Ind. Eng. Chem. Res. 1995,34, 1799-1809
1799
Liquid Membrane Processes for Gallium Recovery from Alkaline Solutions Fu-Fang Zha, Anthony G. Fane,* and Christopher J. D. Fell UNESCO Centre for Membrane Science and Technology, University of New South Wales, Sydney 2052, Australia
In this paper, we examine the possibility of using membrane extraction and supported liquid membranes to recover gallium from alkaline solutions. Membrane extraction proves to be an alternative process to recover gallium from such liquors. In order to maximize mass transfer, highly hydrophilic membranes should be used in both the membrane extraction and stripping processes. The optimum composition of the membrane extractant is 10-15% Kelex 100, 10% n-decanol, 5%Versatic 10, and kerosene (~01%).The highest gallium permeability was obtained when the feed solution contained about 1.5 m o m sodium hydroxide. The supported liquid membrane used failed to transport gallium because of instability. The dominant mechanisms for failure are considered to be spontaneous formation of a water-in-oil emulsion and formation of precipitates, causing membrane pore obstruction.
Introduction Gallium is an important material in the semiconductor industry. Intermetallic compounds with gallium have applications as high-temperature rectifiers and transistors, solar batteries, and other devices where the photovoltaic effect can be used. Gallium also finds uses in superconductors and other areas (Wade and Banister, 1975; Wilder and Katrak, 1982). Because gallium-rich minerals containing over 0.5% gallium are so scarce, most gallium is recovered as a by-product from the processing of other minerals. Currently the world's most important source of gallium is from bauxite and other aluminum ores, such as alunite and nepheline. Some gallium is produced as a by-product from the processing of zinc and lead (Wilder and Katrak, 1982). There are two main processes for the leaching of aluminum ores (Grayson et al., 1979): the Bayer process, for ores of a high N S i ratio, and the sintering process, for ores with a low N S i ratio. Gallium is comparatively rich in the spent liquors from the Bayer process with a concentration ranging from 100 to 300 ppm. In addition, such solutions also contain 80-110 g/L A l 2 0 3 , and 150-240 g/L Nan0 (Leveque and Helgorsky, 1979; Pesic and Zhou, 1988). In the sintering process, the concentrations of gallium, aluminum, and sodium in the solutions are much lower than those in the Bayer process. A common characteristic of the liquors from both processes is the high alkalinity. Recovery of gallium from such liquors requires highly selective processes. The chelating agents which have high selectivity to gallium and low solubility in aqueous solutions are very limited. Kelex 100, whose active component is 7-(4-ethyl-1-methyloctyl)-8-quinolinol is the only known chelating agent for an industrial process of gallium recovery from strong alkaline solutions. Extraction of gallium with Kelex 100 has been studied by several researchers (Leveque and Helgorsky, 1979; Wang et al., 1986; Pesic and Zhou, 1988; Cote and Bauer, 1986; Sat0 and Oishi, 1986), and detailed techniques have been patented (Helgorskyand Leveque, 1976-1982; Wang et al., 1986; Decerle and Masson, 1988). Another source of gallium is from processing of zinc and lead ores. Gallium is usually recovered from the
* To whom correspondence should be addressed. 0888-588519512634-1799$09.00/0
acid leaching solutions by solvent extraction with ether or cupferron, followed by distillation of the organic extract. Sheka et al. (1966) reviewed different agents used in acidic solutions. Recently, Joyger and Kolarik (1993) reported the possibility of separating gallium from potentially accompanying elements by solvent extraction with tri-n-butyl phosphate in HC1 media. Very few references have been found up to now regarding the recovery of gallium by membrane processes. Some lab research work has been conducted by two research groups in Japan exploring the use of liquid membranes to recover gallium from acidic media. Shimidzu and Okushita (1986)tested the transport of gallium through supported liquid membranes (SLMs) with alkylcupferrons as the carrier, which were synthesized by themselves. However, the gallium flux was extremely low under normal conditions. Considerable research into the separation and recovery of gallium from acid leaching solutions has been carried out by Hozawa et al. (1990) investigating the