FR2706440A1 - Process for the crystallisation of sodium chlorate and solid granular mass based on sodium chlorate - Google Patents

Abstract

Process for the crystallisation of sodium chlorate, according to which a supersaturated aqueous sodium chlorate solution (14) is made to move from the bottom upwards through a bed of crystals (6), so as to fluidise the latter. Solid granular mass based on sodium chlorate, comprising granules in the form of balls (lumps) substantially without sharp edges.

Description

Process for the crystallization of sodium chlorate and
granular solid mass based on sodium chlorate
The invention relates to the crystallization of sodium chlorate.

 It relates more particularly to a process for the crystallization of sodium chlorate in the form of granules of high purity and of particular morphology.

According to a usual process for the production of sodium chlorate particles, an aqueous sodium chloride solution is electrolyzed and an aqueous sodium chlorate solution is collected from the electrolysis which is successively subjected to evaporation to concentrate and separate it. sodium chloride, and cooling to crystallize sodium chlorate (The Chemical Process Industries, R. Norris Shreve,
McGraw-Hill Series in Chemical Engineering, Second Edition, 1956,
Pages 326-327).

 To crystallize sodium chlorate, it has also been proposed to put a supersaturated aqueous solution of sodium chlorate in contact with a bed of sodium chlorate crystals (SU-A-0280438 - KHOROSHILIN; Khimivheskaya Promyshlennost, V.44, nO 6 , pages 457-458). In this known process, the crystallization of sodium chlorate proceeds by desaturation of the solution in contact with the crystals of the bed and growth of these.

 In the known methods described above, sodium chlorate is obtained in the form of a granular solid mass.

This is generally a fine powder formed of irregular granules and having a wide particle size dispersion, which can be a disadvantage for certain applications. This granular mass also contains sodium chloride and water and is generally contaminated with impurities comprising calcium, magnesium, sodium sulfate and sodium chromate. Among these impurities, calcium, magnesium and sodium sulfate usually come from the rock salt used to prepare the aqueous solution of sodium chloride subjected to electrolysis.

Sodium chromate is an additive that is incorporated into the aqueous sodium chloride solution to avoid parasitic reduction of hypochlorous ions on the cathode of the electrolysis cell (The Chemical Process Industries, R. Norris Shreve,
McGraw-Hill Series in Chemical Engineering, Second Edition, 1956, page 326). These impurities can be harmful for the applications for which the sodium chlorate granules are intended, which constitutes another disadvantage of the known methods described above.

 The invention aims to remedy these disadvantages of the known methods described above, by providing a method for obtaining granular solid masses based on sodium chlorate, which contain a high content of sodium chlorate and a very low content of impurities, and which moreover have a regular morphology.

 Consequently, the invention relates to a process for the crystallization of sodium chlorate, by bringing a supersaturated aqueous solution of sodium chlorate into contact with a bed of crystals, the process being characterized in that bringing the supersaturated solution into contact with the crystal bed is obtained by circulating said solution from bottom to top through the crystal bed so as to fluidize it.

 In the process according to the invention, the bed crystals serve as seeds for the crystallization of sodium chlorate by desaturation of the supersaturated solution. Although crystals of a mineral material other than sodium chlorate can be used, it is preferred, according to the invention, to use a bed of crystals of sodium chlorate. These can be obtained by any suitable known technique, for example by one of the techniques of the prior art described above.

According to the invention, the crystal bed is a fluidized bed, in accordance with the generally accepted definition (GIVAUDON,
MASSOT, and BENSIMONT - "Precis of chemical engineering" - Tome 1 - 1960 - Berger-Levrault, Nancy - pages 353 to 370). To this end, the technique described and claimed in patent application EP-A-0352847 (SOLVAY & Cie) is advantageously used, which consists in passing the supersaturated solution through a distributor placed under the bed and designed to distribute the solution oversaturated in vertical fillets, the dispenser being kept at an appropriate temperature to prevent sodium chlorate spontaneously crystallizing on its surface. An apparatus suitable for implementing this embodiment of the method according to the invention comprises a vertical cylindrical tank and a vertical tube which is arranged axially in the tank and opens out in the immediate vicinity of the bottom thereof; the vertical annular chamber which is thus delimited between the axial tube and the cylindrical wall of the tank, is divided in two by the distributor of the fluidized bed. In the operation of this apparatus, the crystal bed is used in the annular chamber, above the distributor, and the supersaturated solution is introduced into the axial tube, so that it penetrates radially into the annular chamber, near the bottom of the latter, crosses the distributor and fluidizes the crystal bed.

