Purification of waste water from sulphate pulp bleaching plants

Claim 1

A process for decolouring an aqueous effluent from the bleaching of sulphate pulp with chlorine or chlorine containing compounds, said effluent containing dark coloured lignin degradation products, said bleaching including at least one alkali extraction stage, which consists essentially of: a. mixing the aqueous effluent from at least one alkali extraction stage in the bleaching with a sufficient quantity of at least one acid selected from the group consisting of sulphuric acid, sulphurous acid, sulphur dioxide and hydrochloric acid to reduce the pH of the effluent to a value not exceeding about 2 thereby forming a precipitate, said precipitate consisting essentially of dark coloured lignin degradation products and other organic material, b. separating the acidic aqueous effluent from the precipitate, c. dissolving the precipitate which has been separated, by bringing the precipitate into contact with an alkaline solution, said solution being at least partly black liquor from the sulphate pulp process and d. passing the acidic aqueous effluent which has been separated from the precipitate through a bed of particulate phenolic anion exchange resin having pendant amino groups which are substantially only secondary and/or tertiary amino groups which resin is activated by the acidic effluent, said activated resin binding the residual dark coloured lignin degradation products, and separating a decoloured effluent from the loaded resin bed.