Recycling of papermaking black liquor - spray drying

The main components of plant stems are cellulose, hemicellulose and lignin, cellulose accounts for about 50%, hemicellulose accounts for 15% to 25%, and lignin accounts for 15% to 30%. The current chemical pulping method utilizes only cellulose and a part of hemicellulose, and abandons lignin and a part of hemicellulose, which not only wastes resources but also pollutes the environment.

At the end of the 1990s, the Chinese government forced the closure of paper mills with a size of less than 10,000 tons/year, and required paper mills with a size of 34,000 tons/year or more to use alkali recovery methods. This method must concentrate the black liquor to more than 40% before the black liquor enters the alkali recovery furnace. However, due to the large amount of silicon contained in the black liquor, it is easy to cause fouling in the tube during evaporation and concentration, and it is difficult for the black liquor to evaporate to over 40%.

The currently recognized good solution to this problem is spray (outside tube) horizontal thin film evaporation plus spray drying. The law has been widely used in Guangzhou Paper Mill, Hebei Jingxian Paper Mill, Henan Wugang Steel Mill, Henan Wenlin Paper Mill, Jiangsu Double Light Paper Mill, Jiangsu Xinyu Paper Mill, Guangdong Maoming Paper Mill, etc. Paper mills are still wood pulp mills, large or small, and can be applied. The resulting lignosulfonates are more valued by companies.

Due to the different papermaking raw materials and the different papermaking methods, black liquor (or red liquor) is very different. Therefore, before dealing with black liquor, you must clarify the source of raw materials. One is lignin raw materials, such as pine, hardwood, bamboo; the other is hemerocallis, such as wheatgrass, straw, reed, bagasse or several mixtures.

The papermaking methods commonly used at the present include alkaline methods (using sodium hydroxide or sodium sulfide, hydrazine), acid methods (using calcium bisulfite, magnesium bisulfite, sodium sulfite, etc.), ammonium methods (using ammonium sulfite) and other new methods. Method (SFP method, mechanical refining method, blasting method).

The characteristics of the various black liquors (or red liquors) before spray drying are as follows: the liquid concentration is 40% to 55%; the amount of treatment is relatively large; 1 set of 10,000 tons/year scale paper mill, the black liquor treatment capacity is greater than 3 t/h; containing more ash, the so-called silicon interference; black liquor without desugar treatment, containing a lot of sodium saccharate and its derivatives; containing many small staple fibers; high viscosity at room temperature; lignin spontaneous combustion Sex.

Therefore, the following issues should be noted when performing spray drying: preheat the concentrated black liquor to 70°C or above to reduce the viscosity of the solution; use a rotary disc atomizer to avoid clogging of the orifice of the pressure atomizer. With the air atomizer, due to the low dry powder loading density, the atomization energy consumption increases the cost; in the paper mill with the power generation device, the power station vapor can be used as the heat source, and the coal-fired hot blast stove can be directly used as the heat source after dusting with the flue gas. When the inlet air temperature is higher than 280°C, it may easily cause combustion of accumulated material in the drying tower. Therefore, the inlet air temperature must not exceed 280°C. The feed rate must be strictly controlled to prevent poor atomization and sticky walls due to excessive feeding rate, leading to burning. The phenomenon occurs.