Modelling the effect of mill length on the relationship between slurry hold-up and flowrate

Abstract

The performance of a grinding mill can be greatly influenced by the slurry hold-upninside. As more and more autogenous (AG) and semi-autogenous (SAG) grindingnmills are operated in closed circuit with cyclones, the limitations that can be imposednby this are becoming increasingly apparent. A literature review was performed at thenJulius Kruttschnitt Minerals Research Centre (JKMRC) during the early 90's. Thisnrevealed that relevant literature on the subject was almost non-existent. Hence anresearch program was initiated at the JKMRC to study the variables which affect thentransport and hold-up of slurry inside a mill.nnnnnnnnn A literature review conducted at the beginning of this project revealed that previousninvestigators neglected to study the effects of mill aspect ratio on the relationshipnbetween flowrate and hold-up. Another aspect that has been neglected in the past isnthe effect of slurry rheology on the previously mentioned relationship.nnnnnnnn This research project was conducted with the aim of exploring the effect of millnaspect ratio and slurry rheology on the relationship between flowrate and hold-up.nThe investigations were conducted at the laboratory and pilot scales and experimentsnwere performed using water and slurries. As well as conducting dynamicnexperiments, tests were performed using a static bath. This was designed to measurenthe axial water profiles and how these are affected by design and operating conditions.nnnnnnnnnn Experiments were conducted at the laboratory scale using a variable length mill, thusnallowing experiments to be conducted at three aspect ratio (D/L) configurations: high,nsquare and low. The experimental conditions for each aspect ratio configurationsnincluded five levels of flowrate, three charge levels and three mill speeds. These werenrandomised to avoid the effects of unwanted but significant variables, such as, timendependency. Also two repeat tests were performed for each experimental condition.nnnnnnnnnnnn Experiments at the pilot scale were also conducted using three aspect rationconfigurations, with the same number of levels for each variable. Due to timenconstraints the experimental program was designed using a Central CompositenRotational Design (CCRD). The use of this technique reduced the numbers ofnexperimental conditions significantly, whilst still producing a statistically valid setnresults.nnnnnnnnnnn The analysis of the experimental results revealed that aspect ratio did have an effectnon the flowrate - hold-up relationship under both dynamic and static conditions. Itnwas found that mean hold-up tended to decrease with mill aspect ratio. Static bathnexperiments later revealed that the decrease in hold-up was caused by a decrease innthe mean level of water inside the mill as the mill length was increased. However thenmaximum level of water increased with increasing mill length.nnnnnnnnnnnnn Experiments were also conducted using slurries. The solids in the slurry comprised anmixture of minerals containing roughly 60% by weight of a nickel concentrate andn40% by weight of cyclone underflow from the Mt. Isa Copper Concentrator. Thisnslurry showed pseudoplastic with yield behaviour. The results showed that thenfractional hold-up appeared to increase linearly with the apparent viscosity of thenslurry. It was also shown that hold-up increased with charge level in a non-linearnfashion.nnnnnnnnnnnn The flowrate - hold-up relationship developed by Latchireddi (2002) was used tondetermine if this model was capable of predicting the results obtained from thenexperiments. This exercise showed the Latchireddi model was only able to predict thenresults for a high aspect ratio configuration. It over-predicted the hold-up for thensquare and low aspect ratio configurations.nnnnnnnnnnn The Latchireddi model was modified to take into consideration the effects of millnaspect ratio. Non-linear least square techniques were used to incorporate an aspectnratio term (AR). The new model parameters g1 to g7, were fitted using a total of 633ndata points collected during the experimental stages of this project.nnnnnnnnn Two further models were constructed, the Maximum Height model and the MaximumnFractional Hold-Up model. Their aim was to correlate the maximum height of thenwater level and the fractional maximum hold-up corresponding to the maximum height of the profile, to the mean hold-up. These models were constructed from datancollected at the laboratory scale only and therefore, experiments should be conductednat pilot and industrial scales in order to validate them.n