Coal Preparation » Gravity Separation
Multi-slope (Banana) screen failures have been prevalent in the coal industry since the introduction of this type of screen in the 1980s. Increasing the size of these screens while benefitting the industry in terms of plant capacity and production has resulted in an increasing failure rate, some with serious consequences effecting the screen support structures, discharge chutes and feed chutes. CSIRO with its pilot multi-slope screen conducted a series of experiments in ACARP project C8042 (Improved Performance of Banana Screens in Drain and Rinse) showed that the g force can be reduced on drain and rinse screens without effecting the efficiency of operation. Further fundamental modelling work has also indicated that reducing the g force will have no negative effect and could improve the efficiency of operation. Reducing the screen's operational g force will reduce the forces that are transferred to the screen components and support structures providing an opportunity to reduce failure rates.
The g force chosen for a particular screening application is based on the material to be screened and screen duty, however the industry appears to have adopted a combination of frequency and stroke that is within a “normal” range of approximately 4 to 5 g. The source of the values for this range of g forces is linked to the dry quarrying industry which has performed and published a considerable amount of work in this area using inclined or horizontal screens. No publically available information can be found for the optimsation of Multi-sloped screens for the wet screening of coal.
Operational g force on a drain and rinse screen at a Bowen Basin plant was reduced and the data from samples taken before and after the g force change shows that there was no effect on the efficiency of the screen at least for this single experiment. Additionally early indications are that the magnetite recovery on this screen may have improved possibly due to the longer residence time which gives the finer particles a greater chance to be hydraulically removed from the bed.
There has been no reported beam failures since the g force was reduced 12 months ago, this is encouraging but the lack of failures in the previous 12 months suggest that other factors may also be involved.
The reduction of the g force should be instigated by changing the weights rather than changing the frequency of the screen operation as this may have negative issues in regards to screen natural frequencies and or the progression of the particles along the screen deck, the OEM should be consulted and included in discussions at all stages.