HK1113783A - Wet gypsum accelerator and methods, composition, and product relating thereto - Google Patents

Abstract

A wet gypsum accelerator comprising ground product having a median particle size of from about 0.5 micron to about 2 microns and calcium sulfate dihydrate, water, and at least one additive selected from the group consisting of (i) an organic phosphonic compound, (ii) a phosphate-containing compound, or (iii) a mixture of (i) and (ii), is disclosed. Also disclosed are a method of preparing a wet gypsum accelerator, a method of hydrating calcined gypsum to form an interlocking matrix of set gypsum, a set gypsum-containing composition, and a set gypsum-containing product.

Description

Wet gypsum accelerator and methods, compositions, and products related thereto

Technical Field

The present invention generally relates to gypsum compositions. More particularly, the present invention relates to wet gypsum accelerators for accelerating the hydration of calcined gypsum to calcium sulfate dihydrate, and to methods, compositions and products related thereto.

Background

Set gypsum, which includes calcium sulfate dihydrate, is a well-known material that is commonly included in many product types. For example, set gypsum is a major component of the final product produced by using traditional plaster of paris (e.g., plaster of paris surfaced internal building walls), as well as gypsum board used in the construction of typical drywall of the interior walls and ceilings of buildings. In addition, set gypsum is a major component of gypsum/cellulose fiber composite boards and products, and is included in products that fill and smooth the seams between the edges of the gypsum board.

Typically, the gypsum-containing product is prepared by forming a mixture of calcined gypsum (in other words, calcium sulfate hemihydrate and/or calcium sulfate anhydrite) and water, as well as other components, if desired. The mixture is typically cast into a predetermined shape or onto the substrate surface. The calcined gypsum reacts with water to form a matrix of crystalline hydrated gypsum or calcium sulfate dihydrate. The desired hydration of the calcined gypsum is a hydration that is capable of forming an interlocking matrix of set gypsum crystals, thereby imparting strength to the gypsum structure in the gypsum-containing product. Generally, the hydration rate and% conversion rate can affect the final strength and production speed of gypsum-containing products. Mild heating can be used to drive off unreacted water to obtain a dry product.

Regardless of the type of gypsum-containing product being made, accelerator materials are typically included in the mixture comprising calcined gypsum and water to enhance hydration efficiency, control set time, and maximize production speed. Typically, the accelerator material comprises finely ground dry calcium sulfate dihydrate, commonly referred to as "gypsum seeds". The gypsum seeds enhance the nucleation of set gypsum crystals, thereby increasing their crystallization rate. Conventional gypsum seed accelerator materials, as is well known in the art, gradually lose their effectiveness upon aging, even under normal conditions. In this regard, some efficacy of the accelerator is lost even during grinding, and the gypsum seeds lose efficacy over time during handling or storage. The loss of acceleration efficiency of conventional accelerator materials is exacerbated when the accelerator materials are exposed to heat and/or moisture.

To address the loss of the gypsum seed efficiency over time, especially under hot and/or humid conditions, the calcium sulfate dihydrate accelerator material is typically coated with any of a number of conventional coating agents, such as sugars (which include sucrose, glucose, and the like), starch, boric acid, or long chain fatty acids, and salts thereof. Because the accelerator loses effectiveness upon contact with moisture (e.g., because the accelerator particles undesirably agglomerate and/or because both gypsum and coating agents typically absorb substantially water and also absorb moisture), commonly used heat resistant accelerator materials are both ground and provided in dry form.

Wet gypsum accelerators are disclosed in commonly assigned U.S. Pat. No. 6,409,825. However, there remains a need in the art for wet gypsum accelerators having superior properties, as well as new techniques and systems for making such accelerators.

Disclosure of Invention

The present invention provides wet gypsum accelerators, methods of making wet gypsum accelerators, methods of hydrating calcined gypsum to form an interlocking matrix of set gypsum, set gypsum-containing compositions, and set gypsum-containing products.

The wet gypsum accelerator of the invention comprises a ground product having a median particle size of from about 0.5 microns to about 2 microns, wherein the ground product comprises calcium sulfate dihydrate. The wet gypsum accelerator further comprises water, and at least one additive selected from the group consisting of (i) an organophosphonic acid compound, and (ii) a phosphate-containing compound. Mixtures of (i) and (ii) may also be used. The wet gypsum accelerator is prepared by wet grinding. The water, additives and gypsum are combined in any order to form a mixture, with other optional components added if desired. When combined with water, the gypsum can be in the form of calcium sulfate dihydrate, or, at least some of the gypsum can be in the form of calcined gypsum (i.e., calcium sulfate hemihydrate and/or calcium sulfate anhydrite). The calcined gypsum is at least partially converted to calcium sulfate dihydrate in the presence of water. Excess water is desirable in wet gypsum grinding to facilitate grinding. Preferably, the gypsum is in the form of calcium sulfate dihydrate when grinding is initiated, but grinding can be initiated before all of the calcined gypsum is converted to calcium sulfate dihydrate. The calcium sulfate dihydrate is wet ground in the presence of the additive to form a wet gypsum accelerator.

The wet gypsum accelerator of the invention is useful for preparing set gypsum-containing compositions, and for products including the set gypsum-containing compositions. In particular, the wet gypsum accelerator of the invention can be combined with water and calcined gypsum in any order to form an aqueous mixture, wherein the calcined gypsum is hydrated to form an interlocking matrix of set gypsum. Preferably, the calcined gypsum is first mixed with water and then with the wet gypsum accelerator.

According to the present invention, the wet gypsum accelerator comprises a ground product. The ground product includes calcium sulfate dihydrate. The ground product has a median particle size of about 0.5 microns to about 2 microns. It has been found that the wet gypsum accelerator improves efficiency in preparing set gypsum-containing compositions and products by increasing the rate at which the calcined gypsum hydrates to form an interlocking matrix of set gypsum, which can be measured by the time to 50% hydration. The present invention can be used to make any of a variety of set gypsum-containing products formed from calcined gypsum, such as, but not limited to, ceiling materials, boards (e.g., wallboard), plaster, joint compounds, flooring materials, specialty materials, and the like.

