Method of separating rare earth elements from rare earth polishing powder wastes

Abstract

The present invention relates to a method of separating rare earth elements from rare earth polishing powder waste by a hydrometallurgical process, the method comprising the steps of: synthesizing sodium rare-earth double sulfates by adding sulfuric acid and sodium hydroxide to the rare earth polishing powder waste; converting the sodium rare-earth double sulfates into rare earth hydroxides; and separating cerium (Ce), lanthanum (La), praseodymium (Pr), and neodymium (Nd) from the rare earth hydroxides by adding hydrochloric acid and sulfuric acid. The present invention makes it possible to recover 99% or more of rare earth elements, including cerium (Ce), lanthanum (La), praseodymium (Pr), and neodymium (Nd) from rare earth polishing powder waste, and enables the recovered rare earth elements to be recycled, thereby achieving great industrial economic benefits.

Claims

1. A method of separating rare earth elements from polishing powder waste comprising the rare earth elements, said method comprising: (a) synthesizing a solution of sodium rare earth sulfate, NaRe(SO.sub.4).sub.2.xH.sub.2O wherein Re represents rare earth elements and x is a rational number from 0 to 2, by adding the polishing powder waste to an aqueous solution of Na.sub.2SO.sub.4 and H.sub.2SO.sub.4; (b) removing a leachate A containing liquefied impurities from the synthesized sodium rare earth sulfate solution of step (a) by solid-liquid separation, and then converting the sodium rare earth sulfate (NaRe(SO.sub.4).sub.2.xH.sub.2O) contained in a remaining solid material into trivalent rare earth hydroxides (Re(OH).sub.3) and obtaining a solution containing solids Ce(OH).sub.3, La(OH).sub.3, Nd(OH).sub.3 and Pr(OH).sub.3 by adding the remaining solid material to an aqueous solution of NaOH; (c) oxidizing trivalent cerium hydroxide (Ce(OH).sub.3) into tetravalent cerium hydroxide (Ce(OH).sub.4) and obtaining a solution containing solids Ce(OH).sub.4, La(OH).sub.3, Nd(OH).sub.3 and Pr(OH).sub.3 by injecting O.sub.2 containing air into the solution containing solids Ce(OH).sub.3, La(OH).sub.3, Nd(OH).sub.3 and Pr(OH).sub.3 of step (b); (d) dissolving trivalent lanthanum hydroxide (La(OH).sub.3), praseodymium hydroxide (Pr(OH).sub.3), and neodymium hydroxide (Nd(OH).sub.3) by adding HCl to the solution containing solids Ce(OH).sub.4, La(OH).sub.3, Nd(OH).sub.3 and Pr(OH).sub.3 resulting from step (c) and then separating by solid-liquid separation of the solution, obtaining Ce(OH).sub.4 solid and a leachate B wherein lanthanum (La), praseodymium (Pr) and neodymium (Nd) are dissolved, and subjecting separation of lanthanum (La), praseodymium (Pr) and neodymium (Nd); and (e) adding H.sub.2SO.sub.4 to a solid material Ce(OH).sub.4 remaining after the separating in step (d) to dissolve the tetravalent cerium hydroxide (Ce(OH).sub.4), separating by solid-liquid separation to remove solid impurities and obtain a leachate C wherein cerium (Ce) is dissolved, and subjecting separation of cerium (Ce) from the leachate C.

2. The method of claim 1, wherein step (a) is performed at a temperature of at least 95 C.

3. The method of claim 1, wherein concentrations of Na.sub.2SO.sub.4 and H.sub.2SO.sub.4 in step (a) are 0.4 to 2.5 mol/dm.sup.3 and 6 to 14 mol/dm.sup.3, respectively.

4. The method of claim 1, wherein step (d) is performed at a pH of 1.5 to 5.5.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

(2) FIG. 1 is a schematic diagram showing a method of separating rare earth elements from rare earth polishing powder waste according to the present invention.

(3) FIG. 2 is a graph showing the effect of Na.sub.2SO.sub.4 concentration on the synthesis of NaCe(SO.sub.4).sub.2.H.sub.2O from CeO.sub.2 in a Na.sub.2SO.sub.4H.sub.2SO.sub.4H.sub.2O solution in the separation method of the present invention.

(4) FIG. 3 is a graph showing the effect of sulfuric acid concentration on the synthesis of NaCe(SO.sub.4).sub.2.H.sub.2O from CeO.sub.2 in in a Na.sub.2SO.sub.4H.sub.2SO.sub.4H.sub.2O solution in the separation method of the present invention.

(5) FIG. 4 is a graph showing the results of XRD analysis of precipitates, performed to examine the effect of sulfuric acid concentration on the synthesis of NaCe(SO.sub.4).sub.2.H.sub.2O from CeO.sub.2 in in a Na.sub.2SO.sub.4H.sub.2SO.sub.4H.sub.2O solution in the separation method of the present invention.

(6) FIG. 5 is a graph showing the effect of reaction temperature on the synthesis of NaCe(SO.sub.4).sub.2.H.sub.2O from CeO.sub.2 in a Na.sub.2SO.sub.4H.sub.2SO.sub.4H.sub.2O solution in the separation method of the present invention.

