Combustion of bleaching effluents with kraft black liquor. I. Thermogravimetric analysis

Recycling of bleaching effluents into a black liquor recovery system has become an important step of a closed cycle technology that, in a kraft pulp mill, will result in reducing pollutants discharge in the receiving waters and in the decrease of fresh water consumption. In the present work, the combustion of pre-dried solid samples of black liquor (BL), chlorination effluent (C/D) and extraction effluent (E 1 ), as well as of two-and three-component mixtures of the three liquors was experimentally investigated in a thermogravimetric analyzer (TGA). The thermogravimetric analyses showed that the solids from the C/D stage were similar to those of the black liquor solids in terms of volatile matter, char and ash contents, while the solids from the E 1 stage, containing less volatiles, had a richer ash content. Results also revealed that both effluents were devolatilized and gasified at lower rates than the black liquor. Addition of up to 20% of either effluent had no significant impact of the TGA behaviour of the black liquor. Furthermore, mixtures containing solids from the three liquors were very close to black liquors in terms of their devolatilization and gasification rates, a weight-loss trend similar to that of the black liquor following.