The ENVIROPLAS process is being developed at Mintek in Randburg, South Africa, for the treatment of solid wastes from the metallurgical industry, especially steel-plant dusts, slags, and leach residues that contain zinc oxide. The process is based on the carbothermic reduction of selected metal oxides (e.g. zinc oxide), at high temperatures in a DC plasma-arc furnace. In most countries, the dumping of steel-plant dusts is regarded as presenting an environmental hazard because toxic metals may be leached into drinking-water supplies. The main objectives of the ENVIROPLAS process are to produce disposable slag, and in the case when electric arc furnace (EAF) dust or zinc-containing waste is processed, to recover metallic zinc in a lead-splash condenser. When dust from a stainless-steel operation (e.g AOD dust) is smelted, another objective is to recover chromium and nickel into a crude ferroalloy.

Initial testwork at the 50 to 500 kW scales of operation [1-3], was conducted during 1989 to 1993 on the smelting of EAF dust (high-zinc, carbon-steel dust), zinc-bearing alloy-steel dust, lead blast furnace slag, ISF (Imperial Smelting Furnace) slag, and primary leach residue. The volatilized zinc and lead were burnt in a combustion chamber, and a mixed oxide of zinc and lead was recovered in a bag filter. High extractions of zinc, chromium and nickel (each more than 95 per cent) were achieved, and the slags produced met the criteria of the US EPA for safe disposal. This warranted the building of a 3 MW demonstration plant, including a plasma-arc furnace and an ISP (Imperial Smelting Processes Ltd, UK) lead-splash condenser. The plant was successfully commissioned during 1994/95 using about 1000 t of lead blast-furnace slag [4,5].

In 1994 a hydrometallurgical process was developed at Mintek for the dehalogenation of EAF dust. It is known that high levels of halogens, especially chlorine, interfere with the proper operation of the condenser due to the formation of large quantities of dross. In 1995 about 300 t dehalogenated dust was prepared for demonstration-scale work. During the latter part of 1995, about 130 t of dehalogenated EAF dust containing about 25 per cent zinc oxide, was smelted at a typical dust feed-rate of 1 t/h. About 100 kg of coke was used as reducing agent per ton of dust. The operating power was about 1.3 MW and the zinc fuming rate was between 150 and 200 kg/h, at operating temperatures of 1450 to 1500°C. Zinc oxide levels of 1 per cent were achieved in the slags tapped from the furnace, which corresponded with zinc extraction efficiencies of about 97 per cent. The slags were submitted for TCLP leaching tests, and were found to conform with US EPA disposal regulations. The zinc tapped from the lead-splash condenser met PW (Prime Western) grade specifications in most cases. During steady-state operation, zinc condensing efficiencies of 70 to 75 per cent were achieved. Equipment problems were identified during the demonstration-scale testwork, and these are at present being addressed. A follow-up smelting campaign is planned for 1996, after implementation of the necessary equipment changes, to demonstrate consistent operation for longer periods of time, at acceptable zinc recoveries.


1. A.F.S. Schoukens, L.R. Nelson, and N.A. Barcza, "Plasma-arc treatment of steel-plant dust and zinc-containing slag - theoretical and practical considerations", Recycling of Lead and Zinc the Challenge of the 1990s, Proceedings of an international conference organized by the International Lead and Zinc Study Group Sub-Committee on Recycling, Rome, June 1991, pp. 361-370.

2. A.F.S. Schoukens, F. Shaw, and E.C. Chemaly, "The Enviroplas process for the treatment of steel-plant dusts", Journal of the South African Institute of Mining and Metallurgy, vol. 93, no. 1, January 1993, pp. 1-7.

3. N.A. Barcza, C.J. Hutton, M.J. Freeman, and F. Shaw, "The treatment of metallurgical wastes using the Enviroplas process", International symposium on Extraction and Processing for the Treatment and Minimization of Wastes, 1994 TMS Annual Meeting, 27 February - 3 March 1994, San Francisco. Publication of the Minerals, Metals and Materials Society, Warrendale, Pennsylvania, 1993, pp. 941-962.

4. N.A. Barcza et al, "Enviroplas technology for the recovery of lead and zinc from lead blast furnace slag". Presented at Recycling lead and zinc into the 21st Century : 6th International Conference of the International Lead and Zinc Study Group", Madrid, Spain, 23-28 June, 1995.

5. A.F.S. Schoukens, G.M. Denton, and R.T. Jones, "Pilot-plant production of Prime Western grade zinc from lead blast-furnace slags using the Enviroplas process". Proceedings of the third international symposium on recycling of metals and engineered materials, Point Clear, Alabama, 12 to 15 November 1995. Publication of the Minerals, Metals and Materials Society, 1995, pp. 857-868.

Copyright © 1996-97 Albert Schoukens, Mintek,
19 June 2001

Pyrometallurgy Division, Mintek,
200 Hans Strijdom Drive, Randburg, 2125, South Africa
Private Bag X3015, Randburg, 2125, South Africa.

Phone: +27 (11) 709-4644
Fax: +27 (11) 793-6241