On Spent Liquor of Semichemical Pulping (I)

Abstract

This paper describes species variation of pulping timbers in color and COD of spent liquors from neutral sulfite pulping in connection with water pollution problems. Thorough investigation were made with certain hardwood species like Shii (Castanopsis cuspidata), Shide (Carpinus sp..) and Kagonoki (Actinodaphne lanciforia), common in Japan and certain southern tropical wood species like Red lauan (Shorea sp.), White lauan (Pentacme sp.) and Kapur (Dryobalanops sp.).Spent liquor from the cooking of both individual species and species mixture were analysed in each stage of the pulping processes. The digested chips were separated from cooking liquors (black liquor) and washed with one liter of distilled water (washings). And they were refined on a laboratory defibrator in three steps (defibration liquors) and each spent liquor was subjected to the routine spent liquor analysis searching for the main features of pollution characteristics.As a summary of the tests carried out, it may be stated that the effect of the species variation of pulping timbers in color and volume of the black liquor is considerably of high value, but there are no great differences in COD. Especially, volume of the black liquor has been found to be highly proportional to the apparent specific gravity of timber species. The distribution of solid, total color and COD in each stage of the pulping processes differed among timber species, because of the diffence of the apparent specific gravity of wood species. In the case of Lauan species, for example, color and COD were chiefly originated from the washings and the first defibration liquor rather than black liquor.On all wood species used, the contribution of black liquor reaches at most 50% of total color and COD of over all spent liquors produced, and the recovery of black liquor and especially the first defibration liquor are very important for the treatment of water pollution problems. It should also be mentioned that color of spent liquors and the pulp yield increase, as the pH of black liquors of hardwood cooking increases. Southern tropical wood, however, dose not show this relationship. Under the constant pulp yield, variation of the maximum temperature of digestion dose not influence on color and COD of spent liquors. But bark removal has the effect of lower coloration and higher COD of spent liquors. In the case of cooking of species mixture, the volume, color and COD of black liquors are manifestly represented as the mean values of those obtained from the individual cooking of each species.