Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Alkali metal
Patent
1986-12-16
1988-04-19
Sneed, Helen M. S.
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Alkali metal
423179, 423208, 423210, 423641, 423DIG3, 423DIG6, 423DIG10, 162 301, 162 3011, 48197R, C01D 102
Patent
active
047388358
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method of recovering alkali chemicals from a material containing inorganic dissolved compounds, which contain sodium and/or potassium by which method the material is gasified in a reactor by an external heat source at a temperature of preferably over 1000.degree. C. thus producing gas in which the sodium and the potential potassium are substantially in a gaseous state, which gas is then cooled.
BACKGROUND ART
When spent pulping liquor containing sodium or potassium salts is combusted in order to recover the heat content of the organic material dissolved during the cooking and the chemicals contained in the spent liquor, the alkali metals begin to vaporize to a gas phase when the temperature exceeds 1000.degree. C.; the smaller the air coefficient and the higher the temperature is, the intenser the vaporization becomes. By supplying the strongly substoichiometric combustion with additional heat energy by preheating with plasma, auxiliary fuel or other combustion gas, it is theoretically possible to reach conditions where e.g. the combustion of sulfate black liquor produces gas the temperature of which is over 1000.degree. C., even 1500.degree. C., and which contains CO, CO.sub.2, H.sub.2 O and H.sub.2 S and in which the alkali metals are substantially in the form of a monotomic gas. This is disclosed by the Swedish patent application No. 8302245.9.
The application also discloses a method of condensing the alkali salts to a melt or to an aqueous solution from the gas phase by cooling the gas produced by the combustion as described above whereby the cooking chemicals are directly recovered as NaOH and Na.sub.2 S.
In order to recover the cooking chemicals directly, it is necessary to cool the gas very quickly to prevent the CO.sub.2 contained in the gas from reacting with NaOH or Na.sub.2 O to form sodium carbonate. If sodium carbonate is formed, the advantage provided by the method, i.e. the direct production of cooking chemicals, e.g. NaOH, is lost.
In conventional cooling the NaOH/Na.sub.2 O/Na is condensed onto the cooling surface in the melt temperature range concerned and forms a layer the thickness of which increases until the surface temperature reaches the melting point. The condensed material is removed slowly from the cooling surfaces, providing the melt with a long contact time with the CO.sub.2 in the gas. The heat transfer is decreased. The melt layer is corroding and decreases the heat exchange capacity, it also shortens the endurance expectancy of the device. Finally, the gas is cooled slowly and the Na.sub.2 C.sub.3 content in the melt increases with the result that it is no longer an advantage to use the melt directly as a cooking chemical.
DISCLOSURE OF THE INVENTION
The present invention provides a remarkable improvement to the above. The cooling of the vaporized alkali metal compounds is controlled and rapid whereby the formation of sodium carbonate is minimized. No corroding melt condensates are produced.
The method of the invention is characterized by the feature of cooling the gas from the gasification reactor by bringing it into contact with an adequate amount of cooled recirculated solid particles, which are separated from the gas, to decrease the temperature of the gas rapidly below the sublimation temperature of sodium or potassium compounds.
The present invention is particularly applicable to the recovery of alkali chemicals from the spent liquor of cellulose cooking.
The method of the invention provides very rapid and efficient cooling of the gas. When the gas is mixed with the small circulating bed particles which move fast, the heat is quickly divided evenly between the particles and the gas. Further, the combined area of the cooled particles mixed with the hot gas flow is very large thus providing a large heat exchange area which results in very rapid cooling of the gas below the sublimation temperature. In general the cooling speed is of the order of 500.degree.-1000.degree. C./s or greater.
When the cooling is carried
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A. Ahlstrom Corporation
McFarlane Anthony
Sneed Helen M. S.
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