Pulp is washed after the digestion step to remove cooking liquor chemicals and organic compounds dissolved from the wood chips.1
Brown stock can be used in this washing step, resulting in a higher level of chemical recovery and minimising dilution of black liquor. Optimising the dilution factor lowers the amount of water that must be evaporated from weak black liquor, thereby reducing steam consumption in the evaporators. At one plant, it was estimated that these improvements could reduce natural gas usage by 315,000 GJ/yr, resulting in costs savings of around AUD$400,000-AUD$600,000/yr. 2
Utilising pressure diffusion or wash presses for brown stock washing instead of conventional vacuum pressure units, can reduce electricity and steam use further while also reducing chemical use. Electricity savings are estimated at approximately 12 kWh/tonne of production. 3
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