Title of Invention

PROCESS FOR RECOVERING GALLIUM

Abstract

This invention relates to a process for recovering gallium metal from the Bayer liquor which comprises an electrodialysis step where the inverse capturing solution containing gallium is electrodialyzed to concentrate gallium and recover the acid, an iron removal step where iron is removed from the concentrated gallium solution, an ultrafiltration step where the iron-free gallium solution is neutralized and the resulting gallium hydroxide slurry is subjected to ultrafiltration to concentrate the slurry, a redissolution step where the concentrated gallium hydroxide slurry is dissolved in an alkaline solution, and an electrolysis step where the alkaline electrolytic solution of gallium obtained in the redissolution step is electrolyzed to recover gallium metal and which consequently makes it possible to reuse the acid without generating waste acid in large amounts, increase the concentration of gallium in an alkaline electrolytic solution of gallium thereby raising the current efficiency in the electrolysis step, reduce the amount of waste liquor, and minimize the formation of spongy gallium, and completed this invention.

Full Text

SPECIFICATION Title of the Invention Process for Recovering Gallium Field of Technology This invention relates to a process for recovering gallium from the Bayer liquor (an aqueous solution of sodium aluminate) generated in the Bayer process for producing alumina from bauxite. More particularly, in a process for recovering gallium which comprises bringing the Bayer liquor into contact with a chelating agent composed of a water-insoluble substituted quinolinol to let the chelating agent to capture gallium contained in the Bayer liquor, bringing an inverse capturing solution containing a substituted quinolinol into contact with the gallium-containing chelating agent to extact gallium into the inverse capturing solution, and recovering gallium metal from the gallium-containing inverse capturing solution, this invention relates to a process for recovering gallium metal efficiently from the inverse capturing solution containing gallium in extremely low concentrations. Background Technology According to the Bayer process, alumina is extracted from bauxite with an aqueous solution of sodium hydroxide. Gallium contained in bauxite in a minute quantity is extracted together with alumina in the sodium hydroxide extract of bauxite or the Bayer liquor also contains a minute quantity of gallium. On account of its usefulness as a raw material for semiconductor compounds, scintillators, and the like, gallium has been recovered from the Bayer liquor thus far and several processes including one based on amalgamation have been proposed for the recovery. Of those proposals, a process regarded as most effective at the present time comprises preparing an adsorbent by supporting a substituted quinolinol as a chelating agent capable of forming a chelate with gallium on a porous resin substrate, bringing the Bayer liquor into contact with the adsorbent to let the adsorbent capture gallium contained in the Bayer liquor, extracting gallium from the adsorbent by an inverse capturing solution composed of an aqueous solution of a strong acid such as hydrochloric acid and sulfuric acid, and recovering gallium from the gallium-containing inverse capturing solution (reference should be made, for example, to JP 60-42,234 A, JP4-38,453 B, and JP7-94,324 B). A process proposed for the commercial recovery of gallium metal from the gallium-containing inverse capturing solution obtained in the aforementioned manner comprises bringing this gallium-containing inverse capturing solution into contact with an ion exchange resin, either anionic or cationic, to let the ion exchange resin capture gallium selectively, eluting gallium from the ion exchange resin by an aqueous solution of hydrochloric acid or sulfuric acid or by pure water, concentrating the eluted acidic solution of gallium, and electrolyzing the acidic solution to obtain gallium metal (reference should be made, tor example, to Ullmann's Encyclopedia of Industrial Chemistry, 5th Ed., Vol. A12,pp. 163-167, 1989). In the aforementioned process for recovering gallium, however, a strongly acidic aqueous solution is used not only as an inverse capturing solution but also as an eluting solution in large amounts during concentration by the use of an ion exchange resin and this inevitably generates waste acid in large amounts. The waste acid contains a variety of metallic and other impurities originating from the Bayer liquor besides gallium and it cannot be