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Ind. Eng. Chem. Res. 2005, 44, 1922-1927
Extraction and Separation of Gallium Using Cyanex 301: Its Recovery from Bayer’s Liquor Bina Gupta,* Niti Mudhar, and S. N. Tandon Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee-247 667, Uttranchal, India
This paper describes the extraction behavior of Ga(III) along with associated metal ions, viz., V(V), Ge(IV), Ti(IV), V(IV), Tl(III), In(III), Al(III), Fe(III), Cd(II), Zn(II), Ni(II), and Mn(II), by bis(2,4,4-trimethylpentyl)dithiophosphinic acid (Cyanex 301) from different mineral acid media. The effect of different variables such as equilibration time, temperature, nature of diluent, and concentration of acid, metal ion, and extractant on the extraction of Ga(III) has been investigated. The extracting species has been identified. It has been possible to attain separations of Ga(III) from commonly associated metal ions by selective extraction/stripping. The partition behavior has been successfully extended to the recovery of gallium in good yield with high purity from Bayer’s liquor. The commercial feasibility of the proposed procedure is supported by the fact that the extractant has good hydrolytic stability, regeneration power, and reasonable loading capacity. Introduction The demand for gallium is increasing at a very fast rate because of its use in the electronics and semiconductor industries. There are no primary sources of the metal, and it is mainly obtained as a byproduct from metallurgical processing of aluminum and zinc. Its recovery from Bayer’s liquor encounters problems due to the large bulk of aluminum present in the solution and the level of purity of gallium desired. Direct electrolysis or amalgamation techniques used for the separation suffer from the limitation of high cost. Moreover, the amalgamation process poses environmental hazards. Solvent extraction offers a better alternative for the separation and recovery of high-purity gallium. Carboxylic acids,1-3 high molecular weight amines,4-6 oximes,7,8 DEHPA,9,10 and chelating agents11,12 have been used for the extraction and separation of gallium. But they invariably suffer from one or more drawbacks, including emulsion formation, long equilibration time, extractant loss, and rigid control of phase variables for separation. These limitations may restrict large-scale use of these extractants. The selectivity of Kelex 10013,14 and alkanoyl oxines15 for gallium over aluminum is reported, and these extractants have been investigated for the recovery of gallium from Bayer’s liquor at bench level. In both cases, the addition of modifiers is required, and the process has to be carried out at elevated temperatures with a contact time ranging from 1 to 3 h. Some of our preliminary studies indicated selectivity of di(2-ethylhexyl)phosphoric acid (DEHPA, pKa ∼ 1.5), bis(2,4,4-trimethylpentyl)phosphinic acid (Cyanex 272, pKa ) 6.4), and its sulfur analogue bis(2,4,4-trimethylpentyl)dithiophosphinic acid (Cyanex 301, pKa ) 2.6) for gallium over aluminum. Cyanex 301 scores over DEHPA in terms of its reported higher recycling capacity and hydrolytic stability.16 Cyanex 301 is a better choice than Cyanex 272 because of its lower pKa. A lower pKa will permit extractions at * Author to whom correspondence should be addressed. Phone: +091-1332-285326. Fax: +091-1332-273560. E-mail:
[email protected].
