Determination of drainage rates of heavy media for different aperture sizes on a vibrating screen
Thesis (MEng)--Stellenbosch University, 2018.
ENGLISH SUMMARY: Media losses are a significant contributor to the operational cost in a dense media separation (DMS) circuit. Of these losses, up to 80 wt% can occur on the drain and rinse vibrating screens. Although these screens are an integral part of any DMS circuit, surprisingly little work can be found in open literature regarding the effect of different screening panels on medium recovery, and particularly on ferrosilicon. Hence, considering the cost implication of heavy medium to the DMS circuit, the project focused on the recovery of medium particles. In this case, the effect of slurry density, volumetric flowrate and slot size variation were investigated. To execute the thesis, experimental works were conducted on a 0.6 x 1.2 m vibrating screen with polyurethane, rubber, and poly-wedge slot apertures at slurry density between 1.6 – 2.7 kg/L and volumetric flowrate of 18 – 26 m3/h for both magnetite and ferrosilicon. Medium drainage rates were established with and without ore material for the entire underflow stream. Samples from the feed, underflow and overflow streams were collected for particle size distribution analysis, percent moisture, medium carryover and mass balance calculations. Results obtained showed that increasing volumetric flowrate from 19.9 – 23.7 m3/h led to an increase in ferrosilicon drainage rate, percent moisture and medium carryover. However, once a critical volumetric flowrate was exceeded, a further increase in volumetric flowrate led to a decrease in drainage rate with a sharp increase in moisture and medium bypass to the oversize stream. A shift in the critical volumetric flowrate from 23.7 m3/h for fresh ferrosilicon to 24.5 m3/h for degraded material was observed. Comparable results obtained on magnetite showed different critical volumetric flowrates for different screen panels with 1x13 mm and 0.8x8.8 mm at 20.8 m3/h, 1x12 mm rubber panels and 0.63 mm poly-wedge at 21.3 m3/h, and 0.63x12 mm and 0.63x8.8 mm at 20.4 m3/h. The increase in ferrosilicon slurry density from 1.9 to 2.45 kg/L led to a gradual decrease in medium drainage rate with increase in moisture and medium bypass to the oversize stream. However, a sharp drop in the drainage rate coupled with a significant increase in moisture and medium carryover was observed at slurry density between 2.45 – 2.7 kg/L. Conversely, increase in magnetite slurry density from 1.64 to 1.84 kg/L led to a gradual decrease in medium drainage rate across all the panels tested. On the other hand, aperture size increase from 0.63x8.8 mm to 0.8x8.8 mm to 1.0x13 mm resulted in an increase in ferrosilicon drainage rate of about 1.4 - 1.9 m3/m2/h with a reduction in moisture and medium carryover of about 1.0 – 1.2 w/w% and 14.1 – 20.2 kg/t/m respectively depending on the volumetric flowrate. The increase in slot width from 0.63 to 0.8 mm led to an increase in magnetite drainage rate of about 1.0 - 1.4 m3/m2/h at volumetric flowrate between 20.4 – 22.8 m3/h. Howbeit, the increase in slot length from 8.8 to 12 mm led to an increase in magnetite drainage rate of about 0.1 - 0.5 m3/m2/h at volumetric flowrate between 19.94 – 22.8 m3/h. Increase in slurry density from 1.9 to 2.45 kg/L led to a steady decrease in the sharpness of separation with the cut sizes becoming finer postulating a rise in moisture and medium carryover to the oversize stream. Beyond 2.45 kg/L, a sharp decrease in the efficiency of separation and cut sizes with an increase in water split was observed. Increase in volumetric flowrate from 21.8 to 24.5 m3/h led to a drop in the sharpness of separation and cut size values with an upsurge in water carryover to the oversize stream. However, higher volumetric flowrate and slurry density led to a sharp decrease in the sharpness of separation and cut sizes with a marked increase in moisture bypass.
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