AU620857B2 - Process for preparing calcium sulphate of elongated crystalline structure - Google Patents

Description

OPI DATE 16/10/89 ORGAI AOJP DATE 09/11/89 APPLN- ID 32940 89

PCT

PCT NUMBER PCT/EP89/00300 DEMANDE INTERNATIONALE PUBLILc Cnn vO rau Lu in±i j ui oiV i u-/ (51) Classification internationale des brevets 4 (11) Numero de publication internationale: WO 89/ 09187 CO0F 11/46, D21H 5/18 Al 5 o e 19 (5.10.

CO9C 1/02, C04B 14/38 A e f a nt ion 5 octobre 1989 (05.10.89) C08K 3/30, 7/08 I (21) NumBro de la demande internationale: PCT/EP89/00300 (81) Etats disign6s: AU, BR, FI, JP, US.

(22) Date de d6p6t international: 21 mars 1989 (21.03.89) Publiee Avec rapport de recherche internationale.

(31) Numero de la demande prioritaire: 88/04338 Avant l'expiration du dilai prevu pour la modification des revendications, sera republie si de telles modifica- (32) Date de priorit6: 24 mars 1988 (24.03.88) tions sont recues.

(33 Pavs d. nrinrit.:

FR

I SECTION 3' 1 DIRECTION SEE FO IO. (71) NAME DIRECTED i APA A C coPpr- sA.

Q d, ru- e- M f 'r sllt r ''is Fr, nrPice- (72) Inventeur/D6posant (US seulement) PETIT, Alain [FR/ c FR]; 11, rue Mozart, Lotissement Saint Joseph, F- v 26200 MontBlimar (FR).

(74) Mandataire: EGGERT, Hans-Gunther; Riderscheidtstrasse 1, D-5000 K61n 41 (DE).

(54)Title: PROCESS FOR PREPARING CALCIUM SULPHATE WITH AN ELONGATED CRYSTALLINE STRUCTURE, THE LENGTH AND SHAPE FACTOR OF WHICH ARE CONTROLLED (54) Titre: PROCEDE DE PREPARATION DE SULFATE DE CALCIUM A STRUCTURE CRISTALLINE LONGI- LIGNE DOTEE D'UNE LONGUEUR ET D'UN FACTEUR DE FORME MAITRISES (57) Abstract 1. S 10o V.

Process for preparing dihydrous calcium sulphate with the appearance of elongated crystalline structures from hemihydrated calcium sulphate of the or variety, characterized in that in order to obtain a controlled length and shape factor, it includes: grinding and/or selecting said hemihydrated calcium sulphate within a given granulometric range such that the specific granulometric size to produce elongated cryst-lline structures with a predetermined length and shape factor is situated within the range delimited by extremes of 5 micromieters and 200 micrometers; the presence of at least one recrystallization regulating agent, in the proportion of between 0.01 and 6.66 weight per cent, relative to the total mass of the suspension to be formed in stage preparing by dispersion from materials of stages and a water suspension at an isothermic transformation temperature set to a maximum of 60 0 C, with a proportion of dry matter of no more than 33.33 weight per cent relative to the total mass of the suspension; maintaining the temperature at the fixed level for no longer than 60 minutes and possibly separating the elongated crystalline structures with controlled length and shape factor. The elongated crystalline structures obtained by said process are used in particular for a wide variety of fields of application, including those of paper, paints and synthetic materials.

-I, I I d WO 89/09187 PCT/EP89/00300 PROCESS FOR THE PREPARATION OF CALCIUM SULPHATE OF ELONGATE CRYSTALLINE STRUCTURE ENDOWED WITH CONTROLLED LENGTH AND SHAPE FACTOR FIELD OF THE INVENTION The invention relates to a process for the preparation of calcium sulphate dihydrate having the appearance of elongate crystalline structures endowed with a length and a shape factor which are controlled, and the elongate crystalline structures obtained.

The expression "elongate crystalline structure" as employed denotes the domain consisting of individual crystals having the appearance of needles or else consisting of the said crystals associated at most in threes along their long axis, by partial overlapping.

Similarly, the expression "shape factor" is intended to define the dimensional ratio existing between the length of the individual crystal or of the crystals associated at most in threes, and their longest diameter.

BACKGROUND OF THE INVENTION The various chemical and crystalline forms of calcium sulphate have for a long time been extensively described in the specialized literature.

The most widely known chemical forms are calcium sulphate dihydrate of formula CaSO 4 .2H 2 0, called gypsum in the mineral kingdom, be it of natural or synthetic origin (such as phosphogypsum, desulphogypsum or gypsum originating from chemical neutralization), calcium sulphato hemihydrate of formula CaSO 4 .1/2H 2 O, resulting from the i heat treatment of gypsum (dry or in the presence of water in the form of liquid or steam) between 90 0 C and 250°C, anhydrites I, II and III of formula CaSO 4 one being socalled soluble (anhydrite III) when the treatment is carried out at a temperature chosen in the range 100 0

C

to 250"C, the others being so-called insoluble, when the heat treatment exceeds the temperature of 250 0 C (deadburnt or anhydrite II) or 1200"C (anhydrite I).

Apart from the different chemical formulae just Sreferred to, calcium sulphate can also be identified by its crystalline structure. Thus, it may have a

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WO 89/09187 2 -PCT/EP89/00300 polymorphous appearance or may be in the form of distinct crystalline structures such as the cubic, hexagonal, monoclinic or clinorhombic and orthorhombic.

These crystalline structures, or at least some of them, result in elongate shapes which, individually or in combination, are known by the names of acicular fibres, needles, trichites, or whiskers, for which it has been established that the calcium sulphate can be found therein in a dihydrate or hemihydrate or anhydrous state.

Now, depending on the conditions of the heat treatment applied to gypsum, that is to say to calcium sulphate dihydrate, in order to produce its dehydration, the resulting calcium sulphate hemihydrate can exist in either of the varieties (heat treatment under a steam pressure at least equal to atmospheric pressure) or "p" (heat treatment at atmospheric pressure), these two varieties being distinguished from each other by differences, for example, in density variety, approximately 2.75 g/cm 3 and variety approximately 2.60 g/cm 3 in the heat of hydration, in the temperature of chemical conversion or the like.

As a general rule, the variety of the hemihydrate is the more preferred of the two varieties because, being obtained by virtue of continous dehydration processes, it is made attractive industrially by its low production cost and is consequently employed very widely for the manufacture of plaster. However, this "p" variety of the hemihydrate is not generally likely to be available with a length and a shape factor which are controlled and is consequently of no interest for some applications.

However, the variety of the hemihydrate, although generally produced by more complex and costly noncontinuous processes, is not devoid of interest because, in addition to numerous properties which are well known to the specialist, it is recognized to be capable of possessing a shapj factor (of which the p"" variety is in most cases deprived) which is advantageous tc to specific uses, since it can be obtained with the N Q 1 I I e.r^ WO 89/09187 15 PCT/EP89/00300 WO 89/09187 3 PCT/EP89/00300 appearance of an elongate crystalline structure.

The entire advantage of a structure of this kind can be perceived as soon as it is used in industrial fields where this special feature enables it to act, for example, as a reinforcement (in materials of construction such as beams, sheathing, floors, etc.) or as a filler (for example in polymeric materials) to improve their mechanical characteristics.

