IE51013B1 - Gypsum calcination method and composition - Google Patents

Abstract

The water demand of gypsum plaster is reduced by adding a water demand reducing agent to powdered gypsum rock before or during calcination. The preferred water demand reducing agent are natural or synthetic polymeric dispersing agents, such as sulphonated naphthalene- formaldehyde condensates or sulphonated melamine-formaldehyde condensates. Ligno sulphonates can also be used for this purpose.

Description

This Invention relates to the calcination of gypsum and the production of gypsum wallboard. More particularly, this invention relates to a novel method and compositions for reducing the water demand of gypsum plaster which Is used in producing gypsum wallboard.

Calcium sulphate hemihydrate (plaster) is commonly used in the form of an aqueous slurry, to produce gypsum wallboard. Usually, a large excess of water has to be added to the calcined plaster to ensure that sufficient slurry fluidity is obtained to allow rapid and even spreading of the slurry across the width of the moving paper sheets. The amount of water required by a given plaster to obtain a plaster slurry of a standard consistency is known as the water demand.

With rapidly escalating energy prices, the cost of drying wallboard to remove the excess water has become a major undesirable factor in the overall cost of the wallboard manufacturing process. With lower plaster water demand the wallboard can be dried at lower temperatures thereby decreasing energy consumption and wallboard drying costs per unit area.

Lower drying temperatures during the drying process have the added advantage of reducing the incidence of wallboard edge and end burning, which is a problem in modern high speed wallboard manufacturing plants where elevated dryer temperatures are employed.

A further advantage is realised if the wallboard dryer is operating at maximum drying capacity for a process using a normal water demand plaster slurry. In this case the drying stage is the rate limiting factor governing tha overall speed of the wallboard manufacturing process. Clearly a low water demand plaster increases the dryer capacity potential and permits a boardline speed increase thereby improving wallboard production output.

Various ways to reduce the plaster water demand are known 5 by the gypsum industry. One is the process of aridisation, in which a gypsum plaster of reduced water demand is obtained by calcination in the presence of a salt or other coapound of high affinity for water, notably water-soluble inorganic chlorides such as calcium chloride. Unfortunately, aridised plasters produce undesirable side effects in a wallboard manufacturing process and in the finished wallboard product. For these reasons, the process of plaster aridisation is not generally used in the wallboard manufacturing industry.

Another method of reducing water demand and thereby increasing the fluidity of plaster slurries is through the addition of water reducing agents, such as naturally occurring lignin byproducts of the pulp and paper industry, to the plaster slurry.

The major disadvantage with these products is that generally they have a retarding effect on the gypsum setting rate and tend to decrease gypsum board core strength, both of which are detrimental to the manufacture of good quality gypsum wallboard. Another family of water reducing agents conprises potassium salts of condensation products of naphthalene sulphonic acid and formaldehyde as disclosed in U.S. Patent No. 4,184,887.

It is also known that the water demand of a plaster slurry can ba reduced, whilst maintaining slurry fluidity, by adding a synthetic chemical dispersant or superplasticiser” to the slurry after the calcining process.

Superplasticiser* are natural or synthetic, polymeric dispersing agents which have found application in the concrete industry by improving the workability of fluidity of cement mixtures. They appear to exert their action by decreasing the surface tension of water, increasing the surface charge of the solid particles, or producing a lubricating film at the particle surface.

Superplasticisers of the synthetic type may be classified into two main types: Type 1 - Sulphonated naphthalene-formaldehyde condensates Typo 2 - Sulphonated melamine-formaldehyde condensates.

Type 1 superplasticisers are sold under the trade marks LOHAR □ (available from Diamond Shamrock Corporation) or DARVAN (available frcm R.T. Vanderbilt Company) and Type 2 Plasticiser under the trade mark HELMET HO.

Unexpectedly, it has now been found that gypsum plaster of reduced water demand can be produced by calcining powdered gypsum rock in the presence of a nuperplasticiser added to the gypsum before or during calcination, as opposed to being added after calcination.

