IE42025B1 - Method for producing calcium sulphate hemihydrate and/or anhydrite - Google Patents

Abstract

1529435 Calcium sulphate hemihydrate and/or anhydrite FISONS Ltd 21 Oct 1975 [26 Oct 1974] 46413/74 Headings C1A Calcium sulphate hemihydrate and/or anhydrite crystals are obtained by the conversion of gypsum crystals of which at least 50 weight per cent have a length to breadth ratio of from 1:1 to 1:3, and which have a mean thickness of from 3 to 10 micrometres, and wherein initially at least 15%, typically 40 to 80%, by weight of the crystals are in the form of twinned crystals. The gypsum crystals may be those obtained in the process of U.K Specification 1,520,273. The conversion is preferably achieved by wet or dry calcination. The resulting hemihydrate or anhydrite crystals may be slurried in water for use in building products, e.g. plaster, plaster blocks and board. The addition of fibres prior to shaping the product is described and the products obtained possess high strength and fire resistance.

Description

British Patent Specification No. 1,520,273 . describes an improved process for producing phosphoric acid in which a phosphatic material is treated with a mineral acid, calcium sulphate is precipitated and separated off as calcium sulphate dihydrate which process is improved by incorporating a source of aluminium and a source of silica into the phosphatic feed material and/or into the phosphoric acid reaction system so that the weight ratios of total Al^Og.-P^,. and total reactive SlO2;total F in the reaction mixture at the point of initial precipitation of the calcium sulphate dihydrate have values within the range 1:75 to 1:10 and from 0.1:1 to 1:1 respectively.

A preferred method of operation is to add an aluminium silicate (notably a ball clay) to the phosphoric acid reaction system preferably in such amount as to raise the A12O3:P2Os weight ratio either to within the range 1:66 to 1:15 (notably 1:55 to 1:30) or, if already within that range, to a higher value within the range? and to raise the reactive Si02*F weight ratio to within the range 0.5:1 to 0.1:1 (notably 0.5:1 to 0.14:1). For convenience the process described in the complete specification relating to the above mentioned specification for the production of phosphoric acid and gypsum crystals will be denoted herein as the 'above improved process'.

We have found that the gypsum crystals produced by the above improved process are of particular use in the production of plaster articles, e.g. building products, notably plaster products (e.g. plaster board and the like) and building blocks, since the gypsum crystals are usually produced in a form which is readily washed and hence may be purified for use. This is in contrast to the gypsum crystals produced by other processes which may be difficult to wash or otherwise purify and are therefor unsuitable for commercial use in making building products. Furthermore, we have surprisingly found that the gypsum crystals produced by the above improved process contain a smaller amount of co-crystallised aluminium than expected, usually less than that present in crystals obtained without the addition of ball clay. This is most unexpected since one is adding Al^O^ to system and yet obtaining a lower Al^O^ ·*·η crYstals produced therefrom. The lower co-crystallised Alg^ level is of benefit in building products in that the slurry of hemihydrate and/or anhydrite obtained from such crystals becomes desensitized to setting retarders if large amounts of Al^O^ are Present therein. By using the gypsum from the above improved process one can obtain a slurry with improved sensitivity to setting retarders.

Accordingly, the present invention provides a process for producing a slurry suitable for use in making a building product which comprises converting the gypsum crystals produced by the above improved process into an aqueous slurry of calcium sulphate hemihydrate and/or anhydrite. To form the building product or other article, this slurry is moulded or cast into the desired shape.

The crystals from the above improved process are in general characterised in that at least 50% by weight of the calcium sulphate dihydrate crystals have a length - 4 to breadth ratio of from 1:1 to 3:1, in that the calcium sulphate dihydrate crystals have a mean thickness of from 3 to 10 micrometres, and in that at least 15%, typically 40 to 80%, by weight of the calcium sulphate dihydrate crystals are in the form of X shaped twins and/or clusters of crystals radiating from a nucleus (hereinafter collectively termed ‘twinned' crystals).

Accordingly, the present invention also provides a process for producing a slurry suitable for use in making a building product which comprises converting gypsum crystals which are characterised in that they have the above characteristics ihto an aqueous slurry of calcium sulphate hemihydrate and/or anhydrite crystals. This slurry is then moulded or cast into the desired shape to form the building product·or other article.

In preparing the slurry it is necessary to convert the gypsum into calcium sulphate hemihydrate and/or anhydrite and the invention also provides a process for preparing calcium sulphate hemihydrate and/or anhydrite which comprises treating gypsum crystals produced by the above improved process and/or characterised in that at least 50% by weight of the calcium sulphate dihydrate crystals have a length to breadth ratio of from 1:1 to 3:1, in that the calcium sulphate dihydrate crystals have a mean thickness Of from 3 to 10 micrometres, and in that at least 15%, typically 4o to 80%, by weight of the calcium sulphate dihydrate crystals are in the form of twinned crystals, so as to convert the calcium sulphate dihydrate into calcium sulphate hemihydrate and/or anhydrite.