This is why the variety of the hemihydrate, provided with a shape factor, is at the source of many processes and products, with each process, as shown in the literature, desiring to make its contribution to the control of the conditions for obtaining one or other elongate crystalline structure and the dimensional characteristics of these structures.

A first type of process consists in heat-treating gypsum under pzessure in an aqueous medium, in order to convert it, in most cases in the presence of various substances, into calcium sulphate hemihydrate of "a" variety, provided with a shape factor.

A process of this kind, intended for the production of fibres of the trichite type and illustrated by French Patent FR 2,179,760, consists in a heat treatment under pressure of an aqueous suspension of calcium sulphate dihydrate at a temperature of between 105"C and 150 C until the said fibres of the calcium sulphate hemihydrate type of variety are formed. Once formed, these fibres are collected hot and are stabilized to prevent a destructive rehydration of the shape factor, the stabilization being effected either by a calcination or else by a treatment with a waterproofing agent such as protein hydrolysates, polycarboxylic acid polymers or other polymers (German Patents DE 2,702,097 and DE 2,702,100).

Another process relating to this first type of treatment (French Patent FR 2,415,077) recommends introducing a dispersing agent (such as tannin or tannic acid) into the aqueous gypsum suspension, with the objective of producing fibres of calcium sulphate hemihydrate of "a" a £T C" WO 89/09187 16 PCT/EP89/00300 WO 89/09187 4 PCT/EP89/00300 variety. After this addition, the treatment is carried out under steam pressure at a temperature of the order of 120 0 C for at least one hour.

At the end of this treatment the fibres of "a" variety which are obtained are separated, necessarily dried (at approximately 400"C) and are optionally calcined (at approximately 600 to stabilize them.

However while the purpose of this first type of process is essentially to obtain calcium sulphate hemihydrate of variety of elongate crystalline structure, the product thus obtained can retain its fibrous nature in the presence of water only at the cost of a costly calcination or else of a protective coating, which is never completely watertight, making it less sensitive to the rehydration phenomenon.

The penalty for a poor thermal stabilization or for incomplete protection using a coating is the rehydration of the fibre and the destruction of its elongate shape.

A second type of process, contrary to the first, proposes to employ calcium sulphate hemihydrate, and not gypsum, as starting material. This hemihydrate, dissolved in an acidic aqueous medium deliberately free from crystalline nuclei, is converted into calcium sulphate dihydrate fibres which enjoy an elongate shape factor by virtue of the slow cooling of the said solution until it reaches, but is not taken beyond, the saturation concentration permitting crystallization to be initiated.

A noncontinuous process relating to this second type is described in French Patent PR 2,377,970, which proposes the preparation, in an acidic medium, of monocrystals (whiskers) of calcium sulphate hydrate having a shape factor.

This process consists in using an acidic (pH 4) aqueous solution of calcium sulphate, which is heated to a temperature of between 75 0 C and 100°C and which is freed from all crystalline nuclei. The said hot solution is then cooled slowly over a period of more than minutes, to a temperature of between 65°C and 40"C, until T i, Q^ m WO 89/09187 -5 PCT/EP89/00300 the saturation concentration is attained. As soon as this concentration is reached, the cooling is continued until acicular monocrystals of calcium sulphate dihydrate are obtained, and these are isolated and dried according to known methods.

When implemented, a process of this kind, requires a considerable know-how, which makes its use critical, casting doubt on its industrial nature. In fact, not only must the acidic and hot solution of calcium sulphate be free from any crystalline nucleus, but also the said solution must be cooled in such a way that it is never placed in a supersaturated state, which would result in the disturbing appearance of polycrystalline aggregates (in star form) or of crystals (in flake form), at the same time as that of the desired acicular monocrystals.

Finally, a third type of process is distinguished from those mentioned above by the essential fact that it converts the initial material (calcium sulphate) isothermally into calcium sulphate dihydrate fibres which are endowed with a shape factor.

A process of this type, described in French Patent FR 2,395,965, prepares inorganic fibres which have a length of at least 200 micrometres and which have a shape factor by converting calcium sulphate isothermally in acidic or saline aqueous solutions, at a temperature which is preferably chosen around 100°C in the presence of a solid phase.

When the process is preferably performed in a saline medium, the saline solutions employed (but prepared beforehand) are highly concentrated, it being possible for the concentration to reach values as high as per cent by weight. Such concentrations can make the industrial exploitation of the said process prohibitive.

Moreover, whether practised in acidic and/or saline medium, this process can be at the source of severe corrosion phenomena, to overcome which complex and much more costly plants are required.

Furthermore, this process exhibits the major

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WO 89/09187 6 PCT/EP89/00300 disadvantage of requiring a high temperature level and, despite this, fibre preparation times which are long, since in practice they lie between 2 hours and, at best,, minutes. For, starting with a reaction solution which.

has a calcium sulphate concentration of between 5 and per cent by weight, the formation of calcium sulphate dihydrate fibres takes place (isothermally) between 90 0

C

and 100°C over a time of between several hours and minutes, whereas the formation of anhydrous calcium sulphate fibres takes place (isothermally) between 100°C and 105"C over a time of between several hours and minutes. Below the temperature of 90°C, the lengthening of the time needed to form the fibres is such that it is incompatible with an industrial process.

Finally, depending on the conditions which are chosen for its implementation, this process exhibits reaction yields which are very low (for example of the order of 24 per cent).

Thus, although the prior art has recommended, via the specialized literature, means to be employed for preparing inorganic fibres provided with a shape factor, from calcium sulphate, these means have been frequently found difficult to apply on an industrial scale. For, when used in the present processes, some of them are found to consume great quantities of heat energy (process of type and are ipso facto costly when the formation of the fibres takes place in a medium under pressure; others, quite difficult to exploit because they require conditions which are frequently critical (process of type finally, the latter ones (process of type 3) being not only high consumers of heat energy but also of time, and quite frequently resulting in poor hydroreaction yields.

SUMMARY OF THE INVENTION Conscious of the magnitude of the abovementioned disadvantages, the Applicant Company has aimed, through its investigations, at creating a process for the prepar- "^yA ation of calcium sulphate dihydrate having the appearance V of elongate crystalline structures endowed with a length ii u S. -7and a shape factor which are controlled, and at the elongate crystalline structures obtained.

According to the present invention, there is provided a process for the preparation of calcium sulphate dihydrate having the appearance of elongate crystalline structures from calcium sulphate hemihydrate, of and/or variety, characterized in that in order to endow them with a length and a shape factor (as hereinbefore defined) which are controlled, it comprises: a) the grinding and/or the selection of the said calcium sulphate hemihydrate according to particle size, the required particle size being in the range of 5 micrometres to 200 micrometres, b) the presence of at least one recrystallization 15 regulator in a proportion of 0.01 to 6.66 per cent by weight, relative to the total mass of the suspension to be formed in stage c, c) the formation by dispersion, starting with the materials from stages a and b, of an aqueous suspension at a fixed isothermal conversion temperature not exceeding 60 0

C

which has a solids content not exceeding 33.33 per cent by weight relative to the total mass of the suspension, d) maintaining the temperature at the set value for a period not exceeding 60 minutes, and e) optionally, the isolation of the elongate crystalline structures of controlled length and shape factor.

