Production of scandium-containing concentrate and further extraction of high-purity scandium oxide from the same

Abstract

The invention relates to a method for producing a scandium-containing concentrate from the wastes of alumina production and extracting high-purity scandium oxide from the same. Provided is a method for producing a scandium-containing concentrate from a red mud, wherein the Sc.sub.2O.sub.3 content therein is least of 15 wt. %, the TiO.sub.2 content not more than 3 wt. %, the ZrO.sub.2 content not more than 15 wt. %, and wherein scandium in the concentrate is in form of a mixture of Sc(OH).sub.3 hydroxide with ScOHCO.sub.3.4H.sub.2O. Also provided is a method for producing high-purity scandium oxide, with a purity of approximately 99 wt. %.

Claims

1. A method for producing a scandium-containing concentrate from a red mud, said method comprising the steps of: a) filtering the red mud from a liquid phase to form a first red mud cake; b) repulping the first red mud cake with a first sodium hydrocarbonate solution to form a red mud pulp, wherein the first sodium hydrocarbonate solution comprises a mixture of Na.sub.2CO.sub.3 and NaHCO.sub.3; c) gassing the red mud pulp from step b) with carbon dioxide until a pH of ≤9 is reached; d) leaching the red mud pulp from step c) with a second sodium hydrocarbonate solution; wherein the second sodium hydrocarbonate solution comprises a mixture of Na.sub.2CO.sub.3 and NaHCO.sub.3; e) filtering the red mud pulp from step d) to obtain a second red mud cake and a filtrate and washing the second red mud cake with water to obtain a washing solution; f) sorbing scandium from the filtrate on a phosphorous-containing ion exchanger; g) desorbing scandium from the phosphorous-containing ion exchanger with a soda solution at 20-80° C. to obtain a strippant rich in scandium, wherein the soda solution comprises a sodium carbonate solution having a Na.sub.2CO.sub.3 concentration of 160 to 450 g/dm.sup.3; and h) performing at least a first hydrolysis stage on the strippant rich in scandium obtained from step g) to obtain a scandium-containing concentrate wherein the scandium-containing concentrate has a Sc.sub.2O.sub.3 content that is at least 15 wt. % in terms of dry matter, has a TiO.sub.2 content not more than 3 wt. % in terms of dry matter, and has a ZrO.sub.2 content not more than 15 wt. % in terms of dry matter, and wherein scandium in the scandium-containing concentrate is a mixture of Sc(OH).sub.3 hydroxide with ScOHCO.sub.3.4H.sub.2O.

2. The method according to claim 1, wherein the leaching step d) is performed at a temperature of 80-85° C.

3. The method according to claim 1, wherein in the first or second sodium hydrocarbonate solution, the concentration of Na.sub.2O.sub.total in the second sodium hydrocarbonate solution is ≥60 g/dm.sup.3 and wherein Na.sub.2O bicarbonate amounts to 50-100% of Na.sub.2O.sub.total.

4. The method according to claim 1, wherein the scandium is sorbed in step f) at a temperature of 40-100° C.

5. The method according to claim 1, wherein the desorbing in step g) occurs at a temperature of 40-45° C.

6. The method according to claim 1, further comprising a second hydrolysis stage performed after the first hydrolysis stage wherein in the first hydrolysis stage, impurity components are precipitated at a pH of 10.5-12.0 and at a temperature of 60-80° C., and at the second hydrolysis stage, scandium concentrate is precipitated at a pH of 12.5-13.5 and at a temperature of 70-80° C.

7. The method according to claim 1, wherein the solution resulting from the leaching of the red mud in step d), once scandium is sorbed from the solution, is gassed with an air-gas mixture containing CO.sub.2 at 30-40° C. and is recycled for leaching of a fresh batch of red mud.

8. The method according to claim 1, wherein a weight ratio between Sc.sub.2O.sub.3 and TiO.sub.2 in the scandium-containing concentrate is further adjusted to at least 5:1 by weight.

9. The method according to claim 1, wherein a weight ratio between Sc.sub.2O.sub.3 and ZrO.sub.2 in the scandium-containing concentrate is adjusted to at least 1.5:1 by weight.

10. The method according to claim 1, wherein the washing solution in step e) is recycled for repulping in step b).

11. A scandium-containing concentrate produced by a carbonate sorption hydrolysis process, comprising a mixture of scandium, titanium, zirconium, iron, sodium oxides, hydroxides and carbonates, characterized in that the concentrate has a Sc.sub.2O.sub.3 content that is at least 15 wt. % in terms of dry matter, has a TiO.sub.2 content not more than 3 wt. % in terms of dry matter, has a ZrO.sub.2 content not more than 15 wt. % in terms of dry matter, wherein scandium is present in the concentrate as a mixture of Sc(OH).sub.3 with ScOHCO.sub.3.4H.sub.2O.

12. The scandium-containing concentrate according to claim 11, wherein a weight ratio between Sc.sub.2O.sub.3 and TiO.sub.2 in the concentrate is at least 5:1 by weight.

13. The scandium-containing concentrate according to claim 11, wherein a weight ratio between Sc.sub.2O.sub.3 and ZrO.sub.2 in the concentrate is at least 1.5:1 by weight.