The wet gypsum accelerator of the invention can be used to prepare set gypsum products prepared by any of a variety of processes known in the art by adding the wet gypsum accelerator to an aqueous calcined gypsum mixture. A suitable method FOR incorporating the wet GYPSUM accelerator into an AQUEOUS GYPSUM mixture is described in U.S. patent application Ser. No. _________________, having application and commonly owned herewith, "METHOD OF AND SYSTEMS FOR ADDING A HIGH VISCOSITY GYPSUDDITIVE TO A POST-MIXER AQUEOUS DISPERSION OF CALCINED GYPSUM" (attorney reference 234910).

Advantageously, the wet gypsum accelerator of the invention exhibits substantially long life and maintains its efficacy over time so that it can be manufactured, stored prior to use, and even transported over long distances. Due to its particular nature, wet gypsum accelerators are considered to be highly heat and/or moisture resistant materials, and therefore maintain all or most of their effectiveness even when exposed thereto. In preferred embodiments, the present invention also reduces manufacturing costs because the additive is preferably provided in relatively small amounts and the water to stucco ratio of the stucco slurry is reduced when a wet gypsum accelerator is used as compared to a dry gypsum accelerator. The present invention further reduces manufacturing costs when a second accelerator material (e.g., caustic potash or aluminum sulfate) is not required, typically because the wet gypsum accelerator maintains its high efficiency over time and when exposed to high humidity. However, a second accelerator material may be utilized if desired. In the manufacture of certain set gypsum-containing products, such as, but not limited to, gypsum-cellulosic fiber wallboard, the present invention also allows for easy and efficient manufacture by wet mixing the accelerator with calcined gypsum and other components used.

Wet gypsum accelerators used in gypsum board production and fiber panel production are used to increase stucco hydration rate, increase stucco efficiency, and reduce manufacturing costs. Stucco effectiveness can be measured by the rate of conversion.

These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention. The invention may best be understood by reference to the following detailed description of a preferred embodiment.

Drawings

Detailed Description

The present invention provides a wet gypsum accelerator comprising a ground product. The ground product is the result of wet grinding a calcium sulfate material in the presence of various additives. The additive comprises a compound selected from organic phosphonic acids; a phosphate-containing compound; and an additive in a mixture of an organophosphonic acid compound and a phosphate-containing compound. The ground product has a median particle size of about 0.5 microns to about 2 microns and includes at least calcium dihydrate and water, and may further include one or more additive compounds included during grinding. More than one of each type of additive may be used in the practice of the present invention. It is believed that the median particle size range of the ground product is responsible for the necessary processability of the wet gypsum slurry for use in a continuous or batch production process without sacrificing the acceleration efficiency of stucco hydration.

The wet gypsum accelerator of the invention is preferably prepared by wet grinding calcium sulfate dihydrate in the presence of an additive under conditions sufficient to produce a ground product having a median particle size of from about 0.5 microns to about 2 microns. Once prepared, the wet gypsum accelerator of the invention is used to enhance the efficiency of making set gypsum-containing products. The wet gypsum accelerator is combined with calcined gypsum and water, as well as other components, if desired, to form a mixture that is cast into a predetermined shape or onto the surface of a substrate during the manufacture of a set gypsum-containing product. As is understood in the gypsum industry, calcined gypsum is hydrated in the presence of water to form crystalline hydrated gypsum. When a sufficient amount of hydrated calcined gypsum is present, an interlocking matrix of set gypsum is typically formed.

The inclusion of the wet gypsum accelerator of the invention in the mixture of calcined gypsum and water increases the rate at which the calcined gypsum hydrates to calcium sulfate dihydrate in the desired set gypsum-containing product, as well as the predictability of the time required for it to hydrate. It is believed that the wet gypsum accelerator of the invention provides nucleation sites by increasing the crystallization rate of the interlocking matrix of the resulting set gypsum. The wet gypsum accelerator of the invention can be used to make any of a variety of set gypsum-containing products, such as conventional gypsum board or gypsum-cellulosic fiber board (e.g., FIBEROCK ® composite panels available from USG corporation), as well as ceiling tile materials, flooring materials, joint compounds, plaster of paris, specialty products, and the like.

The wet gypsum accelerator of the invention exhibits a substantially long life such that it maintains all or most of its effectiveness over an extended period of time. Preferably, all or most of the efficacy of the wet gypsum accelerator of the invention is maintained for at least several weeks, and more preferably for at least several months (e.g., 3 months, and still more preferably for at least 6 months or even longer). Thus, the wet gypsum accelerator can be prepared and then stored and/or transported even long distances before use. The wet gypsum accelerator of the invention preferably maintains efficiency even when exposed to high temperatures and/or high humidity. Moreover, since the wet gypsum accelerator of the invention maintains its efficiency over time even when exposed to high humidity, a second accelerator material (e.g., caustic potash or aluminum sulfate) is not required in the practice of the invention, but may be included if desired for certain applications and practices. The wet gypsum accelerator of the invention can be used to make set gypsum-containing products prepared by either dry or wet feed systems. For example, dry feed systems are used to make gypsum board, and wet feed processes are used to make gypsum-cellulosic fiber composite boards. In some embodiments, while the wet gypsum accelerator is prepared in the presence of water, the accelerator can be dried after preparation.

The wet gypsum accelerator of the invention is prepared by wet grinding. The gypsum feed material used in the grinding process can have any suitable initial median particle size. In some embodiments, the gypsum feed material has an initial median particle size of 50 microns or greater. In some embodiments, the gypsum feed material is natural gypsum and has an initial median particle size of about 20 to about 30 microns. In some embodiments, the gypsum feed material is synthetic gypsum and has an initial median particle size of about 40 to about 100 microns. According to the present invention, gypsum, water, and at least one additive are combined to form a mixture. In some embodiments, the gypsum used to form the wet ground mixture is calcium sulfate dihydrate. In other embodiments, when the gypsum is combined with water, it can be in the form of calcined gypsum. If calcined gypsum, it is believed that the calcined gypsum hydrates with a portion of the water to form calcium sulfate dihydrate. Preferably, the gypsum is in the form of calcium sulfate dihydrate when wet grinding is initiated, but it is not necessary that all of the calcined gypsum be converted to calcium sulfate dihydrate. The mixture preferably includes a sufficient amount of water in excess of that required to hydrate the calcined gypsum for the wet grinding step after the calcium sulfate dihydrate is formed. In such cases, the additive is preferably added after most, and more preferably all, of the calcium sulfate dihydrate is formed to maximize exposure of the additive to the calcium sulfate dihydrate.

