Zinc ferrites are one of the major forms of zinc in some wastes, such as steel mill EAF dusts and the leaching residues of roasted zinc sulfide concentrates. These ferrites can be very difficult to chemically decompose so that the zinc can be recovered. This decomposition is the key to the recovery of zinc in these solid wastes. In this work, the recovery of zinc from synthetic zinc ferrite was investigated. It was found that around 75-80% of the zinc in zinc ferrite can be extracted after being fused directly with NaOH pellets and dissolved in an alkaline leaching solution. The recovery increased to over 90% when the ferrite was hydrolyzed with water or dilute NaOH solution prior to the fusion step. 02000 Elsevier Science Ltd. All rights reserved.

Zinc and lead are usually concomitantly present in Zn-Pb ores and solid wastes, such as electric arc furnace (EAF) dusts. One of the treatment methods is to extract zinc and lead from the ores and wastes by an alkaline leaching process if applicable. The selective and quantitative separation of lead from zinc is an important step in these processes. In this work, the separation of lead from zinc in alkaline zinc solution, in leaching solutions of EAF dusts, and in oxidized zinc ores by sodium sulfide precipitation was studied. When the weight ratio of sodium sulfide (average molecular weight 222) added to the lead present in alkaline solution was over 1.8-2.0 (molar ratio approximately 1.5-1.7), lead could be separated selectively and quantitatively, while the zinc remained in the solution without concomitant loss. The residues from the precipitation step were identified as mixtures of PbS and Na2Pb (OH)2S, with PbS being the predominant compound.

In this work, a production process for Zn powder by alkaline treatment of an oxidized Zn ore, smithsonite ZnCO, was studied. It was found that over 85% of both Zn and Pb, and less than 3 10% of Al can be leached from the ore when the leaching operation is conducted at over 958C using 5 M NaOH solution as leaching agent. The dissolution of impurities such as Fe, Ca, etc., was negligible. Leaching of Pb can be improved remarkably with addition of NaCl to the leaching systems. Typical composition of the leach solution is 22-25 grL Zn, 2.96-3.05 grL Pb and 0.5-0.7 grL Al. Zn and Pb contents in the leaching residue were found to be lower than 2.4% and 0.05-0.2%, respectively. Pb present in the leach solution is separated with the addition of sodium sulfide. The Pb-free solution is then used for the electrolysis of metallic Zn using stainless steel electrodes. Zn metal powder with purity higher than 99.5% is obtained. The specific energy for the Zn electrolysis in alkaline leach solution is around 2.4-2.6 kWhrkg Zn, which is lower than the energy consumption of 3.3 kWhrkg Zn required in the conventional process of Zn electrolysis in acidic sulfate electrolytes. The process developed is considered to be cost-effective, simple and easy to be operated and managed, and the flowsheet for the production of Zn from ores is presented. It is proposed that ores be broken into particles smaller than 0.1-0.5 mm. Then, w leached with 5 M NaOH solution at a 90-958C temperature for 1.5 h at a phase ratio NaOH. x solution added ml to the ore leached g of 6-7. After filtration, the leach solution is subjected to electrolysis for the production of metallic Zn after Pb is separated by sodium sulfide. The NaOH solution is recycled to the next leaching operation after most of the Zn is electrowon. To save energy and keep heat loss minimal, all operations can be conducted in an adiabatic environment, including leaching, filtration and electrolysis. Loss of NaOH was estimated to be less than 50 g for the production of 1 kg of Zn, assuming that most NaOH left in the leaching residues and precipitates of lead sulfide could be recovered by washing with dilute NaOH solution and water. However, loss of NaOH is dependent on the type and composition of the ores. The presence and leaching of silicates, carbonates and phosphates in ores would increase the loss of NaOH and deteriorate filtration of leach residues in the alkaline solutions. Therefore, most of the operational cost for this process comprises the consumption of electricity for electrolysis, NaOH, and energy required for heating.

Settlement and changes in refuse compositkm for total sugar, raw cellulose, biological degradable matter, volatile solid, organic carbon, were monitored in a large-scale experimental landfill unit and closed landfill units at Shanghai Refuse Landfill from April 1995 through to October 1998. Mathematical simulation formulae between these parameters and refilse age were developed based on the data obtained from the expcrmrental land-fill unit and validated by data from landfill units closed in April of 199I, I992, 1993 and 1994. Long-term pre-dictions for these parameters for the landfill were made using these formulae. It was predicted that the refiJse might be fully mineralized and the cumulative settlement might be over 30% of the initial height after around 22 to 30 years. The stabilization process and reuse approaches for landfills based on the compositions and settlement are discussed.

The induction of micronuclei (MN) in polychromatic erythrocytes (PCE) of mouse bone marrow by municipal landfill leachate was studied in vivo. Results showed that mouse exposure via drinking water containing various concentrations of leachate caused a significant increase of MN frequencies in a concentration (Chemical oxygen demand measured with potassium dichromate oxidation, CODCr)-dependent manner. MN induction in female and male mice was different at higher concentrations. This implies that leachate is a genotoxic agent in mammalian cells and that exposure to leachate in an aquatic environment may pose a potential genotoxic risk to human beings.