Eco-techno-economic synthesis of process routes for the production of zinc using combinatorial optimization
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IN 1992, 6.8 3 106 tons of primary zinc were produced worldwide. About 80 pct of this quantity was produced hydrometallurgical by roasting, various leaching steps, and electrowinning, as is depicted in Figure 1. The iron that is dissolved in the leaching steps is precipitated mostly as jarosite and sometimes as goethite or as hematite. Precipitating these iron residues also prevents the buildup of some of the other impurities, e.g., Ge and As.[1] Today, the iron containing residues are still usually dumped into ponds; only hematite is reported to have found an application in the cement industry.[2] Jarosite is regarded as a hazardous chemical waste in some countries, e.g., The Netherlands,[1] due to the possibility of hazardous elements being leached from it. Figure 1 shows the unit operations of the various hydrometallurgical production routes for zinc, including a roasting step (1) with its off-gas cleaning units (4,6) and different leaching (5,7,29) and iron precipitation steps (10,11,12,13). Included in Figure 1 are some rather uncommon unit operations such as flotation of lead silver residue (9) and different processes for the hydrometallurgical treat¨ LTER, Postdoctoral Student and Research Metallurgist, and S. SUDHO ¨ GER, Professor and Head, are with the Institute of Nonferrous J. KRU Process Metallurgy, Technical University of Aachen, 52056 Aachen, Germany. M.A. REUTER, Professor, is with the Faculty of Mining and Petroleum Engineering, Delft University of Technology, Mijnbouwstraat 120, 2628 RX Delft, The Netherlands. Manuscript submitted August 7, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS B
ment of zinc containing residues (8,14). Some unit operations such as the pressure leaching of zinc concentrates (2), an activating grind and leach step (3), and the direct pyrometallurgical recovery of zinc from concentrates such as the Warner process[3] could not as yet be included in the synthesis model due to a lack of suitable data. For this reason, these processes were only included in Figure 1 and connected to other unit operations by dotted lines. The zinc industry has to cope with the increasing environmental pressure.[1,4–6] This, and rather severe economic constraints, makes rethinking of zinc process technology imperative. The processing of various concentrates from new mines, which are characterized by varying amounts of iron, silica, manganese, and several minor elements such as germanium[5] and zinc containing residues (secondary raw materials) from other industries only complicate matters more. Figure 2 depicts various process routes for the treatment of the different residues produced during the hydrometallurgical recovery of zinc, i.e., the neutral leach residue, the lead and silver residue, and the iron residue (jarosite, goethite). This part of the overall flow sheet depicts dumping of the residues as well as flotation of jarosite/goethite (35) or neutral leach residue (15) as well as different unit operations for the pyrometallurgical treatment of the residues, i.e., the Isasmelt process (IS
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