Ferro-reduction of ZnO using concentrated solar energy

Abstract

In recent years, the production of zinc from its oxide using solar energy has been attracting increasing interest. This is a promising process for the conversion of solar radiation to its chemical form and storage. When carbon is used as a reducing agent, the main product gases are zinc, CO, and CO 2. A major problem is the need for rapid cooling and separation of the zinc to avoid back reaction and reoxidation. In industry, lead splash condenser is used to remove the zinc vapor rapidly. The lead is circulated out of the condenser and chilled, so that the solubility of the dissolved zinc is reduced and part of the molten zinc is separated and floats on the lead. Because lead is capable of dissolving only a small amount of zinc, the amount of lead to be circulated is about 400 times as much as the amount of produced zinc. This technology is complicated and cumbersome. This paper describes a new approach for two-step process for the reduction of ZnO that can potentially solve this problem. The two-step process can be characterized by the following equations: ZnO(s)+ Fe(l)→ Zn(g)+ FeO(l), ΔH 0 = 177 kJ/mol, FeO(l)+ C(s)→ Fe(l)+ CO(g), ΔH 0 = 153 kJ/mol. Both steps are endothermic, require temperatures in the range of 1300–1600 °C, and can be carried out using concentrated solar energy. In the first step, iron reduces ZnO and Zn vapors are distilled out (zinc is miscible in iron, but at the relevant temperatures, and at atmospheric pressure it volatilizes). In the second step, carbon is injected into the FeO melt and reduces it back to iron. The CO obtained in the second step is separated from the zinc vapors. Basically, this is a gasification process. The carbon is converted to CO. When using coal, the ashes form slag on the surface of the melt and can be removed. The advantages of this process compared to the direct carboreduction are: (i) high rates of heat and mass transfer mechanisms between the iron melt and the ZnO powder; (ii) avoiding the necessary preparation of the feed as required in the direct process, mixing ZnO and carbon in a measured proportion, preparation of briquettes; if ZnO is recycled from zinc/air batteries, there is no need for size reduction and, in principle, large-size fractions can be processed; (iii) avoiding the major difficulty of separation of the CO and providing the possibility for simpler zinc condenser compared to the lead splash condenser. The process has higher thermodynamic gain and higher contribution of solar radiation. This paper analyzes the thermodynamics of the two-step process and compares it to the direct carboreduction of ZnO. The kinetics and mechanism of the reactions are discussed. Experimental results with solar energy for the first step are described. A mixture of ZnO and iron was heated at the Weizmann Institute of Science (WIS) solar furnace. At 1600 °C, 90% of the theoretical yield was obtained after 5 min of testing. The zinc vapors were condensed and X-ray diffraction analysis showed very high purity of zinc and crystalline. The second step is known in the literature. Finely divided carbonaceous materials, e.g. coal are injected into the FeO melt (which floats above the iron as it forms). The coal is dissolved in the FeO and reduces it, thus creating CO gas. The gasification of coal takes place very rapidly owing to the high temperature, the carbon content of the melt and the mixing created by the evolving gas. A conceptual scheme of the solar reactor is shown.