possibility of utilizing both SLMs and emulsion liquid membranes (ELMs). The gallium system they studied is the acid leaching solution of gypsum which is a residue of zinc recovery from black ores. Both a high distribution coefficient and selectivity were obtained using isopropyl ether (IPE) as the extractant for hydrochloric acid concentrations higher than 6 mol&. However, the SLMs were found to be unstable due to the high solubility of IPE in water (Fujinawa et al., 1989). A comparison was then made for both SLMs and ELMs, employing 2-bromodecanoic acid (2BDA)as a carrier (Shono et al., 1989,1992). They found that the application of SLMs was difficult because of low gallium permeability, and the use of ELMs seemed promising. Based on these experimental results, they proposed a three-stage hydrometallurgical process for gallium and indium from such a leaching solution (Hozawa et al., 1990). The first stage employs an ELM with 2BDA as a carrier t o concentrate both gallium and indium. In the second stage, gallium is separated from indium by an SLM impregnated with IPE. The third stage is the separation of indium by an SLM with tributyl phosphate as a carrier. All the reported systems in the membrane processing of gallium have focused on acid leaching solutions. The alkaline liquors from the processing of aluminum ores are actually better sources for gallium because of the
0 1995 American Chemical Society
1800 Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995
Table 1. Properties of Membranes Used membrane Durapore GVHP Accurel 2E-PP K-100 Durapore GVWP Celgard-3501
material PVDF PP PTFE PVDF PP
philicity hydrophobic hydrophobic hydrophobic hydrophilic hydrophilic
higher concentration of gallium contained. However, no references have been found up to now regarding the recovery of gallium from such solutions by membrane processes. Although solvent extraction is currently used to recover gallium from such solutions, loss of agents is serious in the process, especially in the presence of modifiers in the extractant. In this paper we examine the possibility of using SLMs and membrane extraction to recover gallium from alkaline solutions. Membrane extraction is able t o recover gallium from such solutions. The factors that affect gallium transfer performance are investigated, including the selection of the membrane phase and the stripping agent, the effect of substrate properties, and the feed alkalinity. The SLM used, however, failed to transport gallium because of its instability. The mechanisms were found t o be spontaneous formation of a water-in-oil emulsion and formation of precipitates which caused probable pore obstruction.
Experimental Section The experiments for both membrane extraction and SLMs were carried out in stirred cells described elsewhere (Zha et al., 1994). A membrane was sandwiched between two cylindrical perspex cells 36 mm in diameter and 76 mm long. The effective volume of each chamber was 74 mL, and the contact area of the membrane with solutions was 10.2cm2. The solutions in both cells were stirred at 330 rpm, and the apparatus was in a constant temperature room a t 25 & 1 "C. To perform a membrane extraction process, the membrane substrate was first soaked with the impregnating liquid. The organic and aqueous solutions were placed on either side of the membrane. The metal concentrations of the solution in each cell were measured at definite time intervals by removing known volumes with a pipette for analysis and replacing them with fresh solutions. In the gallium systems, the concentrations of sodium and aluminum were hundreds of times higher than that of gallium. To eliminate the influence of such concentration differences, the transport behavior was evaluated in terms of permeability. The permeability was calculated from the concentration versus time curve after correction for sampling as in the following:
p = - - -V dC AEC dt where V and C are the volume and the concentration of the feed solution and A and 6 represent the membrane area and porosity. An SLM was prepared by impregnating the microporous substrate in the membrane liquid for at least half an hour to ensure a complete wetting of pores. The SLM was then clamped between the two cells. The feed and stripping solutions were filled respectively in the two chambers. The organic membrane phase was selected according t o the results of conventional solvent extraction. Equal
thickness (mm) 110 130-170 PP backing 110 25
pore size (mm) 0.22 0.2 0.02 0.22 0.075 x 0.2
