Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Alkali metal
Patent
1999-04-19
2000-12-19
Bos, Steven
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Alkali metal
423208, 162 301, C22B 2610, D21C 1112, D21C 1114
Patent
active
06162408&
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to purifying the fly ash which is collected from recovery boilers and utilized and which principally consists of sodium sulphate containing contaminants such as chlorides, potassium salts and carbonates.
BACKGROUND AND SUMMARY OF THE INVENTION
When spent liquors from the pulp industry, in particular so-called black liquor, which is present in concentrated form, are combusted in a recovery boiler, the water evaporates while the organic material in the liquor, mainly lignin residues, is combusted and inorganic chemicals, in smelt form, run down along the walls and collect in the bottom of the boiler. These chemicals are recovered and used again in the pulp cooking.
However, during the combustion, it is impossible to prevent dust from accompanying the flue gases and becoming stuck to the tubes in the heat exchanger which the flue gases have to flow through. This dust is brushed off and collected as ash. It consists princi pally of sodium sulphate to an extent of about 90%, chlorides and carbonates to an extent of 7-8% and potassium to an extent of 3-4%. While the sodium sulphate can be reused, the potassium, which is principally derived from the wood, and the chlorides and carbonates, have to be removed.
Several processes for purifying this ash have been disclosed. Most of them are based on leaching the ash in a container, in association with which chlorides, potassium salts and the like go in solution. This leaching can take place both at an elevated temperature and at a lower temperature, for example less than 20.degree. C. Processes of this nature are described, for example, in WO96/12847, the published Japanese Patent Application No. 311462/1994 and the U.S. Pat. No. 5,352,332.
If a leaching is to provide the desired result, such that the contaminants are leached out from the substance which it is desired to retain in a solid state, the contaminants have to be soluble in the leaching medium, in this case water, while the solid substance which it is desired to retain has to be of lower solubility. However, sodium sulphate is also soluble in water and there will therefore be losses during the leaching. However, the solubility of sodium sulphate decreases as the temperature decreases, in contrast to the solubility of potassium salts, which is virtually independent of the temperature. The chlorides are also readily soluble at both high and low temperatures. The problem with the above-disclosed processes has been that it has not been possible to achieve the desired selectivity between potassium and sodium salts, i.e. such that the sodium sulphate can be recovered from the leaching process and supplied to black liquor for combustion while the potassium salts remain in solution.
Another problem with the known leaching methods is that, if these methods are carried out in lukewarm or cold water, i.e. at less than approx. 32.degree. C., when the solubility of the sodium sulphate is low, the sodium sulphate will become surrounded by water molecules and form sodium sulphate heptahydrate or, more usually, sodium sulphate decahydrate. If the sodium sulphate is returned to the black liquor in this form, the concentrated black liquor will then be diluted with this water, something which naturally constitutes an undesirable problem.
There has therefore long been a great need to be able to utilize the ash from the recovery boiler by being able to return practically all the sodium sulphate, virtually free of potassium salts and chlorides, to the black liquor, without dilution water being supplied to the black liquor, and a process has been developed, in accordance with the invention, for purifying ash from a recovery boiler, which ash consists principally of sodium sulphate, by leaching or evaporation-crystallizing contaminants consisting mainly of chlorides and potassium salts in water, which process is characterized in that the leaching or the evaporation-crystallization takes place in an aqueous solution at a temperature of greater than approximately 32.degree. C., prefer
REFERENCES:
patent: 5352332 (1994-10-01), Maples et al.
patent: 5938892 (1999-08-01), Maples et al.
patent: 5980717 (1999-11-01), Pudas et al.
File WPI, Derwent accession No. 81-57548D, Nippon Chem Plant, "Sepn. of Glauber's salt and sodium chloride from soln.--by crystallisation of anhydrous glauber's salt, crystallising the deca-hydrate and crystallising salt"; & JP,A, 56073620 810618; & Patent Abstract of Japan, vol. 5, No. 140, abstract of JP,A,56-073620 (Tokuyama Soda Co Ltd), Jun. 18, 1981; & JP,A,56-073620, 810618, see figure, table, col. 4, line 19, col. 10, line 20--col. 11, line 1.
Bos Steven
Fasth Rolf
Kvaerner Pulping AB
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