Method for recovering scandium from red mud left from alumina production

Abstract

The present invention relates to rare earth metallurgy, in particular a method for recovering scandium from the red mud byproduct of alumina production. The method includes repulping red mud, sorption leaching scandium therefrom with the use of an ion-exchange sorbing agent to obtain a rich-in-scandium ion exchanger and depleted-in-scandium pulp, desorbing scandium with a solution of sodium hydrocarbonate to obtain a desorbed ion exchanger which is returned to the sorption leaching stage and a solution of industrial reclaim scandium which is transferred to obtain a deposited concentrated scandium, wherein scandium is continuously sorption-leached from red mud pulp in the phosphorous-containing ion exchanger in a countercurrent mode upon direct contact of the pulp with the ion exchanger, scandium is desorbed from the organic phase of the ion exchanger by a solution with a concentration of Na.sub.2CO.sub.3 of 200-450 g/dm.sup.3 to obtain industrial reclaim scandium, from which a scandium concentrate is recovered.

Claims

1. A method for recovering scandium from red mud to obtain a scandium-containing solution suitable for extraction of scandium therefrom including the following steps: repulping red mud with a mother solution comprising a mixture of sodium carbonate of Na.sub.2CO.sub.3 and sodium bicarbonate of NHCO.sub.3 to form raw red mud pulp having a solid phase and a liquid phase; sorption leaching scandium from the raw red mud pulp with a desorbed ion-exchanger without using acidic reagents to obtain a scandium-rich ion exchanger having more scandium compared with the desorbed ion exchanger and a scandium-depleted red mud pulp having a solid phase containing less scandium as compared with the solid phase of the raw red mud pulp, thereby extracting scandium from the red mud solid phase into the liquid phase from which scandium is sorbed on the surface of the desorbed ion exchanger, wherein the ion-exchanger is an ion-exchange resin having phosphorous-containing functional groups, wherein the sorption leaching is carried-out in a countercurrent mode comprising direct contact of the raw red mud pulp with the desorbed ion exchanger by adding the desorbed ion exchanger into the raw red mud pulp without separating the solid phase of the raw red mud pulp from the liquid phase of the raw red mud pulp; separating the scandium-depleted red mud pulp and the scandium-rich ion exchanger; filtering the scandium-depleted red mud pulp to obtain a spent mother solution and red mud cake; desorbing scandium by treating the scandium-rich ion exchanger with a solution of sodium carbonate to obtain the desorbed ion exchanger which is returned to the step of sorption leaching and a scandium-containing solution suitable for extraction of scandium therefrom; wherein scandium is desorbed from the surface of the ion exchanger by a solution of sodium carbonate with a concentration of 200-450 g/dm.sup.3 to obtain the scandium-containing solution.

2. The method of claim 1, characterized in that the mother solution used to repulp the red mud comprises a total concentration of NaHCO.sub.3 and Na.sub.2CO.sub.3 of 40-80 g/dm.sup.3, wherein the NaHCO.sub.3 concentration is greater than or equal to 50% of the total of NaHCO.sub.3 and Na.sub.2CO.sub.3 concentrations.

3. The method of claim 1, characterized in that sorption leaching of scandium is carried out at the temperature of 40-90° C.

4. The method of claim 1, characterized in that sorption leaching of scandium is carried out during 1-8 hours and the ratio of the mass of the raw red mud pulp to the mass of the ion exchanger is 20-120:1.

5. The method of claim 1, characterized in that sorption leaching of scandium is carried out at the mass ratio of the solid and liquid phases in the raw red mud pulp of 1:2.5-5.0 (solid/liquid ratio).

6. The method of claim 1, characterized in that scandium is desorbed from the surface of the ion exchanger by a solution of sodium carbonate with a concentration of Na.sub.2CO.sub.3 from 200 to 450 g/dm.sup.3.

7. The method of claim 1, further comprising aerating the spent mother solution at a temperature of 15-50° C. with a gas-air mixture containing CO.sub.2 to form the mother solution having a NaHCO.sub.3 concentration that is greater than or equal to 50% of the total of NaHCO.sub.3 and Na.sub.2CO.sub.3 concentrations and using at least a portion of the mother solution in the repulping step.

