A Bond Work index mill ball charge and closing screen product size distributions for grinding crystalline grains

Abstract

Ball mill charge size distributions originally specified for Bond grindability tests to determine the Bond Work index (BWi) are not commercially available. Those proposed to date do not match all of Bond's (1961) original specifications of total ball mass, number and surface area. An alternative mill ball charge is proposed that closely approximates Bond's original total ball mass, number of balls and ball surface area.

Results of 30 Bond Work index tests of six pure materials (calcite, magnesite, labradorite (feldspar), quartz, andalusite and glass) using closing screen apertures (P₁) values of 500, 250, 125, 90 and 63 μm are analysed. The samples were selected on the basis of having distinct hardness's (Mohs hardness 3 to 7.5), being relatively free of crystallographic defects and having distinct cleavage properties The 80 percentile size of the fines produced (P₈₀) concur with those of published values.

The trend based of P₈₀ values for P₁ values of 150 to 44 μm recommended by Bond poorly fit with published P₁ values > 250 μm. It is demonstrated that the BWi test P₁ values of mono-mineralogical or mono-material samples determines the P₈₀ value obtained. Based on test results of this investigation a polynomial exponential equation relating P₈₀ to P₁ values with an R² correlation factor of 0.9977 is presented. This relation is independent of the tested material hardness and crystallographic defects and planes of weakness.