Product quality parameters in the reaction crystallization of metastable iron phases from zinc-rich solutions

Abstract

Iron is often present in leach liquors produced in chemical and hydrometallurgical processes. It is known that voluminous iron precipitates with high impurity values are formed if the conditions during its formation are not controlled well. These products are also often difficult to treat in downstream processes. This study therefore focused on the determination of product quality parameters for the production of good quality iron precipitates from zinc-rich solutions. Special attention was given to the quality of metastable phases such as ferrihydrite and schwertmannite formed at elevated temperatures and in the pH range 1.5 to 3.5 in a continuous crystallizer. These phases are produced over a range of supersaturation levels with the best quality products formed at lower supersaturation. It was shown that most industrial processes are operated well above the metastability limit at relatively high supersaturation. However, stagewise precipitation of iron, even above the metastability limit, yielded better quality products. It was also shown that localized supersaturation levels could be controlled through changes in the micro and macromixing environments. The three-zone model approach was used to improve the quality of ferrihydrite and schwertmannite precipitates. Changes in the reactor design and the position of reagent feed points also impacted on the quality of the precipitates. Control over the localized supersaturation not only ensures the production of good quality nuclei, but also impacts on particle growth, which is required to make downstream processing of precipitates possible. In precipitation processes, growth mainly takes place through agglomeration as the rate of molecular growth is generally low. The final quality of iron precipitates is greatly influenced by the quality of the agglomerates formed during iron precipitation. A Hadamard matrix was used to indicate the relative importance of the most relevant operating parameters for the formation of good quality iron precipitates.