Gallium and/or indium separation and concentration method

It is possible to recover gallium and indium efficiently and at a low cost from solutions containing traces of gallium and indium. In particular, jarosite is produced by performing a specific treatment on a solution obtained by a two-stage neutralization treatment during the zinc leached residue treatment step of wet zinc refining, or on another solution containing traces of gallium and indium; the gallium and indium are separated and concentrated; an alkali is added to the jarosite; and the gallium is separated and concentrated by leaching. Calcium hydroxide or magnesium hydroxide is optionally added to the jarosite leached solution to perform purifying, sulfuric acid is added to the purified solution, neutralization is performed, basic gallium sulfate is precipitated, the precipitate is subjected to alkali leaching, and the gallium in the leached solution is electrolytically extracted, yielding metallic gallium.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a method for separating and concentrating
 gallium or indium from gallium- and indium-containing solutions.
 2. Description of the Related Art
 Gallium, which is a metal element obtained in trace amounts as a byproduct
 of zinc or aluminum smelting, is widely used in compound semiconductors.
 In the field of compound semiconductors, high purity gallium purified to
 6N (99.9999%) or higher is used in the production of GaAs, and GaP, which
 are, in turn, used for light-emitting diodes, ICs, LSIs, and the like.
 Similar to gallium, indium is a metal element obtained in trace amounts as
 a byproduct of zinc or aluminum smelting and mostly used as ITO to form
 transparent electrode films for liquid crystals.
 In conventional practice, ion exchange, solvent extraction, and other
 techniques are used to selectively separate gallium and indium from
 solutions containing traces of gallium and indium, and to concentrate
 these elements. For example, the method disclosed in Japanese Unexamined
 Patent Application (Kokai) 59-193230 is known as such an ion-exchange
 technique. According to this technique, a solution containing traces of
 gallium and indium is passed through a layer of chelating ion-exchange
 resin under an appropriate pH, the gallium and indium are selectively
 adsorbed, and these elements are then eluted using a mineral acid.
 The following method is also well known as solvent extraction technique: a
 carboxylic acid-based or phosphoric acid-based chelate extraction chemical
 is added to an organic solvent, the pH of the aqueous phase is adjusted,
 and the product is brought into close contact with the aforementioned
 organic solvent, whereby the gallium and indium in the aqueous phase are
 selectively extracted as chelates into the organic phase.
 The above-described ion-exchange technique, however, requires resin columns
 and other bulky equipment, irrespective of the recovery volume of gallium
 and indium. This technique is also disadvantageous in that when large
 amounts of iron, aluminum, and other impurities are present, failure to
 remove them in advance will lower the removal efficiency of the resin,
 block the resin column, and the like.
 Solvent extraction is disadvantageous in that large amounts of organic
 chelating agents and organic solvents are needed for the reactions, so
 high running costs are incurred and explosion-proof equipment must be used
 because of safety considerations, resulting in much higher costs in terms
 of initial investment.
 Thus, all these conventional methods are difficult to integrate into future
 industries in terms of cost, and recovery of trace amounts of gallium and
 indium at minimal cost is desired.
 SUMMARY OF THE INVENTION
 It is an object of the present invention to make it possible to recover
 gallium and indium efficiently and at a low cost from solutions containing
 traces of gallium and indium.
 A distinctive feature of the present invention is that, in order to attain
 the stated object, gallium-containing jarosite is formed from a solution
 containing at least gallium, and the gallium is separated from other
 components and concentrated by dividing the jarosite into solid and liquid
 fractions.
 Another distinctive feature of the present invention is that
 gallium-containing jarosite is formed from a solution containing at least
 gallium, and the gallium is separated from other components and
 concentrated by adding an alkali to the jarosite and leaching the
 material.
 Yet another distinctive feature of the present invention is that gallium-
 and indium-containing jarosite is formed from a gallium- and
 indium-containing solution, and the gallium and indium are separated from
 other components and concentrated by dividing the jarosite into solid and
 liquid fractions.
