Abstract: The present invention relates to a method of preparing a polyrotaxane, said method comprising: performing a radical copolymerization of at least (a) a first polymerizable monomer having a stopper group, and of at least (b) a second polymerizable monomer, wherein said second monomer is complexed by a ring-shaped molecule; wherein during said copolymerization a copolymer threading said ring-shaped molecule is formed, wherein during said copolymerization said first monomer having a stopper group is incorporated into the chain of said copolymer at least partially between the ends thereof, and wherein said stopper groups prevent said ring-shaped molecule from disassembling from the copolymer; and wherein the amount of said first monomer having a stopper group is of from 0.1 mol % to 20 mol % based on 100 mol % of the total amount of polymerizable monomers. The present invention also relates to polyrotaxanes which can be prepared by using such a method.

Abstract: In a process for the continuous conditioning of gypsum plaster, the gypsum plaster is passed from a calcination facility connected upstream to a gypsum plaster cooler in the form of particles. In the gypsum plaster cooler, soluble calcium sulphate anhydrite is initially converted to calcium sulphate hemi-hydrate and calcium sulphate dihydrate to calcium sulphate hemi-hydrate, and crystal defects are eliminated. Subsequently, the gypsum plaster is brought into contact with ambient air and dehumidified by the latter and thereby simultaneously cooled indirectly.

Abstract: An air pollution control system 10A according to the present invention includes: a boiler 11 that burns fuel; NOx removal equipment 12 that decomposes nitrogen oxides in flue gas 25 discharged from the boiler 11; a desulfurizer 15 that causes sulfur oxides in the flue gas 25 having passed through the NOx removal equipment 12 to be absorbed by an absorbent, thereby reducing sulfur oxides in the flue gas 25, a waste-water treatment device 16 including a solid-liquid separating unit 31 that separates desulfurized waste water 28 discharged from the desulfurizer 15 into a solid fraction and a liquid fraction, and a mercury removing unit 32 that removes mercury in the desulfurized waste water 28; and a treated waste-water returning unit (a makeup water line) 17 that returns at least a part of treated waste water 40 treated by the waste-water treatment device 16 to the desulfurizer 15.

Abstract: A system (100) for calcining natural gypsum, synthetic gypsum (112) or a combination thereof, the system including: a mill (114) for grinding and drying natural gypsum, synthetic gypsum (112) or a combination thereof, to produce dried gypsum (116); a flash calciner (118) for calcining the dried gypsum to produce an exhaust gas (130) and calcined gypsum (128); and a mechanism (134) for transporting at least a portion of the exhaust gas (130) produced by the flash calciner (118) to an air heater (126) that supplies hot gas (126a) to the flash calciner (118).

Abstract: A process and system for recovering waste heat from a kiln used for lime or cement production. The system unifies the kiln, a waste heat recovery and power generation circuit and a dry scrubber for scrubbing the pollutants from the kiln offgas. Essentially, the system employs the boiler component of the waste heat recovery and power generation circuit as a heat exchanger to recover the waste heat from the kiln, which is used to drive the steam turbines. The heat absorption from the latter stage lowers the temperature of the kiln offgas sufficiently for optimum performance from the scrubber. The presence of lime particles in the offgas effectively protects the boiler tube surfaces from corrosion which would occur at optimum scrubber temperatures, and subsequently provides the lime required as a scrubbing medium for the dry scrubber. Further, the efficient scrubbing allows for the use of any fuel for firing the kiln inclusive of high sulphur content compounds.

Abstract: A process for producing alpha-hemihydrate gypsum from dihydrate gypsum includes feeding a slurry comprising the dihydrate gypsum and water into a heating tube, heating the heating tube at a temperature effective to generate steam and pressure from the water, wherein the steam and pressure are effective to convert the dihydrate gypsum to the alpha-hemihydrate gypsum, and withdrawing the alpha-hemihydrate gypsum from the heating tube.

