WET-GRINDING GYPSUM WITH PQLYCARBOXYLATES

FIELD OF THE INVENTION

This invention relates to wet grinding of gypsum More specifically, it relates to grinding of a slurry of gypsum with added dispersants

BACKGROUND OF THE INVENTION

Finely ground gypsum has many uses. It is added to cosmetics, paper or coatings as a filler. Pure, fine ground gypsum is also useful as a calcium supplement in foods or pharmaceuticals Preparation of the ground gypsum is done using any known grinding method, particularly wet grinding The finely ground solid product is useful as a filler in coatings, paper or cosmetics. Ground gypsum is also useful as an additive to food or pharmaceuticals.

Wet grinding of gypsum is an economical method of producing a powder having average particle sizes less than 10 microns A slurry of gypsum in water flows through a comminution or grinding device, such as a ball mill, and can be recycled through the comminution device as much as necessary to obtain the desired particle size range. It is desirable to grind a slurry that is highly loaded for use in paper coatings However, highly loaded slurries are difficult to pump. For example, a slurry having a 70% loading by weight (70 wt % gypsum and 30 wt % water), for all useful purposes does not flow at all

Even when a pumpable slurry is attained, it is difficult to keep it flowing It has been found that as grinding progresses, the slurry thickens and becomes viscous as the average particle size decreases While not wishing to be bound by theory, it is believed that grinding exposes fresh, reactive gypsum faces that tightly bind water molecules, causing the slurry to thicken Thickening of the siurry requires additional energy to pump it through the grinding device, increasing the cost of the entire process

Dispersants are known for reducing the viscosity of a slurry or solution, but the response of particular dispersants to a specific slurry or solution is unpredictable For example, dispersants known for use with cements are not necessarily effective in dispersing gypsum solutions Of the dispersants known to be useful for gypsum slurries, some cause severe retardation in the setting of the hemihydrate form Knowledge that a particular dispersant is effective in dispersing calcium sulfate slurries does not ensure that the dispersant is suitable for a particular application

Processes are also known for milling gypsum and a titanium dioxide pigment together in the presence of a dispersant, as shown in U S Patent No 5,338,354 However, thus patent fails to teach or suggest the problem identified and solved by Applicants, the problem of thickening of the gypsum slurry during grinding There is no suggestion that polycarboxyiate dispersants are useful for wet grinding of gypsum in the absence of titanium oxide or that natural landplaster or terra alba are suitable feed materials

Cement additive compositions that include polycarboxyiate dispersants are disclosed in U S Patent No 6,800, 129 ('"129 Patent") Combining at least two stable suspensions makes the additive composition The additive composition may be

used with clinker or gypsum before or during the grinding operation However, this reference fails to disclose wet grinding of the feed material, and it would not be understood by an artssan that wet grinding was contemplated Addition of water to clinker would cause the cement to start to set The teachings of the '129 Patent would not be suitable for finely grinding the gypsum slurry of this invention

SUMMARY OF THE INVENTION

Improvements to wet-grinding of gypsum are found in a method of making a finely ground gypsum product that uses a dispersant to grind particles to a smaller particle size than is achievable without it The dispersant is a polycarboxylate dispersant that is highly effective for fiusdizing the gypsum slurry, reducing the product pressure and allowing flow through the grinding media with less power utilization by the pump

More specifically, the process of the present invention includes obtaining a feed material having an initial median particle size A solution of a polycarboxylate dispersant in water is formed, then the feed material is added to the solution to form a slurry The slurry is wet-ground to reduce the median particle size of the feed material Optionally, the total amount of polycarboxylate dispersant is divided into a plurality of doses that are added periodically over the total grinding time Use of the polycarboxyiate dispersant allows the slurry to contain higher loadings of solids compared to other wet-grinding processes Grinding of a slurry with higher solids content reduces the grinding time needed to produce a specific amount of product This reduces power consumption since less grinding is required In

facilities that are limited by the grinding step, this can also result in overall improvement in process efficiency Slurries with high solids loading also have other particular uses such as in paper coatings applications Where needed, use of this method also allows for longer grinding times to produce a finer product As grinding proceeds, the viscosity of the slurry increases and places more of a load on the pump Eventually it is no longer economical to pump the slurry to further reduce the average particle size of the feed material The efficacy of the polycarboxylate dispersant allows grinding to a higher degree than other dispersants, so that a higher value, more finely ground, product is obtainable

