Abstract: The present disclosure is directed to a method of manufacturing gypsum board as well as the resulting gypsum board. The method comprises providing a gypsum slurry with a shrinkage-reducing additive comprising a metal salt of an acid, a silica, or a mixture thereof. As a result, the gypsum board includes gypsum and a shrinkage-reducing additive comprising a metal salt of an acid, a silica, or a mixture thereof. The gypsum board exhibits an area shrinkage of less than 10%.

Abstract: Various embodiments disclosed relate to weighted compositions for treatment of a subterranean formation. In various embodiments, the present invention provides a method of treating a subterranean formation. The method can include placing in the subterranean formation a coated weighting agent. The coated weighting agent can include a weighting agent and an inorganic coating material on the weighting agent.

Abstract: The present disclosure relates to a thermosensitive self-plugging bentonite, a preparation method thereof, and a water-based drilling fluid.

Abstract: A method for preparing calcium sulphate comprising a production of DCP by the attack of a source of phosphate by an acid, a digestion of the isolated DCP by the sulphuric acid under conditions giving rise to the formation of a first slurry of gypsum suspended in an acidic aqueous phase having a content of free SO3 equal to or less than 1.5% and a content of free P2O5, a conversion of at least part of said first slurry by heating to a temperature greater than 80° C. and potentially by adding sulphuric acid, with solubilisation of the gypsum crystals and recrystallisation of the solubilised calcium sulphate in a second slurry of ?-calcium sulphate hemihydrate crystals suspended in an aqueous phase based-on phosphoric acid, wherein the content of free SO3 is less than 10% by weight, and a separation between said aqueous phase and a filter cake based on particularly pure ?-calcium sulphate hemihydrate.

Abstract: Additives including sulfur-containing compounds are used in methods of treating synthetic gypsum. The additives can thermally stabilize heavy metals, such as mercury. This thermal stabilization reduces mercury release from the synthetic gypsum. Illustrative, non-limiting examples of sulfur-containing compounds include inorganic sulfides, organic sulfides, organic compounds containing nitrogen and sulfur, organic compounds containing oxygen and sulfur, and polymers containing sulfur.

Abstract: A method for preparing ?-calcium sulfate hemihydrate with calcium sulfate dihydrate includes steps of: uniformly mixing the calcium sulfate dihydrate with an additive solution, and obtaining a mixture, wherein weight percentages of the calcium sulfate dihydrate and the additive solution in the mixture are respectively 90.00-95.00% and 5.00-10.00%, and the additive solution contains water, inorganic salt, organic salt, organic acid, surfactant, and seed crystal; rising a temperature of the mixture to 130-150° C., keeping for 20-120 minutes, and the calcium sulfate dihydrate in the mixture transforming to the ?-calcium sulfate hemihydrate; drying the mixture after reaction at 105-160° C., and thereafter obtaining ?-calcium sulfate hemihydrate product. The used calcium sulfate dihydrate can be natural raw materials and industrial by-products. The industrial by-products can be directly applied.

Abstract: This invention refers to a novel process to obtain magnesium hydroxide and sodium sulfate from a solid raw material, which contains sodium and magnesium soluble salts, preferably in the form of sulfates, coming from a natural source or as a byproduct of an industrial process. The process consisting in the conditioning of the raw material to guarantee the correct concentrations of sodium and magnesium sulfates that is subjected to a salting-out crystallization when mixed with sodium sulfate obtaining sodium sulfate; the resulting solution is subjected to an alkali treatment to precipitate the magnesium hydroxide and the mother liquor is fed to a regeneration stage of the alkali used in the precipitation of the magnesium hydroxide as well in the sodium sulfate solution for the crystallization. The crystallization can be performed as a multistage process countercurrent to increase the purity of sodium sulfate.

Abstract: The invention relates to a process for conditioning material for pyrometallurgical conversion to magnesium, the process comprising carrying out de-su'fation of the material in a slurry to reduce sulfur content of the material; and carrying out de-ferration of the de-sulfated material in a slurry to reduce iron content of the material to produce a conditioned material suitable for pyrometallurgical conversion to magnesium.

