Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With regeneration – reclamation – reuse – recycling or...
Patent
1995-06-05
1997-05-13
Chin, Peter
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With regeneration, reclamation, reuse, recycling or...
162 3011, 423DIG3, D21C 1102, D21C 1104
Patent
active
056288751
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method of producing green liquor having good clarifying and/or filtering properties in chemical recovery in sulfate and sulfite pulp mills. The prepared (clarified and/or filtered) green liquor is used in both sulfate and sulfite pulp mills to produce fresh cooking liquor with the aid of which ligno-cellulosic material, wood chips for instance, is digested to cellulose pulp. According to the sulfate method, green liquor is converted to white liquor (fresh cooking liquor) by means of a causticizing process. The predominant chemicals in white liquor are sodium hydroxide and sodium sulfide in that order. Since sodium is included in both of these chemicals, sodium is designated as the base in the cooking liquor. In the case of the sulfite method, fresh cooking liquor is produced from green liquor in another way. The predominant chemicals in a sulfite cooking liquor which has sodium as the base are NaHSO.sub.3 and SO.sub.2. One speaks of free and bound SO.sub.2 in this connection.
BACKGROUND ART
When manufacturing sulfate pulp for instance, lignocellulosic material, e.g. wood, finely divided to chips, and cooking liquor are charged to a digester. The cooking liquor may consist entirely of white liquor, i.e. fresh cooking liquor, or may consist of white liquor that has been mixed with a given quantity of cooking liquor that has already been used, i.e. waste cooking liquor. The chemicals present in the cooking liquor are caused to react with the wood chips at elevated temperature and elevated pressure. Subsequent to the cook, the resultant cellulose pulp is separated from the used cooking liquor, i.e. the waste cooking liquor, in diverse washing and/or pressing stages, these material flows being treated in the manner described below.
The cellulose pulp is normally screened and then subjected to at least one bleaching process as a rule. The pulp is normally bleached in several stages, often with intermediate alkali stages (extraction treatments). The pulp is normally washed after each bleaching stage, resulting in bleaching waste liquors. Waste liquor that derive from treatment stages in which the bleaching agents used contain no chlorine, such as oxygen, peroxide and ozone, are normally passed in counterflow to the pulp. treatment chain and finally mixed with the cooking waste liquor. The same applies to waste liquors that derive from alkali stages. When bleaching is effected with chlorine-containing bleaching agents, such as chlorine, hypochlorite and chlorine dioxide for instance, it is not normally possible to recover these bleaching waste liquors, but these liquors must be discharged to the recipient.
As above mentioned, the waste cooking liquor, also referred to as black liquor, may be mixed with recycled bleaching waste liquor. Irrespective of whether the black liquor consists exclusively of cooking waste liquor or of a mixture thereof, the black liquor is treated (for instance evaporation) so as to increase its solids content to 60-70% for instance. This liquor is then referred to as concentrated waste liquor. The concentrated waste liquor is burned in a recovery boiler in which the organic constituents of the concentrated waste liquor are converted essentially to carbon dioxide, water and energy, while the inorganic constituents of the concentrated waste liquor form a smelt in the bottom of the recovery boiler. The smelt is conducted along a drain or spout down into a soda dissolver, where the smelt is dissolved in weak liquor and/or water. The resultant solution is green liquor. The smelt is mainly comprised of sodium carbonate and sodium sulfide, which are both readily dissolved in water. The smelt also includes other elements, which originate mainly from the lignocellulosic material used as starting material in the digestion (cooking) process. Calcium and magnesium predominate among these other elements although iron, manganese, aluminium and silicon are also present in readily measured quantities. A number of other elements are also present in tra
REFERENCES:
patent: 3366535 (1968-01-01), Cann
patent: 4941945 (1990-07-01), Pettersson
patent: 5509998 (1996-04-01), Strid et al.
Andersson Leif I.
Liden Jan G.
Chin Peter
Mo och Domsjo Aktiebolag
Nguyen Dean T.
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