Energy usage and effluents from chemichal and mechanical pulping systems

Authors: J.M. Uprichard, S.R. Carson
Publication: New Zealand Journal of Forestry, Volume N.Z.J.For. 1976, Issue N.Z.J.For. 21(1) 1976, pp 21-35, May 1976
Publisher: New Zealand Institute of Forestry

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Abstract: An examination of the energy demands and the effluent characteristics of the kraft and refiner pulping processes indicates that the refiner process has a much lower environmental impact than the kraft process, but has a greater electrical energy demand.
The minimum economic size of a refiner mill is one with a production rate of about 200 tonnes of pulp per day while that of a kraft mill is generally considered to be between 400 and 500 tonnes. As the water demand of refiner pulp is of the order of 20 m3/tonne and the kraft pulp demand is approximately ten times as much, there is a significant difference in the fresh water consumption and resulting discharge rates.
Also, the large effluent flow from the kraft process must be treated to remove 25 to 45 kg BOD,/'tonne of pulp whereas the refiner mill effluent normally has a loading of only 10 kg/ tonne. While the levels may be reduced with the implementation of recently developed technology, it is likely that the relative magnitudes of each will remain similar.
The atmospheric discharge of the kraft process has an offensive odour due to sulphur compounds. In contrast there is no significant discharge from the refiner process.
Electrical energy is used intensively by the refiner process and 1 500 to 2 000 kWh are required per tonne of pulp. This is approximately twice the electrical energy demand of the kraft process. However, the kraft process also requires 3 500 k Wh/ tonne of heat energy which is obtained by burning waste products of the process in a recovery furnace.

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