Article Recovery of Germanium from Sulphate Solutions Containing Indium and Tin Using Cementation with Zinc Powder
Michał Drzazga * , Grzegorz Benke, Mateusz Ciszewski , Magdalena Knapik, Adrian Rado ´n, Sylwia Kozłowicz, Karolina Goc, Patrycja Kowalik and Katarzyna Leszczy ´nska-Sejda Łukasiewicz Research Network-Institute of Non-Ferrous Metals, ul. Sowi´nskiego5, 44-100 Gliwice, Poland; [email protected] (G.B.); [email protected] (M.C.); [email protected] (M.K.); [email protected] (A.R.); [email protected] (S.K.); [email protected] (K.G.); [email protected] (P.K.); [email protected] (K.L.-S.) * Correspondence: [email protected]; Tel.: +48-32-2380-277
Received: 14 March 2020; Accepted: 14 April 2020; Published: 17 April 2020
Abstract: Cementation of germanium from sulphate solution obtained after the leaching of GeIn dross using zinc dust was investigated. The composition of the examined solution was 5.15 Ge, 1.52 In, and 5.81 g/dm3 Zn. In order to resemble the solution before detinning, Sn concentration between 2–10 g/dm3 was also investigated. It was found that >99% of germanium may be precipitated from the solution. In order to achieve high selectivity, a detinned solution should be used because the precipitation yields of germanium and tin from the solution containing Sn were similar. For cementation with Zn powder at 75 ◦C for 2 h with a final pH of 2.0, over 99% of the germanium was removed from the solution, while the indium precipitation yield was 12%. The obtained cementate contained 50% Ge, mainly in elementary form.
Keywords: zinc metallurgy; germanium recovery; indium recovery; hydrometallurgy; cementation; zinc dust
1. Introduction Germanium is a rare metalloid which is important to several sectors of the global industry. It is applied in optical fibers, used for high-speed telecommunication, infrared optics for military applications, solar cells (mainly in artificial satellites), polymerization catalysts for the production of polyethylene terephthalate (PET), in plastic bottles and synthetic textiles, high brightness light emitting diodes (LEDs), transistors for high speed wireless telecommunication, γ-ray detectors, and phosphors [1]. Over 100 tons of refined germanium is produced each year, and over 60% of the world’s production comes from China [2]. Germanium does not occur in nature in the form of economically-mineable ores, but is recovered as a by-product. Its main sources are ashes generated after the combustion of Ge-rich coals found in China and far-east Russia [3,4] and residues of zinc metallurgy [4–9]. However, there is still huge room for the improvement of germanium recovery. It is estimated that no more than 3% of Ge contained in zinc concentrates is recovered worldwide [2,10]. Typical methods of germanium-bearing material processing to obtain suitable Ge-enriched products consist of its leaching in a mineral acid solution, mostly sulphate solution, followed by precipitation, usually with tannic acid [8,11–15]. However, this process is expensive, mainly due to the large amounts of tannin required for precipitation—at least 8–10 g per 1 g of dissolved Ge [15,16], or even >300 g [17], depending on the germanium concentration and chemical composition of solutions. A promising method of sulphate solution processing is also solvent extraction [18–20]. However, the production of LIX 63, one of the most promising agents, has been discontinued, according to
Minerals 2020, 10, 358; doi:10.3390/min10040358 www.mdpi.com/journal/minerals Minerals 2020, 10, 358 2 of 10 information obtained from its supplier. Therefore, it is necessary to investigate other methods of germanium recovery from sulphate solutions. In zinc, hydrometallurgy cementation processes are mainly used to purify leachates prior to zinc electrowinning operations. The application of zinc powder allows undesirable elements to be removed from solutions, including Ge [11]. However, the cementation technique using iron powder is also considered as a possible method for germanium recovery from sulphate solutions [21]. The objective of the research was to determine the parameters of Ge cementation using zinc dust from an industrial solution also containing zinc, indium, and tin. The investigated solution originated from the processing of zinc metallurgy by-products [5,14]. The influence of temperature, time, the amount of zinc powder added, and final pH on germanium recovery was examined. The reduction potentials of the investigated elements are presented in Table1. Based on the data, it is expected that germanium cementation with zinc is favoured over Sn and In.