Reduction in the load on the pump or grinding time also results in a cost savings on the basis of energy usage Less pumping required less energy Reduction sn fuel that produces energy results in a real savings at a time when fuel costs are very high

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to preparation of a finely ground feed material Feed material is added to water to form a slurry, then fed to a grinder or other comminution device A polycarboxylate dispersant is added to the slurry. Unless otherwise specified, concentrations used in this discussion are weight percents based on the total weight of solids present

Numerous feed materials are used with the process of this invention A preferred feed material is gypsum, also known as landplaster or calcium sulfate dihydrate. Other useful feed materials include calcium sulfate anhydrite or calcium carbonate The use of

combinations of the above-listed materials with each other or with other fillers, such as clay, are also contemplated In some embodiments, the feed material is essentially iandplaster

Preliminary grinding of the feed material may take place prior to introduction to this process Some grinding is advantageous to allow the feed material to be transported and moved by conveyor conveniently After the preliminary grinding, the feed material to the present process has an initial particle size distribution The initial particle size distribution of the feed material depends upon the capacity of the wet grinding mill that is available Typically, the median particle size of the feed material is from about 10 microns to about 15 microns or from about 12 microns to about 15 microns Tailings of the feed material are preferably within the 30 to 40 micron range The present process reduces the initial median particle size of the feed material to a lesser median feed partele size distribution

Either natural gypsum or synthetic gypsum is used Natural gypsum required no physical or chemical treatment to render it useful in the intended applications An especially preferred feed material is terra alba, a high purity, white gypsum having an initial median particle size of about 12-15 microns (United States Gypsum Company, Chicago, I L) Since the feed mateπai preferably has an initial median particle size of about 12-15 microns, less grinding is required to reduce the preferred median particle size distribution of 2 microns or less Wet-grinding the slurry reduces the average particle ssze of the feed material compared to the initial average particle ssze before grinding The slurry is formed by combining the feed mateπai with water Any amount of water can be used that produces a flowabSe slurry When the amount of water is minimized, energy costs can be

reduced since less water will need to be evaporated when the finely ground product ss dried in preferred embodiments, the solids loading is at least 40% by weight Loading of the feed material to 55% by weight or higher is more preferred Preferably, the feed material is ground to an average particle size of less than 2 microns, with the largest particles less than 10 microns

Polycarboxylate dispersants are used to reduce the viscosity of the gypsum slurry The polycarboxylate dispersant includes one or more carboxylate or carboxylic acid repeating units Examples of suitable repeating units are vinyl groups, acrylic groups, maleic acid groups, and the like Useful co-polymers are polymers that include two or more repeating units that can be arranged in any order along the length of the polymer chain The dispersant is preferably a comb-branched polyether polycarboxylate In this arrangement, long chain repeating units are separated by one or more shorter repeating units Any polycarboxylate having dispersant properties appropriate to the feed material is usable in this invention

Particularly preferred polycarboxylates have at least three repeating units, an acrylic unit, a maleic acid repeating unit and a long-chain polyether repeating unit Polycarboxylates of this type are disclosed in U S Patent No 6,777,517, herein incorporated by reference and hereafter referenced as the "2651 -Type Dispersant " The 2651-Type Dispersant has been found to be especially effective in reducing the viscosity of the slurry as it passes through the comminution device These dispersants are marketed under the names MELFLUX 2641 , MELFLUX 2651 and MELFLUX 3L (BASF Construction Polymers GmbH, Trostberg, Germany) High dispersion efficacy allows reduction of the amount of dispersant to be used This is beneficial to process economics since polycarboxylate components

are relatively expensive It is anticipated that any comb-branched polycarboxylate would be useful in this process Other useful commercially available dispersants include MELFLUX 1641 (BASF Construction Polymers GmbH, Trostberg Germany) Preferably the dispersant is used in amounts of about