DETAILED DESCRIPTION OF THE INVENTION m Thus, the process according to the invention is

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-I a- -i WO 89/09187 20 PCT/EP89/00300 distinguished from those described in the prior art by the fact that it makes use of a source of calcium sulphate consisting of particles which are correctly sized, as a result of being ground and/or selected according to a specific particle size cut beforehand, that the isothermal conversion takes place in a nonacidic aqueous medium, in the presence of at least one recrystallization regulator, and that, finally, the isothermal conversion S

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WO 89/09187 8 PCT/EP89/00300 temperature is always lower than those announced previously.

According to the process of the invention, the source of starting material to be used consists of calcium sulphate hemihydrate of and/or variety, resulting from the heat treatment of natural or synthetic gypsums such as phosphogypsum, desulphogypsum or gypsum originating from chemical neutralizations using the means known to the specialist. Before it is used, the calcium sulphate hemihydrate is subjected to a grinding and/or selection operation so that the ground and/or selected particles correspond to the specific particle size cut defined previously.

Depending on the choice made of the particle size which the calcium sulphate hemihydrate is to be given by grinding and/or selection, the elongate crystalline structures resulting from the isothermal conversion according to the invention have size characteristics which evolve in the same direction as those of the ground and/or selected calcium sulphate hemihydrate, all the remaining conditions of the process according to the invention being kept constant, that is to say at their initial values.

In other words, the size characteristics of the elongate crystalline structures of calcium sulphate dihydrate resulting from the process according to the invention can be correlated with those of the ground and/or selected calcium sulphate hemihydrate used in the said process.

In general, the ground and/or selected particles of calcium sulphate hemihydrate have a specific particle size cut chosen in the domair defined by the limits of micrometres and 200 micrometres and preferably i.L the domain defined by the limits of 5 micrometres and 150 micrometres.

The grinding and/or the selection of the calcium sulphate hemihydrate particles are carried out by means known to the specialist, such as by the use of hammer, ball, pin or other mills, in a dry atmosphere or in a -1 W I.

WO 89/09187 9 PCT/EP89/00300 controlled atmosphere and/or of static or dynamic selectors such as, for example, those with stationary blades, rotors or the like.

Consequently, the ground and/or selected partides of calcium sulphate hemihydrate can be used, according to the process of the invention, immediately after grinding and/or the selection, or else after a more or less extended period at rest resulting in a controlled venting (or aging) of the said particles, which is well known to the specialist involved in the control of nucleation of crystallization.

The procesr according to the invention also comprises the presence of at least one recrystallization regulator which has the property of being involved in the kinetics of formation of the elongate crystalline structures and consequently in their length and their shape factor. The regulator for this crystallization may be chosen from adjuvants which have little or no influence on the pH of the aqueous suspension of ground and/or selected calcium sulphate hemihydrate, as a result of their chemically neutral nature or else which have an influence on the said pH because of their clearly alkaline nature.

When it is desirable that the recrystallization regulator should be chemically neutral in nature, in order preferably to promote the formation of elongate crystalline structures of small sizes, it is generally chosen from the group consisting of the salts of monovalent or polyvalent metals and preferably from lithium, sodium, potassium, ammonium, calcium, magnesium and aluminium halides, sulphates, nitrates, silicates and halosilicates, by themselves or in combination.

However, when it is desirable that the recrystallization regulator should be chemically alkaline in I nature, preferably to promote the formation of elongate crystalline structures of larger sizes, it is generally chosen from the group consisting of alkali metal hydroxides, ammonium hydroxides, alkaline-earth metal hydroxides, magnesium or calcium hydroxides, portland WO 89/09187 10 PCT/EP89/00300 cements, alumina cements, alkali metal aluminates and aluminosilicates, by themselves or in combination.

In some cases it may be desirable, or even preferable, to combine at least two recrystallization regulators originating from both of the abovementioned groups with the aim of still better controlling the dimensions of the elongate crystalline structures being formed, and thus of modifying their length or their shape factor.

In other cases it may be desirable, within the scope of the invention, to introduce the abovementioned calcium sulphate into the reaction medium, in the finely ground and/or selected and unvented (or unaged) dihydrated form, or else in the form of elongate crystalline structures obtained according to the invention, optionally ground and/or selected, as seeding nuclei, simultaneously or independently of the other types of recrystallization regulators.

The nature of the water involved as a liquid phase in the formation of the aqueous suspension can be of some importance with regard to the control of the process for the preparation of the elongate crystalline structures. This is why it may be desirable to employ distilled or demineralized water, although any water available may be used without major changes being involved in the settings of the other parameters of the process.

When the process according to the invention for obtaining elongate crystalline structures is implemented, an aqueous suspension of ground and/or selected calcium sulphate hemihydrate is generally prepared by dispersing a pulverulent solid phase consisting wholly or partly of the said calcium sulphate, in an aqueous liquid phase heated to a temperature such that the calcium sulphate hemihydrate suspension thus formed is at the predetermined conversion temperature, not exceeding However, in some special cases it may be adsa vantageous to prepare the said aqueous suspension by P A dispersing in the liquid phase some of the pulverulent Q \c w 4 i 1. i-;i I i ;il u-

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WO 89/09187 24 PCT/EP89/00300 WO 89/09187 11 PCT/EP89/00300 solid phase heated to a temperature such that the suspension of ground and/or selected calcium sulphate hemihydrate thus formed is at the predetermined isothermal conversion temperature.

During its formation, the aqueous suspension of ground and/or selected calcium sulphate hemihydrate is subjected to a stirring which is sufficient to prevent the settling of the said sulphate, to promote its dissolution and to facilitate the development of elongate crystalline s' uctures according to the invention.

When the pulverulent solid phase referred to above consists partly of the ground and/or selected calcium sulphate hemihydrate of the chosen size, it is made up of a physical mixture of the said sulphate with the recrystallization regulator(s).

The solid phase is then introduced into the liquid phase, which is being stirred, to be dispersed therein and to form an aqueous suspension, the said liquid phase being in a suitable enclosure provided with means of stirring and of control which are needed to hoat it to the desired temperature and to maintain the suspension which is formed at the said temperature throughout the hydrothermal reaction.

Other agents, whose functions are diverse and known to the specialist, may also be introduced into the suspension at an optimized time, such as, for example, dispersing agents which have the property of preventing the agglomeration of the elongate crystalline structures being formed and of promoting their uniform development.

The composition of the aqueous suspension thus formed, expressed in per cent by weight of the total mass, comprises: a) from 0.85 to not more than 33.33 per cent by weight, preferably from 2.5 to 17 per cent by weight, but very preferably from 4.0 to 11.0 per cent by weight of ground and/or selected calcium sulphate hemihydrate, b) from 0.01 to 6.66 per cent by weight, and preferably _ss from 0.02 to 2.5 per cent by weight of the recrystallizac tion regulator, i g

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WO 89/09187 12- PCT/EP89/00300 c) from 0 to 33.3 per cent by weight and preferably from 0 to 0.85 per cent by weight of various known agents.

d) from 98.75 to at least 66.66 per cent by weight of aqueous phase.