Abstract: A gypsum dryer/calciner includes a calcining space; a first pipe having an inlet connected to a source of hot gases and an outlet emerging in the calcining space; a second pipe having an inlet connected to a source of gypsum and an outlet emerging in the calcining space, the second pipe being concentric with the first pipe; and a force-feeding screw positioned at least partially in the second pipe, the screw carrying the gypsum along in the calcining space.

Abstract: A process is disclosed for making dead burn calcium sulfate anhydrite. The process includes a slurry calcination step to produce calcium sulfate anhydrite followed by removing free water from the anhydrite while avoiding substantial rehydration. Also disclosed are dead burn calcium sulfate anhydrite particles with high aspect ratios, e.g. fibers having an average aspect ratio of at least 4:1, or very fine particle size, e.g., at or below 2 micrometers average, combined with very low water content, e.g., less than 0.5, preferably less than 0.3, wt. % water. These particles are typically useful as fillers.

Abstract: A process for synthesizing a hydraulic binder based on natural calcium sulfate (gypsum), the process consisting in heating the calcium sulfate to obtain a large proportion of anhydrite III or &agr; anhydrite, and then in quenching. The process consists in: completely dehydrating the calcium sulfate, and suddently cooling the matter thus dehydrated. The present invention finds a preferred application in the construction industry in obtaining a hydraulic binder that can be used as cement.

Abstract: A method of preparation and use of lightweight, high-strength gypsum wallboard, as well as a core composition suitable for use therein, are disclosed. The core composition includes a slurry of calcium sulfate hemihydrate (stucco), water, acid-modified starch, and a starch cross-linking agent, having a pH of about 9 to about 11. The composition and method provide a wallboard having a lower density than conventional wallboard, and with equivalent or better strength characteristics than conventional wallboard.

Abstract: A method of purifying a flue gas desulfurization aqueous waste slurry containing calcium-sulfur salts and residual solid grits, fly ash, carbon and oils by supplying the aqueous waste slurry to a screening station to remove the grits, passing the grit-free aqueous waste slurry to a magnetic separator to remove fly ash components, and transferring the aqueous slurry to a floatation cell to remove carbon and oils therefrom. A purified aqueous slurry of calcium-sulfur salts is removed from the floatation cell which can be used in producing quality plaster, wallboard, or feedstock in producing &agr;-hemihydrate gypsum.

Abstract: An apparatus for continuously calcining gypsum includes a flash calcining impact mill, a feed bin adjacent the impact mill for holding gypsum, and a gypsum feed control mechanism for supplying gypsum from the feed bin to the impact mill at a substantially constant rate. A method for continuously producing calcined gypsum includes the steps of providing a flash calcining impact mill and providing a feed bin adjacent the impact mill, the feed bin containing gypsum to be calcined. A gypsum feed control mechanism is provided for supplying gypsum from the feed bin to the impact mill. Gypsum is supplied from the feed bin to the impact mill at a substantially constant rate, and the impact mill is operated to dry, grind, and calcine the gypsum.

Abstract: A method and apparatus is disclosed for the continuous calcining of gypsum material in a high-efficiency, refractoryless kettle preferably heated by a multiple series of separate immersion tube burner coils, each coil operating within a specific calcining zone inside the kettle. The lowest, i.e., initial, burner tube coil is formed with a low profile to permit use of a small initial gypsum charge, and hence, a quick kettle start-up cycle. That low profile initial burner design also helps keep the agitator motor's load at a minimum during cycle start-up. Due to the immersion tube burner coil construction, no refractory structure for the kettle is required. Also, no separate hot pit structure is required, as the kettle is used to merely hold the material being calcined and no residual heat is present. A relatively thin kettle can be constructed, as it need only withstand relatively low operating temperatures.