Table 1. Standard reaction potentials of germanium, tin, indium, and zinc [22].
Half-Reaction Reduction Potential [V] Ge4+ + 4e Ge(s) 0.12 − → 2H+ + 2e H (g) 0.00 − → 2 Sn2+ + 2e Sn(s) 0.14 − → − In3+ + 3e In(s), 0.34 − → − Zn2+ + 2e Zn(s) 0.76 − → −
2. Materials and Methods The investigated detinned solution was obtained by leaching GeIn dross with sulphuric acid in accordance with the procedure described in the previous paper [14]. Its composition is presented in Table2. In each test, 500 dm 3 of the solution was poured into the flask, except in the last tests, in which 2.5 dm3 solution was investigated. In the first test series, the original solution was enriched 3 with tin to 6 g/dm to resemble leachate before detinning by adding 5.43 g SnSO4. In order to obtain other Sn concentrations, a proportional amount of tin(II) sulphate was added. Tin(II) sulphate (min. 95.5%, Alfa Aesar, Kandel, Germany) was used as the tin source. Cementation was done by adding the proper amount of zinc powder (superfine 3.0–4.5 µm, NorZinco, Goslar, Germany) to the solution while mixing (300 rpm) with a mechanical stirrer equipped with a propeller. Zinc was added in portions due to intensive gas formation. Tests were performed at initial temperatures of 60 ◦C for the solution containing tin, and at 75 ◦C for the detinned solution. It was found that at these temperatures, cementation reactions start to occur: gas formation is noticeable. During the addition of zinc dust, the temperature increased by up to 10 ◦C. The suspensions obtained after adding the set Zn powder amount were mixed for 2 h to allow for a complete reaction. In order to maintain the final pH at a set level (0.25, 0.5, 0.75, 1.0 and 2.0 were investigated), 30% H2SO4 solution obtained by the dissolution of concentrated sulphuric acid (98%, Avantor, Gliwice, Poland) in deionised water was added. The temperature was kept at its initial level. The obtained suspension was filtered using a Buchner funnel and washed with deionised water. The precipitates were dried for 10 h at 50 ◦C.
Table 2. Composition of detinned solution used in cementation tests.
Composition (g/dm3) pH Ge In Zn Sn 5.15 1.52 5.81 0.02 0.21
Filtrates and solids were analysed at the Łukasiewicz-IMN’s Department of Analytical Chemistry. The concentration of elements was measured using ICP MS (Inductively Coupled Plasma—Mass Spectroscopy; Nexion, PerkinElmer, Waltham, MA, USA). Filtrates were diluted and acidified using Minerals 2020, 10, 358 3 of 10
Minerals 2020, 10, x FOR PEER REVIEW 3 of 10 hydrochloric acid (35%, Avantor) prior to analysis. In the case of the solids, a 0.2 g portion of the material was first melted with 1 g Na O (p.a., Avantor) at 600 C for 30 min in a corundum crucible. into which 10 cm3 of 1:1 mixture of concentrated2 2 nitric (65%, Avantor)◦ and hydrofluoric (40%, VWR, The crucible containing the resulting alloy was placed in a polypropylene vessel into which 10 cm3 of Lutterworth, UK) acid was added. The vessel was heated to 110 °C, until the alloy was completely 1:1 mixture of concentrated nitric (65%, Avantor) and hydrofluoric (40%, VWR, Lutterworth, UK) acid dissolved. The crucible was washed with water, and the resulting solution was filled with water to was added. The vessel was heated to 110 C, until the alloy was completely dissolved. The crucible 100 cm3 and analysed. ◦ was washed with water, and the resulting solution was filled with water to 100 cm3 and analysed. X‐ray diffraction (XRD) patterns were collected using an X‐ray diffractometer Rigaku MiniFlex X-ray diffraction (XRD) patterns were collected using an X-ray diffractometer Rigaku MiniFlex 600 (Rigaku, Tokyo, Japan) with a copper tube Cu Kα (λ = 0.15406 nm), a tube voltage of 40 kV, and 600 (Rigaku, Tokyo, Japan) with a copper tube Cu Kα (λ = 0.15406 nm), a tube voltage of 40 kV, and a a current of 15 mA, using a D/teX Ultra silicon strip detector. current of 15 mA, using a D/teX Ultra silicon strip detector. 3. Results 3. Results
