Optimal Precipitation Of Zn+₂ and Ni+₂ From Aqueous Solution: Influence Of Rapid Mixing Parameters

Abstract

Wastewater containing Zn+₂and Ni+₂is normally treated by chemical precipitation, coagulation, flocculation followed by clarification.The metal precipitation is influenced by chemical (wastewater pH, coagulant type and dose) and physical (rapid mixing speed and time) parameters. The process usually consists of the rapid dispersal of a coagulant into the wastewater followed by an intense agitation commonly defined as rapid mixing. This study focused on the most important parameters of rapid mixing design: mixing intensity and duration. Simulated aqueous solutions containing 50 ppm Zn+₂and 50 ppm Ni+₂were treated with aluminum sulphate, ferrous sulphate and ferric chloride coagulants at different doses and different rapid mixing times and speeds. Experimental results obtained indicate that ferric chloride at 30 mg/l dose was superior over aluminum sulphate and ferrous sulphate at the same dose in Zn+₂and Ni+₂removals. Rapid mixing time had a strong influence on the metal removal. An optimal combination of rapid mixing parameters was determined as: 60 s at 100 rpm for Zn+₂and 30 s at 80 rpm for Ni+₂removals. Scanning electron microscopy images for Zn+₂and Ni+₂flocs at optimum parameters of rapid mixing show that ferric chloride addition compacts the surface texture of the metals flocs. Flocs formed by Zn+₂are denser and larger than flocs formed by Ni+₂.