J. Cent. South Univ. Technol. (2011) 18: 1074-1079

DOI: 10.1007/s11771-011-0806-3

Extracting indium and preparing ferric oxide for soft magnetic ferrite materials from zinc calcine reduction lixivium

HE Jing(何静), TANG Mo-tang(唐谟堂), ZHOU Cun(周存), WU Sheng-nan(吴胜男),

CHEN Yong-ming(陈永明), WANG Tao(王涛), HUANG Ling(黄玲)

School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2011

Abstract: A hydrometallurgical process for indium extraction and ferric oxide powder preparation for soft magnetic ferrite material was developed. Using reduction lixivium from high-acid reductive leaching of zinc oxide calcine as raw solution, copper and indium were firstly recovered by iron powder cementation and neutralization. The recovery ratios of Cu and In are 99% and 95%, respectively. Some harmful impurities that have negative influences on magnetic properties of soft magnetic ferrite material are deeply removed with sulfidization purification and neutral flocculation method. Under the optimum conditions, the content of impurities like Cu, Pb, As, Al in pure Zn-Fe sulfate solution are less than 0.004 g/L, but those of Cd, Si, Ca and Mg are relatively high. Finally, thermal precipitation of iron is carried out at 210 °C for 1.5 h. The precipitation ratio of Fe is 93.33%. Compared with the quality standard of ferric oxide for soft magnetic ferrite materials, the contents of Al and Mg in obtained ferric oxide powder meet the requirement of YHT1 level of ferric oxide, and those of Si, Ca meet the requirement of YHT3 level of ferric oxide. XRD and SEM characterizations confirm that the obtained sample is well-dispersed spindle spherule with regular α-Fe₂O₃ crystal structure. The length-to-diameter ratio of α-Fe₂O₃ powder is (3-4):1 with an average particle size of 0.5 μm.

Key words: zinc concentrates; zinc hydrometallurgy; indium extraction; deep purification; ferric oxide powder

1 Introduction

Iron is presented as a common undesirable constituent of zinc concentrates. The leaching of zinc roasted calcine results in the solubilisation of desired metal together with iron. Iron is a severe impurity in pregnant solution and must be removed before zinc electrolysis [1]. Nowadays, iron is often removed from zinc leaching solutions with precipitation method in the form of jarosite [2-4], goethite [5-6] or hematite residue [7]. These wastes, which typically contain 25%-50% of iron oxides, are considered hazardous and toxic due to the presence of heavy metals like Pb, Cd, Cu, As and Zn. To avoid leaching of iron waste residues, they are usually stored in closed containers or in sealed reservoirs, which is a costly and environmentally unsatisfactory procedure. In recent decades, many hydrometallurgical and pyrometallurgical techniques have been developed to recycle jarosite and goethite wastes as building materials, catalysts, pigments or refractories [8-14].

The reserves of sphalerite concentrate bearing high In-Fe are abundant in Guangxi Province, China. At present, “hot-acid leaching-jarosite precipitation” hydrometallurgical process is adopted for zinc and indium extraction [15-16], and the recovery ratios of zinc and indium are relatively low.

In addition, the pollution can be caused by low-concentration sulfur dioxide fume and large amount of secondary iron slag during indium extraction from jarosite residue. On the other hand, Mn-Zn soft magnetic ferrite materials, in which iron is the basic constituent, are highly valuable and widely used products [17-18]. Therefore, in the view point of value-added utilization of iron resource in zinc concentrate and Zn-In clean metallurgy [19-20], a hydrometallurgical process has been proposed to extract indium and prepare ferric oxide for soft magnetic ferrite materials from zinc-oxide calcine reduction lixivium.

2 Experimental

2.1 Raw solution

Zinc-oxide calcine reduction lixivium was used as raw solution, and its chemical composition is listed in Table 1. It is shown that the main constituents of raw solution are Zn, Fe and H₂SO₄. Concomitant valuable metals, such as In, Cu and Cd, should be recovered in consideration of economic benefit. Some harmful impurities like Ca, Mg, Al or Si, which have negative influence on the magnetic properties of soft magnetic ferrite material, must be completely removed from pregnant solution before ferric oxide powder preparation.