Abstract: A process and plant for drying and then separating moist salts, such as magnesium sulfite, in which the moist magnesium sulfite is heated to a temperature greater than or equal to about 200.degree. C. in a dryer and the passed through a solid materials lock into a separating plant. The dryer is heated by a circulating stream of exhaust vapors which is heated in a heat exchanger by exhaust gas generated in the separating plant so that the water of crystallization of the magnesium sulfite as well as residual humidity is eliminated during the drying process. Vapor-free acid anhydride, usually sulphur dioxide, is prepared in the separating plant from the dehydrated salt and a metal oxide, e.g., MgO, is recovered or otherwise used. The acid anhydride may be liquified and has a large degree of purity.

Abstract: Both the drying and also the removal of the water crystalization take place carefully in a respective fluidized bed apparatus (A, B) without any danger of overheating. This is achieved by a complete homogenous fluidization through indirectly heated hot gas. A substantial quantity of energy is transferred via heat exchanger systems (4 or 12) in the fluidized layer. These are heated by heat carrying media. The constructional design of the fluidized bed apparatuses (A, B) which have a rectangular base outline ensures a narrow dwelltime spectrum and a pronounced temperature/moisture profile without the aid of differentially fluidized chambers. Because of the comparatively cold infeed zone and the careful heating which thereby arise the characteristics of the crystals are favourably influenced. The product is characterized by high quality homogenity. In the case of the calcining of moist gypsum no undesired gypsum modifications arise.

Abstract: A process, and the product produced thereby, for the preparation of a waste solidification material from hardened fluorogypsum as a by-product of the manufacture of hydrofluoric acid by reacting fluorospar with sulfuric acid, removing hydrogen fluoride from the reaction product, slurrying the by-product with water, placing the slurried by-product fluorogypsum in settlement ponds until the fluorogypsum hardens, the hardened fluorogypsum having a pH of greater than about 5 to about 13, including heating the hardened fluorogypsum to evaporate substantially all water physically mixed with the fluorogypsum and further heating the fluorogypsum to reduce the water chemically bound to calcium sulfate in the fluorogypsum to form about 0.5% to about 9% by weight.

Abstract: A method for treating produced gypsum and, in particular, a method for treating flue-gas desulfurization gypsum forms highly active, modifiable gypsum binders of high strength with simultaneously drying, deagglomerating and dehydrating. Pursuant to the invention, produced gypsum (flue-gas desulfurization gypsum), with a particle size ranging up to 200 .mu.m, to which 10% by weight of milled natural gypsum is optionally added, is dehydrated at a material temperature of 335.degree. to 363.degree. K. and at a pressure ranging from 2.5 to 8 kpa, the dehydration being concluded when anhydrite III constitutes 50% to 100% by weight.

Abstract: A method for enhancing the efficiency of a thermal process such as calcination. The incoming solid particulate material is preheated in the conveyor by partially cooled exhaust vapors from the thermal processor. In passage through the conveyor, the cooled, intermediate temperature vapors contact, fluidize and heat the particulate solids to increase the available throughput of the system.

Abstract: In a process for preparing calcium sulphate alpha hemihydrate essentially the form of primary crystals, the hemihydrate is milled to specific surface area of 1200 to 4000 cm.sup.2 /g, the fines are removed by screening, and a retarder and liquefier in the form of at least one fruit acid and/or its or their salt(s) are admixed. The product obtained is characterized by delayed onset of rigidity, rapid development of resistance and high resistance, and is used in particular as an injection mortar.

Abstract: A method for production of sulfur dioxide from calcium sulfate by high temperature thermal reduction. Calcium sulfate, preferably in the form of calcined phosphogypsum, is desulfurized using a fossil fuel as a heat source, the combustion products resulting therefrom being used as a reductant. The fuel and reductant may be coal, coke, liquid hydrocarbons or natural gas; high sulfur, medium or low rank coal is preferred. The fuel is combusted with air or oxygen-enriched air. The solid feeds are introduced as finely-divided particles entrained in reactant gases, and the temperature is sufficiently hot to cause residual processed materials to melt and drain from the reaction vessel in the form of a liquid slag, yet not hot enough to produce significant amounts of fine silicon oxide dust. Iron pyrite, sand and/or clay can be added to reduce the slag melting point and to control its viscosity.

