Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With regeneration – reclamation – reuse – recycling or...
Patent
1993-10-28
1996-04-23
Lacey, David L.
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With regeneration, reclamation, reuse, recycling or...
162 301, 162 3011, 423DIG3, D21C 1100
Patent
active
055099980
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a process and a device in a chemicals recovery plant in a sulphate pulp mill.
From SE-B-8700549-2 there is known a process for treating green liquor, more particularly in such a way that the green liquor's content of particularly inorganic impurities and soot obtains better sedimentation and filtration properties and, owing to that, can be separated from the green liquor and accordingly removed from the chemicals cycle in a simpler and more effective way than before. This is achieved by adding to the non-clarified green liquor, while stirring quick lime comprising from 0.5 to 10%, preferably from 1 to 3% of the amount of quick lime necessary for complete causticizing of the green liquor.
According to SE-B-8700549-2 the quick lime is added either to a soda dissolver, in which melt from a soda recovery unit is dissolved in weak liquor, and from this to a mixing tank or directly to the mixing tank. From the mixing tank the green liquor formed is passed to a filter, from which solid particles are removed for dumping whereas clarified green liquor is passed to a lame slaker. From the lime slaker the mixture of lime and green liquor is passed to a causticizing tank from which the mixture of liquor and lime sludge formed is passed to a filter. The filtrate formed therein, white liquor, is passed to the digester house whereas the material filtered off, the lime sludge, is passed to a lime sludge silo. From this the lime sludge is passed to a washing filter from which the weak liquor formed is added to the above-mentioned soda dissolver whereas the lime sludge is passed into a lime kiln where it is burnt to lime. The quick lime is passed to the lime slaker as well as, according to the above, to either the soda dissolver or the mixing tank.
SUMMARY OF THE INVENTION
The object of the present invention is to simplify and cheapen the known process and owing to that also make a simplified and considerable more cheap device possible. The invention is based upon the understanding that it is the slaked lime formed in the soda dissolver and in the mixing tank, respectively, that is active in the process in order to obtain the desired result. According to the present invention it is therefore suggested that the addition of lime is in the form of already slaked lime, that is the slaked lime that is already present in the process and that is formed in the lime slaker. Thus, from the flow of lime and green liquor to the causticizing tank there is taken a partial flow which is passed to the mixing tank or alternatively to the soda dissolver. This is done before the causticizing process has been completed, i.e. that uncausticized liquor (lime milk) is passed into the mixing tank.
By using in this way slaked lime that is already present in the plant, the expensive and long conveyors, which, in plants according to SE-B-87005492, are required to convey slaked lime to the soda dissolver and the mixing tank respectively, are avoided. Instead there is obtained a closed system for lime circulation from the lime slaker through a line, provided for this purpose, to the mixing tank, alternatively the soda dissolver, from the mixing tank to the green liquor filter and then back to the lime slaker.
On the apparatus side there is required for this only that there is provided, from the existing line between the lime dissolver and the causticizing tank, a branch line to the mixing tank and the soda dissolver, respectively, optionally together with a pump, a pump vessel and means for regulating the partial flow withdrawn.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is explained in more detail in the following with reference to the enclosed schematic drawing showing an example of a chemicals recovery plant in a sulphate pulp mill set up to carry out the process of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
In the drawing 1 designates a running shute which passes, from a soda recovery unit not shown, melt to a soda dissolver 2 in which there is a stirre
REFERENCES:
patent: 4941945 (1990-07-01), Pettersson
Oswaldson Rolf
Strid Kent
Gotaverken Energy Aktiebolag
Lacey David L.
Nguyen Dean T.
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