Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With regeneration – reclamation – reuse – recycling or...
Patent
1987-11-18
1989-12-26
Kratz, Peter
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With regeneration, reclamation, reuse, recycling or...
162 3011, 162 49, 162240, 162DIG10, 422108, 422111, 422185, 423175, 423183, 423DIG3, D21C 1104, C04B 204
Patent
active
048895920
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method for controlling processes in connection with burning and slaking of lime and causticizing, said processes comprising lime burning in a kiln, washing of lime sludge on a lime sludge filter and a slaking and causticizing process. These processes constitute the so-called lime cycle which is an essential part of the process for production of paper pulp according to the sulphate process. The invention also comprises a device for carrying out the above-mentioned control processes.
TECHNICAL FIELD--USE OF THE INVENTION
It is the object of the invention to maintain optimal operating conditions in production of lime having a desired quality and in using this lime. The following specification concerns the use of the invention in the sulphate pulp industry. Besides this use the invention can be utilized in burning of lime for other purposes where an intended lime quality according to the following specification is desired.
In production of sulphate pulp lime, i.e. CaO, is used for preparation of boiling liquid. The lime is handled in a cycle of processes, i.e. the lime cycle. Lime is batched together with green liquor in a slaker which is the first vessel in a causticizing plant. Green liquor is causticized to white liquor in the slaker and in the following reaction vessel, also called causticizing tank, meaning that carbonate in the solution is converted into hydroxide and that the lime forms CaCO.sub.3, so-called lime sludge. The lime sludge is separated from the white liquor, is dewatered and washed e.g. on a lime sludge filter and thereafter burnt to lime which is utilized again for causticizing.
Today lime sludge is almost exclusively dried and reburnt in rotary furnaces, so-called lime kilns. This is a process requiring much energy which is of decisive importance for the energy economy of the sulphate pulp mill. Fuel, often oil, is fired in such kilns. Normally the energy consumption is about 200 kg oil per metric ton lime corresponding to 40-45 kg per metric ton produced pulp.
Lime sludge (CaCO.sub.3) is disintegrated in the lime kiln to carbon dioxide (CO.sub.2) and lime (CaO) at temperatures above 900.degree. C. When the burnt lime is subjected to higher temperatures (1100.degree.-1400.degree. C.) its structure is changed--it sinters. If the lime sludge contains impurities, preferably sodium, sintering may occur at lower temperatures than 1100.degree. C. The impurity content of the lime sludge in this respect is decided by the efficiency of the lime sludge filter.
Sintering means that the porosity of the lime and its specific surface are reduced and the reactivity of the lime is impaired in slaking. Sintering also involves influence on the mechanical properties of the lime such as hardness and modulus of elasticity. This is not specific to burning of lime sludge but always occurs when burning materials containing CaCO.sub.3 and/or MgCO.sub.3.
The quality of the burnt lime is of a great importance for the pulp production. An even and good lime quality is a prerequisite for a satisfactorily working causticizing process. The result of that process decides the quality of the white liquor, i.e. that of the boiling liquid, which in turn will influence both quality and production costs of the pulp. The lime must not be burnt too hard nor too loose. Lime that has been burnt too hard has a bad reactivity in slaking, e.g. the slaking rate is low which will influence the preparation of white liquor negatively while lime that has been burnt too loose has a low CaO-content and, moreover, yields a lime sludge that is difficult to separate from the white liquor and to dewater e.g. on a lime sludge filter.
Thus, both the reactivity of the lime and its CaO-content will influence the result of the causticizing. The CaO-content in lime sludge will not vary inconsiderably under normal circumstances provided the lime is finally burnt, e.g. its CaCO.sub.3 -content is low. On the other hand, the reactivity of the lime can vary considerably. As mentioned above the reactivity of the lime is prim
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Andersson Birgitta S.
Gustafsson Jan-Erik V.
Herngren Torbjorn
Zethraeus Malin C.
Kratz Peter
STFI
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