High temperature CTMP from birch

Sammanfattning: This thesis is intended to contribute to understanding of the chemithermomechanical pulping of birch intended for high freeness grades. It focuses on the effects that conditions during pre‐treatment, i.e. the chemical addition of sodium sulphite and sodium hydroxide and the temperature in pre‐heater, have on the energy consumption and process runnability in terms of disc clearance. Pulp properties are evaluated in regard to brightness and the relation between bulk and internal bond strength. Pilot trials showed that pre‐heating birch chips to high temperature prior to refining (HT CTMP > 140°C), facilitated defibration and considerably lowered energy consumption. This made it possible to produce pulp with very high freeness. Despite the low energy input at high pre‐heating temperature, shive content remained low or was even reduced in the high freeness range. Mill trials confirmed the positive effect of a high pre‐heating temperature on energy consumption and on pulp properties. Furthermore it was shown that the internal bond strength in sheets from birch CTMP, in terms of Scott‐Bond at a given bulk, compared well with that of Spruce CTMP. Moreover, the shive content of birch CTMP produced using the high temperature technique was lower than that of spruce CTMP at a given bulk. A new laboratory technique ʹthe shavings methodology was used in combination with multivariate data analysis to investigate the effect of various pre‐treatments on native wood brightness. This method looks directly on the changes in brightness of the green wood as such. It revealed that the brightness of green birch wood is sensitive to increases in relative humidity and temperature. It also indicated that using a relatively high pre‐heating temperature (~140–155°C) when manufacturing birch CTMP is not necessarily detrimental to pulp brightness, provided the chemical charge is properly adjusted. However, at very high temperature (>160°C), the time in the pre‐heater should be kept short. Measurement of frictional behaviour, at simulated CTMP conditions, showed that the coefficient of friction of birch was greatly affected by chemical modification. Thus extraction raised the coefficient of friction. This rise can probably be attributed to reduced lubrication by the extractive substances and to the higher moisture content in the extracted samples. Sulphonation of the birch samples with 3 % Na2SO3 and 2 % NaOH (pH 13.5) gave a local maximum around 140–155°C. The local peak may be correlated with the reduction in energy consumption when the pre‐heating temperature is increased in the production of birch CTMP. Birch wood and spruce wood are also shown to have distinct differences in frictional performance. The coefficient of friction between birch and steel is higher than that between spruce and steel. The high stiffness and density of the birch wood and differences in the amount and composition of birch and spruce extractive substances probably account for the observed variations.

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