Wood chips, sawdust, bark, textiles, fabrics, and other small pieces are the ingredients used as a basic raw material in the manufacture of pulp in the paper pulping machine. The bark and stems are burned to generate a fuel that powers the working of electric machines at the factories.
The wood chips are first sorted according to their size. The pieces that are either too big or too small are removed and used in the fuel-making. The sorted chips are washed and sent to the digester.
When delivered, the content of the wood chip is approximately 50 percent water and 25 percent cellulose fiber. This cellulose fiber is what the pulp is made of, which is the main ingredient in the manufacturing of paper.
The leftover liquid material is called lignin which works as a glue to hold the fibers together. We need to remove this lignin to obtain separates fibers for the papermaking process.
In the paper pulping machine, the Digesters come into play for this motive. This separation of fibers is done in a machine called Digester. The digester is a large pressure cooker like structure in which the wood chips are placed along with white liquor. White liquor is a wood cooking chemical. The lid of the vessel (digester) is then closed and steam is led into it. With the increase in the heat and pressure inside the vessel, the lignin begins to dissolve and the wood chips begin to breakdown. The white liquor works as a soap in the separation of the fibers from the glue.
The cooking liquid that now contains the broken down lignin is called black liquor. This black liquor is ejected from the digestor in a diluted form with a lot of water. This water is evaporated out concentrating the remaining lignin and cooking chemicals.
This mixture is now burned in a recovery boiler which helps in the generation of steam and electricity for the mill. After burning, all the remains of the black liquor are the spent chemicals and are termed as green liquor. Finally, some mills prefer to add lime for the restoration and recycling of original chemical composition.
This is a recycling process of turning white liquor to black liquor to green liquor and is environmentally friendly and cost-effective.
The remaining cellulose fiber in the digestor isn’t yet totally pure. We need to get rid of the remaining lignin from the mixture. For this purpose, the fiber is transferred to the bleaching station (deinking system) and undergoes the bleaching process. In this stage, the pulp is cleaned with bleach and water assuring all living lignin has been removed.
The dark wood fiber has now become whiter and is ready to be transformed into paper. From here, this cellulose fiber is sent to the mills of tissues, or paper boards, or crafts, etc. It can be immediately used or can be stored for future use. This is the basic working of the paper pulp making process using the paper pulping machines.
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