The weight ratio of the liquid phase to the solid phase must advantageously lie in the range from 2 to but preferably in the range from 5 to 30, and very desirably in the range from 8 to The recrystallization regulator(s), as already stated, is (are) preferably introduced into the suspension in the form of a solid mixture with calcium sulphate hemihydrate, it being possible for the quantity of the said regulator to vary from 0.2 to 20 per cent by weight of the said solid mixture.

However, in some cases, the recrystallization regulator may be employed in a dissolved form in an aqueous solution when the mechanism of action of the said regulator requires its conversion into solution.

Similarly, in the case where agents which have various and known functions are used, they may be introduced into the abovementioned solid mixture or into the aqueous phase, in a quantity which can go up to not more than 10 per cent by weight of the solid mixture.

The suspension is generally heated to an isothermal conversion temperature which may not exceed 60°C and which is chosen in the broad range from 5 0 C to 55 0 C and preferably in the narrower range of 10 0 C to 50 0

C.

Depending on the size characteristics of the desired elongate crystalline structures, the isothermal conversion temperature is generally chosen within the low values of the abovementioned range in order to promote the formation of small elongate crystalline structures, while this temperature is generally chosen within the high values of the said range to promote the formation of large elongate crystalline structures, the said temperature being chosen between these extremes to promote the formation of elongate crystalline structures of intermediate lengths.

N WO 89/09187 13 PCT/EP89/00300 The suspension of ground and/or selected calcium sulphate hemihydrate and of the recrystallization regulator, thus formed, is kept stirred and at the chosen temperature for the time needed for the hydroreactional formation and aging of the elongate crystalline structures according to the invention, this time not exceeding minutes, being generally chosen shorter than minutes and preferably set between 30 minutes and 3 minutes in industrial operation.

In practice, the treatment temperature is chosen within the abovementioned domain so as to make the hydroreactional conversion of the ground and/or selected calcium sulphate hemihydrate into the elongate crystalline structure of calcium sulphate dihydrate take place at a minimum temperature of formation and in a time which is desirably as short as possible.

Consequently, and contrary to the teaching of the prior art recommending acidic and/or saline hydroreaction media and high temperatures (90 0 C to 100 0 C) to obtain fast kinetics and a reasonable reaction time (at best between 10 minutes and 30 minutes), it has unexpectedly been found that it is possible to form elongate crystalline structures endowed with a length and a shape factor which are controlled, by virtue of the use of a hydroreactional medium changing from a neutral pH towards an alkaline pH, maintained at a constant and well-known low temperature, and this with an excellent yield despite a very short reaction time. The formation of the said structures consequently takes place through the intermediacy of the dissolved phase of ground and/or selected calcium sulphate hemihydrate present in the suspension, the said sulphate entering the solution only in step with the appearance of the elongate crystalline structures according to the invention.

At the end of the hydroreactional conversion, the elongate crystalline structures of calcium sulphate dihydrate are obtained in an excellent conversion yield a of the calcium sulphate hemihydrate employed.

The crystallized solid phase thus obtained may be

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*v i 1 WO 89/09187 14 PCT/EP89/00300 kept in aqueous suspension without any damage to the elongate crystalline structures of controlled length and shape factor, or else may be isolated from the said suspension.

When the crystallized solid phase thus obtained is not separated from the aqueous phase at the end of the hydroreaction conversion, the aqueous suspension of the said elongate crystalline structures according to the invention may be used industrially as it is or may be subjected, before this use, to an adjustzment of its solids content, by dilution or by concentration. In the latter case, the increase in the solids content of the said suspension can be produced by any known means such as evaporation, settling, electroconcentration, or the like.

When the crystallized solid phase thus obtained is separated from the liquid phase, optionally hot, this separation can be carried out by any known means such as settling, filtration under pressure or under vacuum, the use of hydrocyclones, electrophoresis, centrifuging, etc.; it is then dried according to any one of the processes known to the specialist.

The aqueous phase obtained during the liquidsolid separation may be recycled to the stage of the formation of the aqueous suspension of ground and/or selected calcium sulphate hemihydrate, with the addition thereto of the quantity of water needed to make up the losses during the separation and with the adjustment, by additions, of the concentrations in the case of the various agents involved or otherwise in the process according to the invention.

The solid phase collected after the drying stage is made up of elongate crystalline structures, well individualized and endowed with a length and shape factor which are controlled. These elongate crystalline structures require neither thermal stabilization nor coating, as recommended by the prior art, and when brought into contact with water saturated with calcium sulphate they retain all their characteristics and in particular their 1|i Ti 0., WO 89/09187 15 PCT/EP89/00300 length and their shape factor.

The elongate crystalline structures produced in accordance with the process according to the invention consist of crystals which are individual or associated at most in threes along their long axis by partial overlapping. The said structures have, according to the requirement, a mean length of between 2 micrometres and 200 micrometres, a narrow length scatter and a controlled shape factor (ratio of the mean length to the longest mean apparent diameter) which can vary as required in the range 5/1 to 50/1.

The elongate crystalline structures of calcium sulphate dihydrate may, if it is found desirable for certain uses, be converted thermally into elongate crystalline structures of calcium sulphate hemihydrate when the heat treatment is carried out at a temperature of between 90°C to 250 0 C, or else insoluble anhydrites when the heat treatment exceeds the temperature of 250"C (dead-burnt or anhydrite II) or of 1200°C (anhydrite I), the said elongate crystalline structures retaining at the end of the heat treatment all the size characteristics possessed by the dihydrate structures before the heat treatment, and in particular their length and their shape factor.

The elongate crystalline structures according to the invention may be employed in many industrial fields, as long as their controlled length and shape factor allow them to act, for example, as a reinforcement in some materials or as a filler in other materials in order to improve their mechanical, optical, thermal, surface appearance or other characteristics.

Thus, since they possess qualities as diverse as lightness in weight, whiteness, opacity, gloss and ease of dispersion, the elongate crystalline structures according to the invention are more especially intended for fields of application as diverse as those of paper, of paint and of plastics. The advantage and the scope of G At/ the invention will be better understood by virtue of the examples which illustrate it and which compare it with U:4 T i" WO 89/09187 16 PCT/EP89/00300 the known art.

EXAMPLE 1 (case illustrated by Figure 1) In order to make manifest the technical progress acquired by the crystalline structures according to the invention and to set out properly the creative step linked with the process resulting in the said structures, this example is aimed at illustrating the prior art by the preparation of crystalline structures having a disordered shape factor when at least one of the conditions according to the invention is not met.

With this idea in mind, a number of experiments (numbers 1.1 to 1.4) were carried out by using as a source of raw material calcium sulphate hemihydrate containing no adjuvants (of "Prestia Selecta" trademark, marketed by Platres Lafarge) produced by heat treatment of natural gypsum.

All the experiments concern the formation of aqueous suspensions (in distilled water) of the said calcium sulphate hemihydrate, in variable concentrations, at variable but accurate temperatures in the case of each one, the said formed suspensions being free from recrystallization regulator, All the experiments were carried out according to the same experimental criteria: excluding those referred to above, and in the same apparatus, so as to make the results obtained comparable.

