RAW MATERIALS FOR MAKING KRAFT PAPER:
Softwood and hardwood can be used for making kraft paper. Kraft paper can also use non-wood agri-residues and waste paper in the manufacturing process.
Pulp cooking chemicals are also called “white liquor.” This mixture has active ingredients, like NaOH and Na2S, that are used for cooking the wood chips in the digester for pulp production. While there isn’t a hard and fast rule on the ratio of chemicals, in general, 25 to 35% sulfidity is maintained. Any lower, especially below 15%, affects the reaction rate and pulp quality of the resulting products.
KRAFT PULPING PROCESS:-
The kraft process, sometimes called “Kraft Pulping” or the “Sulfate Process,” is the process by which wood is turned into wood pulp, the main component of the paper. Wood chips are treated with a hot mixture of water, sodium hydroxide (NaOH), and sodium sulfide (Na2S) in large pressure vessels called digesters. These large devices can hold 10-20 tons of pulp and come in two processing types: batch and continuous. This chemical mixture breaks down the bonds that link lignin, hemicellulose, and cellulose. One of the major benefits of kraft pulping is that roughly 90% of the pulping chemicals can be recovered and reused in an economical manner. This makes the process more sustainable and affordable at the same time.
In general, “kraft pulp” and “sulfate pulp” are synonymous but many countries around the world deem the two terms different based on the pulp manufacturing process. In these countries, the paper is also sold using different names to mark the subtle differences in production, and thus the end products.