Cotton linter is the short fibers peeled off from the cottonseed after ginning. The cotton linter can be turned into cotton pulp through the pulp cooking and bleaching process. The cotton pulp is the primary raw material to produce cotton paper, which is also known as rag paper.
Cotton Paper Features
Cotton paper can last many years without deterioration, and is superior in both strength and durability to wood pulp paper, so it is usually used as the printing paper for important documents. In addition to high-grade paper, cotton pulp is also a basic industrial raw material for the manufacture of synthetic fibers and chemical products (cellulose ether series, plastics, pharmaceutical additives, etc.).
Cotton Pulp Making Process
In the continuous development of pulping technology, there have appeared many kinds of cotton linter cooking processes. However, no matter what kind of process is adopted, the raw materials should be purified before cooking to make the impurity content less than 3%. The raw materials should be pre-impregnated with hot alkali solution, then extruded by screw press, and then put into the digester for cooking.
Alkaline Cooking of Cotton Pulp
Alkaline pulping is the most traditional pulping process of cotton linter pulp. The classic cooking process of alkaline cotton pulping is as follows:
- Alkali dosage: 10%~15%
- Liquor ratio: 1:3~1:6
- Maximum cooking temperature: 140~170℃
- Soaking time: 2~5 hours
A small amount (0.5%~1.0%) of lubricants and surfactants, such as alkyl sulfonate, sulfonated castor oil, can be added in the cooking process to improve the pulp quality.
Although high temperature, high alkali cooking process and long soaking time can effectively remove fat, wax, pectin, and lignin in cotton linter, the degradation and loss of fiber are serious, resulting in the reduction of pulp yield and strength. In addition, pectin in cotton linter reacts with alkali at high temperature to form brown substance adsorbed on the surface of fiber, resulting in low whiteness of cotton pulp.
Moreover, the traditional alkaline pulping causes serious pollution. The black liquor generated in the alkaline cotton linter pulping contains high concentration of organic matter. For example, the concentration of CODCr exceeds 5 x 104mg/L, of BOD5 exceeds 2.5 x 104mg/L, and the pH value is higher than 13, which makes the black liquor hard to treat.
The Continuous Cooking of Cotton Pulp
The continuous cooking process has been used to cook cotton pulp since the 1990s. Compared with alkaline pulping, it reduces the alkali dosage in the pulp cooking process and achieves higher pulp yield and pulp quality.
The equipment used for continuous pulping was firstly the horizontal tube continuous cooking digester. The cotton pulp produced by this machine has decent evenness.
With the development of pulping technology, now we can realize the pulping process of cooking, bleaching, and washing in the twin screw extruder. With this machine, the continuous pulping can produce cotton pulp which reaches the quality level of batch cooking pulp. In addition, the fiber loss, the comprehensive energy consumption, and the wastewater pollution during the continuous pulping process are all considerably reduced.
CNBM affiliate Anyang Machinery has been focused on the development and manufacturing of paper pulping machines for five decades. We have rich experience in the design and construction of cotton pulp mill plant. Please feel free to drop us an e-mail and tell us your needs in the paper pulp industry!