simply reused for the recovery of gallium even in the same process for recovering gallium and the disposal of waste acid generated in large amounts becomes a burden. According to another process proposed, for example, in JP56"38,661 B, the acidic solution of gallium is not electrolyzed as it is but first neutralized with an aqueous alkaline solution to form a slurry of gallium hydroxide, the slurry is filtered to recover gallium hydroxide, the gallium hydroxide is dissolved in an alkaline solution to give an alkaline electrolytic solution of gallium, and the alkaline electrolytic solution of gallium is subjected to direct current electrolysis to recover gallium metal. In this process, however, the crystals of gallium hydroxide formed in the neutralization with alkali are extremely fine on the order of 2pm at most; thus the slurry becomes gel-like and the normal procedure for concentration by sedimentation or filtration of the slurry requires a long time. Moreover, according to this process for electrolyzing the alkaline electrolytic solution of gallium to recover gallium metal, spongy gallium forms inevitably unless the concentration of iron ions in the alkaline electrolytic solution of gallium is kept below 5 ppm, preferably below 2 ppm, and the yield of recovery of gallium metal drops and spongy gallium must be disposed of or returned to the first treating step. Now, a process proposed in JP2001-97,716 A for electrolyzing an alkaline solution of gallium to obtain gallium metal aims at preventing gallium hydroxide from forming fine crystals by coprecipitating gallium hydroxide and a substance that is effective for improving the sedimentation of gallium, for example, jarosite [Fes(S04)2(OH)6], "eluting gallium by dissolving the coprecipitate in caustic soda" to prepare an alkaline electrolytic solution of gallium, and electrolyzing this electrolytic solution. However, in this process as well, the steps until the preparation of the alkaline electrolytic solution of gallium become complex and the recovery and waste disposal of the additives such as jarosite create new problems. Disclosure of the Invention Under the circumstances, the inventors of this invention have undertaken intensive studies to solve the problems confronting the conventional processes for recovering gallium from the Bayer liquor, found that the adoption of an electrodialysis step where an inverse capturing solution containing gallium is concentrated and an acid is recovered, an iron removal step where iron in the concentrated gallium solution is separated and removed as iron hydroxide, and an ultrafiltration step where the iron-free gallium solution obtained in the iron removal step is neutralized and then subjected to ultrafiltration to obtain a concentrated slurry of gallium hydroxide makes it possible to reuse the acid without generating waste acid in large amounts, increase the concentration of gallium in an alkaline electrolytic solution of gallium thereby raising the current efficiency in the electrolysis step, reduce the amount of waste liquor, and minimize the formation of spongy gallium, and completed this invention. Accordingly, this invention relates to a process for recovering gallium which comprises bringing the Bayer liquor into contact with a chelating agent composed of a water-insoluble substituted quinolinol to let the chelating agent capture gallium contained in the Bayer liquor, bringing an inverse capturing solution containing a substituted quinolinol into contact with the gallium-containing chelating agent to extract gallium into the inverse capturing solution, and recovering gallium metal from the gallium-containing inverse capturing solution and an object of this invention is to provide a process for recovering gallium which makes it possible to reuse the acid without generating waste acid in large amounts, increase the concentration of gallium in the alkaline electrolytic solution of gallium thereby raising the current efficiency in the electrolysis step, reduce the amount of waste liquor, and minimize the formation of spongy gallium. Thus, in a process for recovering gallium which comprises bringing the Bayer liquor into contact with a chelating agent composed of a water-insoluble substituted quinolinol to let the chelaging agent capture gallium contained in the Bayer liquor, bringing an inverse capturing solution composed of an acidic aqueous solution of a substituted quinolinol into contact with the gallium-containing chelating agent to extract gallium into the inverse capturing solution, and recovering gallium metal from the gallium-containing inverse capturing solution, this