METHOD FOR TREATING LIQUID FLOWS AT A CHEMICAL PULP MILL

摘要

The invention relates to a method of treating liquid flows at a chemical pulp mill comprising at least an alkaline cooking process utilizing cooking liquor for producing pulp, brown stock treatment with essentially closed liquid cycles, a pulp bleaching plant using ECF-bleaching wherein chloride- containing effluents are generated, a chemical recovery plant comprising a causticizing plant and chemical recovery boiler plant, and effluent purification. A characteristic feature of the method is that chloride-containing effluents of the bleaching plant are introduced to the effluent purification where they are treated for decreasing the organic matter content thereof, at least 20 % of the purified effluent is led back to a pulp mill process, purified effluent is used in at least one stage of causticizing as raw water source so that the chloride-containing liquid formed in the process stage is a weak white liquor fraction that is led to the recovery boiler plant as a smelt dissolving liquid component and therefrom further via regeneration of cooking liquor, pulp cooking and pulp washing as a waste liquor component to evaporation, wherefrom the concentrated waste liquor is led for treatment into a recovery boiler process, wherein a separation process for chlorides is arranged in order to control the chloride-level of the liquor cycle.