Reduction smelting on bismuth oxide residue in FeO SiO 2 CaO ternary slag system

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1 J. Cent. South Univ. (2016) 23: DOI: /s Reduction smelting on bismuth oxide residue in FeO SiO 2 CaO ternary slag system ZHANG Du-chao( ), ZHANG Xin-wang( ), YANG Tian-zu( ), WEN Jian-feng( ), LIU Wei-feng( ), CHEN Lin( ), RAO Shuai( ), XIAO Qing-kai( ), HAO Zhan-dong( ) School of Metallurgy and Environment, Central South University, Changsha , China Central South University Press and Springer-Verlag Berlin Heidelberg 2016 Abstract: Reduction smelting of the bismuth oxide residue from pressure leaching of bismuth sulfide was investigated in the FeO SiO 2 CaO ternary slag system. The results show that all the recovery ratios of Bi, Ag, Cu and Pb increase with the increase of reductive coal proportion, reaction temperature and time, while too much reductive coal would help Fe enter metal phase; CaO/SiO 2 and FeO/SiO 2 of the chosen slag system should be and , respectively, for the reason that the slag system has the optimum mobility and is beneficial for the recovery of metals. The corresponding optimum conditions are determined as follows: the added coal proportion is 7% of the leaching residue, CaO/SiO 2 mass ratio in the chosen slag system is 0.5 and FeO/SiO 2 is 1.5, the reaction temperature is 1300 C and the reaction time is 40 min. Under the above conditions, the recovery ratios of Bi, Ag, Cu and Pb are 99.6%, 99.8%, 97.0% and 97.3%, respectively. Key words: bismuth oxide residue; reduction smelting; FeO SiO 2 CaO ternary slag system; recovery ratio 1 Introduction Bismuth is a kind of green metals whose reserves in China are 75% of the world total reserves [1]. The bismuth-bearing materials can be processed with hydrometallurgical or pyrometallurgical method. In the traditional hydrometallurgical methods, the bismuth sulfide concentrate is leached in the hydrochloric system with oxide agent, while the bismuth oxide-containing materials are leached by directly hydrochloric acid [2 3], and then Bi in the leaching solution is replaced with iron powder or recovered by electro-winning in diaphragm electrolytic bath to produce crude bismuth [4]. Besides, bismuth can be separated by hydrolysis precipitation from the leaching solution in the form of bismuth oxychloride [5] and then reduced to produce crude bismuth. Crude bismuth can be transformed to pure bismuth by pyro-refining or electro-refining. Although the hydrometallurgical process has the advantages of high leaching ratio, low reagent costs and simple operation, the hydrochloric acid system causes serious corrosion and a large number of metal impurities transfer into the leaching solution resulting in difficult subsequent separation, poor metal recovery and serious environmental pollution [6]. In addition, leaching bismuth oxide with fluosilicic acid as the leaching agent has been investigated and the results show that the presence of Fe and Pb in leaching solution is harmful to recover Bi [7 9]. Now, pyrometallurgical smelting is the mainstream technology in bismuth production industry. For bismuth sulfide concentrate, SWINBOURNE has researched the thermodynamic model of bismuth smelting with the coal/iron reduction method [10] and the technology has been used to produce crude bismuth by Hunan Shizhuyuan Nonferrous Metals Co., Ltd. Another traditional method of producing bismuth is reverberatory or blast furnace reduction smelting after oxidizing the bismuth sulfide concentrate with sintering method [11]. The above processes have short flowsheet and high metal recovery, but they have such shortcomings as high costs and serious environmental pollution [12 13]. In recent years, YANG et al [14] have studied bismuth separation from bismuth glance concentrate through lowtemperature molten salt smelting. Although the method has high metal recovery, the soda would cause strong corrosion to the refractory. CHEN et al [15] have studied bath smelting of bismuth sulfide concentrate in oxygen side-blown furnace. According to this technology, concentrates are smelted in oxidation furnace, and then bismuth liquid molten slag flows into reduction furnace for reduction smelting. The method can get higher metal recovery, but the effects of technological conditions Foundation item: Project(134414) supported by the Postdoctoral Funded Program of Central South University, China Received date: ; Accepted date: Corresponding author: YANG Tian-zu, Professor; Tel: ; tianzuyang@csu.edu.cn

