Solid-Phase Interaction of Zinc Ferrite with Calcium Oxide

Zinc ferrite is a stable compound, slightly soluble in weak sulfuric acid that makes it difficult to extract zinc in solution during processing of metallurgical semi-products using hydrometallurgical technologies. Previous researchers have shown the possibility of preliminary destruction of zinc ferrite by high-temperature treatment in the presence of calcium oxide to form zinc oxide and calcium ferrite. The mechanism of this exchange interaction was studied using the method of diffusion annealing in air atmosphere (1050 °C for 4 h) of ground and tightly pressed pellets of synthesized zinc ferrite and calcium oxide, 13 mm in diameter and 3–5 mm in height. After annealing, the pellets were cut along the diameter and the cross-section was examined with a micro X-ray spectrometer. A distinct zonal structure was observed in the pellet contact area, the analysis of which shows that calcium diffuses from the calcium oxide pellet into the zinc ferrite pellet. Close to the pellet contact area, the calcium reacts with the zinc ferrite to form calcium ferrite and displaces the zinc to the depth of the zinc ferrite pellet. Thus, the unreacted zinc ferrite is continuously enriched with zinc in the course of annealing and is probably blocked by it, preventing contact between the zinc ferrite and the diffusing calcium. Practical conclusion: To ensure that zinc is completely displaced from its ferrite by calcium under industrial conditions, a highly developed and dense contact surface between zinc ferrite and calcium oxide particles should be achieved.