 Still another distinctive feature of the present invention is that gallium
 and indium are separated from other components and concentrated by means
 of the following steps:
 a first step, in which one or more of iron(III) ions, sulfate ions, and
 monovalent cations are optionally added to a solution containing traces of
 gallium and indium to form a solution containing iron(III) ions, sulfate
 ions, and monovalent cations; and a mineral acid or an alkali reagent is
 optionally added to this solution to adjust the pH to 2-4;
 a second step, in which the temperature of the solution obtained in the
 first step is raised to 70-100.degree. C. under vigorous agitation, the
 system is allowed to react for 10 to 24 hours to give jarosite, and the
 gallium and indium are coprecipitated with jarosite particles; and
 a third step, in which the reaction product obtained in the second step is
 divided into solid and liquid fractions, and the gallium- and
 indium-containing jarosite is recovered.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 FIG. 1 is a flowchart depicting the overall progress of a method for
 separating and concentrating gallium and indium in a solution in
 accordance with an embodiment of the present invention. Following is a
 description, given with reference to FIG. 1, of the manner in which
 gallium and indium in a solution are separated and concentrated in
 accordance with an embodiment of the present invention. This embodiment is
 described with reference to an example in which the solution obtained
 during the zinc leached residue treatment step of hydrometal logic zinc
 refining is used as a solution containing traces of gallium and indium.
 The method of this embodiment comprises (1) a first step for adjusting the
 solution containing traces of gallium and indium to a pH of 2 to 4, (2) a
 second step for reacting the solution obtained in the first step to
 precipitate gallium and indium together with jarosite particles, and (3) a
 third step for separating as a solid the reaction product obtained in the
 second step, and recovering the gallium- and indium-containing jarosite.
 As used herein, the term "jarosite" refers to a substance expressed by the
 chemical formula
EQU M--Fe.sub.3 (SO.sub.4).sub.2 (OH).sub.6
 (where M is a monovalent cation).
 (1) First Step (pH Adjustment Step)
 In this step, an alkali agent or a mineral acid is added to the solution
 containing traces of gallium and indium to adjust its pH to 2-4. Here, the
 solution containing traces of gallium and indium is, above mentioned, the
 solution obtained during a zinc leached residue treatment step of the
 hydrometal lugic zinc refining.
 Iron(III) ions, sulfate ions, and monovalent cations are commonly added to
 the solution containing gallium, indium. These components are important
 structural elements of jarosite. The present invention involves forming
 jarosite, which is an iron oxide, and coprecipitating jarosite particles
 with gallium and indium. It is common knowledge that when an iron(III) ion
 precipitate is deposited from a weakly acidic solution, the gallium and
 indium ions present in trace amounts in the solution are captured by the
 precipitate and separated from the solution. The present invention is
 based on the fact that gallium and indium can be selectively precipitated
 and adequately separated/concentrated through the use of jarosite as the
 iron(III) ion precipitate.
 Iron(III) ions, sulfate ions, and monovalent cations (Na.sup.+, K.sup.+,
 NH.sub.4.sup.+, and the like), which are structural elements of jarosite,
 must first be added to the solution in an amount equal to or greater than
 a specific proportion. These must therefore be replenished as needed to
 maintain their content at a specific level. The content of iron(III) ions
 in the solution should preferably be 0.2-5 g/L. If the content is less
 than 0.2 g/L, the collection efficiency of the gallium and indium ions
 present in trace amounts in the solution falls below 60% for gallium ions,
 and if the content exceeds 5 g/L, the effect remains the same, and an
 incommensurate increase in costs results.
 The sulfate ion content, which depends on the iron(III) content, should be
 0.2 g/L or higher. The content of monovalent cations should be 0.01 to 0.1
 mol/L, which is 5 to 10 times the theoretical amount of the
 above-described chemical formula.