invention provides a process for recovering gallium which comprises an electrodialysis step where the aforementioned inverse capturing solution containing gallium is electrodialyzed to increase the concentration of gallium and the acid is recovered, an iron removal step where the concentrated gallium solution obtained in the electrodialysis step is adjusted to a given pH, the precipitated iron hydroxide is separated and removed to give an iron-free gallium solution, an ultrafiltration step where the iron-free gallium solution is neutralized to form a slurry of gallium hydroxide and the slurry is concentrated by ultrafiltration, a redissolution step where the concentrated slurry of gallium hydroxide is dissolved in an alkaline solution to prepare an alkaline electrolytic solution of gallium, and an electrolysis step where the alkaline electrolytic solution of gallium is electrolyzed to recover gallium metal. According to this invention, any of the conventional processes involving the use of a water-insoluble substituted quinolinol as a chelating agent, for example, the one described in JP7-94,324 A, can be applied as it is to the step for capturing of gallium where the Bayer liquor is brought into contact with a chelating agent composed of a water-insoluble substituted quinolinol and to the step for inverse capturing of gallium where an inverse capturing solution composed of an acidic aqueous solution containing a substituted quinolinol is brought into contact with the gallium-containing chelating agent to extract gallium into the inverse capturing solution. In the electrodialysis step of this invention where the aforementioned inverse capturing solution containing gallium is electrodialyzed, the concentration of acid in the solution usually ranges from 0.4 to 2 N and it is advantageous to perform the electrodialysis so that the concentration of acid becomes normally 0.4 N or less, preferably 0.2 N or less. In this manner, it becomes possible to otain a concentrated solution of gallium in which the concentration of gallium is increased 1.2 times or more, preferably 1.5 times or more, and achieve the recovery of acid to a significant degree to allow reuse of the acid. No restriction is imposed on the apparatus for this electrodialysis step and a commercial apparatus, for example, Model TS-50-740 available from Tokuyama Corporation, can be used as it is. The concentrated gallium solution obtained in the electrodialysis step normally contains 5 ppm or more of iron ions. The solution is then supplied to the iron removal step, the concentration of iron ions is reduced to 0.5 ppm or less, preferably 0.2 ppm or less, and an iron-free solution is recovered. In this iron removal step, an alkali is added to the concentrated gallium solution to adjust the pH to a value between 10 and 13, preferably between 12 and 12.5, and a precipitate of iron hydroxide formed as a result of this pH adjustment is separated and removed by a means such as a thickener. When the concentration of iron ions in the iron-free gallium solution obtained in this iron removal step is higher than 0.5 ppm, signifcant control over the formation of spongy gallium becomes difficult to exercise. The iron-free gallium solution obtained in the iron removal step is then supplied to the ultrafiltration step, the solution is neutralized to form gallium hydroxide, and the resulting gallium hydroxide slurry is concentrated by ultrafiltration to give a concentrated slurry of gallium hydroxide. The neutralization of the iron-free gallium solution in the ultrafiltration step is carried out by adjusting the pH to 7 ± 0.6, preferably 7± 0.3, by addition of an acid and an apparatus for the ultrafiltration of the gallium hydroxide slurry may be chosen from a variety of modules such as membrane module, tubular module, spiral module, hollow-fiber (capillary) module, monolith module, immersion tank type module, and rotary disk membrane module. A rotary disk membrane module is used preferably as it is capable of concentrating the solution to a high degree while preventing deposition of a solid matter on the surface of a membrane. As the initial solid content of the gallium hydroxide slurry is 0.8 to 2.6 g/L (or 0.5 to 1.5 g/L as gallium), it is advantageous here to concentrate this solid content at least 30 times, preferably from 40 to 70 times. When the degree of concentration is less than 30 times, the current efficiency becomes difficult to raise in the electrolysis step and the amount of waste liquor increases. The concentrated slurry of gallium hydroxide prepared in this manner is dissolved in the redissolution