Sulfur Release in High Dry Solids Black Liquor Concentration and Heat Treatment, 1998 International Chemical Recovery Conference Proceedings

The energy efficiency of a pulp mill can be increased by increasing the dry solids content of the heavy black liquor. The concentrated black liquor from a modem evaporator has typically maximum concentration of 75-80%. To improve still the energy efficiency and to reduce size of recovery boiler Ahlstrom Machinery Corp. has developed a new lamella evaporator for black liquor concentrations up to over 90%.

In high black liquor concentrations viscosity of liquor becomes a limiting factor in evaporation. The viscosity can be reduced to a proper level by ncreasing evaporation temperature and even more effectively by thermal Liquor Heat Treatment (LHT). When liquor is heated above temperatures of 160 - 170° volatile organic sulfur compounds will start to form and will be released from black liquor during the evaporation.

In extensive pilot tests black liquor was concentrated from about 75% to concentrations of 85% - over 90% in temperatures of 150 - 180°C with Liquor Heat Treatment temperatures of 170 - 200°C. The sulfur release varied between 1 and 4 kg S / ton DS in black liquor.