(7) FIG. 6 is a graph showing the synthetic behavior of NaCe(SO.sub.4).sub.2.H.sub.2O from La.sub.2O.sub.3, Pr.sub.2O.sub.3 and Nd.sub.2O.sub.3 in a Na.sub.2SO.sub.4H.sub.2SO.sub.4H.sub.2O solution at varying reaction times in the separation method of the present invention.

(8) FIG. 7 is a graph showing the yield (Y.sub.c) and purity (P.sub.c) of Ce(OH).sub.4 in HClH.sub.2O solutions containing Ce(OH).sub.4, La(OH).sub.3, Pr(OH).sub.3 and Nd(OH).sub.3 at varying pHs in the separation method of the present invention.

(9) FIG. 8 is a graph showing the changes in percent separation (S.sub.c) of Ce, La, Pr and Nd from (a) leachate B and (b) leachate C in step 4 of the separation method according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

(10) Unless defined otherwise, all the technical and scientific terms used herein have the same meaning as those generally understood by one of ordinary skill in the art to which the invention pertains. Generally, the nomenclature used herein and the experiment methods, which will be described below, are those well-known and commonly employed in the art. Description of the same elements that not important will be omitted to avoid redundancy.

(11) In the present invention, it was found that rare earth elements can be effectively separated from rare earth polishing powder waste by adding sulfuric acid and sodium hydroxide to the rare earth polishing powder waste to synthesize sodium rare-earth double sulfates, converting the sodium rare-earth double sulfates into rare earth hydroxides, and separating cerium (Ce), lanthanum (La), praseodymium (Pr), and neodymium (Nd) from the rare earth hydroxides by addition of hydrochloric acid and sulfuric acid.

(12) Therefore, in one aspect, the present invention is directed to a method of separating rare earth elements from rare earth polishing powder waste, the method comprising the steps of:

(13) (a) synthesizing a solution of sodium rare earth double sulfates (NaRe(SO.sub.4).sub.2.xH.sub.2O, wherein Re represents rare earth elements, and x is a rational number of 0 to 2) by adding the polishing powder waste to an aqueous solution of Na.sub.2SO.sub.4 and H.sub.2SO.sub.4;

(14) (b) removing an leachate from a synthesized solution of step (a) by solid-liquid separation, and then converting the sodium rare earth double sulfates (NaRe(SO.sub.4).sub.2.xH.sub.2O) into trivalent rare earth hydroxides (Re(OH).sub.3) by adding a remaining solid material to an aqueous solution of NaOH;

(15) (c) converting trivalent cerium hydroxide (Ce(OH).sub.3) into tetravalent cerium hydroxide (Ce(OH).sub.4) by injecting air into a solution containing the trivalent rare earth hydroxides (Re(OH).sub.3) of step (b);

(16) (d) dissolving trivalent lanthanum hydroxide (La(OH).sub.3), praseodymium hydroxide (Pr(OH).sub.3), and neodymium hydroxide (Nd(OH).sub.3) by adding HCl to a solution resulting from step (c) and then separating lanthanum (La), praseodymium (Pr) and neodymium (Nd) by solid-liquid separation; and

(17) (e) obtaining a leachate containing cerium dissolved therein by adding H.sub.2SO.sub.4 to a solid material remaining after the separating in step (d) to dissolve the tetravalent cerium hydroxide (Ce(OH).sub.4), and then separating cerium (Ce) from the leachate by solid-liquid separation.

(18) The present invention can provide an effective wet process for separating rare earth elements from polishing powder waste containing rare earth oxides such as cerium oxide (CeO.sub.2), lanthanum oxide (La.sub.2O.sub.3), praseodymium oxide (Pr.sub.2O.sub.3), and neodymium oxide (Nd.sub.2O.sub.3). The wet process consists of five steps, and each step will now be described in detail.

(19) (a) Step of Synthesizing Sodium Rare-Earth Double Sulfates

(20) Rare earth polishing powder waste (REPPW) contains CeO.sub.2, La.sub.2O.sub.3, Pr.sub.2O.sub.3, Nd.sub.2O.sub.3, CaO, Al.sub.2O.sub.3, SiO.sub.2, MgO and other impurities, which generally have a particle size of less than 0.5 m.

(21) In the first step of the method of separating rare earth elements from rare earth polishing powder waste according to the present invention, the polishing powder waste is added to an aqueous solution of Na.sub.2SO.sub.4 and H.sub.2SO.sub.4 to synthesize rare earths contained in the polishing powder waste into sodium rare-earth double sulfates (NaRe(SO.sub.4).sub.2.xH.sub.2O, wherein Re represents rare earth elements, and x is a rational number of 0 to 2).

(22) Step (a) may be performed at a temperature of at least 95 C., preferably 95 C. to 200 C., more preferably 95 C. to 125 C.

(23) In the above temperature range, the dissolution rate of CeO.sub.2 and the synthesis rate of NaCe(SO.sub.4).sub.2.H.sub.2O increase, and thus the yield of NaCe(SO.sub.4).sub.2.xH.sub.2O increases as the reaction temperature increases.

