Fundamentals of zinc recovery from metallurgical wastes in the Enviroplas process

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Abstract

The Enviroplas process was developed at Mintek to treat certain metallurgical wastes, such as lead blast furnace slag, electric arc furnace dust, and neutral leach residues from the zinc industry. The process relies on a DC arc furnace and an ISP lead splash condenser for the direct recovery of the contained zinc as prime western grade metal, with the simultaneous production of a disposable slag.

This paper outlines certain fundamental issues of the process, including feed preparation and specifications, operating temperatures, coke (coal) requirements, flux addition, and recycling of the fumes and the condenser dross. In addition, the chemical reactions involved and the associated kinetics are presented and discussed, as well as the major factors that influence the condensation process. Selected test results are also included to highlight the significance of the most important factors that affect the process.

Introduction

Over the past ten years, Mintek has been involved in an extensive R&D program in order to develop a viable and cost-effective method for the treatment of certain metallurgical wastes as a means of minimising, if not eliminating, the negative environmental impact of such materials. The success of the program has been demonstrated through the development of the Enviroplas process to a stage where it is ready for commercial implementation.

The Enviroplas process involves the smelting of electric arc furnace (EAF) dust, lead blast furnace (LBF) slag or neutral leach residues (NLR) in a DC arc furnace and the subsequent recovery of the volatilised zinc in an ISP lead splash condenser (LSC) as Prime Western grade metal. To date, more than 2000 tons of LBF slag and 300 tons of EAF dust has been processed, whereby high zinc extraction (of 85% from LBF slag and 98% from EAF dust) was consistently achieved, resulting in zinc fuming rate of more than 100 kg Zn/hm2 (LBF slag) and 130 kg Zn/hm2 (EAF dust). The zinc condensation efficiency at the pilot scale ranged from 65% to 75% and the produced slag consistently met the USA EPA TCLP test for safe disposal.

Recyclability of the generated fume and the condenser drosses was proven in a top blown rotary convertor (TBRC), whereby more than 1000 kg of typical dross was treated for lead metal recovery. Lead recovery was 90% and greater, giving a metal phase containing more than 99% Pb that is suitable for re-use in the condenser.

Throughout the developmental program, a great deal of experience and knowledge was gained with regard to the process fundamentals and their roles either in the smelting stage (fuming) or the condensation stage. In addition, the understanding of these fundamentals clarified many aspects concerning raw materials specifications, and thus the requirements of any pre-treatment step that could be needed. This paper is intended to highlight certain fundamental issues that are believed to be important in zinc recovery in the Enviroplas process. Selected test results are also included to emphasise the significance of the most critical factors that affect the process.

Section snippets

Description of the Enviroplas process

Depending on the material to be treated, the Enviroplas process can be described as follows: for LBF slag, feeding of liquid slag from a lead blast furnace through a holding furnace is an attractive energy-saving option (Schoukens et al., 1995). However, dry and granulated slag (stockpiled in dumps) can be directly charged into the fuming furnace. Metallurgical coke is employed as the reducing agent and is delivered to the fuming furnace at a controlled rate. Coals, charcoal or other

Chemistry of zinc fuming

Due to the wide range of chemical species that are present, the reactions taking place in the fuming furnace are numerous and complex. However, the chemical reactions that lead to the extraction of zinc and its subsequent volatilisation can be described as follow, Dal and Rankin (1993):ZnO+CO(g)=Zn(g)+CO2(g)2FeO+ZnO=Zn(g)+Fe2O3It is widely believed that these reactions take place at the slag/gas interface in contact with the coke particles (Dal and Rankin, 1993). As a result, the local

Feed preparation

The proceeding section highlighted certain requirements and specifications that the raw materials should meet. Other requirements include the following:

Condensation

The pilot-scale ISP condenser consists of a rectangular box that contains the molten lead and a rotor, a pump sump to allow circulation of Pb–Zn metal, a cooling launder, a flux bath and separation bath. The operating philosophy is as follows: the rotor speed is adjusted to attain a low condenser off-gas temperature (400–450 °C). Lead–zinc solution (about 3.5–4.5% Zn, balance Pb, at 550 °C) is continuously pumped from the condenser into the cooling launder, where it flows into the flux bath and

Conclusions

Pilot-scale testwork has proven the technical feasibility of the Enviroplas process for the recovery of zinc from EAF dust and LBF slag. The produced slag consistently met the USA EPA TCLP test for safe disposal. Very good zinc extraction, fuming rates, and condensation efficiency were achieved during the testwork.

Chemical and physical factors that affect the fuming process include: temperature, reductant addition and reactivity, slag basicity, Fe3+/Fe2+ ratio, bath stirring, and arc attachment

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Presented at Pyromet ’02, Cape Town, South Africa, March 2002.

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