14. A method for producing a high-purity scandium oxide, said method comprising: a) dissolving a scandium-containing concentrate in sulfuric acid to form a scandium-containing concentrate solution; b) removing an acid-insoluble precipitate that formed in the scandium concentrate solution in step a) c) precipitating the scandium concentrate solution after step b) with sodium sulfate to obtain Na.sub.3Sc(SO.sub.4).sub.3; d) filtering and washing the Na.sub.3Sc(SO.sub.4).sub.3 obtained in step c); e) dissolving the Na.sub.3Sc(SO.sub.4).sub.3 with water; f) adding NaOH to the Na.sub.3Sc(SO.sub.4).sub.3 dissolved with water to cause precipitation of Sc(OH).sub.3; g) filtering and washing to obtain a cake of Sc(OH).sub.3; h) adding oxalic acid to the cake of Sc(OH).sub.3 to form scandium oxalate; i) filter and washing the scandium oxalate; j) calcining the scandium oxalate to obtain scandium oxide having a purity of approximately 99 wt. %; wherein Sc.sub.2O.sub.3 content in the scandium-containing concentrate is at least 15 wt. % in terms of dry matter, TiO.sub.2 content is not more than 3 wt. % in terms of dry matter, ZrO.sub.2 content is not more than 15 wt. % in terms of dry matter, and scandium in the scandium-containing concentrate is a mixture of Sc(OH).sub.3 with ScOHCO.sub.3.4H.sub.2O.

15. The method according to claim 14, wherein the calcining step is performed at a temperature not lower than 650° C.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) The accompanying drawing shows a general flowchart illustrating all process steps for producing a scandium-containing concentrate and scandium oxide, including extracting scandium from the red mud by carbonization and sorption to obtain a scandium-containing strippant; producing a rich scandium-containing concentrate from a scandium-containing strippant; processing the scandium concentrate into scandium oxide with a Sc.sub.2O.sub.3 content of ≥99 wt. %.

(2) The proposed process for producing scandium oxide shown in the flowchart comprises the following stages:

(3) 1—filtering a pulp from a red mud resulting from the main alumina production with a liquid-to-solid ratio of ≥2.5:1 (by weight). An alkaline solution is filter separated and recycled back to the alumina production while a red mud cake with a humidity of ≥25% is supplied for repulping using a sorption mother solution;

(4) 2—repulping the red mud cake using the sorption mother solution and wash water with a NaOH solution added thereto to obtain a predetermined concentration of Na.sub.2O.sub.total in a liquid phase and a predetermined liquid-to-solid ratio;

(5) 3—carbonizing the mud pulp by bubbling through the same an air-gas mixture containing carbon dioxide to turn a part of Na.sub.2CO.sub.3 soda into NaHCO.sub.3 bicarbonate with a pH≤9;

(6) 4—soda-bicarbonate leaching of scandium from red mud to turn scandium into liquid phase;

(7) 5—filtering and filter washing with water the leached pulp to remove the leached red mud cake for storage and to supply the filtrate to the sorption stage;

(8) 6—sorbing scandium from the filtrate on a resin and recycling the sorption mother solution back to repulping stage 2;

(9) 7—desorbing scandium from the resin with a soda solution to supply a scandium-containing strippant to the hydrolysis stage;

(10) 8—first stage hydrolysis of a scandium-containing solution at a pH=10.5÷12;

(11) 9—filtering and filter washing a Ti-containing concentrate to supply the same for storage and to pump the filtrate out to the second stage hydrolysis;

(12) 10—first stage hydrolysis of a scandium-containing solution at a pH≥12.5;

(13) 11—filtering and filter washing a scandium-containing concentrate to supply the same for repurification and to supply the filtrate to carbonization stage 12;

(14) 12—carbonizing the filtrate with an air-gas mixture containing carbon dioxide to reduce its pH from 12.5 to 9-10 for further pumping the resulting solution to red mud leaching stage 4;

(15) 13—dissolving the scandium-containing concentrate in sulfuric acid to turn scandium into solution;

(16) 14—filtering and filter washing an acid-insoluble precipitate comprising a Zr-containing concentrate to supply the same for storage and to pump out the scandium-containing solution to stage 15;

(17) 15—precipitating scandium with sodium sulfate to obtain double sodium and scandium sulfate;

(18) 16—filtering and filter washing a cake of double sodium and scandium salt;

(19) 17—dissolving double sodium and scandium sulfate with water;

(20) 18—precipitating scandium with caustic alkali to obtain a hydroxide;

(21) 19—filtering and filter washing the pulp to obtain a cake scandium hydroxide;

(22) 20—turning scandium hydroxide into scandium oxalate;

(23) 21—filtering and filter washing scandium oxalate;

(24) 22—calcining scandium oxalate at a temperature not lower than 650° C. to obtain commercial scandium oxide with a purity of ≥99 wt. %.

DETAILED DISCLOSURE

(25) One of the objects of the proposed method for producing a scandium-containing concentrate is to achieve maximally high scandium content in the concentrate while processing the red mud.

(26) The above object is achieved by providing the following main innovations in the known process:

(27) 1) increasing the leaching temperature from 60-65° C. to 80-85° C., i.e. the process is performed where the leaching agent, NaHCO.sub.3 sodium bicarbonate, is thermally instable and dissociates into Na.sub.2CO.sub.3 and CO.sub.2. The presence of a free carbonate radical facilitates the increased extraction of scandium the from red mud;

(28) 2) increasing the concentration of Na.sub.2O.sub.total to ≥65 g/dm.sup.3, i.e. the process is performed using solutions oversaturated in terms of bicarbonate;

(29) 3) selecting an efficient phosphate ion exchanger having a high selectivity with respect to scandium and a low selectivity with respect to zirconium.

