Chemistry of inorganic compounds – Carbon or compound thereof – Oxygen containing
Patent
1997-09-12
1999-11-30
Griffin, Steven P.
Chemistry of inorganic compounds
Carbon or compound thereof
Oxygen containing
423165, C01F 524
Patent
active
059937726
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a process for the coproduction of calcium carbonate and sodium hydroxide.
TECHNOLOGY REVIEW
Calcium carbonate and sodium hydroxide are used in large amounts in the paper industry, frequently in the form of aqueous suspensions of calcium carbonate and of caustic lyes. The manufacture of these products directly in these forms in satellite plants on the very site of the paper mills appears advantageous. It has been known for a long time that the causticizing of an aqueous sodium carbonate solution by means of the milk of lime generates calcium carbonate and sodium hydroxide. This process, conventionally known as causticizing, results in the formation of 2 mol of sodium hydroxide per mole of calcium carbonate produced, a ratio fixed by the stoichiometry of the chemical reaction involved (Na.sub.2 CO.sub.3 +Ca(OH).sub.2 .fwdarw.2 NaOH+CaCO.sub.3). Causticizing thus does not make it possible to obtain the desired products in variable proportions suited to the specific requirements of the operating site. In addition, the quality of the calcium carbonate produced by causticizing is not suitable for certain grades of paper, in particular because of an excessively broad particle size distribution and of an excessive residual calcium hydroxide content. The calcium carbonate intended for these grades of paper is most often obtained by carbonation of a milk of lime by means of a gas containing CO.sub.2, a process conventionally known as carbonation.
A process for the manufacture of calcium carbonate which can be used in particular as filler in the manufacture of paper has been provided in U.S. Pat. No. 1,188,505, according to which a calcium carbonate suspension obtained by causticizing is subjected to a carbonation, in order to convert the calcium hydroxide which might remain in the suspension into calcium carbonate. However, this process does not make it possible to obtain calcium carbonate of equivalent quality to that of the calcium carbonate prepared by carbonation nor to vary the ratio of the amounts of calcium carbonate and sodium hydroxide produced.
SUMMARY OF THE INVENTION
The present invention is aimed at overcoming the abovementioned disadvantages by providing a process for the coproduction of calcium carbonate and sodium hydroxide which produces a calcium carbonate/sodium hydroxide ratio greater than the stoichiometric ratio of the causticizing reaction and which, simultaneously, provides calcium carbonate of equivalent quality to that of the calcium carbonate obtained by carbonation.
DETAILED DESCRIPTION OF THE INVENTION
The invention consequently relates to a process for the coproduction of calcium carbonate and sodium hydroxide which comprises the following stages (a) to (e), in which aqueous mixture comprising sodium hydroxide and solid calcium carbonate; carbonate, on the other hand, are separated from the said mixture, sufficient amount of milk of lime, so as to obtain a calcium carbonate and calcium hydroxide slurry which exhibits a ratio by weight of calcium hydroxide to calcium carbonate of at least 0.1; ; and (d).
The invention advantageously combines a causticizing stage (a) and a carbonation stage (d). This is because it has been observed, unexpectedly, that certain properties of calcium carbonate obtained by causticizing, in particular the particle size distribution of the particles, were improved when this calcium carbonate, as a mixture with milk of lime, is subjected to a subsequent carbonation.
The milk of lime employed in the stage (a) is an aqueous calcium hydroxide suspension. The milk of lime can be prepared by any known means, for example by suspending slaked lime or quicklime in an amount of water such that a stable calcium hydroxide suspension is obtained. The calcium hydroxide concentration in the milk of lime is typically from 50 to 400 g/l, advantageously from 100 to 250 g/l. The temperature at which the milk of lime is prepared and stored before being treated in the stage (a) is generally from 10 to 90.degree. C., for exam
REFERENCES:
patent: 1188505 (1916-06-01), Statham
patent: 1862176 (1932-06-01), Church et al.
patent: 2062255 (1936-11-01), Brooks et al.
patent: 2538802 (1951-01-01), Schur et al.
patent: 5364610 (1994-11-01), Merris, Jr.
Criado Claude
Feys Francis
Ninane Leon
Griffin Steven P.
Hendrickson Stuart L.
Solvay ( Societe Anonyme)
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