T. Kojovic

The applications of rock mechanics parameters to the prediction of comminution behaviour

Abstract A wide range of test methods are available to the rock mechanics engineer in order to assess rock strength. The application of these test techniques to the prediction of comminution behaviour is investigated. Previously test methods that rely on inducing tensile failure have been found to correlate well with power consumption and product grading from crushing machinery. Building on this previous work the point load, fracture toughness and Hopkinson Pressure Bar core based tests are examined in relation to breakage. The application of these testing techniques to the prediction of comminution behaviour has tremendous potential if the rock strength parameters can be linked to a measure of breakage. The measure of breakage used in this study is the drop weight apparatus developed at the Julius Kruttschnitt Mineral Research Centre. The A, b and t10 parameters provided by the drop weight test describe the ore-specific rock breakage in the context of comminution. This approach has been successfully used in the simulation and optimisation of numerous crushing and grinding circuits. As such this paper seeks to investigate the relationship between the parameters of tensile based tests and those of the drop weight test. Given the expense of obtaining drill core samples it is essential that the samples yield the maximum amount of information. Early indications suggest that this technique will enable data required for comminution plant design to be obtained from core samples.

An energy-based model for swing hammer mills

An energy-based swing hammer mill model has been developed for coke oven feed preparation. it comprises a mechanistic power model to determine the dynamic internal recirculation and a perfect mixing mill model with a dual-classification function to mimic the operations of crusher and screen. The model parameters were calibrated using a pilot-scale swing hammer mill at various operating conditions. The effects of the underscreen configurations and the feed sizes on hammer mill operations were demonstrated through the fitted model parameters. Relationships between the model parameters and the machine configurations were established. The model was validated using the independent experimental data of single lithotype coal tests with the same BJD pilot-scale hammer mill and full operation audit data of an industrial hammer mill. The outcome of the energy-based swing hammer mill model is the capability to simulate the impact of changing blends of coal or mill configurations and operating conditions on product size distribution. Alternatively, the model can be used to select the machine settings required to achieve a desired product

Some causes of medium loss in dense medium plants

Abstract Medium consumption is a significant component of the cost of operating a dense medium plant. This study aimed to identify and quantify many of the factors contributing to overall consumption, through extensive surveys of four widely differing dense medium mineral separation plants, and associated laboratory experiments. Particular attention was given to the effect of operating density on medium loss. It was concluded that quite modest increases in operating density significantly increased loss from all sources. Density should therefore be minimised, commensurate with process requirements. There is also unequivocal evidence that consumption increases significantly with plant downtime, probably due to housekeeping wastage and overloading of regeneration circuits during the return of dumped medium to circuit. Other conclusions related to the effects of feedrate, ore porosity, medium viscosity, screen washing configuration, screen drainage efficiency and medium size on medium loss. Recommendations for operating practice are given.

Modelling and simulation of large diameter autogeneous and semi-autogeneous mills

Abstract In recent years there has been an increasing trend within Australia to instal large diameter (> 9.0 m) AG/SAG mills. A number of these mills have been extensively surveyed by the JKMRC under a wide range of operating conditions. Using the JKMRCs AG/SAG mill model, the results from these surveys together with those from a large data base of pilot scale mills were analysed in terms of their breakage rate distributions. The variations in these distributions were found to be similar at both the pilot scale and in mills up to 9.6 m in diameter and were related to operating conditions such as mill speed, ball charge volume, ball size and ore feed size. The data indicated that the effect of increasing speed, ball charge or ball size in SAG mills was to increase the degree of coarse particle breakage, often with an associated reduction in the degree of fine particle breakage. The effect of increasing the feed size was found to be dependent upon the mode of mill operation ie AG or SAG, with AG mill throughput increasing as the feed F 80 was increased. The opposite was true for SAG mills providing the ball charge was in excess of about 5% by volume.