Calcium sulfate dihydrate is wet ground in the presence of the additive component to form the wet gypsum accelerator either when combined with water or after forming calcium sulfate dihydrate from calcined gypsum in water. Generally, the smaller the median particle size of the resulting ground product, the better the acceleration efficiency for making set gypsum-containing compositions and products. However, as the median particle size decreases, the viscosity of the wet gypsum accelerator slurry increases, and thus the slurry becomes increasingly difficult to handle and process. The high viscosity slurry may be diluted with additional water or aqueous solution after grinding, or in a subsequent grinding pass to make it easier to handle and process. Thus, the particle size of the calcium sulfate dihydrate can be as small as desired to effectively produce set gypsum. In some embodiments where no additional dilution step is performed, a sufficiently large particle size can be used to produce a wet gypsum accelerator slurry of sufficiently low viscosity for slurry pumps and other processing machinery to effectively handle the slurry during the grinding and set gypsum formation process. In other embodiments, the particle size is kept small and the slurry is diluted prior to use.

The mixture comprising calcium sulfate dihydrate, water, and additives is preferably milled under conditions sufficient to provide a slurry in which the ground product has a median particle size of about 0.5 microns to about 2 microns. Preferably, the ground product has a median particle size of about 1 micron to about 1.7 microns. More preferably, the ground product has a median particle size of from about 1 micron to about 1.5 microns. Desirably, the standard deviation of the particle size distribution of the calcium sulfate dihydrate particles is less than 5 microns. Preferably, the standard deviation is less than 3 microns. The particle size of the wet gypsum accelerator can be measured using laser light scattering analysis and/or other suitable techniques. Suitable laser scattering instruments are commercially available from Horiba, Microtrack and Malvern. A Horiba instrument was used for the measurements in the examples section.

Alternatively, the mixture comprising calcium sulfate dihydrate, water, and additive is desirably milled under conditions sufficient to provide a slurry having a viscosity of between about 1000cP or above at a temperature between room temperature and about 150 ° f. Typically, the wet gypsum accelerator has a viscosity between about 1000cP to about 5000 cP. Preferably, the wet gypsum accelerator has a viscosity of between about 2000cP to about 4000 cP. More preferably, the wet gypsum accelerator has a viscosity of between about 2500cP to about 3500 cP. In some embodiments, the viscosity ranges from about 2800cP to about 3200 cP. The above viscosity ranges are ranges measured in the absence of dispersants or other chemical additives that have a significant effect on their viscosity or measurement.

It has been found that the milled product of the milling process is essentially an irregularly shaped and amorphous product. Calcium sulfate dihydrate formed by conventional wet milling processes is generally highly crystalline. Milling in the presence of additives can counter recrystallization to form defined crystalline gypsum particles. Thus, the milled product being substantially amorphous means that the milled product comprises little or no defined crystalline shape. Typically, about 60% or more of the ground product is amorphous. Preferably, about 75% or more of the milled product is amorphous. More preferably, about 90% or more of the milled product is amorphous.

The ground product can have about 20,000cm²A surface area of/g or greater as determined by laser light scattering analysis in water. Preferably, the ground product has about 30,000cm²A/g or greater, or about 40,000cm²A surface area of/g or greater. Typically, the milled product has about 100,000cm²A surface area of/g or less. In a preferred embodiment, the ground product has about 20,000cm²G to about 80,000cm²In terms of/g, or about 40,000cm²G to about 80,000cm²Surface area in g.

According to the present invention, the mixture of calcium sulfate dihydrate, water, and additives is wet ground in a mill assembly. First, calcium sulfate dihydrate, water, and additives are combined in any order and then pumped into the mill assembly. The mill assembly can be any suitable wet mill assembly. Typically, the mill assembly comprises a grinding chamber containing a grinding shaft fitted with a disc and a gasket and a plurality of beads. The disks and gaskets are constructed of any suitable material, for example, the disks and gaskets are constructed of at least one of stainless steel, premalony ®, nylon, ceramic, and polyurethane. The disk and pads are preferably constructed of PREMALLOY ®. The disks selected for the grinding chamber can have any suitable shape. Typically, the discs are standard round discs or pin discs, particularly pin discs designed to improve the axial flow of media through the mill. The milling shafts and the respective milling chambers may be oriented horizontally or vertically. In a preferred embodiment, the milling shaft is oriented horizontally. Typically, the grinding chamber is jacketed so that it can be water cooled. Preferably, the grinding chamber is water cooled to maintain a constant grinding temperature.

The mill assembly may comprise any suitable beads (e.g., balls and/or spheres). The beads may comprise any suitable material, for example the beads may comprise one or more metals or one or more ceramics. Suitable metals include stainless steel, carbon steel, chrome alloy steel, and the like. Suitable ceramic materials include zirconia, alumina, ceria, silica, glass and the like. As shown by laboratory tests, the sulfate groups of calcium sulfate dihydrate create a corrosive environment within the mill. Therefore, corrosion resistant beads are preferably used. The corrosion-resistant beads include stainless steel beads or steel beads coated with a corrosion-resistant material and ceramic beads. In a particularly preferred embodiment, the beads comprise ceria-stabilized zirconia comprising 20% ceria and 80% zirconia, for example, ZIRCONOX ® beads available from Jyoti ceramics, Nashik, India.

The beads used in conjunction with the mill assembly may be of any suitable size and density. Generally, the size and density of the beads should at least partially determine the size of the calcium sulfate dihydrate particles and the corresponding viscosity of the wet gypsum accelerator produced by the milling process. To obtain calcium sulfate dihydrate having a median particle size of from about 0.5 microns to about 2 microns, it is desirable to use beads having an average bead diameter of from about 0.5mm to about 3 mm. Preferably, the beads have an average bead diameter of about 1mm to about 2 mm. Desirably, the beads have about 2.5g/cm³Or a greater density. Preferably, the beads have about 4g/cm³Or a greater density. More preferably, the beads have about 6g/cm³Or a greater density. In particularly preferred embodiments, the beads have a median particle diameter of about 1.2mm to about 1.7mm and a density of about 6.1g/cm³Or larger ZIRCONOX ® ceramic beads. Desirably, the beads are present in the mill assembly in an amount of about 70% by volume or more. Preferably, about 70% to about 90% by volume of beads are present in the mill assembly. More preferably, the beads are present in the mill assembly from about 75% to about 85% by volume.

