J. Cent. South Univ. Technol. (2008) 15: 835-839

DOI: 10.1007/s11771-008-0154-0

Recovery of indium and lead from lead bullion

HE Jing(何 静)¹, WANG Rui-xiang(王瑞祥)^{1, 2}, LIU Wei(刘 维)¹

 (1. School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China;

2. School of Materials and Chemical Engineering, Jiangxi University of Science and Technology,

Ganzhou 341000, China)

Abstract: Lead and indium were recovered by electrolysis and nonequilibrium solvent extraction process from lead bullion. The effects of current density, electrolytic period and circle amount of electrolyte on the electrochemical dissolution of lead and indium were investigated. The effects of extraction phase ratio and mixing time on solvent extraction of indium and striping phase ratio and stripping stage on the loaded organic phase stripping were also investigated. The experimental results indicate that under optimum conditions, the purity of lead deposited on cathode is 98.5% and the deposit rate of lead is 99.9%, the dissolution rate of indium is 94.28%, the extraction rate of indium is 98.69%, the stripping rate of indium is almost 100%, and the impurity elements, such as Zn, Fe and Sn can be removed.

Key words: lead bullion; indium; electrochemical dissolution; nonequilibrium solvent extraction

1 Introduction

In the process of zinc pyrometallurgy, some lead bullions with many impurity elements will generate in the rectifying tower when crude zinc is rectified. The pure lead is produced by affining. Usually the content of indium in the lead bullion is high. Indium is a typical rare metal, and distributes very sporadically in the lithosphere. Independent indium mineral is rare, which always associates with lead and zinc ores. So indium must be recovered during zinc and lead metallurgy process. 60% indium enters into spelter during zinc pyrometallurgy process and then richens in lead bullion at the bottom of the rectifying tower. Indium can be recovered during the refine process of lead bullion^[1-6]. The main techniques of treating the lead bullion are electrolyte method^[7], vacuum distilling^[8], electric process^[9-10] and flame furnace process^[11-12]. After electrolysis—extraction—cementation, the electrolytic dissolution rate of indium is 94% and the high purity lead is generated. Under the conditions that the vacuum is 66-133 Pa, temperature is 1 000 ℃ and distillation time is 60 min, indium yield is higher than 90%^[8]. However, there are some disadvantages in these techniques, such as expensive cost, high alkali consumption, low thermal utilization factor and serious pollution. To solve these problems, electrolysis—non- equilibrium solvent extraction process is used to purify lead bullion and recover indium. Based on Ref.[13], the process of indium recovery from lead fluosilicate electrolyte by nonequilibrium solvent extraction was researched in this work.

2 Experimental

2.1 Experimental materials and process

The materials used in this study were taken from Pb-Zn smelter in the south of China. The chemical components of lead bullion are listed in Table 1.

Table 1 Chemical components of lead bullion (mass fraction, %)

Fig.1 shows the process of electrolysis—non- equilibrium solvent extraction of indium in this work.

Fig.1 Flow sheet of electrolysis—nonequilibrium solvent extraction of indium

2.2 Experimental principle

2.2.1 Lead electrolysis principle

Electrochemical system of lead electro-refining in the silicofluoric acid electrolyte consists of Pb_(pure)|PbSiF₆, H₂SiF₆, H₂O|Pb_(crude).

Ions of Pb²⁺, SiF₆^2-, H⁺, OH^- etc are produced when the components of electrolyte ionize.

Main electrode reactions are as follows.

Cathode reaction:

Pb²⁺+2e=Pb                                                 (1)

Anode reaction:

Pb-2e=Pb²⁺                                                    (2)

In the process of lead electrolysis, the metals such as In, Fe, Zn and Sn, whose standard electrode potential is more negative than that of lead, will electrochemically dissolve with lead. But the metals such as Ag, Cu, As, Sb and Bi, whose standard electrode potential is more positive than that of lead, cannot dissolve, which form the main elements of anode mud settled in the hydroelectric bath.

2.2.2 Principle of indium extracted by P204

In the process of indium extraction, P204 (D2EHPA) is used as extraction agent. In the nonpolar solvent, P204 exists as dual-molecule associate. In the silicofluoric acid system, the reaction of extracting indium using P204 is cation exchange reaction, as follows:

In³⁺_(A)+3(HA)_2(O)=3H⁺+In(HA₂)_3(O)               (3)

The loaded organic phase is stripped by hydrochloric acid. The stripping process can be expressed as follows:

In(HA₂)_3(O)+4HCl_(A)=3(H₂A₂)_(O)+HInCl_4(A)         (4)

3.1 Process of lead electrolysis and dissolving indium

3.1.1 Effects of current density on lead electro-refining process

The effects of current density on lead electrolysis process were investigated in the conditions that the temperature was 25 ℃, the electrode distance was 4 cm, the additive consumption was 0.5 kg/t_Pb, and electrolytic period was 48 h. The results are shown in Table 2.