step to prepare an alkaline electrolytic solution of gallium and this electrolytic solution is then electrolyzed in the electrolysis step to recover gallium metal. In the redissolution step, an aqueous alkaline solution, preferably a highly concentrated one, is added to the concentrated gallium hydroxide slurry whose gallium content is 30 to 70 g/L and the gallium hydroxide in the slurry is dissolved completely to give an alkaline electrolytic solution of gallium whose gallium content is 23 to 53 g/L, preferably 35 to 45 g/L. The electrolysis is performed in the usual manner at a current density of 0.1 to 0.16 A/cm2, preferably 0.12 to 0.14 A/cm2, at a voltage of 2 to 5 V, preferably 3 to 4 V, and at a bath temperature of 40 to 70 °C, preferably 50 to 60 °C, while keeping the current density or voltage constant. According to this invention, the inverse capturing solution containing gallium is electrodialyzed to increase the concentration of gallium and the acid is recovered in the electrodialysis step, the pH of the concentrated gallium solution is adjusted and the precipitated iron hydroxide is separated and removed in the iron removal step, the iron-free gallium solution is neutralized and the resulting gallium hydroxide slurry is concentrated by ultrafiltration in the ultrafiltration step, and a highly concentrated alkaline electrolytic solution of gallium prepared from the concentrated gallium hydroxide slurry is electrolyzed to recover gallium metal; as a result, it is possible to reuse the acid without generating waste acid in large amounts, raise the current efficiency in the electrolysis step, reduce the amount of waste liquor, and minimize the formation of spongy gallium thereby markedly raising the rate of recovery of gallium. Brief Description of the Drawing Fig. 1 is a flow sheet illustrating a mode of reduction to practice of the process for recovering gallium according to this invention: SIM, capturing step; ST-2, inverse capturing step; ST-3, electrodialysis step; ST-4, iron removal step; ST-5, ultrafiltration step; ST-6, redissolution step; and ST-7, electrolysis step. Preferred Embodiments of this Invention A preferred mode of reduction to practice of this invention is described below with reference to the flow sheet in the appended drawing. A flow sheet in Fig. 1 illustrates an example of the mode of reduction to practice of this invention. An adsorption tower 1 is provided for operating the capturing step (ST-l) where the Bayer liquor is brought into contact with a chelating agent composed of a water-insoluble substituted quinolinol to let the chelating agent capture gallium contained in the Bayer liquor and the inverse capturing step (ST-2) where an inverse capturing solution composed of an acidic aqueous solution containing a substituted quinolinol is brought into contact with the gallium-containing chelating agent obtained in ST-1 to extract gallium into the inverse capturing solution and the tower is packed with an adsorbent prepared by supporting a water-insoluble chelating agent, typically 7-(4-ethyM-methyloctyl)-8-quinolinol (Kelex 100, tradename of Schering Corporation), on a synthetic substrate (Diaion HP20, tradename of Mitsubishi Chemical Corporation). The adsorption tower 1 normally comprises two or more towers connected in parallel and the capturing step (ST-l) and the inverse capturing step (ST-2) are operated alternately or successively and, as a whole, the capturing step and the inverse capturing step can be operated continuously. In this mode of reduction to practice, the Bayer liquor containing gallium is introduced to the adsorption tower 1 from a line 2, the liquor comes into contact with the adsorbent in the adsorption tower 1, gallium in the liquor is adsorbed on the adsorbent, and the Bayer liquor is supplied through a line 3 to the Bayer process not shown in the flow sheet. The adsorption tower 1 is washed with water in the usual manner when the operation of capturing gallium in ST-l is over, an inverse capturing solution or a 5.5 wt% aqueous hydrochloric acid containing 2 xlO"3 wt% of substituted quinolinol (Kelex 100, a tradename of Schering Corporation) is introduced to the adsorption tower 1 to extract gallium adsorbed on the adsorbent into the inverse capturing solution, the inverse capturing solution containing gallium is taken out from a line 4 and introduced to the following electrodialysis step (ST-3). In the electrodialysis step (ST-3), the inverse capturing solution containing gallium is electrodialyzed in such a manner as to reduce the acid concentration to 0.2 N or less and increase the gallium concentration 