Ultra-high-yield pulping. Part VII. The effect of pH during impregnation on the quality of lightly sulphonated CTMP

Black spruce chips, impregnated with sodium sulphite solutions at pH 4.2, 5.9, and 8.6, were refined in a pilot plant two-stage TMP system in order to determine the effect of treatment on pulp and handsheet properties of lightly sulphonated (0.4-0.5% SO3 on o.d. pulp) CTMP. Reducing the pH of the chip impregnation liquor from 8.6 to 4.2 decreased the refining energy required to obtain a given freeness. It was found that, at a given specific energy, the CTMP made from chips sulphonated at pH 4.2 had the highest strength and best optical properties. At a given freeness, CTMP handsheet properties, with the exception of specific scattering and hence opacity, were superior to corresponding TMP properties. Therefore, higher production rates can be obtained from a given system by lowering the pH of the solution.