The wet gypsum accelerator of the invention can be prepared in a batch operation or a continuous operation. In a typical wet gypsum accelerator production system for wallboard applications, calcium sulfate dihydrate, water, and additives are first mixed in a feed tank. In some embodiments, this mixing is performed for about 8 minutes. The mixing time will depend in part on the batch size and feed rate. In some embodiments, it is desirable to add the calcium sulfate dihydrate to the mill assembly via an automatic feed system. The resulting mixture was then conveyed by a feed pump into a water-cooled mill assembly. The mixture is continuously milled and circulated through the closed loop circulation system for about 10 minutes or more. The actual grinding time will depend, at least in part, on the desired final median particle size of the calcium sulfate dihydrate particles and/or the desired viscosity of the wet gypsum accelerator slurry, as well as the size and density of the grinding beads used to grind the calcium sulfate dihydrate particles. Typically, the mixture is milled for about 15 minutes to about 50 minutes. Preferably, the mixture is milled for about 20 minutes to about 40 minutes. More preferably, the mixture is milled for about 25 minutes to about 35 minutes. After the desired median particle size is obtained, the mixture exits the mill assembly. In some embodiments, a median particle size of about 0.5 microns to about 2 microns is obtained. For batch operation, the mixture is delivered to a holding tank. When batch operations are implemented, the mixture is typically multi-pass milled through the milling assembly via a closed loop system. In some embodiments, about 4 to 5 strokes are performed at a flow rate of about 10 to 15 gallons per minute. In the continuous mode of operation, the mixture is fed directly to the plate mixer. When continuous operation is practiced, the mixture is typically milled in a single pass at a flow rate of about 2-3 gallons per minute.

The wet gypsum accelerator of the invention is desirably added to the aqueous calcined gypsum mixture in an amount effective to accelerate and/or control the rate at which the calcined gypsum mixture is converted to set gypsum. Typically, the rate of hydration is estimated from the "time to 50% hydration". The time to 50% hydration can be shortened by using more accelerator. Gypsum accelerator provides nucleation sites so that more dihydrate crystals are formed and one will obtain a larger number of thinner gypsum crystals. Other accelerators, such as caustic potash and aluminum sulfate, cause faster growth of existing gypsum crystals, which results in fewer, fuller crystals. A larger number of thinner gypsum crystals gives a stronger matrix than fewer, less abundant gypsum crystals.

Since the hydration of calcined gypsum to set gypsum is an exothermic process, the time to 50% hydration can be calculated by determining the temperature increase due to hydration and then measuring the amount of time required to generate the temperature rise. The time midpoint has been found to correspond to the time to 50% hydration, as is known to those skilled in the art. Preferably, the wet gypsum accelerator of the invention provides a time to 50% hydration of the calcined gypsum of about 8 minutes or less, more preferably 6 minutes or less. Even more preferably, the use of the wet gypsum accelerator of the invention results in a time to 50% hydration of the calcined gypsum of from about 5 minutes or less to about 4 minutes or less. The time to 50% hydration can be affected by many different factors, such as the amount of accelerator used, the amount of calcium sulfate hemihydrate and water used, the initial slurry temperature, and the mixing energy used during mixing. When measuring hydration, a control experiment can be performed using fixed variables in addition to the variable to be tested (e.g., the amount or type of WGA). This procedure can be used to compare various types of commonly used accelerators as well as specific types of WGA.

The amount of wet gypsum accelerator added to the aqueous calcined gypsum mixture will depend on the components of the aqueous calcined gypsum mixture, including, for example, set retarders, dispersants, foams, starches, paper fibers, and the like. For example, the wet gypsum accelerator of the invention can be provided in an amount from about 0.05% to about 3% by weight of the calcined gypsum, more preferably in an amount from about 0.5% to about 2% by weight of the calcined gypsum.

The calcined gypsum used to prepare the calcium sulfate dihydrate included in the wet gypsum accelerator of the invention can be in the form of calcium sulfate alpha hemihydrate, calcium sulfate beta hemihydrate, water-soluble calcium sulfate anhydrite, or mixtures of these various calcium sulfate hemihydrate and anhydrite forms. The calcined gypsum can be fibrous or non-fibrous. In addition, the wet gypsum accelerator of the invention can be used to accelerate the hydration of calcined gypsum (e.g., fibrous and non-fibrous calcined forms of gypsum) in any of those forms of calcium sulfate hemihydrate and anhydrite, as well as in various mixtures of forms of calcium sulfate hemihydrate and anhydrite.

While not wishing to be bound by any particular theory, it is believed that the desirable additives of the present invention bond with the newly created outer surface of the calcium sulfate dihydrate when ground, which provides at least a partial coating on the calcium sulfate dihydrate. It is believed that the additive adsorbs strongly and instantly to the active sites on the freshly ground calcium sulfate dihydrate surface, where otherwise undesirable recrystallization would occur. Thus, by adsorbing on the active sites, it is believed that the additive protects the size and shape of the active sites to prevent the gypsum from recrystallizing when the ground gypsum is exposed to water and heat and protects the active sites of the ground gypsum itself during the wet grinding process.

The organophosphonic acid compounds suitable for use in the wet gypsum accelerator of the invention have at least one RPO₃M₂A functional group, wherein M is a cation, phosphorus or hydrogen, and R is an organic group. Examples include organic phosphonates and phosphonic acids. Organic polyphosphonic acid compounds are preferred, but organic monophosphonic compounds can also be used according to the invention. Preferred organic polyphosphonic acid compounds include at least two phosphonate salt or ionic groups, at least two phosphonic acid groups, or at least one phosphonate salt or ionic group and at least one phosphonic acid group. The monophosphorus compounds of the invention comprise a phosphonate or ionic group or at least one phosphonic acid group.

The organic group of the organophosphonic acid compound is directly bonded to the phosphorus atom. Organic phosphonic compounds suitable for use in the present invention include, but are not limited to, compounds characterized by the following structures:

or

In these structures, R refers to an organic moiety comprising at least one carbon atom directly bonded to the phosphorus atom P, and n is a number from about 1 to about 1,000, and preferably from about 2 to about 50.