3.1. Ge Ge Cementation Cementation from from Solution Solution Containing Containing Tin InIn the the first first step, step, germanium germanium cementation cementation from from the the solution solution containing containing tin was tin investigated. was investigated. This solutionThis solution resembled resembled the solution the solution obtained obtained after after leaching leaching GeIn GeIn dross, dross, which which was was described described in in the previous paper [14]. [14]. The The solution solution was was enriched enriched in in Sn using solid tin(II) sulphate directly before the tests.tests. This This was was due due to to the the instability instability of of the the solution solution containing containing Sn Sn caused caused by by the the oxidation oxidation of Sn(II) to to Sn(IV) and precipitation of tin(IV) oxide. During the firstfirst test,test, the the amount amount of of zinc zinc dust dust required required for for the the precipitation precipitation of germanium, of germanium, tin, andtin, and indium was determined. Tests were done at 60 °C for 2 h. The initial tin concentration was 63 indium was determined. Tests were done at 60 ◦C for 2 h. The initial tin concentration was 6 g/dm . 3 g/dmPrecipitation. Precipitation yields ofyields the investigated of the investigated elements elements are presented are presented in Figure in1 Figure. 1.
Figure 1. InfluenceInfluence of of Zn Zn dust dust amount amount on on precipitation precipitation yields yields of of Ge, Ge, In, In, and and Sn, Sn, and and final final pH pH (60 (60 °C,◦C, 3 2 h, 6 g/dm g/dm3 SnSn in in initial initial solution). solution).
The precipitation yields of germanium and tin increased as the amount of zinc dust added was increased.increased. Moreover, thethe levelslevels of of precipitated precipitated Sn Sn and and Ge Ge were were comparable comparable for for diff differenterent masses masses of the of reducing agent. Only for the smallest investigated amount of Zn added (5 g per 1 dm3 solution) was the reducing agent. Only for the smallest investigated amount of Zn added (5 g per 1 dm3 solution) wasthe Sn the precipitation Sn precipitation yield significantly yield significantly larger than larger that than of Ge that (29% of vs. Ge 1%). (29% For vs. the 1%). highest For investigatedthe highest amount of zinc dust (70 g dm3), the complete precipitation of all investigated elements was achieved. investigated amount of zinc/ dust (70 g/dm3), the complete precipitation of all investigated elements was achieved.It was observed that the complete separation of germanium could not be achieved when the finalIt pH was of observed the cementation that the wascomplete not kept separation below 2.0.of germanium Therefore, could in the not next be experimentalachieved when series, the finalthe investigation pH of the cementation of the final was pH not on Ge,kept In, below and Sn2.0. precipitation Therefore, in yields the next was experimental examined. Testsseries, were the 3 investigationperformed at 60of ◦theC for final 2 h, andpH 100on gGe,/dm In,of and zinc Sn dust precipitation was added. Ayields final cementationwas examined. pH inTests the rangewere performed at 60 °C for 2 h, and 100 g/dm3 of zinc dust was added. A final cementation pH in the range 0.25–2.0 was maintained by adding sulphuric acid. The influence of the final pH on the precipitation yield is presented in Figure 2.