In the case of each experiment, the aqueous suspension of calcium sulphate hemihydrate was prepared by introducing the solid phase into the hot liquid phase contained in a 1300-litre vessel equipped with means of stirring (which had a circumferential velocity capable of being varied from 10 to 20 metres per second) and suitable heating means allowing first the aqueous phase alone and then the suspension formed to be maintained at the chosen temperature.

The measured recrystallization time corresponded to the virtually complete conversion of calcium sulphate hemihydrate into calcium sulphate dihydrate.

iolP j ii 4: .JJL hJ L.W =I=AL OJ CILLA VU UILL..L WO 89/09187 17 PCT/EP89/00300 The crystalline structures obtained within the scope of each experiment, isolated from the aqueous phase and then dried, were subjected to an examination in a scanning electron microscope (magnification 500). The characteristics and the results pertaining to each experiment have been collated in Table A, while the crystalline structures obtained according to one of the experiments belonging to the prior art (experiment 1.2) are the subject of Figure 1.

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i TABLE A Experiment Preliminary CaSO,. 1/2H.0 suspension No. grinding and/or selection of CaSO 4 l/2H 2 0 -Recrystallization Isothermal Crystal- CaSQ,.l1/2H 2 0 in by regulator in by conversion lization time weight weight temeerature in min in C 1.1 no 16.67 0 45 1.2 no 9.09 0 45 1.3 no 6.25 0 45 1.4 no 4.76 0 45 0D qCO rr~~ WO 89/09187 19 PCT/EP89/00300 When examined microscopically, the crystalline structures obtained within the scope of each experiment are seen to be a jumble of needles of all lengths and of all diameters, of twins and/or of agglomerates of many sizes, giving the shape factor a disordered nature (disordered crystallization).

According to Figure 1, which illustrates the known art, and which relates to the photographic observation in the scanning electron microscope of the crystalline structures obtained under the conditions described in the case of experiment 1.2, the disordered nature of the said structures is clearly visible in the region observed, since it is possible to see needles of very short length and of very small diameter, needles of greater length but of the same diameter as needles of substantial length and of greater diameter, agglomerates of needles, twins and "spearheads" EXAMPLE 2 (case illustrated by Figure 2) This example is an illustration of the influence of the size characteristics of the particles of calcium sulphate hemihydrate containing no adjuvant on those of the elongate crystalline structures formed according to the process of the invention.

In practice, calcium sulphate hemihydrate resulting from the heat treatment of gypsum (marketed under the trademark "Prestia Selecta" by Platres Lafarge) is subjected to a grinding and/or selection operation so that the particles have, within the scope of three experimemnts, particle sizes which are well defined by the cuts: 2.1: 16-micrometre particle size cut 2.2: 60-micrometre particle size cut 2.3: 120-micrometre particle size cut The various grinding and selection operations were performed in the same mill to make the results comparable.

SThe apparatus employed was a hammer mill of the Forplex No. 2 type (marketed by the Forplex company) WO 89/09187 20 PCT/EP89/00300 equipped with a recirculating dynamic selector SD1 allowing the particles to be selected according to their size.

The mill-selector system was fed with calcium sulphate hemihydrate at a rate of 200 to 300 kilograms per hour and made it pqssible to obtain the desired particle size cuts as a function of the settings show below.

Particle size cut (pm) 16 60 120 Speed of rotation of the mill (rev/min) 6,000 5,400 2,400 Speed of rotation of the selector (rev/min) 3,000 1,350 640 At the end of the grinding, the ground and selected calcium sulphate hemihydrate was treated with an adjuvant in the form of a recrystallization regulator (Portland Cement CPA 45 marketed by Ciments Lafarge) in a proportion of 5.26 per cent by weight relative to the calcium sulphate hemihydrate, the mixture forming the pulverulent solid phase intended to be introduced into the preheated liquid phase.

The solid phase was consequently introduced into a crystallization vessel equipped with means of stirring and of controlling the temperature of the liquid phase which it contained previously. The liquid phase consisting of distilled water was maintained at the isothermal conversion temperature set at 45 0 C. During its formation, the hot aqueous suspension of ground and selected calcium sulphate hemihydrate was kept stirred sufficiently to prevent the said sulphate from settling, to promote its dissolution and to facilitate the development of elongate crystalline structures.

In the case of each experiment, the composition of the aqueous suspension was the following, expressed in per cent by weight: ground and selected calcium sulphate hemihydrate 8.64 recrystallization regulator 0.45 (Prtan C i I rootinof526pr et ywegt'eITv t h WO 89/09187 21 PCT/EP89/00300 distilled water 90.91 The suspension thus formed was kept stirred and at a temperature of 45°C during the period of recrystallization of calcium sulphate dihydrate, which was minutes.

Two minutes after the introduction of the solid phase and throughout the isothermal conversion of the said hemihydrate, the pH of the suspension changed in the range 11.5 to 12.5.

At the end of the isothermal conversion, the elongate crystalline structures were collected by filtering the aqueous phase and were then dried at a temperature of approximately 45 0 C and subjected to a microscopic inspection using a scanning electron microscope (magnification 500) in order to determine their length, diameter and shape factor.

The results pertaining to each experiment and the characteristics of the elongate crystalline structures obtained have been collated in Table B below.

When examined microscopically, the elongate crystalline structures obtained within the scope of each experiment show the existence of needles which are uniform in length and diameter, which are well individualized and which are endowed with a controlled shape factor.

Figure 2, which illustrates the invention, is the photographic inspection, using a scanning electron microscope with a magnification of 500, of the crystalline structures obtained in the conditions of experiment 2.3. Comparison of Figures 2 and 1 shows the controlled nature of the process according to the invention alongside the disorder resulting from a process of the prior art.

i TABLE B Experi- Preliminary CaSO 4 I/2H 2 0 aqueous suspension Elongate crystalline structures ment grinding and No. selection of Concentration Recrym- Isothermal Recrys- Length in Diameter in Shape CaSO 4 ,l/2H 2 0 in X by tallization conversion tallization micrometres micrometres factor cut in weight regulator in temperature time in micrometres Z by weight in *C minutes 2.1 18 8.64 0.45 45 30 25 -35 1.0 -1.5 25 2.2 60 8.64 0.45 45 30 35 -45 1.8 2.2 19.5 20.5 2.3 120 8.64 0.45 45 30 60 80 3.0 4.0 WO 89/09187 23 PCT/EP89/00300 EXAMPLE 3 (case illustrated by Figure 3) This example is an illustration of the influence of the storage time of calcium sulphate hemihydrate before and/or after its grinding and it selection on the size characteristics of the elongate crystalline structures formed according to the process of the invention.

In practice, calcium sulphate hemihydrate containing no adjuvant, originating from heat treatment of gypsum (marketed under the trademark "Prestia Selecta" by Platres Lafarge) was subjected to a grinding-selection operation, preceded or followed by a more or less extended storage period before being subjected to the other stages of the process of the invention.

The various experiments were as follows: 3.1: "Selecta" calcium sulphate hemihydrate originating from heat treatment of gypsum was subjected without preliminary storage to a grinding-selection operation giving a 16-micrometre particle size cut, and was then subjected to the other stages of the process immediately afterwards.