(24) The concentrations of Na.sub.2SO.sub.4 and H.sub.2SO.sub.4 in step (a) are 0.4 to 2.5 mol/dm.sup.3 and 6 to 14 mol/dm.sup.3, respectively, at which the synthesis smoothly occurs. Preferably, the concentrations of Na.sub.2SO.sub.4 and H.sub.2SO.sub.4 are 0.4 mol/dm.sup.3 and 8 mol/dm.sup.3, respectively, which show the highest synthesis efficiency. If the concentration of Na.sub.2SO.sub.4 is higher than 2.5 mol/dm.sup.3, the concentration of H.sup.+ decreases and the dissolution rate of CeO.sub.2 decreases, and thus the yield decreases. If the concentration of H.sub.2SO.sub.4 is lower than 6 mol/dm.sup.3, the dissolution rate of CeO.sub.2 decreases rapidly, and if the concentration of H.sub.2SO.sub.4 is higher than 14 mol/dm.sup.3, the precipitation of Ce(SO.sub.4).sub.2 increases rapidly, thus reducing the yield.

(25) (b) Step of Synthesizing Rare Earth Hydroxides (Re(OH).sub.3

(26) In the second step of the method of separating rare earth elements from rare earth polishing powder waste according to the present invention, the leachate of step (a), which contains primary impurities dissolved therein, is subjected to solid-liquid separation to remove the liquid, and then the remaining solid material is added to an aqueous solution of NaOH to convert the sodium rare-earth double sulfates (NaRe(SO.sub.4).sub.2.xH.sub.2O) contained in the solid material into trivalent rare earth hydroxides (Re(OH).sub.3).

(27) (c) Step of Synthesizing Tetravalent Cerium Hydroxide (Ce(OH).sub.4)

(28) In third step of the method of separating rare earth elements from rare earth polishing powder waste according to the present invention, O.sub.2-containing air is injected into the solid/liquid material resulting from step (b) to convert the trivalent cerium hydroxide (Ce(OH).sub.3) into tetravalent cerium hydroxide (Ce(OH).sub.4).

(29) (d) Step of Separating of Lanthanum (La), Praseodymium (Pr) and Neodymium (Nd)

(30) In fourth step of the method of separating rare earth elements from rare earth polishing powder waste according to the present invention, HCl is added to the solid/liquid material resulting from step (c) to dissolve trivalent lanthanum hydroxide (La(OH).sub.3), praseodymium hydroxide (Pr(OH).sub.3), and neodymium hydroxide (Nd(OH).sub.3), and the leachate containing lanthanum (La), praseodymium (Pr) and neodymium (Nd) dissolved therein is subjected to solid-liquid separation to separate lanthanum (La), praseodymium (Pr) and neodymium (Nd).

(31) Step (d) may be performed at a pH of 1.5 to 5.5. Preferably, step (d) is performed at a pH of 2.5 to 3.5, which shows high separation efficiency. Particularly, step (d) is performed at a pH of 2.83, which shows the highest separation efficiency. At a pH lower than 1.5, the percent purity of the remaining Ce(OH).sub.4 exceeds 99% as pH lowers, whereas the yield decreases rapidly. At a pH higher than 5.5, there is a problem in that La(OH).sub.3, Pr(OH).sub.3, and Nd(OH).sub.3, which are undesirable by-products, interfere with obtaining high-purity Ce(OH).sub.4.

(32) (e) Step of Separating Cerium (Ce)

(33) In the last step of the method of separating rare earth elements from rare earth polishing powder waste according to the present invention, the solid material obtained by solid-liquid separation in step (d) is added to an aqueous solution of H.sub.2SO.sub.4 to dissolve the tetravalent cerium hydroxide (Ce(OH).sub.4), and the leachate containing cerium dissolved therein is subjected to solid-liquid separation to separate cerium from solid secondary impurities.

EXAMPLES

(34) Hereinafter, the present invention will be described in further detail with reference to examples. It will be obvious to a person having ordinary skill in the art that these examples are for illustrative purposes only and are not to be construed to limit the scope of the present invention.

(35) CeO.sub.2, Ce(OH).sub.4, La.sub.2O.sub.3, La(OH).sub.3, Pr.sub.2O.sub.3, Pr(OH).sub.3, Nd.sub.2O.sub.3, Nd(OH).sub.3, CaO, Al.sub.2O.sub.3, SiO.sub.2, and MgO that were used in Examples had a metal purity of >99.9% and a particle size of <5 m. HCl, Na.sub.2SO.sub.4, NaOH, and H.sub.2SO.sub.4 that were used in the present invention were products (purity: 99.0%) manufactured by Wako Pure Chemical Industries, Ltd.

Example 1: Synthesis of NaCe(SO.SUB.4.).SUB.2..xH.SUB.2.O from CeO.SUB.2

(36) All synthetic experiments were performed in a batch-type glass reactor at atmospheric pressure. 20 mmol of CeO.sub.2 powder was added to 100 mL of a sulfuric acid solution (concentration: 6-14 mol/dm.sup.3) containing 0.4-2.5 mol/dm.sup.3 of Na.sub.2SO.sub.4. The solution was heated, allowed to stand at temperatures of 95 C., 105 C., 115 C. and 125 C., and stirred at 650 rpm by use of a magnet stirrer. The experiment was repeated 5 times to obtain each data, and the error range with respect to the standard deviation of the data was measured.

