The fracture and autogenous comminution of quartzite

Doctoral Thesis


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University of Cape Town

Comminution is the process which aims at increasing the surface area and the resultant liberation of a particular constituent from the mass of a solid. The autogenous mill uses tumbling to effect comminution, but instead of special milling bodies being added, pebbles of the material to be comminuted are used. The autogenous comminution process utilises less than 0,1 per cent of the energy input. The principal objective of the present work was to analyse autogenous milling behaviour in terms of the individual comminuting mechanisms and to establish the inter-relationships between the main process variables, namely rock petrography, size distribution of the feed, applied load, relative velocity and environment. In this manner the optimisation of the process and an improvement of its efficiency was sought. In addition the establishment of testing procedures to predict the autogenous milling behaviour of a given type of rock was aimed. In the present work the gold bearing Witwatersrand quartzite was used, although the findings are applicable to other types of rocks. Since fracture phenomena are involved in all comminuting mechanisms of impact-compression, chipping and abrasion, slow compression and Brazilian tests were performed. The grain size and the mineral composition of the rock has been found to have a large influence on the local stresses required for these processes. Indeed the results show that the fragility and therefore ease of comminution increases with increasing grain size of the quartzite. Brazilian tests on drill cores of varying diameters may allow the prediction of the critical size of rock of the mill feed which can survive in a mill of given characteristics.

Bibliography: p.103-116.