The gypsum crystals are preferably treated before conversion so as to remove at least some of the impurities therefrom, e.g. carbonaceous impurities, phosphoric acid and/or fluorosilicates. This treatment usually comprises washing which may be carried out in any suitable manner, e.g. by slurrying the crystals in water one or more times and then separating the crystals, e.g. by filtration, from the water; or by elutriating the impurities from the crystals with an upward stream of washing water. Alternatively, the crystals may be slurried, e.g. with water, and then passed through one or more hydrocyclones or continuous decanting centrifuges. By using a hydrocyclone it is possible to remove some of the very fine and/or very coarse crystals so as to produce washed crystals having a predominant size range 6—16 micrometres which we have found usually improves the strength of the product obtained. If need be, the crystals may be milled at some point during or after the washing process to ensure that the desired crystal size is achieved. Milling may be carried out in a wet mill or in a dry mill, e.g. a ball or hammer mill. It is also preferred to neutralise any free acid in the crystals, e.g. by the addition of an alkali, e.g. lime or calcium carbonate, during the washing and/or milling of t(ie crystals. It will be appreciated that the twinned crystals in the original form of the gypsum may be broken down during these operations and that the gypsum crystals as converted may not contain a significant proportion of twinned crystals.

The washing of the crystals may be carried out over a wide range of temperatures, e.g. up to 90°C, although we prefer to operate at from ambient temperature to 40°C. After washing the crystals may be deliquored, e.g. to a water content of from 10 to 20% using a filter or centri42025 - 6 fuge. Since the liquid, notably water, is readily separated from the crystals, it is possible to produce drier crystals than hitherto, and this may be of benefit where the washed crystals are to be dried subsequently or stored for any time before conversion to the hemihydrate or anhydrite form.

The gypsum crystals, after washing if necessary, are converted, usually by direct dehydration, to the hemihydrate or anhydrite form. This conversion may be achieved by heating the crystals in the substantially dry state. Dry calcination is conveniently carried out by feeding the crystals to a rotating tube heated to 100 to 7O0°C, e.g. i20 to 160°c for the hemihydrate and 400 to 600°C for the anhydrite, to remove much of the water of crystallisation. It is preferred that the overall final water of crystallisation of the crystals should not be reduced to an average of less than 0.4 molar proportions per molar proportion of CaSO^ for the hemihydrate form.

If desired, additives may be mixed with the crystals before they are calcined to assist removal of water and/or to stabilise the hemihydrate form produced. Typical of such additives are keratin, sodium hexametaphosphate, borax and/or calcium acetate. alternatively, the calcium sulphate dihydrate crystals may be subjected to a wet calcination. In this, the dihydrate crystals are mixed with an aqueous acid, typically dilute phosphoric ahd/or sulphuric acid at such a concentration to give a pH in the range 1 to 6 and a solids content of 5 to 30% by weight in mixture; and the mixture is heated. Preferably heating is carried out under 2 superatmospherie pressure, e.g. 2 to' 10 kg/cm , and to 2025 - 7 a temperature in the range 105 to 170, preferably 110 to 150°C. Again, if desired, additives may be incorporated into the mixture to be calcined, e.g. reactive silica compounds, borax, alkylaryl sulphonates, sulphite waste liquors, calcium hydroxide, Portland cement, urotropin and ferric salts. The calcination product is then treated to recover the hemihydrate crystals produced, e.g, by filtration, decantation or by the use of one or more hydrocyclones.

The crystals produced by the above process contain hemihydrate and, in the case of dry calcination, anhydrite crystals. These may be subjected to further treatment before use. Thus, they may be milled and/or classified, notably using a hydrocyclone, to produce crystals within the size range 6“—16 micrometres; or they may be calcined further to produce a product consisting predominantly of anhydrite.

The hemihydrate and/or anhydrite crystals find use in the preparation of a wide range of products, notably building products, e.g. plaster, plaster blocks and board, for which hemihydrate and/or anhydrite crystals from other sources are used. Thus, the crystals may be mixed with conventional thickeners, setting retarders, setting accelerators, pigments and water to form a slurry, usually containing from 20 to 90%, preferably 30 to 75%, by weight of water based on the dry weight of the crystals to form a plaster mix. This slurry is then cast, moulded, extruded or otherwise formed into articles of the desired shape using conventional techniques, under pressure if desired. A particularly preferred use of the crystals is in the preparation of fibre reinforced articles, notably aheets or boards. In this preferred use, a slurry of the crystals in water (optionally also containing other ingredients) is mixed with fibres, e.g. shredded paper or glass fibres, and the mixture poured into a mould where an article is formed, preferably by moulding under pressure. Alternatively, the mixture may be extruded into beams and similar structural members. Such fibre reinforced products possess high strength and fire resistance and find use in the production of fire resistant doors, panels and partitions.