2 J. Cent. South Univ. (2016) 23: during smelting on the behavior of bismuth have not been researched in detail. In this work, the leaching residue is used as the raw material from pressure leaching of bismuth sulfide concentrates to separate tungsten and molybdenum. The leaching residue contains not only such valuable metals as Bi, Ag, Cu and Pb but also some impurity elements of Fe, Si and Ca [16 17]. This work aims to value metal extraction from the leaching residue in the ternary slag system of FeO SiO 2 CaO [18 19], which has been thoroughly studied and used widely in copper and lead pyro-metallurgical process. The behavior of the valuable metals is investigated with various operating parameters, such as coal powder dosage, slag type, reaction temperature and time, so that the optimum reduction smelting conditions can be determined. 2 Experimental 2.1 Materials The raw material used in this work is the leaching residue containing Bi [20], and its chemical composition is listed in Table 1. The fluxes including calcium oxide and silica are analytical reagents. The reductive coal powder has the chemical composition listed in Table 2. Table 1 Chemical composition of bismuth oxide residue w(bi)/% w(cu)/% w(pb)/% w(ag)/(g t 1 ) 39.46% 2.48% 1.25% w(fe)/% w(sio 2 )/% w(cao)/% w(al 2 O 3 )/% 17.84% 10.47% 5.10% 3.49% Table 2 Analysis result of reductive coal used in smelting experiment Proximate analysis Pulverized coal Chemical analysis Pulverized coal Fixed carbon Volatiles Ash Moisture 55.36% 27.34% 16.80% 0.50% Total of Fe SiO 2 Al 2 O 3 CaO 3.17% 3.91% 3.02% 2.21% lower part is crude bismuth alloy. Both the two products were weighed. Furthermore, the smelting slag was crushed, mixed and sampled to analyze its metal content and calculate metal recovery ratio. 2.3 Analysis The main elements in the reactions were bismuth, silver, lead, copper and iron. Gravimetric method, titration method and ICP-AES method were used to quantitatively analyze the proportion of bismuth, lead and copper in the smelting slag. And silver content was determined with fire-assaying method. The phase composition of the leaching residue and the smelting slag was obtained by X-ray diffraction (XRD) using a Rigaku TTTR-III equipment with Cu K radiation (40 kv and 30 ma) and the scanning rate of 10 ( )/min. 3 Results and discussion 3.1 Effect of coal powder amount on reduction smelting The effects of reductive coal powder amount on metal recovery ratio were investigated by reduction smelting at 1200 C for 40 min with CaO/SiO 2 mass ratio of 0.75 and FeO/SiO 2 mass ratio of The results in Fig. 1 show that all the metal recovery ratio increases significantly with the increase of the mass ratio of coal powder to residue. The recovery ratio of bismuth, silver, copper and lead increases approximately from 89.6%, 96.9%, 71.6% and 53.8% to 97.8%, 100%, 84.7% and 88.8% when coal powder amount increases from 4% to 7%. Under the above conditions, the silver content in smelting slag is very low because of easy enrichment of silver in the metal alloy. Although the recovery ratio of most valuable metals has a positive correlation with coal powder amount, such unwanted metals as iron were still reduced into crude 2.2 Experiment procedure The amount of silica and calcium was calculated according to the FeO SiO 2 CaO ternary slag system. All additives containing silica, calcium oxide, reductive coal were mixed with the leaching residue containing Bi in a mortar and pestle. The mixture was put into a clay crucible and then smelted in a pit furnace with a certain heating system. When smelting was finished, the clay crucible was removed, cooled to room temperature and then carefully broken to obtain two parts of smelting products. The upper part is the smelting slag and the Fig. 1 Effect of adding ratio of reductive coal on metal recovery (Conditions: m(feo)/m(sio 2 )=1.25, m(cao)/m(sio 2 )=0.75, smelting time 40 min, 1200 C)

