Coal Preparation » Dewatering
This report summarises the results from a study aimed at improving the solids capture of scroll centrifuges. The main objective was to assess at pilot and commercial scales, the performance of a scroll centrifuge fitted with baskets of finer aperture (ie 250 or 125 ?m) compared with the conventionally fitted size of 375?m. Improved solids capture might allow the scroll centrifuge to be applied to finer feeds. It was intended to monitor the wear of the commercial scale basket which was to be hardened with a coating of tungsten carbide. In addition it was intended to gain a greater understanding of the main parameters which affect the performance of scroll centrifuges.
A literature review showed that there was virtually no prior published work on scroll centrifuges and that the proposed approach in this project had not been attempted before. After some preliminary bench scale tests and modelling, a series of trials were performed at a CMI pilot scale (ca 2 tph feed solids) scroll centrifuge at the Clarence CPP in NSW.
Unfortunately, no meaningful runs were possible with the 375?m basket due to rapid blinding with near size particles. In tests with the other baskets, the greatest influence on solids capture was not the basket aperture but the solids density of the feed, suggesting that the kinetics of bed formation is the controlling variable, and that the particle bed itself is the main medium for capturing particles and not the basket. Product moisture was determined primarily by feed size and centrifuge spin speed, which is not surprising but the extent of moisture reduction that could be achieved for relatively modest increases in spin speed was unexpected. Since there seemed little difference in the performance of the 250 and 125?m baskets, it was decided to opt for the coarser aperture for the commercial scale trial.
A 250?m tungsten carbide lined basket was trialed at Rix's Creek, CPP in the first half of 1998. Sampling runs were conducted in March and May.
The first objective, to improve fine coal dewatering, was not achieved. Moistures ranged from 20% to 43%. This may have been due in part to excessively fine feed, having 20-38% <100?m. The corresponding product contained 16% -30% <100?m.
The second objective, to improve solids capture, was achieved when the basket was in good condition. Sampling in March showed solids recovery 92% - 94%. However in May solids recovery had fallen to 75% - 79.8%. Over this period the mean aperture had worn from 287?m to 731?m.
The third objective, to extend basket life, was not achieved. On completion of the trial the basket had run for 611.5 hours, handling 16,896 tonnes of feed at an average rate of 27.6 tph. All the tungsten carbide coating had worn off, and the basket had failed in a number of places. Some anecdotal evidence is cited that much longer basket life has been achieved, but the plant in question could not locate operational records to confirm this belief.
The report recommends further, as the results of the pilot testing are quite encouraging. An alternative wedge wire profile is suggested.