Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Including recovery of organic by-product
Patent
1999-01-12
2000-12-26
Nguyen, Dean T.
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Including recovery of organic by-product
162 29, 162 55, 162239, D21C 1100
Patent
active
061653160
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to a method and apparatus for intensifying an alkaline pulping process according to which at least soap-containing spent liquors from digestion and soap-containing washing filtrates from brown stock washing are recirculated in the digester house, the brown stock washing plant or between these. The invention relates in particular to treatment of at least the soap fraction separated from liquors and filtrates in order to separate fiber material therefrom.
In a chemical alkaline pulping process, lignocellulose-containing material, such as wood chips, is treated by digesting it in a solution the active chemicals of which are mainly sodium sulphide and sodium hydroxide, or sodium hydroxide alone. After digestion, spent liquor is separated from the pulp which is carried to further treatment such as washing, screening and bleaching. Chemicals are recovered from the spent liquor, i.e. black liquor, for production of fresh cooking solution.
During the digestion process, greases and resin acids of wood saponify and form sodium soaps, sulphate soap. Conventionally, soap is separated from black liquor prior to feeding the liquor to evaporation and combustion to form green liquor and further white liquor. As soap is lighter than liquor it rises to the surface and is readily decanted off. It is important to remove soap as fully as possible because it is a strong scummy substance and thus causes many problems in the process. However, it should be noted that part of the soap has been dissolved into the black liquor and thus it is not separated to the surface of the liquor.
In addition to the digestion, soap is transferred from the pulp to the liquor also during washing of the pulp. The solubility of soap is different from that of other dry solids and thus it may not be as easily washed as other dry solids. In filter washing, soap is removed from the pulp at a later stage than other dry solid. Foaming soap also hampers filter washing of pulp.
As known, chemical pulp may be produced by an alkaline cooking process both in batch digesters and in continuous digesters. Instead of always feeding the black liquor from the digester to recovery of chemicals, hot black liquor may be used to pretreat wood chips before the digestion stage itself in order to improve the process. In a continuous digestion, hot black liquor may be recirculated to pretreat wood chips in the impregnation stage as has been described for example in EP patent no. 527 294. Hot, pressurized black liquor may also be flashed to produce steam and to raise the dry solids content of the black liquor. In batch digestion, black liquor may be stored at different temperatures in liquor tanks, "liquor batteries", for use in a pretreatment of chips.
Recirculation of black liquor provides several advantages. For example, the heat economy of the process is improved and, when sulphur is present in the digestion, black liquor augments the volume of sulphur compounds in the cellulose pulp which in the impregnation stage have a very favourable influence of decreasing the Kappa number of the pulp. Recirculation of black liquor brings about problems, also, as the soap contained in the black liquor accumulates in the process if it is not efficiently removed from the solutions recirculated.
Efficient removal of soap in the processes described above has not been paid necessary attention to before. EP patent application no. 520 452 discloses a method having three tanks for black liquor. At the beginning of displacement with washing filtrate, which takes place after digestion, black liquor is introduced into a first hot liquor battery essentially at the cooking temperature and pressure and at the dry solids content reached during the digestion. Black liquor, the temperature of which corresponds to the boiling temperature of liquor in an atmospheric pressure, is fed into a second black liquor battery. The dry solids content of the black liquor in the second battery is lower than that of the black liquor in the first
REFERENCES:
patent: 2186295 (1940-01-01), Hasselstrom
patent: 3109839 (1963-11-01), Keith
patent: 4058433 (1997-11-01), Fuller
patent: 4495095 (1985-01-01), Lawson et al.
patent: 5164480 (1992-11-01), Huibers et al.
Iivonen Mauno
Koskinen Kari
Kurkio Jarl
Ramark Hannu
Turunen Esko
Andritz-Ahlstrom Oy
Nguyen Dean T.
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