0 1 % to about 5% based on the weight of the feed material More preferably, the amount of the dispersant is about 0 4% to about 2 5% Many embodiments use less than 1% dispersant by weight The amount of dispersant selected depends primarily on the loading of feed material used in the slurry and the length of grinding needed to reach a specific particle size Generally, the amount of dispersant increases as the amount of grinding increases More dispersant is also needed as the loading of the slurry increases Addition of the dispersant can be in a single dose or a series of small doses Use of a plurality of doses is discussed in greater detail below

Use of two or more dispersants in combination is contemplated However, care must be taken in combining dispersants of different chemical types For example, combinations of polyethers and polyacrylates are known to gel, limiting the effectiveness of both dispersants

Where a powdered dispersant is used, a solution is formed by dissolving the polycarboxylate in water Upon adding the dispersant powder to the water and stirring until the powder is dissolved, a satisfactory solution is obtained However, preferably, the solution is aged overnight or about 24 hours When the dispersant solution is aged prior to use, it has been more effective at reducing the viscosity of the gypsum slurry While not wishing to be bound by theory, it is believed that the dispersant molecule relaxes and becomes at least partially uncoiled Uncoiling of the molecule is likely

to expose more or different reaction sites, changing the reactivity and/or selectivity of the dispersant Although the aged solution is more effective in dispersing the gypsum particles, solutions that are not aged are also useful in this invention The gypsum and the dispersant are added to water to form a slurry Preferably the dispersant is added to the water before the feed material, however, there is not believed to be a significance in the order of addition Using the dispersant of this invention, relatively high loadings of solids are possible Solids are loaded into the solution up to 55% by weight on the basis of the total weight of the solution Preferred solids loadings are approximately 40 weight % to about 55 weight %

Optional additives can be added to the slurry to alter one or more properties of the slurry or the converted feed material Any additives known to alter the properties of the slurry or the ground feed material are added in their conventional amounts Where a slurry low in calcium is desired, a chelating agent is able to be added to tie up calcium ions, reducing their concentration in solution The addition of sodium carbonate also reduces the dissolution of calcium ions Sodium carbonate has also been found to further lower the viscosity of the slurry, and is therefore a preferred additive when used in amounts of about 0 5% to about 0 25% by weight based on the weight of the gypsum solids

Optionally, a pH modifier is added to enhance aging stability With some polycarboxylates, control of pH reduces the viscosity increase with time Increasing the slurry pH to a range of about 8 to about 10 slows the rate of viscosity increase during agsng Control of pH had the largest effect on Melflux 2651 dispersant, while Melflux 3L dispersant showed no response to changes in pH

In some embodiments, it is advantageous to add the dispersaπt as a plurality of small doses As grinding progresses, the slurry becomes more viscous as the grinding exposes new surfaces The increase in viscosity is often equated with the pressure needed to push the feed material through the grinder and piping to and from the grinder, and referred to as the "product pressure " Even slurries that had good initial flowability increase in viscosity as the grinding continues In such cases, the addition of a subsequent dose of the dispersant is optionally added to the slurry The viscosity of the slurry drops again, allowing the slurry to be pumped more easily and economically if grinding continues and the slurry again becomes viscous, the slurry is optionally redosed with dispersant until a satisfactorily reduced median particle size ^s obtained After an initial dose, additional dispersant is suitably added at intervals of about 5 to about 10 minutes Use of longer or shorter intervals is contemplated including continuous addition of the dispersant The intervals need not be regular or time based Dispersant dosage is also determinable based on the slurry viscosity or pump pressure

