GB1605240A - Process for the preparation of pure dry crystals - Google Patents

Description

(54) PROCESS FOR THE PREPARATION OF PURE DRY CRYSTALS (71) We, PRODUITS CHIMIQUES PECHINEY-SAINT-GoBAIN, a company registered under the laws of France, of 16 Avenue Matignon, Paris 8, France, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a process for the preparation of pure dry crystals and is concerned with the preparation of such crystals from crystals having liquid retained therein as a result of their precipitation from solution.

It is quite usual for crystals having an inorganic or sometimes organic nature and obtained by precipitation from solution followed by separation from the mother liquor by filtration or centrifuging to still contain an appreciable proportion of mother liquor.

Under the influence of moisture fluctuations and the surrounding medium temperature, the mother liquor retained on the crystals tends to either crystallise out on the crystals and in the interstices therebetween or dissolve or redissolve the crystals. Thus, a succession of recrystallizations occurs as a result of which the crystals and more particularly the finer crystals agglomerate forming lumps, sometimes very hard lumps, which are very inconvenient for the users of the product.

It is already known to avoid such agglomeration by adding to the crystals various different inorganic or organic substances generally known as, for example, "anti clod", "anti binder" and "water repellent" substances.

However the addition of such substances generally leads to contamination of the final product. When the crystals are, for example, crystals of ammonium perchlorate the addition of such substances also can catalyze spontaneous and unexpected decomposition thus provoking ignition or explosion. Hence the addition of such substances cannot be made crystals such as ammonium perchlorate.

According to the present invention there is provided a process for producing dry crystals of a readily decomposable substance from crystals of said substance obtained by precipitation from solution and wet with mother liquor, which comprises successively washing the crystals with a series of organic liquids each having a flash point greater than 50"C. in such a manner as to substantially completely replace the liquid wetting the crystals before each washing by each of the series of organic liquids in turn, and evaporating off from the crystals the final liquid of the series whereby dry crystals free of retained liquid are obtained, wherein each organic liquid of the series is miscible with the liquid already retained by the crystals and with the next succeeding organic liquid of the series and the final organic liquid of the series has a high vapour pressure at ordinary temperatures, is essentially non-flammable and non-explosive and is such that no appreciable residue is left on evaporation thereof.

By means of the present invention, it is possible to separate in the pure state crystals, for example readily decomposable crystals such as ammonium perchlorate, obtained by precipitation from an aqueous solution, and to avoid recrystallisation leading to agglomeration of said crystals without adding to the crystals foreign substances prejudicial to their purity. Crystals obtained by the process of the present invention do not tend to agglomerate during storage i.e. they retain their free-running properties. This is of importance if the crystals are of ammonium perchlorate and are to be used in the preparation of solid propellents for use in rocket propulsion.

The choice of the series of organic liquids used to eliminate the liquid retained by the crystals is determined according to the physical properties of mutual miscibility of these liquids and of the solubility of the crystals in these liquids and particularly the ignition and explosion dangers that the crystals may show when hot and in contact with the said organic liquids. Organic liquids which have a low flash point i.e. less than 50"C. should be carefully avoided. Preferably all the organic liquids used to treat the crystals are non inflammable and non-explosive and they should be such that they only dissolve very few or better none at all of the treated crystals and leave practically no residue after evaporation.In addition, the final liquid of the series has a high vapour pressure at room temperature whereby it can be easily evaporated and maintain the initial appearance of the crystals.

Advantageously, the crystals for example of ammonium perchlorate, are filtered or centrifuged after their initial precipitation from aqueous solution and are then treated with a series of three organic liquids. Firstly the initial crystals having aqueous mother liquor retained therein, are treated with a water-miscible liquid, for example ethylene glycol or propylene glycol. After filtration, the crystals do not contain any mother liquor, this being replaced by the glycol saturated with ammonium perchlorate. To eliminate the glycol retained therein the crystals are then treated with for example n-butanol which dissolves the glycol without substantially dissolving the crystals. After filtration again the crystals do not contain glycol, this having been replaced by n-butanol containing in solution about 0.3% of ammonium perchlorate.