(30) Fundamentally new techniques of scandium desorption from a phosphate ion exchanger using strong soda solutions at a high temperature enable a desorption value to be achieved of up to 95% without using chloride solutions, which is one of the main advantages of the invention. At the same time, a rich scandium eluate is obtained from which a scandium-containing concentrate comprising a mixture of scandium, titanium, zirconium, iron, sodium oxides, hydroxides and carbonates is produced by the carbonate sorption-hydrolysis process with the scandium content in terms of oxide being from 15 to 75 wt. % in the form of Sc(OH).sub.3 hydroxide or in a mixture with ScOHCO.sub.3 basic salt.

(31) An optimum ratio between Sc.sub.2O.sub.3 and TiO.sub.2 in the concentrate of at least 5:1 (by weight) was further selected, allowing a simple and low-cost process to be used for obtaining pure scandium oxide from the same with a Sc.sub.2O.sub.3 content of ≥99 wt. % (in the calcined product). The ratio between Sc.sub.2O.sub.3 and ZrO.sub.2 in the concentrate should be preferably at least 1.5:1 (by weight), also allowing a simple and low-cost process to be used for obtaining pure scandium oxide from the same with a Sc.sub.2O.sub.3 content of ≥99 wt. % (in the calcined product).

(32) The obtained scandium-containing concentrate, subject to its chemical and phase composition and component ratio, allows a simple and low-cost process to be further used for obtaining pure scandium oxide from the same with a Sc.sub.2O.sub.3 content of ≥99 wt. % (in the calcined product) with practically no rare-earth metals contained therein, including radionuclides (uranium and thorium).

(33) The scandium-containing concentrate composition obtained by the proposed method allows pure scandium oxide to be produced from the same according to a simple flowchart without using strong acids and expensive acid-resistant equipment and without using the extraction with venomous organic extractants.

(34) In general, the final composition of the scandium-containing concentrate is dependent on selecting and optimizing a number of the following red mud processing modes: temperature of carbonate leaching, sorption-desorption, hydrolysis; concentration and composition of solutions used for leaching, sorption-desorption, hydrolysis; time of leaching, sorption-desorption, hydrolysis; production of pulps and sorbents including a liquid-to-solid ratio, gassing with carbon dioxide, linear feed rate of sorption and desorption solution, etc.; selecting a pH value for leaching, sorption-desorption, hydrolysis.

(35) Selecting and optimizing said modes enable a scandium-containing concentrate comprising a mixture of scandium, titanium, zirconium, iron, sodium oxides, hydroxides and carbonates to be obtained by the carbonate sorption-hydrolysis process.

(36) It is essential for the concentrate composition that the Sc.sub.2O.sub.3 content should be at least of 15 wt. % (in terms of dry matter), the TiO.sub.2 content not more than 3 wt. % and the ZrO.sub.2 content not more than 15 wt. %. Scandium is present in the concentrate in the form of a mixture of Sc(OH).sub.3 hydroxide and ScOHCO.sub.3.4H.sub.2O basic salt. The weight ratio between Sc.sub.2O.sub.3 and TiO.sub.2 in the concentrate is above 5:1 (by weight), allowing a simple and low-cost process to be used for obtaining pure scandium oxide from the same with a Sc.sub.2O.sub.3 content of ≥99 wt. % (in the calcined product). Said TiO.sub.2 content of ≤3 wt. % allows a simple and low-cost process to be used in the process of further repurification for obtaining pure scandium oxide from the same with a Sc.sub.2O.sub.3 content of ≥99 wt. % (in the calcined product).

(37) The proposed method for repurification of a scandium-containing concentrate is unique in that scandium is precipitated from a sulfuric solution using sodium sulfate in the form of a sodium and scandium sulfate double salt rather than using a strong acid in the form of scandium sulfate.

(38) At a Sc.sub.2O.sub.3 content in the concentrate of less than 15 wt. % (in terms of dry matter), the process for repurifying the concentrate to a SO 99 commercial product (i.e. with a scandium oxide content of ≥99 wt. %) becomes significantly more complicated, the secondary losses of scandium while being repurified together with repurification tailings amount to 30% and more, and the operating costs for repurification exceed 300 US/kg of SO 99 as shown below in Table 1.

(39) TABLE-US-00001 TABLE 1 Sc.sub.2O.sub.3 Sc.sub.2O.sub.3 content losses, in the % (of concentrate, Repurification initial wt. % cost, content Number of Example (in terms of US$/kg of in the repurification No. dry matter) SO 99 concentrate) steps 1 10 450 40 5 2 50 180 10 3

(40) When using a lean concentrate (Sc.sub.2O.sub.3<15 wt. %), it will be needed to increase the number of the steps of dissolving the concentrate in sulfuric acid and again precipitating scandium with strong sulfuric acid in order to achieve the required purity of scandium oxide. Each additional process step is a source of secondary losses of scandium with tailings and makes the process considerably more expensive.

(41) At a TiO.sub.2 content of >3 wt. %, no concentrate with required purity can be obtained so that so that it will be needed to dissolve again the double salt in sulfuric acid and to precipitate again sodium sulfate. It results in increased losses and a more expensive process as shown below in Table 2.