Organic phosphonic compounds include, for example, aminotri (methylenephosphonic acid), 1-hydroxyethylidene-1, 1-diphosphonic acid, diethylenetriamine penta (methylenephosphonic acid), hexamethylenediamine tetra (methylenephosphonic acid), and any suitable salt thereof (e.g., pentasodium, tetrasodium, trisodium, potassium, sodium, ammonium, calcium, or magnesium salts of any of the foregoing acids, and the like), or combinations of the foregoing salts and/or acids. In some embodiments, DEQUEST ® phosphonate available from Solutia corporation (st. louis, Missouri) is utilized in the present invention. Examples of DEQUEST ® phosphonates include DEQUEST ® 2000, DEQUEST ® 2006, DEQUEST ® 2016, DEQUEST ® 2054, DEQUEST ® 2060S, DEQUEST ® 2066A, and the like. Other examples of suitable organic phosphonic compounds can be found, for example, in U.S. Pat. No. 5,788,857.

Any suitable phosphate-containing compound that provides the advantages of the present invention may be utilized. For example, the phosphate-containing compound may be an orthophosphate or polyphosphate, and further the phosphate-containing compound may be in an ionic, salt, or acid form.

Suitable examples of phosphates according to the invention will be clear to the skilled person. For example, any suitable orthophosphate-containing compound may be utilized in the practice of the invention, including, but not limited to, monobasic phosphates such as ammonium dihydrogen phosphate, sodium dihydrogen phosphate, potassium dihydrogen phosphate, or combinations thereof. The preferred monobasic phosphate is sodium dihydrogen phosphate. Also, multi-basic orthophosphates may be utilized according to the invention.

Likewise, any suitable polyphosphate may be used in accordance with the present invention. Polyphosphates may be cyclic or acyclic. Examples of cyclic polyphosphates include trimetaphosphate salts, which include double salts (i.e., trimetaphosphate salts having two cations). The trimetaphosphate salt can be selected from, for example, sodium trimetaphosphate, potassium trimetaphosphate, calcium trimetaphosphate, sodium calcium trimetaphosphate, lithium trimetaphosphate, ammonium trimetaphosphate, aluminum trimetaphosphate, and the like, or combinations thereof. Sodium trimetaphosphate is the preferred trimetaphosphate salt. Moreover, any suitable acyclic polyphosphate may be utilized in accordance with the present invention. Preferably, the acyclic polyphosphate has at least two phosphate units. For example, acyclic polyphosphates suitable according to the present invention include, but are not limited to, pyrophosphates, tripolyphosphates, sodium hexametaphosphate having from about 6 to about 27 repeating phosphate units, potassium hexametaphosphate having from about 6 to about 27 repeating phosphate units, ammonium hexametaphosphate having from about 6 to about 27 repeating phosphate units, and combinations thereof. Preferred acyclic polyphosphates suitable for the present invention are available as CALCON ® from Solutia corporation (St. Louis, Mo.), which is sodium hexametaphosphate having from about 6 to about 27 repeating phosphate units. Further, the phosphate-containing compound can be in the acid form of any of the above salts. The acid may be, for example, phosphoric acid or polyphosphoric acid.

Preferably, the phosphate-containing compound is selected from the group consisting of: tetrapotassium pyrophosphate, sodium acid pyrophosphate, sodium tripolyphosphate, tetrasodium pyrophosphate, sodium potassium tripolyphosphate, sodium hexametaphosphate salt having from 6 to about 27 phosphate units, ammonium polyphosphate, sodium trimetaphosphate, and combinations thereof.

The ingredients in the wet gypsum accelerator of the invention can be provided in any suitable amounts. For example, the calcium sulfate dihydrate may be present in an amount of at least about 20% by weight of the accelerator, preferably at least about 30% by weight of the accelerator. The calcium sulfate dihydrate may be present, for example, in an amount of from about 35% to about 45% by weight of the accelerator, more preferably from about 38% to about 42% by weight of the accelerator. Generally, lower solids content provides higher efficiency, but also significantly increases grinding time, which results in reduced throughput and/or production rate.

The additives are preferably provided in amounts that minimize cost as much as possible, but still obtain the desired benefits of enhanced life, so that the wet gypsum accelerator maintains its efficiency over time and resists exposure to water and heat. Preferably, the additive component, whether a single additive or a combination of additives, is provided in an amount of from about 0.1% to about 10% by weight of the calcium sulfate dihydrate, more preferably in an amount of from about 0.1% to about 2% by weight of the calcium sulfate dihydrate, and even more preferably in an amount of from about 0.1% to about 1% by weight of the calcium sulfate dihydrate.

In a preferred embodiment, at least one organic phosphonic compound is used as an additive. The organic phosphonic compounds are generally superior in enhancing accelerator efficiency, even when included in relatively small amounts. More preferably, at least one phosphate-containing compound is used in combination with at least one organic phosphonic compound. For example, it is believed that, depending on the size and shape of the different active sites, the organophosphonic acid compound may enhance nucleation of some active sites while the phosphate-containing compound may act at other sites, and thus such a combination is desirable. Furthermore, in a preferred embodiment, a phosphate-containing compound, particularly a cyclic compound including at least one ion and/or salt (e.g., a trimetaphosphate compound), is added in combination with the organic phosphonic compound to enhance aging resistance. It is believed that the inclusion of the phosphate-containing compound stabilizes and maintains the wet strength of the accelerator to improve the aging properties of the wet gypsum accelerator.

In embodiments of the invention that include more than one additive, the various additives are preferably included in amounts suitable to achieve a long life and/or a desired time to 50% hydration, but preferably the total amount of additives is within the ranges set forth above. For example, in embodiments in which at least one phosphate-containing compound is used in combination with at least one organophosphonic acid compound, the organophosphonic acid compound is preferably included in an amount of from about 0.05% to about 9.95% by weight of the calcium sulfate dihydrate, and likewise the phosphate-containing compound is preferably present in an amount of from about 0.05% to about 9.95% by weight of the calcium sulfate dihydrate. In some embodiments, the additive is present up to 10% by weight of the calcium sulfate dihydrate. In some embodiments, the additive is present in an amount of about 0.05% to about 4.95% by weight of the calcium sulfate dihydrate. In a particularly preferred embodiment, the additive is a mixture of about 0.5% pentasodium salt of aminotri (methylenephosphonic acid) by weight of the calcium sulfate dihydrate and about 0.5% sodium trimetaphosphate by weight of the calcium sulfate dihydrate.

