15-11-2012, 02:08 PM
The Recovery System
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During the cook the cooking chemicals have been transforms. Recovery of these chemicals is absolutely essential for both environment and economic reasons. This is accomplished in the chemicals recovery department at the mill. The black liquor coming from the pulp wash also contain lignin from wood and other dissolleved organic matter. This, too is taken care of in the mill’s chemical recovery system and is used in the recovery boiler.
The sulphate process than is based on a cycle principle ensuring that the raw materials are used efficiently and that emissions and energy losses are minimized.
The recovery system at the sulphate pulp consist of:
• Evaporation in cause of which the black liquor is thickened into so called concentrated or thick waste liquor.
• The soda recovery boiler, where the organic matter in the thick waste liquor is converted into steam with the chemicals being recovered at the same time.
• White liquor preparation (with causticization and lime sludge reburning kiln), in which the chemicals are converted to that they once again become cooking chemicals.
The dry solid content of the concentrated liquor waste liquor is of importance for the operation of the soda recovery boiler. The higher the dry solids contents the higher the temperature in the furnace. This increase the degree of reduction and lowers the contents of environmentally harmful sulphur dioxide (SO₂) and hydrogen sulphide (H₂S) in the outgoing flue gas. A high dry solids contents also increase steam production. Development in evaporation technique in recent years leading towards high dry solids contents, will therefore be beneficial in several ways.
The soda recovery boiler, in principle, is a traditional steam boiler with tubes in the walls, bottom and ceiling of the furnace and with superheaters and convection tubes after the furnace. Feed water is supplied which boils off the heat from combustion. The steam is superheated at about 450˚C. The steam pressure is normally is 6MPa.
The melt and the flue gases are corrosive. A water leak can cause of explosion if the water comes into contact with the melt. Because of this tubes in the lower boiler is usually have a protective coat of stainless steel (compound tubes).
To prevent dust accompanying the flue gases passing out through the chimney and dirtying the surrounding. It is trapped in electrostatic precipitators. The dust is returned to the soda recovery boiler via the mixing tank. Sometimes there is also a flue gas scrubber after the filters. Here, any remaining dust is washed away, together with sulphur dioxide and white liquor preparation. Green liquor from the soda recovery boiler continues to the white liquor preparation facility which is the final process stage in the recovery system.
White liquor preperatio: Green liquor from the melt dissolvers is first cleaned sludge contaminated. In the arrangement sketch. This is done in a green liquor clarifier which separates the sludge by sedimentation. Pressure filter have also come into use in recent years. One recently develop apparatus is a disc filter with rotating felt-clad filter discs. Any green liquor remaining in the separated sludge is washed out on a sludge filter. The filtrate (weak Liquor ) is pumped to the melt dissolver in the soda recovery boiler.