(42) TABLE-US-00002 TABLE 2 Sc.sub.2O.sub.3 content in the Sc.sub.2O.sub.3 concentrate, losses, % wt. % (in Repurification (of initial Number of TiO.sub.2 in Example terms of dry cost, US$/kg content in the repurification OC 99, No. matter) Sc.sub.2O.sub.3:TiO.sub.2 of SO 99 concentrate) steps wt. % 3 30 5 400 30 4 >0.1 4 30 10 250 10 3 ≤0.05 5 15 5 350 20 3 ≤0.1 6 10 2 500 40 5 >0.15

(43) At a ZrO.sub.2 content in the concentrate not exceeding 15 wt. %, a simple and low-cost process may be used for producing pure scandium oxide from the same with a Sc.sub.2O.sub.3 content of ≥99 wt. % (in the calcined product) during further repurification. At a ZrO.sub.2 content of >15 wt. %, no concentrate with required purity can be obtained so that it will be needed to dissolve again the double salt in sulfuric acid and to precipitate again sodium sulfate. It results in increased losses and a more expensive process as shown below in Table 3.

(44) TABLE-US-00003 TABLE 3 Sc.sub.2O.sub.3 content in the Sc.sub.2O.sub.3 concentrate, losses, % wt. % (in Repurification (of initial Number of ZrO.sub.2 in Example terms of dry cost, US$/kg content in the repurification OC 99, No. matter) Sc.sub.2O.sub.3:ZrO.sub.2 of SO 99 concentrate) steps wt. % 7 20 0.5 280 30 3 ≥0.5 8 20 1.5 550 10 5 <0.2 9 15 1 350 20 3 0.3 10 10 1.5 650 50 5 ≥0.5

(45) The method for producing a scandium concentrate from red mud is illustrated by the following examples.

(46) Carbonization leaching of scandium is carried out using an industrial pulp of the source red mud with the following average chemical composition:

(47) solid phase, wt. %: 41.0 Fe.sub.2O.sub.3total; 13.0 Al.sub.2O.sub.3; 7.5 CaO; 13.0 SiO.sub.2; 4.50 TiO.sub.2; 5.5 Na.sub.2O; 0.0140 Sc.sub.2O.sub.3; 0.14 ZrO.sub.2;

(48) liquid phase, g/dm.sup.3: 5.5 Na.sub.2O.sub.total; 3.0 Al.sub.2O.sub.3; pH 12.5; liquid-to-solid ratio in the pulp is on average 3.0 (by weight).

Example 1

(49) In a carbonizing apparatus (V.sub.effective=30.0 m.sup.3) having a gas bubbler, a steam register and a mixer, scandium was leached from the red mud at a liquid-to-solid ratio of at least 4 using a soda-bicarbonate solution, with a NaHCO.sub.3 content in the pulp=80÷110 g/dm.sup.3, a Na.sub.2CO.sub.3 content=45÷60 g/dm.sup.3 and at a temperature=80÷85° C. The total duration of the leaching process was 3 hours, wherein before leaching, the mud pulp was gassed with an air-gas mixture containing 97-99% (by volume) of CO.sub.2 at a pulp temperature of 35-45° C.

(50) Once the general leaching process of scandium was competed, the carbonized mud pulp was filtered and the resulting primary SC-containing solution with the following chemical composition, g/dm.sup.3: 65.0 Na.sub.2O.sub.total; 97.0 NaHCO.sub.3; 50.0 Na.sub.2CO.sub.3; 0.007 Al.sub.2O.sub.3; 0.012 Sc.sub.2O.sub.3; 0.140 TiO.sub.2; 0.180 ZrO.sub.2; 0.020 Fe.sub.2O.sub.3; pH 8.8÷9.2 was supplied to the stage of scandium sorption extraction and concentration (see Example 2).

(51) Table 4 shows the experimental results of the red mud carbonization leaching and extracting scandium into a solution in accordance with the parameters of the claimed invention and also going beyond the optimum parameters.

(52) The optimum conditions of scandium oxide carbonization leaching from the red mud (1-4) are the following: liquid-to-solid ratio of at least 4, NaHCO.sub.3 sodium hydrocarbonate and Na.sub.2CO.sub.3 soda concentration in the mud pulp liquid phase, respectively, of 80÷110 g/dm.sup.3 and 45÷60 g/dm.sup.3, duration of 3 hours, process temperature of 80-85° C.

(53) Under these conditions, a considerable increase by ˜4.0÷9.0% in the Sc.sub.2O.sub.3 extraction from the red mud was achieved compared to the prototype (from the initial Sc.sub.2O.sub.3 content).

(54) Below the optimum parameters (5-10), no positive effect consisting in Sc.sub.2O.sub.3 extraction from the red mud is present, i.e. it is either lower than that of the prototype (5, 6, 8, 9 and 10) or comparable to the prototype (7).

(55) When exceeding the optimum parameters (11-12), despite a certain increase in Sc.sub.2O.sub.3 extraction to 30.1÷30.5%, the process is inexpedient because the concentration of Na.sub.2O.sub.total has to be considerably increased to 70 g/dm.sup.3, which will result in impaired sorption characteristics.

