PRIMARY PRODUCTION PROCESSES
There are three different types of primary zinc production. The first method is a metallurgical process called electro-thermic distillation. The process is used to combine roasted concentrate and secondary zinc products into a sinter feed that is burned to remove zinc, halides, cadmium, and other impurities. The resulting zinc oxide-rich sinter is combined with metallurgical coke in an electric retort furnace that reduces the zinc oxides and produces vaporized zinc which is captured in a vacuum condenser. The reduction results in the release of non-energy carbon dioxide (CO2) emissions. The electro-thermic distillation process is used in the United State and in Japan. (Sjardin, 2003; European IPPC Bureau, 2001)
The second method of zinc production is a pyrometallurgical process involving the use of an Imperial Smelting Furnace, which allows for the simultaneous treatment of lead and zinc concentrates. The process results in the simultaneous production of lead and zinc and the release of non-energy CO2 emissions. The metallurgical coke/coal reductant used in this process must be allocated to lead and zinc production in order to perform an emission calculation without double counting. A mass based allocation results in a factor of 0.74 tonnes coke/tonne zinc. (Sjardin, 2003; European IPPC Bureau, 2001)
The third zinc production method is the electrolytic process, which is a hydrometallurgical technique. In this process, zinc sulphide is calcined, resulting in the production of zinc oxide. The zinc oxide is then leached in sulphuric acid and purified to remove iron impurities, copper, and cadmium. The zinc is then drawn out of the solution using electrolysis. The electrolytic process does not result in non-energy CO2 emissions. (Sjardin 2003; European IPPC Bureau 2001)
There are more than 40 hydrometallurgical and pyrometallurgical technologies that can be used to recover zinc metal from various materials. The preferred method for a given situation depends on the zinc source (contamination level and zinc concentration) and the desired end use for the recovered zinc. The process frequently consists of zinc concentration (through physical and/or chemical separation), sintering, smelting, and refining. In some cases, high grade zinc is removed from this process after physical concentration and consumed by other industries, including iron and steel manufacture, brass manufacture, and zinc die-casting, without going through the rest of the process steps. (Sjardin, 2003)
The sintering, smelting, and refining steps are identical to the steps used in the primary zinc production process, so certain smelting processes are considered emissive, while the sintering and refining steps are considered non emissive from the perspective of non-energy CO2 emissions. When the concentration step involves the use of a carbon-containing reductant and high temperatures to volatilize or fume zinc from the source materials, the process could result in non-energy CO2 emissions. The Waelz Kiln and slag reduction or fuming processes are two such concentration methods. The Waelz Kiln process, which is used to concentrate zinc in flue dusts, sludges, slags, and other zinc-containing materials, involves the use of metallurgical coke as a reductant. However, the reduced zinc is re-oxided during the processes and the metallurgical coke also serves as a heat source during the process. The slag reduction or fuming process, which is used strictly to concentrate zinc in molten slags from copper and zinc smelting, involves the use of coal or another carbon source as a reductant. (Sjardin, 2003; European IPPC Bureau, 2001)
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