Evaluation of graphite depressants in a poly-metallic sulfide flotation circuit

Synthetic dyes are commonly used for graphite depression in poly-metallic flotation circuits; however, these dyes can be very expensive. The aim of this study is to evaluate performance of certain low-cost alternative depressants for a complex lead-zinc (Pb-Zn) ore rich in graphite (Gr-C) on a conventional mini pilot-scale flotation circuit. The reagents used were commercial and industrial grade starch; agro-based waste-sugarcane bagasse and charred (burnt) bagasse powder. The primary evaluation criteria were quality (grades) of lead and zinc concentrates, their recoveries (%), and graphite rejection (%) in the tails. Benchmark tests using nigrosine as graphite depressant showed 94.3% rejection of Gr-C. The results with commercial starch were found as effective with 93.8% graphite rejection. Furthermore, bagasse powder showed potential in improving product quality (36.4% and 65.6% Pb grade and recovery) with an intermediate effectiveness in graphite rejection (85.6%). The order of effectiveness in Gr-C rejection follows nigrosine ≈ commercial starch > bagasse > industrial starch > charred bagasse. In addition, the effect these depressants on silver (byproduct) grade and recovery was also investigated.