8. The method of claim 1, characterized in that during sorption leaching of scandium from the raw red mud pulp in the phosphorous-containing ion exchanger, the raw red mud pulp is aerated with a gas-air mixture containing CO.sub.2.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) The present invention will be further described in the detailed description with reference to the accompanying drawing, which is given by way of illustration only, and thus, does not limit the present invention.

(2) FIG. 1 a shows a process flow scheme for scandium recovery from red mud.

DETAILED DESCRIPTION OF THE INVENTION

(3) It is desired to optimize suggested method by following:

(4) Scandium is sorbed at each stage during 1-8 hours at a mass ratio of pulp to ion exchanger of 20-130:1. After sorption, the red mud pulp depleted in scandium is transferred to the filtration stage, the resulted solution of the mixture of sodium carbonate and sodium bicarbonate is aerated at the temperature of 15-50° C. with a gas-air mixture containing CO.sub.2 until the percentage of Na.sub.2Obicarb of Na.sub.2Ototal is 50-100% and then is returned for sorption leaching of a new red mud portion.

(5) When scandium is sorbed from the pulp in the phosphorous-containing ion exchanger, the pulp is aerated with a gas-air mixture containing CO.sub.2.

(6) Due to the recovery of scandium from red mud by means of sorption leaching in a phosphorous-containing ion exchanger on a continuous contact of the ion exchanger with the red mud pulp in a countercurrent mode and at an elevated temperature at which sodium bicarbonate is decomposed to form an ultradispersed carbon dioxide, the degree of scandium recovery from red mud is increased by means of a shift of a reaction equilibrium towards formation of soluble scandium complex compounds with carbonate ions and its transformation into a solution due to the continuous reduction of scandium concentration in the solution in the result of its sorption in a phosphorous-containing ion exchanger having under optimal process conditions the higher scandium capacity.

(7) Scandium desorption from a saturated phosphorous-containing ion exchanger by means of a carbonate solution under optimal parameters of desorption allows to provide an industrial scandium reclaim having an increased content of scandium oxide and to further recover therefrom a concentrated scandium having an increased scandium content.

(8) Sorption leaching of scandium enables to reduce ion exchanger loading if compared with the scandium sorption from solutions in a compressed layer of the ion exchanger, thus, reducing process costs. This process is connected with the use of carbonate-containing reagents and doesn't make use of any acidic reagents which makes it possible to simplify the process flow and to reduce costs for process implementation by excluding stages of neutralization of a leached red mud, washing out an acid from an ion exchanger and disposing of acidic wash-offs.

(9) Carrying out the scandium sorption leaching from red mud in a continuous countercurrent mode by means of a solution having a mixture of sodium carbonate and sodium bicarbonate with a concentration of Na.sub.2Ototal of 40-80 g/dm.sup.3, wherein the percentage of Na.sub.2Obicarb is from 50 to 100% of Na.sub.2Ototal, the temperature is 40-90° C., a liquid/solid ratio is 1:2.5-5 and a mass ratio of pulp to ion exchanger is 20-130:1 makes it possible to achieve the highest degree of scandium recovery from red mud and to concentrate it in the phosphorous-containing ion exchanger with optimal reagent consumption.

(10) Carrying out the scandium desorption in a phosphorous-containing ion exchanger by means of a solution of sodium hydrocarbonate with a concentration of Na.sub.2CO.sub.3 from 200 to 450 g/dm.sup.3 makes it possible to achieve the highest degree of reclaiming of the ion exchanger with the maximum scandium content in the industrial reclaim which is transferred for concentrated scandium recovery.

(11) Aeration of a solution of a sodium carbonate and sodium bicarbonate mixture after scandium sorption leaching (a sorption mother solution) by means of a gas-air mixture at a temperature of 15-50° C. containing CO.sub.2 enables restoring of the relationship of Na.sub.2Obicarb and Na.sub.2Ototal up to 50-100% at minimum CO.sub.2 consumption rate and using the sorption mother solution for repulping a new red mud portion, thus, minimizing sodium carbonate and sodium bicarbonate consumption for the process implementation, as well as preventing losses of scandium and sorption mother solution.