 The pH of the solution is important for forming a jarosite precipitate. The
 pH of the solution should preferably be 2 to 4. When the pH is greater
 than 4, elements other than gallium and indium ions (that is, aluminum,
 zinc, and other impurities) precipitate together with the iron
 precipitate, making it impossible to separate gallium and indium from
 these impurities. When pH is less than 2, a precipitate composed of iron
 alone forms, and gallium and indium cannot be coprecipitated.
 (2) Coprecipitation Step (Second Step)
 Jarosite is formed by heating and ripening. Specifically, the solution is
 heated to 70-100.degree. C. under vigorous agitation. The solution is then
 reacted and cured for 10 to 24 hours in this state. If the temperature is
 too low in this case, the jarosite does not form, iron(III) hydroxide is
 produced, and filterability is adversely affected. If the reaction time is
 too short, the rate of coprecipitation of gallium into the jarosite
 becomes inadequate.
 Jarosite can be produced in a shorter reaction time (2 to 6 hours) when
 separately prepared jarosite particles are added to the reaction layer in
 a pulp concentration of 50-150 g/L. These separately prepared jarosite
 particles are added solely at the start of treatment. After the treatment
 has been started and jarosite recovered in the third step, part of the
 jarosite recovered in the third step is added back. Such addition is
 repeated in order to set the gallium and indium concentration in the
 gallium- and indium-containing jarosite obtained in the third step to a
 level of 1-5% for each element.
 (3) Jarosite Recovery Step (Third Step)
 In the third step, the reaction product of the second step undergoes
 solid-liquid separation in a thickener or the like, the resulting solution
 is discharged, and the gallium- and indium-containing jarosite is
 recovered. With the exception of the portion recirculated to the second
 step in the manner described above, the recovered jarosite is fed to an
 alkali leaching step or SO.sub.2 reductive leaching step.
 Following is a description of specific examples in which gallium and indium
 were separated and concentrated by the above-described method for
 separating and concentrating gallium and indium.
 EXAMPLE 1
 A solution obtained by leaching gypsum (produced by the zinc leached
 residue treatment step of zinc refining) and removing most of the indium
 in advance was used as the solution containing traces of gallium and
 indium. The primary components were gallium (100 mg/L) and indium (100
 mg/L), and 30-g/L zinc and 15-g/L aluminum were contained as impurities.
 The solution was acidic due to sulfuric acid, so no sulfate ions were
 added, K.sup.+ (monovalent cations) were added in an amount of 2.5 g/L
 (0.06 mol/L), iron(III) ions were added in two amounts (0.2 g/L and 4.0
 g/L), and each solution was introduced into a stirred reaction tank.
 The pH of the aforementioned two solutions was adjusted to 3.0 with calcium
 carbonate, the solutions were vigorously agitated, the temperature was
 raised to 90.degree. C., and a reaction was conducted in this state for 24
 hours. The reaction product was filtered and precipitated; the gallium,
 indium, and impurities (aluminum and zinc) in the filtrate were
 quantified, and the precipitation ratio of each was determined. The
 results are shown in Table 1 (FIG. 2).
 EXAMPLE 2
 The same solution as in Example 1 was used, the concentration of iron(III)
 ions was set to 2 g/L, the concentration of monovalent cations (K.sup.+)
 was set to 0.3 g/L (0.008 mol/L) or 3.0 g/L (0.08 mol/L), and the same
 procedures as in Example 1 were performed in each case. The corresponding
 precipitation ratios are shown in Table 2 (FIG. 2).
