Investigation of the reasons for copper and gold loss in the cleaner tail, at Ok Tedi, Papua New Guinea

Abstract

Ok Tedi Mining Limited generates a copper and gold concentrate from its porphyry and skarn ore deposits located at Mt. Fublian, Western Province, PNG. The predominant porphyry ore-type is blended with high grade skarn ores to optimize copper and gold feed grades to the concentrator. Current operation (2003) is to blend 80% porphyry with 20% skarn ores, resulting in an acceptable concentrate grade and recovery. However, when the proportion of skarn ore in the plant feed exceeds 20%, low flotation recovery is often observed. Approximately 20% of the copper and 30% gold losses occur through the rougher flotation circuit. However, losses via the cleaner tailing stream are 8% and 9% for copper and gold, respectively, and augment value mineral losses to the final tailing. It may be noted that the cleaner tailing stream contains value minerals which have already been successfully floated in the rougher section of the concentrator, and are therefore recoverable. This study has the objective of determining the reasons for copper and gold loss in the cleaner tailing. The focus is therefore on the cleaner flotation bank and related streams. The project strategy commenced with characterization of the cleaner circuit performance under various operating conditions. This would provide evidence for any proposed mechanisms to explain the losses of valuable minerals to the cleaner tailing. The second part of the project strategy was to evaluate potential methods for improvement of cleaner flotation performance via laboratory flotation tests. To complete the work, plant trials of methods showing benefit in the laboratory were conducted.Characterisation surveys of the cleaner flotation bank were conducted as a function of ore blend, pulp and surface chemistry, mineralogy, flotation kinetics, particle size distribution, cleaner flotation cell hydrodynamic characterisation and residence time. These studies indicated that for greater than 20% pyrite skarn in the concentrator feed blend, lower than expected gold and copper recoveries were achieved in the cleaner bank. Pulp and surface chemical analytical techniques indicated layers of oxide coatings existed on all particles, depressing flotation. Oxide coatings had also resulted in the copper activation of pyrite (CuS rimming), making separation from copper sulfides difficult. Losses included coarse composite particles, also suggesting possible liberation issues. However, liberated copper sulfides were also lost, supporting the notion of depression via oxide coatings. Residence time calculations for the cleaner flotation bank suggested that the flotation capacity was less than adequate, particularly at high mill throughput rates. Hydrodynamic characterisation indicated improvements in cell hydrodynamics were warranted. In contrast to the oxidised ore feed blend described above, when less than 20% pyrite skarn was contained in the ore blend, high copper and gold recoveries were achieved in the cleaner bank. Laboratory tests suggested that increasing cleaner feed pH from 10.5 to 11.5, and 1 or collector addition to the cleaner feed, would result in improved cleaner flotation performance. These changes were trialed in the plant, with the pH adjustment being successful. High cleaner feed pH continues as a permanent modification to plant operating conditions.