(37) In order to measure the weights of CeO.sub.2 residue and synthesized NaCe(SO.sub.4).sub.2.xH.sub.2O, a solution sampled at a suitable time point of reaction was filtered using a pressure filtration unit on a membrane having a pore size of 0.5 m, and then washed three times with ethanol to remove sulfuric acid. Then, the sample was dried at a temperature of 100 C. for 24 hours, and then the sample weight (a) was measured using a balance. Thereafter, the sample was added to a sulfuric acid solution (3-4 mol/dm.sup.3) at 90 C. to dissolve a cerium (IV) sulfate (Ce(SO.sub.4).sub.2) precipitate. CeO.sub.2-containing residue and synthesized NaCe(SO.sub.4).sub.2.xH.sub.2O were filtered from the solution and washed with ethanol. After drying at a temperature of 100 C. for 24 hours, the weight (b) of the precipitate composed of CeO.sub.2 residue and synthesized NaCe(SO.sub.4).sub.2.xH.sub.2O was measured using a balance.

(38) The precipitate was added to 1000 mL of an alkaline solution containing 0.7 mol NaOH to concert NaCe(SO.sub.4).sub.2.xH.sub.2O into Ce(OH).sub.3. Then, the solution was continuously added to 1000 mL of an acid solution containing 1 mol HCl to dissolve Ce(OH).sub.3 and was filtered and dried at a temperature of 100 C. for 24 hours, and then the weight (c) of CeO.sub.2 residue was measured using a balance. The weight difference between (a) and (b) and the weight difference between (b) and (c) indicate the weight of precipitated Ce(SO.sub.4).sub.2 and the weight of synthesized NaCe(SO.sub.4).sub.2.xH.sub.2O, respectively: The weight of Ce(SO.sub.4).sub.2 was used to describe the effect of H.sub.2SO.sub.4 concentration on the synthesis of NaCe(SO.sub.4).sub.2.xH.sub.2O.

Example 2: Purification of Ce(OH).SUB.4

(39) The initial concentration of each of Ce(OH).sub.4, La(OH).sub.3, Pr(OH).sub.3, and Nd(OH).sub.3 was set at a constant level of 0.1 mol/dm.sup.3, and 100 mL of an aqueous solution having such initial concentrations was prepared. At room temperature (25 C.), the aqueous solution was adjusted to a pH of 1.5-5.5 by addition of a dilute HCl solution, and stirred with a magnet stirrer at 650 rpm. After stirring, the residue was separated using filter paper. After filtration, the filtrate was analyzed by inductively coupled plasma atomic emission spectrometry (OPTIMA 5300DV, Perkin Elmer) to measure the concentrations of Ce, La, Pr and Nd. In addition, the residue was dissolved in an acid solution of HCl, and then analyzed by inductively coupled plasma atomic emission spectrometry to determine the chemical composition of the residue.

Example 3: Five-Step Process for Separating Rare Earth Elements from Rare Earth Polishing Powder Wastes (REPPWs)

(40) The chemical compositions of various kinds of polishing powder wastes are as shown in Table 1 below.

(41) TABLE-US-00001 TABLE 1 Elements Ce La Pr Nd Ca Al Si Mg 33-94 0-33 0-5 0-14 5 1.5 1.5 Tao X et al., J. Rare Earths, 2009, 27: 1096 8.34 24.02 Yoon et al., J. Kor. Inst. Res. Rec. 2005, 14: 3 22.1 17.8 2.3 5.1 24.8 Kato et al., Ind. Eng. Res., 2000, 39: 943 51 32 2 5.3 5.3 1.5 Chong et al., VATTEN, 2009, 65: 193 39.40 20.00 0.22 0.22 4.30 0.11 Kim et al., J. Rare Earths, 2011, 29: 1075 22.49 17.72 1.41 0.21 9.27 9.27 11.53 3.65 Wang et al., J. Ther. Anal. Calorim., 2011, DOI 10.1007/s10973-011-1500-2

(42) Since REPPWs contain not only rare earth oxides (CeO.sub.2, La, Pr, and Nd.sub.2O.sub.3), but also impurities such as non-rare earth elements (Ca, Al, Si, and Mg), a process of efficiently separating rare earths from REPPWs is needed. Thus, based on the result data shown in FIGS. 2, 3, 5, 6, and 7, an effective process was designed which is a five-step process including: synthesis of NaRe(SO.sub.4).sub.2.xH.sub.2O from rare earth oxides in a Na.sub.2SO.sub.4.H.sub.2SO.sub.4.H.sub.2O solution (step 1); conversion into Re(OH).sub.3 (step 2); and separation of REEs by oxidation and acid leaching (steps 3, 4, and 5).

(43) FIG. 1 schematically shows the five-step process, and each step of the process will not be described in detail.

