Please remember I am an old man and have not been on the operating floor of a paper mill machine room in many years, but I will attempt to describe this as I experienced it. I would like to start the narrative in a Pulp Mill, -- the North Caroline Pulp and Paper Company at Plymouth, North Carolina because "Pulp" is where paper gets its start and that is where I was first introduced to the Pulp making process.
Plymouth is located in an area that is well adapted to the growth of Southern Yellow Pine forests. The trees were, at that time, cut into five-foot lengths and the logs were brought to the "Wood Yard" where they were stacked adjacent to conveyors that would eventually take them to the "Barking Drums". These were large cylindrical, slotted, tubes that rotated, and in so doing, tumbled the logs, knocking the bark from the logs.
The bark was taken by conveyer, to the "Boiler Room" where it became fuel for the bark burning "Boiler". This Boiler, by the way, was rated at 60,000 pounds per hour, which means that at full capacity, it could evaporate 60,000 pounds of water into steam at 650 pounds pressure.
The logs, after having had the bark removed, were conveyed to the "Chippers" where they were sliced, transversely, at a very steep angle. The "Chips" would then be conveyed to the "Chip Bin" which was above the "Digester" Building. The Digesters were upright cylinders that when filled with chips, through a hatch at the top, and Liquor was added the mixture was cooked with steam at 150 psi.
From this point on, "Liquor" becomes a very important part of the process. There was "Strong Black Liquor", "Black Liquor", "Weak Black Liquor", "Weak Weak Black Liquor", "White Liquor" and "Green Liquor", So please try to keep these different liquors in mind. The White Liquor was a strong caustic material that was compounded in the "Caustisizing" Building. I have no idea what the formula was, but I do know that a lot of lime and Green Liquor was used in its preparation.
After the chips had been cooked, the mixture was blown into a "Cyclone Separator" where the liquid and the pulp were separated. The liquid was then Strong Black Liquor and was sent to the "Evaporators". The pulp went to "Washers" where it was washed, first with Weak Black Liquor that produced Black Liquor to be used in future cooks. It was then washed with Weak Weak Black Liquor that produced Weak Black Liquor then with water that produced Weak Weak Black Liquor.
After the Evaporators, which operated at 35 psi, had removed most of the water the Liquor went to "Concentrators" that operated at 150 psi and more water was removed. It then was passed over a "Cascade Evaporator" that was located in the flue gas passage of the "Recovery Boiler" where some of the noncombustible elements of the flue gas were recovered and even more water was removed. The now highly concentrated Black Liquor was sprayed into the furnace of the Recovery Boiler.
The Recovery Boiler performs a very important function in the production of pulp for the paper industry. It not only produces an appreciable amount of steam essential to the process but it recovers the noncombustible elements that remain in the concentrated Liquor. The noncombustible elements, along with Soda Ash and other chemicals, are melted and become a molten mass that is collected, and dissolved with water, in the "Smelt Tank" that is located beneath the furnace. This particular boiler was rated at 100,000 lb.per hr. and was said to be the first conventional boiler ever to be used in the recovery process. Prior to this the Liquor was burned only for the purpose of recovering the chemicals that were in the liquor.
The liquid that is produced in the Smelt Tank is called "Green Liquor" and is pumped to the "Caustisizing" Building where it is treated with lime and other ingredients, and becomes "White Liquor" to be used in the cooking process.
If you have been in Sarajevo when the Serbs and the Croats were going at it, or have attended the final few minutes of the fire works show at the County Fair, you would have some idea what it was like to be in a Boiler Room when one of these Recovery Boilers were operating at their peek efficiency. It was one constant chain of explosions, in that Smelt Tank.
This is a description of the paper making process as remembered by an old man who spent a large part of his early working life in paper mills as a steam plant laborer, a turbine room oiler, a steam engine operator, a turbine operator, an Assistant Chief Engineer, a Chief Engineer, then an Inspector for an Insurance Company that insured many large Pulp and Paper Mills.
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