The effect of regrinding chemistry and particle breakage mechanisms on subsequent cleaner flotation

Abstract

Regrinding rougher flotation concentrates is typically used to liberate valuable minerals from gangue prior to the cleaner separation stage in processing of low grade ores. Compared to the rougher flotation after primary grinding, it is usually more challenging to achieve a satisfactory performance in post-regrind cleaner flotation especially when fine particles are generated. One main factor which results in this reduction in flotation is the unsuitable particle surfaces produced after regrinding. However, this factor is not usually considered when designing and optimizing the regrinding process. Extensive studies have demonstrated that grinding chemistry can influence mineral floatability. However, earlier studies focused on primary grinding and rougher flotation rather than regrinding and cleaner flotation. In addition, different types of regrind mills are used in industry, and these provide different particle breakage mechanisms which may also influence mineral floatability. Therefore, the overall objective of this thesis study is to investigate the effects of regrinding chemistry and particle breakage mechanisms on the cleaner flotation. The implementation of regrinding in the copper and pyrite flotation circuits at Telfer gold mine was taken as a case study and four research areas were addressed with the objective of developing fundamental understanding to provide practical guidance for the plant operation. The four areas investigated are: 1. Effect of regrinding chemistry on copper activation of pyrite and its flotation; 2. Separation of different copper sulphide minerals from pyrite after regrinding; 3. Importance of pulp chemistry during regrinding rougher concentrates; 4. Effect of particle breakage mechanisms on subsequent flotation. In this study, it was found that copper activation of pyrite and its flotation in the cleaner stage were affected by regrinding. In general, a decrease in surface concentration of collectors due to the increased surface area after regrinding contributed to the low pyrite recovery in the cleaner. Surface analysis revealed that both the surfaces that were carried from the rougher flotation concentrate and those which were freshly created were modified by the strong electrochemical reactions occurring inside the regrind mill. Stainless steel media promoted pyrite oxidation, especially on the copper activated surfaces. Mild steel media produced a greater amount of iron oxidation species which could adsorb on the carried surfaces. It was also found that mild steel media generated a reducing condition which favoured the copper activation on the freshly created surfaces. The research has also shown that the separability of chalcopyrite and chalcocite from pyrite was significantly different after regrinding, which was related to their electrochemical properties. Chalcocite was more active and therefore presented a stronger galvanic interaction with pyrite. Consequently chalcocite was more oxidised, which not only depressed its flotation but also produced sufficient copper ions to activate pyrite, making the separation of chalcocite from pyrite more difficult. In comparison, chalcopyrite was less active. Its oxidation generated hydrophobic surfaces which improved chalcopyrite floatability. At the same time pyrite flotation was depressed due to the insufficient copper ions for activation, leading to an effective separation of chalcopyrite from pyrite. However, these flotation differences were not significant at the rougher stage and this was thought to be due to weak galvanic interactions at the coarser particle sizes present in this stage of the process. A pulp chemistry survey at Telfer indicated extremely low dissolved oxygen concentration and Eh after regrinding of pyrite concentrate resulting in poor subsequent copper and gold flotation. To develop a remedial strategy, the plant flotation conditions and performance were replicated in the laboratory and several methods were examined to increase the oxidising condition. It was found that oxidising conditions were beneficial to copper-gold flotation and pyrite depression. The surface analysis showed that the improved flotation was attributed to a combined effect of several factors, including pyrite surface oxidation which reduced its floatability, increased collector adsorption on copper-gold surfaces, and also the oxidation of chalcopyrite producing hydrophobic species. Apart from the regrinding chemistry, another important factor considered in this study was the type of grinding mills which provide different particle breakage mechanisms. A significantly lower recovery was observed after regrinding in a stirred mill than in a rod mill. After examining all possible factors influencing the flotation, it was found that the predominating factor was the different distribution of collectors remaining from rougher flotation concentrates. In the tumbling mill, impact breakage dominated, causing the collector to remain on the surface of newly produced particles. In the stirred mill, attrition breakage removed collector from the surface, and decreased particle floatability. This was further confirmed by the analysis of collector intensity on mineral surfaces on a size-by-size basis by ToF-SIMS. Overall, the results of this thesis study clearly show that both regrinding chemistry and particle breakage mechanisms play critical roles in changing pulp chemistry and the formation and distribution of surface species and hence affect mineral flotation and separation in the subsequent cleaner flotation.