a higher acidity and restrict the possibility of hydrolysis of metal ions. This paper reports the extraction behavior of Ga(III) along with commonly associated metal ions such as V(V), Ge(IV), Ti(IV), V(IV), Tl(III), In(III), Al(III), Fe(III), Cd(II), Zn(II), Ni(II), and Mn(II) in Cyanex 301 from different mineral acid media. The effect of different aqueous- and organic-phase variables on the extraction of Ga(III) has been investigated. The extracted Ga(III) species has been identified. On the basis of the partition behavior, binary separations of Ga(III) from different associated metal ions have been achieved. The data have been extended to the recovery of pure gallium from Bayer’s liquor. The loading capacity, hydrolytic stability, and regeneration power of the extractant have been assessed. Experimental Section Reagents and Materials. Cyanex 301 (average mol wt 322, assay 77%) was obtained from Cytec Inc., Niagara Falls, ON, Canada, and used without purification. Gallium sulfate of analytical grade from SigmaAldrich was used. All other chemicals were AR/GR grade reagents from T. Baker/E. Merck (India). Kerosene of the boiling fraction 160-200 °C was used. The metal ion solutions were prepared by dissolving their salts in a minimum amount of suitable acid and making up the solution to a known volume by double-distilled water. The metal ion solutions were standardized by the usual complexometric titrations. The Bayer’s liquor was procured from HINDALCO, Renukoot (U.P.), India. The composition of alkaline Bayer’s solution is 160 ( 2 g/L Na2O, 20 ( 1 g/L Al(III), and 150 ( 5 mg/L Ga(III). Extraction and Analysis. For the distribution studies, equal volumes (10 mL) of the aqueous phase (metal ion solution in appropriate mineral acid) and organic phase (Cyanex 301 solution in toluene) were shaken manually at room temperature (25 ( 3 °C) for 5 min to ensure complete equilibration. The two phases were separated, and a suitable aliquot of the aqueous phase was assayed for the metal ion concentration by ICPAES (Plasmalab, 8440) or AAS (Perkin-Elmer, 3100).
10.1021/ie049730w CCC: $30.25 © 2005 American Chemical Society Published on Web 02/18/2005
Ind. Eng. Chem. Res., Vol. 44, No. 6, 2005 1923 Table 1. Effect of the Nature of Diluents on the Percent Extraction of 1.0 × 10-3 mol L-1 Ga (III) diluent
dielectric constant
% extraction
n-hexane kerosene (160-200 °C) xylene benzene toluene nitrobenzene
2.0 2.0 2.2 2.2 2.4 35.74
80 81 72 77 79 48
The other experimental conditions are mentioned along with the corresponding data. Each data point represents an average of three observations, and the variance at 70% extraction is (2. Results and Discussion Equilibration Time. The equilibration time was varied from 30 s to 30 min, and the results showed that equilibrium was attained in about 2 min and that prolonged shaking had no adverse effect on extraction. In all subsequent studies, the solution was shaken for 5 min. Temperature. The effect of temperature on the extraction of Ga(III) (1.0 × 10-3 M) from 5.0 × 10-2 M HCl in 0.20 M toluene solution of Cyanex 301 has been investigated from 10 to 70 °C by shaking the two phases in a thermostatically controlled ((0.1 °C) water bath (Julabo, Germany). The extraction increases with an increase in temperature, thus depicting the process to be endothermic, and the ∆H value is 22.0 × 10-3 kJ mol-1. All other studies were carried out at room temperature (25 ( 3 °C). Nature of Diluent. Different diluents such as toluene, xylene, benzene, n-hexane, nitrobenzene, and kerosene (160-200 °C) of varying nature and dielectric constant were used for the extraction of Ga(III) (Table 1). No correlation in the extent of extraction with dielectric constant or chemical nature was observed. Toluene, n-hexane, and kerosene showed higher and comparable extractions. In the case of toluene, a quicker phase separation was observed, and this was used as a diluent for all the subsequent studies. Kerosene can replace toluene for commercial purposes. Effect of Metal Ion Concentration. The effect of varying Ga(III) concentration (1.0 × 10-3-0.10 M) on its extraction has been investigated by keeping the HCl molarity constant at 5.0 × 10-2 M HCl and employing 0.50 M solution of Cyanex 301 (Figure 1). A linear curve is obtained up to 9.7 × 10-3 M Ga(III), indicating that the extracted species does not change in this region. Beyond 9.7 × 10-3 M, Ga(III) saturation condition starts setting in, and the results indicate that the extractant can hold the metal ion up to one-fiftieth of its molar concentration. Effect of Concentration of Extractant. The effect of increasing Cyanex 301 concentration (1.0 × 10-20.50 M) on the extraction of Ga(III) (1.0 × 10-3 M) in 5.0 × 10-2 M hydrochloric acid is shown in Figure 2. The extraction of Ga(III) increases with increasing concentration of the extractant. The log-log plot between the extractant concentration and distribution ratio gives a straight line with a slope approximating 3, indicating the involvement of three molecules of extractant in the formation of the extracted species. Effect of H+ and Cl- Ion Concentration. The effect of H+ concentration (pH 1-3) on the distribution of Ga(III), equilibrium pH vs log D, is shown in Figure 3. A
Figure 1. Effect of concentration of metal ion on the extraction of Ga(III) from hydrochloric acid by Cyanex 301. Conditions: [acid] ) 5.0 × 10-2 mol L-1, [Cyanex 301] ) 0.50 mol L-1.
