Gallium is relatively abundant in nature but is not naturally concentrated. It is usually associated with aluminium in bauxite, nephelines and other ores. It has been also found in the ashes of certain kinds of ore. A major resource for the recovery of gallium is gallium bearing aluminium ores. The spent caustic solution from Bayer process, which is recycled, builds up the gallium concentration approximately to 200 ppm. Gallium is also obtained from the iron mud or residues that results from the purification of zinc sulphate solutions, in zinc production.
Gallium is recovered from Bayer process liquors by the process of
1) direct electrolysis
2) solvent extraction and
3) ion exchange.
In the direct electrolysis process the gallium is generally recovered from Bayer process liquor by using mercury cathode. The drawbacks of this process are that when the organic content of the Bayer process liquor is high, the process becomes uneconomical due to low current efficiency and the use of the mercury is highly toxic to the environment. In such case preliminary separation of gallium with partial enrichment is carried out either by fractional precipitation (by neutralising the alkali with CO2) or by removal of part of alumina by addition of lime and subsequent recovery of gallium by passing CO2. Reference is made to Bhat and Sundarajan, J. Less Common Metals, 1967, 12 pp: 231-238 wherein they have studied the solvent extraction method for the recovery of gallium from the enriched fraction as mentioned above. In this study about 50% of the alumina content of the liquor was precipitated as calcium aluminate by the addition of lime. The gallia and the remaining part of alumina were then co-precipitated by neutralising the alkali with CO2. This fraction containing about 0.6% Ga2O3 was dissolved in hydrochloric acid and maintaining free acid at 3N. From this solution gallium was extracted by contacting an equal volume of 20% TBP and then recovered by back extraction with water. Gallium was precipitated with ammonia and the gallium hydroxide dissolved in 10% sodium hydroxide, and from which gallium metal was finally obtained by electrolysis using a gallium cathode and nickel anode. The gallium thus obtained was found to be 99% pure with an overall recovery of 90%. The drawbacks are the destruction of the alumina liquor which cannot be recycled back into the Bayer process.
Reference is made to Varadhraj et al., J. Appl. Electrochemistry, 1989, 19(1) pp: 61-64, wherein their investigations on the effect of organics employing linear stripping voltammetry techniques on glassy carbon electrodes in alkaline gallate solutions revealed the inhibitory effect of those compounds on the electrodepositions of gallium and hence on gallium recovery from aluminate solutions.
Reference is made to Dorin and Frazer, J. Appl. Electrochemistry, 1988, 18(1), pp: 134-141, wherein they have electrodeposited gallium from a synthetic Bayer process liquor comprising 4.5M NaOH/0.2M Na2CO3/0.3M NaCl and 1.7M Al(OH)3. The deposition was in part controlled by the mass transfer and in part by electron transfer step. Heavy metal impurities, such as Fe and V, usually found in these liquors, promote the hydrogen evolution reaction, completely inhibiting gallium production if present above certain critical concentrations, i.e. 3 ppm for Fe and 30 ppm for V. The drawbacks of the above mentioned two process are that the direct electrowinning of gallium from Bayer process liquor is not possible if the liquors contain iron and vanadium above their critical limits.
Reference is made to Leveque and Helegorsky, International Solvent Extraction Conference 1977, pp: 439-442, wherein the solvent extraction of gallium from concentrated Bayer process liquors using Kelex 100 was first reported. The organic phase was made up of 8.5 vol % of Kelex 100, 10 vol % of n-decanol and 81.5 vol % of kerosene. When this organic phase was contracted with a Bayer process liquor containing 75 g/L of Al2O3, 194 g/L of Na2O and 270 ppm of Ga, at 1.0:1.0 aqueous to organic phase ratio at 28° C., 80% of gallium was reported to be extracted in 3 h. The drawback of this process is the slow kinetics where the time taken to reach equilibrium was reported to be 3 h.
Reference is made to Pesic and Zhou. J. Metals, 1988, 40 pp: 24-26, wherein 80% of gallium extraction was obtained in 4 h from synthetic aluminate solutions containing 200 ppm of gallium. The drawback of this process is again slow kinetics. Reference may be made to Borgess and Mason wherein they have studied the solvent extraction of gallium from a weak Brazilian Bayer process liquor containing 110 ppm Ga, 16-25 g/L of Al2O3 and 108-120 g/L of Na2O using 10.0 vol % of Kelex 100, 5.0 vol % of Versatic 10, 8.0 vol % of n-decanol and 77 vol % of kerosene and showed 90% recovery in 2 min. Though the problem associated with the slow kinetics of gallium extraction is overcome by incorporating Versatic 10 acid into the organic phase the process is not addressed the actual recovery of the gallium metal.
Reference is made to Swift, J Am. Chem. Soc, 1924, 46, 2375-2381, wherein from 6.0M HCI gallium can be loaded selectively onto diethylether over virtually any probable co-existing elements excepting germanium and Fe(III). The presence of HCl promotes formation of HGaCl4 which is extracted by solvation, but above 6.0M HCl competition with acid extraction reduces its recovery. Ether extraction was preceded by removal of heavy metal impurities and Fe(III) reduction through aluminum. The drawback of this process is its non selectivity to Fe (III), where aluminium is added to reduce Fe(III) to Fe(II).
Reference is made to Mihalov, I and Distin, P. A. Hydrometallurgy, 1992, 28, 13-27, wherein a detailed review on the solvent extraction of gallium from HCl solutions was given where several organic agents such as organophosphorous compounds, D2EHPA, carboxylic acids, ketones, alkyl amines and quarterly ammonium salts are discussed with respect to their extractability of gallium from HCl solutions. Gallium is extracted as GaCl4 into quarterly ammonium salts (eg., Tricapryl mono methyl ammonium chloride—Aliquat 336) by anion exchange. The extraction of gallium is rapid and increases with increasing chloride concentration.
U.S. Pat. No. 5,204,074 teaches the recovery of gallium from basic aqueous solutions thereof such as Bayer liquors by contacting with a medium comprising a gallium extractant. The gallium values are transferred to the extractant which is then contacted with a basic aqueous solution and the gallium then back-extracted into the basic aqueous solution. This solution is then further contacted with a second medium containing a gallium extractant to transfer the gallium values thereto. The gallium enriched second medium is then contacted with a second aqueous solution which can be either acidic or basic to back-extract the gallium values. This is then directly electrolyzed to produce gallium.
U.S. Pat. No. 5,008,016 discloses the recovery of gallium by liquid/liquid extraction from basic aqueous solutions using an organic phase containing a substituted hydroxyquinoline and caustic soda.
U.S. Pat. No. 4.169,130 discloses a the recovery of gallium by liquid/liquid extraction with a water immiscible organic phase comprising an organic solvent and a dissolved water insoluble substituted hydroxyquinoline. The recovery of gallium is conducted under inert atmosphere.
The above patents suffer from the disadvantages that the processes are kinetically slow, require inert atmosphere thereby not being feasible on large scale and low purity of gallium recovered.