Ind. Eng. Chem. Res. 1998, 37, 2093-2098
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Precipitation Stripping of Yttrium Oxalate Powders from Yttrium-Loaded Carboxylate Solutions with Aqueous Oxalic Acid Solutions Yasuhiro Konishi,* Yoshiyuki Noda, and Satoru Asai Department of Chemical Engineering, Osaka Prefecture University, 1-1 Gaknen-cho, Sakai, Osaka 593, Japan
Precipitation stripping is a combined process of the stripping and precipitation stages in a conventional solvent extraction process for separation and purification of rare earths. Crystalline yttrium oxalate powders were precipitated by emulsifying yttrium-loaded carboxylate solutions with aqueous oxalic acid solutions at 30 °C and atmospheric pressure. The yttrium in the organic solution was completely stripped and precipitated within the first 5 min of mixing with the aqueous solution. The particle size distributions of the oxalate powders were markedly dependent on processing parameters, such as mixing time, liquid-phase stirring speed, initial organic-phase concentration of yttrium carboxylate, and ionic strength of the aqueous oxalate solution. In addition, the nature of the two solvent extractants, carboxylic acid and phosphoric acid systems, had little effect on the size distribution of yttrium oxalate. Introduction 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 mineral acid. After adjusting the pH of the aqueous strip solution, the rare earths are precipitated as insoluble powders of oxalates or carbonates, from which oxides can be produced by calcination. Such conventional processing can be simplified when the stripping and precipitation stages are combined by emulsifying rare-earth-loaded solvent extractant with aqueous oxalic acid solution. Assuming that LnR3‚3RH is the predominant rare earth species in the organic phase, the following reactions can be represented for the single-stage precipitation stripping of rare earth Ln with an aqueous oxalic acid solution:
Stripping: 2LnR3‚3RH + 6H+ ) 2Ln3+ + 6R2H2 (1) Precipitation: 2Ln3+ + 3(COO)22- ) Ln2[(COO)2]3 (2) Overall reaction: 2LnR3‚3RH + 3(COOH)2 ) Ln2[(COO)2]3 + 6R2H2 (3) where overbars denote species present in the organic phase and R2H2 represents extractant dimer. Oxalic acid, which dissociates into hydrogen ions and oxalate anions in the aqueous phase, acts as both a stripping agent and a precipitating agent; the rare earth in the organic phase is stripped with the hydrogen ions and subsequently precipitated by oxalate anions. Because the rare earth ions in the aqueous phase are readily consumed for precipitating oxalate powders, a combination of the stripping and precipitation stages, eqs 1 and * To whom all correspondence should be addressed. Telephone: 81-722-52-1161. Fax: 81-722-59-3340. E-mail:
[email protected].
2, tends to shift the position of equilibrium in eq 1 to the right and achieves complete stripping of rare earth ions at rather lower aqueous acidity than for the conventional stripping. Thus, it is likely that the singlestage precipitation stripping results in a marked decline in the consumed amounts of precipitating and neutralizing agents, thereby saving on both running and capital costs. Yoon and Doyle (1989) first demonstrated that oxalate powders of lanthanum and yttrium are directly precipitated from rare-earth-loaded carboxylic acid solvent extractant when contacted with aqueous oxalic acid solution at ambient temperature and atmospheric pressure. Yttrium oxalate powders were also prepared by precipitation stripping of di-(2-ethylhexyl)phosphoric acid (D2EHPA) extractant using oxalic acid-hydrochloric acid solutions (Lee and Doyle, 1991). For the precipitation stripping from D2EHPA systems, the precipitation rate and yield were influenced by the compositions of the aqueous and organic solutions, and faster precipitation rates produced finer precipitates. Recently, neodymium oxalates were directly precipitated from organic solutions of tertiary aliphatic monocarboxylic acid and 2-ethylhexyl phosphoric acid mono2-ethylhexyl ester using aqueous oxalic acid solutions, and powder characteristics of the resulting oxalates were compared with those of powders by the conventional solvent extraction routes (Konishi et al., 1993a, 1993b). Moreover, for the direct precipitation of rare earth oxalate from the carboxylate system, a quantitative criterion for oxalate precipitation was derived theoretically, and the rate-controlling step was determined (Konishi et al., 1993a). However, little attention has been directed towards the influence of processing parameters on the particle size distributions of oxalate precipitates. In this paper we describe a precipitation stripping in which yttrium ions are stripped from carboxylate extractant in organic solvent and simultaneously precipitated as yttrium oxalate using aqueous oxalic acid solution. The effects of process conditions on the