3.2: "Selecta" calcium sulphate hemihydrate originating from heat treatment of gypsum was subjected after a storage of three months to a grinding-selection operation giving a 16-micrometre particle size cut and was then subjected to the other stages of the process immediately afterwards.

3.3: "Selecta" calcium sulphate hemihydrate originating from heat treatment of gypsum was subjected without preliminary storage to a grinding-selection operation giving a 16-micrometre particle size cut and was then subjected to the other stages of the process after a post-grinding storage of three months.

3.4: "Selecta" calcium sulphate hemihydrate originating from heat treatment of gypsum was subjected after a storage of three months to a grinding-selection operation giving a 16-micrometre particle size cut, and was then subjected to the other stages of the process immediately afterwards.

The various grinding-selection operations were V

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1 WO 89/09187 24 PCT/EP89/00300 carried out in the Forplex mill already used in Example 2, using the same operating procedure.

At the end of the grinding-selection operation, followed by the storage period or otherwise, the calcium sulphate hemihydrate was treated with adjuvants using two recrystallization regulators: lime and a mixture of gypsum and starch described in French Patent FR 2,22P,735). The proportion of recrystallization agent is betwejn 0.8 and 1.5 per cent by weight relative to the calcium sulphate hemihydrate employed, this mixture being the pulverulent solid phase intended to be introduced into the preheated liquid phase.

The solid phase thus formed was introduced into the crystallization vessel equipped with means of stirring and of controlling the temperature of the liquid phase which it contained previously.

The liquid phase consisted of distilled water maintained at the isothermal conversion temperature, set at 50 0

C.

During its formation, the hot aqueous suspension of the abovementioned solid phase was kept stirred sufficiently for the same reasons as those set out in Example 2.

4 In the case of each experiment, the composition of the aqueous suspension, expressed in per cent by weight, was as follows: ground and selected calcium sulphate hemihydrate from 9.02 to 8.95 recrystallization regulator from0.07 to0.14 distilled water 90.91 The suspension thus formed was kept stirred and at a temperature of 50 C during the calcium sulphate dihydrate recrystallization period, which was 15 minutes.

After being collected by filtration, the elongate crystalline structures were dried at a temperature of approximately 50"C and subjected to a microscopic inspection using a scanning electron microscope (magnification 500) in order to determine their length, diameter and shape factor.

I J1 WO 89/09187 25 PCT/EP89/00300 The results linked with each experiment and the characteristics of the elongate crystalline structures obtained have been collat;ed in Table C, which shows that storage after grinding is the only one to have an influence.

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0 TABLE C Experi- Grinding and selection of CaSO 4 *1/2H 2 0 CaSO 4 .1/2H20 aqueous suspensionl Elongate crystalline structures mient No. Shape__ Storage Grinding Storage Concen- Recrys- IISO- Recrys- Length Diameter Sae before end selec- after tration talliz-a- thermal talliza- in micro- in micro- factor tion cut in in Z by tion regu- conver- tion metres metres micrometres weight lator in x aion tem- time in by weight perature min relative to in *C the sulphate CaD 3.1 no 16 no 8.95 50 15 20 -40 2 -5 10 -8 BHA 1 CaD 3.2 3 months 16 no 8.95 50 15 20 -40 2 -5 10- 8 BMA 1 CaD 3.3 no 16 3 months 9.02 50 15 55 -65 4 -6 13.75 BMA 0.1 10.8 CaO 3.4 3 months 16 no 9.02 50 15 30 -50 2 -4 15 -12.5 BMA 0.1 00

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WO 89/09187 27 PCT/EP89/00300 When examined microscopically, the elongate crystalline structures obtained within the scope of each experiment show the existence of needles which are uniform in length and diameter, which are well individualized and which are endowed with a length and a shape factor which are controlled.

Figure 3, which illustrates the invention, is the photographic inspection using the scanning electron microscope (magnification 500) of the crystalline structures obtained in the conditions of experiment 3.2.

Comparison of Figures 1 to 3 shows the controlled nature of the process, which permits the production of elongate crystalline structures of desired length and shape factor (Figures 2 and while the prior art (Figure 1) results in the production of disordered crystalline structures.

EXAMPLE 4 (case illustrated by Figure 4) This example illustrates the influence of the type and/or of the concentration of the recrystallization regulator employed. To this end, calcium sulphate hemihydrate containing no adjuvant, from the same source as in Example 3, was subjected to a grinding and selection operation according to a specific particle size cut not exceeding 120 micrometres, in the abovementioned Forplex mill.

At the end of the grinding and of the selection, the calcium sulphate hemihydrate was treated with an adjuvant in the form of a recrystallization regulator pertaining to each of the experiments 4.2 to 4.10 and in given concentrations, expressed in per cent by weight relative to the total mass.

In the case of experiment 4.1, the recrystallization regulator (aqueous ammonia) was introduced directly into the liquid phase before the dispersion of the ground and selected calcium sulphate hemihydrate.

Experiments 4.1 to 4.3 concern the use of various recrystallization regulators (aqueous ammonia, Portland Cement CPA 45, mixture of lime and of the latter VZ being described in French Patent FR 2,228,735).

w t WO 89/09187 28 PCT/EP89/00300 Experiments 4.4 to 4.6 concern the use of various concentrations of ground and selected gypsum as a recrystallization regulator, in a particle size which has a 16micrometre cut.

Experiments 4.7 to 4.10 concern the use of various concentrations of Portland Cement CPA 45 as recrystallization regulator.

All the experiments were performed in the same apparatus as previously to make the results obtained comparable, and according to the experimental criteria set out in Table D below together with the results pertaining to each experiment.

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i .Nr O Experi- Prelimi- Recrystallization CaSO 4 .1/2H20 Elongate crystalline structures ment No. nary grind- regulator aqueous suspension o ing and to selection Type Concentration Concen- losthermal Recrys- pH at 2 Length in Diameter Shape of CaS0 4 in by tration conversion talliza- min after micro- in micro- factor 00 1/2H20 weight of in by temperature tion time the forma- metres metres the suspension weight in 'C in min tion of Cut in the susaicro- pension metres 4.1 120 pure 0.32 8.77 45 15 11.6 15 30 1 2 aqueous ammonia 4.2 120 Portland 0.45 8.64 45 30 11.5 60 80 3 4 cement CPA 4.3 16 CaO 0.045 8.95 45 15 11.7 20 40 2 5 10 8 BMA 0.091 4.4 16 Ground 0.27 8.82 25 15 8 5 10 1 and (twins) 5 selected 16 gypsum 0.45 8.64 25 15 8 7 10 1 7 below 4.6 16 16 a 0.91 8.18 25 15 8 5 7 0.5 10 14 4.7 16 Portland 0.27 8.82 45 30 11.5 30 50 1.5 3.5 20 14 0 4.8 16 0.45 8.64 45 30 11.5 40 60 1 1.5 40 3 cement 4.9 16 0.91 8.18 45 30 11.5 40 50 2 3 20 16 tv 0 o 4.10 16 CPA 45 1.36 7.73 45 30 11.5 40 60 2 3 20 tO 1 -1 r- a at -1

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WO 89/09187 30 PCT/EP89/00300 When examined microscopically, the crystalline structures obtained within the scope of each experiment show the existence of needles which are uniform in length and diameter and which are well individualized and endowed with controlled length and shape factor. In particular, the fact of changing the type of recrystallization regulator, or else changing the concentration of a particular agent, makes it possible to obtain elongate crystalline structures whose length changes within the range of 5 micrometres to 80 micrometres, depending on the desired length and shape factor.