 EXAMPLE 3
 Iron(III) ions and K.sup.+ were added to the same solution as in Example 1
 in concentrations of 0.5 g/L and 0.7 g/L (0.018 mol/L), respectively, and
 the solution was introduced into a stirred reaction tank. The pH of the
 solution was adjusted to 3.0 with calcium carbonate, separately prepared
 jarosite particles were added in a pulp concentration of 102 g/L, the
 temperature was raised to 90.degree. C., and a reaction was conducted in
 this state for 4 hours. The reaction product was filtered and
 precipitated; the gallium, indium, and impurities (aluminum and zinc) in
 the filtrate were quantified, and the precipitation ratio of each was
 determined. The results are shown in Table 3 (FIG. 2).
 Following is an example of a gallium separation and concentration method
 pertaining to another embodiment of the present invention. Until a certain
 intermediate step, the gallium separation and concentration method
 pertaining to this embodiment is performed in exactly the same manner as
 the gallium/indium separation and concentration method pertaining to the
 previously described embodiment. In more-simple terms, this separation and
 concentration method first involves forming jarosite from a
 gallium-containing solution. This jarosite is subsequently leached by the
 addition of an alkali. The previously described gallium/indium separation
 and concentration method is performed completely unchanged as this
 jarosite production method.
 FIG. 3 is a flowchart depicting the steps of the method for separating and
 concentrating gallium in accordance with this embodiment. The steps from
 "gallium-containing solution" to "jarosite production" in FIG. 3 are the
 same as the steps in the area enclosed within the dotted line in FIG. 1.
 The method for separating and concentrating gallium in accordance with
 this embodiment will now be described with reference to FIG. 3.
 The method of this embodiment comprises (1) a first step (jarosite
 production step) for producing jarosite from a gallium-containing solution
 and dividing the jarosite into solid and liquid fractions, (2) a second
 step (alkali leaching step) for leaching the jarosite by the addition of
 an alkali and removing iron hydroxide from the jarosite leached solution,
 (3) a third step (purification step) for purifying the jarosite leached
 solution by adding calcium hydroxide or magnesium hydroxide and removing
 the purified residue, (4) a fourth step (neutralization step) for adding
 sulfuric acid to the solution, performing neutralization, and
 precipitating basic gallium sulfate, (5) a fifth step (second alkali
 leaching step) for recovering the precipitate and performing alkali
 leaching, and (6) a sixth step (electrowinning step) for the
 electrowinning of gallium from the leached solution.
 (1) First Step (Jarosite Production Step)
 As noted above, this step extends from the "gallium-containing solution" to
 "jarosite production" in FIG. 3, contains a process for dividing the
 resulting jarosite into solid and liquid fractions, and is the same as the
 steps in the area enclosed within the dotted line in FIG. 1.
 (2) Second Step (Alkali Leaching Step)
 In this step, the jarosite produced in the above-described first step is
 leached by the addition of an alkali, and iron hydroxide is removed from
 the jarosite leached solution. Caustic soda or caustic alkali is used as
 the alkali.
 Specifically, caustic soda having a specific minimum concentration (150
 g/L) is added to the jarosite obtained in the first step, and the product
 is leached for 0.5 hour or longer at a temperature (60.degree. C. or
 higher) above a specific temperature. Because iron, which is a principal
 component of jarosite, forms iron hydroxide or another poorly soluble
 precipitate as a result of such leaching, iron and gallium can be readily
 separated by filtration.
 In this case, the indium present in the jarosite together with gallium does
 not dissolve in the solution at higher pH values (about 13), so the metals
 dissolved in the alkali leached solution are limited to gallium and
 jarosite impurities (aluminum, zinc, and the like).
 The leaching ratio of the gallium-containing jarosite is limited to about
 60% when reductive leaching rather than alkali leaching is performed using
 sulfuric acid, hydrochloric acid, sulfurous acid gas, or the like, so
 large amounts of iron are contained as impurities in the leached solution.