3 1328 bismuth alloy. The iron content in crude bismuth alloy increases from 0.2% to 10% with coal powder amount increasing from 7% to 10%. In order to avoid unwanted metals in crude bismuth alloy, the coal powder amount should be controlled. 3.2 Effect of ternary slag type on reduction smelting The effects of CaO/SiO 2 mass ratio and FeO/SiO 2 mass ratio on metal recovery ratio were investigated by reduction smelting at 1200 C [21] for 40 min with a coal addition of 6%. The results in Fig. 2 show that FeO-to-SiO 2 mass ratio has an obvious influence on metal recovery. The maximum bismuth recovery ratio is 95.2% with FeO-to-SiO 2 mass ratio of The bismuth content in the smelting slag decreases approximately from 5.7% to 3% with FeO-to-SiO 2 mass ratio increasing from 1.0% to 1.25%. The further increase of FeO-to- SiO 2 mass ratio has no obvious influence on bismuth recovery. The recovery ratios of lead and copper are only 43.6% and 75.1%, respectively, with FeO-to-SiO 2 mass ratio of 1.0. When FeO-to-SiO 2 mass ratio ranges from 1.25 to 1.75, the recovery ratios of both lead and copper increase a little and then almost keep constant. Fig. 2 Effect of change of m(feo)/m(sio 2 ) on metal recovery (Conditions: m(cao)/m(sio 2 )=0.75, smelting time 40 min, 1200 C, coal additional proportion 6%) The effect of FeO-to-SiO 2 mass ratio on metal recovery ratio is shown in Fig. 3 with CaO-to-SiO 2 mass ratio of 0.5. From Fig. 3, the maximum bismuth recovery ratio is 97.2% with the lowest bismuth content of 2.18% in the smelting slag. The increase of FeO-to-SiO 2 mass ratio improves the bismuth recovery, which can be explained that the increase of FeO-to-SiO 2 mass ratio can significantly lower the melting point and viscosity of the smelting slag resulting in easier separation of Bi from the slag. For the ternary system of FeO SiO 2 CaO, when FeO-to-SiO 2 mass ratio ranges from 1.5 to 1.75, the melting temperature of the smelting slag is about C. Therefore, the experimental temperature 1200 C is beneficial for increasing slag fluidity. It can J. Cent. South Univ. (2016) 23: Fig. 3 Effect of change of m(feo)/m(sio 2 ) on metal recovery (Conditions: m(cao)/m(sio 2 )=0.5, smelting time 40 min, fining time 20 min, 1200 C, coal additional proportion 6%) be concluded that the reduction smelting process would be improved by decreasing the melting temperature and viscosity of smelting slag. The effect of CaO-to-SiO 2 mass ratio on metal recovery is shown in Fig. 4 with FeO-to-SiO 2 mass ratio of 1.5. The bismuth and lead recovery ratios are only 90.5% and 49.7%, respectively, with CaO-to-SiO 2 ratio of 1.0. For the ternary system of FeO SiO 2 CaO, the melting temperature of the smelting slag is about 1250 C when FeO-to-SiO 2 and CaO-to-SiO 2 mass ratios are 1.5 or 1.0. However, too high melting temperature of the smelting slag is harmful for the smelting process resulting in direct bismuth recovery ratio decreased. When CaO-to-SiO 2 mass ratio is 0.35, higher bismuth recovery ratio (96.3%) is obtained. Therefore, CaO-to-SiO 2 ratio should be controlled at a lower level during smelting. Phase compositions of the typical reduction smelting slag were analyzed by XRD. Figure 5 shows XRD pattern of the slag with FeO/SiO 2 mass ratio of 1.5 Fig. 4 Effect of change of CaO/SiO 2 on metal recovery (Conditions: m(feo)/m(sio 2 )=1.5, smelting time 40 min, fining time 20 min, 1200 C, coal additional proportion 6%)

4 J. Cent. South Univ. (2016) 23: Fig. 5 XRD pattern at m(feo)/m(sio 2 )=1.5 and m(cao)/ m(sio 2 )=0.35 and CaO-to-SiO 2 mass ratio of 0.35, where the main phases are Fe 2 O 3, CaO Al 2 O 3 2SiO 2 and 2CaO Al 2 O 3 SiO 2. The formation of CaO Al 2 O 3 2SiO 2 and 2CaO Al 2 O 3 SiO 2 is attributed to the slagging reaction between Al 2 O 3 in leaching residue and the additives of CaO and SiO 2. Although the melting temperature of CaO Al 2 O 3 2SiO 2 and 2CaO Al 2 O 3 SiO 2 is relatively high, CaO Al 2 O 3 2SiO 2 and 2CaO Al 2 O 3 SiO 2 could still form due to their relatively low content in the slag. The XRD pattern of the reduction smelting slag with FeO/SiO 2 mass ratio of 1.0 and CaO-to-SiO 2 mass ratio of 0.75 is shown in Fig. 6, where the main phases are FeO Fe 2 O 3, CaO FeO 2SiO 2 and SiO 2. The high CaO content can promote the transformation of the complex anion (Si x O y ) z into smaller anions. So, the characteristic peaks of SiO 2 can be observed in Fig. 6. The existence of CaO FeO 2SiO 2 is consistent with SiO 2 CaO FeO phase diagram. The characteristic peaks of Fe 2 O 3 can be observed from Fig. 5 and Fig. 6, showing that reductive coal addition is inadequate to reduce Fe 2 O 3 into FeO. Therefore, a low amount of reductive coal is not conducive to smelting. A series of optimal experiments were conducted with various mass