| This thesis presents the results of an investigation on the impact of continuous circuit and solution parameters on coprecipitation of arsenic with ferric iron from acidic sulphate solution. The techniques employed were selected, or developed, to emulate industrial practice. The concept behind the work was to better understand the link between the process of precipitation and the stability of arsenic in the resultant coprecipitates. This was performed by examining the role of circuit design and co-ions on both arsenic removal during coprecipitation and arsenic retention during ageing. The parameters/factors investigated included continuous versus batch operation, number of stages (pH profile), recycling and Ca2+, Ni2+ and Al 3+ as co-ions.Arsenic removal was found to be greatly improved by continuous rather than batch coprecipitation. In addition, the presence of calcium (introduced as slaked lime) was found to be instrumental in the removal and retention of arsenic. Arsenic retention during ageing (up to 300 days) at various temperatures (3, 22, 40 and 70°C) was observed to reach an "equilibrium" that was strongly affected by the continuous circuit design, as well as the co-ions present during coprecipitation. Evidence is presented of the partitioning of arsenic within the coprecipitates in two principal phases, namely ferric arsenate (FeAsO4·xH2O) and arsenic adsorbed ferrihydrite. Continuous circuit design parameters, such as staging, that resulted in enhanced stability appear to yield coprecipitates with higher ferric arsenate content. Analysis of the kinetic and "equilibrium" arsenic retention data yielded activation energy (&sim 60 kJ/mol) and enthalpy (&sim -38.5 kJ/mol) values that suggest a reaction controlled exothermic dissolution mechanism. |