(12) Optimal parameters of the sorption leaching process have been found from numerous experiments by varying concentrations of carbonate and sodium bicarbonate in the solution, contact time of the pulp with a resin, the pulp temperature, the solid/liquid ratio, and the mass ratio of pulp to ion exchanger. When the sorption process was carried out in the time interval of interest, the pulp was separated from the ion exchanger on a sieve; the ion exchanger was washed with distilled water and selected for analysis. The pulp was filtered after sorption and the filtrate sample was taken. The deposit was washed out from the sorption mother solution and was taken, too, for analysis.

(13) Table 1 shows results of experiments studying the impact of the concentration of Na.sub.2Ototal in a carbonate-bicarbonate solution for leaching and the content of Na.sub.2Obicarb therein on the degree of scandium recovery from red mud where at the first stage of sorption leaching the temperature is 60° C. the solid/liquid ratio is 1:4, the mass ratio of pulp to ion exchanger is 90:1 and the contact time is 2 hours.

(14) TABLE-US-00001 TABLE 1 Recovery degree of Na.sub.2Ototal, Na.sub.2Obicarb/ Sc at the stage of No. g/dm.sup.3 Na.sub.2Ototal, % sorption leaching, % 1 40 100 25.3 2 50 23.8 3 25 21.2 4 50 100 29.1 5 50 26.6 6 25 24.3 7 55 100 39.4 8 50 37.1 9 25 31.9 10 65 100 47.8 11 50 42.4 12 25 39.3 13 75 70 33.1 14 50 30.9 15 25 25.2 16 80 70 23.2 17 50 21.3 18 25 18.4

(15) Numbers of the Table 1 show that with increase of the concentration of Na.sub.2Ototal in the carbonate-bicarbonate solution of red mud in the presence of the ion exchanger and with increase of the sodium bicarbonate content therein as well, the scandium recovery degree also increases, and at 65 g/dm.sup.3 of Na.sub.2Ototal (100% Na.sub.2Obicarb) reaches 47.8%. When concentration of Na.sub.2Ototal becomes lower than 55 g/dm.sup.3, the degree of scandium recovery is reduced by reducing the amount of a leaching agent in the reaction zone, wherein the more Na.sub.2Obicarb in relation to Na.sub.2Ototal in the solution of the sodium carbonate and sodium bicarbonate mixture, the more efficient the leaching process is, because the thermal decomposition of sodium bicarbonate produces an ultradispersed carbon dioxide which intensifies the process of scandium transition into a solution to form carbonate complexes that are removed from the reaction zone by means of the scandium sorption in the phosphorus-containing ion exchanger, thus, shifting the equilibrium towards the formation of new scandium complexes. The increased sodium carbonate content in the leach solution (more than 50% of Na.sub.2Ototal) results in a partial desorption of scandium sorbed in the ion exchanger and consequently to a decrease in the degree of scandium recovery per ion exchanger. With an increase of the Na.sub.2Ototal concentration above 75 g/dm.sup.3 the amount of scandium recovered from red mud decreases due to a partial suppression of the sorption process due to the high salt background in the solution. Moreover, when the concentration of Na.sub.2Ototal is more than the optimal one and the sodium carbonate content is increased, the risk of supersaturation of the leach solution and deposition of sodium bicarbonate crystals in the solid phase becomes higher.

(16) Table 2 shows results of experiments studying the impact of temperatures on scandium capacity in a phosphorous-containing ion exchanger for sorption scandium from a red mud pulp in a phosphorous-containing ion exchanger, where the concentration Na.sub.2Ototal in a carbonate-bicarbonate solution (percentage of Na.sub.2Obicarb is 80%) is 65 g/dm.sup.3, the contact time is 2 hours, the solid/liquid ratio is 1:4 and the mass ration of the pulp to ion exchanger is 90:1.

