PROPERTIES OF SOUTH-AFRICAN SILICA SAND USED FOR RAPID FILTRATION

The paper reports the finding of an investigation into the properties of silica sand for use in rapid sand filtration, as supplied by variou s South African producers. The properties that were investigated are g rain size, uniformity coefficient, grain shape, porosity, density, dur ability, silica content and acid solubility. Each property is defied, its role and influence on the filtration process are pointed out, the method(s) of measurement is explained, and typical values for the prop erty are finally presented. From the results, a general profile of the commercially available media is derived to guide filter designers tow ards media specifications that will guarantee consistent filter medial of the higher quality within the constraint of economic attainability . In general, South African silica media, from all sources, are charac terised by a high degree of chemical and mechanical stability, as evid enced by low values for attrition losses (median 0.149%) and acid solu bility (median 0.11%), a high value for silica content (median 99.47%) and (median 2632 kg . m(-3)) which is close to the generally accepted 2650 kg . m(-3) for pure silica. The aeolian deposits yield media gra ins that are rounder (median 0.82) than the other deposits (median 0.5 8). The porosity and sphericity relationship at minimum fluidisation c losely agrees with correlations published by earlier workers. The meas ured effective sizes d(10) conformed closely to the values claimed by the suppliers, but not so for the coefficient of non-uniformity, which emphasises the need to use a clear, unambiguous sieve analysis proced ure.