2.2 Experimental method

The principle flow sheet for extracting indium and preparing ferric oxide is illustrated in Fig.1. Reduction lixivium from high-acid reductive leaching of zinc oxide calcine bearing high In-Fe was used as raw solution. After copper cementation and indium neutralization, some harmful impurities were deeply removed by sulfidization purification and neutral flocculation. And then, Zn-Fe separation and sulfuric acid regeneration were achieved with thermal precipitation method. Simultaneously, ferric oxide powder for soft magnetic ferrite material was prepared.

According to former research [20], copper cementation and indium precipitation were carried out respectively under optimum conditions. The reaction vessel containing 3 L of past-reaction solution was placed in thermostatic water bath. When the solution temperature reached the desired value, iron powder or basic zinc carbonate was added at a constant mechanical stirring speed. After 1 h, the post-reaction slurry was filtered using a pressure filter, and the solid material was vacuum dried until constant mass at 70 °C.

During sulfidization purification and neutral flocculation, effect of temperature and reactant dosage on the impurities removal was investigated with single-factor experimental method. The reaction vessels containing 500 mL of pregnant solution were placed in thermostatic water bath, and then heated to the desired temperature. After pH value adjustment, predetermined amounts of (NH₄)₂S and PAM flocculent were added at a constant mechanical stirring speed. After 30 min, the slurry were filtered using a pressure filter for solid-liquid separation, and the contents of Cu, As, Si, Zn in filtrate were measured respectively.

Thermal precipitation of iron was conducted in FYXD2-type autoclave made by Weihai Huixin Chemical Industry Co. Ltd. 500 mL of pure Zn-Fe sulfate solution and hydrogen peroxide were added into autoclave. The mixed solution was heated at 210 °C for 1.5 h, and then cooled to ambient temperature by cooling water. The post-reaction slurry was filtered using a pressure filter. The solid material was washed two times with deionized water, and the obtained ferric oxide was vacuum dried until constant mass at 70 °C.

Table 1 Chemical compositions of zinc-oxide calcine reduction lixivium (g/L)

Fig.1 Principle flow sheet for extracting indium and preparing ferric oxide powder

The ferric oxide powder was characterized by X-ray diffractometry (XRD) to confirm its crystal structure. XRD analysis was performed on a Rigaku D/max-2500 type diffractometer equipped with a rotation anode, a copper target (Cu K_α, λ=1.540 6 ?), and a monochromator composed of graphite crystal. The diffractometry was conducted at 50 kV, 100 mA and a scan rate of 1 (°)/min. Scanning took place between 10° and 80° with a 0.04° step. JEOF JSM-5600LV type scanning electron micro- scope (SEM) was employed to monitor the microscopic morphology and particle size of ferric oxide powder.

The content of Zn in sample was measured by EDTA titration, and that of Fe by potassium dichromate oxidation-reduction titration. The contents of In, Cu, Cd, Pb, As, Si, Ca, Mg and Al were analyzed by atomic absorption spectroscopy or inductively coupled plasma atomic emission spectroscopy (ICP-AES).

3 Results and discussion

3.1 Cementation of copper

In order to recover valuable metal and conduct deep reduction of Fe³⁺ ion in pregnant solution, cementation of copper by iron powder was carried out at 55 °C for 1 h with iron power dosage equal to 2.1 times of theoretical consumption. The experimental results are shown in Table 2. It is indicated that cementation ratio of Cu is about 99%, and the grade of obtained copper residue is higher than 33%. Calculated by post-reaction filtrate, the loss ratios of Zn and In are 1.3% and 0.3%, respectively.

3.2 Neutralization of indium

Using basic zinc carbonate as neutralizer, the neutralization of indium was conducted at 50 °C for 1 h with terminal pH value of 4.5-5, and the experimental results are listed in Table 3.

As shown in Table 3, the precipitation ratio of In is about 95%, and the grade of obtained indium concentrate is higher than 3%. During indium neutralization, most aluminum is precipitated into indium concentrate, and the loss ratios of Fe and Zn calculated by filtrate are 4.63% and 1.29%, respectively. The contents of Zn, Fe, Al in obtained indium concentrate are 24.47%, 20.59% and 4.13%, respectively.