Abstract: The process makes calcium sulfate alpha-hemihydrate or essentially plaster of paris from finely divided gypsum made by desulfurizing flue gas obtain from a power plant fired by brown coal by transforming the calcium sulfate dihydrate in the presence of saturated steam. A molded body is formed from the calcium sulfate dihydrate which has a plurality of pores. The pore volume amounts to 15 to 60% by volume of the total volume of the entire molded body. More than 5% by volume of the pore volume contains air. The molded body is fed to an autoclave. In the presence of a sufficient water quantity in the pores, the crystal habit and the crystal growth of the calcium sulfate alpha-hemihydrate crystals which grow in an aqueous solution phase are controlled by a process temperature in the range between 110.degree. to 180.degree. C. by the pressure of the treatment atmosphere in the autoclave. The molded body is removed from the autoclave after recrystallization transformation and delivered for use.

Abstract: The process makes calcium sulfate alpha-hemihydrate from finely divided calcium sulfate dihydrate by transforming the calcium sulfate dihydrate in the presence of saturated steam. A molded body is formed from the calcium sulfate dihydrate which has a plurality of pores having a pore volume amounting to 15 to 60% by volume of the total volume of the entire molded body. More than 5% by volume of the pore volume contains air. The molded body is fed to the autoclave. In the presence of a sufficient water quantity in the pores the crystal habit and the crystal growth of the calcium sulfate alpha-hemihydrate crystals which grow in an aqueous solution phase are controlled by a process temperature in the range between 110.degree. to 180.degree. C. by the pressure of the process atmosphere in the autoclave. The molded body is removed from the autoclave after recrystallization transformation and delivered for use. Key products of this process and their derivatives are also described.

Abstract: In a method and apparatus for calcining calcium sulphate dihydrate or like heat sensitive material, in which a bed of the material is heated in a vessel (41) by the direct introduction of a hot gas through a tube (21) extending downwardly into the bed, the proportion of insoluble anhydrite (II) in the product is reduced by lowering the temperature of the hot gas before it contacts the material of the bed. This is done by indirect heat exchange through the wall of the tube 21 with a relatively cool substance, for example, powdered gypsum or recycled exhaust gases, in an outer tube (26). The cooling material in the outer tube contacts the material in the bed before the latter is contacted by the hot gas.

Abstract: A method of producing cement or concrete, comprising the step of adding fly ash to the cement or concrete at the production stage, the fly ash containing lime and reaction products from the reaction occurring between lime and sulphur oxides in connection with the desulphuration of flue gases. To avoid the disadvantageous effects of calcium sulphite and entringite reactions and to increase the used amount of fly ash containing desulphuration waste, the calcium sulphite is oxidized by hydrogen peroxide either before the production of cement or concrete or during the production.

Abstract: The present invention relates to the coproduction of a combustible gas stream usable as an energy source, a sulfur-dioxide-containing second gas stream usable as a source of oxidant in the gasification of coal, and a sulfur-dioxide-containing third gas stream usable as a feedstock for the production of sulfuric acid. The process includes heating coal in a coal gasification zone in the presence of an oxygen and sulfur dioxide-containing atmosphere under partial coal gasifying conditions to produce a carbonaceous char and a crude coal gas stream. Sulfur-containing compounds are removed from the coal gas stream and converted to elemental sulfur. The carbonaceous char is combined with gypsum to form a feed mixture. The non-gypsum portion of the feed mixture contains sufficient reducing potential to release substantially all of the sulfur in the gypsum as gaseous compounds of sulfur in a +4 or lower oxidation state.

Abstract: A pelletized mixture of gypsum, carbonaceous material and pyrite is charged to a travelling grate where the charge is heated under suitable conditions to produce a solid sintered material which has a broad spectrum of applications due to its chemical and physical properties and a gaseous effluent containing sulfur dioxide, sulfur or mixtures thereof.