Finally, to eliminate n-butanol from the crystals, they are treated with the third of the series of organic liquids for example trifluorotrichloroethane commercially available under the designation "Flugene 113". This liquid rapidly volatilizes at low temperature and only leaves a negligible residue. The n-butanol retained by the crystals is thus replaced by the Flugene 113. The crystals are then filtered off and dried to evaporate the Flugene 113.

If the crystals are at least of the order of a few microns (i.e. from 1 to 10 microns) in size and do not contain more than about 20% of residual mother liquor, the first stage of the treatment, i.e. the treatment with, for example, ethylene or propylene glycol, can be omitted.

In this case, the crystals obtained after the initial precipitation and having mother liquor retained therein are treated firstly with an alcohol having a flash point higher than 50"C.

for example with n-butanol. Thereafter, the liquid phase is removed to obtain crystals wet with the alcohol. These crystals are then treated with a halogenated organic compound having a flash point greater than 50 C.which has a high vapour pressure at normal temperatures, and which does not appreciably dissolve the crystals and which leaves a negligible deposit on evaporation. The halogenated compound is preferably trifluorotrichloroethane. The crystals thus wetted with the halogenated organic compound are then dried to evaporate the halogenated compound to obtain the pure dry crystals.

The following Examples illustrate the invention.

EXAMPLE 1.

An aqueous suspension of ammonium perchlorate crystals was filtered to separate the crystals from the liquid phase. A filter cake composed of from 80 to 82% of crystals having a size of from 1 to 40 microns and from 20 to 18% of mother liquor containing 175 g/l of dissolved ammonium perchlorate at 15"C. was obtained.

After a conventional drying of a part of this cake, a compact mass of strongly agglomerated crystals was obtained. Furthermore an increase of the medium size of the crystals was observed as a result of about 5 g. ammonium perchlorate, per 100 g. of crystals, being precipitated from the mother liquor retained by the crystals during the drying.

The remainder of the cake was then treated in accordance with the present invention. The cake wet with mother liquor was first put again in suspension by agitating it in a sufficient quantity of ethylene glycol. The suspension was then filtered and the cake twice washed on the filter with ethylene glycol.

After the second washing, the filtered cake contained from 78 to 80% of ammonium perchlorate crystals and from 22 to 20% of ethylene glycol containing about 10% of dissolved perchlorate. This cake was then suspended with agitation in a sufficient quantity of n-butanol. After filtration followed by two washings on the filter with butanol, a cake was obtained which was composed of from 82 to 84% of crystals of ammonium perchlorate and from 18 to 16% of butanol having up to 0.3% of its weight of perchlorate dissolved therein.

This cake was then suspended in a sufficient quantity of Flugene 113 with agitation. The suspension was then filtered, the cake being washed twice with Flugene 113. The cake obtained was composed of from 85 to 87% of ammonium perchlorate crystals and from 15 to 13% of Flugene 113 containing in solution only a negligible quantity (of up to 0.05% at 15"C.) of ammonium perchlorate.

The cake was then rapidly dried (1 hour at 100"C.). The obtained crystals were not agglomerated and their initial size was unchanged. Even after six months storage in a closed container, the product retained its flow properties, this being necessary when it is destined to be used in the preparation of a solid propellent for use, for example, as a rocket propellant since the product must then be regularly distributed so as to ensure a uniform combustion.

EXAMPLE 2.

An aqueous suspension of very fine crystals of ammonium perchlorate was filtered under vacuum using a rotary filter. The cake obtained was composed of from 80 to 82% of crystals of 1 to 10 microns in size and from 20 to 18% of mother liquor containing 175 g/liter of ammonium perchlorate at 15"C.

When a part of this cake was dried directly in the conventional manner, a compact mass of highly agglomerated crystals was obtained.

