To enhance pulp yield and reduce load in the chemical recovery process
Increased yield has always been one of the main targets in pulp production. While there are many ways to increase yield, production has often been bottlenecked by the capacity available in the recovery process, or it has been proven to be too costly due to the required capital investment. This has now caused many mills to look at an old and proven, yet relatively rarely used solution.
While the polysulfide cooking process has been around for many years, and is proven in select mills around the world, it has not been used that often. The use of polysulfide cooking liquor improves pulp qualities that offer real benefits to papermakers – reduction in refining energy and enhancements in fiber bonding. Equally important, polysulfide cooking increases fiber yield for the pulp producer, especially for softwood pulps.
In polysulfide cooking, the polysulfide liquor is added to the impregnation stage at the beginning of the cooking process. The liquor oxidizes the end groups of the hemicelluloses, and slows down the alkaline peeling reactions, resulting in higher hemicellulose yield.
The characteristically orange-colored polysulfide liquor is made by modifying conventional white cooking liquor by oxidizing sodium sulfide in the liquor to polysulfide with the help of an active coal catalyst and air, without harmful substances. This process is carried out in a MOXY plant, which ANDRITZ has installed in several mills around the world.
For the papermaker, pulp produced with polysulfide liquor has increased tensile strength as well as stiffer pulp fibers, which increases the internal bond strength (Scott bond). Papermakers have documented reduced refining energy consumption due to the improved beatability of the pulp.
Polysulfide cooking also helps to reduce solids in the black liquor, which in turn reduces the load on the recovery boiler. With increases in yield, pulp quality, and overall pulp production, this makes investment in polysulfide cooking economically very beneficial.