Improving fines recovery by grinding finer

Abstract

It is commonly believed that fine particles have low flotation recoveries. Indeed, size–recovery graphs often are ‘hill shaped’ with high recovery in the mid sizes and low recovery at the fine and coarse ends. Yet rather than being due to ‘overgrinding’, fines recovery may be low because the grind is not fine enough! The different flotation needs of fine and coarse particles have long been recognised, for example, in the old design for separate ‘sand and slimes’ circuits. Yet the principles may be overlooked in the desire for simpler circuits and larger equipment. Most plants now treat all particles together in one wide size distribution. Reagent conditions are set for the dominant coarser particles, so fines are starved of collector. Worse still, if there are significant mid sized composites, they often have to be rejected in cleaning to achieve target concentrate grade. Yet the conditions which reject mid size composites (collector starvation and high depressants) also reject fine liberated particles. In contrast to this common result, fines flotation can be excellent when flotation chemistry is tailored to them. After first recovering fast floating liberated particles, it is essential to adequately grind remaining composites to enable fines recovery. First, this allows lower depressant and higher collector additions since composites do not have to be rejected from concentrate. Second, finer grinding in appropriate equipment may be managed to narrow the size distribution to flotation, allowing reagent conditions to be set to suit the majority of particles. Third, grinding in an inert attritioning environment like an IsaMill increases fine flotation rates by producing clean mineral surfaces. An excellent case study is the installation of IsaMills in the Mount Isa lead zinc concentrator to grind lead and zinc rougher concentrate to P₈₀ of 12 μm and zinc cleaner tailings to 7 μm. Most plant losses had previously been in the sub 10 μm fraction, yet the finer grinding increased plant recovery by 5% lead and 10% zinc and also increased concentrate grades. Circulating loads dropped, reagent additions dropped in spite of the much higher particle surface area, and the plant became much more operable and responsive.