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Abstract

A study was undertaken to assess the suitability of wheat straw to be pulped by means of the alkali-oxygen cooking process. The effects of some cooking variables, such as oxygen presence, type and amount of alkali, cooking temperature and time, on yield and Kappa number were evaluated.

The alkaline cooking of wheat straw in the presence of oxygen results in increased delignification. This is especially marked when the alkaline agent has a poor delignification capacity (sodium carbonate or bicarbonate). Thus oxygen seems to be very well suited to use in cooking processes with sodium carbonate liquors.

The use of such liquors can bring about a very interesting simplification of the recovery process.

In the second part of the work the influence of oxygen on pulp properties was evaluated as a function of the alkali used and of some cooking variables.

It is shown that the use of oxygen in alkaline cooking results in fibre degradation which affects the strength properties of the unbleached pulps only in the case of caustic soda cooking. The presence of oxygen during carbonate (or bicarbonate) cooking has favourable effects on pulp strength properties. This behaviour can be attributed to the fact that, in the case of carbonate cooking, the increment of delignification due to oxygen is far superior to that obtained in caustic cooking.

At equal delignification levels, the oxygen-carbonate pulps show better strength properties than the oxygen-caustic pulps. Long cooking times usually result in negative effects on strength properties.

The use of magnesium ions as inhibitors of carbohydrate degradation, had no definite effect on yield, viscosity, or strength properties.


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