Pyrometallurgical recovery of zinc and valuable metals from electric arc furnace dust – A review

Approximately 20 kg of dust and sludges are produced per ton of liquid steel produced via the blast furnaces (BF)/basic oxygen furnace (BOF) production route. Many of these dusts are recycled through the sinter plant or blast furnace route without issue, but high zinc content dusts are routinely landfilled. Hydrometallurgical techniques, such as alkaline leaching, that are often utilized to remove zinc from electric arc furnace dusts are inappropriate for recovery of material from BOS dust due to the lower zinc concentration present and extra post-processing steps to utilize the separated iron product. Pyrometallurgical treatment through a rotary hearth furnace (RHF), in processes such as FASTMET®, can currently be considered as the most commercially attractive option for the processing and recovery of iron and zinc units when employed as part of an integrated steelworks. The crude zinc oxide produced is suitable for sale to zinc smelters, and the direct reduced iron produced provides process benefits through use, such as reduced blast furnace coking rates and increased productivity. The advantages and disadvantages of variations and alternatives are reviewed with regard to future developments.

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Pyrometallurgical removal of zinc from basic oxygen steelmaking dust – A review of best available technology

Solid waste generation is one of the main problems in the steelmaking process. One of the most problematic waste products is the electric arc furnace dust, which is a by-product rich in iron and zinc and is present as zincite (zinc oxide) or franklinite (zinc ferrite). This work focuses on the reduction kinetics of synthetic zinc ferrite by gases containing hydrogen and carbon monoxide. This process was examined via forced stepwise isothermal analysis. The test was conducted at temperatures ranging from 500 to 950 °C. Reduction of zinc was accomplished using a mixture of hydrogen and carbon monoxide in order to simulate reformed natural gas. The results indicated that reduction of zinc ferrite occurred in two stages (550–750 °C and 800–900 °C). The first stage was characterized by iron oxide reduction, where a mix control between nucleation and diffusion was determined. The apparent activation energy obtained was 71.5 kJ mol−1. The second stage was characterized by zinc oxide reduction, where the controlling mechanism was identified as a mixed control between diffusion and phase boundary reaction. The apparent activation energy was 135.5 kJ mol−1. The formation of a dense layer of metallic iron around the unreacted core may have caused the apparent activation energy to increase.

Synthetic zinc ferrite reduction by means of mixtures containing hydrogen and carbon monoxide

This paper aims to study the dielectric properties and carbothermic reduction of zinc oxide (zincite, ZnO) and zinc ferrite (franklinite, ZnFe 2 O 4 ) by microwave heating. To achieve this aim, the dielectric properties were measured with an open-ended coaxial method to understand the behaviour of the samples under microwave irradiation. The effects of microwave power, duration time and sample mass on the heating rate, and the effects of the stoichiometric amount of graphite on the reduction of ZnO and decomposition of ZnFe 2 O 4 were investigated. The results show that ZnFe 2 O 4 has significantly higher dielectric properties compared to ZnO. Generally, for both samples, the dielectric values at room temperature were quite low, indicating that both ZnO and ZnFe 2 O 4 are poor microwave absorbers. It was found that the temperatures have a more significant effect on the imaginary permittivities than on the real permittivities. The heating rate showed that the sample temperature increased with increase in microwave power and sample mass. Using 700 W of microwave power and two times the stoichiometric amount of graphite, almost complete reduction of ZnO was achieved in 12 min, while ZnFe 2 O 4 completely decomposed to zincite and wustite in 3 min.

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Dielectric properties and carbothermic reduction of zinc oxide and zinc ferrite by microwave heating.

Microwave-assisted self-reduction of composite briquettes of zinc ferrite and carbonaceous materials

The carbothermal reduction of zinc ferrite was studied using X-ray diffractometer (XRD), wet chemical analysis, scanning electron microscope (SEM), surface area meter, and thermogravimetrical analysis (TGA) systems. Zinc ferrite was found to be decomposed to ZnO and Fe2O3 initially and carbothermal reduction of ZnO and Fe2O3 took place simultaneously. The results of the surface area measurement indicated that the surface area of the solid sample increased with reaction time. Pore volume and average pore diameter were found to increase, reach a maximum, and then decrease with reaction time. These results were explained by considering the escape of zinc vapor, the expansion of the iron oxide grain, and the sintering of the iron. A mechanism and a model were proposed to explain the reaction. The experimental results of the thermogravimetrical analysis indicated that the reaction rate can be increased by increasing either the argon flow rate or the reaction temperature. Furthermore, the reaction rate was found to increase with a decrease in either the sample height, the molar ratio of ZnFe2O4/C, the size of the carbon agglomerate, or the initial bulk density.

Hsi-Kuei Chen

Papers

Carbothermal reduction of zinc ferrite