(56) TABLE-US-00004 TABLE 4 Experimental results of scandium extraction under optimum conditions of the red mud carbonization leaching, all other conditions being equal (see the text) Leaching parameters NaHCO.sub.3 Na.sub.2CO.sub.3 Experiment concentration, Liquid-to-solid Leaching Leaching Sc.sub.2O.sub.3 No. g/dm.sup.3 ratio in the pulp temperature, ° C. time, hours extraction, % According to the prototype 21.0 Optimum parameter limits 1 80 4.0 80 3.0 25.0 60 2 95 4.5 85 3.0 28.0 50 3 110  4.5 80 3.0 28.0 45 4 100  5.0 85 3.0 30.0 50 Beyond optimum parameter limits 5 105  4.0 80 3.0 19.0 35 6 110  3.5 85 3.0 20.0 45 7 100  4.5 80 2.0 22.0 50 8 100  4.0 80 1.0 18.0 50 9 110  4.5 70 3.0 10.0 45 10 110  4.5 60 3.0 5.5 45 11 100  4.5 80 6.0 30.1 50 12 135  4.0 85 3.0 30.5 50

Example 2

(57) A SC-containing solution was produced at the first stage under optimum conditions described in Example 1.

(58) At the second stage, scandium was sorbed from a solution containing, g/dm.sup.3: 65.0 of Na.sub.2O.sub.total; 97.0 of NaHCO.sub.3; 50.0 of Na.sub.2CO.sub.3; 0.007 of Al.sub.2O.sub.3; 0.012 of Sc.sub.2O.sub.3; 0.140 of TiO.sub.2; 0.180 of ZrO.sub.2; 0.020 of Fe.sub.2O.sub.3; pH 8.8-9 on LEWATIT® TP-260 phosphorous-containing ion exchanger.

(59) Other equal conditions are: optimum mode of scandium oxide carbonization leaching of the red mud; scandium desorption conditions: eluting solution −320-350 g/dm.sup.3 of Na.sub.2CO.sub.3, linear feed rate of the solution through a resin layer—0.25-0.3 m/hour, temperature—40-45° C.; (see Table 3); precipitation conditions of Ti—Zr concentrate are the following: τ=70-80° C., pH-10.0÷10.5, τ=1.0÷1.5 hour (see Table 4); precipitation conditions of Sc concentrate: pH=12.0÷12.5, t=70-80° C., τ=0.5÷1.0 hour.

(60) Table 5 shows the experimental results of sorption extraction, wherein the linear feed rate of a mother solution through a resin layer and its temperature, Sc.sub.2O.sub.3 content in the concentrate and the yield of the latter are according to the parameters of the claimed invention and also beyond the optimum parameter limits.

(61) According to Table 5, the optimum conditions of scandium sorption from a mother solution are the following: linear feed rate of a Sc-containing mother solution: 1.0÷2.0 m/hour; process temperature: 70-80° C.

(62) At the same time, Sc.sub.2O.sub.3 content in a primary concentrate is ˜25.0-60.0% with Sc.sub.2O.sub.3 extraction being of ˜29.5 g against ˜20.7 g of Sc.sub.2O.sub.3/t of red mud (dry) according to the prototype, i.e. on average 1.4 less than in the claimed invention.

(63) Below the optimum sorption parameters as related to the process temperature (5 and 6), the end-to-end extraction of scandium oxide into an end product (concentrate) is 17.1÷18.5%, which is lower than according to the prototype while at a lower linear feed rate of the solution through a resin layer (7 and 8), the end-to-end extraction scandium oxide into an end product (concentrate) is 29.7÷29.9%, which is higher than according to the prototype, but these sorption rates are inexpedient for carrying out the process because of a significant enlargement of the sorption equipment.

(64) When exceeding the optimum sorption parameters as related to the linear feed rate of the solution through a resin layer or the process temperature (9 and 10), the end-to-end extraction of scandium oxide into a concentrate is either significantly lower (9) or higher (10) than according to the prototype with the scandium oxide content in an end product (concentrate) being 59.9% against 27.0% according to the prototype.

(65) TABLE-US-00005 TABLE 5 Experimental results of scandium sorption from for a carbonate solution using Lewatit TP-260 under optimum conditions, all other conditions being equal (see the text) Scandium concentrate characteristic Process parameters Concentrate Experiment Linear rate of Temperature, Sc.sub.2O.sub.3 yield, Sc.sub.2O.sub.3 No. solution, m/h ° C. Content, % g/t of red mud Extraction, % According to the prototype 27.0 108.9 20.7 Optimum parameter limits 1 1.0 70 55.0 73.8 29.0 2 1.5 80 45.5 88.6 28.8 3 2.0 75 25.7 155.3 28.0 4 1.0 80 59.8 69.1 29.5 Beyond optimum parameter limits 5 1.5 40 23.5 101.9 17.1 6 1.5 50 26.0 99.6 18.5 7 0.25 70 60.5 69.2 29.9 8 0.50 80 60.2 69.1 29.7 9 4.5 80 20.5 86.7 12.7 10 1.5 90 59.9 69.6 29.8

Example 3

(66) A SC-containing solution was produced under optimum conditions described in Example 1 and scandium was sorbed from said solution on LEWATIT® TP-260 ion exchanger under optimum conditions described in Example 2.

(67) Table 6 shows the experimental results of scandium desorption from LEWATIT® TP-260 ion exchanger phase using an elution solution containing Na.sub.2CO.sub.3 under counter-flow conditions and with the following process parameters: Na.sub.2CO.sub.3 concentration in desorption solution, linear feed rate of the desorption solution through an ion exchanger layer and its temperature being under optimum conditions in accordance with the parameters of the claimed invention and also beyond the optimum parameter limits.

