Australian Newsprint Mills Limited has operated a Cold Caustic Soda pulping process at its mill at Boyer in Tasmania since 1957. This paper describes commercial operations in which 7 tonnes o.d./hr are relined to 110 Csf using 700 kW hr/tonne o.d.
The process is best suited to E.regnans and E.delegatensis of 30-100 years of age, although eucalypts as young as 10 years have been used successfully. Process operations are: atmospheric steaming, impregnation with 9% NaOH on o.d. fibre, two stages of low consistency relining with Sprout Waldron 36-2 refiners, washing, bleaching with calcium hypochlorite, washing, a third stage of low consistency refining and finally screening and thickening.
This paper describes experiments in which one Sprout Bauer 45-1B refiner was used as a single stage in an attempt to equal the effect of five smaller refiners arranged in first and second refining stages. This trial refiner was run for four weeks total and covered a range of refining consistencies from 5% to 35% and was run in parallel with the 36-2 refiners. It was possible to fully load the 3MW machine while operating in the range of 35 to 10% o.d. solids, although plate changes were needed. The full range of operating conditions are discussed in the paper.
In normal operation of the plant approximately 230 kW hr/tonne is applied in each refining stage; approximately 700
kW hr/tonne being required to produce a pulp at 110 Canadian Standard Freeness.
High consistency refining in the 45-1B refiner provided a maximum energy saving of 8% (43 kW hr/tonne) over the low consistency refining. The high consistency pulp had latency, which was not a characteristic of the 36-2 pulp, 16% lower Tensile Index and 1.7 units lower IS0 brightness which also affected the pulp after third stage refining.
Previous pilot refiner and commercial studies by other workers had led us to expect significant increases in energy
requirements and some changes to pulp properties from high consistency refining. This work showed that a commercial plant using larger refiners than the Sprout Waldron 36-2 refiner can now be designed, still using a very low specific unit energy of 700 kW hrltonne o.d.
The success of this process depends on one or more of three factors; the uniform impregnation of the eucalypt chips with caustic soda, the care taken to ensure a uniform feed flow to the refiner and the open discharge from the refiner.
Work at CSIRO and this laboratory has shown that the addition of sulphite up to 1% on o.d. fibre has not improved pulp quality. The impacts of the process variables and the use of young eucalypts are discussed.
Low Energy Commercial High Yield Pulp from Eucalypts, 1991 International Mechanical Pulping Conferen