Moreover a notable increase of the average crystal size was observed as a result of a deposit of about 5 g. of the salt per 100 g. of crystals occurring during the evaporation of the mother liquor said increase being chiefly obvious for the smallest crystals.

The remainder of the cake was treated in accordance with the present invention by suspending it with strong agitation in a convenient quantity of n-butanol. 4 litres of alcohol per kg. of crystals of ammonium perchlorate were used. Then the liquid phase was removed from the suspension thus obtained by a centrifugal drier. The centrifuged cake thus obtained was composed of from 95 to 97% of crystals of ammonium perchlorate and from 5 to 3% of n-butanol containing about 0.3% of its weight of perchlorate dissolved therein. The direct drying of this cake would give rise to crystals of the required characteristics i.e. free from agglomeration; however this drying could not be carried out without any danger. To avoid this drawback, the cake was directly washed on the centrifugal drier using a sufficient quantity of trifluorotrichlorethane.In this case, 0.4 liter of this halogenated organic compound per kg. of crystals of perchlorate were used. Then the liquid phase was separated from the crystals on the centrifugal drier.

After this second washing-drying, the cake contained 96-97% of crystals of ammonium perchlorate which retained 5 to 3% of trifluorotrichlorethane containing in solution only a negligible quanity of ammonium perchlorate (up to 0.05% at temperature of 15 C.).

After a rapid drying (for from 30 to 60 minutes at a temperature of 100"C.) the crystals of ammonium perchlorate obtained were not agglomerated. Further, their initial sizes were not modified. After 2 months storage in a closed container it is observed that the crystals had retained their initial characteristics.

Although the invention has been described with reference to ammonium perchlorate, the process is applicable to any crystals which are easily decomposable, which are precipitated as fine crystals in a saturated aqueous solution medium and which show towards the organic liquids used physical and chemical properties similar to those of ammonium perchlorate. The organic liquids used can of course be liquids other than the liquids used in the Examples.

Thus the first of the series of liquids used to remove and eliminate the aqueous solution from which the crystals were initially precipitated and which is to be subsequently replaced by the second of the series may be any liquid which is miscible with water and the second liquid of the series provided that it has a flash point higher than 50"C. Hence the first of the series may be, for example trimethyleneglycol; 1,3 butane diol; 1,5 pentanediol; 2,4 pentanediol; 1,6 hexanediol; 2,5 hexanediol; 2,4 hexanediol; 2 methyl-2,4 pentanediol; 2,4 heptanediol; diethylene glycol; triethylene glycol; tetraethylene glycol and dipropylene glycol in addition to ethylene glycol and propylene glycol.

The second organic liquid of the series used to displace and to eliminate from the crystalline mass the first of the series and to be subsequently displaced and then eliminated by the third of the series may be hexyl alcohol, furfuryl alcohol or other liquid instead of n-butanol on condition that it is miscible with the first and the third of the series. Moreover, its flash point should be higher than 50"C. The secondary butyl alcohol, for instance, is not suitable because its flash point is too low.

The third organic liquid of the series used to displace and eliminate the second of the series and subsequently to be evaporated off to obtain the pure dry crystals should be noninflammable and non-explosive. Preferably it should evaporate without leaving a fixed residue of more than 10 p.p.m, dissolve less than 0.2% of the crystals and have a boiling point less than 100 C. Thus instead of Flugene 113, the third organic liquid of the series may be for example carbon tetrachloride, 1,1,1trichlorethane, trichloroethylene, monofluorotrichloromethane (commercially available under the trade name Flugene 11), monofluorotrichloromethane in admixture with trifluorotrichloroethane, and tetrafluorodichloroethane (commercially available under the trade designation Flugene 114) in admixture with trifluorotrichloroethane.