(17) TABLE-US-00002 TABLE 2 Temperature, ° C. 25 40 60 80 90 Sc capacity in an 0.11 0.19 0.26 0.31 0.33 ion exchanger, %

(18) As can be seen from Table 2, when the temperature rises, the scandium capacity in the phosphorous-containing ion exchanger increases which is associated with an improvement in the kinetic characteristics of the sorption and leaching process. The maximum temperature of the sorption leaching process of 90° C. is a result of the thermal stability temperature of the phosphorous-containing ion exchanger, when the temperature rises above 90° C. the ion exchanger breaks down (elimination of functional phosphorus-containing groups). Thus, the maximum scandium capacity in the ion exchanger of 0.33% is achieved at the temperature of 90° C.

(19) Table 3 shows results of experiments studying the impact of a solid/liquid ratio on the scandium capacity in a phosphorous-containing ion exchanger for sorption leaching from a red mud pulp in a phosphorous-containing ion exchanger, where the concentration of Na.sub.2Ototal in a carbonate-bicarbonate solution (percentage of Na.sub.2Obicarb is 80%) is 65 g/dm.sup.3, the contact time is 2 hours, the temperature is 80° C. and the mass ration of pulp to ion exchanger is 90:1.

(20) TABLE-US-00003 TABLE 3 Solid/liquid ratio 1:2 1:2.5 1:3 1:4 1:5 1:6 Sc capacity in an ion 0.35 0.33 0.325 0.31 0.26 0.19 exchanger, %

(21) The analysis of Table 3 has shown that when the solid/liquid ratio in the leaching pulp decreases the scandium capacity in the ion exchanger rises, at the same time the low solid/liquid ratio makes pulp separation from the ion exchanger difficult because of the viscosity of the pulp; when the solid/liquid ratio increases the concentration of scandium in the leaching solution decreases, thus, reducing the ion exchanger capacity.

(22) Table 4 represents results of experiments studying the impact of the mass ratio of a pulp and ion exchanger on the degree of scandium recovery from a red mud pulp by sorption leaching, where the Na.sub.2Ototal concentration in the carbonate-bicarbonate solution (the percentage of Na.sub.2Obicarb is 80%) is 65 g/dm.sup.3, the contact time is 2 hours, the solid/liquid ratio is 1:4 and the temperature is 80° C.

(23) TABLE-US-00004 TABLE 4 Mass ration of a pulp and ion exchanger 10:1 20:1 40:1 60:1 80:1 100:1 120:1 140:1 Recovery 50.1 49.8 49.3 48.1 47.9 41.2 35.6 29.2 degree of Sc, %

(24) As can be seen from Table 4, the maximum recovery degree of scandium, which is 47.9-50.1%, is in the range of the pulp to ion exchanger ratio of 10-80:1. When the ion exchanger dose decreases below the optimal value (pulp:ion exchanger≥100:1), the recovery degree of scandium is decreased, as the amount of ion exchanger at the stage of leaching reduces which leads to the faster establishing of the equilibrium concentration of scandium in the ion exchanger phase and to stopping reduction of scandium concentration in the pulp liquid phase, therefore the transition of scandium from red mud into a solution is stopped. When the ion exchanger dose increases above the optimal value (pulp:sorbing agent 10-60:1) as the result of larger amounts of ion exchangers in the pulp, the base material (scandium) capacity in the ion exchanger decreases.

(25) Table 5 shows results of experiments studying the impact of the contact time of the pulp with the ion exchanger on the degree of scandium recovery from red mud, where, at the first stage of sorption leaching, the concentration of Na.sub.2Ototal in a carbonate-bicarbonate solution (percentage of Na.sub.2Obicarb is 80%) is 65 g/dm.sup.3, the mass ratio of the pulp and the ion exchanger is 90:1, the solid/liquid ratio is 1:4 and the temperature is 80° C.

(26) TABLE-US-00005 TABLE 5 Contact time, h 0.5 1 2 4 6 8 10 Recovery 23.5 35.7 47.8 47.9 47.95 48.0 48.1 degree of Sc, %

(27) As can be seen from Table 2, when the contact time of the red mud pulp with the ion exchanger, the degree of scandium recovery increases to 48.1%. When the contact time is above the optimal value, the process performance reduces, when the contact time is lower than the optimal value the degree of scandium recovery goes down due to the failure to achieve the scandium equilibrium in the pulp-ion exchanger system for a short period of time.

