Chemistry: physical processes – Physical processes – Crystallization
Reexamination Certificate
1999-01-28
2001-09-04
Hendrickson, Stuart L. (Department: 1754)
Chemistry: physical processes
Physical processes
Crystallization
C423S203000, C423S421000
Reexamination Certificate
active
06284005
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the production of sodium carbonate monohydrate crystals from anhydrous sodium carbonate containing impurities.
BACKGROUND OF THE INVENTION
One common method of purifying a compound is to crystallize the compound in a solution. Methods of crystallization typically involve controlling macroscopic external variables such as evaporating solvent to create supersaturation or adjusting the temperature of the solvent to affect solubility. These crystallization methods are generally directed to achieving maximum solids recovery and/or purification without any regard to the size or shape of the crystals.
Therefore, there is a need for a crystallization process that can effectively control or influence the ratio of crystal growth to formation of new crystals at low energy costs.
SUMMARY OF THE INVENTION
The present invention is based on the discovery that sodium carbonate has an unexpectedly high stable supersaturation capacity under appropriate conditions that can be rapidly relieved by the introduction of sodium carbonate monohydrate crystal surfaces to produce relatively large crystals of sodium carbonate monohydrate at high rates of crystal growth. The resulting crystals can be readily separated from insoluble impurities on a size separation basis.
More particularly, the process of the present invention is for producing sodium carbonate monohydrate from a feedstream which includes anhydrous sodium carbonate and insoluble impurities. The process includes adding the feedstream to a saturated sodium carbonate brine solution under conditions to create supersaturation of at least about 5 g/l. The process further includes processing within parameters that preferentially relieve the supersaturation by rapid growth of existing sodium carbonate monohydrate crystals rather than by nucleation. In this manner, the particle size distribution of crystals is controlled to achieve a desired distribution of crystal size product. The sodium carbonate monohydrate crystals produced by the process are recovered from the saturated brine solution.
The process can include the use of a high feed rate of at least about 100 grams of feedstream per minute for each liter of solution in the crystallizer. The process can also include relieving the supersaturation preferentially by rapid growth of existing sodium carbonate monohydrate crystals over nucleation by adding sodium carbonate monohydrate seed crystals to the saturated sodium carbonate brine solution. Such seed crystals can be produced by removing sodium carbonate monohydrate crystals from the brine solution and sizing the removed crystals to produce a seed crystal size fraction for reintroduction to the brine solution. In a preferred embodiment, the particle size of the feedstream is less than about 150 mesh and the particle size of the seed crystals is from about 100 mesh to about 150 mesh.
Relief of supersaturation preferentially by rapid growth of existing sodium carbonate monohydrate crystals over nucleation can alternatively be achieved by a variety of methods. Such methods can include maintaining a solids content of at least about 40% in the crystallizer, agitating the brine solution at an agitation index of at least about 6, periodically lowering the temperature of the brine solution by at least about 5° C., or pausing feedstream addition at least about 60% of the time of crystallization.
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Baughman David R.
Denham, Jr. Dale Lee
Hazen Wayne C.
Pruszko Rudolph
Tacoma Ralph B.
Environmental Projects, Inc.
Hendrickson Stuart L.
Sheridan & Ross P.C.
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