INFLUENCE OF PROCESSING CONDITIONS ON ATTRITION OF NACL-CRYSTALS

Industrial solution-grown NaCl crystals exhibit different attrition propensities depending on the processing route and conditions. Pure Dried Vacuum (PDV) salt is produced by ICI plc via three routes following the crystallisation stage: in routes 1 and 2, the crystals are centrifuged and fluid-bed dried, and in route 3, they are rotary vacuum filtered and dried in situ on the filter. The output from this stage is conveyed to the storage vessels by belt conveyors in routes 1 and 3, and by pneumatic conveying, preceded by fluidised bed cooling, in route 2. The attrition propensity of salt samples from these routes is examined by single-particle impact testing. It is shown that the salt produced by vacuum filtration and drying has less attrition propensity than that produced via the first two routes. This is found to be due to differences in their surface hardness. Measurements of microhardness made at several depths show that, for the pneumatically conveyed salt from route 2, the hardness at a few hundred nanometres into the crystal is substantially greater (by up to ten times!) than that at several micrometres deep. Smaller variations are observed for the vacuum filtered and dried salt. It is suggested that 'rough' conditions during processing as occurring in centrifuges, fluidised beds and pneumatic conveyors may cause microplastic deformation which in turn work-hardens the surface, thus increasing their propensity to attrition.