The straw pulp is a common paper pulp just like wood pulp. Straw pulp can be made from various fiber materials, such as wheat straw, rice straw, eulaliopsis binata and etc. Compared to wood fiber, the straw fibers are easy to get, so that straw pulp cannot be overlooked.
Straw pulp making methods include soda pulping, sulfite pulping, sulfite pulping, chlorine pulping, etc. virgin straw pulp can be used to manufacture package paper and paperboard, or mixed into other kinds of pulp to make cultural paper. Bleached straw pulp can be applied to manufacture writing paper and typing paper, etc.
Straw pulp processing
Straw pulp processing can be generally summed into following steps: material preparation, pulp cooking, pulp washing, pulp screening, and pulp bleaching. Related equipment includes displacement digester system, blow tank, vacuum drum washer, twin roll press, single screw press, disc filter and etc.
6 rules for straw pulp making
Rule1: strengthen materials preparation
Material preparation is a vital step for straw pulp making. Knot, sheath, leaves, and stachys totally take 48% weight of straw, and stalk takes about 52%. The most of silt are mixed in the sheath. Each part of straw has different fiber morphology and chemical composition. In a word, Strengthen material preparation is good for straw pulp making. The advantages of material preparation as follows:
- It is the most cost-effective way to reduce silicon content of black liquor and reduce viscosity of black liquor.
- Benefit to reduce chemicals consumption.
- Benefit to decrease the parenchyma cell, fine fibers of paper pulp and increase the drainability of pulp.
- Increase the pulp yield, bleaching, and strength of pulp.
Rule2: lignin of straw pulp making
Alkaline pulping of straw fibers will dissolve 70% lignin, straw fiber contain plenty of lignin with alkaline solubility and low molecular mass, and lignin has high content of hydrophilic group, which means more instability
Rules3: residual lignin of straw pulp
Residual lignin of straw pulp has certain degree of condensation. As for the pieces of low molecular lignin, a part of them is diffused from cell wall, and it can be dissolved under alkali extraction.
Rule4: silicon content of straw pulp
The silicon content is not the only factor that affects viscosity of black liquor. Only in the high concentration, silicon content has no influence on viscosity of black liquor.
Rule5: displacement cooking: produce black liquor with low viscosity
Displacement cooking is a common pulp cooking process, with a lot of advantages, it widely applied to straw pulp making.
Adopt cold blow and high-temperature recycled black liquor, which can recovery heat and save energy consumption.
- In the primary period of cooking, dissolved lignin and carbohydrate from straw fibers are displaced outside digester and reduce the alkaline consumption of final stage of cooking.
- Large liquor ratio is good for even soak of alkali liquor, the evenness of cooking is better.
- Assign alkali concentration uniformly and adopt cold blow, reduce carbohydrate decomposition, increase pulp yield and strength.
- High-temperature washing inside digester, high washing efficiency.
- Pulp hardness is lower and easy to bleach, fewer chemicals required and reduce pollution.
Rule6: improve equipment and processing of straw pulp making
To apply for straw pulp making, improve the extraction equipment and processing. Ensure vacuum drum washer can be operated well under vacuum conditions, strengthen operation management.
About black liquor of straw pulp
The black liquor extracting of straw pulp is the bottle-neck of straw pulp pollution. The black liquor of straw pulp has high content of ash, sugar and big viscosity, which brings difficulties of pulp cooking. Besides, black liquor extracting equipment is limited.
Physicochemical characteristics of black liquor
Straw fibers has high content of ash, hemicellulose and parenchyma cell, which results in high silicon content, large viscosity and bad drainability of black liquor of straw pulp. For the composition of black liquor solid, important standard is the value between organics and nonorganic. In addition to, lignin of black liquor is another vital factor. Compared to wood fibers, the lignin content of straw fiber is higher.
Viscosity of black liquor
In general, rice straw has largest viscosity of black liquor, wheat straw takes the second place, and it goes sugarcane bagasse, reed, bamboo, and etc. Wood is has least viscosity of black liquor. There are some factors to affect viscosity, such as concentration, temperature, alkaline content, etc. high viscosity of black liquor of straw pulp is directly caused by high content of silicon and hemicellulose.