 The optimum degree of supersaturation of the supersaturated solution depends on various parameters, in particular its temperature and its content of impurities. In practice, all other things being equal, it is advantageous to achieve a maximum degree of supersaturation, this must however be limited to avoid accidental crystallizations on the walls of the crystallization installation, upstream of the bed, or growth anarchic bed crystals, detrimental to their morphology and mechanical strength.

 The temperature of the supersaturated solution is not critical.

However, it has been observed in practice that the rate of growth of the bed crystals is influenced by the temperature of the solution. It is also necessary for the temperature of the solution to remain below its boiling point at the pressure prevailing in the crystallization installation. For example, it is advantageously possible, according to the invention, to use a supersaturated solution having a degree of supersaturation with sodium chlorate of 1 to 6 g / kg (preferably 3 to 5.5 g / kg), at a temperature from 10 to 100 OC (preferably from 20 to 50 OC), when normal atmospheric pressure prevails in the crystallization plant. By definition, the degree of supersaturation expresses the mass of sodium chlorate, per kg of solution, which exceeds the mass corresponding to the saturation of the solution.

The means used to obtain the supersaturated sodium chlorate solution is not critical. One commonly used means is to gradually cool a substantially saturated aqueous solution of sodium chlorate. The latter can be obtained in a conventional manner by electrolysis of an aqueous solution of sodium chloride, in the manner described in the abovementioned document "The Chemical Process Industries,
R. Norris Shreve, McGraw-Hill Series in Chemical Engineering,
Second Edition, 1956, Pages 326-327 ".

 At the end of the process according to the invention, sodium chlorate is collected in the crystallized state. In accordance with a preferred embodiment of the process according to the invention, crystals obtained by grinding part of the sodium chlorate originating from crystallization are used in the bed. To this end, said part of the sodium chlorate originating from the crystallization is ground and recycled in the bed of crystals.

 The sodium chlorate collected at the end of the process according to the invention comes in the form of regular granules, substantially devoid of sharp and sharp edges. These granules also have a high purity, mainly with regard to the usual impurities comprising magnesium, calcium, sodium sulfate and sodium chromate.

 The invention therefore also relates to a granular solid mass based on sodium chlorate, comprising granules being balls substantially devoid of sharp edges.

 According to the invention, the term “ball” is intended to denote a solid body which is essentially delimited by curved and convex surfaces and which is inscribed inside a regular tetrahedron. In the ideal case, the balls forming the granular mass according to the invention are spheres.

 The granular mass according to the invention generally comprises at least 50% by weight (preferably at least 75% by weight) of granules in the form of balls in accordance with the definition given above. In the ideal case, the entire granular mass is formed of granules in the state of balls. In practice, the granular mass according to the invention advantageously comprises from 75 to 95% by weight of granules in the state of balls.

dans lesquelles ni désigne la fréquence pondérale des granules de diamètre Di et dgg (respectivement d50 et d10) représente le diamètre pour lequel 90 Z (respectivement 50 Z et 10 %) des granules (exprimées en poids) ont un diamètre inférieur à dgg (respectivement d50 et d10). Dans la définition qui précède, les diamètres des granules ont été mesurés par la méthode de tamisage normalisée TYLER.The granular mass according to the invention generally has a particle size characterized by an average diameter Dm greater than 0.5 mm, usually at least equal to 0.75 mm, and a standard deviation o less than 2 and usually not exceeding 1, 5. In the above definition, the mean diameter Dm and the standard deviation o are defined by the following relationships

in which ni denotes the weight frequency of the granules of diameter Di and dgg (respectively d50 and d10) represents the diameter for which 90 Z (respectively 50 Z and 10%) of the granules (expressed by weight) have a diameter less than dgg (respectively d50 and d10). In the above definition, the diameters of the granules were measured by the standardized TYLER sieving method.

 According to an advantageous embodiment of the invention, the granular mass has an average diameter substantially equal, at most, to 3 mm (preferably 2.5 mm). Preferably, the granular mass according to the invention has an average diameter of 1 to 2 mm and a standard deviation of 0.25 to 1.08.