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 method for producing of ultra-dispersed carbon comprises the steps of introducing gaseous methane and chlorine into passageways (3 and 4) of a burner (2) communicating with a chamber (1) of a reactor, combusting the mixture of methane and chlorine resulting in the formation of a diffusion flame, depositing products of a process of oxidative coupling of methane, separating a suspension containing solid carbon particles, and extracting a desired product. The greater part of the methane stream is fed to the outer boundary of the flame through a branch pipe (6) provided within the reactor chamber. Internal walls of the reactor chamber (1) in the combustion zone of the diffusion flame are washed by means of water stream. The desired product is extracted from the crude product through thermal processing of the solid particles suspension. Utilization of the method essentially increases the mono-dispersed carbon yield in the process of oxidative coupling of methane.

Abstract: The present invention provides an improved process for the preparation of MgO of high purity >99% from salt bitterns via intermediate formation of Mg(OH)2 obtained from the reaction of MgCl2 and lime, albeit indirectly, i.e., MgCl2 is first reacted with NH3 in aqueous medium and the slurry is then filtered with ease. The resultant NH4Cl-containing filtrate is then treated with any lime, to regenerate NH3 while the lime itself gets transformed into CaCl2 that is used for desulphatation of bittern so as to recover carnallite and thereafter MgCl2 of desired quality required in the present invention. The crude Mg(OH)2 is dried and calcined directly to produce pure MgO, taking advantage of the fact that adhering impurities in the Mg(OH)2 either volatilize away or get transformed into the desired product, i.e., MgO.

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: The invention pertains to a method of inertising free soluble impurities of phosphates and/or fluoride ions, in phosphogypsum, for use in commercial and industrial applications. The Phosphogypsum is pre-conditioned over an extended period of time separately or treated along with standard constituents by spraying, interblending or intergrinding during the process of manufacture, with alkyl, alkenyl and/or alkanol derivatives of ammonia, either individually or in combination with one another, to form stable intermediary phosphates and/or fluoride salts of the derivatives of ammonia.

Abstract: A process of recovering magnesium oxide from a source of magnesium sulfate includes the steps of providing a source of magnesium sulfate in solution that is derived from part of a process that is associated with the leaching of a metal containing ore or concentrate; converting the magnesium sulfate in solution to solid magnesium sulfate; contacting the solid magnesium sulfate with elemental sulfur in a reducing atmosphere; and recovering the magnesium as magnesium oxide, and the sulfur as sulfur dioxide gas.

Abstract: This invention generally relates to a barium sulfate product particularly useful as an X-ray contrast agent in preparations administered for examination of the gastrointestinal tract and to processes for making the product from naturally occurring barite ore containing barium sulfate crystals and gangue materials. The process includes treating or contacting the barium sulfate-containing particles obtained from barite ore with a fluidizing agent in a liquid medium to remove gangue materials present therein.

Abstract: The present invention provides methods and apparatus for treating flue gas containing sulfur dioxide using a scrubber, and more particularly relates to recovering gypsum and magnesium hydroxide products from the scrubber blowdown. The gypsum and magnesium hydroxide products are created using two separate precipitation reactions. Gypsum is crystallized when magnesium sulfate reacts with calcium chloride. Magnesium hydroxide is precipitated when magnesium chloride from the gypsum crystallization process reacts with calcium hydroxide. The process produces a high quality gypsum with a controllable pH and particle size distribution, as well as high quality magnesium hydroxide.

Abstract: A process for reducing a level of radionuclei in a phosphogypsum material having a radioactive contaminant is provided, wherein the process includes the steps of: (a) mixing the phosphogypsum material with an acid decontamination solution to form a mixture; (b) separating the mixture into a liquid stream having a large portion of the radioactive contamination and a solid stream having a residual component of the acid decontamination solution; and (c) removing the residual component of the acid decontamination solution from the solid stream, thus providing a gypsum material that meets or exceeds federal standards for level of radioactive nuclei and providing a process that is environmentally friendly and capable of avoiding the generation of phosphogypsum stacks, as well as eliminating existing or abandoned phosphogypsum stacks, further providing for recovery of the radioactive nuclei if desired.

