Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With regeneration – reclamation – reuse – recycling or...
Patent
1995-03-23
1997-03-04
Czaja, Donald E.
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With regeneration, reclamation, reuse, recycling or...
162 301, 159 473, D21C 1104
Patent
active
056075480
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION
When producing pulp in accordance with the sulphate pulp method, the wood is digested in the form of chippings in an alkaline solution chiefly consisting of sodium hydroxide and sodium hydrosulphide. The relative proportions of hydroxide ions [OH.sup.- ] and sulphide ions [HS.sup.- ] vary from plant to plant but normally within the interval 25-40% sulphide [2HS.sup.- ], calculated on the basis of the sum of the contents of sulphide and hydroxide ions.
An increased content of sulphide ions in relation to hydroxide ions in the cooking liquid increases the pulp yield and normally improves the important properties of the final product.
At the beginning of the cooking process, the so-called impregnation phase, an increased proportion of sulphide ions provides the possibility of achieving a more efficacious dissolution of lignin during the cooking process itself. A lower content of hydroxide ions in this part of the cooking process decreases the breakdown of cellulose, thereby increasing the yield and improving the quality of the final product.
Recently, the cooking process has been modified in order to increase the yield of the pulp and to improve its properties. This has been done by dividing up the alkali charge which is required by adding a part of the cooking liquid (the white liquor) in the conventional manner together with the wood and then adding the remaining quantity later in the cooking process. However, the ratio between the sulphide and hydroxide ions remains the same in the white liquor which is added. In certain cases, an increased recirculation of spent liquor (black liquor) to the beginning of the cooking process has been effected with a view to obtaining a higher content of suliphide ions in this stage of the cooking.
The present invention provides the possibility of apportioning sulphide and hydroxide ions within wide limits by dividing up the available alkali after the liquor combustion into two or more constituent streams.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 schematically illustrates the system utilized in the practice of the present invention.
DETAILED DESCRIPTION OF THE DRAWING
The major portion of the cooking chemicals are present, together with organic material released from the wood, in the spent liquor (black liquor) which is conveyed, after evaporation down to combustible dry manner, to the liquor combustion process (15), which is normally a conventional recovery boiler (1) but which can also be a liquor gasification plant. The greater part of the cooking chemicals are recovered in the combustion process and usually leave the process in smelted form (16). The recovered chemicals are present in the main as sodium carbonate (Na.sub.2 CO.sub.3) and sodium sulphide (Na.sub.2 S) and are dissolved in a weakly alkaline washing liquid (17) (weak liquor) which was obtained by washing calcium carbonate (CaCO.sub.3), usually termed lime sludge, which separated off in the causticization process. After dissolution in the soda dissolver (2), the strongly alkaline liquid is termed green liquor.
On dissolution, the sodium sulphide reacts in accordance with the formula:
If metals other than sodium are present in the liquor combustion process and with the aim of producing a so-called autocausticization, this also provides an additional contribution of hydroxide ions in the green liquor.
The weak liquor in which the alkali smelt is dissolved contains varying quantities of hydroxide ions (OH.sup.-) depending on the systems for separating off and washing the green liquor sludge (18) and the lime sludge (19).
The content of hydroxide ions in the green liquor after dissolving the alkali smelt is normally within the range 0.8-1.2 kmol, with a cation content of 4.0-4.5 kmol per m.sup.3. The solid particles in the green liquor, consisting of elements which are foreign to the process (EFP), are normally separated off either by means of sedimentation (6) or filtration. The purified green liquor (20) then proceeds to the lime-slaker (8) where quicklime from the l
REFERENCES:
patent: 1906886 (1933-05-01), Richter
patent: 3617434 (1971-11-01), Nakafuri et al.
patent: 4093508 (1978-06-01), Henricson
patent: 4941945 (1990-07-01), Pettersson
Czaja Donald E.
Kvaerner Pulping Technologies
Nguyen Dean T.
LandOfFree
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