by J. E. CLAY* (M.I.M.M.) (Visitor) and G. P. SCHOONRAAD*. Pr. Eng.. B.Sc. Eng. (Rand) (Member)
A review is given of the Waelz process. The chemistry of the process and the mechanism of crust formatbn cretions) are briefly dealt with. The plant of Kiln Products Limited and its operation are described. The problems encountered andmethods for their elimination are presented.
'n Oorsig van die Waelz proses word gegee. Die chemie van die proses en die meganisme van die kors formasie word saaklik behandel. Die aanleg te Kiln Products Limited en die operasie daarvan word beskryf. Die probleme wat teengekom is asook die oplossing daarvan word gestel.
INTRODUCTION Kiln Products Limited, which is adjacent tothe Berg Aukas Mine, is situated approximately 21 kilometres north-east of Grootfontein in South West Africa. The mine, which is operated by the South West Mrica Company Limited, produces lead-zinc vanadate, lead and zinc sulphides, and zinc silicate concentrates by froth flotation. The production of zinc silicate (willemite) concentrate, which contains about 45 per cent zinc, amounts toapproximately 3000 tonnes per month. An appreciable amount of zinc silicate in the form of a fine slime, which is not recoverable by froth flotation, is also produced. The slime, which contains about 18 per cent zinc, is collected in settling ponds for later treatment in the Waelz Kiln. In March 1969, an associated company, Kiln Products Limited, commissioned a Waelz Kiln at Berg Aukas to recover the zincfrom the willemite concentrate, current slime tailings, and tailings from a dump previously accumulated. The zinc oxide produced is transported by road and rail over a distance of 2800 kilometres to Zinc Corporation of South Mrica Limited, near Springs, where ingot zinc is produced by an electrolytic process. The original pilot-plant investigations were carried out in West Germany by MessrsFrederick Krupp Forschungsinstitut. Additional testwork was subsequently done by Messrs Lurgi Gesellschaft fUr Chemie und Htittenwesen M.RH., and the latter company was the successful bidder for the commercial plant. The following conclusions were drawn from the testwork. (a) It is possible to produce a zinc oxide averaging 63,3 per cent zinc and 10,2 per cent lead, with a magnesium content rangingfrom 0,4 per cent to 1,4 per cent. (b) The burden starts softening at about 1100°C. (c) To avoid excessive formation of accretions, the operating temperature should not exceed 1100 °c. (d) At least 90 per cent of the zinc present can be volatilized without exceeding 1100°C. (e) As a source of additional heat, gas is preferable to either oil or pulverized coal owing to its softer flame. (f) Althoughaccretions are to be expected, it should be possible to treat the raw materials tested if the operating conditions are controlled. (g) To overcome dust losses during transport and to
minimize health hazards, the final product should be pelletized. Further testwork indicated that, although the magnesium and fluorine might prove troublesome, high recoveries of zinc couldbe expected. THE W AELZ PROCESS The use of a revolving tubular furnace for the volatilization of zinc was proposed for the first time in 1881 by George Druyeel. However, it was not until 1923 that Krupp Grusonwerk, in collaboration with Metallgesellschaft A.G., started developing the process. The process can treat ores, tailings, middlings, slimes, ashes, slags, and residues of any kind. Thematerial containing zinc in the form of zinc oxide, zinc ferrite, zinc silicate, or zinc sulphide is mixed with any carboncontaining fuel. When heated in a horizontal rotary kiln at temperatures ranging from 1000 to 1500 °c, the zinc is reduced, volatilized, and oxidized to zinc oxide. The zinc oxide is then separated from the exhaust gases by bag filters or electrostatic precipitators. It is commonly...