Abstract: A process, and the product produced thereby, for the preparation of a waste solidification material from hardened fluorogypsum as a by-product of the manufacture of hydrofluoric acid by reacting fluorospar with sulfuric acid, removing hydrogen fluoride from the reaction product, slurrying the by-product with water, placing the slurried by-product fluorogypsum in settlement ponds until the fluorogypsum hardens, the hardened fluorogypsum having a pH of about 3 to about 5, including heating the hardened fluorogypsum to evaporate substantially all water physically mixed with the fluorogypsum and further heating the fluorogypsum to reduce the water chemically bound to calcium sulfate in the fluorogypsum to from about 0.5% to about 9% by weight.

Abstract: An active metal bed provided with a filter unit of sintered metal having a wide surface in the horizontal direction, enclosing an active metal powder running a reversible hydrogenation reaction therein and, which is single or plural combined in series, a heater attached thereto and an airtight container housing them, and characterized by, at the time of absorbing hydrogen isotope gas, contacting or flowing the gas to the active metal through the filter, while preventing the scattering of active metal powder, to obtain a large hydrogen absorption velocity by a wide gas contacting area and a decreased pressure loss at the time of gas flowing, and characterized by, at the time of releasing hydrogen isotope gas, preventing the overheating of air tight container and the permeating of hydrogen thereby while heating only the filter unit by the heater.

Abstract: Disclosed herein is a method for producing .alpha.-form gypsum hemihydrate, comprising heating an aqueous slurry containing gypsum dihydrate and a salt of sulfosuccinic acid as the catalyst for crystallization, thereby converting gypsum dihydrate into .alpha.-form gypsum hemihydrate.

Abstract: Gypsum is calcined by contacting it with the hot gases spewing out of a combustion tube submerged in the mass of gypsum. The formation of dead-burned anhydrite is reduced by surrounding the combustion tube with a second tube called a draft tube. Gypsum passing through the annulus between the tubes is heated by the hot gases and is ejected from the annulus to prevent repeated excursions into the hot gas zone at the muzzle of the combustion tube.

Abstract: The present invention relates to the coproduction of a combustible feed gas stream useable as an energy source and a sulfur-containing second gas stream useable as a feedstock for the production of sulfuric acid. The process includes heating coal in the presence of an oxygen-lean atmosphere under partial coal gasifying conditions to produce a solid carbonaceous char and a crude coal-gas stream. Sulfur-containing compounds are removed from the coal gas stream and converted to solid sulfur-containing materials. The solid sulfur-containing materials are combined with the solid carbonaceous char and gypsum to form a feed mixture. The non-gypsum portion of the feed mixture contains sufficient reducing potential to release substantially all of the sulfur in the gypsum as gaseous compounds of sulfur in a +4 or lower oxidation state. The feed mixture is heated under reducing conditions to produce a sulfur-containing second gas stream and a solid sintered product.

Abstract: Hydrodynamic pumping of the fluidized calcining mass in a kettle is set up by the continuous flow of the mass to and from heating rods immersed therein which are constantly emitting at least about 1 British Thermal Unit of heat per minute per square inch of the surface of the rods. The transfer of heat from the walls and bottom of the kettle is enhanced so that more of the available heat is actually absorbed than in a kettle lacking such heating rods. The rate of calcination may be increased while the rate of heat input is decreased.

Abstract: Gypsum is calcined by contacting it with the hot gases spewing out of a combustion tube submerged in the mass of gypsum. The formation of dead-burned anhydrite is reduced by surrounding the combustion tube with a second tube called a draft tube. Gypsum passing through the annulus between the tubes is heated by the hot gases and is ejected from the annulus to prevent repeated excursions into the hot gas zone at the muzzle of the combustion tube.

