Precipitation Stripping of Rare-Earth Carbonate Powders from Rare

Tertiary carboxylate solutions of lanthanum and yttrium gave precipitates of lanthanum carbonate and yttrium carbonate, when contacted with both water...
0 downloads 0 Views 99KB Size
Download PDF
Ind. Eng. Chem. Res. 2001, 40, 1793-1797

1793

APPLIED CHEMISTRY Precipitation Stripping of Rare-Earth Carbonate Powders from Rare-Earth-Loaded Carboxylate Solutions Using Carbon Dioxide and Water Yasuhiro Konishi* and Yoshiyuki Noda Department of Chemical Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan

This paper describes a chemical processing route of a rare-earth oxide precursor, in which rareearth carbonate powders are prepared directly from disperse systems of rare-earth-loaded carboxylate solutions and water, using carbon dioxide gas as a precipitant. This synthetic route of rare-earth carbonate is a combined process of the stripping and precipitation stages in a conventional solvent extraction. Tertiary carboxylate solutions of lanthanum and yttrium gave precipitates of lanthanum carbonate and yttrium carbonate, when contacted with both water and pure carbon dioxide gas at 10-80 °C and 0.1-3.0 MPa for 2 h. The rates of carbonate precipitation in a batch autoclave were sensitive to processing parameters such as carbon dioxide pressure, temperature, and organic-phase composition. The particle size distributions of the carbonate powders were markedly dependent on the operating temperature: the mean particle size of yttrium carbonate drastically decreased from 41 µm at 80 °C to 7.2 µm at 10 °C. Introduction It is generally accepted that solvent extraction is the most appropriate commercial technology for separation and purification of rare earths. In typical commercial processes, the rare earths are stripped from loaded solvent extractants using aqueous solutions of inorganic acids. After neutralization of the strongly acidic aqueous strip solution, the dissolved rare earths are precipitated as insoluble oxalates and carbonates (oxide precursors), from which oxides are recovered by calcination, heating the resultant precipitates and driving off both carbon dioxide and water. Such conventional solvent extraction processing of rare earths, which involves the stripping stage and consequent precipitation stage, can be simplified by emulsifying rare-earth-loaded solvent extractant with an aqueous solution of precipitant for rare-earth ions. It is likely that a combination of the stripping and precipitation stages offers considerable savings in precipitating and neutralizing agents over the conventional solvent extraction process. Doyle and co-workers1,2 first demonstrated that lanthanum and yttrium oxalates are directly precipitated from rare-earth-loaded solvent extractants [decanoic acid and bis(2-ethylhexyl)phosphoric acid] when contacted with an aqueous oxalic acid solution at ambient temperature and atmospheric pressure. Moreover, neodymium oxalates were prepared from organic solutions of tertiary aliphatic monocarboxylic acid and 2-ethylhexylphosphonic acid mono(2-ethylhexyl) ester using aqueous oxalic acid solutions, and powder characteristics of * To whom all correspondence should be addressed. Telephone: 81-722-54-9297. Fax: 81-722-54-9911. E-mail: [email protected].

the resulting oxalates were compared with those of powders by the conventional solvent extraction routes.3,4 Recently, attention has been directed toward the influence of processing parameters on the particle size distributions of yttrium oxalate precipitates.5 Alternatively, rare-earth carbonate powders can be recovered directly from disperse systems of rare-earthloaded solvent extractant and water, using carbon dioxide gas as the precipitant for rare earth. Figure 1 shows a conceptual diagram for precipitation stripping of rare-earth-loaded solvent extractant using carbon dioxide and water. Assuming that LnR3‚3RH is the predominant rare-earth species in the organic phase, the following reactions can be represented for the singlestage precipitation stripping of rare-earth Ln with carbon dioxide and water:

Dissolution: 3CO2(g) + 3H2O(aq) ) 6H+(aq) + 3CO32-(aq) (1) Stripping: 2LnR3‚3RH(org) + 6H+(aq) ) 2Ln3+(aq) + 6R2H2(org) (2) Precipitation: 2Ln3+(aq) + 3CO32-(aq) ) Ln2(CO3)3(s) (3) Overall reaction: 3CO2(g) + 3H2O(aq) + 2LnR3‚3RH(org) ) Ln2(CO3)3(s) + 6R2H2(org) (4) where R2H2 represents a free extractant dimer. As shown by eq 1, carbon dioxide gas, which dissociates into hydrogen ions and carbonate anions in the aqueous phase, acts as both a stripping agent and a precipitating

10.1021/ie0007668 CCC: $20.00 © 2001 American Chemical Society Published on Web 03/15/2001

1794

Ind. Eng. Chem. Res., Vol. 40, No. 8, 2001

Figure 1. Conceptual diagram for precipitation stripping of the rare-earth-loaded solvent extractant using carbon dioxide and water.

agent: the rare earth in the organic phase is stripped with the hydrogen ions and subsequently precipitated by carbonate anions. Because the rare-earth ions in the aqueous phase are readily consumed for precipitating carbonate powders, a combination of the stripping and precipitation stages, eqs 2 and 3, tends to shift the position of equilibrium in eq 2 to the right. This achieves stripping of rare-earth ions at rather lower aqueous acidity than for the conventional stripping, thereby saving on reagent costs. This paper describes a new processing of a rare-earth oxide precursor, in which rare-earth carbonate powders are prepared from disperse systems of rare-earth-loaded carboxylate solutions and water, using carbon dioxide as the precipitating agent. The rare earths used were lanthanum and yttrium, which were considered to be representative of low and medium weight lanthanides, respectively. A quantitative criterion for precipitating carbonate powders was derived from thermodynamic analysis. The effects of process conditions on the precipitation rate and the particle size distribution of carbonate powders were examined. Experimental Section Materials. The commercially available Versatic 10, a synthetic tertiary aliphatic monocarboxylic acid (Shell Chemical Co., Tokyo, Japan), was used as a solvent extractant. The synthetic carboxylic acid contained at least 98% C9H19COOH and had an acid value of 320 mg of KOH/g. The Versatic 10 was diluted to desired concentration levels using commercial Exxsol D80, an aliphatic hydrocarbon diluent (Exxon Chemical Co., Tokyo, Japan). These organic materials were used without further purification. The rare earths used in this work were lanthanum and yttrium. Aqueous solutions of the individual rare earths were prepared by dissolving their pure oxides (Santoku Metal Industry Co., Ltd., Kobe, Japan) in a 2.0 kmol/m3 hydrochloric acid solution, evaporating the excess hydrochloric acid in the aqueous solution. Rare-earth-loaded carboxylate solutions were prepared by solvent extraction from the aqueous rare-earth solutions into the diluted Versatic 10 solutions. During the extraction operation, the aqueous solution pH was adjusted to about 6.0 by the addition of a dilute sodium hydroxide solution. The rare-earth-loaded organic solutions were washed with distilled water to remove residual anions and then passed through glass fiber

paper and phase-separating paper to remove physically entrained water. The initial concentrations of rare earths in the organic solution were 0.053 and 0.104 kmol/m3, and the initial concentrations of free carboxylic acid were 0.80 and 1.21 kmol/m3. Apparatus and Procedure. A stainless steel autoclave lined with glass was used to precipitate carbonate powders at elevated carbon dioxide pressures. The autoclave was 7-cm i.d. and 16-cm height, and a sixblade turbine impeller of 5.0-cm diameter was placed 3 cm above the bottom of the vessel. A 100-cm3 volume of the rare-earth-loaded organic solution was charged into the autoclave with an equal volume of distilled water. The organic and aqueous solutions were mixed by the impeller at 500 rpm and were maintained at an experimental temperature. The experimental temperature was varied from 10 to 80 °C. After continuous gassing with 98% carbon dioxide was performed, the total pressure over the starting solution in the autoclave was immediately increased to an experimental pressure. This time was taken as zero time. The total pressure in the autoclave was maintained at a desired constant value in the pressure range of 0.1-3.0 MPa by adjusting the regulating valve on the carbon dioxide cylinder and the exit valve on the autoclave. The duration of the precipitation tests was 120-240 min. A solution sample of 5 cm3 was withdrawn from the autoclave and centrifuged for analysis. The organic samples were mixed with a 6 kmol/m3 hydrochloric acid solution to strip completely the rare-earth species in the organic phase, and the aqueous solutions were analyzed for rare earth by ethylenediaminetetraacetic acid titration. The resulting precipitates were filtered, washed with distilled water and acetone, and dried for 5 h at 50 °C. The precipitates were characterized by X-ray diffraction (XRD) analysis. The particle size distributions of the precipitate powders were measured with a Leeds and Northrup Microtrac analyzer. Before the particle size distributions were measured, the precipitate particles were dispersed in a 0.2% sodium hexametaphosphate solution for 3 min, using an ultrasonic bath. Results and Discussion Precipitation of Rare-Earth Carbonate. The precipitation condition of rare-earth carbonate is

