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Pulping
In a chemical pulping process, heat and chemicals are added to wood chips in a pressure cooker called the digester. In the kraft process, an aqueous solution of sodium hydroxide and sodium sulfide, known as white liquor, selectively dissolve the lignin and make it soluble in the cooking liquid. After 2 to 4 hours, the mixture of pulp, spent pulping chemicals and wood waste is discharged from the digester. The pulp is washed to separate it from the black liquor - the pulping chemicals and wood waste. Kraft pulping is a low yield process - only 45% of the wood used becomes pulp. The pulp, called brownstock at this point in the process, is ready to be bleached. Softwood pulp from a conventional cooking process contains about 4.5% lignin. This lignin will be removed and the pulp will be brightened during the bleaching process.
Efficient pulp washing is very important because it ensures the maximum recovery of the pulping chemicals and it minimizes the amount of organic waste carried over with the pulp into the bleaching process. Poorly washed pulps require higher bleaching chemical doses, thus increasing the cost and the amount of organic waste discharged in the bleach plant effluent.
The chemical recovery process has four steps:
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The water from the washer goes to the evaporator, where the black liquor is concentrated to 65% to 75% solids prior to its entry to the recovery boiler.
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In the recovery boiler, a specially-designed furnace, the used pulping chemicals are separated from the wood waste. The pulping chemicals form a lava-like smelt at the base of the recovery boiler; the wood waste is burned at the top of the recovery boiler. This heat is used to generate high pressure steam, that can be used to meet the mill 's steam and electricity requirements.
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The smelt is poured into a large tank of water to form green liquor, a mixture of sodium sulfide and sodium carbonate.
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In the next step, lime (calcium oxide) is added to the green liquor to convert the sodium carbonate into sodium hydroxide, and thus remake the white liquor.
The resulting calcium carbonate is converted back to lime in a lime kiln.
Improved Pulping: Oxygen and Extended Delignification
Conventional pulping processes remove about 95% of the lignin from the pulp. In the past, the remaining lignin was removed during the bleaching process. Oxygen has been used to remove lignin from pulp on a mill scale since the late 1970s. Scandinavian mills first installed oxygen delignification in response to concerns about the amount of organic waste in the effluent. Union Camp installed the first oxygen delignification system in the US in 1980 at Franklin, VA.
The pulp is washed after it leaves the tower where it has reacted with oxygen. The waste water or filtrate can either be used in the brownstock washers or it can be routed directly to the evaporator where it will be combined with the black liquor. The degraded lignin products will be burned for energy in the recovery boiler, so less organic waste will be discharged in the waste water from the bleach plant. Today, a well run oxygen system can remove 55% of the lignin from the unbleached pulp.
Digesters can be modified to remove more lignin from the pulp without damaging the cellulose by using the same amount of white liquor, but adding it at several points during the cooking process. This additional wood waste is also sent to the recovery system.
Extended delignification and oxygen delignification are proven technologies that can remove as much as 70% of the lignin before the bleaching process removes the remaining lignin and coloured substances. To produce high quality totally chlorine-free pulp (TCF), mills will have to install oxygen delignification, extended delignification or both in order to minimize the use of chemicals in the bleach plant. More organic waste is burned in the recovery boiler and smaller quantities of bleaching chemicals are required. These technologies make environmental and economic sense.