There are two primary processes for the production of rough lead bullion from lead concentrates. The first type is sintering/smelting, which consists of sequential sintering and smelting steps and constitutes roughly 78 percent of the primary lead production. The second type is direct smelting, which eliminates the sintering step and constitutes the remaining 22 percent of primary lead production in the developed world. (Sjardin, 2003)
In the sintering/smelting process, the initial sintering blends lead concentrates with recycled sinter, lime rock and silica, oxygen, and high-lead-content sludge to remove sulphur and volatile metals via combustion (Metallurgical Industry, 1995). The process, which produces a sinter roast that consists of lead oxide and other metallic oxides, results in the emission of sulphur dioxide (SO2) and energy-related carbon dioxide (CO2) from the natural gas used to ignite the lead oxides (DOE, 2002). The sinter roast is then put in a blast furnace along with ores containing other metals, air, smelter by-products, and metallurgical coke (Metallurgical Industry, 1995). The coke burns as it reacts with air and produces carbon monoxide (CO) that actually performs the reduction of the lead oxide by chemical reaction (DOE, 2002). The smelting process occurs in either a traditional blast furnace or an Imperial Smelting Furnace, and it is the reduction of the lead oxide during this process that produces CO2 emissions (Sjardin, 2003). The sintering process produces molten lead bullion (Metallurgical Industry, 1995).
In the direct smelting process, the sintering step is skipped, and the lead concentrates and other materials are entered directly into a furnace in which they are melted and oxidized (Sjardin, 2003). A variety of furnaces are used for the direct smelting process, with the Isasmelt-Ausmelt, Queneau-Schumann-Lurgi, and Kaldo furnaces used for bath smelting and the Kivcet furnace used for flash smelting. A number of reducing agents, which include coal, metallurgical coke, and natural gas, are used in the process in different quantities for each furnace, which results in different levels of CO2 emissions for each type of furnace (Sjardin, 2003; LDA, 2002). The direct smelting process offers significant environmental and potential cost saving benefits through the avoidance of the sintering process and is therefore expected to constitute a growing portion of primary refinery lead production in the future (LDA, 2002).
The secondary production of refined lead amounts to the processing of recycled lead to prepare it for reuse. The vast majority of this recycled lead comes from scrapped lead acid batteries. The lead acid batteries are either crushed using a hammer mill and entered into the smelting process with or without desulphurization or they are smelted whole (Sjardin, 2003). Traditional blast furnaces, Imperial Smelting Furnaces, electric arc furnaces, electric resistance furnaces, reverbatory furnaces, Isasmelt furnaces, Queneau-Schumann-Lurgi furnaces, and Kivcet furnaces can all be used for the smelting of these batteries and other recycled scrap lead (Sjardin, 2003). As with the furnaces used for primary lead bullion production, these furnaces generate different levels of CO2 emissions from their use of differing types and quantities of reductants. The primary reductants are coal, natural gas, and metallurgical coke, although the electric resistance furnace uses petroleum coke (Sjardin, 2003).
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