1.5 times or more, and the concentrated gallium solution is introduced through a line 5 to the iron removal step (ST-4) while the recovered aqueous hydrochloride acid is taken out from a line 6. In this mode of reduction to practice, a portion of the gallium-containing inverse capturing taken out from the line 4, normally 40 to 60 wt% of the total, is circulated through a line 11 to a mixer 9 for reuse of hydrochloric acid and some of the aqueous solution of hydrochloric acid taken out from the line 6 in the electrodialysis step (ST-3) is branched off and introduced through a line 10 to the mixer 9. Moreover, the chelating agent is recovered in a chelating agent recovery tower 8 provided in the line 4 for the gallium -containing inverse capturing solution and introduced through a line 11 to the mixer 9 and, if necessary, a fresh portion of the chelating agent is introduced in such an amount as to make up for the deficit through a line 12 to the mixer 9 and they are mixed there homogeneously to provide an acidic aqueous solution containing the substituted quinolinol (that is, the inverse capturing solution) and introduced through a line 13 to the inverse capturing step (ST- 2). In the aforementioned iron removal step (ST-4), the concentrated gallium solution is first introduced to a pH adjusting tank 14 and a 48 wt% aqueous solution of sodium hydroxide as a pH adjusting agent is introduced from a line 15 to the pH adjusting tank 14 and iron hydroxide precipitates out when the gallium solution and the pH adjusting agent are mixed homogeneously to a pH of 10 to 13. The slurry obtained in the pH adjusting tank 14 is sent to a thickener 16, the solid is separated from the liquid in the thickener 16, and the iron-free gallium solution is taken out from a line 17 and supplied to the following ultrafiltration step (ST-5) while iron hydroxide is taken out from a line 18. By means of this iron removal operation, the concentration of iron ions in the concentrated gallium solution is normally reduced to 0.2 ppm or less. In the ultrafiltration step (ST-5), the iron-free gallium solution is first introduced to a neutralization tank 19, an aqueous solution of hydrochloric acid is supplied to the tank from a line 20 to neutralize the gallium solution to pH 7 thereby forming gallium hydroxide, and the gallium hydroxide slurry formed in the neutralization tank 19 is sent to an ultrafiltration apparatus 21 comprising a rotary disk membrane module, the concentration of gallium is increased 30 to 70 times in the ultrafiltration apparatus, and the concentrated gallium hydroxide slurry is taken out from a line 22 and supplied to the following redissolution step (ST-6) while a practically neutral waste liquor is taken out form a line 23. In the redissolution step, a 48 wt% aqueous solution of sodium hydroxide is added from a line 24 to the concentrated slurry to dissolve the solid (gallium hydroxide) in the slurry to prepare an alkaline electrolytic solution of gallium whose gallium concentration is generally 35 to 45 g/L, this electrolytic solution is introduced to the following electrolysis step (ST-7) and electrolyzed, and gallium metal is taken out from a line 25 while the aqueous solution of sodium hydroxide is taken out from a line 26 and introduced to a waste liquor disposal tank 28. In the aforementioned inverse capturing step (ST-2), a cleaning liquor consisting of a dilute aqueous solution of hydrochloric acid with an acid concentration of 0.04 to 1.8 wt % is introduced to the step and the waste liquor after the cleaning operation is taken out from a line 29, introduced to the aforementioned waste liquor disposal tank 28, and neutralized by the aforementioned aqueous solution of sodium hydroxide introduced from the line 26. The neutral waste liquor formed by the neutralization treatment in the waste liquor disposal tank 28 is taken out from a line 30. The operations performed according to the flow sheet shown in Fig. 1 are described in an example and a comparative example below. [Example l] The process of this invention was reduced to practice in the mode shown in Figure 1 and the flow rate, concentration of gallium, concentration of iron ions, amount of gallium treated, and amount of iron treated are shown in Table 1 for each of the Bayer liquor introduced to the capturing step (ST-l), the inverse capturing solution containing gallium taken out from the inverse capturing step (ST-2), the concentrated gallium solution taken out from the electrodialysis step (ST-3), the iron-free gallium solution taken out from the iron removal step (ST-4), the gallium hydroxide slurry taken