An additional benefit of the present invention is that the wet gypsum accelerator can serve as a means of providing organophosphonic acid compounds and/or inorganic phosphate compounds as a pretreatment to enhance various properties (e.g., strength, dimensional stability, resistance to permanent deformation, and the like) of the resulting set gypsum-containing compositions and products (e.g., wallboard, ceiling tile, and the like), as described in commonly assigned U.S. application No. 08/916,058 (abandoned) and commonly assigned U.S. patent nos. 6,342,284, 6,409,824, and 6,632,550 (the entire contents of which are incorporated herein by reference).

The following examples further illustrate the invention but should not be construed as in any way limiting its scope.

Example 1: rate of hydration

This example illustrates the preparation of the wet gypsum accelerator and demonstrates that the enhancement in hydration rate and efficiency of calcined gypsum compared to utilizing a dry gypsum accelerator is a result of the use of the wet gypsum accelerator of the present invention.

To prepareVarious Wet Gypsum Accelerators (WGA), using a PremierHM-45 wet bead mill equipped with PREMALLOY ® disks and a liner, were used to initially wet grind calcium sulfate dihydrate obtained from the Galena Park plant of United states gypsum in the presence of one or more additives. The calcium sulfate dihydrate starting material had an initial median particle size of about 55 microns. Specifically, 50 gallons of process water, 400lbs of calcium sulfate dihydrate, and 0.5 wt% (based on the weight of the calcium sulfate dihydrate) each of aminotri (methylenephosphonic acid) pentasodium salt (Dequest ® 2006) and sodium trimetaphosphate (NaTMP) were combined and cycled 4-5 times at a flow rate of 13-15 gallons/minute for 10 minutes, 20 minutes, and 25 minutes, respectively, in a spiral fluted stainless steel grinding chamber containing 75-82 volume% of water having a diameter of 1.2mm to 1.7mm and a density of 6.1g/cm³ZIRCONOX ® ceramic beads. The longer the grinding time of the WGA composition, the smaller the median particle size of the ground product. The resulting median particle diameters for the various WGA compositions are shown in table 1 below.

Each WGA sample was then tested to determine the rate of hydration. For each test, 300g of calcium sulfate hemihydrate from a United stateS gypsum Southard plant was combined with 300ml of tap water (70F.). WGA was added to the calcium sulfate hemihydrate slurry at a dry weight basis of 1 gram and the slurry was saponified for 10 seconds, followed by mixing at low speed for 7 seconds using a Waring blender. The resulting slurry was poured into polystyrene foam cups, which were then placed in insulated Styrofoam containers to minimize heat loss to the environment during the hydration reaction. A temperature probe was placed in the middle of the slurry and the temperature was recorded every 5 seconds. Since the setting reaction is exothermic, the extent of the reaction is measured by the temperature rise. The time to 50% hydration was determined to be the time to reach a temperature half way between the lowest and highest temperatures recorded during the test. The results are provided in table 1 below.

TABLE 1 Wet Gypsum Accelerator preparation and evaluation
								Preparation of WGA				Laboratory scale TRS evaluation
Numbering	Grinding time (min)	Median particle diameter (μm)	Acceleration efficiency (%)	Time to 50% hydration (min)	Time to 98% hydration (min)	Initial slurry temperature (F.)	Total temperature rise (F)
								1	10	2.2±4.4	120	6.75	12.08	74.3	35.5
2	20	1.7±3.4	180	5.83	11.33	72.0	35.9
								3	25	1.4±2.4	210	5.42	10.92	72.5	35.6

The results in table 1 show that as the median particle size of the calcium sulfate dihydrate decreases, the time to 50% hydration decreases and the acceleration efficiency (expressed as a percentage of the Galena Park standard dry Heat Resistant Accelerator (HRA) efficiency) increases. A higher standard deviation of the median particle size means a large particle size distribution (broad range), and a lower standard deviation of the median particle size means a smaller particle size distribution (narrow range). Since the feed material is a narrow range of synthetic gypsum (about 50 microns), the median particle size of the WGA product should generally have a large particle size distribution with high standard deviation. Generally, the longer the milling time, the narrower the particle size distribution of the WGA product with a smaller standard deviation.

Example 2: rate of hydration

This example illustrates the preparation of WGA and demonstrates that the enhancement in efficiency is a result of using the WGA of the invention.

To prepare each WGA, eight different calcium sulfate dihydrate were initially wet ground from United States Gypsum company factories using a Premier HML-1.5 wet bead mill (laboratory ultra-precision mill) in the presence of one or more additives. The calcium sulfate dihydrate starting material is a material that varies in impurities from high impurity mined gypsum to pure synthetic gypsum. In particular, 4000ml of process water, 3000 grams of calcium sulfate dihydrate (43% solids), and 0.75 wt% (based on the weight of the calcium sulfate dihydrate) each of aminotri (methylenephosphonic acid) pentasodium salt (Dequest ® 2006) and sodium trimetaphosphate (NaTMP) were combined and ground in a spiral grooved stainless steel grinding chamber containing 75 to 82 volume% of each having a diameter of 1.2mm to 1.7mm and a density of 6.1g/cm with 4-5 passes at 0.6 gallons per minute³ZIRCONOX ® ceramic beads. The relationship between grinding time and viscosity for each wet gypsum accelerator formulation is shown in table 2 below.

TABLE 2WGA preparation and evaluation
			Numbering	Grinding time (min)	WGA viscosity (cP)
1	101520	100028004240
			2	10152025	1000210034804600
3	101520	104025204680
			4	101520	120025604960
5	1020	14405760
			6	10152025	760208034805840
7	101520	124036807360
			8	101520	3000584010100

Each WGA formulation was then tested to determine the rate of hydration. For each test, 300g of calcium sulfate hemihydrate from a United stateS gypsum Southard plant was combined with 300ml of tap water (70F.). WGA was added to the calcium sulfate hemihydrate slurry at a dry weight basis of 1 gram and the slurry was saponified for 10 seconds, followed by mixing at low speed for 7 seconds using a Waring blender. The resulting slurry was poured into polystyrene foam cups, which were then placed in insulated Styrofoam containers to minimize heat loss to the environment during the hydration reaction. A temperature probe was placed in the middle of the slurry and the temperature was recorded every 5 seconds. Since the setting reaction is exothermic, the extent of the reaction is measured by the temperature rise. The time to 50% hydration was determined to be the time to reach a temperature half way between the lowest and highest temperatures recorded during the test. The results are provided in table 3 below.

