Waelz process

Cupola filter dust is re-injected into the cupola furnace. The aim of this technique is to achieve an accumulation of zinc in the dust, up to a level that allows reprocessing with Zn recovery (>18 %). Zn recovery is performed using the Waelz process. 

Over the period 1955-1961, the electrolytic plant processed only calcined zinc oxide having a composition of 60% Zn, 3-4% Pb and 0.03% Cd. The zinc oxide was produced by calcination of the fumes from the Waelz process at temperatures from 1420 to 1470 K. Zinc carbonate ores, Zn-Fe residues and other zinc-bearing materials were used as raw materials in the Waelz process. After enhancement of the electrolytic zinc plant in 1962, the importance of roasted sulphide concentrates was continuously growing in the feed. Currently, only the calcine from fluid-bed roasters is processed. Table I shows the analysis of the raw materials processed over the period 1998-1999. 

Electric arc furnace (EAF) dusts have become a significant source of feed for the ISP. These contain up to 35% Zn and environmental legislation in developed coxmtries is discouraging the previous practice of dumping EAF dusts. In a number of countries the Waelz process to produce zinc oxides treats these dusts. In some cases these oxides are hot briquetted as a direct feed for the ISF. An alternative method of treating steel plant and other dusts is to inject them directly into the furnace through the tuyeres. 

Waelz process. In this process, after the calcine is leached, the zinc ferrite residue is filtered, washed, dried, and heated with coke in a rotary hearth furnace. The zinc is reduced and fumed off as metallic vapor, reoxidized, and collected as a pure zinc oxide in the bag-house dust and normally leached in a separate step. Part of the lead and silver is recovered, and the iron remains in the slag. 

Waelz process A process used for the extraction of zinc and lead from ores using a rotary Idln. Zinc is still largely extracted from ores although around 40 per cent of zinc is recycled from galvanized steels and other scrap metals. A high-temperature rotary Waelz Uln is used to convert zinc dust from electric arc furnaces into zinc oxide, which is then converted to zinc metal. The process of zinc volatilization was first proposed in 1881. 

Waelz zinc oxide process, 26 612 Waferboard bonding, phenolic resins in,... 

The Waelz kiln operation is subjected to the same stringent environmental standards as the EAF steelmaking process. Trials have been undertaken in Europe and the USA to determine whether these same standards are met when operating with mercury free primary batteries as part of the feed. 

The Waelz plant processed 400,000 tonnes/year of zinc carbonate ore typically containing 10 % Zn and 2 % Pb and Zn-Fe residues with zinc contents of about 16-18 %. After running out of zinc carbonate ore deposits and because of environmental reasons, nine kilns were liquidated in 1989. Currently, six kilns are operated 4 as Waelz kilns and 2 for ealeination of the Waelz fumes. 

Neutral slurry is classified in hydrocyclones (3500/12°). Underflow fiom the hydrocyclones is pumped to acid leaching, and the overflow to five Dorr thickeners, each with a diameter of 9.5 meters. Magnafloc 338 is used for this process (0.017 kg per toime of cathode zinc). Underflow from each Dorr thickener is pumped to low acid leaching, and after classification and washing, to Larox filters. The filter cake with a maximum moisture content of 17 % is fed to the Waelz kilns. 

Besides the metallic iron, a Waelz slag produced from lead residues is advantageous to add into the feed of the Kivcet process. The size of the Waelz slag should be minus 20 mm its moisture content should be no more than 3%. The composition of the mixed feed using Waelz slag is shown in Table I. 

The need to add Waelz slag to the charge is dictated by two circumstances. First, Waelz slag is a good reductant since it contains free unreacted carbon second, iron contained in the Waelz slag is also a good reductant and serves to remove lead from the matte. Thus, a fundamental possibility to smelt a lead-copper-zinc feed achieving satisfactory process values was found in the course of the laboratory tests. 

C. Mattich, K. Hasselwander, H. Lommert and A.N. Beyzavi, Electrolytic Zinc Manufacture with Waelz Treatment of Neutral Leach Residues , Zinc and Lead Processing. J.E. Dutrizac, J.A. Gonzalez, G.L. Bolton and P. Hancock, Eds., Canadian Institute of Mining, Metallurgy and Petroleum, Montreal, Canada, 1998, 561-578. 

De la Cuadra (1991) has surveyed the treatment of zinc-coated steel sheet scrap including the wet processes (Waelz, Contop, St. Joe, Plasmadust, and Inmeco) and the dry processes (Cebedeau and Cardiff) and outlined the new CENIM-LNETI process (developed by the Spanish metals research body (CENIM) with others) involving leaching with ammonium chloride, all of which demonstrate that zinc-coated steel scrap gives no undue problems in treatment. 

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