GB2086869A - Reducing water demand of gypsum plaster - 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

SPECIFICATION Gypsum calcination method and composition 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 the 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 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 compound 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 by-products 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 comprises 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 be reduced, whilst maintaining slurry fluidity, by adding a synthetic chemical dispersant or "superplasticiser" to the slurry after the calcining process.

Superplasticisers are natural or synthetic, polymeric dispersing agents which have found application in the concrete industry by improving the workability or 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.

Type 2 -Sulphonated melamine-formaldehyde condensates.

Type 1 superplasticisers are sold under the trade marks LOMAR D (available from Diamond Shamrock Corporation) or DARVAN # $ (available from R. T. Vanderbilt Company) and Type 2 superplasticiser under the trade mark MELMET L10.

Unexpectedly, it has now been found that the reduction in plaster water demand can be significantly improved by adding a water demand reducing agent or superplasticiser to the gypsum before or during calcination rather than adding it after calcination. Further, the low water demand plaster produced by in situ calcination with a superplasticiser surprisingly does not significantly affect the setting characteristics of the aqueous plaster slurry, neither are the resultant wallboard core properties adversely affected.

The invention thus provides a method of reducing the water demand of gypsum plaster produced from powdered gypsum rock comprising adding a water demand reducing agent, such as a synthetic polymeric dispersing agent or superplasticiser, to the powdered gypsum rock before or during calcination. The invention has application to either batch or continuous calcination of gypsum.

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

The superplasticiser may be selected from sulphonated naphthalene-formaldehyde condensates and sulphonated melamine-formaldehyde condensates. Alternatively, a lignosulphonate may be used as water demand reducing agent for the purposes of the invention.

The superplasticiser may be added to the powdered gypsum rock in a concentration ranging from about 0.01 to about 1.0 percent by weight of the gypsum, and preferably from 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 gypsum plaster; FIGURE 2 illustrates in graphical form the effect of the superplasticiser LOMAR D in aqueous form added to gypsum plaster; and FIGURE 3 illustrates in graphical form the effect of the superplasticiser DARVAN # 6 added to gypsum 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 U. K. Patent Nos. 1,018,464 and 1,087,549.

The laboratory method comprised dry blending various weights of superplasticiser with ground gypsum using a Patterson-Kelly twin shell dry blending apparatus. Each mixture was then calcined continuously in the laboratory kettle apparatus using a constant gas firing rate and a calcination temperature of 147 + 1 OC.

The plaster produced by in situ calcination with a superplasticiserwas analysed to determine: a) The chemical analysis of the plaster; b) The effect of 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 superplasticisers added after calcination were used as controls.

Typical results for LOMAR D type 1 superplasticiser are shown in TABLES 1 and 2, which 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 superplasticiser was added after calcination. The results indicate that the LOMAR D aqueous solution having only 33% superplasticiser reduced water demand more effectively on an equivalent basis than the LOMAR D powder.

The surprising effect of before calcination compared with after calcination of superplasticisers to plaster is also graphically demonstrated in FIGURES 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 name DARVAN # 6.

TABLE 1 LOMAR D (POWDER FORM) - SUPERPLASTICIZER ADDITION TO GYPSUM PLASTER

Chemical Analysis, % % Water Water Slurry Reduction Plaster Hemi Soluble Residual Demand Fluidity at std Type Hydrate Anhydrite Gypsum mls /100 g mm spread Consistency Normal Plaster without addition of superplast icizer 64.6 7.1 2.5 70 68 Normal Plaster LOMAR D superplasticizer added after calcination 0.05% LOMAR D* 64.6 7.1 2.5 67 79 6.8 0.1% LOMAR D* 64.6 7.1 2.5 66 88 11.1 0.15% LOMAR D* 64.6 7.1 2.5 64 93 12.6 Plaster with LOMAR D superplasticizer added before calcination 0.05% LOMAR D* 64.3 7.9 2.3 66 84 8.9 0.1% LOMAR D* 62.6 9.5 1.0 64 94 16.3 0.15% LOMAR D* 64.6 7.5 2.5 62 107 21.6 * LOMAR D Powder Analysis: 84% Naphthalene sulphonate polymer 11% Sodium sulphate 5% Free moisture TABLE 2 LOMAR D (SOLUTION FORM) - SUPERPLASTICIZER ADDITION TO GYPSUM PLASTER