Table 2 Effect of current density on electrolysis process

The results given in Table 2 show that current density has a little effect on the purity of lead deposited on cathode and deposit rate of lead. The purity of lead deposited on cathode slightly decreases with the increase of current density. The deposit rate of lead fluctuates with the current density and its fluctuant range is very narrow when the current density is lower than 200 A/m². The current efficiency decreases rapidly when the current density excesses 155 A/m². The electrolytic dissolution rate of indium increases rapidly with the increase of current density. When the current density increases from 145 to 155 A/m², the electrolytic dissolution rate of indium increases from 61.68% to 98.19%, and the current efficiency increases too. So the optimum current density is 155 A/m².

3.1.2 Effects of electrolytic period on electrolysis process

The effects of electrolytic period on electrolysis process were investigated with the current density of 155 A/m². The results are shown in Table 3.

Table 3 Effect of electrolytic period on electrolysis process

The results given in Table 3 show that the purity of lead deposited on cathode decreases slightly as the electrolytic period prolongs. But lead deposit rate and current efficiency are almost changeless. Electrolytic period has some effects on electrolytic dissolution rate of indium. When the electrolytic period is 24 h or 48 h, the electrolytic dissolution rate of indium is relatively high. In consideration of effects of the electrolytic period on the purity of lead deposited on cathode and the production rate, the optimum electrolytic period is 24 h.

3.1.3 Effects of circle amount of electrolyte on electrolysis process

The effects of circle amount of electrolyte on electrolysis process were investigated under the conditions that current density was 155 A/m² and electrolytic period was 24 h. The results are given in Table 4.

Table 4 Effect of circle amount of electrolyte on electrolysis process

The results given in Table 4 show that lead deposit rate and electrolytic dissolution rate of indium increase with the increase of circle amount of electrolyte, and the current efficiency increases from 86.18% to 99.91% when the circle amount of electrolyte increases to 200 mL/h. The purity of lead deposited on cathode maintains almost about 99%, and its fluctuant range is less than 0.2%. Considering all the factors, the optimum circle amount of electrolyte is 100 mL/h.

3.2 Extraction of indium by P204

To investigate the effect of extraction phase ratio and stage on the indium extraction and stripping, lead fluosilicate electrolyte was used as extraction aqueous feed, whose composition is listed in Table 5. Extraction agent consists of P204 and sulfonic kerosene with the volume ratio of P204 to sulfonic kerosene being 3?7. Hydrochloric acid was used as stripping agent, whose concentration is 6 mol/L. Countercurrent extraction and stripping were carried out according to Refs.[14-15].

Table 5 Typical composition of extraction water phase c

3.2.1 Effect of extraction phase ratio on indium extraction

The effect of extraction phase ratio (V(O)?V(A)) on indium extraction was studied under the conditions that extraction and stripping stages were kept constant of 3 and 6, respectively, and the concentration of indium in the extraction aqueous feed was 4.444 g/L. The results are listed in Table 6.

Table 6 Effect of phase ratio on indium extraction

The results in Table 6 show that the extraction rate maintains at more than 98% with little changes when extraction phase ratio changes from 1?1 to 1?3. The extraction rate decreases sharply to 86.06% when extraction phase ratio changes from 1?3 to 1?4. The reason is that when the concentration of indium in the extraction aqueous feed is 4.444 g/L and the extraction phase ratio is 1?3, indium in the solution cannot reach the saturated capacity of organic phase even if it is extracted completely. When extraction phase ratio is 1?4, indium in the solution cannot be extracted completely even if it reaches the operating saturated capacity of organic phase (15 g/L).

From the above analysis it can be seen that extraction phase ratio has a little effect on the process of indium extraction before the concentration of indium in the organic phase reaches the operating saturated capacity. On the contrary, when the concentration of indium in the organic phase reaches the operating saturated capacity the extraction rate of indium decreases with increasing extraction phase ratio. So, extraction phase ratio should be adjusted according to the initial concentration of indium in the extraction aqueous feed.

3.2.2 Effect of mixing time on indium extraction

The effect of mixing time on indium extraction was studied under the condition that extraction phase ratio is 1?3. The results are given in Fig.2.

Fig.2 Effect of mixing time on indium extraction

The results in Fig.2 show that the extraction rate of indium increases with prolonging mixing time before 5 min. After mixing for 5 min, the process of extraction approximates equilibrium. So the suitable mixing time is 5 min.

3.3 Indium stripping experiments

3.3.1 Effect of stripping phase ratio on indium stripping

The effect of stripping phase ratio on indium stripping was studied under the conditions that extraction phase ratio was 1?4, extraction and stripping stages were both kept at 3, and the concentration of indium in the extraction aqueous feed was 4.444 g/L. The results are listed in Table 7.