The feed material is ground by any known grinding or comminution apparatus Examples of suitable grinding apparatus include ball mills, rotary grinders or any aqueous grinding apparatus where low viscosity is beneficial in the product slurry Where a ball mill is used, silica beads are the preferred grinding media The slurry is recycled through the grinding apparatus until the desired median average particle size is obtained

In the following examples, feed material was processed wsth a Supermill 1 5 wet-grinding rrull from Premier Mill, an SPX Process Equipment Company (Delevan Wl) The grinding media used was a high density, high crush strength ceramic bead,

ZIRCONOX brand cerium-stabilized zirconia (Techno Ceramic lnc , Lower Makefielcf, PA) The feed material, polycarboxylate dispersant, water and additives, if any, were mixed in a five gallon bucket, then charged to the feed tank The pump was started on low speed and the speed was gradually increased The mill was started and a rag was used to cover the feed tank to reduce splashing therefrom Mill speed was adjusted to about 1750 fpm The first liter of slurry was measured and discarded The flow rate was measured with a timer and beaker The material was reαrculated prior to taking samples AfI viscosity data was taken using a Brookfield RV viscometer at 50 RPM unless otherwise noted (Brookfieid Engineering Laboratories, lnc , Middleboro, Massachusetts)

EXAMPLE 1

A slurry of 43% solids was prepared from 4500 g of Terra Alba No 1 powder (United States Gypsum Company, Chicago, I L) and 600Og tap water Median particle size for the Terra Alba was 5 2 microns with a top size of 31 microns Particle size data were measured on a Microtrac X100 Analyzer (Microtrac, lnc , North Largo FL) Weight percents of additives were measured on the basis of the dry terra alba

Water was added to the feed tank and 0 5 wt % sodium tπmetaphosphate was added The gypsum powder was added to the slurry chamber Finally, 0 5 wt % Dequest 2006 dispersant, penta sodium salt of amino tπ(methylene phosphonic acid), was added to the slurry The slurry was ground for a total of 20 minutes Grinding was stopped because the product pressure had πsen to 6 1 psi Samples of the slurry were taken at 10, 15 and 20 minutes From 10 minutes to

20 minutes, the viscosity of the sample rose from about 1700 cps to almost 7000 cps Although the median particle size after 20 minutes of grinding was 2 59 microns, the top particle size was 22 microns, much larger than desired

EXAMPLE 2

A five-gallon pail of terra alba was milled in two trials described below Sodium carbonate, approximately 1 1 g, was added to 600O g water Next 37 5 g Melflux 2651 dsspersant was added to the mixture Terra alba, in amounts of approximately 7333 g, was introduced into the feed tank

Additional doses of the Melflux 2651 dispersant were added at 15, 23, 36, and 40 minutes of milling Each additional dose of 37 5 g of the dispersant reduced the product pressure about 0 1 to 0 2 psi The maximum product pressure during the trial was 1 7 psi The total dose of Melflux 2651 dispersant was 1 0 wt% based on the weight of the terra alba The slurry flow rate after 45 minutes when the trial was terminated was 33 3 mL/s In a second trial, the same procedure was used for mixing and loading the slurry into the feed tank The slurry flow rate was 33 mL/s Dispersant was dosed, however, the timing of the dosing on this portion of the trial was unclear Samples from the first trial and the second trial were removed for viscosity measurements at 25, 35 and 45 minutes into grinding Results of the viscosity testing is shown in Table I below

TABLE

After one day, the slurries from both trials were combined into one five-gallon bucket The one day viscosities were measured on each batch as well as the combined slurry These results are presented in Table Ii below

