PROCESS FOR MANUFACTURING ULTRA LOW CONSISTENCY ALPHA- AND BETA- BLEND STUCCO |
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申请号 | EP07811478.2 | 申请日 | 2007-08-22 | 公开(公告)号 | EP2069243B1 | 公开(公告)日 | 2017-06-21 |
申请人 | United States Gypsum Company; | 发明人 | YU, Qiang; LYNN Michael,R.; SONG Weixin,David; CLOUD, Michael, Lee; LIU, Qingxia; | ||||
摘要 | |||||||
权利要求 | |||||||
说明书全文 | This invention relates to an improved method of making calcined gypsum which results in an ultra-low consistency alpha- and beta- blend stucco. In particular, the present invention provides a process which comprises a slurry calcination step in a first reactor to produce alpha calcium sulfate hemihydrate followed by a calcination step, for example a fluidized bed calcination step, in a second reactor to produce beta calcium sulfate hemihydrate. Gypsum calcium sulfate dihydrate, CaS04.2H20 comes from a variety of sources. Land plaster is a term for natural gypsum which is any mixture containing more than 50% calcium sulfate dihydrate, CaS04.2H20 (by weight). Generally, gypsum-containing products are prepared by forming a mixture of calcined gypsum phase (i.e., calcium sulfate hemihydrate and/or calcium sulfate soluble anhydrite) and water, and, optionally, other components, as desired. The mixture typically is cast into a pre-determined shape or onto the surface of a substrate. The calcined gypsum reacts with the water to form a matrix of crystalline hydrated gypsum, i.e., calcium sulfate dihydrate. It is the desired hydration of calcined gypsum that enables the formation of an interlocking matrix of set gypsum, thereby imparting strength to the gypsum structure in the gypsum-containing product. Stucco is defined as chemically calcium sulfate hemihydrate and is a well-known building material used to make building plasters and gypsum wallboard. Stucco is typically made by crushing the gypsum rock with and then heating the gypsum at atmospheric pressure to calcine (dehydrate) the calcium sulfate dihydrate into calcium sulfate hemihydrate. In addition to natural gypsum rock the use of Flue Gas Desulphurization gypsum or gypsum from chemical processes can be used as well. Traditionally, the calcining of gypsum has occurred in a large atmospheric pressure kettle containing a mixture of the various phases of the gypsum This "dispersed consistency", also known in the art as "consistency" or "water demand", is an important property of stucco. Stuccos of lower consistency generally result in stronger casts. The normal consistency of stucco (gypsum plaster) is a term of art and is determinable according to ASTM Procedure C472, or its substantial equivalents. It is defined as the amount of water in grams per 100 grams of stucco. For example, as explained in Low consistency stucco is particularly advantageous in automated gypsum board manufacture, in which a large portion of the processing time and processing energy is devoted to removing excess water from the wet board. Considerable excess water is required in gypsum board manufacture to properly fluidize the calcined gypsum and obtain proper flow of the gypsum slurry. [0012]A dispersed consistency value of 100-150 cc. indicates a water requirement of about 85 - 100 parts of water per 100 parts of the calcined gypsum for a typical slurry in a gypsum wallboard plant. The theoretical water required to convert the calcined gypsum (calcium sulfate hemihydrate or stucco) to set gypsum dihydrate is only 18.7% by weight on a pure basis. This leaves about 67 to about 82% of the water present in the gypsum slurry to be removed in drying the board. Ordinarily, gypsum board dryers in a gypsum board manufacturing line will remove this water, for example, by maintaining the air temperature at about 400 °F (204 °C) and requiring a drying time of about 40 minutes. There is a need for stuccos having low consistency and good strength characteristics. The present invention relates to a process according to claim 1. All embodiments described in this application, which may not fall under the scope of claim 1, are herewith indicated as not in accordance with the present invention. It is an object of the invention to provide a process for making a stucco composition comprising alpha calcium sulfate hemihydrate and beta calcium sulfate hemihydrate. The present process starts with 50-75 % gypsum-containing solids by weight in aqueous slurry. Direct injection of steam of a quality between 100 to 200 psig (7,9 to 14,8 bar), into the slurry in or prior to the first continuous stirred tank reactor, at 60 psig (5,2 bar), converts 50 to 95% wt. % of the gypsum solids to alpha calcium sulfate hemihydrate. This forms