Recycling of bismuth oxides

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 3 2021 magnesium addition to the molten bath [6‒8]. The obtained bismuth drosses consisting of 3–5 % Bi and 80–85 % Pb are processed by smelting at 500–600 °С with the addition of NaNO 3 and NaOH. The result of the process is alkali flux (bismuth oxides of %: 1–5 Bi; 60–70 Pb) which is sent to the lead refining cycle, and Pb-Bi alloy (1–12 % Bi) of grades SSV-1, SSV-3, SSV-6, SSV-9, SSV-12, which is shipped to the customers. The turnover of bismuth oxides causes accumulation of valuable metals in the technological cycle. Whereas the demand for Pb-Bi alloy is limited, it is necessary to expand the range of commercial products as well as to increase the content of the target product and to enhance the added value of the product. The production of rough bismuth during the processing of bismuth drosses and oxides is possible using pyro [9–11], hydro [12–14], pyroelectric [15–17] and hydroelectrometallurgical [18–20] methods. Electrometallurgical technologies implemented through two main production methods can be considered promising and can be applied for modernization of Non-ferrous Alloys Production branch. The first variant of the implementation is the hydroelectrometallurgical method, which includes: – reduction smelting of oxides with the addition of sodium carbonate, quartz and coke to produce slag which can be recycled in the copper smelting process, dust returned to smelting, and bismuth lead, cast into anodes, followed by electrolysis in a silicofluoride solution to obtain recycled cathode lead and anode sludge subjected to washing and drying; – melting of sludge mixed with coal and sodium carbonate to obtain recycled slag and ingots of commercial rough bismuth; – melting of recycled slag in a mixture with coal and sodium carbonate to obtain recyclable waste slag and ingots of commercial rough bismuth [21]. The second variant of the implementation is the pyroelectrometallurgical method, which includes: – reduction smelting of oxides to produce slag, dust and ingots of bismuth lead; – electrolysis of the latter in a chloride melt to produce cathode lead and ingots of enriched bismuth lead; – secondary electrolytic processing of ingots in a chloride melt to produce cathode lead and ingots of commercial rough bismuth [22]. Bismuth from its oxygen compounds is obtained by reduction smelting in a molten sodium hydroxide in the presence of elemental sulfur. reduction smelting is carried out in the presence of coal at a ratio of bismuth : alkali : coal : sulfur equal to 1 : (0.8 – 1.8) : (0.04 – 0.16) : (0.02 – 0.08) [23]. Low temperature reduction smelting includes mixing of bismuth-bearing material and starch followed by melting to obtain rough bismuth. Starch as a reducing agent makes it possible to lower the reduction temperature to 800 – 850 ºС and to increase the direct extraction of bismuth to ~ 95 % [24]. Both approaches have a great interest for processing of oxidized drosses. The aim of this study is to study and develop fundamental methods and cost-effective integrated technologies of bismuth drosses and oxides processing, which are obtained at rough lead refining. Bismuth drosses and oxides are processed applying reduction smelting of raw material and bismuth-enriched slime, and electrolysis of lead bismuth resulting in production of rough bithmuth containing ≥ 90 % Bi. The direct recovery of bismuth from processed products amounts to ≥ 70 %. The objectives of this study are as follows: assessment of the material composition of the original oxides and the products obtained; experimental simulation of the basic operations, namely oxides smelting with the addition of sodium carbonate, silicon dioxide and a carbon-bearing reducing agent; justification of the main redevelopments of the proposed technological scheme. Methodology In order to produce lead bismuth the charge containing bismuth oxides, sodium carbonate, silicon diox- ide and carbon was melted at 1,100–1,150 ºС (Table 1). The charge components were mixed and placed in an alundum crucible, which was installed into an electric resistance furnace with graphite heating elements, and covered with a graphite crucible. The furnace was heated to a preset temperature.