WHAT WE CLAIM IS: 1. Process for producing dry crystals of a readily decomposable substance from crystals of said substance obtained by precipitation from solution and wet with mother liquor, which comprises successively washing the crystals with a series of organic liquids each having a flash point greater than 50 C. in such a manner as to substantially completely replace the liquid wetting the crystals before each washing by each of the series of organic liquids in turn, and evaporating off from the crystals the final liquid of the series whereby dry crystals free of retained liquid are obtained, wherein each organic liquid of the series is miscible with the liquid already retained by the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (22)

**WARNING** start of CLMS field may overlap end of DESC **. EXAMPLE 2. An aqueous suspension of very fine crystals of ammonium perchlorate was filtered under vacuum using a rotary filter. The cake obtained was composed of from 80 to 82% of crystals of 1 to 10 microns in size and from 20 to 18% of mother liquor containing 175 g/liter of ammonium perchlorate at 15"C. When a part of this cake was dried directly in the conventional manner, a compact mass of highly agglomerated crystals was obtained. Moreover a notable increase of the average crystal size was observed as a result of a deposit of about 5 g. of the salt per 100 g. of crystals occurring during the evaporation of the mother liquor said increase being chiefly obvious for the smallest crystals. The remainder of the cake was treated in accordance with the present invention by suspending it with strong agitation in a convenient quantity of n-butanol. 4 litres of alcohol per kg. of crystals of ammonium perchlorate were used. Then the liquid phase was removed from the suspension thus obtained by a centrifugal drier. The centrifuged cake thus obtained was composed of from 95 to 97% of crystals of ammonium perchlorate and from 5 to 3% of n-butanol containing about 0.3% of its weight of perchlorate dissolved therein. The direct drying of this cake would give rise to crystals of the required characteristics i.e. free from agglomeration; however this drying could not be carried out without any danger. To avoid this drawback, the cake was directly washed on the centrifugal drier using a sufficient quantity of trifluorotrichlorethane.In this case, 0.4 liter of this halogenated organic compound per kg. of crystals of perchlorate were used. Then the liquid phase was separated from the crystals on the centrifugal drier. After this second washing-drying, the cake contained 96-97% of crystals of ammonium perchlorate which retained 5 to 3% of trifluorotrichlorethane containing in solution only a negligible quanity of ammonium perchlorate (up to 0.05% at temperature of 15 C.). After a rapid drying (for from 30 to 60 minutes at a temperature of 100"C.) the crystals of ammonium perchlorate obtained were not agglomerated. Further, their initial sizes were not modified. After 2 months storage in a closed container it is observed that the crystals had retained their initial characteristics. Although the invention has been described with reference to ammonium perchlorate, the process is applicable to any crystals which are easily decomposable, which are precipitated as fine crystals in a saturated aqueous solution medium and which show towards the organic liquids used physical and chemical properties similar to those of ammonium perchlorate. The organic liquids used can of course be liquids other than the liquids used in the Examples. Thus the first of the series of liquids used to remove and eliminate the aqueous solution from which the crystals were initially precipitated and which is to be subsequently replaced by the second of the series may be any liquid which is miscible with water and the second liquid of the series provided that it has a flash point higher than 50"C. Hence the first of the series may be, for example trimethyleneglycol; 1,3 butane diol; 1,5 pentanediol; 2,4 pentanediol; 1,6 hexanediol; 2,5 hexanediol; 2,4 hexanediol; 2 methyl-2,4 pentanediol; 2,4 heptanediol; diethylene glycol; triethylene glycol; tetraethylene glycol and dipropylene glycol in addition to ethylene glycol and propylene glycol. The second organic liquid of the series used to displace and to eliminate from the crystalline mass the first of the series and to be subsequently displaced and then eliminated by the third of the series may be hexyl alcohol, furfuryl alcohol or other liquid instead of n-butanol on condition that it is miscible with the first and the third of the series. Moreover, its flash point should be higher than 50"C. The secondary butyl alcohol, for instance, is not suitable because its flash point is too low. The third organic liquid of the series used to displace and eliminate the second of the series and subsequently to be evaporated off to obtain the pure dry crystals should be noninflammable and non-explosive. Preferably it should evaporate without leaving a fixed residue of more than 10 p.p.m, dissolve less than 0.2% of the crystals and have a boiling point less than 100 C. Thus instead of Flugene 113, the third organic liquid of the series may be for example carbon tetrachloride, 1,1,1trichlorethane, trichloroethylene, monofluorotrichloromethane (commercially available under the trade name Flugene 11), monofluorotrichloromethane in admixture with trifluorotrichloroethane, and tetrafluorodichloroethane (commercially available under the trade designation Flugene 114) in admixture with trifluorotrichloroethane. WHAT WE CLAIM IS:

