Avaliação dos produtos de oxidação e ocorrência do efeito preg-robbing da oxidação sob pressão em autoclave de bancada e industrial para o minério sulfetado da Mina I de Córrego do Sítio, Minas Gerais

Abstract

This work studied samples of sulfide ore from Mine I of Córrego do Sítio (“CdS plant”, in Santa Bárbara, Minas Gerais), owned by the Anglogold Ashanti company. The gold from this ore is partially contained in a sulfide matrix, which was submitted to the oxidation stage under pressure. Chlorides present in this stage leach the gold which is subsequently adsorbed by carbonaceous matter in the oxidized product from the ore, a phenomenon which is called the preg-robbing effect. The main objective of this work is to present methodology of oxidation under pressure at a bench scale that allows a comparative evaluation with an industrial autoclave, as well as to correlate the preg-robbing occurrence with the parameters used in the pressure oxidation stage. In this sense, it was investigated the influence of pH, oxidation kinetics, the characterization of oxidation products, the occurrence of the preg-robbing effect and the metallurgical recovery in the cyanidation stage. To achieve the objectives, the experimental methodology on tests of oxidation under pressure on a bench scale was used. The initial tests took place with a totally closed system, however, low degrees of oxidation of the concentrate were obtained in them, in comparison with the results achieved in the same period in the industrial autoclave. A new methodology was developed, called the semi-open system, which improved the degrees of oxidation. In addition, tests were performed with variation in the pH value of the fed concentrate and leaching tests, which indicated a metallurgical recovery of oxidation products with an alkaline pH below those of acid pH. The characterization by MLA (Mineral Liberation Analyzer) indicated that gold particles after oxidation are predominantly associated with minerals from rocks and transformed phases, such as oxide and sulfates. The results of the study demonstrated that the oxidation under pressure in bench with a semi-open system generates oxidation products compatible with the products of the industrial autoclave, however, the system is sensitive to the variation of the oxygen flow. The oxidized concentrate under analysis had the presence of the preg-robbing effect, in terms of the impact on the gold global recovery in the metallurgical plant. Finally, the importance of controlling the pH value in the autoclave feed was emphasized, because, although a high degree of oxidation was achieved in tests with both acid and alkaline feeds, in the alkaline one there was a negative impact in the gold recovery.