(44) Step 1: Synthesis of NaRe(SO.sub.4).sub.2.xH.sub.2O from Rare Earth Oxides

(45) In the case of CeO.sub.2, the reaction rate of reaction (1) is very slow, and thus significantly affects the synthesis rate of NaRe(SO.sub.4).sub.2.xH.sub.2O. Such results are as reported by Um and Hirato (Um N et al., Mater Trans., 2012b, 53: 1986), and indicate that a period of time of 48 hours or more was taken to completely dissolve 0.02 mol of CeO.sub.2 powder having an average particle size of 2.5 m in 100 ml of sulfuric acid (8 mol/dm.sup.3) at 25 C. In addition, unlike other REEs present in a trivalent state, cerium is present in a tetravalent state and changes into a trivalent state during dissolution in acid, and thus undergoes a complex process during synthesis. According to Um and Hirato (Um N et al., Mater. Trans., 2012a, 53: 1992), a process of synthesizing NaRe(SO.sub.4).sub.2.xH.sub.2O from CeO.sub.2 in a Na.sub.2SO.sub.3.H.sub.2SO.sub.4.H.sub.2O solution is expected to include the following chemical reactions (1), (2) and (3). Dissolving CeO.sub.2 in a sulfuric acid solution can be represented by the following reaction equation:
CeO.sub.2+4H.sup.+.fwdarw.Ce.sup.4++2H.sub.2O(1).

(46) Thereafter, cerium (IV) cation is reduced into cerium (III) cation:
Ce.sup.4++H.sub.2O.fwdarw.Ce.sup.3++H.sup.++O.sub.2(2).

(47) A cerium precipitate as NaRe(SO.sub.4).sub.2.xH.sub.2O in the Na.sub.2SO.sub.4.H.sub.2SO.sub.4.H.sub.2O solution undergoes the following reaction:
Ce.sup.3++2Na.sub.2SO.sub.4+xH.sub.2O.fwdarw.NaCeSO.sub.4).sub.2.xH.sub.2O+3Na.sup.+(3).

(48) However, since Ce.sup.4+ dissolved in high-concentration sulfuric acid is easily oversaturated, does not undergo reaction (2), and is formed directly into cerium (IV) sulfate (Ce(SO.sub.4).sub.2) as shown in the following reaction (4), it does not change into Ce.sup.3+, and thus interferes with the synthesis of NaCe(SO.sub.4).sub.2.xH.sub.2O (Um N et al., Mater Trans., 2012b, 53: 1986):
Ce.sup.4++2SO.sub.4.sup.2.fwdarw.Ce(SO.sub.4).sub.2(4).

(49) This fact is shown in FIG. 3.

(50) The yield of NaCe(SO.sub.4).sub.2.xH.sub.2O synthesized from CeO.sub.2 in a Na.sub.2SO.sub.4.H.sub.2SO.sub.4.H.sub.2O solution was evaluated as a function of Na.sub.2SO.sub.4 concentration, sulfuric acid concentration and reaction temperature, and the results are shown in FIGS. 2, 3 and 5, respectively. First, the effect of Na.sub.2SO.sub.4 concentration was tested by changing the concentration of Na.sub.2SO.sub.4 in the range from 0.4 mol/dm.sup.3 to 2.5 mol/dm.sup.3. In these tests, initial CeO.sub.2 concentration, reaction temperature and initial sulfuric acid concentration were set at 0.2 mol/dm.sup.3, 125 C. and 8 mol/dm.sup.3, respectively, and a reaction was performed for 36 hours. As shown in FIG. 2, the dissolution rate of CeO.sub.2 and the yield of NaCe(SO.sub.4).sub.2.xH.sub.2O decreased as the concentration of Na.sub.2SO.sub.4 increased. The change in the Na.sub.2SO.sub.4 concentration affected the synthesis of NaCe(SO.sub.4).sub.2.xH.sub.2O. According to reaction equation (3), an initial Na.sub.2SO.sub.4 concentration of 2 mol was required per mol of cerium, and thus a Na.sub.2SO.sub.4 concentration of only 0.4 mol/dm.sup.3 was sufficient for an initial CeO.sub.2 concentration of 0.2 mol/dm.sup.3. However, an excessive amount of Na.sub.2SO.sub.4 interferes with the dissolution of CeO.sub.2. For example, as the concentration of Na.sub.2SO.sub.4 increases, the frequency of the reaction Na.sub.2SO.sub.4.fwdarw.2Na.sup.++SO.sub.4.sup.2 increases to increase the concentration of SO.sub.4.sup.2, and the H.sup.+ ion and SO.sub.4.sup.2 ion required for the dissolution of CeO.sub.2 cause the reaction H.sup.++SO.sub.4.sup.2.fwdarw.HSO.sub.4 to reduce H.sup.+. For this reason, the dissolution rate of CeO.sub.2 decreases, and thus the yield of NaRe(SO.sub.4).sub.2.xH.sub.2O synthesized decreases (Um N et al., Mater. Trans., 2012a, 53: 1992). Meanwhile, the effect of sulfuric acid concentration is shown in FIG. 3. The effect of sulfuric acid concentration was tested by changing the concentration of sulfuric acid in the range from 6 mol/dm.sup.3 to 14 mol/dm.sup.3. In these tests, initial CeO.sub.2 concentration, reaction temperature and initial Na.sub.2SO.sub.4 concentration were set at 0.2 mol/dm.sup.3, 125 C. and 0.4 mol/dm.sup.3, respectively, and a reaction was performed for 36 hours. As shown in FIG. 3, the dissolution rate of CeO.sub.2 increased as the concentration of sulfuric acid increased, but the yield curve of NaCe(SO.sub.4).sub.2.xH.sub.2O showed a -shape at a sulfuric acid concentration of 6 to 14 mol/dm.sup.3. The increase in the yield of NaCe(SO.sub.4).sub.2.xH.sub.2O with the increase in the sulfuric acid concentration can be explained by the well-known common ion effect. As mentioned above, the increase in the sulfuric acid concentration with the increase in the SO.sub.4.sup.2 ion concentration induces an increase in a precipitate of cerium (IV) phosphate (Ce(SO.sub.4).sub.2) as shown in reaction equation (4), and thus reduces Ce.sup.4+ into Ce.sup.3+ (trivalent) which interferes with the synthesis of NaCe(SO.sub.4).sub.2.xH.sub.2O. At a sulfuric acid concentration of 8 mol/dm.sup.3, NaCe(SO.sub.4).sub.2.xH.sub.2O is obtained in a high yield of 190 mmol or more, but at a sulfuric acid concentration of 14 mol/dm.sup.3, the yield of NaCe(SO.sub.4).sub.2.xH.sub.2O decreases rapidly to 20 mmol at the same reaction time. Furthermore, at a sulfuric acid concentration lower than 8 mol/dm.sup.3, the dissolution rate of CeO.sub.2 decreases, resulting in a decrease in the yield of NaCe(SO.sub.4).sub.2.xH.sub.2O. FIG. 4 shows XRD patterns the yield of Na.sub.2SO.sub.4H.sub.2SO.sub.4H.sub.2O, precipitated Ce(SO.sub.4).sub.2, and CeO.sub.2 residue remaining after the reaction, at varying sulfuric acid concentrations. The results in FIG. 4 support the results shown in FIG. 3. Next, in order to examine the effect of reaction temperature, tests were performed by changing the reaction temperature in the range from 95 C. to 125 C. In these tests, initial CeO.sub.2 concentration, initial Na.sub.2SO.sub.4 concentration and sulfuric acid concentration were set at 0.2 mol/dm.sup.3, 0.4 mol/dm.sup.3 and 8 mol/dm.sup.3, respectively, and a reaction was performed for 36 hours. As shown in FIG. 5, the dissolution rate of CeO.sub.2 and the synthesis rate of NaCe(SO.sub.4).sub.2.H.sub.2O all increased as the reaction temperature increased.

