as the reducing agent in an amount not less than 0.05 wt% and not more than 5 wt% of the raw cobalt compound containing ammonia and nitrate impurities. P:\WPDOCS\GRS\Junc\78267533 WH daims.doc-30/06/04 -24-
6. A process for producing a solution of cobalt sulfate which comprises adding to a solution containing a cobalt compound produced by a process according to any one of claims 1 to 5, sulfuric acid together with at least one reducing agent selected from methanol, ethanol, sodium sulfite, and sulfur dioxide gas, thereby effecting dissolution at a temperature not lower than 50 0 C.
7. A process for producing a solution of cobalt sulfate as defined in Claim 6, wherein sulfur dioxide gas is used as the reducing agent such that the solution containing the cobalt compound has an oxidation-reduction potential not lower than 300 mV and not higher than 700 mV.
8. A process for producing a cobalt compound substantially as hereinbefore defined with reference to the examples. DATED this 30th day of June, 2004. SUMITOMO METAL MINING CO., LTD. By Its Patent Attorneys DAVIES COLLISON CAVE