The utilisation of the ash disposal system as a salt sink : enhancement and optimisation of chemical interactions

[摘要] ENGLISH ABSTRACT: The fine ash produced at the Sasol Secunda Petrochemical Plant is disposedof through a wet ash disposal system. Other process waste streams with highsalt concentrations are co-disposed of in the Sasol Secunda ash disposalsystem. This has led to a steady rise in the salt concentrations of the recycledclear ash effluent (CAE) over the past 17 years. To combat this increase insalt concentrations, the capability of the Sasol Secunda ash disposal systemto act as a salt sink, needs to be enhanced.This investigation focussed on ways to enhance the salt removal/retentioncapabilities of the Sasol Secunda ash disposal system and consisted of thefollowing:• A literature survey of relevant information.• The mixing of different combinations of fine ash, brine and CAE.• Adding CO2 to the fine ash and CAE mixtures.• Investigation to enhance salt precipitation in the CAE and Evaporationdams.• Salt balances and a residence time calculation over the CAE andEvaporation dams.From these investigations it were concluded that the Sasol Secunda ashdisposal system could be used as a salt sink for SO4 ions. Up to 43% of theSO4 is removed from the brines after the initial ash/water contact. It was alsofound that the tubular reverse osmosis (TRO) brine could be used as a carriermedium for the ash. The large amounts of Ca that is leached into the ashwater during the mixing of the CAE and fine ash can be prevented by theaddition of CO2 to the mixing point. There is usually an increase of 240% inthe Ca concentration and this is reduced to only an 8% increase with the CO2addition. The most feasible precipitation enhancement for the CAE and Evaporationdams is an increase in evaporation. This enhances CaCO3 precipitation,which is the main mechanism for salt removal in the CAE and Evaporationdams.Ca, Na and Cl are retained in the evaporation and CAE dams. SO4 is leachedfrom solid phases in the dams. There is however an overall decrease in thetotal dissolved solids (TDS) of the ash water. The salt removal of the CAEand Evaporation dams is approximately 57 tons per day.The capability of the Sasol Secunda ash disposal system to act as a salt sinkcan be enhanced by the addition of CO2 at the mixing point and by increasingthe evaporation rate in the CAE and Evaporation dams. Using the TRO brineas carrier medium may also increase the SO4 precipitation capabilities of theSasol Secunda ash disposal system.

[发布机构] Stellenbosch University