(28) Optimal parameters of scandium desorption from a phosphorous-containing ion exchanger were established based on results of experiments comprised in varying a sodium carbonate concentration in a desorption solution.

(29) To desorb scandium from the phosphorous-containing ion exchanger a sodium carbonate solution was used, since it effectively desorbs scandium from an ion exchanger surface to obtain industrial reclaims having concentrated scandium and doesn't lead to any wastes from any possible return of the solution to the technological process after removal of impurities and scandium therefrom which makes it possible to operate in a closed cycle. For these experiments, a scandium-saturated phosphorous-containing ion exchanger was used with the following composition, in % by weight: 0.46 Sc.sub.2O.sub.3, 0.056 ZrO.sub.2, 0.53 TiO.sub.2, 3.0 Fe.sub.2O.sub.3, 0.1 Al.sub.2O.sub.3.

(30) Table 6 comprises results of experiments studying the impact of sodium carbonate concentration in a desorbing solution on desorption of scandium and impurities from a phosphorous-containing ion exchanger. The desorption process was carried out at 50° C. in a compressed layer of an ion exchanger at the solution-feed linear speed 0.3 m/h.

(31) TABLE-US-00006 TABLE 6 Concentration of Na.sub.2CO.sub.3, g/dm.sup.3 150 200 300 400 450 Degree of Sc desorption from an 35.1 45.2 77.5 93.1 96.0 ion exchanger, % Concentration of Sc.sub.2O.sub.3 87 107 186 249 275 in a pregnant solution, mg/dm.sup.3

(32) As can be seen from Table 6, when the concentration of sodium carbonate in the desorbing solution increases the degree of scandium desorption from resin increases as well, and when the scandium oxide concentration in the desorbing solution is 275 mg/dm.sup.3 this degree reaches 96% at 450 g/dm.sup.3 of Na.sub.2CO.sub.3. The upper limit of sodium carbonate concentration in the desorbing solution is defined by the solubility of sodium carbonate at the given temperature; decrease in sodium carbonate concentration below 200 g/dm.sup.3 is unpractical due to the low degree of scandium desorption from the ion exchanger and production of a scandium-depleted industrial reclaim which results in the lower quality of the concentrated scandium and higher reagent consumption needed for recycling such solutions to obtain concentrated scandium.

(33) Researches in the field of sorption leaching of scandium from red mud have defined optimal modes for the following process operations:

(34) a) Preparing a red mud pulp for sorption: initial red mud is filtered and repulped with a solution having a mixture of sodium carbonate and sodium bicarbonate where the concentration of Na.sub.2Ototal is 40-80 g/dm.sup.3 and the percentage of Na.sub.2Obicarb is from 50 to 100% of Na.sub.2Ototal; a solid/liquid ratio is 1:2,5-5, preferably 1:4; prior to sorption, a pulp is heated to 40-90° C., preferably 60-90° C.

(35) b) Sorption of scandium from a red mud pulp in a phosphorous-containing ion exchanger: a continuous countercurrent mode; sorption is carried out in the phosphorous-containing ion exchanger; the contact time of the ion exchanger with the pulp at each stage is 1-8 hours, preferably 2-3 hours; the mass ratio of pulp to ion exchanger is 20-120:1, preferably 60-100:1; the process temperature is 40-90° C., preferably 60-90° C.

(36) c) Desorption of scandium from a phosphorous-containing ion exchanger to obtain an industrial reclaim for concentrated scandium recovery: desorption is carried out with the use of a solution of Na.sub.2CO.sub.3 with a concentration of 200-450 g/dm.sup.3 in the relation to Na.sub.2CO.sub.3.

(37) d) Aeration of a solution of a mixture of sodium carbonate and sodium bicarbonate obtained after filtering a pulp of a leached red mud to restore the Na.sub.2Obicarb to Na.sub.2Ototal ratio of 50-100% and its return to the stage of leaching: aeration of a gas-air mixture containing CO.sub.2; the process temperature is 15-60° C., preferably 20-30° C.