(68) TABLE-US-00006 TABLE 6 Experimental results of scandium desorption using a carbonate solution under optimum conditions, all other conditions being equal (see the text) Process parameters Linear rate Concentration Sc.sub.2O.sub.3 Sc.sub.2O.sub.3 Experiment of solution, Na.sub.2CO.sub.3 in Temperature, concentration in extraction No. m/h solution, g/dm.sup.3 ° C. eluate, g/dm.sup.3 into eluate, % Optimum parameter limits 1 0.3 320 40 0.53 98.5 2 0.25 350 40 0.79 99.5 3 0.25 340 40 0.73 99.0 4 0.3 340 45 0.75 99.2 5 0.3 350 45 0.80 99.8 Beyond optimum parameter limits 6 0.25 300 45 0.50 97.0 7 0.25 250 45 0.27 90.0 8 0.25 200 45 0.07 46.0 9 0.25 150 45 0.01 25.0 10 1.0 350 45 0.23 86.0 11 0.3 350 60 0.81 99.9 12 0.3 340 30 0.79 99.5

(69) Optimum conditions of scandium desorption: linear rate of elution solution (320-350 g/dm.sup.3 of Na.sub.2CO.sub.3) is 0.25-0.30 m/hour; Na.sub.2CO.sub.3 concentration in the solution is 320-350 g/dm.sup.3; eluate temperature is 40-45° C., wherein at higher temperatures, the solution concentrates by evaporation, its concentration increases and, as a consequence, the solution crystallizes, and at lower temperatures, the saturated solution is instable, which also results in its crystallization so that the process becomes impracticable.

(70) Below the optimum parameter limits (6, 7, 8 and 9,) the concentration of scandium in the resulting eluate becomes significantly lower and its extraction into eluate goes down to ≤90.0% (7, 8 and 9), a minimally acceptable extraction ratio of a valuable component by desorption. At a lower temperature of eluate (12), the desorption process becomes impracticable because of the risk of crystallization of the desorption solution.

(71) When exceeding the optimum limits as related to the linear rate of the elution solution (10), both the Sc.sub.2O.sub.3 concentration in eluate and the extraction ratio into eluate of <90% are also insufficient, which is associated now with an excessively high specific load in terms of the elution solution, leading first of all to a lower Sc.sub.2O.sub.3 concentration in eluate due to an increased volume of the elution solution and also to a diffused front line of the desorption process. At a higher eluate temperature (11), the desorption process becomes impracticable because of the risk of crystallization of the desorption solution due to evaporation.

Example 4

(72) A SC-containing solution was produced as described in Example 1, scandium was sorbed with LEWATIT® TP-260 ion exchanger under optimum conditions described in Example 2, and scandium was desorbed from the ion exchanger phase with a carbonate solution under optimum conditions described in Example 3.

(73) After desorption, the resulting eluate containing, g/dm.sup.3: 0.35 of TiO.sub.2, 0.17 of ZrO.sub.2 and 0.78 of Sc.sub.2O.sub.3 was purified coarsely purified of impurity elements (Ti, Zr) concomitant with scandium to further obtain a scandium concentrate with a high Sc.sub.2O.sub.3 content as the end product.

(74) Table 7 shows the experimental results of the purification of a Sc-containing eluate under optimum conditions according to the present invention as well as beyond the optimum parameter limits.

(75) The optimum conditions of precipitation of a Ti—Zr concentrate (1-5) are: temperature=70÷80° C.; pH=10.0÷10.5; duration of 1÷1.5 hour.

(76) As a result, a maximum purification factor is achieved, i.e. a ratio of a scandium concentration in eluate being purified to an overall concentration of impurity components (Ti+Zr) of 14÷19.5 against 1.5 in the stock eluate.

(77) TABLE-US-00007 TABLE 7 Experimental results of the purification of a Sc-containing eluate of impurity components (titanium and zirconium) under optimum conditions, all other conditions being equal (see the text) Process parameters Concentration of Experiment t, τ, components, g/dm.sup.3 MeO.sub.2:Sc.sub.2O.sub.3 ratio Purification No. ° C. pH hour TiO.sub.2 ZrO.sub.2 Sc.sub.2O.sub.3 Sc.sub.2O.sub.3:TiO.sub.2 Sc.sub.2O.sub.3:ZrO.sub.2 factor*.sup.) Stock eluate 0.35 0.17 0.78 2.23 4.59 1.5 1 80 10.0 1.0 0.015 0.04 0.778 51.9 19.45 14.15 2 75 10.5 1.0 0.005 0.035 0.775 155.0 22.14 19.38 3 70 10.0 1.5 0.018 0.043 0.779 43.28 18.12 12.73 4 70 10.3 1.5 0.014 0.039 0.774 55.29 19.85 14.60 5 75 10.2 1.0 0.014 0.037 0.776 55.43 20.97 15.22 Beyond optimum parameter limits 6 60 10.5 1.0 0.09 0.14 0.779 8.66 5.56 3.39 7 80 8.5 1.5 0.32 0.17 0.78 2.44 4.59 1.59 8 80 9.0 1.5 0.28 0.17 0.78 2.79 4.59 1.73 9 80 9.5 1.5 0.30 0.17 0.78 2.60 4.59 1.66 10 75 10.5 3.0 0.013 0.031 0.773 59.46 25.94 17.57 11 80 10.0 0.5 0.014 0.13 0.772 55.14 5.94 5.36 12 95 10.5 1.5 0.013 0.032 0.70 53.85 21.88 15.56 *.sup.)Purification factor - a ratio of scandium concentration in eluate to an overall concentration of impurity components (Ti + Zr).