Ksp < [Ln3+]2[CO32-]3

(5)

where Ksp is the solubility product. The aqueous-phase concentration [Ln3+] of rare-earth ions can be expressed in terms of the equilibrium constant Kex for the solvent extraction, i.e., the reverse process of eq 2:

[Ln3+]1/2 ) (1/Kex)[LnR3‚3RH][H+]3/[R2H2]3

(6)

To define the carbonate concentration [CO32-] in eq 5, the dissociation equilibria of the dissolved carbon dioxide must be considered because the dissolved gas may exist in the liquid phase in any of four forms: CO2, H2CO3, HCO3-, and CO32-. The equilibrium relations

KA1 ) [H+][HCO3-]/([H2CO3] + [CO2])

(7)

KA2 ) [H+][CO32-]/[HCO3-]

(8)

Ind. Eng. Chem. Res., Vol. 40, No. 8, 2001 1795

indicate that the aqueous-phase carbonate concentration is pH sensitrive:

[CO32-] ) CT/{1 + [H+]/KA2 + [H+]2/KA1KA2} (9) with

CT ) [CO2] + [H2CO3] + [HCO3-] + [CO32-]

(10)

where KA1 and KA2 denote the first and second dissociation equilibrium constants for carbonic acid in the aqueous solution. The total dissolved carbon concentration CT in the aqueous phase is equal to the solubility of carbon dioxide in water, which can be related to the partial pressure pCO2 of carbon dioxide over the solution. When Henry’s law is valid, the total dissolved carbon concentration CT is given by

CT ) HpCO2

(11)

where H is the Henry’s law constant. Substituting eqs 6 and 9 into eq 5 and rearranging give a criterion for the precipitation of rare-earth carbonate:

CT > JP

(12)

where JP is defined by

JP ) Ksp1/3Kex1/3{1/[H+]2 + 1/KA2[H+] + 1/KA1KA2}[R2H2]2/[LnR3‚3RH]2/3 (13) When the rare-earth ions are immediately consumed for precipitating carbonate powders, the concentration of hydrogen ions in the aqueous phase can be expressed through the following electroneutrality relation:

[H+] ) 2[CO32-] + [HCO3-]

(14)

For a given concentration CT of total dissolved carbon, the hydrogen ion concentration is given by combining eqs 7, 8, 10, and 14:

[H+]3 + KA1[H+]2 + (KA1KA2 - CTKA1)[H+] 2CTKA1KA2 ) 0 (15) To find conditions for the precipitation of rare-earth carbonate, the thermodynamic constants appearing in eq 13 were obtained from previous literatures. The extraction equilibrium constant Kex was estimated as 3.18 × 10-17 for the lanthanum-Versatic 10 system at 20 °C and 8.71 × 10-16 for the yttrium-Versatic 10 system at 20 °C.6 The first and second dissociation equilibrium constants KA1 and KA2 for dissolved carbon dioxide are 4.45 × 10-7 and 4.69 × 10-11 kmol/m3 at 25 °C and infinite dilution.7 The solubility product Ksp is 3.98 × 10-34 (kmol/m3)5 for lanthanum carbonate and 2.51 × 10-31 (kmol/m3)5 for yttrium carbonate at 25 °C and infinite dilution.7 Data on the solubility of carbon dioxide in water at different temperatures and pressures were obtained from a useful source.8 For typical organicphase concentration ratios [H2R2]2/[LnR3‚3HR]2/3 used in this work, the JP values were calculated from eq 13 using the aforementioned values of Kex, KA1, KA2, and Ksp. These calculated results indicated that the quantitative criterion for precipitation, eq 12, is satisfied under the experimental conditions covered in this work.