TABLE 3WGA preparation and evaluation
							Preparation of WGA			Laboratory scale TRS evaluation
Numbering	Grinding time (min)	WGA viscosity (cP)	Time to 50% hydration (min)	Time to 98% hydration (min)	Initial slurry temperature (F.)	Total temperature rise (F))
							1	20	4240	4.67	10.58	76.1	34.3
2	25	4600	4.83	10.75	75.0	35.4
							3	20	4680	4.92	10.42	76.5	36.2
4	20	4960	4.75	10.25	76.4	36.1
							5	20	5760	4.33	9.67	76.7	36.1
6	25	5840	4.42	10.17	74.7	34.4
							7	20	7360	4.17	9.08	75.2	35.8
8	20	10100	4.25	9.92	74.5	37.8

The results of tables 1-3 clearly show that the time to 50% and 98% hydration decreases with increasing viscosity and with decreasing median particle size of the ground product. Generally, the longer the grinding time, the finer the median particle size of the WGA ground product will be, the higher the viscosity of the WGA will be, and the higher the acceleration efficiency will be.

Example 3: efficiency of

This example illustrates the preparation of WGA and demonstrates that the enhancement in hydration rate is a result of using WGA of the invention.

To prepare each of the WGAs, calcium sulfate dihydrate from the United States Gypensum Southard plant was initially wet ground using a Premier HML-1.5 wet bead mill (laboratory ultra-precision mill) in the presence of one or more additives. Specifically, three WGA formulations were tested that included (1) 43% solids, (2) 33% solids, and (3) 22% solids. Formulation (1) included 4000ml of tap water, 3000 grams of calcium sulfate dihydrate, and 0.75 wt% (based on the weight of the calcium sulfate dihydrate) of each of aminotri (methylenephosphonic acid) pentasodium salt (Dequest ® 2006) and sodium trimetaphosphate (NaTMP). Formulation (2) included 4690ml of tap water, 2310 grams of calcium sulfate dihydrate, and 0.5 weight percent (based on the weight of the calcium sulfate dihydrate) of each of aminotri (methylenephosphonic acid), pentasodium salt (Dequest ® 2006) and sodium trimetaphosphate (NaTMP). Formulation (3) included 5460ml of tap water, 1540 grams of calcium sulfate dihydrate, and 0.5 wt% (based on the weight of the calcium sulfate dihydrate) of each of aminotri (methylenephosphonic acid) pentasodium salt (Dequest ® 2006) and sodium trimetaphosphate (NaTMP).

To obtain WGA samples for viscosity measurement and efficiency testing, the WGA formulations were combined and milled at 0.6 gallons per minute for a specified time interval simultaneously through 4-5 strokes in a spiral fluted stainless steel milling chamber containing 75-82 volume% of particles having a diameter of 1.2mm to 1.7mm and a density of 6.1g/cm³ZIRCONOX ® ceramic beads. The relationship between milling time, viscosity, hydration time and efficiency for each WGA formulation is shown in table 4 below.

TABLE 4WGA preparation and evaluation
					Preparation of WGA			Laboratory scale TRS evaluation
Numbering	Grinding time (min)	WGA viscosity (Cp)	Time to 50% hydration (min)	Efficiency (%)
					1 (43% solids)	1015	17604320	5.424.50	87171
1 w/dispersant added	152025	2560416010000	4.584.253.58	160210390
					2 (33% solids)	01015202527	405601280272043204880	8.175.174.584.253.923.75	20103160210281330
	10152025	120240400720	------4.33	------196

3 (22% solids)

30354045505560

920120015602000248030403400

4.00--3.58--3.42--3.25

261--390--460--553

The results in table 4 show that WGA formulations with low solids content according to the invention can have excellent efficiency without sacrificing slurry workability. However, WGA throughput is significantly reduced with low solids content. Therefore, a solids content of at least about 30% is desirable to optimize the production rate, process efficiency and workability of WGA. In some embodiments, the solids content is about 38% to about 42%.

All references, including patents, patent applications, and publications, cited herein are hereby incorporated by reference in their entirety.

While the invention has been described with a preferred embodiment in mind, it will be apparent to one skilled in the art that variations of the preferred embodiment may be used and it is intended that the invention may be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications encompassed within the scope of the invention as defined by the following claims.

Claims (38)

1. A wet gypsum accelerator, comprising:

(a) a ground product having a median particle size of from about 0.5 microns to about 2 microns, wherein the ground product comprises calcium sulfate dihydrate;

(b) water; and

(c) an additive selected from the group consisting of:

(i) an organic phosphonic compound;

(ii) a phosphate-containing compound; and

(iii) (iii) a mixture of (i) and (ii).

2. The wet gypsum accelerator of claim 1, wherein the ground product is a substantially amorphous product.

3. The wet gypsum accelerator of claim 1, wherein the ground product has a median particle size of from about 1 micron to about 1.7 microns.

4. The wet gypsum accelerator of claim 1, wherein the ground product has a median particle size of from about 1 micron to about 1.5 microns.

5. The wet gypsum accelerator of claim 1, wherein the additive is present in an amount from about 0.1% to about 10% by weight of the calcium sulfate dihydrate.

6. The wet gypsum accelerator of claim 1, wherein the additive is a mixture of at least one organophosphonic acid compound and at least one phosphate-containing compound, wherein the organophosphonic acid compound is present in an amount from about 0.05% to about 9.95% by weight of the calcium gypsum dihydrate, and wherein the phosphate-containing compound is present in an amount from about 0.05% to about 9.95% by weight of the calcium gypsum dihydrate.

7. The wet gypsum accelerator of claim 1, wherein the additive is a mixture of about 0.5% pentasodium salt of aminotris (methylenephosphonic acid) by weight of calcium gypsum dihydrate and about 0.5% sodium trimetaphosphate by weight of calcium gypsum dihydrate.

8. The wet gypsum accelerator of claim 1, wherein the calcium sulfate dihydrate is present in an amount of at least about 20% by weight of the accelerator.