Chemical Analysis, % % Water Water Slurry Reduction Plaster Hemi Soluble Residual Demand Fluidity at std Type Hydrate Anhydrite Gypsum mls/100 g mm spread Consistency Normal Plaster without addition of superplast icizer 68.0 5.9 2.8 71 62 - Normal Plaster LOMAR D superplasticizer added after calcination 0.05% LOMAR D* 68.0 5,9 2.8 70 66 4.6 0.1% LOMAR D* 68.0 5.9 2.8 69 72 7.2 0.2% LOMAR Dt 68.0 5.9 2.8 67 80 11.8 Plaster with LOMAR D superplasticizer added before calcination 0.05% LOMAR D* 67.3 6.6 2.9 70 72 6.7 0.1% LOMAR D* 65.8 7.0 3.3 68 89 12.8 0.2% LOMAR D* 59.5 7.8 3.1 65 100 16.4 * LOMAR D Aqueous Solution Analysis: 33% Naphthalene sulphonate polymer < 1% Sodium sulphate TABLE 3 DARVAN &num;6 - SUPERPLASTICIZER ADDITION TO GYPSUM PLASTER

Chemical Analysis, % % Water Water Slurry Reduction Plaster Hemi Soluble Residual Demand Fluidity at std Type Hydrate Anhydrite Gypsum mIs /100 g mm spread Consistency Normal Plaster without addition of superplast icizer 64.5 7.7 2.1 68 74 Normal Plaster DARVAN #6 superplasticizer added after calcination 0.05% DARVAN #6* 64.5 7.7 2.1 66 82 4.9 0.1% DARVAN #6* 64.5 7.7 2.1 65 87 8.1 0.15% DARVAN &num;6* 64.5 7.7 2.1 64 90 9.7 Plaster with DARVAN &num;6 superplasticizer added before calcination 0.05% DARVAN #6* 59.2 11.7 1.1 64 94 11.9 0.1% DARVAN &num;6* 64.4 6.6 2.4 63 96 13.0 0.15% DARVAN #6* 64.1 6.8 1.4 62 97 13.5 * DARVAN &num;;6 - Polymerized alkyl naphthalene sulfonic acid, sodium salt 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 improved.

While the invention has been described way of example with reference to the calcination process of U. K. Patents No. 1 018 464 and 1 087 549, it is equally applicable to other calcination methods, such as batch calcination, for example in conventional kettles, or for continuous calcination by process not employing the kettle type of vessel, for example that of US Patent No. 4 052 149.

Claims (9)

1. A method of reducing water demand of gypsum plaster produced from powdered gypsum rock comprising adding a water demand reducing agent to the powdered gypsum rock before or during calcination of the gypsum.

2. A method according to claim 1 wherein the water demand reducing agent is a superplasticiser.

3. A method according to claim 1 or 2 wherein the superplasticiser is a synthetic polymeric dispersing agent.

4. A method according to claim 1 wherein the water demand reducing agent is selected from sulphonated naphthalene-forma Idehyde condensates, sulphonated melamine-formaldehyde condensates and ligno-sulphonate.

5. A method according to any of claims 1 to 4 wherein the superplasticiser is added to the powdered gypsum rock in a concentration ranging from 0.01 to 1.0 percent by weight of the gypsum.

6. A method according to claim 5 wherein the superplasticer is added to the powdered gypsum rock in a concentration ranging from about 0.05 to about 0.5 percent by weight of the powdered gypsum rock.

7. A method according to any of claims 1 to 6 wherein the gypsum plaster is used in the production of gypsum wallboard.

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

9. Gypsum wallboard produced by the method of claim 7.