(51) La.sub.2O.sub.3, Pr.sub.2O.sub.3, and Nd.sub.2O.sub.3 show very high dissolution rates in an acid solution, and thus are easily converted into NaRe(SO.sub.4).sub.2.xH.sub.2O. In order to confirm this fact, examination was performed of the rates at which La.sub.2O.sub.3, Pr.sub.2O.sub.3, and Nd.sub.2O.sub.3, each having a particle size of <5 m, was converted into NaRe(SO.sub.4).sub.2.xH.sub.2O in a Na.sub.2SO.sub.4H.sub.2SO.sub.4H.sub.2O solution. Initial sulfuric acid concentration, Na.sub.2SO.sub.4 concentration and reaction temperature were set at 8 mol/dm.sup.3, 0.4 mol/dm.sup.3 and 125 C., respectively (see FIGS. 2, 3, and 5). In addition, the initial concentrations of La.sub.2O.sub.3, Pr.sub.2O.sub.3, and Nd.sub.2O.sub.3 were 0.2 mol/dm.sup.3. The results are shown in FIG. 6. As a result, it was shown that La, Pr, and Nd oxides were dissolved rapidly, and the dissolved La, Pr or Nd ion was synthesized into NaLa(SO.sub.4).sub.2.xH.sub.2O within about 2 minutes. The following reaction equations (5) to (10) are reaction equations for describing a process of converting La, Pr, and Nd oxides into NaRe(SO.sub.4).sub.2.xH.sub.2O:
La.sub.2O.sub.3+6H.sup.+.fwdarw.La.sup.3++3H.sub.2O(5)
Nd.sub.2O.sub.3+6H.sup.+.fwdarw.Nd.sup.3++3H.sub.2O(6)
Pr.sub.2O.sub.3+6H.sup.+.fwdarw.Pr.sup.3++3H.sub.2O(7)
La.sup.3++2Na.sub.2SO.sub.4+xH.sub.2O.fwdarw.NaLa(SO.sub.4).sub.2.xH.sub.2O+3Na.sup.+(8)
Nd.sup.3++2Na.sub.2SO.sub.4+xH.sub.2O.fwdarw.NaNd(SO.sub.4).sub.2.xH.sub.2O+3Na.sup.+(9)
Pr.sup.3++2Na.sub.2SO.sub.4+xH.sub.2O.fwdarw.NaPr(SO.sub.4).sub.2.xH.sub.2O+3Na.sup.+(10).

(52) Steps 2, 3, 4, and 5: Re(OH).sub.2 Conversion, Oxidation and Acid Leaching

(53) In order to effectively separate and recover cerium (Ce), which is present as a main phase and the highest content component in REPPWs, and other rare earth elements (lanthanum (La), praseodymium (Pr) and neodymium (Nd)), from REPPWs, it is highly effective to apply the following four steps, including Re(OH).sub.2 conversion, oxidation and acid leaching, after the step of synthesizing NaRe(SO.sub.4).sub.2.xH.sub.2O:

(54) Conversion of NaRe(SO.sub.4).sub.2.xH.sub.2O into Re(OH).sub.3 by use of NaOH (step 2);

(55) Oxidation of Ce(OH).sub.3 into Ce(OH).sub.4 by injection of O.sub.2-containing air (step 3);

(56) Acid leaching with HCl and sulfuric acid (steps 4 and 5).