Further, the low water demand plaster produced by in situ calcination with a suparplaeticiser surprisingly does not significantly affect the setting characteristics of the aqueous plaster slurry, neither are the resultant wallhoard core properties adversely affected. The invention has 5101 application to either batch or continuous calcination of gypsum.

Ihe gypsum plaster of the invention may be used in the production of gypsua wallboard.

Ihe superplasticiser aay be selected trots sulphonated S naphthalene-formaldehyde condensates and sulphonated melamineformaldehyde condensates. Alternatively, a lignosulphonate may be used as water demand reducing agent for the purposes of the invention ihe superplasticiser may be added to the powdered gypsum rock in a concentration ranging from about O.Ol to about 1.0 percent by weight of the gypsua, and preferably iron about 0.05 to about 0.5 percent.

In the drawings: FIGURE 1 illustrates in graphical form the effect of the superplasticiser LOMAR D in powder form added to gypsua plaster: FIGURE 2 illustrates in graphical form the effect of the superplasticiser LOHAR D in aqueous form added to gypsua plaster; and FIGURE 3 illustrates in graphical form the effect of the supezplasticiser DARVAN A4, 6 added to gypsua plaster.

Continuous calcination experiments were carried out in a laboratory kettle apparatus which is essentially a scaled down model of -the kettle described in detail in Patent Specification No. 26961 and British Patent Specification No, 1,087,549.

Ihe laboratory method conprised dry blending various weights of superplasticiser with ground gypsum using a Patterson-Kelly twin shell dry blending apparatus. Each aixture was then calcined continuously in the laboratory kettle apparatus using a constant gas firing rate and a calcination tenperature of 147 + 1°C. 31013 Hie plaster produced by in situ calcination with a superplasticiser was analysed to determine: a) The chemical analysis oi the plaster? b) The effect on the fluidity of the aqueous plaster slurry? c) The effect on the plaster water demand? and d) The percentage of water reduction in the aqueous plaster slurry at a standard consistency.

A calcined plaster produced without superplasticiser addition and a plaster containing suparplasticisers added after calcination were used as controls.

Typical results for LOMAS D type 1 superplasticiser are shown in TABLES 1 and 2, uhich clearly demonstrate the particular effectiveness of the superplasticisers added before or during calcination in comparison with addition after calcination. Adding the superplasticiser before or during calcination unexpectedly produces plasters with a water demand lower than plasters in which the superplasticer was added after calcination. The results indicate that the LOfffiH 0 aqueous solution having only 33% auperplasticiser reduced water demand more effectively on an equivalent basis than the LOMAH D powder.

The suprising effect of before calcination compared with after calcination addition of superplasticisers to plaster is also graphically demonstrated in FIGOBES 1 and 2.

