The dewatering of a mining sludge containing hexavalent chromium using a tubular filter press - a South African development

A chromium smelting process can result, under oxidising conditions, in the production of a dust which when slurried and pumped to a waste dump heap is likely to pollute the environment. South Africa is a large producer of chromium metal and the problem of the release of hexavalent chromium (Cr⁶⁺) to the environment is a serious concern. The department of Water Affairs and Forestry in South Africa monitors all environmental discharges and regulate the levels of toxic chemicals and pollutants. The ‘Tubular Filter Press’ a South African developed sludge dewatering system was proposed for the recovery of dust in this effluent. The filter press was operated for the dewatering of waterworks sludges and promises to be successful for this application. A single tube pilot study shows that chromium discharge to the environment can be eliminated. Feed solids concentrations of between 40 and 50 g/l were fed to the tubular filter resulting in cake solids concentrations in excess of 50%. The operating pressure of the system was between 200 and 300 kPa and the flux reduced to 200 L/m².h during the filtration cycle. The filtration was modelled using a variable pressure, internal cylindrical compressible cake filtration model and operating parameters for a full scale plant were proposed.