(57) The following reaction equations 11 and 12 are related to steps 2 and 3:
NaRe(SO.sub.4).sub.2.xH.sub.2O+3NaOH.fwdarw.Re(OH).sub.3+2Na.sub.2SO.sub.4+xH.sub.2O(11)
(ReCe, La, Pr and Nd)
2Ce(OH).sub.3+O.sub.2+H.sub.2O.fwdarw.2Ce(OH).sub.4(12).

(58) In step 3, injection of O.sub.2-containing air into a mixture of Ce, La, Pr, and Nd(OH).sub.3 makes it possible to recover high-purity Ce from La, Pr and Nd. This is because Ce(OH).sub.4 which is converted from Ce(OH).sub.3 is less soluble than La, Pr, and Nd(OH).sub.3 in a dilute acid solution. In order to evaluate the effect of pH on the separation of Ce(OH).sub.4 from Ce, La, Pr, and Nd(OH).sub.3 in a dilute acid solution in step 4 (acid leaching with HCl) after the oxidation reactions in steps 2 and 3, the yield and percent yield of residual Ce(OH).sub.4 in a HClH.sub.2O solution containing Ce(OH).sub.4, La, Pr, and Nd(OH).sub.3 at varying pHs were examined, and the results are shown in FIG. 7. The yield and percent purity (Pc) of residual Ce(OH).sub.4 were calculated using the following equations:
Y.sub.c(Y.sub.t/Y.sub.0)100(13)
P.sub.c(Y.sub.t/P.sub.t)100(14)

(59) wherein Y.sub.0 represents the initial concentration (mol) of Ce(OH).sub.4, and Y.sub.t and P.sub.t represent the concentration (mol) of Ce(OH).sub.4 remaining after stirring and the total concentration (mol) of residual Ce(OH).sub.4, La(OH).sub.3, Pr(OH).sub.3, and Nd(OH).sub.3, respectively. As shown in FIG. 7, at a pH between 1.5 and 2.5, the percent purity of residual Ce(OH).sub.4 exceeded 99%, but at a pH of 1.5, the yield decreased rapidly below 80%. In addition, at a pH higher than 3.5, La(OH).sub.3, Pr(OH).sub.3, and Nd(OH).sub.3 remained as undesirable by-products that interfered with obtaining Ce(OH).sub.4 with high purity.

Example 4: Experiment for Separating Rare Earth Elements from Simulated Polishing Powder Waste to Confirm Applicability of Actually Devised Five-Step Process: Simulated Polishing Powder Waste Composed of a Mixture of CeO.SUB.2., La.SUB.2.O.SUB.3., Pr.SUB.2.O.SUB.3., Nd.SUB.2.O.SUB.3., CaO, Al.SUB.2.O.SUB.3 .SiO.SUB.2 .SiO.SUB.2., and MgO Having a Particle Size of <0.5 m

(60) According to Kim et al. (Kim et al., J. Rare Earths, 2011, 29: 1075) and Hoshino et al. (Hoshino et al., J. Non-Cryst. Solids, 2001, 283: 129), it was reported that polishing powder waste has a particle size distribution (D.sub.50/m) of 1.689 and all particles in the polishing powder waste are fine powder having a particle size of <5 m. Thus, in this experiment, simulated polishing powder waste was prepared by mixing CeO.sub.2, La.sub.2O.sub.3, Pr.sub.2O.sub.3, Nd.sub.2O.sub.3, CaO, Al.sub.2O.sub.3, SiO.sub.2, SiO.sub.2, and MgO, each having a particle size of <5 m. Regarding the mixing ratio, 23.20 g of CeO.sub.2 (the initial amount of CeO.sub.2 relative to the initial amount of the mixture: 46.4 wt %, Ce 18.89 g), 11.50 g of La.sub.2O.sub.3 (23.0 wt %, La 9.81 g), 1.50 g of Pr.sub.2O.sub.3 (3.0 wt %, Pr 1.28 g), 2.60 g of Nd.sub.2O.sub.3 (5.2 wt %, Nd 2.23 g), 2.80 g of CaO (5.6 wt %, Ca 2.00 g), 5.70 g of Al.sub.2O.sub.3 (11.4 wt %, Al 3.02 g), 2.10 g of SiO.sub.2 (4.2 wt %, Si 0.98 g) and 0.60 g of MaO (1.2 wt %, Mg 0.36 g) were used to make a total mass of 50 g. The mixing ratio was applied to a chemical composition similar to the data shown in Table 1 above. On the simulated polishing powder waste, an experiment was performed as shown in FIG. 1.

