GB838026A - Improvements in or relating to the disposal of sludge - Google Patents

Abstract

Sludge formed by softening hard water with e.g. lime and soda, alum or sodium aluminate, to precipitate calcium and magnesium in insoluble form, is treated in aqueous suspension with carbon dioxide and/or sulphur dioxide to reduce the undissolved solids content of the sludge. As sources of the gases, combustion products of sulphur-containing carbon fuels are suitable, e.g. flue gas deriving from the combustion with air of coal, coke or fuel oil. Passage of the flue gas through the suspension results in conversion of insolubles to soluble bicarbonates, and sulphites and bisulphites which are then largely converted to sulphates and bisulphates by oxygen present in the gas. The final pH of the treated suspension is between 5,0 and 6,0. The suspension and gas are preferably contacted countercurrently by allowing the suspension to fall under gravity down a tower packed with e.g. granite chips or Raschig rings and passing the gas up the tower. In the lower part of the tower a pool of suspension is maintained to effect more efficient scrubbing of the gas. Drawings accompanying the Provisional Specification illustrate suitable apparatus. In an example 80% of water previously softened was drained off and the remaining 20% including the sludge was diluted with hard water and maintained in suspension with compressed air. The suspension was then passed down a tower countercurrent to flue gas containing carbon dioxide, sulphur dioxide, nitrogen and oxygen, and had a pH of about 5,5 when drawn off, with an undissolved solids content of 400 parts per million, less than one third of that in the untreated suspension. In general, aqueous suspensions of sludge containing from 800 to 2000 parts per million of undissolved solids may be treated.