A study of comminution in a vertical stirred ball mill.
A 20 litre experimental batch and continuous test rig and 5 litre batch and 50 litre continuous test rigs for stirred ball milling were built at the University of Natal and Mintek respectively. All the mills featured a grinding vessel with a central shaft equipped with pins and a torque measurement system. A washed chrome sand from the Bushveld Igneous Complex was used for the grinding experiments. Particle size analysis of products was performed using standard sieves and a Malvern Particle Sizer. Batch tests were run in the 20 litre stirred ball mill to achieve efficient grinding conditions. The effects of grinding conditions such as pulp density, media size, media density and shaft rotation speed and mill design parameters such as ball load, pin spacing and pin diameter on product size, power consumption and media wear were studied. It has been shown that the median size of the product can be calculated by the Charles' Energy-Size Equation. The stirred ball mill has been found to be more energy efficient than the tumbling ball mill. An energy reduction of 50% was possible for a product size of 6 microns when the stirred ball mill was employed instead of the tumbling ball mill. The energy input per ton of grinding media in the stirred ball mill could be 10 times higher than for the tumbling ball mill. Although during coarse grinds the media wear was higher in the stirred ball mill than in the tumbling mill, it became less so as grinding proceeded and for a product median size of 4.8 microns it was the same. Using a 5 litre batch mill, an experimental programme was designed to study the comminution characteristics of the stirred mill. A factorial design was prepared with the following parameters, which influence grinding in the stirred ball mill: pulp density, pin tip velocity and ball density and size. The energy required for grinding the chromite sand in the stirred ball mill was determined by the use of Charles' Equation. The findings were in agreement with the results predicted by this equation. It was shown that the Rosin-Rammler size distribution equation was a suitable procedure for presenting and comparing grinding data obtained from the stirred ball mill. The factors that had the greatest effect on grindability were, in order of importance: ball size, pin tip velocity and ball density. Interactions between grinding parameters were negligible. results implied that accurate predictions can be made to determine the grinding conditions required to achieve a desired product specification. An attempt was made to study the grinding kinetics the chromite are using the mass population·balance model. Grinding tests were performed with two mono size fractions ·53+38 and -38+25 microns and natural feed ·100 microns using various pin tip velocities, ball densities and within the normal stirred ball milling operating range. relationship between the ball diameter and the particle was explained by the "angle of nip" theory which applied for roller crushers. It was shown that the particle giving the maximum breakage rate was directly proportional to the ball diameter. Estimated grinding kinetic parameters from monosize provided a good basis for predictions of natural feed. However, the breakage rate obtained from monosize tests appeared to be lower than those from the natural feed It was found that if the selection and breakage functions were determined by monosize tests, it was possible to modify selection function parameters by back-calculation which gave the best fit to the natural feed size. A good correlation was obtained between the experimental and product distributions using a population-balance model. The links between the empirical model combining Charles' and Rosin-Rammler equations and the first-order batch grinding equation were also shown. The stirred ball mills were operated in batch and continuous mode. The median size of the products from the batch stirred ball mill experiments closely matched those of the continuous grinding experiments under similar grinding conditions. Using a salt solution as a tracer material, an attempt was made to estimate the residence time distribution based on a simplified analysis of the motion of the water in tile mill. The current scale-up methods for the stirred ball mill are discussed. A torque model was developed for given shaft geometry and ball relating the power rements of the stirred ball mill to the following prime design and operating parameters : mill diameter, mill height, pin tip velocity and effective density of the mill load. The basic assumptions underlying the model were that the mill content behaved as a fluidised bed, consequently a P effg h type model for the pressure was applied throughout the grinding media bed the effective charge velocity was proportional to the pin tip velocity. It was found that pin spacing, pin diameter and ball diameter significantly affected the mill torque. A semi-empirical torque model was derived to include these parameters. The relationships formulated from these models were shown to be in excellent agreement with experimental results.