3.3 Sulfidization purification

After copper cementation and indium neutralization, the chemical compositions of post-reaction solution are (g/L): Zn 78.66, Fe 14.27, Cu 0.135, Cd 0.433, Ca 0.305, Mg 0.217, Al 0.010 8, Si 0.054, Mn 0.287 and As 0.002 7. Sulfidization purification process was adopted to completely remove heavy metals like Pb, Cd and Cu. The effects of (NH₄)₂S amount and sulfidization temperature on the impurities removal are investigated.

3.3.1 Effect of (NH₄)₂S amount

Effect of (NH₄)₂S amount on the precipitation ratio of impurities is investigated under the following conditions: flocculent concentration of 5 mg/L, initial pH value of 2.5, and temperature of 40 °C. The experimental results are presented in Fig.2. As shown in Fig.2, (NH₄)₂S amount has obvious influence on the impurities removal. The precipitation ratios of Cu, As, Si and Zn are enhanced as (NH₄)₂S amount is increased. In order to decrease the lose ratio of Zn, 120 mg/L of free S^2- is the optical condition. Under the optical condition, the precipitation ratios of Cu, As, Zn and Si are about 99%, 73%, 1% and 27%, respectively.

3.3.2 Effect of sulfidization temperature

Effect of temperature on the precipitation ratio of impurities is illustrated in Fig.3 under the following conditions: flocculent concentration of 5 mg/L, initial pH value of 2.5, free S^2- ion concentration of 120 mg/L.

Table 2 Experimental results of copper cementation

Table 3 Experimental results of indium neutralization

Fig.2 Effect of free S^2- concentration on precipitation ratio of impurities

Fig.3 Effect of sulfidization temperature on precipitation ratio of impurities

It is demonstrated in Fig.3 that the precipitation ratios of Cu, Si and Zn are gradually enhanced with increasing the temperature, but that of As is quickly decreased on the contrary. Considering all, 40 °C is the optimal temperature. The precipitation ratios of Cu, As, Zn and Si are 98.8%, 72.4%, 1% and 26.6%, respectively.

3.4 Neutral flocculation

3.4.1 Effect of flocculent addition

The effect of flocculent (PAM) addition on the precipitation ratio of impurities is investigated at 40 °C for 30 min with a terminal pH value of 5.0-5.2, and the experimental results are presented in Fig.4.

As shown in Fig.4, the precipitation ratios of As and Si decline with the increase of flocculent concentration. In contrast, it has slight influence on the precipitation ratios of Cu and Zn. It is because that flocculent provides the backbone for Fe(OH)₃, and Fe(OH)₃ can adsorb Si [21]. Overall, 5 mg/L is the optimal flocculent concentration with larger precipitation ratios of Cu, As, Si and lower precipitation ratios of Zn and Fe.

Fig.4 Effect of flocculent addition on precipitation of impurities

3.4.2 Effect of flocculation temperature

The effect of flocculation temperature on the precipitation ratio of impurities is shown in Fig.5 at a terminal pH value of 5.0-5.2. It is indicated that the elevation of flocculation temperature is favorable to impurities removal. When the flocculation temperature is increased from 40 °C to 80 °C, the precipitation ratios of As, Pb, Al and Si are enhanced correspondingly from 75.61%, 51.09%, 36.93% and 31.52% to 94.80%, 51.36%, 47.73% and 51.98%, respectively. Henceforth, the further elevation of flocculation temperature induces the decrease of the precipitation ratio of impurities. It is because high temperature makes flocculent degrade. In the temperature range of 40-90 °C, the precipitation ratios of Zn and Fe are about 1% and 2%, respectively. Considering all, 80 °C is the optimal temperature.