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0.25–2.0 was maintained by adding sulphuric acid. The influence of the final pH on the precipitation Minerals 2020, 10, x FOR PEER REVIEW 4 of 10 yieldMinerals is presented 2020, 10, x inFOR Figure PEER 2REVIEW. 4 of 10
Figure 2. Influence of final pH on precipitation yields of Ge, In, and Sn (60 °C, 2 h, 100 g/dm33 Zn used, FigureFigure 2. Influence2. Influence of finalof final pH pH on on precipitation precipitation yields yields of Ge,of Ge, In, In, and and Sn Sn (60 (60◦C, °C, 2 h, 2 100h, 100 g/dm g/dmZn3 Zn used, used, 6 g/dm3 Sn in initial solution). 6 g6/dm g/dmSn3 Sn in in initial initial solution). solution). It was observed that for the investigated dose of zinc powder, almost complete precipitation of It wasIt was observed observed that that for for the the investigated investigated dose dose of of zinc zinc powder, powder, almost almost complete complete precipitation precipitation of of germanium (>96%) was achieved for all of the investigated final pH values. Tin precipitation yields germaniumgermanium (> 96%)(>96%) was was achieved achieved for for all all of of the the investigated investigated final final pH pH values. values. Tin Tin precipitation precipitation yields yields increased with higher final pH values—from 79% at 0.25 to 93% at 2.0. Indium precipitation yields increasedincreased with with higher higher final final pH pH values—from values—from 79% 79% at 0.25at 0.25 to 93%to 93% at 2.0.at 2.0. Indium Indium precipitation precipitation yields yields were below 25%, but varied in the final pH. The most In (20%–22%) precipitated at 0.5–0.75. No werewere below below 25%, 25%, but variedbut varied in the in final the pH. final The pH. most The In most (20%–22%) In (20%–22%) precipitated precipitated at 0.5–0.75. at No0.5–0.75. indium No indium cementation was observed for pH 0.25. cementationindium cementation was observed was for observed pH 0.25. for pH 0.25. Due to the observed co‐cementation of tin and germanium in the first tests, in the third test, the DueDue to to the the observed observed co-cementation co‐cementation of of tin tin and and germaniumgermanium in the first first tests, tests, in in the the third third test, test, the influence of tin concentration on Ge and In precipitation yields was investigated. This was done by theinfluence influence of of tin tin concentration concentration on Ge andand InIn precipitationprecipitation yields yields was was investigated. investigated. This This was was done done by by adding different amounts of the tin source to the investigated solution. All tests were done at 60 °C addingadding diff differenterent amounts amounts of the of the tin sourcetin source to the to investigatedthe investigated solution. solution. All tests All weretests were done done at 60 ◦atC 60 for °C for 2 h. In all tests, 50 g/dm3 3 Zn dust was added. The results of the tests are shown in Figure 3. 2 h.for In 2 all h. tests,In all 50tests, g/dm 50 g/dmZn dust3 Zn wasdust added. was added. The results The results of the of tests the aretests shown are shown in Figure in Figure3. 3.
FigureFigure 3. InfluenceInfluence ofof SnSn concentration concentration in in the the solution solution on on precipitation precipitation yields yields of Ge, of Ge, In, and In, and Sn, and Sn, finaland Figure 3. Influence of3 Sn concentration in the solution on precipitation yields of Ge, In, and Sn, and finalpH (60 pH◦ C,(60 2 °C, h, 50 2 h, g/ dm50 g/dmZn3 used). Zn used). final pH (60 °C, 2 h, 50 g/dm3 Zn used). It was observed that the increase of tin concentration in the initial solution improved the It was observed that the increase of tin concentration in the initial solution improved the germanium precipitation yield. For the solution without tin, the Ge precipitation yield was 39%, while germanium precipitation yield. For the solution without tin, the Ge precipitation yield was 39%, while for the one with 10 g/dm3 Sn, it was 88%. The final pH of the solution after cementation was lower for for the one with 10 g/dm3 Sn, it was 88%. The final pH of the solution after cementation was lower for higher tin concentrations—1.28 for the initial solution without tin and 0.93 when the initial Sn higher tin concentrations—1.28 for the initial solution without tin and 0.93 when the initial Sn concentration was 10 g/dm3. The precipitation yield of tin also increased for its higher initial amounts. concentration was 10 g/dm3. The precipitation yield of tin also increased for its higher initial amounts.
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It was observed that the increase of tin concentration in the initial solution improved the germanium precipitation yield. For the solution without tin, the Ge precipitation yield was 39%, while for the one with 10 g/dm3 Sn, it was 88%. The final pH of the solution after cementation was lower for higher tin Mineralsconcentrations—1.28 2020, 10, x FOR PEER for theREVIEW initial solution without tin and 0.93 when the initial Sn concentration5 of was 10 10 g/dm3. The precipitation yield of tin also increased for its higher initial amounts. For the smallest For the smallest investigated Sn concentration, less than 1% of tin was precipitated, while when 8–10 investigated Sn concentration, less than 1% of tin was precipitated, while when 8–10 g/dm3 tin was in g/dm3 tin was in the initial solution, 67% of tin was precipitated. The indium precipitation yield was the initial solution, 67% of tin was precipitated. The indium precipitation yield was also affected by the also affected by the change of Sn concentration—the lowest (10%) was in the detinned solution, while change of Sn concentration—the lowest (10%) was in the detinned solution, while the highest (42%) the highest (42%) was observed for the highest investigated tin concentrations (8–10 g/dm3). was observed for the highest investigated tin concentrations (8–10 g/dm3).