A further example of the unexpected advantage of superplasticiser addition before or during calcination is presented in TABLE 3 and FIGURE 3. The superplasticiser used in this case was the powder sold under the trade mark DAEVAN r? 6· TABU 1 UMAR P (POWDER FORM) - SUPERPLAST1C12ER ADDITION TO CYPSUH PLASTER Plaster Type Chemical Analysis, 1 Water Demand mls/100 g Slurry Fluidity mm spread 1 Water Reduction at std Consistency Heroi Hydrate Soluble Anhydrite Residual Gypsun Normal Plaster without addition of superplasticizer 64.6 7.1 2.5 70 68 -- Normal Plaster LOMAR 0 superplasticizer added after calcination 0.051 LOMAR D* 64.6 7.1 2.5 67 79 6.8 0.11 LOMAR D* 64.6 7.1 2.5 66 88 11.1 0.151 LOMAR 0* 64.6 7.1 2.5 64 93 12.6 Plaster with LOMAR D superplasticizer added before calcination 0.051 LOMAR D· 64.3 7.9 2.3 66 84 8.9 0.11 LOMAR D* 62.6 9.5 1.0 64 94 16.3 0.151 LOMAR D* 64.6 7.5 2.5 62 107 21.6 * LOHAR D Powder Analyst» , 841 Naphthalene sulphonate polymer 111 Sodium sulphate Free moisture TABLE a LOMAR D (SOLUTION FORM) - SUPERPLASTICIZER ADDITION TO GYPSUH PLASTER plaster Type Chemical Analysis, % Water Demand mis/100 g Slurry Fluidity mm spread 4 Water Reduction at std Consistency Hemi Hydrate Soluble Anhydrite Residual Gypsum Normal Plaster without addition of superplasticiser 68.0 5.9 2.8 71 62 Normal Plaster LOMAR D cuperplasticiscr . added after calcination 0.05% LOMAR D* 68.0 5.9 2.8 7® 66 4.6 0.1% LOMAS D4 68.0 5.9 2.8 69 72 7.2 0.3% LOMAR D4 68.0 5.9 2.8 67 80 11.8 plaster with LOMAR 0 superplasticizer added before calcination 0.05% LOMAR D4 67.3 6.6 2.9 70 72 6.7 0.1% LOMAR D4 65.8 7.0 3.3 68 89 12.8 0-34 LOMAR D® 59.5 7.S 3.1 6S 100 16.4 • LOMAR B Aqueous Solution Analysis» 33% Napthalene sulphonate polymer TABLE 3 PABVAH «6 - SOPERPLASTlCltER ADDITION TO GYPSJM PLASTER plaster Type chemical Analysis, t water Demand mis/100 g Slurry Fluidity mm spread t Water Reduction at std Consistency He mi Hydrate soluble Anhydrite RealquaI Gypsum Normal Plaster without addition of superplasticizcr 64.5 7.7 2.1 68 74 — Normal Plaster DARVAN *6 suoerplast( citer added after calcination 0.05» DARVAN <6* 64.5 7.7 2.1 66 82 4.9 0.1» PARVAN 16' 64.5 7.’ 2.1 65 87 8.1 0.15« PARVAN 16* 64.5 7.7 2.1 64 90 9.7 plaster with PARVAN <6 superplasticizer added Before calcination 0.05» PARVAN J6* 59.2 11.7 1.1 64 94 11.9. 0.1« PARVAN i6* 64.4 5.6 2.4 63 96 13.0 0.15« DARVAN »6* 64.1 6.6 1.4 62 97 13.5 * DARVAN #6 - PolywsTiXed alkyl naphthalene sulfonic acid, sodium «alt Surprisingly, the superplasticiser did not lose effectiveness by being subjected to the conditions of the calcination process. Moreover, the fact that the performance of the superplasticiser was superior when added before or during calcination compared to adding the superplasticiser after calcination was even more surprising.

While the applicants do not wish to be bound to any theories, it appears from preliminary testing that the superplasticiser is advantageously modified by the conditions of the calcination process, which is completely unexpected. It is apparent from the foregoing data that regardless of whether the superplasticiser is modified or not, the performance of the superplasticiser when added before or during calcination is distinctly ismroved.

While the invention has been described way of example with reference to the calcination process of Patent Specification No, 26961 and 15 British Patent Specification No. 1 087 549, it is equally applicable to other ralr-ίnation methods, such as batch calcination, for exanple in conventional kettles, or for mptiminus calcination by process not employing the kettle type of vessel, for exanple that of US Patent Ito. 4 052 149.

Claims (6)

1. A method oi producing gypsum plaster ot reduced water demand, which comprise* calcining powdered gypsum rock in the presence of a superplasticiser added to the gypsum before or 5 during calcination.

2. λ method according to claim 1 wherein the superplasticiser is selected from sulphonated naphthalene-formaldehyde condensates, sulphonated melamine-formaldehyde condensates and ligno-sulphonate.

3. A method according to claim 1 or 2 wherein the super10 plasticiser is added to the powdered gypsum rock in a concentration ranging from 0.01 to 1.0 percent by weight of the gypsum.

4. A method according to claim 3 wherein the supexplasticiser is added to the powdered gypsum rock in a concentration ranging from 0.05 to 0.5 percent by weight of the powdered 15 gypsum rock.

5. A method according to any of claims 1 to 4 wherein the gypsum plaster is used in the production of gypsum wallboard.

6. Calcined gypsum plaster having a water demand reduced by a method according to any of claims 1 to 4. S10 13