partially calcined gypsum slurry which contains calcium sulfate dihydrate and alpha calcium sulfate hemihydrate. In particular, about 80-90% wt. % or 70-85 wt. % of the gypsum is calcined to alpha calcium sulfate hemihydrate. The partially calcined gypsum slurry is then dewatered, for example in a filter press to produce a filter cake of dewatered solids of 95 to 98 % solids. The filter cakes temperature is maintained above 170 °F (77 °C) during the separation. Then the dewatered hot solids are fed to an atmospheric kettle to complete the calcination process by converting the calcium sulfate dihydrate of the dewatered solids into beta calcium sulfate hemihydrate. The hot water (recovered without significant cooling) is returned to the feed of the process to minimize the energy used in the process. Alternately, the heat from the water can be used along with the waste heat from the kettle process to preheat the feed slurry of gypsum at the start of the process. The present process for making a blend of alpha- and beta-stucco results in near theoretical water demand for use in a board manufacturing process. The theoretical amount of water to hydrate 100 % pure CSH (calcium sulfate hemihydrate) to the gypsum form would be 21 parts of water to 100 parts of CSH. This process results in a water demand down to 21 parts with a minimum amount of dispersants or fluidizers required. Beta stucco alone has a water demand up to 140 parts of water and requires a large amount of dispersant to reach the flow characteristic of the alpha-beta blend stucco. Alternately a blend of alpha and beta hemihydrate can be made using powders. The resulting material requires more total energy if made by stand alone processes. Also, the resulting material requires a higher percentage of alpha to beta to achieve the same results. Therefore, the present invention provides a more economical calcining method to produce the alpha-beta stucco. The alpha hemihydrate aids in fluidity while the beta-hemihydrate aids in reactivity. The process can also be energy efficient because it can recycle hot water recovered from dewatering. Also, the solids are kept hot during dewatering to ensure the material does not hydrate back to gypsum. A crystal modifier 14 may also be fed to the reactor 12 if desired. The crystal modifier 14 controls the crystal morphology of the calcium sulfate alpha hemihydrate to achieve a desired particle size, e.g., 50 to 20 microns (dso) average particle size. Prior to the dewatering of the alpha hemihydrate slurry additives may be added that will aid in the filtration, act as a hydration accelerator, and/or provide added fluidity to the final material. The slurry 10 is held in the reactor 12 at conditions for calcining the gypsum to partially convert it to alpha calcium sulfate hemihydrate, for example 55 psig (4,8 bar) at 300 °F (149 °C). Typically, 50 to 95%, or 80 to 95 %, or 80 to 90% of the gypsum is converted by calcination to alpha calcium sulfate hemihydrate, alpha-CaS04.0.5 H20 with a residence time of 5 minutes. The conversion can be controlled by changing the residence time or temperature of the reactor discharge. The higher the temperature the faster the conversion takes place. The longer the residence time the higher the conversion rate is achieved. Typically, the reactor 12 is a continuous stirred tank reactor (CSTR) operating at a pressure of 15 to 100 psig (29.7 to 114.7 psia, 2.0 to 7.9 bar), preferably 25 to 75 psig (39.7 to 89.7 psia, 2.7 to 6.2 bar) or 35 to 55 psig (49.7 to 69.7 psia, 3.4 to 4.8 bar). The temperature of the reactor 12 corresponds to the temperature of saturated steam at the operating pressure. For example, a pressure of about 52 psig (66.7 psia, 4.6 bar) corresponds to a temperature of about 300 of (149 "C). The residence time of the slurry in the reactor 12 generally ranges from 2 to 30 minutes, preferably 5 to 15 minutes. For example, in a typical embodiment, after the reactor 12 is closed, hot steam 13 is delivered to the jacket around the reactor 12 to heat the reactor 12 for about 5 minutes. The change in temperature and pressure inside the reactor are monitored as a function of time. Then after about 10 minutes, the delivery pressure of the steam 13 was increased to bring the reaction to completion in about 5 additional minutes. The crystal modifiers 14 could, for example; be added to the slurry 10 before heating begins or while the slurry 10 is being heated or maintained at a desired temperature in the reactor 12. The partially calcined gypsum product 16 discharges from the reactor 12 as a slurry comprising calcium sulfate dihydrate and alpha calcium sulfate hemihydrate and feeds an