The Behavior of Zinc and Iron in Neutralized Residue During Pressure Leaching

In two-stage zinc pressure leaching process, zinc calcine is used to neutralize the acid pressure leaching solution. The residue usually contains a certain amount of zinc, iron, and other elements. The dissolved iron usually affects the mineral leaching and impurity removal as well as solution purification process. However, the behaviors of iron oxidation and migration are not fully understood and those behaviors impact zinc recovery from the residue. Thus, the behaviors of zinc and iron in the neutralized residue during pressure and hot acid leaching were investigated for varying temperature, acidity, leaching time, and oxygen partial pressure. The results showed that acid to zinc mole ratio is the key factor for the extraction process, and the zinc extraction rate accelerates with an increase in solution acidity. At temperature 150 °C, acid to zinc mole ratio 2.7, residence time 2 h, and oxygen partial pressure 0.6 ~ 0.7 MPa, the zinc extraction rate can reach 98.0%. The iron extraction, however, varied: (A) it increased with temperature and then decreased and (B) displayed an initial increase followed by a decrease with respect to the leaching time. In actual production, iron in marmatite and pyrrhotite is leached into solution and then oxidized and precipitated into residue as hematite or ferrovanadium. In order to achieve the decomposition of ferrite in the neutralized residue, a high initial acidity is essential.