Major stages of pulp production
Wood chip production
Approximately 4—5 m3 of wood are required to manufacture 1 ton of pulp for different purposes. Therefore 2—2,5 million m3 of wood will be used, for instance, to manufacture 500,000 tons of pulp. Such a significant quantity of wood is supplied as logs, 4—6 m long. Then logs are cut into 2-3-meter sections and are fed to debarking drums where bark is separated during drum rotation owing to friction of logs against each other and drum walls. After debarking logs are supplied to the chipping machine where they are cut into pulp chip. This chip is screened to have sawdust, wood dust, and large particles separated.
Pulp cooking
Screened pulp chip is supplied to digesters, for instance continuous cycle digesters, for cooking. The so-called «white liquor», which is a mixture of hydroxide and sodium sulfide, is fed to the digester. When the mixture is heated, wood lignin is diluted and unbleached pulp is produced. Chip is steamed, mixed with white liquor, and supplied to the upper part of the digester. When chip moves downwards, unbleached pulp is cooked and subjected to diffusion washing. Pulp is supplied for screening from the lower part of the digester, since it contains various pollutants such as knots and undercooked fiber. Separated liquid is called «black liquor» containing diluted wood organic substances and mineral substances presented by different sodium and sulfur salts is fed to the recovery system. Produced pulp mass has a brownish tint and is supplied for bleaching to achieve required brightness.
Pulp bleaching:
Screened and washed unbleached pulp is supplied to the storage silos and from there on to the bleaching shop. Pulp mass is bleached gradually using different bleaching agents until the required level of pulp brightness is achieved.
Pulp drying
Produced bleached pulp is cleaned at screeners and supplied for drying to the drier. Pulp is de-watered in the drier until a specific dryness value is reached and is then squeezed at presses and steam dried. Then a pulp web is cut into sheets, baled, pressed, and wrapped into large sheets of similar pulp, and afterwards wired. Packaged bales are delivered to the warehouse and then to consumers.
Recovery of cooking chemicals
Incineration of black liquor requires removal of excess water. Therefore it is fed to evaporators where excess water evaporates. Evaporated black liquor is incinerated in recovery boilers where its organic part gets burnt and its mineral part is reduced to sodium carbonate and sodium sulfide the solution of which is called «green liquor». Then this liquor is supplied to the caustisation shop where caustic lime (calcium hydroxide) is added, converting it into white liquor. Calcium hydroxide is produced in extinguishers by dilution of caustic lime (calcium oxide) in water. Spent lime is recovered after caustisation through burning in rotating furnaces. Lime losses are compensated by lime (calcium carbonate) supply to the burning furnace.