 In a preferred embodiment of the granular mass according to the invention, the granules are monolithic.

 The term “monolithic granule” is intended to denote a granule formed of a unitary solid, not agglomerated.

 The granular mass according to the invention has the advantageous characteristic of having a relatively low content of impurities. It generally contains more than 99% by weight of sodium chlorate. It usually includes sodium chloride, water and impurities, the impurities being present in an amount less than 0.5% by weight and comprising calcium, magnesium, sodium sulfate and sodium chromate. According to an advantageous embodiment of the invention, the granular mass comprises, per kg, from 994 to 999 g of sodium chlorate, from 0.5 to 3.0 g of sodium chloride, from 0.10 to 0, 50 g of water and 0.02 to 2.5 g of impurities. In a preferred variant of this embodiment of the invention, the impurities comprise, per kg of the granular mass, less than 0.20 g (for example from 0.02 to 0.15 g) of sodium sulfate, less from 0.0080 g (for example from 0.0020 to 0.0075 g) of sodium chromate, less than 0.0050 g (for example from 0.0008 to 0.0030 g) of calcium and at most 0.00010 g of magnesium, the balance being made up of elements and chemical compounds usually accompanying sodium chlorate, such as chlorides, sulfates and metallic carbonates.

 The granular mass according to the invention, based on sodium chlorate, can advantageously be obtained by means of the process according to the invention, defined above.

 The granular solid mass according to the invention generally has a high hardness and a high resistance to impact and abrasion. It has the additional advantage of being little subject to agglutination in the presence of ambient air, which simplifies its storage and handling.

 The granular solid mass according to the invention finds applications in particular in the bleaching industry for paper pulp and textiles as well as in metallurgy.

 Special features and details of the invention will emerge from the following description of the accompanying drawings.

FIG. 1 is the general diagram of an installation implementing a particular embodiment of the method according to the invention;
Figures 2 and 3 show, in vertical axial section, two fluidized bed crystallization apparatus, usable in the installation of Figure 1;
FIGS. 4 and 5 are photographs, at 10 × magnification, of a granular solid mass based on sodium chlorate, in accordance with the invention;
FIG. 6 is a diagram reproducing cumulative particle size curves of granular solid masses based on sodium chlorate, in accordance with the invention. In this figure, the abscissa scale represents the diameter of the crystals (expressed in mm and measured by the standard sieving method
TYLER) and the ordinate scale represents the cumulative fractions, expressed in Z of the weight of the mass of crystals.

 In these figures, the same reference notations designate identical elements.

 The installation shown diagrammatically in FIG. 1 comprises an electrolysis cell 1, an evaporator 2, a decantation chamber 3, a coolant 4, a circulation pump 5 and a crystallization chamber 6. The electrolysis cell 1 is of the type known per se for the production of sodium chlorate by electrolysis of an aqueous solution of sodium chloride.

 During the operation of the installation, an aqueous solution, substantially saturated with sodium chloride 7 is electrolyzed in the electrolysis cell 1 and there is collected therefrom, on the one hand, hydrogen 8 and, on the other hand, an aqueous solution of sodium chlorate 9. The aqueous solution of sodium chlorate 9 is treated in the evaporator 2, in order to concentrate it in sodium chlorate and to precipitate at least part of the sodium chloride which 'it contains. Water vapor 10 and an aqueous suspension 11 are withdrawn from the evaporator 9 and are transferred to the decantation chamber 3 where a precipitate 12, consisting mainly of sodium chloride, takes place, and of a substantially saturated aqueous solution 13 of sodium chlorate. The sodium chlorate solution 13 is transferred to the condenser 4 where it is cooled to a set temperature to supersaturate it with sodium chlorate, without the formation of a precipitate.

 A supersaturated aqueous solution of sodium chlorate is collected from the refrigerant 4, which is sent to the crystallization chamber 6, via the pump 5. In the crystallization chamber 6, the supersaturated solution 14 passes vertically, from bottom to top, a bed of sodium chlorate crystals.