Figure 2. Rate data for precipitation of lanthanum carbonate at different carbon dioxide pressures: (0) 3.0 MPa; (b) 1.1 MPa; (O) 0.1 MPa. Conditions: 30 °C, 0.104 kmol/m3 lanthanum carboxylate, and 0.80 kmol/m3 carboxylic acid.

Identification of Resulting Precipitates. The rare-earth-loaded organic solutions gave precipitation when contacted with carbon dioxide gas and distilled water at different temperatures and carbon dioxide pressures, 10-80 °C and 0.1-3.0 MPa. The resulting precipitates were identified by XRD analysis. Main XRD peaks observed for the precipitates were consistent with standard patterns of lanthanum carbonate [La2(CO3)3‚ 8H2O; JCPDS 25-1400] and yttrium carbonate [Y2(CO3)3‚3H2O; JCPDS 25-1010], and the sharpness of peaks indicated the highly crystalline nature of the carbonate products. In the presence of dissolved carbon dioxide that dissociates into carbonate and hydrogen ions in water, the rare earth in the organic carboxylate solutions was found to be stripped and precipitated as carbonate powders. Because the rare-earth carbonate powders were prepared in the presence of the organic phase, there was a fear of unfavorable contamination of the carbonate products with the organic starting materials, the Versatic 10-hydrocarbon diluent. Thermogravimetry (TG) and differential thermal analysis (DTA) was performed to examine the organic contamination of the carbonate powders. The thermal analysis revealed that the carbonate powders did not exhibit an exothermic peak and a weight loss at 300 °C on the TG-DTA curve, which would result from combustion of the organic material. This indicates that the rare-earth carbonates are uncontaminated by the organic starting materials. Rate of Carbonate Precipitation. The rates of carbonate precipitation were followed by measuring the organic-phase concentrations of rare earths as a function of time, to examine the effect of various processing parameters, such as the carbon dioxide pressure, temperature, and organic-phase composition. Rate data collected at different operating conditions are shown in Figures 2-5, where the precipitation percentages are plotted against time. The percentages of precipitation were determined from the concentration of the rare earth in the organic solution at any time divided by the initial organic-phase concentration. Figure 2 shows rate data for the precipitation of lanthanum carbonate at different carbon dioxide pressures in the gas phase at 30 °C. The lanthanum carboxylate solution gave the precipitation of carbonate even at atmospheric pressure, 0.1 MPa. The precipitation rate was markedly enhanced as the carbon dioxide pressure was increased from 0.1 to 3.0 MPa. The sensitivity to carbon dioxide pressure probably reflects a higher carbon dioxide solubility at higher pressures. At 3.0 MPa and 30 °C, the precipitation of lanthanum carbonate was achieved within 10 min. Figure 3 compares the precipitation rates of lantha-

1796

Ind. Eng. Chem. Res., Vol. 40, No. 8, 2001

Figure 3. Rate data for precipitation of two different carbonates: (O) lanthanum carbonate; (b) yttrium carbonate. Conditions: 30 °C, 0.2 MPa carbon dioxide pressure, 0.104 kmol/m3 rareearth carboxylate, and 0.80 kmol/m3 free carboxylic acid.

