(729j) Carbothermal Zinc Recycling in a Packed-Bed Solar Reactor – Experimental Demonstration and Dynamic Modeling

Carbothermal Zinc Recycling in a Packed-Bed Solar Reactor – Experimental Demonstration and Dynamic Modeling

Waelz oxide (WOX) is produced in the dominating Zn recycling pyrometallurgical process, the so‑called Waelz process. The crude WOX generated contains 55–65% Zn in oxidic form, along with considerable amounts of chlorine, fluorine and lead compounds. A washing treatment for the production of dehalogenized WOX (washed WOX) is thus required before using WOX as a feedstock along with ZnS concentrates for the electrolytic production of primary Zn.

In the present study, an environmentally cleaner process route for the purification of crude WOX is experimentally investigated using concentrated solar process heat [1]. The solar-driven process comprises the thermal purification of WOX via volatilization of the Cl-, F- and Pb-compounds at temperatures between 1000 and 1285 ^oC to yield solar-clinkered WOX, which contains approximately 80 wt.% ZnO and can be used as feedstock for the subsequent carbothermal reduction.

Experimentation was carried out using a 10 kW_th packed-bed solar reactor [2]. It consists of two cavities in series separated by a thin ceramic plate, with the upper cavity serving as the solar radiative absorber and the lower cavity containing the reacting packed bed of solid feedstock. The laboratory scale reactor was subjected to solar flux concentrations of up to 2,500 suns (1 sun = 1 kW/m²). Solar-driven purification of WOX at above 1265°C reduced the amount of Cl- and Pb-compounds to levels lower than those of washed WOX (< 0.1 wt.%) and led to ZnO contents of approximately 88 wt.% in the product.

The second process investigated was the solar-driven carbothermal reduction of crude, washed, and solar-clinkered WOX as well as commercial ZnO powder. Beech charcoal was used as reducing agent for a CO₂-neutral carbothermal process. Solar experimental runs were carried out with the 10 kW_th packed bed reactor at temperatures up to 1170‑1320^oC and ZnO:C molar ratios between 1:0.8 and 1:1.25. Condensed products contained 90 wt.% Zn.

A dynamic reactor model was developed to solve numerically the transient energy conservation equation coupling heat transfer with the reaction kinetics. Model validation was accomplished by comparison of packed-bed temperatures and Zn production rates with experimentally measured data obtained with the 10 kW_thpacked-bed solar reactor. The validated model was further applied for optimizing the solar reactor configuration.

References

1. Wieckert C, Frommherz U., Kraeupl S., Guillot E., Olalde G., Epstein M., Santen S., Osinga T., Steinfeld A., “A 300 kW solar chemical pilot plant for the carbothermic production of zinc”, ASME Journal of Solar Energy Engineering, Vol. 129, pp. 190-196, 2007.
2. Tzouganatos N., Matter R., Wieckert C., Antrekowitsch J., Gamroth M., Steinfeld A., “Thermal Recycling of Waelz Oxide using Concentrated Solar Energy”, Journal of Metals, Vol. 65, pp. 1733-1743, 2013.