The present invention will now be illustrated by the following Example in which all parts and percentages will be given by weight unless stated otherwise: Example Phosphoric acid was prepared in a commercial scale plant by the gypsum route to give gypsum crystals A with a long needle-like shape (i.e. with a length to breadth ratio greater than 1:1) The test run was repeated, this time with the addition of ball clay to the phosphate rock B feed. The gypsum erystalsTrh this case were short squat *1 crystals with a length to breadth ratio of 1:2, a mean thickness Of about 10 micro metres; and So% Of the crystals were in the form of twinned crystals. These crystals B contained 0.015% co-erystallised Al, whereas crystals A contained 0.034% co-crystallised Al as determined by analysis of a sample of the crystals which had been washed in water to remove surface adherent Al and then air dried.

Plasters were prepared from crystals A and B by washing the crystals in water, and filtering them, then calcining the crystals in an oven at 160°C until the overall water of hydration of the cryatals had been reduced to 7% by weight. The calcined crystals were then slurried in water (0.7 parts of water per 1 part of crystals) to give a plaster slurry. Varying amounts of a commercial plaster setting retarder (diethylene triamine penta acetic acid) were incorporated into the plasters and thp setting time of the plasters recorded using a Vicat long needle apparatus.

The results of these tests are set out below: Amount of retarder % w/w Setting time in seconds Crystals A Crystals B 0.0 0.02 0.04 0.06 0.10 85 120 180 265 525 85 145 245 350 660

Claims (19)

1. CLAIMS:1. A process for preparing calcium sulphate hemihydrate and/or calcium sulphate anhydrite which comprises converting the gypsum crystals produced by the above im5 proved process as hereinbefore defined into calcium sulphate hemihydrate and/or calcium sulphate anhydrite crystals. 2. Wherein calcium sulphate dihydrate is dehydrated directly to calcium sulphate hemihydrate and/or anhydrite.

2. A process for preparing calcium sulphate hemihydrate and/or calcium sulphate anhydrite which comprises

3. Wherein the conversion of the gypsum crystals is by 25 dry calcination.

4. A process as claimed in any one of claims 1 to

5. A process as claimed in any one of claims 1 to 3 wherein the conversion of the gypsum crystals is by wet calcination.

6. A process according to any one of the preceding 30 claims wherein the gypsum crystals are washed before conversion. - 11

7. A process according to claim 6 wherein washing is carried out in a hydrocyclone.

8. A process according to either of claims 6 or 7 wherein washing is carried out at a temperature of less than 40°C.

9. A process according to any one of the preceding claims wherein the calcium sulphate dihydrate is dehydrated to give a calcium sulphate containing not less than an average of 0.4 molar proportions of water of crystallisation per molar proportion of CaSO^.

10. A process according to claim 1 for preparing calcium sulphate hemihydrate and/or anhydrite crystals substantially as hereinbefore described. 10 converting gypsum crystals produced by the above improved process which are characterised in that at least 50% by weight of the calcium sulphate dihydrate crystals have a length to breadth ratio of from 1;1 to 3:1, in that the calcium sulphate dihydrate crystals have a mean

11. A process according to claim 1 for preparing calcium sulphate hemihydrate and/or anhydrite crystals substantially as hereinbefore described in the Example. 12. - 12 slurry is cast, moulded or extruded under pressure.

12. A process for producing a slurry suitable for use in making a building product which comprises forming an aqueous slurfy of the calcium sulphate hemihydrate and/or anhydrite crystals produced by a process as claimed in any one of claims 1 to 11. I

13. A process as claimed in claim 12 wherein the slurry contains from 20 to 90% hy weight of water based on the dry weight of the crystals.

14. A process as claimed in claim 12 wherein fibres are incorporated into the slurry.

15. A process as claimed in any one of claims 12 to 14 wherein the slurry is extruded, moulded or cast to form an article of the desired shape. 15 thickness of from 3 to 10 micrometres, and in that initially at least 15%, typically 40 to 80%, by weight of the calcium sulphate dihydrate crystals are in the form of twinned crystals, into calcium sulphate hemihydrate and/or calcium sulphate anhydrite crystals. 20 3. A process as claimed in either of claims 1 or

16. A process as claimed in claim 15 wherein the 420 2 5

17. A process according to claim 12 for forming a slurry of calcium sulphate hemihydrate and/or anhydrite substantially as hereinbefore described. 5

18. A process according to claim 12 for forming a slurry of calcium sulphate hemihydrate and/or anhydrite substantially as hereinbefore described in the Example

19. An article whenever produced by a process as claimed in either of claims 15 or 16.