Figure 4. Rate data for precipitation of yttrium carbonate at different temperatures: (O) 10 °C; (b) 30 °C; (0) 50 °C; (9) 80 °C. Conditions: 0.2 MPa carbon dioxide pressure, 0.104 kmol/m3 yttrium carboxylate, and 0.80 kmol/m3 carboxylic acid.

num carbonate and yttrium carbonate at 30 °C and a carbon dioxide pressure of 0.2 MPa. The precipitation rates were similar, regardless of whether lanthanum or yttrium was used as the rare earth for the preparation of an organic carboxylate solution. Figure 4 shows rate data for the precipitation of yttrium carbonate at different temperatures and a carbon dioxide pressure of 0.2 MPa. There was a marked decrease in the precipitation rate of carbonate powders as the operating temperature was changed from 10 to 80 °C. However, the carbonate precipitation at 80 °C showed a lag period (60 min), followed by a significant increase in the percentage precipitation. In general, a rise in temperature has a marked effect on the solubility of carbon dioxide: the carbon dioxide solubility in water greatly decreases from 0.105 kmol/m3 at 10 °C to 0.0135 kmol/m3 at 80 °C when the total pressure (the sum of the partial pressures of carbon dioxide and water vapor) was 0.2 MPa. Thus, a decrease in the aqueous-phase concentrations of carbonate and hydrogen ions, reflected by a decrease in the carbon dioxide solubility, appears to be responsible for slower carbonate precipitation. Figure 5 shows that the precipitation rates of yttrium carbonate powders are influenced by the initial organicphase compositions. The precipitation rate decreased with a decrease in the initial concentration of yttrium carboxylate in the organic phase. Moreover, a marked decrease in the precipitation rate occurred when the initial concentration of free carboxylic acid was increased from 0.80 to 1.21 kmol/m3. A decrease in the rare-earth carboxylate concentration and an increase in the free carboxylic acid concentration shift the position of equilibrium in eq 4 to the left and appear to suppress the precipitation of carbonate. Because the overall precipitation process depletes the rare-earth carboxylate in the organic phase and produces the free carboxylic acid, the precipitation stripping system appears to be approaching equilibrium as the carbonate precipitation proceeds. Therefore, the precipitation of carbonates was

Figure 5. Rate data for precipitation of yttrium carbonate at different initial organic compositions: (O) 0.104 kmol/m3 yttrium carboxylate and 0.80 kmol/m3 free carboxylic acid; (4) 0.053 kmol/ m3 yttrium carboxylate and 0.80 kmol/m3 free carboxylic acid; (b) 0.104 kmol/m3 yttrium carboxylate and 1.21 kmol/m3 free carboxylic acid. Conditions: 30 °C and 0.2 MPa carbon dioxide pressure.

Figure 6. Particle size distributions of lanthanum carbonate prepared at different carbon dioxide pressures. Conditions: 30 °C, 0.104 kmol/m3 lanthanum carboxylate, and 0.80 kmol/m3 free carboxylic acid.

initially rapid and then gradually decreased the rate, as shown by Figures 2-5. Particle Size Distribution of Rare-Earth Carbonates. Particle size distributions of carbonate powders precipitated at different operating conditions for 2 h are shown in Figures 6-8, where the frequency size distribution is plotted against the logarithm of the particle size. The log-normal distribution frequency function was used to represent carbonate size distributions, and the particle size distribution data were graphically analyzed to determine the two parameters of the volume distribution, the geometric mean diameter dpg, and the geometric standard deviation σg. Figure 6 shows particle size distributions of lanthanum carbonate precipitated at different carbon dioxide pressures between 0.1 and 3.0 MPa. The particle size distribution was dependent on the carbon dioxide pressure. dpg of the carbonate particles increased from 7.0 to 12.2 µm as the carbon dioxide pressure was changed from 0.1 to 3.0 MPa. However, σg was virtually unchanged at σg ) 1.8-1.9 at 0.1-3.0 MPa. Figure 7 compares the particle size distributions of the carbonate powders prepared using the two different rare earths. The yttrium carbonate particles prepared at 30 °C and 0.2 MPa, which had a dpg of 15.3 µm and a σg of 1.6, were larger than the lanthanum carbonate particles of dpg ) 7.0 µm and σg ) 1.8. Figure 8 shows the size distributions of the yttrium carbonate particles prepared at different liquid temperatures. As the temperature was raised from 10 to 80 °C, dpg was drastically increased from 7.2 to 41 µm and σg was decreased from 1.7 to 1.4. The rise in temperature exhibited a lower precipitation rate (Figure 4), which suggests the slow formation of primary particles (nuclei) over a wide time interval. Such a slower nucleation during the precipitation process is likely to