Abstract: An apparatus and process for calcining gypsum is disclosed which utilizes a flash furnace for rapidly calcining fine particulate gypsum while the material is suspended in a stream of hot gas. The method and apparatus utilize a two-stage suspension preheater for utilizing waste gases from the calcining furnace for preheating raw material to be calcined and a two-stage suspension cooler for cooling hot calcined gypsum which is discharged from the calcining furnace. Spent cooling gas is supplied to the preheater for use in preheating the fresh gypsum to be calcined. The invention also contemplates recirculating a portion of the gypsum discharged from the furnace back to the furnace for further calcination. Hot product may be discharged directly from the furnace.

Abstract: Wet chemical gypsum cake may be directly fed to a continuous kettle calciner to produce a stucco having lowered dispersed consistency. Pre-drying of the chemical gypsum is eliminated and the thus produced stucco may be used in the formulation of building plasters and in gypsum wallboard manufacture that will use less fuel for drying excess gauging water.

Abstract: Cold, wet gypsum is admixed to a moving bed of hot solid particles of a heat-retaining material. The mixture is fed to a moving bed type drier where it is agitated while the released vapors are withdrawn. After discharge from the drier, dry gypsum is separated from the heating particles and the latter are recirculated into a heater. Preferably, flue gases from a combustion chamber are employed for heating the particles, which are preferably in the form of steel balls.

Abstract: By-product gypsum, which is formed during a flue gas desulfurization process, (FGD gypsum) is carefully formed as an aqueous slurry of crystals having a mean particle diameter of between about 35 microns and 80 microns and filtered, which filter cake is substantially completely dried before calcination, without any substantial grinding action, the crystals are then flash calcined to a hemihydrate, then ground, to reduce consistency, and then formed into an aqueous slurry from which a paper-covered gypsum board is formed of superior core quality.

Abstract: A calcium sulfate anhydrite material is prepared from a spent absorbent resulting from desulfurization of flue gases. First the sulfite in the spent absorbent is oxidized into sulfate, preferably by heating in an oxygen containing gas, and thereafter the chloride content of the spent absorbent is reduced by heating the spent absorbent at a temperature of 600.degree.-950.degree., preferably 750.degree.-950.degree. C. together with fly ash. Said fly ash may be recovered together with the spent absorbent in the desulfurization process, or it may have been recovered separately and mixed with the spent absorbent.The product may be used as cement additive or for other purposes in which calcium sulfate anhydrite is conventionally utilized.

Abstract: A method for heat treating a material such as gypsum mineral, consists in burning a fuel in a combustion bed which is in heat exchange contact through bounding walls with beds of the material to be heated. Combustion gases are passed from the combustor through the heat treatment vessels containing beds and these gases then serve to fluidize at least partly and add heat to the beds through the agency of distributors.

Abstract: Alpha gypsum hemihydrate is obtained by calcining calcium sulfate dihydrate, generated with flue-gas desulfurization of power plants or by chemical process, e.g. in the production of phosphoric acid. The calcining process comprises producing initially a dispersion from calcium sulfate dihydrate and diluted, sulfuric acid solution and heating same at a predetermined rate to about 373 K with industrial, dust-free waste heat, whereupon the solid components are separated from the aqueous sulfuric acid and are dried. An alpha gypsum hemihydrate practically free from anhydrite II is obtained from a solution of sulfuric salts in the diluted sulfuric acid. An addition of magnesium oxide yields a hemihydrate with a high degree of fineness and avoids blooming effects.

Abstract: A process for producing a healed stucco of low water demand is disclosed in which calcined gypsum is treated with an aqueous solution of a gypsum solubilizing agent, preferably in an impacting blender while grinding the calcined gypsum to increase its surface area. The product can be dried for storage or sent directly to forming gypsum board.

Abstract: Compositions useful for lightweight structural units such as wallboard panels and the like are provided in which low density expandable thermoplastic granules or particles that have been expanded are used in conjunction with a cementitious base material, such as, gypsum (generally calcined to hemihydrate form, i.e. CaSO.sub.4.1/2 H.sub.2 O); a surfactant; an additive which acts as a frothing agent to incorporate an appropriate amount of air into the mixture to aid in obtaining a light density without decreasing strength or causing undue brittleness of the finished product; a film forming component to provide better flow and consistence of the mixture, as well as greater strength, flexibility, water resistance and adherence of the final product when the mixture is cured; and a starch which may be added as a binding agent and to increase the adhesive properties of the mixture.