The dimensions of the crystal bed, the particle size of the crystals and the rate of rise of the supersaturated solution 14 are adjusted so as to fluidize the entire bed. The solution 14 is therefore gradually desaturated when it crosses the bed, the crystals of which grow accordingly. The large particle size fractions occupying the bottom of the crystallization chamber 6 are withdrawn periodically or continuously by a withdrawal conduit 15 and constitute the granular solid mass according to the invention. The height of the crystal bed in the chamber 6 is adjusted so that the mother liquor 26 of the crystallization, collected at the top of the chamber 6, is saturated or slightly supersaturated with sodium chlorate. It is sent to the electrolysis cell 1.

 In the installation of FIG. 1, the crystallization chamber 6 must be designed to allow the implementation of a stable bed of crystals. For this purpose, it is advantageous to use the crystallization apparatus shown in FIG. 2.

This conforms to the device described in patent application EP-A-0352847 (SOLVAY & Cie). I1 comprises a vertical cylindrical tank 16 in which a vertical tube 17 is arranged axially. The tank 16 is closed by a cover 19 and this is crossed by the vertical tube 17 which also opens in the vicinity of the bottom of the tank 16. An annular, horizontal, perforated partition 22 divides the tank into an upper annular chamber 24 and a lower chamber 25. The upper chamber 24 constitutes the crystallization chamber and contains the crystal bed 20. The vertical tube 17 serves for the admission of the supersaturated solution of sodium chlorate 14 into the chamber 25; a conduit 21 opening into the upper part of the chamber 24 serves to remove the mother liquor 26 from the crystallization and a conduit 15 is used for withdrawing the crystals.

 During the operation of the apparatus of FIG. 2, the supersaturated aqueous solution of sodium chlorate 14 descends vertically into the tube 17, penetrates radially into the lower chamber 25 of the tank 16 and successively passes through the partition 22 and the bed of crystals 20. The partition 22 has the function of distributing the supersaturated solution 14 in vertical threads 23, to fluidize the bed of crystals 20. The crystals of the bed will therefore be distributed in layers or strata according to their particle size. The large particle size fractions progress towards the bottom of the bed, from where they are discharged periodically or continuously by the draw-off pipe 15. The desursaturated solution leaving the bed is discharged through the pipe 21.

 Figure 3 shows a modified embodiment of the crystallization apparatus. This differs from the device of FIG. 2 by the arrangement of the tube 17 (which opens vertically into the chamber 25 through the bottom thereof), by the profile of the perforated partition 22 (which is a horizontal disc ) and by the profile of the crystallization chamber 24 (which is cylindrical).

 The examples whose description follows serve to illustrate the invention.

 These examples relate to laboratory tests, in which sodium chlorate was crystallized from aqueous solutions of sodium chlorate containing sodium chloride and dissolved impurities.

Example 1 (according to the invention)
A synthetic aqueous solution was prepared comprising, per kg, when dissolved, the following constituents
NaClO3: 405 g,
NaCl: 87 g,
Na2SO4: 0.033 g,
Na2Cr207: 0.098 g.

 The solution, initially at a temperature of approximately 30 OC, was cooled to approximately 28.5 OC, which had the effect of supersaturing it with sodium chlorate, with a degree of supersaturation equal to 3.3 g / kg , as defined above.

 The crystallization chamber consisted of a cylindrical glass column with an inside diameter of 50 mm and a height of 1000 mm, provided, at its lower part, with a horizontal stainless steel grid with square meshes (0.25 mm side ). 70 g of a bed of sodium chlorate crystals, having an average particle diameter of 630 to 800 mm, were placed on the grid.

 The supersaturated aqueous solution of sodium chlorate was circulated from bottom to top in the crystallization chamber, at a rate of 130 l / h, for about 2 hours, so as to fluidize the bed of crystals. At the end of the test, the crystals were collected from the bed, they were washed on a filter with an aqueous solution of sodium chlorate, then they were dried for 2 hours at 110 ° C. A granular mass of approximately 700 g of crystals was thus collected, which is shown in the photograph in FIG. 4. In FIG. 6, the cumulative particle size distribution of the mass of crystals is represented by curve 27.

The mass of crystals also had the following composition
NaClO3: 994 g / kg,
H2O: 0.24 g / kg,
NaCl: 2.4 g / kg,
Na2SO4: 0.022 g / kg,
Na2Cr207: 0.0025 g / kg,
Ca: 0.0009 g / kg,
Mg: <0.0001 g / kg, the balance being made up of other usual impurities of sodium chlorate.