accumulator tank 20. Accumulator tank 20 acts as a holding tank and permits release of the steam as the slurry's pressure drops to atmospheric pressure. If desired the accumulator tank 20 may be omitted if the separation stage (dewatering unit 30) is direct coupled. The slurry 24 discharges from the accumulator tank 20 and feeds a dewatering unit 30 which removes water to produce a dewatered solids containing product 32 and a removed water stream 34. All or a portion of the removed water 34 may be recycled as a stream 38 to be part of the slurry 10 to assist in recycling water, heat and chemicals (such as the crystal modifiers or other additives) used in the process. Typically the stream 38 is recycled at an elevated temperature, such as 100 to 200 OF (38 to 93 DC). The partially calcined gypsum product 16, the accumulator tank 20, the stream 24, the dewatering unit 30 and the dewatered product 32 are kept at a temperature sufficiently high to prevent the alpha hemihydrate from rehydrating, e.g., kept at elevated temperature of 160-212 °F (71-100 °C). Typically the dewatering unit 30 is a filter press and/or centrifuge and the dewatered product 32 has a 2 to 6 wt. %, typically 4 %, free water moisture content. A typical filter press employs steam to press down on a plate over the partially calcined gypsum product slurry to drive out the water. If desired the process of Baehr The dewatered product 32 is fed to a board stucco kettle calciner 40 at conditions to convert the majority or all of the gypsum in the dewatered product 32 to beta calcium sulfate hemihydrate. The kettle calciner 40 typically is indirectly heated at atmospheric pressure by use of natural gas heating on the bottom and direct fired heated air 42. The material behaves as a fluidization bed due to the free water vapor leaving the solids fed to the kettle reactor 40 as well as the bound water released as the gypsum (calcium sulfate dihydrate) converts to calcined beta gypsum (beta calcium sulfate hemihydrate). Fluidization gas may also be provided by the indirect fired gas heated air or use of direct fired heated air 42. The kettle 40 typically operates at atmospheric pressure, and a temperature of from 150 to 1000 °F (66 to 538 °C), preferably 250 to 650 °F (121 to 343 °C) or 400 to 500 °F (204 to 260 °C) or 285 to 300 °F (140 to 149 °C). The kettle 40 discharges a dry product 44 comprising alpha calcium sulfate hemihydrate and beta calcium sulfate hemihydrate (also known as an alpha and beta stucco blend). Optionally, the dry product 44 is sent to grinding 50 to reduce the particle size of the material. Typically the dry product 44 has less than 5 wt. %, preferably less than 2 wt. %, calcium sulfate anhydrite and less than 5 wt. %, preferably less than 2 wt. %, calcium sulfate dihydrate. Typically the calcium sulfate of the final product is 50-95 wt. % alpha hemihydrates and 50 to 5 wt. % beta hemihydrate; for example, 70-85 wt. % alpha hemihydrates and 30-15 wt. % beta hemihydrate; or 80-90 wt. % alpha hemihydrates and 20-10 wt. % beta hemihydrates. The crystal modifier 14, if employed, is in the solution during the period of calcination to alpha hemihydrate. The pH of the solution is in the neutral range between 6 and 8. The crystal modifiers 14 act in reducing the number of nuclei that form in the solution and also restrain the growth of the crystal in one of its axis. The result is control of the particle size through control of the number of crystals forming and growing. The other result is that the shape of the crystal is cubic like in aspect ratio. With no modifiers in the solution the shape of the alpha hemihydrate would be a long acicular needle shaped crystal of aspect ratio up to 100: 1 in length to diameter. The resulting alpha- beta-stucco blend typically has a number of desirable properties of consistency, compressive strength and density. For example, the typical dry product has a normal consistency of about 30 to 36 as measured by a handmix drop consistency determination. In contrast to normal consistency measured according to ASTM Procedure C472, normal consistency as measured by a hand mix drop consistency method is not ASTM Procedure C472 test. The test method for measuring normal consistency by a handmix drop consistency method is as follows. Weigh a 50 gram sample of the plaster to be tested at 70-80 of (21-27 "C) to 0.1 gram accuracy. Drained the mixing cup and spatula before using such that the mixing cup and spatula contain a maximum of 1/4 cc of adhering droplets of water or are wiped dry. Add water to the mixing cup from a burette (deionized or distilled at 70-80 °F (21-27 °C) unless otherwise specified) in the estimated