ENVIRONMENT - THE BEHAVIOR OF ZINC, CADMIUM, THALLIUM, TIN AND SELENIUM DURING FERRIHYDRITE PRECIPITATION FROM SULFATE MEDIA

Ferrihydrite is often the initial precipitate resulting from the rapid neutralization of Fe(lll) sulphate solutions, and it seems to be a ma jor constituent of zinc plant neutral leach residues. The most importa nt parameters affecting ferrihydrite precipitaton are temperature, sol ution pH and the neutralization rate, low temperatures and rapid neutr alization rates yield poorly filterable ferrihydrite, whereas higher t emperatures and slower neutralization rates produce readily filterable precipitates; in all instances, however, the precipitates are two-lin e ferrihydrite. Significant amounts of Zn are incorporated in the ferr ihydrite precipitates, and the Zn contents increase with increasing Zn concentration, pH or temperature. Generally, the sulphate contents of the precipitates decrease as the Zn contents increase, and this may i ndicate that the Zn is adsorbed on the ferrihydrite. Discrete Zn phase s were not detected. Neither Cd nor TI is precipitated with ferrihydri te, although thallium jarosite co-precipitates with ferrihydrite at hi gh TI concentrations and elevated temperatures. Virtually all of the S n added to the solution precipitates with the ferrihydrite, but the ti n is believed to be present as intimately admired amorphous SnO2.nH(2) O. Although selenite extensively precipitates with the ferrihydrite, s elenate remains mostly in solution, likely because of the close simila rity of the SeO42- ion to the SO42- ion which is present in high conce ntrations and saturates the available sites for anion adsorption.