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2094 Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998
particle size distribution of resulting oxalate powders are considered. In addition, another phosphoric acid solvent extractant was used to compare the particle size distribution of yttrium oxalates obtained from the two different extractants. Experimental Section Materials. The commercially available Versatic 10, a synthetic tertiary aliphatic monocarboxylic acid (Shell Chemical Comapny, Tokyo, Japan), was mainly 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 Company, Tokyo, Japan). In some runs, commercial PC-88A extractant, 2-ethylhexyl phosphoric acid mono2-ethylhexyl ester (Daihachi Chemical Company Ltd., Osaka, Japan), was employed. The acid value for the ester was 180 mg of KOH/g. The PC-88A was diluted using commercial IP-solvent, an isoparaffin hydrocarbon (Idemitu Chemical Company Ltd., Tokyo, Japan). These organic materials were used without further purification. An aqueous solution of yttrium was prepared by dissolving pure yttrium oxide (Santoku Metal Industry Company, Ltd., 99.9%) in 2.0 kmol/m3 hydrochloric acid solution, evaporating the excess hydrochloric acid in the aqueous solution. Yttrium (III)-loaded Versatic 10 solutions were prepared by solvent extraction from the aqueous yttrium solutions into the dilute Versatic solutions. During the extraction operation, the aqueous phase pH was adjusted to ∼6.0 by the addition of dilute NaOH solution. Organic solutions were also prepared by solvent extraction of yttrium using the dilute PC-88A extractant. The yttrium-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 yttrium in the organic solution were 0.054, 0.114, and 0.226 kmol/m3, and the initial concentrations of free Versatic 10 were from 0.99 to 1.99 kmol/m3. The initial concentration of free PC-88A was also 1.44 kmol/m3. The strip solutions were aqueous oxalic acid solutions ranging in concentrations from 0.92 to 1.78 kmol/m3. The total amount of oxalic acid was five times or above the stoichiometric requirement for the amount of yttrium present in the organic solution. In some runs, the ionic strength of the oxalic acid solution was adjusted by adding a neutral salt, sodium chloride. Apparatus and Procedure. A glass vessel with 300-cm3 volume and 7.5-cm i.d. was used as a stirred vessel in the precipitation stripping experiments. A sixblade turbine impeller of 5.0-cm diameter was placed 0.5 cm above the bottom of the vessel. A 100-cm3 volume of the yttrium-loaded organic solution was heated and maintained at 30 °C in the stirred vessel. A 20-cm3 volume of aqueous oxalic acid solution was heated separately in a glass flask and then charged into the stirred vessel. The stirring of the organic and aqueous solutions was started immediately at atmospheric pressure, and this time was taken as zero time. The liquid-phase stirring speed was changed from 100 to 1000 rpm. The duration of precipitation tests was 0.5 to 20 min. A solution sample of 5 cm3 was withdrawn from the stirred vessel and centrifuged for
analysis. To determine the organic-phase yttrium concentration, the organic samples were mixed with 6 kmol/m3 hydrochloric acid solution to completely strip the metal species, and the aqueous solutions were analyzed for yttrium by EDTA 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 morphology was observed by scanning electron microscopy (SEM). The particle size distributions of the precipitates were measured with a Leeds and Northrup Microtrac analyzer. Before measuring the particle size distributions, the precipitate particles were dispersed in a 0.2% sodium hexametaphosphate solution for 3 min, using an ultrasonic bath. Results and Discussion Precipitation of Yttrium Oxalate. Previous work (Konishi et al., 1993) has established a quantitative criterion for the precipitation of rare earth oxalate from a metal-loaded cation-exchange solvent extractant with a aqueous oxalic acid solution. The criterion for oxalate precipitation is