porosity (%) 75 75 NA 70 45
volumes (5 mL each) of organic and aqueous solutions, placed in glass tubes, were shaken on a mechanical shaker (GFL 3016)for the required time at 25 "C. The frequency of the shaker was chosen to give a complete mix of the solutions. After the separation of the two phases, the concentration of metal ions in both phases was analyzed. Membranes. The membranes used in the experiments and their characteristic parameters are listed in Table 1. The first three membranes are hydrophobic with different materials and pore sizes. The last two are hydrophilic membranes according to the manufacturer's specification. The Durapore series of membranes are products of Millipore Corp. The Accurel membranes were supplied by Enka AG in Germany and the Celgard membranes by Hoechst Celanese in the USA. The K-100 membrane was obtained from Separation Systems Pty Ltd. in Australia. Chemicals. Kelex 100 (abbreviated as HL) was supplied by Schering Australia. The concentration of the active component, 7-(4-ethyl-l-methylocty1)-8-quinolinol, was determined as 96% according to the copper loading test of Flett et al. (1975).The commercial Kelex 100 was used without further purification. Versatic 10 (a kind of carboxylic acid used as a modifier) is a product of Shell Chemicals. Anhydrous n-decanol was purchased from Sigma and kerosene from Aldrich Chemicals for laboratory use. Metal gallium with a purity of 99.9999% was first dissolved in hydrochloric-nitric acids and then diluted to the desired concentration. Other chemicals used were pure analytical grade. Preparation of Synthetic Bayer Spent Liquor. Two types of gallium feed solution were used in the experiments. The simple solution was gallium in NaOH-NaC1 buffer solution. The other one was a synthetic Bayer spent liquor (SBSL). The composition of the SBSL was based on that in the spent liquors from the Bayer process and samples from the tertiary tank overflow (TTOF) in a plant of Queensland Alumina Pty Ltd. in Australia. The concentration of metals in TTOF solutions were analyzed with an atomic absorption (AA) spectrophotometer, and the OH- concentration was determined by titration with a standard HC1 solution. The following average compositions were obtained: 0.195 g/L Ga, 35.5g/LAl (67g/L as A l 2 0 3 ) , 132 g/L Na (304 g/L as NazC031, 4.0 mol& OH-. The target composition of the SBSL was set as 170 g/L caustic soda (225g/L if expressed as NazC03), 78 g/Laluminum oxide (as AlzO3, AlzOdNazC03 = 0.35),250 g/Ltotal soda (as Na2C03), 0.20g/T., gallium. To prepare a 2 L stock solution of SBSL, 50 g of Nazcos was dissolved in ca. 1.2 L of hot water. When dissolved, it was cooled to room temperature. Then 340 g of NaOH pellets was slowly added to the solution with stirring. The temperature rose quickly. Into the solution 241 g of Al(OH)3 solid was added in one lot. The solution was then heated to near the boiling point with stirring until all the aluminum hydrate was dissolved. The final hot solution was filtered through a filter paper (Whatman) under vacuum produced by a water ejector.
Ind. Eng. Chem. Res., Vol. 34,No. 5, 1995 1801 Membrane phase
Feed
may behave as a tertiary amine to react with an acid (e.g., HC1) forming an organ0 complex: HL H+ C1- == H2LC1 (6)
Strip
+ +
G
H+ Mass Transfer in SLMs. Figure 1 illustrates the coupled transport of gallium tetrahydroxide ions through an SLM. If the reaction resistance at both interfaces can be neglected, assuming fast chemical reactions, and the diffusion of the chelating agent is much faster than the metal complex in the membrane phase, the permeation rate of the metal ion can be expressed as
- CS
-,X JA
Figure 1. Coupled transport process in SLM.
The filtrate was then transferred into a 2 L volumetric flask. 29.4 mL of gallium stock solution ([Gal = 13.62 g/L) was transferred into the solution which was made up to 2 L by adding water. The final concentration of each component was determined analytically. Analytical Methods. Metal ion concentrations in aqueous solutions were analyzed with an atomic absorption spectrophotometer (Varian AA-275). Metal complexes in organic solutions were analyzed after stripping with 1.5-2.0 m o m HC1. However, when the concentration of sodium ion was 100 times higher than that of gallium, the interference in the AA analysis of gallium was very serious. In those cases, analysis was carried out with inductively coupling plasma (ICP) atomic emission spectrophotometry (Labtam Plasma Lab).