Abstract: An apparatus for effecting the thermal conversion of gypsum to calcium sulfate hemihydrate includes a reactor having a series of fluidized bed compartments containing separate heat exchangers and defined by partitions each provided with an opening which is selected so that the material undergoing treatment progresses, without return movement, through the series of fluidized bed compartments. The apparatus is further constructed so as to permit recycling of fine gypsum particles from the outlet to the middle of the reactor.

Abstract: A method and apparatus for calcining calcium sulphate dihydrate in a calcination vessel in which the dihydrate is heated in the vessel to calcination temperature not only by heat applied to the exterior of the vessel but also by hot gas, especially hot gaseous combustion products, supplied into the interior of the mass of calcining material within the vessel. The supply of hot gas according to the invention can improve the production rate in both batch and continuous calcination operations without adversely affecting the product quality, by permitting increase in the heat input to the vessel without entailing the risk of vessel bottom burn-out.

Abstract: A combined method for granulation and defluoration of phosphogypsum, suitable for subsequent fluidized bed disassociation to lime and sulphur dioxide is described. The new phosphogypsum, obtained from phosphoric acid production, containing residual combined fluorine, is dried to reduce its residual water. It is moistened with sulfuric acid and aqueous solutions of binding agent additives and then granulated. The granules are heat-treated at temperatures between 170.degree. C. and 700.degree. C. During the heat treatment the residual fluorine residue is volatilized and the granules increase in strength sufficient for use in subsequent fluidized bed treatments.

Abstract: Continuous calcination of gypsum is provided by uniformly feeding uncalcined gypsum into the calcining batch in a kettle to a location below the surface but above the uppermost agitator, and allowing calcined gypsum to be discharged from the kettle through a discharge outlet in the wall of the kettle at the top surface of the calcining gypsum.

Abstract: Calcium sulfate is decomposed to CaO and SO.sub.2 by high temperature treatment in a fluidized bed wherein reductive and oxidative conditions are simultaneously maintained. A highly reducing gas is formed in the lower portion of the bed from partial combustion of the fuel in admixture with the primary fluidizing air. The quantity of the primary fluidizing air is limited so that the reducing conditions in the lower zone converts the CaSO.sub.4 to a mixture of CaO and CaS. Secondary air is introduced at a higher level in the bed to create an oxidizing zone in the upper portion of the bed above the reducing zone capable of converting CaS to CaO. The concurrent use of such reducing and oxidizing zones permits reducing conditions to be maintained which favor a high rate of decomposition even though these conditions favor the formation of CaS as well as CaO.

Abstract: A continuous calciner, particularly advantageous in converting gypsum to plaster, with two coaxial cylindrical walls, provides for heating air within the inner cylinder and then directing the air, for conveying and heating fine particles, along a spiral path, between the walls of the two coaxial cylinders, extending from one end of the calciner to an exit at the second end.

Abstract: Alpha type calcium sulfate hemihydrate gypsum is obtained by suspending gypsum in an aqueous solution containing a specific monocarboxylate in a specific concentration and heating the resultant suspension in a specific temperature range under atmospheric pressure. The production of this alpha type calcium sulfate hemihydrate is accomplished at an increased rate by adding a specific monocarboxylic acid to the aqueous solution, suspending gypsum in the resultant solution and heating the resultant suspension in a specific temperature range under atmospheric pressure.

Abstract: A method of preparing plaster by brief contact between gypsum and a hot gas. The method comprises introducing the gypsum through the axial tube of a nozzle, around which a helical stream of hot gases is created, and bringing the two phases into contact in a vessel at the outlet of the nozzle. The method of the invention makes it possible to vary the quality of plaster produced and to use either dry gypsum or dilute gypsum pastes as the starting material.