ratio of coal powder to residue. The experimental conditions and results are given in Table 3. From Table 3, the bismuth and lead recovery ratios are above 97% and 88%, respectively, with FeO/SiO 2 mass ratio of and CaO/SiO 2 mass ratio of The copper recovery ratio is 91.99% with FeO-to-SiO 2 mass ratio of 1.5 and CaO-to-SiO 2 mass ratio of 0.5. Therefore, the optimal slag type is of FeO-to-SiO 2 mass ratio at 1.5 and CaO-to-SiO 2 mass ratio at 0.5. The phase composition of smelting slag in the optimal slag system is determined by XRD, and its XRD pattern is shown in Fig. 7. The characteristic peaks of CaO Al 2 O 3 2SiO 2 and 2FeO SiO 2 appear in Fig. 7. The optimal slag type is around the lowest melting point area in FeO SiO 2 CaO ternary phase diagram, and the melting point of fayalite is relatively low, so it can be concluded that the main phase in smelting slag is 2FeO SiO 2. Table 3 Further selection and optimization of slag constitution Metal recovery /% FeO/SiO 2 CaO/SiO 2 Bi Cu Pb Fig. 7 XRD pattern at m(feo)/m(sio 2 )=1.5, m(cao)/m(sio 2 )= 0. 5 and 1200 C Fig. 6 XRD pattern at m(feo)/m(sio 2 )=1.0 and m(cao)/ m(sio 2 )= Effect of temperature on reduction smelting The effect of smelting temperature on metal recovery is shown in Fig. 8, where the higher the temperature, the higher the bismuth recovery ratio. The recovery ratios of bismuth, silver, copper and lead at 1100 C are 97.1%, 98.1%, 91.4% and 91.3%, respectively. The recovery ratio of bismuth reaches 99% and its content in smelting slag is less than 0.17% at the temperature over 1300 C. Increasing temperature not

5 1330 Fig. 8 Effect of temperature on metal recovery (Conditions: m(feo)/m(sio 2 )=1.5, m(cao)/m(sio 2 )=0.5, smelting time 40 min, fining time 20 min, coal additional proportion 7%) J. Cent. South Univ. (2016) 23: that the composition of the smelting slag is very complex and similar to amorphous structure. 3.4 Effect of reaction time on reduction smelting The effect of smelting time on metal recovery is shown in Fig. 11, where it is can be seen that prolonging reaction time slightly improves metal recovery ratio. The bismuth recovery ratio increases from 96.7% to 99.0% with reaction time increasing from 20 min to 80 min, showing that increasing reaction time is helpful for more complete reduction and settling separation. However, prolonging reaction time decreases production efficiency and increases energy consumption. Therefore, the optimal smelting time is selected at 40 min. The recovery ratios of bismuth, silver, copper and lead are 99.6%, 99.8%, 97.0% and 97.3%, respectively, realizing efficient recovery of bismuth, silver, copper and lead. only accelerates reduction reaction but also decreases the viscosity of smelting slag to promote the settling separation of metal phase from slag phase. The phase composition of smelting slag at 1300 C and 1400 C is analyzed by XRD, having the results in Fig. 9 and Fig. 10. The characteristic peaks of silica appear, but the characteristic peaks of other products are not observed as expected in Fig. 9 and Fig. 10. The results demonstrate Fig. 11 Effect of smelting time on metal recovery (Conditions: m(feo)/m(sio 2 )=1.5, m(cao)/m(sio 2 )=0.5, fining time 20 min, coal additional proportion 7% and 1200 C) 4 Conclusions Fig. 9 XRD patterns at m(feo)/m(sio 2 )=1.5, m(cao)/ m(sio 2 )=0. 5 and 1300 C Fig. 10 XRD patterns at m(feo)/m(sio 2 )=1.5, m(cao)/ m(sio 2 )=0. 5 and 1400 C 1) Reduction smelting of bismuth oxide residue from pressure leaching of bismuth sulfide was investigated in the FeO SiO 2 CaO ternary slag system. The results show that all the recovery ratios of Bi, Ag, Cu and Pb increase with the increased reductive coal proportion, reaction temperature and time. But too much reductive coal would help Fe enter metal phase. 2) The chosen slag system range of m(cao)/m(sio 2 ) and m(feo)/m(sio 2 ) would be and , respectively, for the reason that the slag system in the above range has the optimum mobility and is beneficial for metal recovery. Fayalite phase would appear in smelting slag system according to XRD analysis. 3) The corresponding optimum conditions are determined as follows: smelting temperature of 1300 C, smelting time of 40 min, coal addition of 7%, CaO/SiO 2 mass ratio of 0.5 and FeO-to-SiO 2 mass ratio of 1.5. Under the above conditions, the recovery ratios of

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