Fundamentals of Liquid Membrane Processes for Gallium Chemical Reactions in Processing of Gallium with Kelex 100. In basic sodium aluminum solutions, gallium exists mainly as tetrahydroxy species, Ga(OH)4-, though polynuclear aggregates and species with more than four coordinated hydroxide ions may appear (Wade and Banister, 1975). Extraction of gallium from such solutions involves the following predominant reactions (Sat0 and Oishi, 1986):
+ OH- + 3H20 A.l(OH),- + 3HL A1L, + OH- + 3H20 - Na' + OH- + HL NaL + H20
Ga(OH),-
+3
3 -c GaL,
(2)
(3) (4)
The top bars indicate the organic phase; the others are the aqueous phase. From the above equations, it can be seen that an increase in the OH- concentration in the liquors is unfavorable to chelate gallium but stimulates the reaction with Na+. The results obtained by Sat0 and Oishi (1986)indicate that a high concentration of OH- reduces both the distribution coefficient and the extraction rate of gallium with the chelating agent. The presence of high concentrationsof Na+ and Al(OH)4will compete with gallium to react with the chelating agent. Gallium can be released from its complex form by stripping with acids:
GaL,
-
+ 3H+ == 3HL, + Ga3+
(5)
Sodium and aluminum can also be easily stripped from their complexes by acids. Moreover, the free Kelex
= R(Cf -
1- 1K kf
3.¶)
1 +-+mPm
(7)
mS mPs
where Cf and Cs are the metal ion concentrations in the feed and strip, kf, ks, and km represent the individual mass transfer coefficients in the feed, strip, and membrane phase, respectively. A distribution coefficient is defined as the ratio of the concentration of the metal in the membrane phase over that in the aqueous phase. Therefore, the distribution coefficients a t the interfaces on the feed side and the strip side are, respectively,
Equations 7 and 8 only quantitatively describe the coupled transport through a perfect SLM. If the SLM experiences instability, loss of membrane liquid (ML) or blocking of the membrane pores would alter the permeation rate and, in some special cases, could completely stop the mass transfer (Zha et al., 1994). In the recovery of gallium from alkaline liquors, other factors need to be considered that may influence gallium transfer. First, the large amounts of aluminum and sodium present in the solutions will compete with gallium for transport through the SLM. The second factor is that the chelating agent selected can take up a certain amount of the stripping acid, as shown by eq 6. Therefore, a countertransport of the acid against the direction of gallium transport is possible. This countertransport may cause additional effects on the gallium permeation. These factors will be further discussed later. Mass Transfer in Membrane Extraction Processes. The membrane extraction technique divides an SLM into two stages: membrane extraction and membrane stripping. In the membrane extraction processes, an aqueous phase and an organic phase are separated by a solid membrane. The membrane acts as a barrier to prevent the dispersion of one phase into the other but allows the permeation of the solute. The chemical reactions take place a t the interface created in the pore mouths of the membrane. The extraction and stripping operations are usually conducted in two membrane modules (Prasad and Sirkar, 1987; Kim, 1984) though a single module design has been reported (Sengupta et al., 1988). The major advantages of this technique over conventional dispersion-based processes are (Prasad et al., 1991) (1)less extractant loss, owing to dispersion-
1802 Ind. Eng. Chem. Res., Vol. 34,No. 5, 1995 Table 2. Mass Transfer Resistance in Membrane Extraction Processes membrane process
membrane
total mass transfer resistance
membrane extraction
hydrophobic
1 - 1 + 1 + - 1 KME kf m&m m&org
simplification 1
ifmf >> 1
hydrophilic
membrane stripping
hydrophobic
hydrophilic
-=-+B+B 1 1 " KMS korg k~ ks
free operation; (2) modular design eases scale-up and retrofitting of equipment; (3) independent variation in phase flow rates, avoiding limitations of flooding, loading, and sweeping; (4) no need for density differences and the ability to handle solutions containing solid particles; ( 5 ) high contact area per unit equipment volume available when microporous hollow fibres are used. Compared to SLMs, membrane exaction operation can greatly reduce the instability problem if the pressures of the two phases are properly controlled. Moreover, it offers more flexible choices of membrane support. If a hydrophobic membrane is employed, only the organic phase can wet and impregnate the membrane pores. While in a hydrophilic membrane-based extraction, the aqueous solution should preferentially impregnate and occupy the membrane pores. Table 2 summarizes the mass transfer resistance incurred in different membrane-based extraction processes. It is a common situation that m f > 1 and m,