 FIG. 4 is a diagram depicting in tabular form results obtained by leaching
 a jarosite of a specific grade with sulfuric acid, hydrochloric acid
 (comparisons), and caustic soda (present invention) having specific
 concentrations. It can be seen in the figure that adding caustic soda with
 a concentration of 200 g/L to jarosite (Ga=1.51%, In=2.11%, Al=8.86%,
 Fe=10.34%, Zn=0.16%) and leaching it for 2 hours at 80.degree. C. leaches
 100% of gallium, 76.3% of aluminum, and 0.3% of zinc, but the leaching
 ratio of iron or indium is 0%. Iron and indium can thus be removed.
 (3) Third Step (Purifying Step)
 In this step, calcium hydroxide or magnesium hydroxide is added to the
 jarosite leached solution to precipitate out aluminum and zinc or
 germanium. Specifically, the leached solution obtained in the second step
 contains, in addition to gallium, a trace amount of zinc and a
 comparatively large amount of aluminum as impurities. Adding calcium
 hydroxide to this solution (pH: approximately 13) causes the aluminum and
 zinc in the solution to solidify and precipitate.
 Filtering the pulp after the addition of calcium hydroxide allows a
 solution containing only gallium to be obtained. Aluminum and zinc are
 removed at this stage for the following reasons. Specifically, the
 alkali-leached solution undergoes neutralization in the next step to
 concentrate gallium, and the presence of aluminum and zinc contaminants in
 the solution at this stage increases the amount of neutralization residue
 and drives up the cost of equipment and chemicals.
 If germanium is contained in the alkali leached solution, this germanium
 can be removed by the addition of magnesium hydroxide. Calcium hydroxide
 and magnesium hydroxide cannot form a poorly soluble precipitate with
 gallium, and can thus be used in the above-described method for purifying
 an alkaline gallium solution.
 FIG. 5 is a diagram depicting in tabular form results obtained by adding
 calcium hydroxide in a specific proportion to a jarosite leached solution
 having a specific composition. It can be seen from Tables 6 and 7 in FIG.
 5 that when calcium hydroxide is added in an amount of 60 g per liter of
 jarosite leached solution (Ga: 720 mg/L; In: 0.0 mg/L; Al: 5111 mg/L; Fe:
 0.0 mg/L; Zn: 209 mg/L), a reaction is allowed to occur for 2 hours at
 80.degree. C., and the product is divided into a solid and liquid
 fractions, the purified solution contains 720 mg/L Ga, 0.0 mg/L In, 750
 mg/L Al, 0.0 mg/L Fe, and 47 mg/L Zn. Aluminum and zinc can thus be
 efficiently removed.
 FIG. 6 is a table depicting the results of a study into the aluminum
 removal ratio and the temperature dependence of the reaction at varying
 amounts of calcium hydroxide added. Table 8 in FIG. 6 depicts the aluminum
 removal ratio at varying amounts of calcium hydroxide added, and Table 9
 depicts the temperature dependence of the reaction during the addition of
 calcium hydroxide. It can be seen in the tables that the greater the
 amount of calcium hydroxide added, the higher the aluminum removal ratio,
 and that this removal ratio is 80% or higher when calcium hydroxide is
 added in an amount greater than 60 g per liter of jarosite leached
 solution. It can also be seen that the temperature during the reaction
 must be 60.degree. C. or higher.
 FIG. 7 is a diagram depicting in tabular form the germanium removal effect
 at varying amounts of magnesium hydroxide added to the jarosite leached
 solution. The greater the amount in which magnesium hydroxide is added,
 the higher the germanium removal ratio. A marked removal effect is
 achieved when the amount in which magnesium hydroxide is added exceeds 20
 g per liter of jarosite leached solution.
 (4) Fourth Step (Neutralization Step)
 In this step, sulfuric acid is added to the solution purified in the
 aforementioned third step, neutralization is performed, and basic gallium
 sulfate is precipitated (a precipitate is formed). Here, dissolved gallium
 (Ga(OH).sub.4.sup.- ions) precipitates as gallium hydroxide (Ga(OH).sub.3)
 if sulfuric acid or the like is added to the dealuminized alkaline
 solution and the pH is gradually lowered. It should be noted that gallium
 hydroxide is a substance that has low crystallinity and high moisture
 content, and considerable time and much energy are therefore needed to
 filter this substance. In addition, the high moisture content of the
 resulting gallium hydroxide makes it impossible to raise the gallium
 content of the initial solution obtained by electrowinning during a
 subsequent step.