quantity to produce the proper flow. Sift the plaster into the water and allow the sample to soak undisturbed for 60 seconds. Mix thoroughly for 30 seconds, stirring 90 to 100 complete revolutions with the spatula. Pour the slurry immediately after mixing on to a clean, dry, unscratched PLEXIGLASS sheet from a height of 1~ inch. At the correct consistency, the mix will flow out of the cup without the aid of the spatula. The mix should form a round patty of reasonably uniform thickness. The patty diameters for each specific consistency range are as follows in TABLE 1 (when measured in at least two directions and averaged): TABLE 2 presents typical crystal modifiers. Also, The stucco composition of the invention can be used in both the manufacture of gypsum wallboard and stucco for production of a plaster for interior and exterior applications. One or more additives can be added to the stucco composition to facilitate the desired viscosity, and other optional additives may be added to achieve desired physical characteristics in the final set product, such as, for example, flexural strength, abuse resistance (e.g., chip resistance), water resistance, flame resistance, and the like, or combinations thereof. A plant control and three plant trial examples of the present invention were conducted. In the Control and Examples, 75% solids slurry was fed to one continuous stirred tank reactor (CSTR) of 275 gallons (1041 liters) in size used for the Alpha-portion of the calcinations. A high temperature Tube mill was used for the Beta-portion of the calcinations of the Examples. The Tube mill was a heated ball mill. At a reactor temperature of 298 °F (148 °C), 99% of the gypsum of the feed slurry was calcined to Alpha calcium sulfate hemihydrate, which had a normal consistency of 32 to 34 cc. Normal consistencies in the Control and the following Examples were measured by the above-described hand drop test. At a reactor temperature of 285 °F (141 °C). 90% of the gypsum fed to the first reactor was calcined to Alpha calcium sulfate hemihydrate. The resulting slurry was filtered and the filtered solids were further calcined in the Tube mill at 300 °F (149 °C). The filtered product before being fed to the Tube mill was kept at elevated temperature of 160-212 °F(71-100 °C). The Tube mill converted at least a portion of the calcium sulfate dihydrate of the dewatered solids into beta calcium sulfate hemihydrate. Thus, the resulting product had 90% alpha calcium sulfate hemihydrate and 8.5%-9% beta calcium sulfate hemihydrate for a total hemihydrate yield of 98.5% or higher relative to the amount of gypsum of the feed slurry. In other words, 90% of the gypsum of the feed slurry converted to alpha calcium sulfate hemihydrate and 8.5%-9% converted to beta calcium sulfate hemihydrate. The normal consistency of the resulting product was 32 cc. At a reactor temperature of 280 °F (138 °C), 85% of the gypsum fed to the first reactor was calcined to alpha calcium sulfate hemihydrate. The resulting slurry was filtered and the filtered solids were further calcined in the tube mill at 300 °F (149 °C). The filtered product before being fed to the Tube mill was kept at elevated temperature of 160-212 °F (71-100 °C). The Tube mill converted at least a portion of the calcium sulfate dihydrate of the dewatered solids into beta calcium sulfate hemihydrate. The resulting product had 85% alpha calcium sulfate hemihydrate and 13.5%-14% beta calcium sulfate hemihydrate for a total hemihydrate yield of 98.5% or higher relative to the amount of gypsum of the feed slurry. The normal consistency of the resulting product was 34 cc. At a reactor temperature of 275 °F (135 °C), 80% of the gypsum fed to the first reactor was calcined to alpha calcium sulfate hemihydrate. The resulting slurry was filtered and the filtered solids were further calcined in the tube mill at 300 °F (149 °C). The filtered product before being fed to the Tube mill was kept at elevated temperature of 160-212 °F (71-100 °C). The Tube mill converted at least a portion of the calcium sulfate dihydrate of the dewatered solids into beta calcium sulfate hemihydrate. The resulting product had 80% alpha calcium sulfate hemihydrate and 18.5%-19% beta calcium sulfate hemihydrate for a total hemihydrate yield of 98.5% or higher relative to the gypsum of the feed slurry. The normal consistency of the resulting product was 32 cc. The data shows the present inventive process has the advantage that it results in a combined alpha calcium sulfate hemihydrate and beta calcium sulfate hemihydrate product that has a normal consistency similar to that of an alpha calcium sulfate hemihydrate product. |