(38) The process flow scheme for scandium recovery from red mud is shown in FIG. 1 and includes following operations: filtering the pulp of initial red mud to obtain a red mud cake; repulping red mud with a solution having a mixture of sodium carbonate and sodium bicarbonate with a concentration of Na.sub.2Ototal of 40-80 g/dm.sup.3 to obtain the red mud pulp ready for leaching: sorption leaching of scandium from red mud in a phosphorous-containing ion exchanger; separating the ion exchanger and the depleted in scandium pulp of leached red mud; desorbing of scandium from the phosphorous-containing ion exchanger with the solution of sodium hydrocarbonate to obtain an industrial reclaim of scandium which is transferred for recovering of the concentrated scandium and a reclaimed ion exchanger transferred for sorbing scandium from a new portion of the red mud pulp; filtering the depleted in scandium pulp to obtain a cake of leached red mud and a sorption mother solution; aeration of the sorption mother solution by carbon dioxide to restore the ratio of Na.sub.2Ocarb:Na.sub.2Obicarb and transfer thereof for repulping and sorption leaching of a new portion of red mud.

EXAMPLES

(39) The implementation of the claimed method and its advantages over the prototype are confirmed by the following examples.

Example 1

(40) In a reactor of 1 dm.sup.3, 300 g of wetted red mud (humidity is 32%) obtained at the stage of filtering the pulp of initial red mud of JSC “SUAL” operating as a subsidiary of “UAZ-SUAL”were repulped by a solution having a mixture of sodium carbonate and sodium bicarbonate with a concentration of Na.sub.2Ototal of 65 g/dm.sup.3 (the concentration of Na.sub.2Obicarb is 108.4 g/dm.sup.3, the concentration of Na.sub.2Ocarb is 42.8 g/dm.sup.3) to obtain the solid/liquid ratio of 1:4. Then, during stirring the pulp a swollen phosphorous-containing ion exchanger LewatitTP-260Monoplus in the form of Na.sup.+ was added to the pulp in the amount of 10 cm.sup.3, after that the pulp was heated to the 80° C.

(41) The solid phase of initial red mud has the following chemical composition, in % by weight: 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.

(42) The red mud pulp having the ion exchanger was held in the reactor with stirring during 120 minutes after which the ion exchanger was separated from the pulp on a sieve, washed and analyzed. The pulp was filtered through two layers of Blue ribbon paper, the deposit was washed on a filter with cold distilled water.

(43) Table 7 represents the chemical composition of the washed phosphorous-containing ion exchanger rich in scandium.

(44) TABLE-US-00007 TABLE 7 Specific Composition of saturated ion exchanger, % volume, Element name Sc.sub.2O.sub.3 Al.sub.2O.sub.3 Fe.sub.2O.sub.3 TiO.sub.2 ZrO.sub.2 cm.sup.3/g Percentage, % 0.36 0.18 2.0 3.4 0.015 2.97

(45) The degree of scandium recovery at the stage of sorption leaching was 44%.

(46) The sorption mother solution obtained after the leached red mud pulp had been filtered was used to prepare a solution for leaching a new portion of red mud.

Example 2

(47) Washed ion exchanger LewatitTP-260 in the amount of 50 cm.sup.3 having the chemical composition shown in Table 8, was transferred for desorbing. For this, from bottom to top the ion exchanger was loaded into an ion exchange column into which a desorbing solution was fed with a concentration of Na.sub.2CO.sub.3 of 350 g/dm.sup.3, at the same time the temperature in the column was maintained at 50° C.

(48) Upon completion of the desorption process, the ion exchanger was unloaded from the column and washed out from desorbing solution residues, after which it was transferred to the second stage of the sorption leaching of scandium from the red mud pulp.

(49) The industrial reclaim comprising 220 mg/dm.sup.3 of scandium oxide was transferred for further processing to recover concentrated scandium.

(50) In such a way, the suggested method for recovering scandium from red mud allows to improve the degree of scandium recovery up to 47% by applying the sorption leaching of scandium in a continuous countercurrent mode at an elevated temperature and to improve the concentrated scandium quality thanks to optimal conditions of scandium desorption from a phosphorous-containing ion exchanger to obtain a rich in scandium industrial reclaim, in addition without involving of additional technical complicated operations.