3.2.3.2. Ge Ge Cementation Cementation from from Detinned Detinned Solution Solution AlthoughAlthough the tin tin contained contained in in the the solution solution improved improved the Ge Ge precipitation precipitation yield, in the the next next series series ofof tests germanium cementationcementation fromfrom a a detinned detinned solution solution was was investigated. investigated. This This was was done done in in order order to toobtain obtain a tin-free a tin‐free precipitate, precipitate, which which might might be be better better a product a product from from an an industrial industrial point point of view.of view. InIn the the first first test, test, the the influence influence of of zinc zinc dust dust mass mass on on precipitation precipitation yields yields of of germanium germanium and and indium indium was determined. Tests were performed at 75 °C for 2 h. The results of the tests are shown in Figure 4. was determined. Tests were performed at 75 ◦C for 2 h. The results of the tests are shown in Figure4.
FigureFigure 4. InfluenceInfluence of of Zn Zn dust dust amount amount on on precipitation precipitation yields yields of of Ge Ge and and In, In, and and final final pH pH (75 (75 °C,◦C, 2 2 h, h, detinneddetinned solution).
ItIt was observed that, that, as as in in the case of the solution containing tin, tin, a higher amount of zinc dust increasedincreased thethe precipitation precipitation yields yields of germanium of germanium and indium. and indium. However, However, only for theonly highest for investigatedthe highest 3 investigatedzinc dose (80 zinc g/dm dose) were (80 g/dm precipitation3) were precipitation yields the same yields for boththe same solutions for both (>99%). solutions When (>99%).a lower When mass aof lower Zn dust mass was of added, Zn dust Ge was and added, In precipitation Ge and In yieldsprecipitation from the yields detinned from solution the detinned were solution lower. On were the lower.other hand, On the the other pH hand, of the the solution pH of after the solution adding zincafter toadding the detinned zinc to the solution detinned was solution higher thanwas higher that of thanthe solution that of the containing solution tin. containing tin. ItIt was found found that that without without control control of of the the final final pH, pH, it it was was not not possible possible to to separate separate Ge Ge and and In. In. Therefore, in in the the next next experimental experimental series, series, the the final final pH pH was was kept kept at 1.0 at by 1.0 adding by adding sulphuric sulphuric acid. acid.The influenceThe influence of the of amount the amount of Zn of powder Zn powder on germanium on germanium and and indium indium cementation cementation is demonstrated is demonstrated in Figurein Figure 5. 5.
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Figure 5. Influence of Zn dust amount on precipitation yields of Ge and In (75 °C, 2 h, detinned solution, final pH 1.0). Figure 5. Influence of Zn dust amount on precipitation yields of Ge and In (75 C, 2 h, detinned Figure 5. Influence of Zn dust amount on precipitation yields of Ge and In (75 ◦°C, 2 h, detinned solution,Controlling final the pH final 1.0). pH had a positive impact on the separation of germanium from tin. It was solution, final pH 1.0). observed that as the amount of zinc dust increased, the Ge precipitation yield also increased. When Controlling3 the final pH had a positive impact on the separation of germanium from tin. It was 90–100Controlling g/dm Zn the powder final pH was had added, a positive over impact 97% Geon thewas separation cemented. of Ongermanium the other from hand, tin. theIt was In observed that as the amount of zinc dust increased, the Ge precipitation3 yield also increased. When3 precipitationobserved that yield as the increase amount was of zinclower—from dust increased, 11% for the 60 Ge g/dm precipitation of zinc added yield toalso 17% increased. for 100 g/dmWhen 90–100 g/dm3 Zn powder was added, over 97% Ge was cemented. On the other hand, the In Zn.90–100 g/dm3 Zn powder was added, over 97% Ge was cemented. On the other hand, the In precipitation yield increase was lower—from 11% for 60 g/dm3 of zinc added to 17% for 100 g/dm3 Zn. precipitationIn the next yield test, increase the final was pH oflower—from the process 11%was keptfor 60 at g/dm different3 of zincconstant added levels to 17% between for 100 0.25 g/dm and3 In the next test, the final pH of the process was kept at different constant levels between 0.25 2.0Zn. in order to determine maximum possible Ge precipitation yield and possibility of indium and 2.0 in order to determine maximum possible Ge precipitation yield and possibility of indium separation.In the nextThe test,final the pH final was pH adjusted of the process using wassulphuric kept at acid different solution. constant The levelsobtained between precipitation 0.25 and separation. The final pH was adjusted using sulphuric acid solution. The obtained precipitation yields yields2.0 in areorder presented to determine in Figure maximum 6. possible Ge precipitation yield and possibility of indium are presented in Figure6. separation. The final pH was adjusted using sulphuric acid solution. The obtained precipitation yields are presented in Figure 6.
