Chemistry of inorganic compounds – Halogen or compound thereof – Binary compound containing metal
Reexamination Certificate
2001-08-27
2004-02-17
Nguyen, Ngoc-Yen (Department: 1754)
Chemistry of inorganic compounds
Halogen or compound thereof
Binary compound containing metal
C423S499500, C423S499100, C423S518000
Reexamination Certificate
active
06692720
ABSTRACT:
The present invention relates to a process for the production of sodium chloride crystals.
A widely used technique for producing solid sodium chloride consists in crystallizing it by evaporation of a sodium chloride brine. The sodium chloride brine used in this technique can be, for example, seawater. In an alternative form, it can be obtained by dissolution of rock salt in water. The brines used in this technique for the manufacture of sodium chloride are usually contaminated by undesirable compounds which risk contaminating the sodium chloride. This is in particular the case with potassium chloride, sodium carbonate and sodium sulphate which, if they are not removed from the brine before evaporation of the latter, could easily be encountered in association with the sodium chloride crystals. The removal of these undesirable compounds from the brine moreover presents the problem of their discharge and of their storage (generally in public landfill sites) under conditions suited to preventing them from polluting the environment.
A process for the manufacture of sodium chloride crystals starting from a sodium chloride brine contaminated by potassium chloride and sulphate ions is known (U.S. Pat. No. 4,547,197). According to this known process, calcium hydroxide is added to the brine, to precipitate the sulphate ions in the form of calcium sulphate, and then sodium carbonate is added, to precipitate the calcium ions in the form of calcium carbonate. The aqueous solution collected after separation of the precipitates is subjected to evaporation at a temperature of approximately 120° C. to crystallize sodium chloride, which is collected, and the aqueous mother liquor from the crystallization is cooled to approximately 45° C. (preferably by subjecting it to a reduction in pressure) to crystallize simultaneously sodium chloride, potassium chloride and sodium sulphate. In this known process, the coprecipitation of the three salts (sodium chloride, potassium chloride and sodium sulphate) causes difficulties in the discharge of the latter. A solution suggested in the document U.S. Pat. No. 4,547,197 consists in dispersing the mixture of the three salts in cold water, so as to selectively dissolve the sodium chloride and the potassium chloride, while the sodium sulphate recrystallizes in the form of Glauber's salt.
The abovementioned known process exhibits a disadvantage of great complexity, imposed by the need to dispose of the mixture of the three salts (sodium chloride, potassium chloride and sodium sulphate) without harming the environment. This known process exhibits the additional disadvantage that it consumes a large amount of calcium compound (calcium hydroxide) in extracting the sulphate ions from the brine.
The invention is aimed at overcoming the disadvantages of the known process described above by providing a process for the manufacture of sodium chloride crystals from a brine contaminated by potassium chloride and sulphate ions which makes possible a substantial reduction in the consumption of calcium compound in extracting the sulphate ions from the brine and which, moreover, simplifies thee discharge and the storage of the residual solid materials from the purification.
The invention consequently relates to a process for the production of sodium chloride crystals from a sodium chloride brine contaminated by potassium chloride and sulphate ions, according to which, in a first stage, a calcium compound is added to the brine to precipitate calcium sulphate, which is isolated, and an aqueous solution is collected, in a second stage, the aqueous solution from the first stage is subjected to evaporation, to crystallize sodium chloride, and sodium chloride crystals and an aqueous mother liquor are collected separately, and, in a third stage, the aqueous mother liquor from the second stage is subjected to cooling in order to crystallize at least a portion of the potassium chloride; according to the invention, the amount of calcium compound in the first stage and the cooling in the third stage are adjusted so that the sulphate ions precipitate in the form of glauberite in the first stage and of glaserite in the third stage.
In the process according to the invention, the sodium chloride brine is, by definition, an aqueous sodium chloride solution. It is invariably an aqueous solution which is saturated or unsaturated with sodium chloride. Its content by weight of sodium chloride is advantageously greater than 5%, usually at least equal to 10%. Brines comprising at least 20% by weight of sodium chloride are especially recommended. Brines which are substantially saturated at ambient temperature are preferred.
The sodium chloride brine employed in the process according to the invention is contaminated by impurities. These impurities comprise potassium chloride and sulphate ions. The sulphate ions are, for example, present in the form of dissolved sodium sulphate. The impurities are normally present in brine in an amount of less than the sodium chloride content.
In the first stage of the process according to the invention, the role of the calcium compound is to react with the sulphate ions to form calcium sulphate, which crystallizes. The calcium compound must consequently be chosen from those which are capable of reacting with sulphate ions, in particular with alkali metal sulphates (in particular sodium sulphate), to form calcium sulphate. The calcium compound employed in the first stage is advantageously a water-soluble compound. Calcium chloride is preferred.
On conclusion of the first stage, the calcium sulphate crystals are isolated from the brine. The means employed for this purpose is not critical. It advantageously comprises a filtration or a sedimentation followed by a separation.
In the second stage of the process according to the invention, the aqueous solution collected after separation of the crystals from the first stage is subjected to evaporation in order to crystallize sodium chloride. The parameters of the evaporation (in particular temperature, pressure and the degree of evaporation) are chosen so as to avoid simultaneous crystallization of undesirable compounds, such as potassium chloride or sodium sulphate. The optimum values of these parameters will depend on the concentration of the brine, on its contents of potassium chloride and of sulphate ions and, if appropriate, on the other impurities present. They can be easily determined by routine work, from liquid-solid equilibrium diagrams, in particular the Na—K—Cl—SO
4
—H
2
O diagram, accessible from the information in the literature.
On conclusion of the second stage, the sodium chloride crystals are separated from the aqueous mother liquor. The said separation can be carried out by any appropriate means, for example by filtration, by centrifuging or by sedimentation followed by separation.
In the third stage of the process according to the invention, the aqueous mother liquor from the second stage is subjected to controlled cooling in order to render insoluble and to crystallize the potassium chloride.
In accordance with the invention, the calcium compound is employed in the first stage in an amount which is in deficiency with respect to that which is necessary to convert all the sulphate ions to calcium sulphate. More specifically, the amount of calcium compound employed in the first stage is adjusted according to the potassium content of the brine, so that a fraction of the sulphate ions of the brine precipitates in the form of glauberite (mixed calcium sulphate and sodium sulphate compound of general formula CaSO
4
.Na
2
SO
4
) in the first stage and so that the balance of the sulphate ions precipitates with all the potassium ions in the form of glaserite (mixed potassium sulphate and sodium sulphate compound of general formula Na
2
SO
4
.3K
2
SO
4
) in the third stage. The removal of the sulphate ions and potassium ions by successive crystallizations of glauberite and glaserite is based on a sulphate content in the brine which is greater than that necessary to precipitate all the potassium ions in the f
Gerard Pascal
Humblot Cedric
Ninane Leon
Larson & Taylor PLC
Nguyen Ngoc-Yen
Solvay ( Societe Anonyme)
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