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Effect of polymeric additive and other variables on efficiency of grinding of limestone

Grinding is employed in the minerals industry for size reduction, the production of a large surface area and the liberation of valuable minerals from their matrices. The efficiency of this important unit operation is, however, very low. The energy consumed in grinding represents up to 70% of the energy for the whole mineral beneficiation process. In the past grinding was considered a purely physical operation, controlled by mechanical conditions of the grinding system. Rehbinder1 was the first to report that a reduction in the surface free energy of the solids is induced by the adsorption of surface-active agents and, hence, less energy is required to generate new surfaces. Several chemicals, both organic and inorganic, can influence grinding when added in small percentages and their effects have been the subject of intensive research for the past three decades.2,3 Summaries of the effects of various additives have been presented by Hartley et al.4 and, more recently, Bhima Rao et al.5 Further investigation of the effect attributed by Rehbinder to reduction in surface free energy1 led to the alternative proposal of change in surface hardness due to the influence of the adsorbed species on the mobility of near-surface dislocation,6 but for a variety of reasons neither of these mechanisms is well understood.7 Consequently, the effect of chemical additives on the grinding process has been attributed to individual mechanisms; indeed a review of past work reveals that the same additive can have opposite effects when used with different minerals. Against this background the present investigation was carried to study the effect of a particular additive on the grinding efficiency of limestone. The effect of other variables and of interactions between them was also studied because it is quite likely that in such systems the optimum level of one variable changes when the levels of others are changed.