Figure 6. Influence of Zn final pH on precipitation yields of Ge and In (75 C, 2 h, detinned solution, Figure 6. Influence of Zn final pH on precipitation yields of Ge and In (75 ◦°C, 2 h, detinned solution, 100 g/dm3 zinc dust added). 100 g/dm3 zinc dust added).
ItFigure was observed 6. Influence that of atZn a final lower pH final on precipitation pH, more indium yields of was Ge cemented.and In (75 °C, For 2 pHh, detinned 0.25, 21% solution, of indium It was observed that at a lower final pH, more indium was cemented. For pH 0.25, 21% of indium was removed100 g/dm from3 zinc thedust solution, added). while at pH 1.0 and 2.0 it was 7% and 12%, respectively. On the other was removed from the solution, while at pH 1.0 and 2.0 it was 7% and 12%, respectively. On the other hand, the most germanium (>99%) was removed for the highest investigated final pH (2.0). At a final pH ofIt 0.25 was only observed 74% Ge that was at a precipitated. lower final pH, more indium was cemented. For pH 0.25, 21% of indium was removed from the solution, while at pH 1.0 and 2.0 it was 7% and 12%, respectively. On the other
Minerals 2020, 10, x FOR PEER REVIEW 7 of 10 Minerals 2020, 10, 358 7 of 10 hand, the most germanium (>99%) was removed for the highest investigated final pH (2.0). At a final pH ofThe 0.25 quality only 74% of the Ge solid was precipitated. obtained by cementation was verified by cementation at a larger scale, i.e., usingThe quality a detinned of the solution solid obtained with a volume by cementation of 2.5 dm 3was. The verified cementation by cementation process was at a carried larger outscale, at i.e., using a detinned3 solution with a volume of 2.5 dm3. The cementation process was carried out at 75 ◦C, 100 g/dm Zn dust was added in portions with control of the final pH at 2.0, and the obtained suspension75 °C, 100 g/dm was3 mixed Zn dust for was 2 h. added The results in portions are collected with control in Table of 3the. XRD final analysis pH at 2.0, of and the cementate the obtained is presentedsuspension in was Figure mixed7. for 2 h. The results are collected in Table 3. XRD analysis of the cementate is presented in Figure 7. 3 Table 3. Composition of solution and precipitate obtained after cementation (75 ◦C, 2 h, 100 g/dm Zn dust,Table final 3. Composition pH 2.0). of solution and precipitate obtained after cementation (75 °C, 2 h, 100 g/dm3 Zn dust, final pH 2.0). Ge In Zn Ge In Zn initial solution [g/dm3] 5.15 1.52 5.81 initial solution [g/dm3] 5.15 1.52 5.81 final solution [g/dm3] 0.009 1.09 105 final solution [g/dm3] 0.009 1.09 105 precipitate [%] 50.0 4.7 8.2 precipitationprecipitate yield [%} [%] >99%50.0 18%4.7 n8.2/d precipitation yield [%} >99% 18% n/d Note: n/d—not determined. Note: n/d—not determined
Figure 7. XRD pattern of precipitate obtained after cementation of detinned solution with zinc dust Figure 7. XRD pattern3 of precipitate obtained after cementation of detinned solution with zinc dust (75 ◦C, 2 h, 100 g/dm Zn dust); a.u.—arbitrary units. (75 °C, 2 h, 100 g/dm3 Zn dust); a.u.—arbitrary units. The main precipitate component was germanium, which constituted half of the dry mass. AccordingThe main to XRD precipitate analysis, component most of thewas germanium germanium, was which present constituted in metallic half form. of the Part dry ofmass. the germaniumAccording to was XRD also analysis, present most as of GeO the 2germanium. The cementate was present also contained in metallic 8.2% form. Zn, Part present of the germanium mainly as sulphatewas also monohydrate present as GeO and2. 4.7% The indium. cementate Minor also identified contained metallic 8.2% componentsZn, present were mainly copper, as sulphate gallium, andmonohydrate iron (1.5%–2.5%). and 4.7% indium. Minor identified metallic components were copper, gallium, and iron (1.5%–2.5%). 