Abstract: A chemical process for the purification of CaSO4 utilizes the ability of an aqueous solution of a chelating agent to selectively dissolve CaSO4 in that the initial extraction can be carried out at certain pH and CaSO4 can be recovered by titration to a different pH (usually a a lower pH) following a mechanical treatment, such as centrifugation or filtration, to separate the aqueous chelate solution from insoluble material. Also claimed are an apparatus and the use of chelating agents therefor.

Abstract: Hydrogen is produced from solid or liquid carbon-containing fuels in a two-step process. The fuel is gasified with hydrogen in a hydrogenation reaction to produce a methane-rich gaseous reaction product, which is then reacted with water and calcium oxide in a hydrogen production and carbonation reaction to produce hydrogen and calcium carbonate. The calcium carbonate may be continuously removed from the hydrogen production and carbonation reaction zone and calcined to regenerate calcium oxide, which may be reintroduced into the hydrogen production and carbonation reaction zone. Hydrogen produced in the hydrogen production and carbonation reaction is more than sufficient both to provide the energy necessary for the calcination reaction and also to sustain the hydrogenation of the coal in the gasification reaction. The excess hydrogen is available for energy production or other purposes.

Abstract: Fluorgyp is converted to calcium sulfate by reacting sulfuric acid in the fluorgyp with calcium carbonate having a small particle size. The resulting product is useful as a set retarding agent for cement.

Abstract: Modification of phosphoric acid manufacturing provides improved process waters: decontaminated water pH-adjusted to be only slightly acidic, scrubber process water, gypsum stack water, and a composite of the latter two waters. The composite water undergoes two-stage neutralization with clarification, eliminating precipitatable contaminants, in forming the pH-adjusted water, which in turn is useful in forming scrubber process water or gypsum stack water, as needed, Resulting gypsum stacks and rainfall surge ponds, having been made relatively fluoride-free through such processing modification, and preferably also having been lined, supersede previous gypsum stacks and acid gypsum ponds—whether lined or not.

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: The process for purifying gypsum suspensions, particularly those formed in a flue gas desulfurizing plant, includes feeding the gypsum suspension (9) to a first hydrocyclone (1), which is operated to produce an underflow (11) with a comparatively high underflow concentration and an overflow fed to a collecting tank (2). The underflow (11) of the first hydrocyclone stage is further diluted to a form another underflow with a lower solids content in a suspension tank (3) and then is fed to a second hydrocyclone stage (4), the overflow of which is also fed to the collecting tank (2). Depending on purity of the gypsum required or attained, the gypsum contained in an underflow (14) of the second hydrocyclone stage (4) is either immediately dewatered and removed or is first fed to one or more hydrocyclone stages, which operate analogously. The combined overflows (15) of the first and second or further hydrocyclone stages are fed from the collecting tank (2) to a third hydrocyclone stage (6).

Abstract: A means to increase the reducing potential of natural gas used in the reduction of sulfates such as gypsum. A reducing mixture of natural gas and air (or oxygen) is heated prior to reduction of the sulfate. For gypsum reduction, heating the reducing mixture to 1500.degree. F. (816.degree. C.) prior to gypsum reduction increased the gypsum conversion to calcium sulfide to above 65% at a reactor temperature of 1400.degree.-1500.degree. F. (700.degree.-816.degree. C.) verses less than a 40% conversion rate when an unheated reducing mixture was used.

Abstract: This invention relates to an improved process for producing gypsum calcium sulfate dihydrate flakes from powdered gypsum calcium sulfate dihydrate obtained from the desulfurization of flue gas by the wet limestone process. The flakes have sufficient size and strength such that the gypsum calcium sulfate dihydrate can be handled and processed in conventional equipment commonly used by wallboard manufacturers and others in the construction industry, The intermediate calcium sulfate dihydrate from the desulfurization process is dewatered to about 5 to 12% by weight and then compressed into a thin sheet under a compaction or compression load of 4 tons force to 15 tons force per linear inch of sheet width. A linear speed of from 0.5 to 2.5 feet per second is maintained during compaction providing a compaction residence time of from about 0.2 to 1.2 seconds.