9. The wet gypsum accelerator of claim 1, wherein the calcium sulfate dihydrate is present in an amount of from about 35% to about 45% by weight of the accelerator.

10. The wet gypsum accelerator of claim 1, wherein the viscosity of the wet gypsum accelerator is from about 1000cP to about 5000 cP.

11. The wet gypsum accelerator of claim 1, wherein the viscosity of the wet gypsum accelerator is from about 2000cP to about 4000 cP.

12. The wet gypsum accelerator of claim 1, wherein the organic phosphonic compound is selected from the group consisting of: aminotris (methylene-phosphonic acid), 1-hydroxyethylidene-1, 1-diphosphonic acid, diethylenetriamine penta (methylene phosphonic acid), hexamethylenediamine tetra (methylene phosphonic acid), pentasodium, trisodium, tetrasodium, sodium, ammonium, potassium, calcium, or magnesium salts of any of the foregoing acids, and combinations thereof.

13. The wet gypsum accelerator of claim 1, wherein the phosphate-containing compound is selected from the group consisting of orthophosphates, polyphosphates, and combinations thereof.

14. The wet gypsum accelerator of claim 11, wherein the phosphate-containing compound is selected from the group consisting of: tetrapotassium pyrophosphate, sodium acid pyrophosphate, sodium tripolyphosphate, tetrasodium pyrophosphate, sodium potassium tripolyphosphate, sodium hexametaphosphate salt having from 6 to about 27 phosphate units, ammonium polyphosphate, sodium trimetaphosphate, and combinations thereof.

15. The wet gypsum accelerator of claim 1, wherein the accelerator allows for a time to 50% hydration of calcined gypsum of about 6 minutes or less when added to a mixture comprising calcined gypsum and water for forming an interlocking matrix of set gypsum.

16. The wet gypsum accelerator of claim 13, wherein the accelerator allows for a time to 50% hydration of calcined gypsum of about 5 minutes or less when added to a mixture comprising calcined gypsum and water for forming an interlocking matrix of set gypsum.

17. A method of making a wet gypsum accelerator, comprising:

(a) wet grinding calcium sulfate dihydrate, water, and at least one additive selected from the group consisting of: (i) an organic phosphonic compound; (ii) a phosphate-containing compound; and (iii) a mixture of (i) and (ii); and

(b) wet grinding the gypsum in the presence of the additive to form the wet gypsum accelerator to form a wet gypsum accelerator comprising a ground product having a median particle size of from about 0.5 microns to about 2 microns.

18. The method of claim 17, further comprising:

providing a mill assembly comprising a milling shaft and milling beads, wherein the milling beads have an average bead diameter of about 0.5mm to about 3mm and about 2.5g/cm³Or a higher density.

19. The method of claim 18, wherein the milled beads have an average bead diameter of about 1mm to about 2 mm.

20. The method of claim 18, wherein the milling beads are ceramic beads.

21. The method of claim 18, wherein the milling beads comprise ceria stabilized zirconia.

22. The method of claim 18, wherein the calcium sulfate dihydrate is added to the mill assembly via an automatic feed system.

23. The method of claim 18, wherein the wet grinding is performed in a single pass through the bead mill assembly.

24. The method of claim 18, wherein the wet grinding is performed in multiple passes through the bead mill assembly.

25. The method of claim 17, wherein the wet gypsum accelerator comprises a substantially amorphous ground product.

26. The method of claim 17, wherein the wet gypsum accelerator comprises a ground product having a median particle size of from about 1 micron to about 1.7 microns.

27. The method of claim 17, wherein the wet gypsum accelerator comprises a ground product having a median particle size of from about 1 micron to about 1.5 microns.

28. The method of claim 17, wherein the calcium sulfate dihydrate is present in an amount of about 35% to about 45% by weight of the accelerator.

29. The method of claim 17, wherein the organopolyphosphonic acid compound is selected from the group consisting of: aminotris (methylene-phosphonic acid), 1-hydroxyethylidene-1, 1-diphosphonic acid, diethylenetriamine penta (methylene phosphonic acid), hexamethylenediamine tetra (methylene phosphonic acid), pentasodium, trisodium, tetrasodium, sodium, potassium, ammonium, calcium, or magnesium salts of any of the foregoing acids, and combinations thereof.

30. The method of claim 17, wherein the phosphate-containing compound is selected from the group consisting of orthophosphates, polyphosphates, and combinations thereof.

31. The method of claim 30, wherein the phosphate-containing compound is selected from the group consisting of: tetrapotassium pyrophosphate, sodium acid pyrophosphate, sodium tripolyphosphate, tetrasodium pyrophosphate, sodium potassium tripolyphosphate, sodium hexametaphosphate salt having from 6 to about 27 phosphate units, ammonium polyphosphate, sodium trimetaphosphate, sodium salt, ammonium salt, calcium salt, magnesium salt, or combinations thereof.

32. The method of claim 17, wherein the additive consists of a mixture of about 0.5% pentasodium salt of aminotris (methylenephosphonic acid), calcium sulfate dihydrate, and about 0.5% sodium trimetaphosphate, by weight of calcium sulfate dihydrate.

33. A method of hydrating calcined gypsum to form an interlocking matrix of set gypsum, comprising:

forming a mixture of calcined gypsum, water, and a wet gypsum accelerator comprising a ground product having a median particle size of from about 0.5 microns to about 2 microns, wherein the ground product comprises calcium sulfate dihydrate, the accelerator further comprising water, and at least one additive selected from the group consisting of:

(i) an organic phosphonic compound;

(ii) a phosphate-containing compound; and

(iii) (iii) a mixture of (i) and (ii).

34. The method of claim 33, wherein the time to 50% hydration of the calcined gypsum is about 6 minutes or less.

35. The method of claim 33, wherein the time to 50% hydration of the calcined gypsum is about 5 minutes or less.

36. A set gypsum-containing composition comprising an interlocking matrix of set gypsum formed from at least calcined gypsum, water, and an accelerator comprising calcium sulfate dihydrate having a median particle size of from about 0.5 microns to about 2 microns, water, and an additive selected from the group consisting of:

(i) an organic phosphonic compound;

(ii) a phosphate-containing compound; and

(iii) (iii) a mixture of (i) and (ii).

37. A set gypsum-containing product comprising the composition of claim 36.

38. The set gypsum-containing product of claim 36, wherein the product is a board or panel.