Filed under: Specialty Cellulose Production

What Are Digesters?

by on Aug 4th, 2010

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At the beginning of the month, Fortress Paper Ltd. announced the purchase of equipment that would be necessary to begin the conversion of the Thurso hardwood pulp mill into a dissolving pulp specialist.

Acquired from Finnish company Stora Enso Oyi Cellulose Inc., three digesters (as well as several other pieces of specialized processing equipment) began their journey from Finland to Thurso, Quebec this week.

So what are digesters and why are they so important?

Digesters are essentially huge vats in which wood chips are “cooked” and boiled for several hours at a high temperature (130 to 180 °C) in cooking liquids (combinations of previous pulping runoff and various chemicals). A large part of the digestion process requires removing oxygen from the mix. Doing this allows biodegradable material to break down at a faster pace.

Digesters are where solids are transformed into a more liquid-like sludge that will go through other necessary processes in order to become pulp. They are also the location where regular pulp differs from dissolving pulp.
Whereas hardwood pulp moves from the digesters to the screening, washing and drying process right away, dissolving pulp must go through another process called “the sulfite process” first.

Using various salts of sulfurous acid, the sulfite process helps extract lignin – a complex chemical compound found in the cell walls of plants – from the wood pulp. The chemicals thus remove hemicellulose, a weaker polysaccharide inside the biological composition of individual pulp cells and ensure that the pulp has a higher cellulose content.

More digesters on site at Thurso will mean a higher dissolving pulp output from the mil.

The new digesters are expected to arrive on massive barges in the Ottawa River at the beginning of August.

SOURCES:
Government of Alberta: “Anaerobic Digsters”
The Globe And Mail: “Quebec Mill Sees New Life In Rayon Market”
Fortress Specialty Cellulose: “The Difference Between NBHK Pulp and Dissolving Pulp”
Fortress Specialty Cellulose: “What Is The Kraft Process? Part 1”

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