Student number : 0318567G School of Chemical and Metallurgical Engineering Faculty of Engineering / Comminution is an important process in mineral processing, power plants, cement production and pharmaceutical industries. It is costly and an inefficient process in terms of energy requirements and steel consumption related to grinding media and liners. Spherical grinding media are predominantly used in final stages of ore grinding. The spherical balls change shape through breakage and wear. Though this is universal in milling, its contribution and effect on milling kinetics, load behaviour and mill power is not fully established. One area that is usually ignored is the relationship between media shape and mill power. The objective of this dissertation was to investigate how media shape affects grinding. Ball size distribution inside an industrial mill was analysed in terms of shapes and sizes. Load behaviour, mill power and breakage as affected by media shapes were studied in a pilot laboratory mill. An inductive proximity probe, light emitting diode, phototransistor and video photographs were used to determine the load orientation in terms of toe and shoulder positions. A load beam was used to measure the torque exerted by the charge. The variation in load orientation and mill power with speed among different media shapes was observed. Higher shoulder positions were noted with cylpebs than with worn and spherical balls. The power increased to a maximum with increasing mill speed for all media shapes reaching its peak at different mill speeds for the three shapes studied. Variations of breakage rates with media shapes were found; higher breakage rates were noted with spherical media but the differences narrows with decreasing feed size and increasing material fractional filling U. Considering that worn balls in an industrial mill charge constitute about 15 to 40 percent and that the highest difference in breakage rate observed being nine percent for purely one shape charge; it is very doubtful whether it is worthwhile in attempting to develop techniques for removing worn balls from the mill.

The production of multi-component cement from clinker and two additives such as trass and blast furnace slag has now spread throughout the world. These additives are generally interground with clinker to produce a composite cement of specified surface area. The grinding stage is of great importance as it accounts for a major portion of the total energy consumed in cement production and also as it affects the quality of composite cements by the particle size distribution of the individual additives produced during grinding. This thesis study was undertaken to characterize the breakage properties of clinker and the additives trass and slag with the intention of delineating their grinding properties in separate and intergrinding modes. Single particle breakage tests were conducted by means of a drop weight tester in order to define an inherent grindability for the clinker and trass samples in terms of the median product size ( ). In addition, a back-calculation procedure was applied to obtain the breakage rate parameters ( ) of perfect mixing ball mill model using industrial data from a cement plant. Kinetic and locked-cycle grinding tests were performed in a standard Bond mill to determine breakage rates and distribution functions for clinker, trass and slag. Bond work indices of these cement components and of their binary and ternary mixtures were determined and compared. Attempts were made to use back-calculated grinding rate parameters to simulate the Bond grindability test. The self-similarity law was proved to be true for clinker and trass that their shapes of the self-similarity curves are unique to the feed material and independent of the grinding energy expended and overall fineness attained. The self-similar behaviour of tested materials will enable process engineers to get useful information about inherent grindability and energy consumption in any stage of the comminution process. The parameters, and indicating the degree of size reduction were defined with different theoretical approaches as a function of energy consumption by using single particle breakage test data of clinker and trass. The breakage distribution functions were found to be non-normalizable. On the other hand, the breakage rate functions were found to be constant with respect to time but variable with respect to changing composition in the Bond ball mill. These variations are critical in computer simulation of any test aiming to minimize the experimental efforts of the standard procedure. As a result of the back calculation of breakage rate parameters for clinker and trass samples in the Bond mill, no common pattern was seen for the variation of the rate parameters. Therefore, computer simulation of the Bond grindability test did not result in an accurate estimation of the Bond work index.