Fig.5 Effect of flocculation temperature on precipitation ratio of impurities

3.5 Hydrothermal precipitation of iron

Based on the above-mentioned experimental results, the optimum conditions for deep purification are determined as follows: 40 °C, initial pH value 2.5, and free S^2- ion concentration 120 mg/L for sulfidization purification; 80 °C, 30 min, terminal pH value 5.0-5.2, and PAM dosage 5 mg/L for neutral flocculation. Using the post-reaction filtrate from copper cementation and indium precipitation as raw solution, sulfidization purification and neutral flocculation are conducted in turn under the optimum conditions. The chemical compositions of obtained pure Zn-Fe sulfate solution and purification residue are listed in Table 4. It is shown that the content of impurities like Cu, Pb, As, Al in pure Zn-Fe sulfate solution is less than 0.00 4g/L, but that of Cd, Si, Ca and Mg is relatively higher. Calculated by purification residue, the loss ratios of Zn and Fe are about 0.2% and 0.3%, respectively.

Table 4 Chemical compositions of pure Zn-Fe sulfate solution and purification residue

Using pure Zn-Fe sulfate solution as raw material, thermal precipitation of iron is carried out at 210 °C for 1.5 h with hydrogen peroxide dosage equal to 1.5 times theoretic consumption. The results show that the precipitation ratio of Fe is 93.33%. The chemical compositions of obtained ferric oxide powders are listed in Table 5.

As shown in Table 5, the grade of obtained ferric oxide powder is less than 99% due to high contents of H₂O and S. The contents of Zn and Mn are 0.47% and 0.04%, respectively, but those of other impurities like Cu, Cd, Pb, As, Co, Al, Si, Sb and Mg are very low. Compared with the quality standard of ferric oxide for soft magnetic ferrite materials, the contents of Al, Mg in obtained sample meet the requirement of YHT1 level of ferric oxide, while those of Si and Ca meet the requirement of YHT3 level of ferric oxide. In respect of chemical composition, calcination for S removal or another deep purification technique is necessary for present ferric oxide powder before its utilization in soft magnetic ferrite field.

Table 5 Chemical compositions of obtained ferric oxide powders (mass fraction, %)

XRD pattern in Fig.6 confirms α-Fe₂O₃ crystal structure of obtained ferric oxide powder. As shown in Fig.7, α-Fe₂O₃ powder is well-dispersed spindle spherule with a length-to-diameter ratio of (3-4):1, and its average particle size is about 0.5 μm.

Fig.6 XRD pattern of ferric oxide powder obtained

Fig.7 SEM image of ferric oxide powder obtained

4 Conclusions

1) A hydrometallurgical process for indium extraction and ferric oxide powder preparation from zinc oxide calcine bearing high In-Fe has been developed. After copper cementation, indium is precipitated using basic zinc carbonate as neutralizer. The grade of obtained indium concentrate is 3% with indium precipitation ratio of 95%, and the loss ratios of Fe and Zn are less than 5% and 1.5%, respectively.

2) In order to remove harmful impurities that have negative influence on magnetic properties of soft magnetic ferrite material, sulfidization purification and neutral flocculation are carried out under the optimum conditions. The content of impurities like Cu, Pb, As, Al in pure Zn-Fe solution is less than 0.004 g/L, but that of Cd, Si, Ca and Mg is relatively high. The loss ratios of Zn and Fe are about 0.2% and 0.3%, respectively.

3) Ferric oxide powder for soft magnetic ferrite material is prepared with thermal precipitation method. Simultaneously, Zn-Fe separation and sulfuric acid regeneration are achieved. The precipitation ratio of Fe is 93.33%. The contents of Al, Mg in obtained sample meet the requirement of YHT1 level of ferric oxide, while those of Si, Ca meet the requirement of YHT3 level of ferric oxide. XRD and SEM characterization confirm that the obtained sample is well-dispersed spindle spherule with regular α-Fe₂O₃ crystal structure. The length-to-diameter ratio of hematite powder is (3-4):1 with an average particle size of 0.5 μm.

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(Edited by YANG Bing)

Foundation item: Project(50674104) supported by the National Natural Science Foundation of China; Project(2006BA02B04-4-2) supported by the Planned Science and Technology of China

Received date: 2010-07-06; Accepted date: 2011-01-05

Corresponding author: HE Jing, Associate Professor; Tel: +86-731-88830470; E-mail: he6213@163.com