Chemistry: physical processes – Physical processes – Crystallization
Reexamination Certificate
1999-10-12
2001-07-31
Langel, Wayne (Department: 1754)
Chemistry: physical processes
Physical processes
Crystallization
C023S298000, C023S301000, C423S164000, C423S166000, C423S499400
Reexamination Certificate
active
06267789
ABSTRACT:
The invention relates to a process for enriching crude salt.
It relates in particular to a process for obtaining, starting from crude salt, salt enriched with sodium chloride, of purity sufficient to allow its use in an industrial process or as food.
Industrial salt and food salt are usually obtained starting from mineral salt which is found in the natural solid state in the earth's crust (rock salt) or which is obtained by the evaporation of natural brackish water, such as sea water or water from salt lakes. Crude mineral salt usually contains more than 80% (generally more than 90%) by weight of sodium chloride. In crude mineral salt, the sodium chloride is accompanied by impurities, especially calcium, magnesium and potassium salts (usually sulphates and/or chlorides) and iron oxides. These impurities make crude mineral salt unsuitable, as it is, for most industrial applications or as food.
U.S. Pat. No. 3,655,333 provides a process making it possible to obtain high-purity sodium chloride starting from crude salt, in which the crude salt is dispersed in a saturated solution of sodium chloride, the aqueous suspension thus obtained is cooled, below 0° C., in order to crystallize the sodium chloride dihydrate, the crystals of sodium chloride dihydrate are collected and washed with a pure aqueous solution of sodium chloride. In this known process, the washing of the crystals of sodium chloride dihydrate is a difficult, operation, the effectiveness of which is not guaranteed. It involves the use of a pure aqueous solution of sodium chloride which is then thrown away, thereby constituting a loss and increasing the cost of the sodium chloride produced.
The invention aims to remedy these drawbacks of the known process described above, by providing a novel process making it possible to obtain, starting from crude salt, high-purity sodium chloride without requiring a crystal-washing step.
The invention consequently relates to a process for enriching crude salt, in which the crude salt is cooled in a saturated aqueous sodium chloride solution to a temperature below the anhydrous sodium chloride to sodium chloride dihydrate transition temperature and, after cooling, a stock of a powder comprising sodium chloride crystals is collected, the process being characterized in that the crude salt is ground before it is cooled and the powder is subjected to particle-size fractionation from which a fine particle-size fraction and a coarse particle-size fraction comprising the enriched salt are collected.
The crude salt involved in the process according to the invention is a solid substance consisting of sodium chloride and of impurities, existing in the natural state (salt of mineral origin) or forming a residue of an industrial process. Rock salt, sea salt obtained by solar evaporation of sea water, and salt coming from the evaporation of brine obtained by dissolving rock salt in situ in a deposit constitute examples of crude salt of mineral origin. Another example of crude salt coming within the scope of the invention is that produced as a residue in processes for scrubbing hydrogen-chloride-containing fumes by means of sodium bicarbonate [SOLVAY (Société Anonyme), brochure Br 1566a-B-1-0396]. The sodium chloride weight content of crude salt is generally greater than 80%, usually at least 90% (and frequently at least 95%).
In the process according to the invention, the crude salt is ground at a temperature above the anhydrous sodium chloride to sodium chloride dihydrate transition temperature so that the sodium chloride present in the salt collected from grinding is in the form of anhydrous sodium chloride crystals. The grinding is generally carried out at ambient temperature. The crude salt collected from grinding is cooled to below the anhydrous sodium chloride to sodium chloride dihydrate transition temperature. The purpose of cooling the salt is to recrystallize the sodium chloride into the form of sodium chloride dihydrate. The salt is cooled in a saturated aqueous sodium chloride solution. For this purpose, various operating methods may be employed. According to a first operating method, the aqueous sodium chloride solution is used at ambient temperature, the crude salt is introduced into it at ambient temperature and the resulting aqueous suspension is cooled to below the aforementioned transition temperature.
According to another operating method, the crude salt being at ambient temperature, the saturated aqueous sodium chloride solution is firstly cooled to a temperature below the said transition temperature and then the crude salt is introduced into the cold solution. The cooling temperature of the ground crude salt is chosen so that the anhydrous sodium chloride recrystallizes to sodium chloride dihydrate in the saturated aqueous sodium chloride solution in a time which is technically and economically acceptable. In practice, the ground crude salt is cooled to a temperature which is more than 2° C. (preferably at least 5° C.) below the aforementioned transition temperature. Consequently, the crude salt is generally cooled in the saturated sodium chloride solution to a temperature below −2° C., at standard atmospheric pressure, the temperatures lying between −5 and −15° C. being especially recommended.
After cooling, an aqueous stock of a powder comprising sodium chloride dihydrate crystals is collected. According to the invention, the said powder is subjected to particle-size fractionation. As a variant, before the powder is subjected to particle-size fractionation, the stock is heated to a temperature above the aforementioned transition temperature in order to recrystallize the sodium chloride to the anhydrous state. In this alternative method of implementation of the process according to the invention, the temperature to which the stock is heated is chosen so that the sodium chloride dihydrate recrystallizes to anhydrous sodium chloride in a time which is technically and economically acceptable. In practice, the stock is heated to a temperature which is more than 2° C. (preferably at least 5° C.) above the aforementioned transition temperature. Consequently, the stock is generally heated to a temperature above 2° C., at standard atmospheric pressure, the temperatures lying between 5 and 15° C. being especially recommended.
In the present specification, the expression “powder comprising sodium chloride crystals” denotes, equally well, either a powder comprising anhydrous sodium chloride crystals or a powder comprising sodium chloride dihydrate crystals, depending on whether or not the aforementioned variant of the process is carried out.
According to the invention, it is preferred to carry out the said alternative method of implementation so that the powder subjected to particle-size fractionation is a powder of anhydrous sodium chloride crystals.
In the process according to the invention, the conditions employed in grinding, on the one hand, and in cooling, on the other hand, are set so that the particles of ground crude salt have a smaller mean diameter than the mean diameter of the sodium chloride crystals in the powder of the stock collected from cooling.
The mean diameter of the crude salt particles (namely, sodium chloride crystals) is defined by the mathematical equation
d
=
∑
n
i
d
i
∑
n
i
in which d denotes the mean diameter and n
i
denotes the frequency by weight of the crude salt particles (namely, sodium chloride crystals) of diameter d
i
, the diameters d
i
being measured by screening according to the AFNOR standard.
In the process according to the invention, the purpose of the particle-size fractionation is to split the particles of the stock powder into two separate particle-size fractions (a coarse particle-size fraction containing enriched salt and a fine particle-size fraction containing most of the crude-salt impurities). The particle-size fractionation is set so that the cut-off diameter of the two particle-size fractions is approximately equal to the mean diameter of the sodium chloride crystals of the stock powd
Langel Wayne
Medina Maribel
Schneller Marina V.
Solvay ( Societe Anonyme)
Venable
LandOfFree
Method for enriching raw salt does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for enriching raw salt, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for enriching raw salt will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2558029