Invention of paper played a critical role in the development of human race. From enabling smoother communication, archiving events and sharing knowledge to acting as environment friendly packaging option, paper has transformed the world.
Production of paper has also changed over the years, from being hand made to use of industrial machinery. However, the essence of the process lies in making a dilute suspension of cellulose fibers and draining it through a screen to make an interwoven mat.
a. Wood is converted into chips of size, not more than 30mm.
b. Chips are treated with white liquor in a digester with temperatures of 140 Degree Celsius to form the pulp. It is also known as cooking.
c. Washing follows the cooking stage to remove caustic (NaOH) and lignin. All the washed chemicals(black liquor) are sent back to the chemical recovery plant.
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a. Impurities like sand, silica, and sieves are removed from the washed pulp in this stage.
b. Post-screening stage brown pulp is bleached using a mix of Sodium hypochlorite (fondly called hypo) and chlorine. Oxygen and peroxides can be used for chlorine-free bleaching.
To achieve the desired characteristics of the papermaking furnish, the loading, beating, and refining of the pulp fiber is needed.
a. Pulp mixed with water, fillers, coloring agents is received in a chest. Here the pulp is softened by beating and refining through rotating blades.
b. Different additives are mixed with refined pulp to bring it to desired properties for papermaking.
a. Headbox: This is a stock distribution zone, where prepared stock (slurry) is pushed onto the wire to form a fragile sheet.
b. Wire: This is a mesh conveyor belt used to handle feed from the headbox. At the wire, primary sheet formation occurs.
c. Press: Water is squeezed out from the paper sheet. The purpose of the Press is to provide maximum dryness in a paper sheet for input to dryers.
d. Dryers: Hot paper drying cylinders (The Largest part of a paper machine) are used to evaporate the remaining water in the paper sheet.
e. Size Press: Apply sizing solutions to the surface of the paper sheets. These could be starch solutions or coating colors.
f. Calendar: For the perfect surface finish of a paper sheet. The paper passes through different rollers with different surfaces, from tough steel to soft rubber (According to the paper type), with each applying pressure and temperature.
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Chemical recovery does not contribute directly to paper production but is an integral part of the papermaking process. The pulping process generates a high amount of weak black liquor. Black liquor is produced from wood and is one of the most prominent renewable bio-fuel. Chemical recovery is necessary because it minimizes the environmental impacts of waste, recovers NaOH andNa2S, and generates power for the paper mill. Chemical recovery involves the following steps:
a. Evaporator: Black Liquor from the washer is concentrated to the point that it can be burned in a recovery boiler.
b. Recovery Boiler: Concentrated black liquor is burnt in the absence of oxygen to form Na2S.Sodium and sulfur are recovered as molten smelt, which is dissolved further to create green liquor.
c. Causticizing: Green liquor reacts with lime to produce NaOH, whereas Na2S remains unchanged in this stage. The green liquor is sent back to the digester for pulping.
d. Lime Mud Recovery: The precipitate CaCO3 is heated in a lime kiln to release CaO to reuse later.
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