out from the neutralization tank 19 in the ultrafiltration step (ST-5), the concentrated slurry taken out from the ultrafiltration apparatus 21 in the ultrafiltration step (ST-5), and the alkaline electrolytic solution of gallium obtained in the redissolution step (ST-6). In the electrolysis step (ST-7), a two-chamber electrolytic cell composed of one stainless steel plate (SUS316) as cathode interposed between two nickel plates as anode at an inter-electrode distance of 5 cm was used and the electrolysis was performed by applying a constant current with a current density of 0.13 A/cm2 for a total of 300 A while keeping the voltage at 4.2 V initially and at 3.6 V at 50 °C and the cell temperature at 50 °C during the electrolysis and the rate of recovery of gallium (Ga recovery rate), the average current efficiency, and the amount of neutral salt (NaCl) formed in the waste liquor disposal tank 28 were determined. The results are shown in Table 1. [Comparative Example l] The inverse capturing solution containing gallium obtained in the inverse capturing step was neutralized to pH 7 to form a gallium hydroxide slurry, the slurry was concentrated by evaporation in a wetted wall evaporator, a concentrated aqueous solution of sodium hydroxide was added to the concentrated slurry to prepare an alkaline electrolytic solution of gallium, and the alkaline electrolytic solution of gallium was filtered through a 0.3 pm membrane filter and then electrolyzed as in the aforementioned Example 1 and, likewise, the rate of recovery of gallium (Ga recovery rate), the average current efficiency, and the amount of neutral salt (NaCl) formed were determined. The results are shown in Table 2. [Table 2] Industrial Applicability The process of this invention makes it possible to reuse the acid without generating waste acid in large amounts, raise the current efficiency during electrolysis, reduce the amount of waste liquor, and minimize the formation of spongy gallium thereby markedly raising the rate of recovery of gallium and is extremely useful for commercial recovery of gallium from the Bayer liquor. What is claimed is: 1. In a process for recovering gallium which comprises bringing the Bayer liquor into contact with a chelating agent composed of a water-insoluble substituted quinolinol to let the chelating agent capture gallium contained in the Bayer liquor, bringing an inverse capturing solution composed of an acidic aqueous solution containing a substituted quinolinol into contact with the chelating agent containing gallium to extract gallium into the inverse capturing solution, and recovering gallium metal from the inverse capturing solution containing gallium, a process for recovering gallium which comprises the following steps: an electrodialysis step where the aforementioned inverse capturing solution containing gallium is electrodialyzed to concentrate gallium and recover the acid; an iron removal step where the concentrated gallium solution obtained in the electrodialysis step is adjusted to a given pH and a precipitate of iron hydroxide is separated and removed to obtain an iron-free gallium solution; an ultrafiltration step where the iron-free gallium solution is neutralized to form a gallium hydroxide slurry and the slurry is subjected to ultrafiltration to give a concentrated slurry of gallium hydroxide; a redissolution step where the concentrated slurry of gallium hydroxide is dissolved in an alkaline solution to form an alkaline electrolytic solution of gallium! and an electrolysis step where the alkaline electrolytic solution of gallium is electrolyzed to recover gallium metal. 2. A process for recovering gallium as described in claim 1 wherein the inverse capturing solution is an aqueous solution of hydrochloric acid and the electrodialysis of the inverse capturing solution containing gallium in the electrodialysis step is performed until the concentration of hydrochloric acid in the inverse capturing solution becomes 0.20 N or less. 3. A process for recovering gallium as described in claim 1 or 2 wherein the concentrated gallium solution shows a pH of 10 to 13 after the pH adjustment in the iron remval step. 4. A process for recovering gallium as described in any one of claims 1 to 3 wherein the concentration of iron ions in the iron-free gallium solution is reduced to 0.2 ppm or less in the iron removal step. 5. A process for recovering gallium as described in any one of claims 1 to 4 wherein the concentration of gallium in the gallium hydroxide slurry is increased 30 to 70 times in the utrafiltration step.

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