(78) Below the optimum parameter limits (6, 7, 8, 9 and 11), the purification of Sc-containing eluate of impurity components (Ti, Zr) is inefficient with the purification factor of −1.6÷5.4 so that an end product (concentrate) with a low Sc.sub.2O.sub.3 content is further produced.

(79) When the purification process of Sc-containing eluate exceeds an optimum limit in terms of duration (10), although a higher purification factor of 17.57 is achieved, it does not result in significant purification efficiency and is within the range of 14÷19.5 optimum for the process so that only excessive energy consumption occurs. When purified at higher parameter values: t=95° C., pH=10.5 and duration of 1.5 hour, a purification factor of 15.56 is achieved, however, with significant losses of scandium: Sc.sub.2O.sub.3 concentration in eluate goes down from c 0.78 to 0.70 g/dm.sup.3 or by 10.3%.

Example 5

(80) A SC-containing solution was obtained from red mud under optimum conditions described in Example 1, sorption and desorption of scandium was performed under optimum conditions described in Examples 2 and 3, respectively, and the resulting Sc-containing eluate was purified of impurity components under optimum conditions described in Example 4.

(81) The resulting purified Sc-containing eluate containing, g/dm.sup.3: 0.014 of TiO.sub.2, 0.036 of ZrO.sub.2 and 0.773 of Sc.sub.2O.sub.3, pH of 10.5 was further used to precipitate a primary scandium concentrate.

(82) Table 8 shows the experimental results of scandium precipitation (production of concentrate as the end product) at optimum values of pH, temperature and duration as well as beyond the optimum parameter limits.

(83) TABLE-US-00008 TABLE 8 Experimental results of scandium precipitation from purified eluate, all other conditions being equal (see the text) Scandium concentrate characteristic Process parameters Concentrate Experiment t. Sc.sub.2O.sub.3 yield, g/t Sc.sub.2O.sub.3 extraction No. pH ° C. Duration, hours content, wt. % of red mud into concentrate, % According to the prototype 27.0 108.9 20.7 Optimum parameter limits 1 12.5 75 0.5 35.0 115.6 28.9 2 12.0 80 1.0 60.3 65.0 28.0 3 12.5 80 0.5 25.8 157.4 29.0 4 12.5 75 1.0 45.0 91.8 29.1 Beyond optimum parameter limits 5 12.0 60 1.0 34.3 79.2 19.4 6 11.0 80 1.0 23.6 57.5 9.7 7 11.5 80 1.0 32.3 77.2 17.8 8 12.5 90 0.5 25.0 164.1 29.3 9 12.5 95 1.0 23.5 175.7 29.5 10 12.0 75 3.5 47.0 85.2 28.6

(84) The optimum conditions of production of a primary scandium concentrate are: temperature 70÷80° C.; pH 12.0÷12.5; process duration of 0.5÷1.0 hour.

(85) As a result, the following production figures are achieved: Sc.sub.2O.sub.3 content in the resulting concentrate is on average 25.0÷60.0% with the extraction ratio of 28.0÷29.1% against 27.0% and 20.7% according to the prototype.

(86) Although a high Sc.sub.2O.sub.3 content of 23.6.0÷34.3% in the concentrate exceeding on average the one according to the prototype (27.0%) is achieved below the optimum parameter limits of pH and the process temperature (5, 6 and 7), the end-to-end extraction from the red mud of 15.6% is less than the extraction ratio of 20.7% according to the prototype.

(87) When the process is performed under optimum conditions but its duration is 3.5 hours (10), the Sc.sub.2O.sub.3 content in the resulting concentrate is 47.0% with the extraction ratio of 28.6%, i.e. within the limits of the optimum process conditions (Sc.sub.2O.sub.3 content of 25.0-60.0% with the extraction ratio 28.0-29.1%) so that only excessive energy consumption occurs.

(88) When the process is performed at an optimum pH but at an elevated temperature of 90-95° C. and for 0.5-1.0 hour (8 and 9), the Sc.sub.2O.sub.3 content in the concentrate goes down to 23.5% (9).

(89) Therefore, the proposed method for producing a scandium-containing concentrate enables a maximally high scandium concentration to be achieved in the concentrate while processing the red mud.

(90) It is another object of the proposed invention to provide scandium oxide with a maximally high purity and minimal costs.

(91) To achieve this object, a method is proposed for producing scandium oxide, comprising dissolving a scandium-containing concentrate in sulfuric acid, removing an acid-insoluble precipitate, bringing the concentration of sulfuric acid in the filtrate to 540-600 g/dm.sup.3, precipitating scandium in the presence of an ammonium chloride compound at 50-70° C. while being kept for 1-2 hours with stirring, filtering, washing with ethanol at a volume ratio of 1-10÷11, drying and calcining to produce a scandium oxide precipitate.