Figure 4, which illustrates the invention, is the photographic inspection using the scanning electron microscope (magnification 500) of the crystalline structures obtained in the conditions of experiment 4.6.

Comparison of Figures 2, 3 and 4 shows the completely controlled nature of the process according to the invention, which makes it possible to produce elongate crystalline structures of desired and controlled length and shape factor.

EXAMPLE This example illustrates the influence of the temperature of isothermal conversion. To this end, calcium sulphate hemihydrate containing no adjuvant, from the same source as in the preceding examples, was subjected to a selection operation according to a specific 120-micrometre particle size cut, in the abovementioned Forplex selector.

At the end of the selection, the calcium sulphate hemihydrate was treated with a recrystallization regulator as an adjuvant. All the experiments were performed in the same apparatus as previously, to make the results obtained comparable. The experimental criteria are shown in Table E below together with the results pertaining to each experiment.

Experiments 5.1 to 5.3 concern the influence of the temperature of isothermal conversion on the dimeni sions of the elongate crystalline structures, the other *f characteristics of the process being kept identical from

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c y I 17Us~;r; WO 89/09187 31 PCT/EP89/00300 one experiment to the next.

Experiment 5.4 concerns the influence of the temperature of isothermal conversion, chosen particularly low and applied to ground and selected calcium sulphate hemihydrate according to a specific 16-micrometre particle size cut.

The crystalline structures obtained within the scope of each experiment show, when inspected using the scanning electron microscope (magnification 500), the existence of needles which are uniform in length and in diameter, which are well individualized and which are endowed with a length and shape factor which are chosen in advance and controlled, no other crystalline structures being observed, such as a twin or spearhead, except in the case of experiment 5.3, where the temperature used was outside the limits of the invention.

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TABLE E Experi- Preli- Recrystallization CatSO 4 /2H 2 0 Elongate crystalline structures ment so. minary regulator aqueous suspension selection of CaSO 4 Concen- Concen- Isothermal Recrys- PH at Length in Diameter in IShape 1/2Z20 Type tration tration conversion talliza- 2 min micro- micro- factor in by in by temperature tion time after metres metres Cut in weight weight in 'C in min the formicro- relative mation of metros to the the sussuspension pension Portland 5.1 120 0.45 6.64 45 30 11.5 60 -80 3 -4 5.2 120 0.45 8.64 60 30 11.5 80 -150 8 -10 10-15s Cement 5.3 120 CPA 45 0.45 8.64 80 30 11.5 rods -poor flakes 5.4 16 0.91 8.18 10 5 8 2 -3 0.4-0.5 5- 6 (grinding Ground and and selection) selected gypsum r. I, r- I i WO 89/09187 33 PCT/EP89/00300 EXAMPLE 6 This example illustrates the influence of the nature of the water involved as a liquid phase in the formation of the aqueous suspension.

In practice, calcium sulphate hemihydrate, containing no adjuvant, produced from desulphogypsum at the Ottmarsheim (France) works of Platres Lafarge, was subjected to a grinding and selection operation making it possible to obtain a specific 16-micrometre particle size cut, in the abovementioned Forplex mill.

At the end of the grinding and of the selection, the calcium sulphate hemiihydrate was treated with the same recrystallization regulators (CaO BMA) as adjuvants, in identical concentrations, expressed in per cent by weight relative to the total mass of the suspension.

This mixture was the pulverulent solid phase intended to be dispersed in the preheated liquid phase.

This solid phase was then introduced into the liquid phase contained in the crystallization vessel, equipped with means of stirring and of controlling the isothermal conversion temperature, set at Two experiments (6.1 and 6.2) were performed in succession.

Experiment 6.1 concerned the use as a liquid phase of a drinking water whose analysis resulted in a hardness determination titre (HT) of 36.8 and in a total alkalinity determination titre (TAT) of 32.6, measured according to NF standards T 90 003 and T 90 036.

Experiment 6.2 concerns the use of a distilled water as liquid phase.

In the case of each experiment, the composition of the aqueous suspension expressed in per cent by weight, was as follows: ground and selected calcium sulphate hemihydrate 9.020 recrystallization regulators CaO BMA 0.072 H water 90.908 WO 89/09187 34 PCT/EP89/00300 Each suspension thus formed was kept stirred and at the isothermal conversion temperature (25°C) during the calcium sulphate dihydrate recrystallization period, which was twenty minutes. After having been collected and dried, the elongate crystalline structures were subjected to a visual inspection using the scanning electron microscope at a magnification of 500, in order to determine their length, diameter and shape factor.

The results linked with each experiment and the characteristics of the elongate crystalline structures obtained have been collated in Table F.

Examination of the results shows the existence of needles which are uniform in length and diameter, and well individualized, whose differences are not highly significant when a hard water is substituted for distilled water.

i L11 j.

TIABLE F uxpon- Grinding Recrystallization CaSO 4 l/2H 2 0 Elongate crystalline structures Mw~t No. and regulator aqueous suspension selection Of CaSO 4 Concen- Concen- Isothermal Recrys- PH at Length in Diameter in Shape 1/2H 2 0 Type tration tration conversion talliza- 2 min micro- micro- factor in by in by temperature tion time after metres metres Cut in weight weight in 1C in min the formicro- relative -mation. of metres to the the sussolution pension 6.1 16 CaO 0.045 9.02 25 20 11.7 18-20 1.5 12- 13 BHA 0.027 6.2 16 CaO 0.045 9.02 25 20 11.7 15-17 1,3 11.5 -13 BM& 0.027 Lt ~?4

A

WO 89/09187 36 PCT/EP89/00300 EXAMPLE 7 This example is an illustration of the influence of the concentration of ground calcium sulphate hemihydrate in the aqueous suspension. To this end, calcium sulphate hemihydrate containing no adjuvant was subjected to a grinding operation, so as to make 96% of the particles smaller than 120 micrometres, in the abovementioned Forplex mill.

At the end of the grinding, the calcium sulphate hemihydrate was treated with an adjuvant in the form of a recrystallization regulator (ground and selected gypsum according to a specific 16-micrometre cut).

All the recrystallization operations were performed in the same apparatus as previously, to make the results obtained comparable. The experimental criteria employed have been shown in Table G below, together with the results pertaining to each experiment.

Experiments 7.1 to 7.4 concern the influence of the concentration of ground calcium sulphate hemihydrate, the other characteristics being kept constant. It has not been possible to keep the recrystallization time constant from one experiment to the next because of the change in the concentration of calcium sulphate hemihydrate in the initial suspension.

The crystalline structures obtained within the scope of each experiment show, when inspected using the scanning electron microscope (magnification 500), the existence of uniform, well individualized needles whose lengths, diameters and shape factors are detailed in Table G. Little variation in the length of the needles is observed within the range of the hemihydrate concentrations tested. However, it appears that the lower the concentration of the suspension, the wider the scatter in the size of the said needles.

