Chemistry of inorganic compounds – Miscellaneous process
Patent
1987-10-13
1990-01-16
Heller, Gregory A.
Chemistry of inorganic compounds
Miscellaneous process
423165, 423183, 423430, 423432, 423DIG3, 423641, 162 11, 162 3011, C01F 100, C01D 306, C01B 100, D21C 900
Patent
active
048942170
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for causticizing a water solution containing alkali carbonate, usually green liquor, with calcium oxide and/or hydroxide. This invention relates in particular to a method for causticizing a water solution containing alkali carbonate, in which the alkali carbonate containing water solution is brought into contact with calcium oxide or/and hydroxide, upon which the alkali hydroxide solution produced in the causticizing is separated from the calcium carbonate precipitate. With the method according to the present invention a digestion liquor intended to be used in the preparation process of soda or sulphate pulp can be prepared from green liquor.
In the pulp preparation process, for instance according to the sulphate process, the regeneration of cooking chemicals an important partial process. By the present causticizing process, the sodium carbonate and the sodium sulphide melt are dissolved into weak white liquor, whereby a water solution of sodium carbonate and sodium sulphide and other sodium salts (sodium sulphate), which are more or less inert with regard to the cooking process, as well as to a certain extent other metal salts appearing as impurities, is obtained. When this so-called green liquor has been purified from solid impurities, it is causticized into white liquor. Nowadays the causticizing is carried out as follows.
Calcined lime (CaO) is added to the green liquor, whereby the calcium oxide first is extinguished by reaction with the water of the green liquor, thus forming drowned lime. The calcium ions, which are generated by the dissolving of the calcium hydroxide, then form together with the carbonate ions of the solution a calcium carbonate precipitate, whereby the hydroxide ion concentration grows and its carbonate ion concentration decreases.
The above process of extinction and causticizing can be described in principle with the following balance reaction: heterogeneous system in two senses, as the calcium has first to be dissolved from the solid calcium hydroxide and then to form a precipitate with the carbonate ions of the solution.
Typical of heterogeneous systems is that they pass very slowly towards the state of equilibrium because of the slow interphasial diffusion. These conditions are particularly conspiciuous in the causticizing process because of its double heterogeneity and above all because of the low solubility of calcium hydroxide. The diffusion rate being highly dependent of the temperature, the causticizing rate decreases further as the temperature sinks.
For the above reasons a disadvantage of the causticizing process used at present is that a relatively long retention time, and thus also relatively large causticizing vessels, are required. Still the causticizing degree required by a reaction balance is not achieved, as a relatively large portion of the carbonate solution to be causticized has a much shorter retention time than the average retention time when series connected mixing causticizing receptacles are being used.
The correct batching of the lime quantity is also difficult. If too little lime is batched, the causticizing degree is naturally low, and again, if too much is batched, the settling of the white liquor becomes difficult and the combustion capacity need and heat consumption of the caustic sludge increases.
In addition, the impurities of the above green liquor have to be efficiently removed from the green liquor, for the settling of the white liquor to succeed. For these reasons also, the settling of the green liquor and the white liquor require large clarifiers.
With the present causticizing process, the settling of the causticizing and the green liquor as well as the white liquor has to take place at a temperature that is as high as possible, since the diffusion rate is a decisive factor, and this again is highly dependent on the temperature. The high temperature and the high alkali concentration lead to accentuated material requirements e.g. with regard to the filters and wires in the present process.
The purpose
REFERENCES:
patent: 1754208 (1930-04-01), Bradley et al.
patent: 2211908 (1940-04-01), O'Connor
patent: 2287886 (1942-06-01), Beekhuis, Jr. et al.
patent: 2552183 (1951-05-01), Knight
patent: 3268388 (1966-08-01), Baumann
patent: 4668342 (1987-05-01), Blackwell
Ekono Oy
Heller Gregory A.
LandOfFree
Process for causticizing of an aqueous solution containing alkal does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for causticizing of an aqueous solution containing alkal, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for causticizing of an aqueous solution containing alkal will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-1333826