(92) Once the acid-insoluble precipitate is removed, scandium from the filtrate is precipitated with sodium sulfate in the form of a double salt of sodium and scandium sulfate, filtered out, the resulting precipitate is washed with a sodium sulfate solution, the double salt is dissolved in water and scandium hydroxide is precipitated with caustic soda, then the cake is filtered, washed and added to a solution of oxalic acid to obtain scandium oxalate using oxalic acid, filtered out and washed with water. For producing scandium oxide with a purity of ≥99 wt. %, scandium oxalate is calcined at a temperature not lower 650° C.

(93) The main difference from the prototype is precipitating scandium in the form of a double salt sodium and scandium sulfate and reprecipitating scandium in the form of a hydroxide using caustic alkali. By the proposed method for producing a scandium-containing concentrate, it is possible to optimize the method for extracting scandium oxide from the resulting concentrate of a certain composition. The prototype uses a lean scandium concentrate with a Sc.sub.2O.sub.3 content of about 2 wt. % and for this reason various very complicated multistage repurification patterns are used. In particular, an operating procedure using a strong sulfuric acid is provided, making the requirements more stringent. Unlike the prototype, low-aggressive sodium sulfate is proposed for use as a precipitant (i.e. the equipment is not acid-resistant, working conditions are better, and it can be recycled) and the resulting double salt of sodium and scandium sulfate is highly selective with respect to the remaining impurities. Provided below are the examples of the repurification operation and modes:

(94) Precipitating with Sodium Sulfate Double Salt

(95) Dry sodium sulfate (Na.sub.2SO.sub.4) was added to a filtrate containing 30±5 g/dm.sup.3 of Sc.sub.2O.sub.3 to achieve Na.sub.2SO.sub.4 concentration=250±30 g/dm.sup.3. The double salt was synthesized at 70-80° C. for at least 1 hour and then cooled to a room temperature at which the double salt solubility becomes lower.

(96) Filtering and Washing

(97) The resulting precipitate was filtered at a room temperature and then the precipitate was washed with a sodium sulfate solution at Na.sub.2SO.sub.4 concentration of ˜250±30 g/dm.sup.3. The washing solution consumption was 50 cm.sup.3 per 100 grams of the crystalline precipitate (i.e. at a ratio of 1:2 by weight), the washing temperature was 22±3° C.

(98) Dissolving Double Salt

(99) The double salt was dissolved with distilled water at 80±5° C. to obtain Sc.sub.2O.sub.3 concentration in the solution of ˜20-25 g/dm.sup.3.

(100) Precipitating Scandium Hydroxide

(101) Scandium hydroxide was precipitated using a concentrated (45%) solution of NaOH at a room temperature. The resulting precipitate was washed with distilled water at a room temperature. The water consumption was 50 cm.sup.3 of the solution per 100 grams of Sc.sub.2O.sub.3 (i.e. at a ratio of 1:2 by weight).

(102) Producing Scandium Oxalate

(103) Scandium hydroxide was turned into scandium oxalate by treating the precipitate with a solution of oxalic acid (H.sub.2C.sub.2O.sub.4) at a concentration of 100 g/dm.sup.3 at 70-80° C.

(104) Filtering and Washing Scandium Oxalate

(105) The resulting scandium oxalate was filtered at a room temperature. The precipitate was washed with distilled water at ratio of 1:1 (by weight) at a room temperature.

(106) The method for producing a Sc.sub.2O.sub.3 is illustrated by the following example.

(107) 50 g of a scandium-containing concentrate with the following composition, wt. %: Sc.sub.2O.sub.3—52.1; TiO.sub.2—0.95; Fe.sub.2O.sub.3—1.7; ZrO.sub.2—2.6; Na.sub.2O—3.2; CaO—2.1; Si—0.4 was provided; pulpified in 840 dm.sup.3 of water, 27 cm.sup.3 of 94% sulfuric acid was added and dissolved at 60° C. An acid-insoluble precipitate was removed from the solution and the concentration of sodium sulfate in the filtrate was adjusted to Na.sub.2SO.sub.4=250±30 g/dm.sup.3. The resulting pulp comprising a scandium double salt and a liquid phase was kept while being stirred at 70-80° C. for 1 hour. Then 108.5 g of scandium double salt crystals were filtered, 200 dm.sup.3 of a sodium sulfate solution with a Na.sub.2SO.sub.4 concentration of 250 g/dm.sup.3 were filter washed at a room temperature (22±3° C.). The resulting scandium double salt crystals washed of interstitial moisture containing concomitant impurities were dissolved in 1032 dm.sup.3 of water at 80±5° C.). A 45% solution of NaOH was added to the resulting scandium-containing solution, and scandium hydroxide was precipitated at a room temperature and solution pH of 6.5±7.0. The precipitated scandium hydroxide was separated from the precipitant mother solution by filtration, the hydroxide whose weight is 203 g was filter washed with 100 dm.sup.3 of water following which it was added to 550 dm.sup.3 of an oxalic acid solution with a H.sub.2C.sub.2O.sub.4 concentration of 100 g/dm.sup.3. The resulting pulp comprising a liquid phase and scandium oxalate was kept at 70-80° C., then filtered, scandium oxalate was washed with water, dried at 120° C. for 2 hours until a constant weight was obtained and calcined at 850° C. for 1 hour. Scandium oxide was obtained with a purity of 99.5% and a yield of 98.3%. Scandium losses amounted to 1.7%.

(108) Therefore, the proposed method for producing scandium oxide enables a maximum degree of purity to be achieved at minimum costs.