Method for treating liquid flows at a chemical pulp mill

Abstract

The present invention relates to a method for treating liquid flows at a chemical pulp mill comprising at least an alkaline cooking process for producing pulp, brown stock treatment with essentially closed liquid cycles, a pulp bleaching plant using ECF-bleaching, a chemical recovery plant comprising a chemical recovery boiler, and effluent purification. Chloride-containing bleaching plant effluents are led to the effluent purification, where they are treated in order to decrease the organic matter content thereof. At least 20 % of the purified effluent is returned to a pulp mill process and purified effluent is used in a last washing stage included in brown stock treatment. In the brown stock treatment the liquid flow is passed counter-currently to evaporation, wherefrom it is led for treatment to a recovery boiler process, wherein a separation process for chlorides is arranged for controlling the chloride level of the liquor cycle.

Description

technical field

[0001] The invention relates to a method for the treatment of liquid streams in a chemical pulp mill comprising at least an alkaline cooking process for the production of pulp, brown stock treatment with a substantially closed liquid cycle, bleached using ECF Pulp bleaching plants, chemical recovery plants including chemical recovery boilers, and sewage purification. Background technique

[0002] During the past few years, the size of pulp mills has increased substantially, as today a pulp mill producing 1 million tons / a is the standard size, and it seems that the increase in pulp mill size will not stop. While pulp mills are increasing in size, pulp mills are being built in areas and environments with very strict environmental protection regulations. For example, the amount of water used in pulp mills is strictly limited. A small reduction in the amount of water used by the pulp mill per tonne of pulp due to an increase in the size of the pulp mill will in...

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