Characterisation of wear in natural rubber and polyurethane hydrocyclones used for minerals processing applications

Abstract

Slurry wear is a common problem in the minerals processing industry. Hydrocyclones are widely used in wet mineral classification applications and can be notoriously prone to wear. This study investigated wear in hydrocyclones, with specific attention to wear of spigots in polyurethane- and natural rubber-lined cyclones. Hydrocyclone wear, and more specifically spigot wear and its influence on efficiency (imperfection value), was investigated for a Multotec HC250 (250 mm barrel diameter) hydrocyclone. The HC250 hydrocyclone comprised a metal shell lined with vulcanized natural rubber, with a thermoset polyurethane spigot and vortex finder. Tests were conducted on this hydrocyclone in a pilot-scale recirculating test setup. The wear rate of the spigot was quantified by measuring its volume change with operating time using microcomputed-tomography scans. After 122 days in operation (recirculating feed materials in a closed loop), wear in the hydrocyclone led to an increase of 13.67% in the volume of the spigot. This relatively low wear rate was related to the fact that material recirculation led to particle smoothening and a reduction in particle size. This result also showed the enormous influence of particle shape and size on wear. A large single wear groove at the bottom of the natural rubber lower cone formed due to localized porosity (a compression-moulding defect), as determined by microcomputed tomography. The worn natural rubber lining also showed signs of chemical attack (devulcanization) occurring at microscopic level. In terms of efficiency, it was found that wear in the spigot resulted in a lower imperfection value of 0.48 compared with that of 0.39 for a new spigot. The relative density of the underflow decreased significantly from a value of 1.359 for the new hydrocyclone to 1.183 when worn. Although the worn spigot recovered a larger amount of water to the underflow, misplacement of fines and the fish-hook effect were greater for the new spigot. Multotec VV165 polyurethane (165 mm barrel diameter) hydrocyclones, used at the Exxaro Grootegeluk coal dense medium separation plant (South Africa) for magnetite densification, were also evaluated to determine their wear rate and enable more informed decisions on replacement intervals. It was found that after 90 days of operation in densifying magnetite medium (−45 µm), these hydrocyclones showed no wear, based on the change in volume in the spigot as measured by microcomputed tomography. These findings showed the importance of particle size on the wear rate of a hydrocyclone spigot and confirmed the requirement of a critical diameter at a set velocity to cause wear. The magnetite medium of −45 µm was below the critical particle diameter that would cause wear within this time period.