4. Discussion 4. DiscussionThe final pH of cementation should be kept below 2.0. For higher pH values, the complete precipitationThe final of pH germanium, of cementation tin, and should indium be waskept observed. below 2.0. In For this higher situation, pH itvalues, was not the possible complete to separateprecipitation these of elements germanium, and obtain tin, and a valuable indium solid. was observed. The amount In ofthis zinc situation, powder it used was was not higherpossible than to requiredseparate tothese achieve elements pH 2.0. and Therefore, obtain a pHvaluable adjustment solid. byThe sulphuric amount of acid zinc addition powder was used required. was higher than Therequired mechanisms to achieve of thepH cementation2.0. Therefore, in pH the adjustment presence of by tin sulphuric and in detinned acid addition solutions was seemed required. to be diThefferent. mechanisms In Figure 3of, it the may cementation be observed in thatthe presence the precipitation of tin and yields in detinned of germanium solutions and seemed indium to arebe different. higher when In Figure tin concentration 3, it may be is observed high. Additionally, that the precipitation the final pH yields of the of process germanium was lower, and indium which suggestsare higher that when less tin hydrogen concentration was evolved is high. atAdditionally, a higher tin the concentration. final pH of the The process comparison was lower, of Figures which1
Minerals 2020, 10, 358 8 of 10 and4 may also lead to a similar conclusion. For zinc doses at which the final pH was below 2, lower Ge and In precipitation yields, as well as a higher pH, were observed for detinned solutions. As can be seen in Table1, the standard reduction potential di fference of germanium is slightly higher than that of hydrogen (0.12 V), and much higher than that of indium (0.46 V). Therefore, germanium cementation should be more favourable than hydrogen emission and indium cementation. However, all of these reactions took place simultaneously and were driven by the oxidation of zinc powder. The reduction potentials for these reactions were 0.88 for Ge, 0.76 for H, and 0.42 V for In. A relatively low concentration of germanium in the initial solution (5.15 g/dm3) also had an influence on hydrogen evolution during the process, especially at the beginning of the process. In the case wherein tin was present in the solution, it might act as an intermediate, i.e., tin was reduced with zinc dust, and later germanium was cemented with metallic tin. The relative difference between the reduction potential of hydrogen evolution and tin cementation with zinc was not very high (0.76 for H2, 0.62 V for Sn); therefore, both reactions may have occurred in parallel. However, the relative difference between the reduction potential of germanium and hydrogen with tin was much more significant (0.26 and 0.14 V, respectively). This may have had influence on the lower hydrogen evolution and lower final pH of the suspension (Figure3). It may be noted that the amount of zinc dust consumed for complete Ge cementation was much higher than expected. According to stoichiometry, 1.8 g Zn should be used to precipitate 1 g Ge. In the test it was found that 100 g of zinc powder was required to achieve >99% germanium precipitation, which corresponds to almost 20 g Zn required to cement 1 g Ge. During the experiment, high amounts of evolving gas (hydrogen) were observed right after the addition of each zinc portion. Therefore, most of the zinc was consumed for solution neutralisation and the reduction of hydronium ions. This conclusion was also supported by the results of tests without final pH control, in which a significant increase of the final pH, even >5.0, was observed. In future research, more attention should be paid to the control of reaction kinetics, which may be an effective way to reduce hydrogen formation during cementation. Slower Zn addition does not seem to be a suitable solution. During experiments it was observed that temperatures of 60 ◦C (for solution containing Sn) and 75 ◦C (for detinned solution) were important limits, and for lower values no reaction was observed—when zinc dust was added below those temperatures, gas evolution, precipitate formation, and temperature increase were not observed. The addition of zinc powder also caused a rapid increase in temperature, up to >10 ◦C. The