The Effect of Particle Size Distribution in Electrostatic and Magnetic Separation on Mineral Recovery and Yields

摘要

Namakwa Sands is a mineral processing company based on the West Coast of South Africa and isrnsolely owned by Exxaro Resources Limited. Mineral ore is mined and processed, concentrated andrnboth chemical and mechanically upgraded before being sent for electrostatic and magnetic separationrnunits. The fi nal high value mineral products report as Zircon, Rutile and Ilmenite.rnThe recovery of non-magnetic material is largely dependent upon a number of factors, includingrnbut not limited to dew point, ambient temperature, feed rate, mineral size distribution, specifi crngravity and mineral content in head feed. One important limiting factor to recovery in typical dry millrnoperations is the particle size distribution of the feed. Variance in the particle size has catastrophicrnconsequences for recovery. This is the reason for this study. Until recently large variances in nonmagneticrnproduction were viewed to be a function of machine operation. This view was later rejectedrnwith the feed size distribution determined as the major driver of low recoveries, for Namakwa Sandsrnorebody 50 per cent variance in particle size distribution can be noticed within one day. This isrnbecause of the successive mining blending techniques employed.rnThe predicament is that the two modes were so closely placed together that recovery of a single modernis almost impossible without the consequence of greatly reducing recovery. The second mode wasrnfound at 125 μm while the fi rst was at 90 μm. Interestingly, for the mentioned feed type a cumulativernsize distribution of a unimodal feed type is almost exactly the same as for a bimodal size distribution.rnThus a plot of cumulative size distribution will show no difference due to the modes being so closerntogether. This is the reason why this phenomenon was not noticed before.rnThe study also found that, using an high tension roll (HTR) or rare earth roll, a fine stream with arnsingle mode at 90 μm versus a coarse stream with a single mode at 150 μm would yield completelyrndifferent results interms of yields. Larger conductive particles are thrown from the roll while smallrnparticles loose charge faster (entrainment). Results in terms of the bimodal size distribution showrnthat even though 25 per cent of the feed type is greater than 150 μm the yields followed a similarrnpattern to that of the coarse particles rather than that of the majority (75 per cent) fi ne particles. Thisrnconfi rms what was published by M Ziemski and P N Holtham in 2005 on charge decay rates; theyrnmentioned that particle bed effects played a major role in recovery.