CY > JP
(4)
where JP is defined by
JP ) Ks1/3Kex2/3{1/(KA1KA2) + 1/(KA2[H+]) + 1/[H+]2} × [H2R2]2/[LnR3‚3HR]2/3 (5) where CY is the total oxalate concentration, Ks is the solubility product constant of yttrium oxalate, Kex is the extraction equilibrium constant for the yttrium-extractant system, and KA1 and KA2 denote the first and second dissociation equilibrium constants for oxalic acid in the aqueous solution. For a given concentration CY of total oxalate, the hydrogen ion concentration can be calculated from the following relation:
[H+]3 + KA1[H+]2 + (KA1KA2 - CYKA1)[H+] 2CYKA1KA2 ) 0 (6) To find conditions for the precipitation of yttrium oxalate, the thermodynamic constants appearing in eq 5 were obtained from previous literature. The extraction equilibrium constant Kex for the yttrium-Versatic 10 system was estimated as 8.71 × 10-16 at 20 °C (Preston, 1985). The dissociation constants KA1 and KA2 were 5.60 × 10-2 and 5.42 × 10-5 m3/kmol at 25 °C and infinite dilution (Martell and Smith, 1977). The solubility product constant Ks of yttrium oxalate was 1.23 × 10-29 (kmol/m3)5 at 25 °C (Gmelin Handbook of Inorganic Chemistry, 1984). For typical organic-phase concentration ratios [H2R2]2/[LnR3‚3HR]2/3 used in this work, the JP values were calculated from eq 5 using the aforementioned values of Kex, KA1, KA2, and KS. These calculated results indicated that the quantitative criterion for precipitation, eq 4, is satisfied under the experimental conditions covered in this work. When contacted with aqueous oxalic acid solutions at 30 °C, yttrium precipiated from the yttrium-loaded organic solutions. After vigorous stirring of the aqueous and organic solutions was stopped, the emulsion system was completely separated into the organic and aqueous phases, and the precipitates readily settled at the
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Figure 1. Rate data for precipitation of yttrium oxalate from carboxylic acid extractant using aqueous oxalic acid solution. Conditions: 30 °C, 0.114 kmol/m3 yttrium carboxylate, 1.44 kmol/ m3 free carboxylic acid, 0.920 kmol/m3 oxalic acid, and 1000 rpm.
bottom of the aqueous phase. The resulting precipitates were identified by XRD analysis. Main XRD peaks observed for the precipitates agreed with a standard pattern of yttrium oxalate, and the sharpness of peaks indicated the highly crystalline nature of the oxalate products. The rates of oxalate precipitation were followed by measuring the organic-phase yttrium concentrations as a function of time. Rate data collected under a representative operating condition are shown in Figure 1, where the precipitation percentage is plotted against time. The percentages of precipitation were determined from the concentration of yttrium in the organic solution at any time, divided by the initial organic phase concentration. The precipitation of yttrium oxalate was a fast process; the yttrium being completely stripped and precipitated from the carboxylate solution within the first 5 min of exposure to the aqueous oxalic acid solution. Kinetic tests at different operating conditions also indicated that the precipitation stripping of yttrium oxalate was complete within 5 min. Morphology and Particle Size of the Yttrium Oxalate. A typical scanning electron micrograph for yttrium oxalate powder prepared by single-stage precipitation stripping is shown in Figure 2. The yttrium oxalate formed as angular and flaky particles, and there was a slight agglomeration. Moreover, microscopic examination revealed that the particle morphology was not changed appreciably by various processing parameters, such as the operating time, the liquid-phase stirring speed, and the organic and aqueous phase compositions. However, the particle size of oxalate powders was highly sensitive to the processing parameters. Particle size distributions collected at different operating conditions are shown in Figures 3 to 9, where the cumulative undersize is plotted against the particle size. Cumulative undersize distributions of yttrium oxalate particles, precipitated at four different mixing times, are shown in Figure 3. The particle size distribution was fairly broad at the shortest mixing time of t ) 0.5 min. During the early stages of stirring of the organic and aqueous solutions (t