Table 7 Effect of stripping phase ratio on indium stripping

The results in Table 7 show that small stripping phase ratio (V(O)?V(A)) is of great advantage to indium- stripping rate. When the stripping phase ratio is 4?1, the stripping rate of indium is 84.55%, which is higher than that (65.73%) at the stripping phase ratio of 6?1. But the concentration of indium in the stripping liquor is 53.435 g/L when the stripping phase ratio is 4?1, and it is lower than that (60.83 g/L) at the stripping phase ratio of 6?1.

3.3.2 Effect of stripping stage on indium stripping process

The effect of stripping stage on the indium stripping was studied under the conditions that extraction phase ratio was 1?4, extraction stage was 3, and stripping phase ratio was 6?1. The results are given in Table 8. Table 8 shows that the lowest stripping rate of indium is 92.08% after 4 stages stripping, and the highest value is 97.60% after 6 stages stripping. The higher stripping stage can increase the stripping rate of indium.

Table 8 Effect of stripping stage on indium stripping rate

3.4 Behavior of impurity elements in indium extraction

In the system of lead fluosilicate electrolyte, the main impurity elements influencing indium extraction are Zn, Fe and Sn. The behavior of these impurities in the process of indium extraction was studied in this work.

The impurity elements of Zn, Fe and Sn are also electrochemically-dissolved into the electrolyte with indium and lead. All these metallic ions can be extracted by P204. The relationship between logarithm of distribution coefficient^[14] (lgD) of these metallic ions and pH is given in Fig.3.

Fig.3 Relationship between lg D and pH

Fig.3 shows that the distribution coefficient of indium is the largest and the extraction power of each metallic ion by P204 increases with the increase of pH. So, indium ions in the system of fluosilicate can be selectively extracted at suitable acidity.

Experimental results indicate that in the process of indium extraction, the extraction rate of zinc is high, which is about 73%. But stripping rate of zinc is very low; the highest stripping rate of zinc is only 0.24%. Overall yield of zinc is less than 0.2%. This indicates that indium and zinc can be separated primarily in the extraction process. Both the extraction rate and stripping rate of iron are low (less than 10%), and overall yield of iron is only 0.27%, which satisfies the separation of indium and iron in the extraction process. The extraction behavior of Sn is given in Table 9.

Table 9 Extraction behavior of Sn in indium extraction process

It can be seen that from Table 9 that the maximum extraction rate and stripping rate of Sn are 35.57% and 22.7%, respectively. XU et al^[13] suggested that washing loaded organic phase by fluosilicic acid with concentration of 200 g/L can separate indium and tin. In this experiment, the concentration of fluosilicic acid in the extraction aqueous feed is higher than 200 g/L, thus the loaded organic phase need not to be washed before stripping indium, and the highest separation rate of tin is 97.46%. This indicates that if the concentration of fluosilicic acid in the extraction aqueous feed is higher than 200 g/L, indium and tin can be separated without washing the loaded organic phase.

4 Conclusions

1) Current density, electrolytic period and circle amount of electrolyte have marked effects on electrochemical dissolution of lead bullion bearing indium. In the optimum conditions that current density is 155 A/m, electrolytic period is 24 h, ambient temperature, circle amount of electrolyte is 100 mL/h and the electrode distance is 4 cm, the average dissolution rate of indium is 94.28% and the current efficiency is 89.19%.

2) Indium in the system of lead fluosilicate electrolyte can be extracted by P204. In the conditions that extraction agent consists of 30%P204+70% sulfonic kerosene, extraction stage is 3 and extraction phase ratio (V(O)?V(A)) is 1?3, the extraction rate of indium is 98.6%. Hydrochloric acid is used as stripping agent with concentration of 6 mol/L. In the conditions that stripping phase ratio (V(O)?V(A)) is 6?1 and stripping stage is 6, the stripping rate of indium approximates to 100%.

3) In the process of extraction, both the extraction rate and stripping rate of iron are low, so indium and iron can be separated satisfactorily. The extraction rate of zinc is high, but the stripping rate is low, therefore, the overall yield is also low. Indium and zinc can be separated. The extraction rate and stripping rate of tin are both high, and the highest separation rate of tin is 97.46%. This indicates that if the concentration of fluosilicic acid is higher than 200 g/L, indium and tin can be separated without washing the loaded organic phase.

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Foundation item: Project(2007CB613604) supported by the Major State Basic Research and Development Program of China; Project(50674104) supported by the National Natural Science Foundation of China; Project(GJJ08279) supported by the Jiangxi Provincial Department of Education, China

Received date: 2008-05-28; Accepted date: 2008-09-12

Corresponding author: HE Jing, Associate professor; Tel: +86-731-8830470; E-mail: he6213@163.com

(Edited by ZHAO Jun)