Compositions: coating or plastic – Coating or plastic compositions – Inorganic settable ingredient containing
Patent
1996-09-17
1998-01-27
Marcheschi, Michael
Compositions: coating or plastic
Coating or plastic compositions
Inorganic settable ingredient containing
106738, 423173, 423177, 423430, 423175, 432 14, 432 67, 432 68, 432 69, 432 36, C04B 202
Patent
active
057118027
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a method (hereinafter referred to as of the kind described) and a plant for heat treatment of lime sludge (CaCO.sub.3) formed by the causticizing process during the manufacture of paper pulp. When manufacturing paper pulp according to the sulphate process, green liquor (Na.sub.2 CO.sub.3) is causticized with burned lime (CaO), thereby generating a sodium hydroxide solution (NaOH) or white liquor which is utilized for boiling the wood pulp from which the paper pulp is manufactured and a product with a content of calcium carbonate (CaCO.sub.3) or lime sludge. Typically, the product is dried and burned in a kiln plant for manufacturing burned lime which, in turn, can be used for the causticizing process. This cycle is generally referred to as the lime cycle of the sulphate process.
A previously known and widely used method for lime sludge reburning consists in equipping the uppermost end of a rotary kiln with a number of suspended chains inside the kiln in order to promote the heat transfer rate and the disintegration of the supplied lime sludge in such a way that the drying of the latter within the kiln is promoted. The treated product in the form of burned lime is removed from the kiln after undergoing cooling in the material cooler fitted immediately after the kiln. The kiln is fired by oil, gas or bark powder, and the burned lime reaches a temperature of 1000.degree.-1200.degree. C. during the treatment in the kiln. The drying-out of the substantial water content in the lime sludge results in a relatively low exit temperature of 150.degree.-200.degree. C. in the flue gases, which means that the stack loss will be moderate. The operating efficiency of the kiln, i.e. the portion of the fired calorie quantity being utilized for drying and calcination of the treated product, is quite high, and the thermal economy of the kiln must generally be characterized as being satisfactory for the purpose.
Improved technology for the manufacture and dehydration of lime sludge has resulted in lower water percentage rates in the kiln feed, but the expected fuel saving has not been achieved, instead there has been a rise in the exit temperature of the exhaust gases and an increase in the dust loss from the kiln. The latter phenomenon is due to the chains whirling up dry raw material, thereby generating more dust, which, in turn, increases the load imposed on the dedusting equipment of the kiln.
The kiln reacts in the same way if forced beyond its rated production capacity: rising exhaust gas temperature and increased dust loss, resulting in increased fuel consumption per unit of burned lime produced. Hence, the chain system of the kiln imposes stringent limits for the possibilities of boosting kiln production, and another inherent disadvantage of the system is the need regularly to replace the chains due to wear, which, in addition to the capital outlay on new chains, necessitates shut down of kiln, and hence interruption of production, causing an undesirable increase in the total operating costs.
Another known method for lime sludge reburning consists in replacing the chain system of a rotary kiln with a drier crusher, e.g. of the hammer mill type, which, viewed in the flow direction of the supplied raw material, is mounted before the kiln, hence permitting a boost in the production capacity for a specific size of kiln. In such a plant the sticky, often lumpy, wet lime sludge is fed to the drier crusher together with hot exhaust gas from the kiln, being pulverized and dried in the drier crusher and carried by the drying gas, and suspended therein, as a powder to a separation cyclone, in which the powder is separated from the exhaust gas and fed to the rotary kiln as dry material. Such a drying plant, which is also known in other contexts, see e.g. EP 0 041 113, is, however, associated with other significant disadvantages. A new machine, the drier crusher, is incorporated, which must be designed for withstanding temperatures of up to 700.degree. C., necessitating
REFERENCES:
patent: 5354375 (1994-10-01), Cohen
patent: 5413635 (1995-05-01), Matweecha et al.
F. L. Smidth & Co. A/S
Marcheschi Michael
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