1

TABLE

Experi- Preli- Recrystallization CaSO 4 .l/2H 2 0 Elongate crystalline structures Mont No0. minary regulator aqueous suspension grinding Of CaSO 4 Concen- Concen- Isothermal ReCryB- PH at Length in Diameter in Shape l/2H 2 0 Type tration tration conversion talliza- 2 min micro- micro- factor in X by in Z by temperature tion tim~e after metrea metres G6Z of weight weight in 'C in mini the forthe par- relative mation of ticles to the the susmicro- suspension pension 7.1 120 )Ground 1.67 31.67 45 5 8 5 -8 1 -1.5 5 -5.3 3and 7.2 120 seleoc- 0.45 8.64 45 10 8 5-10" 0.5 1.5 10 6.6 3ted 7.3 120 316-i 0.16 3.06 45 20 8 5- 15 0.5 -1.5 3gypsum 7.4 120 0.07 1.34 45 30 8 5 -15 0.5 -2 10

Claims (3)

1. Process for the preparation of calcium sulphate dihydrate having the appearance of elongate crystalline structures from calcium sulphate hemihydrate, of and/or variety, characterized in that in order to endow them with a length and a shape factor (as hereinbefore defined) which are controlled, it comprises:- a) the grinding and/or the selection of the said calcium sulphate hemihydrate according to particle size, the required particle size being in the range of 5 micrometres to 200 micrometres, b) the presence of at least one recrystallization regulator in a proportion of 0.01 to 6.66 per cent by weight, relative to the total mass of the suspension to be formed in stage c, c) the formation by dispersion, starting with the S materials from stages a and b, of an aqueous suspension at a fixed isothermal conversion temperature not exceeding 600C which has a solids content not exceeding 33.33 per cent by S weight relative to the total mass of the suspension, d) maintaining the temperature at the set value for a period not exceeding 60 minutes, and e) optionally, the isolation of the elongate crystalline structures of controlled length and shape factor.

2. Process of preparation according to claim 1, characterized in that the calcium sulphate hemihydrate has a particle size in the range of 5 micrometres to 150 SAQ micrometres. S[ T> C-i -39- 3) Process of preparation according to either of claims 1 and 2, characterized in that the grinding and/or the selection of calcium sulphate hemihydrate is carried out preferably by the use of hammer, ball or pin mills, in a dry or controlled atmosphere, using static or dynamic means of selecting particle size. 4) Process of preparation according to any one of claims 1 to 3, characterized in that the ground and/or selected calcium sulphate hemihydrate is used immediately after the grinding and/or the selection. Process of preparation according to any one of claims 1 to 3, characterized in that the ground and/or selected S. calcium sulphate hemihydrate is used after controlled aging of the ground and/or selected particles. 6) Process of preparation according to any one of claims 1 to 5, characterized in that the recrystallization regulator is chemically neutral in character and is preferably chosen from the group consisting of lithium, sodium, potassium, ammonium, calcium, magnesium and aluminium halides, sulphates, nitrates, silicates and halosilicates, taken by themselves or in combination. 7) Process of preparation according to any one of claims 1 to 5, characterized in that the recrystallization regulator is chemically alkaline in nature and is preferably chosen I from the group consisting of alkali metal hydroxide, ammonium hydroxide, alkaline-earth metal hydroxides, magnesium hydroxide, calcium hydroxide, portland cements, alumina I i i.

3 i -39a- cements, and alkali metal aluminates or aluminosilicates, by themselves or in combination. 8) Process of preparation according to any one of claims 1 to 7, characterized in that the recrystallization regulator consists of the combination of at least two crystallization regulators originating from groups of neutral nature and of alkaline nature. 9) Process of preparation according to any one of claims 1 to 8, characterized in that the recrystallization regulator is introduced into the suspension in the form of a solid mixture with ground and/or nelected calcium sulphate hemihydrate. Process of preparation according to any one of claims 1 to 8, characterized in that the I I i, ANNEXE AU RAPPORT DE RECHERCHE INTERNATIONALE I n WO 89/09187 40 PCT/EP89/00300 recrystallization regulator is employed in a dissolved form in aqueous medium. 11) Process of preparation according to any one of claims 1 to 10, characterized in that the aqueous suspen- sion of ground and/or selected calcium sulphate hemihy- drate is preferably formed using distilled or deminera- lized water. 12) Process of preparation according to any one of claims 1 to 11, characterized in that the aqueous suspen- sion of ground and/or selected calcium sulphate hemihy- drate is prepared by dispersing the solid phase in the aqueous liquid phase heated to the set isothermal conver- sion temperature not exceeding 13) Process of preparation according to any one of claims 1 to 8, characterized in that a seeding initiator is introduced into the reaction medium, the said in- itiator consisting of optionally ground and/or selected calcium sulphate dihydrate. 14) Process of preparation according to any one of claims 1 to 13, characterized in that the formation of the aqueous suspension by dispersion employs, relative to the total mass: a) from 0.85 to not more than 33.33 per cent by weight, preferably from 2.5 to 17.0 per cent by weight, but very preferably from 4.0 to 11.0 per cent by weight of ground and/or selected calcium sulphate hemihydrate, b) from 0.01 to 6.66 per cent by weight, and preferably from 0.02 to 2.5 per cent by weight of the recrystalliza- tion regulator, 30 c) from 0 to 33.3 per cent by weight and preferably from 0 to 0.85 per cent by weight of various known agents. d) from 98.75 to at least 66.66 per cent by weight of aqueous phase. 15) Process of preparation according to one of claims 1 to 14, characterized in that the isothermal conversion temperature is generally chosen from the wide range of RAAG 5°C to 55 0 C and preferably from the narrower range of 0 C to A To U BjJ i -41- 16) Process of preparation according to any one of claims 1 to 15, characterized in that the suspension of ground and/or selected calcium sulphate hemihydrate is kept stirred and at the isothermal conversion temperature for a period which is generally shorter than 45 minutes and preferably between and 3 minutes. 17) Process of preparation according to any one of claims 1 to 16, characterized in that the elongate crystalline structures originating from the isolation stage e) are subjected to a heat treatment. 18) Process of preparation according to claim 17, characterized in that the heat treatment is a drying operation. 19) Process of preparation according to claim 17, characterized in that the heat treatment is carried out at a S' temperature of between 90°C and 2500C. 20) Process of preparation according to claim 17, characterized in that the heat treatment is carried out at a b temperature of at least 2500°C to obtain anhydrite II, and 9 of at least 1,2000°C to obtain anhydrite I. 21) Elongate crystalline structures prepared by the process of any one of claims 1 to 20, characterized in that they have, according to requirement, a mean length of between 2 micrometres and 200 micrometres and a shape factor within the range of 5/1 to 50/1. I 22) Use of an elongate crystalline structure according to C.) -0 I A claim 21, in solid form or in the form of a dispersion in a liquid phase, in the preparation of paper, paint or plastics. Dated this 11th day of December, 1991. LAFARGE COPPEE S.A. By their Patent Attorneys: PETER MAXWELL ASSOCIATES A I ;r i: r: r I I a :i ii r r