Mechanical Dewatering and Thermal Drying Characteristics of Pulp Mill Sludge Cake

Abstract

Sludge cake is a type of solid waste in pulp and paper mills which may harm the environment if disposed without treatment. Its proximate analysis (adb) are: ash 26.74%; volatile matter 59.09%; fixed carbon 11.04%; moisture 3.13%, while its ultimate analysis (adb) are: C 33.46%; H 4.5%; N 1.14%; S 0.35%; O 33.81%. Having a calorific value of 3000 cal/g (adb), sludge cake may be used as renewable fuel. Unfortunately it has a high water content, so reduction of water content become a main concern in utilizing sludge cake. A combination of mechanical dewatering and thermal drying has been considered for water removal with a minimum energy consumption. Moreover, the application of pressurized mechanical dewatering may also play a role in formation of briquettes. Experiments on mechanical dewatering and thermal drying were carried out using 50 g sludge cake with initial water content 73%. With a pressure of 400 kg/cm2 for compression, water content at the end of mechanical dewatering was 57%. This dewatered slugde was subsequently treated using thermal drying to a moisture content of 15%. Calculated total energy for this combination of mechanical dewatering and thermal drying was about 1080 J/g. This value was lower than the energy consumption of 1520 J/g required for direct thermal drying from its moisture content of 73% down to 15%. Experiments on the use of coal powder as compression aid were also trial to improve sludge dewaterability and briquettes calorific value. Mechanical dewatering of slugde with addition of 20%-w/w coal powder resulting a briquette with moisture content of 38% at compression pressure of 400 kg/cm2. The drying curves of briquette did not affected with its size if its diameter was less than 0.5 cm. Briquette with 1 cm thickness took significantly more time to remove its water content.