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After installing three of these 25-tonne stainless steel drums at its Red Rock kraft pulp mill, Domtar Inc. says efficiency improved to such a degree that it may now expand the plant's capacity
Archived News Item: Washer-drum innovation boosts mill's efficiency

NICKEL, December 1995

Washer-drum innovation boosts mill's efficiency

Three 25-tonne perforated, stainless steel drums, each measuring 3.5 metres in diameter by eight metres long, have been installed by Domtar Inc. of Montreal to upgrade the brown stock washing cycle in its kraft pulp mill at Red Rock, Ont., Canada.

The drums are a key part of a design that improves the efficiency of pulp washing by 25% compared to conventional vacuum washers and reduces the need to vent fumes to the environment by 90%, says manufacturer Tristar Industries Ltd. of Delta, B.C., Canada.

The Red Rock plant had been on the verge of shutting down because the old washer drums had not performed as expected, says Trent James, Domtar's resident manager at the plant. The replacement drums supplied by Tristar allowed the wash system to meet its designed reliability and efficiency. Domtar is now considering expanding the plant's capacity.

Kraft pulp mills separate fibres from the lignin in wood cells by cooking them in an alkaline sodium and sulphur-based cooking liquor. The fibre or pulp is then washed in large displacement-type rotary washers.

The drum, part of the washer, sits partially immersed in the cooking liquor and pulp slurry inside an enclosed vat. The drum rotates at 1-3 revolutions per minute and, as the liquor is forced through the drum's perforated surface, a pulp sheet forms on the outside of the ~ drum. Cleaner liquor is then forced --through the pulp and the drum, displacing the cooking liquor . The process of separating the cooking liquor from the pulp is called brown stock washing. The cooking liquor is recovered and recycled in a closed loop system while the washed pulp moves forward in the mill for further processing.

To form the pulp sheet on the drum and to drive the wash liquor through the sheet, a differential pressure is required. The conventional approach is to form a vacuum inside the drum, under the sheet. Typically three drums in series are required for efficient brown stock washing.

The stainless steel replacement drums allowed the wash system to meet its designed reliability and efficiency.

Alternatively, as in Tristar's design, pressure can be created above the sheet to drive the liquid through and into the drum.

Although vacuum washers are popular because of their simplicity of operation and consequential equipment reliability, pressure washers are significantly more efficient in washing the pulp, largely because less fresh water is required to remove the black liquor from the pulp fibre.

But pressure washers have a history of design problems, particularly with the rotating drum. The Tristar pressure washer drum, developed with the help of research conducted at the University of British Columbia, is made of 531603 stainless steel for corrosion resistance with main structural components flux core welded and thermally stress relieved. It is designed to withstand the heavy loads that have led to fatigue cracking in previous designs.

As a result, Tristar expects that this technology will compete against other high efficiency washing technologies, including the wash-press system. The company has filed for patents on its design and expects to produce 3-5 drums each year for the next 10 years.