Ind. Eng. Chem. Res., Vol. 40, No. 8, 2001 1797

boxylic acid. Moreover, the operating temperature had a considerable effect on the carbonate particle size distribution: the mean particle diameter was changed from 7.2 to 41 µm as the operating temperature was increased from 10 to 80 °C. Acknowledgment

Figure 7. Particle size distributions of lanthanum carbonate and yttrium carbonate. Conditions: 30 °C, 0.2 MPa carbon dioxide pressure, 0.104 kmol/m3 rare-earth carboxylate, and 0.80 kmol/ m3 free carboxylic acid.

This work was supported by Hosokawa Powder Technology Foundation, Japan. The authors thank Messrs. Takashi Goto, Yoshimasa Katayama, Jiro Nakanishi, Michiya Ohashi, and Hirofumi Takemori, Santoku Metal Industry Co., Ltd., Kobe, Japan, for their assistance in characterizing the precipitated powders. Literature Cited

Figure 8. Particle size distributions of yttrium carbonate prepared at different temperatures. Conditions: 0.2 MPa carbon dioxide pressure, 0.104 kmol/m3 yttrium carboxylate, and 0.80 kmol/m3 free carboxylic acid.

produce larger particles. In addition, a rise in temperature is likely to promote agglomeration and growth of primary particles. Thus, it can be concluded that the operating temperature is an important processing parameter for determining the particle size of rare-earth carbonates. Conclusions Upon treatment with both water and pure carbon dioxide gas at 10-80 °C and 0.1-3.0 MPa for 2 h, the carboxylate solutions of lanthanum and yttrium gave crystalline powders of lanthanum carbonate and yttrium carbonate. The precipitation rates of carbonate powders were markedly increased with increasing carbon dioxide pressure and rare-earth carboxylate concentration and decreasing temperature and free car-

(1) Yoon, J. H.; Doyle, F. M. Precipitation of Rare-Earth Powders from Aqueous Solutions and Emulsions. In Innovations in Materials Processing Using Aqueous Colloid, and Surface Chemistry; Doyle, F. M., Raghavan, S., Somasundaran, P., Warren, G. W., Eds.; The Minerals, Metals & Materials Society: Warrendale, PA, 1989; pp 195-211. (2) Lee, J. C.; Doyle, F. M. Precipitation of Yttrium Oxalate from Di-2-Ethylhexyl Phosphonic Acid Solution. In Rare Earths: Resources, Science, Technology and Applications; Bautista, R. G., Jackson, N., Eds.; The Minerals, Metals & Materials Society: Warrendale, PA, 1992; pp 139-150. (3) Konishi, Y.; Asai, S.; Murai, T. Precipitation Stripping of Neodymium from Carboxylate Extractant with Aqueous Oxalic Acid Solutions. Ind. Eng. Chem. Res. 1993, 32, 937-942. (4) Konishi, Y.; Asai, S.; Murai, T. Characterization of Neodymium Oxalate Precipitated from 2-Ethylhexyl Phosphonic Acid Mono-2-Ethylhexyl Ester Solution. Metall. Mater. Trans. B 1993, 24B, 537-539. (5) Konishi, Y.; Noda, Y.; Asai, S. Preparation Stripping of Yttrium Oxalate Powders from Yttrium-Loaded Carboxylate Solutions with Aqueous Oxalic Acid Solutions. Ind. Eng. Chem. Res. 1998, 37, 2093-2098. (6) Preston, J. S. Solvent Extraction of Metals by Carboxylic Acids. Hydrometallurgy 1985, 14, 171-188. (7) Martell, A. E.; Smith, R. M. Critical Stability Constants; Plenum Press: New York, 1977; Vol. 3, p 92; Vol. 4, p 37. (8) Linke, W. F.; Seidell, A. Solubilities of Inorganic and MetalOrganic Compounds, 4th ed.; van Nostrand: New York, 1958; Vol. I, p 459.

Received for review August 18, 2000 Revised manuscript received January 2, 2001 Accepted February 3, 2001 IE0007668