(61) To obtain the yield of NaCe(SO.sub.4).sub.2.H.sub.2O synthesized, the conditions of step 1 were set as follows: the initial amount of the mixture in a Na.sub.2SO.sub.4H.sub.2SO.sub.4H.sub.2O solution according to FIGS. 2, 3, and 5: 50 g/dm.sup.3; the concentration of Na.sub.2SO.sub.4 in the solution: 0.46 mol/dm.sup.3; the initial concentration of sulfuric acid in the solution: 8 mol/dm.sup.3; stirring speed: 650 rpm; reaction time: 36 hours; and reaction temperature: 125 C. The results of step 1 indicated that the product included a mixture of NaRe(SO.sub.4).sub.2.xH.sub.2O, Al.sub.2O.sub.3, and SiO.sub.2.

(62) As shown in Table 2 below, based on the solubility of CaSO.sub.4 and MgSO.sub.4 in the Na.sub.2SO.sub.4H.sub.2SO.sub.4H.sub.2O solution, Ca and Mg can be removed as liquified primary impurities (leachate A) by solid-liquid separation after the reaction in step 1.

(63) TABLE-US-00002 TABLE 2 Results of chemical analysis of liquified primary impurities (leachate A), leachate B containing La, Pr, and Nd dissolved therein, leachate C containing Ce dissolved therein, and solidified secondary impurities, which are separated in each step Amount separated/g (Sc) Liquified Leachate B Leachate C primary containing La, Pr, containing Solidified Initial impurities and Nd dissolved Ce dissolved secondary Elements amount(g) (leachate A) therein therein impurities Ce 18.89 0.20(1.0) 0.75(4.0) 17.94(95.0) <0.01(<0.1) La 9.81 0.16(1.6) 9.59(97.7) 0.06(0.7) <0.01(<0.1) Pr 1.28 0.06(4.7) 1.22(95.0) <0.01(<0.3) <0.01(<0.1) Nd 2.23 0.08(3.6) 2.14(95.9) 0.01(0.5) <0.01(<0.1) Ca 2.00 2.00 <0.01 <0.01 <0.01 Al 3.02 0.07 <0.01 <0.01 2.95 Si 0.98 <0.01 <0.01 <0.01 0.98 Mg 0.36 0.36 <0.01 <0.01 <0.01

(64) Next, in step 2, 1000 mL of an alkaline solution containing 0.7 mol NaOH was added to convert NaRe(SO.sub.4).sub.2.xH.sub.2O into Re(OH).sub.3. In step 3, O.sub.2-containing air was injected into the Re(OH).sub.3-containing solution, and Ce(OH).sub.3 was oxidized into Ce(OH).sub.4 at a stirring speed of 650 rpm and a reaction temperature of 80 C. for 12 hours. In hydrochloric acid leaching in step 4 after oxidation, the effect of pH on the percent separation (S.sub.c) of Ce, La, Pr, and Nd from leachate B (containing La, Pr, and Nd dissolved therein) and leachate C (containing Ce dissolved therein) was examined, and the results are shown in FIG. 8. The pH range shown in FIG. 8 was based on the data shown in FIG. 7. The percent separation (S.sub.c) of Ce, La, Pr, and Nd was calculated using the following equation:
S.sub.c(Re.sub.r/Re.sub.0)100(15)

(65) wherein Re.sub.0 represents the amount (g) of Ce, La, Pr or Nd in the initial mixture, and Re, represents the amount of Ce, La, Pr or Nd separated.

(66) As a result, for effective separation between Ce, La, Pr and Nd in percent separation (Sc), the solution was adjusted to pH 2.83 by addition of 0.31 mol HCl at 25 C. (at least acid leaching should be performed at a pH of 2.5 to 3.5).

(67) After acid leaching with HCl in step 4, Ca(OH).sub.4, Al.sub.2O.sub.3, and SiO.sub.2 residues were separated from leachate B containing Ce.sup.4+ (0.75 g), La.sup.3+ (9.59 g), Pr.sup.3+ (1.22 g), and Nd.sup.3+ (2.14 g). Then, the residues were added to 1000 mL of an acid solution containing 1 mol H.sub.2SO.sub.4 to dissolve Ce(OH).sub.4. After filtration, the concentration of Ce.sup.4+ dissolved in leachate C was 17.94 g, and the concentration of other impurities was only less than 0.08 g. In addition, the final residue (solidified secondary impurities) remaining in the filtrate was composed of 5.57 g of Al.sub.2O.sub.3 (2.95 g of Al) and 2.1 g of SiO.sub.2 (0.98 g of Si). The amount of element separated in each step can be seen in Table 2 above.

(68) A method of recovering rare earth elements from rare earth polishing powder waste by a five-step hydrometallurgical process according to the present invention can recover 95% or more of rare earth elements, including cerium (Ce), lanthanum (La), praseodymium (Pr), and neodymium (Nd), from the polishing powder waste, and rare earth elements are recovered through such an efficient process so that great economic gains can be achieved.

INDUSTRIAL APPLICABILITY

(69) A method of recovering rare earth elements from rare earth polishing powder waste by a five-step hydrometallurgical process according to the present invention can recover and recycle 95% or more of rare earth elements, including cerium (Ce), lanthanum (La), praseodymium (Pr), and neodymium (Nd), from the polishing powder waste, and thus can contribute to achieving economic gains and securing resources.

(70) Although the present invention has been described in detail with reference to the specific features, it will be apparent to those skilled in the art that this description is only for a preferred embodiment and does not limit the scope of the present invention.