1. Process for producing dry crystals of a readily decomposable substance from crystals of said substance obtained by precipitation from solution and wet with mother liquor, which comprises successively washing the crystals with a series of organic liquids each having a flash point greater than 50 C. in such a manner as to substantially completely replace the liquid wetting the crystals before each washing by each of the series of organic liquids in turn, and evaporating off from the crystals the final liquid of the series whereby dry crystals free of retained liquid are obtained, wherein each organic liquid of the series is miscible with the liquid already retained by the

crystals and with the next succeeding organic liquid of the series has a high vapour pressure at ordinary temperatures, is essentially non-flammable and non-explosive and is such that no appreciable residue is left on evaporation thereof.

2. Process according to Claim 1, wherein the liquid initially wetting the crystals before treatment is aqueous mother liquor and a series of three organic liquids is used, the first organic liquid of said series being miscible with water, the second organic liquid of said series being a solvent for the first of said series, and the third organic liquid of said series being miscible with the second of said series.

3. Process according to Claim 2, wherein the first organic liquid of said series is ethylene glycol or propylene glycol.

4. Process according to Claim 2, wherein the first organic liquid of said series is trimethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol or dipropylene glycol.

5. Process according to Claim 2, wherein the first organic liquid of said series is 1,3butanediol.

6. Process according to Claim 2, wherein the first organic liquid of said series is 1,5pentanediol, 2,4-pentanediol or 2 methyl 2,4-pentanediol.

7. Process according to Claim 2, wherein the first organic liquid of said series is 1,6 hexanediol, 2,4-hexanediol or 2,5-hexanediol.

8. Process according to Claim 2, wherein the first organic liquid of said series is 2,4heptanediol.

9. Process according to any one of Claims 2 to 8, wherein the second organic liquid of said series is n-butanol.

10. Process according to any one of Claims 2 to 8, wherein the second organic liquid of said series is hexyl alcohol or furfuryl alcohol.

11. Process according to any one of Claims 2 to 10, wherein the third organic liquid of said series is trifluorotrichloroethane.

12. Process according to any one of Claims 2 to 10, wherein the third organic liquid of said series is carbon tetrachloride, 1,1,1trichloroethane or trichloroethylene.

13. Process according to any one of Claims 2 to 10, wherein the third organic liquid of said series is monofluorotrichloromethane either alone or in admixture with trifluorotrichloroethane.

14. Process according to any one of Claims 2 to 10, wherein the third organic liquid of said series is a mixture of tetrafluorodichloroethane and trifluorotrichloroethane.

15. Process according to Claim 1, wherein the crystals are of a size af from 1 to 10 microns, the liquid initially retained by the crystals before treatment is aqueous mother liquor and a series of two organic liquids is used, the first organic liquid of said series being an alcohol miscible with water and the second organic liquid of said series being a halogenated organic compound miscible with said alcohol.

16. Process according to Claim 15, wherein the alcohol is n-butanol.

17. Process according to Claim 15 or 16, wherein the halogenated organic compound is trifluorotrichloroethane.

18. Process according to any preceding claim, wherein the crystals are crystals of ammonium perchlorate.

19.Process for producing dry crystals in accordance with Claim 1, substantially as hereinbefore described in either of the foregoing Examples.

20. Crystals whenever treated by the process claimed in any one of Claims 1 to 17.

21. Ammonium perchlorate crystals whenever treated by the process claimed in Claims 18 or 19.

22. A solid propellent containing ammonium perchlorate as claimed in Claim 21.