Figure 2. Effect of concentration of Cyanex 301 on the distribution of Ga(III). Conditions: [HCl] ) 5.0 × 10-2 mol L-1, [Ga(III)] ) 1.0 × 10-3 mol L-1.
Figure 3. Effect of equilibrium pH on the extraction of Ga(III). Conditions: [Ga(III)] ) 1.0 × 10-3 mol L-1, [Cyanex 301] ) 0.20 mol L-1.
straight line with a slope value around one is obtained, thus suggesting that only one molecule of the extractant is involved in neutralizing the charge on metal ion and the other two molecules solvate the species. Gallium in the organic phase can thus exist as Ga(OH)2R‚2RH or
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Ind. Eng. Chem. Res., Vol. 44, No. 6, 2005
Figure 4. Effect of concentration of acids on the extraction of Ga(III). Conditions: [Ga(III)] ) 1.0 × 10-3 mol L-1, [Cyanex 301] ) 0.50 mol L-1.
GaCl2R‚2RH. To ascertain the composition of the extracting species, the effect of Cl- on the distribution of Ga(III) was examined at pH 2 by varying the concentration of Cl- ion from 1.0 × 10-2 to 1.0 M using NaCl solution. The results indicate that Cl- ion concentration does not affect the extraction of Ga(III) as extraction remains constant in the investigated range of Cl- ion concentration. Thus, the possibility of existence of the GaCl2R‚2RH species is excluded, and Ga(OH)2R‚2RH is proposed as the extracting species. Cyanex 301 is taken to be monomer in the organic phase.17,18 Equilibrium Treatment. On the basis of the above results, the following extraction equilibria can be proposed:
Ga(OH)2+aq + 3HRorg h [Ga(OH)2R‚2HR]org + H+aq (1) K)
[H+]aq[Ga(OH)2R‚2HR]org [Ga(OH)2]+aq[HR]3org
(2)
Assuming [Ga(OH)2R‚2RH] is the only extracted species in the organic phase and the metal ion in the aqueous phase at pH 1-3 predominantly exists as Ga(OH)22+, the distribution ratio D is given by the expression
D)
[Ga(OH)2R‚2HR]org [Ga(OH)2]+
(3)
Substituting the value of D in eq 2, taking logarithm and rearranging gives
log K ) log D - pH - 3 log [HR]org
(4)
The value of conditional extraction constant K, as calculated using eq 4, is 20. Extraction Behavior The extraction behavior of Ga(III) (1.0 × 10-3 M) was studied from the three mineral acid media (namely, sulfuric, nitric, and hydrochloric acid) and is illustrated in Figure 4. Ga(III) shows quantitative extraction up to 0.1 M acid in HCl medium. Thereafter its extraction decreases sharply and is negligible at 0.50 M HCl, beyond which an increase in extraction is observed. In HNO3 and H2SO4 media, the extraction of Ga(III)
Figure 5. Effect of HCl concentration on the extraction of associated metal ions using Cyanex 301. Conditions: [metal ion] ) 1.0 × 10-3 mol L-1, [Cyanex 301] ) 0.50 mol L-1. Table 2. Efficiency of Different Stripping Reagents for Ga(III) stripping reagent
recovery of Ga(III) from organic phase (%)
3% H2O2 (pH ) 1) 0.1 M citric acid 0.1 M tartaric acid 0.1 M H2SO4 (3 vol) 0.1 M oxalic acid (2 vol) 0.5 M HCl (3 vol)