 During neutralization, the pH is reduced (to 2-3) below the gallium
 hydroxide precipitation region (pH=5-10), and the Ga(OH).sub.4.sup.- ions
 are converted to Ga.sup.3+ and dissolved on the acidic side. Heating the
 solution to 80-90.degree. C. causes highly crystalline basic gallium
 (KGa.sub.3 (OH).sub.6 (SO.sub.4).sub.2 and the like) to precipitate.
 Monovalent cations (K.sup.+, Na.sup.+, NH.sub.4.sup.+, and the like) and
 sulfate ions are needed for this reaction. Consequently, gallium sulfate
 is added prior to heating if these are absent from the initial solution.
 This operation can improve the filterability of the neutralization residue
 and lower the moisture content.
 FIG. 8 is a diagram depicting in tabular form the composition of the
 purified solution used as a starting solution for neutralization, and the
 composition of the residue obtained when neutralization is performed at
 varying pH values during neutralization. As is evident from Table 12 in
 FIG. 8, higher residue grades are obtained at a pH of 1-4 than at a pH of
 6-7.
 (5) Fifth Step (Second Alkali Leaching Step)
 In this step, the precipitate obtained in the fourth step is recovered and
 subjected to alkali leaching. FIG. 9 is a diagram depicting in tabular
 form the conditions of the second alkali leaching and the composition of
 the leached solution. As is evident from Table 13 in FIG. 9, the leaching
 process involves adding 150 g caustic soda per liter of the concentrated
 residue (pulp concentration: 200 g/L) obtained in the fourth step, setting
 the solution temperature to 80.degree. C., and performing leaching for 1
 hour. As a result, the leached solution contains 55 g/L Ga, 14 g/L Al, 450
 mg/L Zn, 2 mg/L Fe, and 2 mg/L In (as shown in Table 14), and the pH
 becomes 13 or higher. The solution thus obtained serves as a starting
 solution for the electrolysis in the next step.
 (6) Sixth Step (Electrowinning Step)
 In this step, gallium is electrolytically extracted from the solution
 obtained in the fifth step. During this electrowinning, the gallium
 content of the starting electrolyte solution is set to 50-150 g/L, the
 temperature of the electrolyte solution is kept at 50 to 60.degree. C.,
 and a current is passed between SUS electrode plates (anode and cathode)
 at a current density of 500 to 1000 A/m.sup.2. Metallic gallium is
 deposited on the cathode as a result, but because the melting point of
 gallium is 29.6.degree. C., it precipitates as liquid gallium on the
 bottom of the electrolytic cell.
 FIG. 10 is a diagram depicting in tabular form the electrolysis conditions
 and the composition of extracted metallic gallium in an example involving
 electrowinning. As is evident from Table 15 in FIG. 10, the electrolysis
 conditions correspond to a solution temperature of 50.degree. C., a
 current density of 500 A/m.sup.2, and a cell voltage of 3.7 V. As can be
 seen in Table 16, the resulting high-grade metallic gallium contains 1 ppm
 or less Al, 5 ppm or less Fe, 1 ppm or less Ni, 10 ppm or less Zn, 2 ppm
 Ge, 22 ppm In, 4 ppm Sn, and 1 ppm or less Pb.
 FIG. 11 is a diagram depicting in tabular form the recovery percentage for
 each of the steps comprising the gallium separation and concentration
 method performed in accordance with the above-described embodiment. The
 recovery percentage of each step is very high, and the total recovery is
 90.3%, providing exceptional value.