Example 2 (reference)
The synthetic starting aqueous solution of Example 1 was treated in an evaporator to concentrate it and precipitate sodium chlorate. The collected crystals were washed and dried as described in Example 1 and their composition was then noted.
NaClO3: 990 g / kg,
H2O: 0.2 g / kg,
NaCl: 4.2 g / kg,
Na2S 4: 3.4 g / kg,
Na2Cr207: 0.4 g / kg,
Ca: 0.4 g / kg,
Mg: 0.002 g / kg, the balance being made up of other usual impurities of sodium chlorate.

 A comparison of the results of Examples 1 and 2 shows the progress made by the invention as regards the purity of the crystals obtained.

Example 3 (according to the invention)
Example 1 was repeated under the following conditions.

Composition of the aqueous sodium chlorate starting solution
NaClO3: 364 g / kg,
NaCl: 90 g / kg,
Na2SO4: 14 g / kg,
Na2Cr207: 4.79 g / kg.

 The solution, initially at the temperature of 30 OC, was cooled to 28.5 OC, so that it has a degree of supersaturation with sodium chlorate equal to 3.3 g / kg.

 At the end of the treatment of the solution in the crystallization chamber under the conditions set out in Example 1, a mass of crystals was collected which was washed and dried as described in Example 1. The mass of The crystals thus obtained are shown in the photograph in FIG. 5, and their particle size distribution is represented by curve 28 in FIG. 6.

Their weight composition was noted
NaClO3: 998 g / kg,
H2O: 0.17 g / kg,
NaCl: 0.8 g / kg,
Na2SO4: 0.104 g / kg,
Na2Cr207: 0.0072 g / kg,
Ca: 0.0021 g / kg,
Mg: <0.0001 g / kg.

Example 4 (for reference)
The starting synthetic aqueous solution of Example 3 was treated in an evaporator to concentrate it and precipitate sodium chlorate. The collected crystals were washed and dried as described in Example 3 and their composition was then noted.
Na103: 997 g / kg,
H2O: 0.45 g / kg,
NaCl: 0.9 g / kg,
Na2SO4: 0.4 g / kg,
Na2Cr207: 0.08 g / kg,
Ca: 0.6 g / kg,
Mg: 0.002 g / kg.

 A comparison of the results of Examples 3 and 4 shows the progress made by the invention as regards the purity of the crystals obtained.

Claims (11)

 1 - Process for the crystallization of sodium chlorate, according to which an supersaturated aqueous solution of sodium chlorate is brought into contact with a bed of crystals, characterized in that the supersaturated solution (14) is circulated from bottom to top at through the crystal bed (6, 20) so as to fluidize it.  2 - Method according to claim 1, characterized in that the bed crystals (6, 20) comprise sodium chlorate crystals.  3 - Method according to claim 1 or 2, characterized in that the supersaturated solution (14) has a degree of supersaturation from 3 to 5.5 g / kg.  4 - Process according to any one of claims 1 to 3, characterized in that one uses a supersaturated solution obtained by cooling an aqueous solution of sodium chlorate produced by electrolysis of an aqueous solution of sodium chloride .  5 - Method according to any one of claims 2 to 4, characterized in that one implements, in the bed, crystals obtained by grinding a fraction of sodium chlorate from the crystallization.  6 - Granular solid mass based on sodium chlorate, characterized in that it comprises granules in the form of balls substantially deprived of sharp edges.  7 - granular solid mass according to claim 6, characterized in that it has a particle size defined by an average diameter of 1 to 2 mm and a standard deviation of 0.25 to 1.08.  8 - granular solid mass according to claim 6 or 7, characterized in that the granules are monolithic.  9 - granular solid mass according to any one of claims 6 to 8, characterized in that it comprises sodium chlorate in an amount greater than 99; K by weight, sodium chloride, water and less than 0.5% by weight of impurities.  10 - granular solid mass according to claim 9, characterized in that it comprises, per kg, from 994 to 999 g of sodium chlorate, from 0.5 to 3.0 g of sodium chloride, from 0.10 to 0.50 g of water and 0.02 to 2.5 g of impurities.  11 - Granular solid mass according to claim 10, characterized in that it comprises, per kg, from 0.02 to 0.15 g of sodium sulfate, from 0.0020 to 0.0075 g of sodium chromate, 0.0008 to 0.0030 g of calcium and a maximum of 0.00010 g of magnesium.