The pulp of paper made from cellulose contains cellulose, lignin, hemicellulose, pigments, etc. Cellulose is the main target product in papermaking and bio-refining. It has the characteristics of high polymerization degree, large molecular weight, and high crystallinity. Its solubility in water is relatively low.
Lignin usually forms the framework of plants together with cellulose and hemicellulose and cannot exist alone. Lignin can be obtained through various separation methods such as alkaline lignin, lignin sulfonate, and enzymatic lignin. Hemicellulose is a type of polysaccharide mainly composed of monosaccharides such as xylose, arabinose, mannose, and glucose, which are connected by β-1,4-glycosidic bonds. The molecular weight of alkali-soluble hemicellulose is mostly above 3000D.
In the traditional paper-making industry, alkaline liquid is often used to wash the pulp at high temperatures, dissolving lignin, hemicellulose, pigments, etc. into the alkaline liquid, thereby obtaining cellulose with higher purity.
The alkaline solution is reused in multiple cycles. The amounts of hemicellulose and pigments accumulate, turning it black and being called papermaking black liquor. The papermaking black liquor with more impurities will affect the quality of high-quality glass paper. Usually, acid neutralization for sewage treatment or recovery of sodium hydroxide is chosen.
The concentration of sodium hydroxide in the papermaking black liquor can reach as high as 20%. Neutralizing with acid would consume a large amount of valuable acid solution, and the resulting waste liquid contains high concentrations of salts, causing significant environmental pressure.
With the increasing demand of enterprises for the recycling of waste acid and waste alkali, and the advancement of acid and alkali recovery technologies, the use of membrane methods for treating sodium hydroxide solutions in papermaking black liquor has become the top choice for enterprises. Some studies have adopted the filtration method using small-pore ceramic membranes to recover lignin from the black liquor, achieving a recovery rate of over 70%.
After process development and research, Wuhan Bona found that the small-pore ceramic membrane was ineffective in retaining low-molecular-weight hemicellulose and pigments in the black liquor from papermaking, and thus failed to achieve the intended application purpose. However, the alkali-resistant organic membrane performed better in retaining hemicellulose.
Through the alkali-resistant organic membrane, 20% concentration of alkaline solution can be permeated, and semi-cellulose and macromolecular pigments can be retained. Among them, the retention rate of semi-cellulose is over 90%. Considering the filtration flux, the alkali-resistant membrane is selected, which can achieve a semi-cellulose retention rate of over 70%. The alkaline solution recovered through the alkali-resistant membrane can be directly used in the production of paper pulp. The semi-cellulose after being concentrated by the alkali-resistant membrane can be prepared into low-molecular semi-cellulose through the recovery process.
Organic membrane membrane filtration has the characteristics of high filtration accuracy, linear scalability, and high automation level. The membrane filtration method adopts the cross-flow filtration mode. The liquid material flows tangentially under the action of the pump, effectively avoiding the formation of a filter cake layer of hemicellulose oligosaccharides on the membrane surface and blocking the membrane pores, thereby maintaining a high filtration efficiency and reducing the frequency of chemical cleaning.
Applying alkali-resistant membranes to recover the black liquor from papermaking enables the recycling of the alkali solution, achieving the goals of waste utilization, cost reduction and efficiency improvement, as well as reducing environmental pressure. This ensures the quality of downstream products and enhances the technical level of the industry.