reduction of the reaction area, e.g., by applying larger zinc particles, might limit reaction kinetics. The presence of tin improved cementation efficiency; however, co-cementation of germanium and tin was not favourable when a Ge-enriched product is desired. Cementation using Zn dust from the solution containing Ge, Sn, and In allowed us to only obtain a solid enriched in germanium and tin. It was not possible to separate germanium and tin. Therefore, it was necessary to remove tin before the cementation step. In the case of the detinned solution, it was not possible to achieve high separation of germanium from indium when the final pH was kept below 1.0. For pH 1.0 and 2.0, precipitation yields of germanium and indium were 97% and 99%, respectively. For the same pH values, 7% and 12% In was cemented. Based on the Ge precipitation yields and sulphuric acid consumption, a pH of 2.0 should be preferred for an industrial process. The cementate was composed mainly of germanium, which was mainly in the elementary form. Part of the germanium was present as dioxide. It is not clear if GeO2 was formed during cementation, or later during drying, despite a drying temperature of 50 ◦C being applied. Zinc was in the form of sulphate. Monohydrate was formed during drying as the hexa- and heptahydrate were more favourable forms of ZnSO4 precipitate from aqueous solutions. The presence of the zinc in sulphate form suggests that washing the cementate with water was not sufficient, and pulping techniques should be applied for the purification of final product. The application of an additional washing step for precipitation and control of the drying process may improve the quality of the final product. Minerals 2020, 10, 358 9 of 10
The solution obtained after germanium cementation contained significant amounts of indium, which might be suitable for recovery. Moreover, high zinc levels were present in the solution. Therefore, there is a possibility of indium and zinc recovery, but further cementation or precipitation using other agents should also be considered before application in an industrial scale.
5. Conclusions The cementation of germanium from sulphate solution containing 5.15 Ge, 1.52 In, 5.81 g/dm3 Zn with and without Sn using zinc dust was investigated. Precipitation yields of the analyzed elements under different conditions were determined. The following conclusions were derived:
1. The final pH of the cementation process should be controlled and kept at 2.0; with a higher ≤ pH, precipitation yields of Ge, Sn, and In were close to 100%; therefore, Ge in the solid was not separated from Sn and In; 2. the presence of Sn in the solution reduced the Zn consumption required to achieve Ge cementation; however, Sn was co-precipitated with Ge, and therefore only separation from In was achieved—20% of the indium present in the solution was precipitated; 3. the almost complete cementation of Ge (>99%) from the investigated solution was achieved with 3 the following parameters: 75 ◦C, 2 h, Zn dust consumption: 100 g/dm solution, final pH: 2.0; under these conditions, ca. 12% of In was precipitated from the solution; 4. the solid obtained after cementation contained 50% Ge, 8.2% Zn, and 4.7% In; germanium was present mainly in the elementary form, and also partially as GeO2; zinc was present mainly as sulphate; 5. the solution obtained after cementation contained 1.09 In and 105 g/dm3 Zn; therefore, it may be a source for indium and zinc recovery.
Author Contributions: Conceptualization, M.D.; investigation, M.D., M.C., M.K., A.R., S.K., K.G., and P.K.; methodology, M.D. and G.B.; supervision, M.D. and K.L.-S.; visualization, M.D.; writing—original draft, M.D. and M.C.; writing—review and editing, G.B. and K.L.-S. All authors have read and agreed to the published version of the manuscript. Funding: The study was funded by the Polish Ministry of Science and Higher Education under the project ST/33/2019/G “Investigation on germanium recovery from post-leaching solutions”. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.
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