Challenges of Studying Effects of Recycled Process Water Quality on the Flotation of Sulphide Minerals

The increasing recirculation of process water in mineral processing operations can substantially alter water chemistry and, consequently, flotation performance. However, the effects of recycled process water on sulphide mineral flotation remain insufficiently understood, particularly under industrial conditions where water chemistry and flotation operating conditions continuously evolve. This study investigated the influence of process water quality on flotation performance at the Kevitsa Cu-Ni concentrator, with particular emphasis on the ability of laboratory-scale tests to reproduce plant conditions. Laboratory-generated and plant process waters exhibited substantial differences in key physicochemical parameters, including pH, oxidation-reduction potential (ORP), dissolved oxygen (DO), and conductivity. These differences were accompanied by markedly different nickel recovery distributions. In the copper circuit, nickel recovery was approximately 10% under plant conditions compared with 68% in laboratory tests, whereas nickel recovery in the subsequent nickel circuit was approximately 60% under plant conditions and only 10% in laboratory tests. These discrepancies were associated with two main factors: inadequate reproduction of the plant conditions required for effective depression of nickel-bearing minerals during copper flotation and the consequent reduction in NiS feed grade to the subsequent nickel flotation stage. NiS feed grade was significantly positively correlated with nickel recovery (r = 0.635, R2 = 0.404, p < 0.001, n = 115), indicating that feed grade accounted for approximately 40% of the observed variation in recovery. The results demonstrate that laboratory flotation responses cannot be attributed solely to process water quality when upstream flotation selectivity, feed characteristics, and the evolving physicochemical conditions of sequential flotation circuits are not adequately reproduced. A plant-centred methodology integrating process water characterisation, plant monitoring, representative sampling, and laboratory and plant flotation data is therefore proposed for evaluating water-quality effects within their operational context. This approach provides a framework for recognising process water quality as a measurable process variable and for improving the interpretation and optimisation of flotation performance in highly water-recirculating mineral processing operations.