A Proposed Mechanistic slurry discharge model for AG/SAG mills

From a study of slurry transport phenomena and discharge mechanisms in AG/SAG mills it was concluded that the slurry discharge is driven by four major independent factors; flow resistance of the charge; flow through the grate; removal from the discharge chamber (via pulp lifters); and flow-back from the discharge chamber into the mill. No existing models cover all these processes, so independent mechanistic models of such sub process are proposed and these are to be implemented into a single dynamic model. A series of laboratory scale experiments, utilising real ores and slurries, have been designed to simulate each component of the discharge. INTRODUCTION The importance of the slurry transport in grinding devices such as AG/SAG mills is broadly understood as well as its influence in mill throughput capacity. The study of slurry transport along mills using tracers was popular during the late seventies and early eighties as explained largely by Austin et al. (1984) and Abuzeid (2000). To describe the residence time distribution (RTDs) some mathematical models were used, such as the axial dispersion and the tank in series models. No studies on AG/SAG mills were reported elsewhere, although Austin et al. (1987) suggest that ‘the simple concept of RTD loses meaning in AG/SAG mills because the RTD is determined by the rate of breakage of feed material to less than the grate size’. III-422 Further, empirical and semi-empirical approaches have been used in order to obtain predictions of mill slurry filling and its relation to flowrate. Some of the investigators are Marchand et al. (1980), who correlate the hold-up in the mill with the flow rate considering other variables to be constant, and Rogers and Austin (1984) who later enhanced the correlation including other variables; such as mill critical speed, slurry density and mill dimensions. One of the most advanced and useful semi-empirical models was developed by Morrell and Stephenson (1996) expanded later by Latchireddi (2002, 2003). They carried out extensive laboratory and pilot scale experiments which together with industrial data was used to correlate slurry hold-up with mill flowrate and mill operating conditions such as: critical velocity, charge filling, diameter, grate open area, radial position of grate apertures and aspect ratio. Slurry rheology and other unknown factors where included in a constant factor. There is limited literature addressing the mechanistic description of hold-up and slurry transport in mills. Hogg (1984) considers two distinct regions for overflow mills; the ball charge and the pool zone; the slurry is continuously interchanged between these zones and flows out of the mill through the pool, ideal settling velocities are considered and the ratio of flow rate to hold-up determines the axial velocity. Moys (1986) used equations similar to the flow of fluids through packed beds and flow through orifices. The incorporation of slurry viscosity and grate discharge parameters are important features in this mechanistic model. Following a similar procedure but considering a dynamic ball charge where the hold-up is related to three zones of ball motion, Shi (1994) developed a mechanistic model describing the hold-up as a function of mill dimensions. Slurry discharge modelling In spite of the importance of the grate discharge and pulp lifter in the removal of slurry from the mill, little published literature has been found on this topic. Mokken et al. (1975) were among the first researchers who studied the grate discharge and pan lifter behaviour. Their study was inspired by slurry pooling issues in the South African style run of mine SAG mills. They developed a theoretical approach to flow patterns as a function of mill critical speed that showed that the only region within which a particle in the pulp chamber could move towards the centre of the mill is where gravitation acceleration exceeds the centrifugal acceleration, and there is only a small portion of the mills revolution where this occurs. The paper reports the use of alternative pan lifters such as curved and spiral designs in order to reduce this problem. DEPARTMENT OF MINING ENGINEERING UNIVERSITY OF BRITISH COLUMBIA Vancouver, B. C., Canada SAG 2 0 0 6

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