TABLE

Sample One-Day Viscosity (cP) of Example 1 at Shear Rate

20 RPM 50 RPM

Trial 1 1492 1033

Trial 2 1950 1090

Combined 2300 1 1 13

The slurry density was also measured and found to be

1 48 g/L

EXAMPLE 3

A five-gallon pail of natural anhydrite was milled in two trials similar to Example 2 above In the first trial, eleven grams of sodium carbonate was added to 6000 g water Thirty-seven and one/half grams of Melflux 2651 solution was added to the mixture Fmally, 7333 grams of natural anhydrite were added and the slurry was put into the feed tank The mill speed was 1750 fpm, and the first liter of diluted slurry was collected and discarded Only one additional dose of Melflux 2651 dispersant solution was added at 26 30 minutes of milling The product pressure decreased 0 4 psi upon its addition The maximum product pressure during the trial was 1 5 psi The trial was terminated at 45 minutes with a final product pressure of 1 1 psi The total dose of Melflux 2651 solids was 0 4% by weight of cement filler

In the second trial, a similar procedure was used for mixing and loading the slurry into the feed tank Again, only one additional dose of Melflux 2651 solution was added, but this time at 27 minutes In this tπal, the product pressures were lower, with the maximum product pressure only being 1 2 psi The Melflux 2651 dose decreased the product pressure from 1 2 pss to 1 0 psi The final product pressure was 1 0 psi

Samples of both trials were removed for viscosity measurements at 25, 35 and 45 minutes The slurries were combined into one bucket, and the combined viscosity was also measured A compilation of viscosity results from both trials is presented in Table SIE below

TABLE

In this experiment, sodium carbonate was added with the mix water prior to the Melflux 2651. Six thousand grams (6000 g) of water were added to the feed tank. One dose of Melflux 2651 powder at 0.2 wt % based on the weight of terra alba (9 g) was added to the tank. Since the lightweight powder tends to float on the surface of the water and does not dissolve easily, the water/dispersant mixture was circuiated through the grinding media to aid in its solution and dispersion. Terra alba, in an amount of 4500 g, was added to the tank for a total grinding time of 50 minutes. Subsequent doses of the Melflux 2651 were added at 10, 20, 30, 40 and 50 minutes during milling. Grinding time was extended to 60 minutes. The viscosity was decreased to between 76 and 140 cP during grinding.

Product pressure and viscosity were monitored between two experiments as detailed above, one with sodium carbonate and the other without it. The sodium carbonate appears to decrease the viscosity until approximately 50 minutes into the trial. This requires

less power to be applied to the pump, and the resulting energy savmgs

COMPARATIVE EXAMPLE 5

Ethacryl G (Lyondell) dispersant was added to the slurry to reduce the viscosity of the gypsum products It is a copolymer of polycarboxylate sn the backbone and polyether in the side chains It is a liquid comprising 40% dispersant solids and a pH of 4 0 A total amount of Ethacryl G was measured so that the dispersant soiids weighed were 4% by weight of the feed material The total amount of Ethacryl G was subdivided into 9 doses that were added initially, at 8 minutes, 13, 18, 22, 28, 32, 37, and 43 minutes The total grinding time was 48 minutes A slurry was made by mixing 4500 g Terra Alba No 1 with 6000 g water The terra alba had an initial median particle size distribution of 5 2 microns, with a top particle size of 32 microns The initial dose of Ethacryl G dispersant was added to the slurry and it was wet-ground to reduce the particle size distribution The dosing schedule resulted in a slurry having a viscosity of approximately 1000 to about 2500 cps over the 48 minutes of grinding Median average particle size was reduced to 1 1 microns and the top particle size was 9 microns

Compared to Ethacryl G dispersant, the Melflux 2651 polycarboxylate dispersant produced greater reduction in viscosity at one-quarter of the dosage The efficacy of the Melflux 2651 polycarboxylate allows for savings in energy due to lower pump loads, resulting in lower savings Although polycarboxylate dispersants are

generally higher in cost, at thus low dose, savings may be realized from the reduction in dispersant use

While a particular embodiment of the method of wet- gπnding a finely ground gypsum product has been shown and described, it will be appreciated by those skilled in the art that changes and modifications may be made thereto without departing from the invention in its broader aspects and as set forth in the following claims