Abstract: Disclosed herein is a method for producing plasters that contain chloride contents of less than 0.5% by weight from calcium-containing impure products or residues having a high chloride content (e.g., about 10% or higher based on the total product or residue weight). The method does not require the addition of fly ash, SiO.sub.2 or Al.sub.2 O.sub.3. The method comprises temperature and water-mediated conversion of calcium chloride to calcium oxide in the calcium-containing product or residue using water, steam and/or substances that generate water to obtain calcium oxide and hydrochloric acid; and sulfating calcium components in the product or residue using oxygen and sulfur-containing, largely calcium-free substances. The product so obtained is an anhydride plaster which can optionally be hydrated.

Abstract: A process for working-up waste materials from barium sulfide or strontium sulfide leaching in which the amount of waste material to be disposed of is substantially decreased.

Abstract: A simple, efficient and inexpensive method of purifying magnesium sulfate from mixtures of epsomite and halite. The method allows for the preparation of substantially pure magnesium sulfate products. The method generally involves heating a mixture of epsomite and halite to form clusters of lower hydrated magnesium sulfate crystals and subsequently applying slight pressure to the clusters so that they collapse to yield fine, less hydrated magnesium sulfate crystals which can then be easily separated from the comparatively coarse halite crystals. The less hydrated magnesium sulfate crystals can be recrystallized to provide highly purified epsomite which can then be dehydrated to provide kieserite or anhydrous magnesium sulfate.

Abstract: Alkaline earth metal scales, especially barium sulfate scale deposits are removed from oilfield pipe and other tubular goods with a scale-removing composition comprising an aqueous alkaline solution having a pH of about 8 to about 14, preferably about 11 to 13, of a polyaminopolycarboxylic acid, preferably EDTA or DTPA and a catalyst or synergist comprising a monocarboxylic acid, preferably a substituted acetic acid such as mercaptoacetic, hydroxyacetic acid or aminoacetic acid or an aromatic acid such as salicylic acid. When the scale-removing solution is contacted with a surface containing a scale deposit, substantially more scale is dissolved at a faster rate than is possible without the synergist.

Abstract: A process for the preparation in an aqueous medium of purified calcium sulfate, with high whiteness, characterized in that it comprises:a) the formation of an aqueous solution by dissolution of impure hemihydrated calcium sulfate, at a concentration at most of 13.0 grams per liter (expressed in dissolved CaSO.sub.4) and at a pH of at least 5.

Abstract: The process for preparing calcium sulfate anhydrite II by heating a flowable powder of flue gas gypsum moistened with less than 20% by weight, and preferably with from 7 to 17% by weight, of diluted sulfuric acid, wherein the sulfuric acid content is from 0.5 to 7% by weight, and preferably 1 to 3% by weight of sulfuric acid, based on the total mixture, is carried out by that the total mixture is direct-fired or is rapidly heated with indirect heat exchangers having relatively high wall temperatures whereby it is heated at temperatures of from 80.degree. C. to 190.degree. C. and releases the water adherent from the diluted sulfuric acid as well as two moles of water of crystallization, and then the virtually anhydrous product is annealed at temperatures of from 80.degree. C. to 190.degree. C. for 0.3 to 24 hours.

Abstract: A process for economically producing tabular acicular gypsum crystals in a continuously stirred reactor is disclosed. The process involves establishing a steady state atmospheric pressure reaction zone of about 20% to about 25% by weight solids of gypsum crystals in water at a temperature of about 50.degree. C. to about 60.degree. C. with continuous mild agitation. Calcium sulfate hemihydrate is fed to the reaction zone at a rate to maintain a low level of supersaturation, and a suspension of gypsum crystals in water containing crystals of a mean particle length at least greater than 45 microns is withdrawn.

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: 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: 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: At present the usual method of recycling of drilling muds by the exclusive application of classifying processes during drilling rig operation leads to the disposal of fine grained barite containing waste materials. In this invention, a simple one-stage flotation process using alkylphosphate based collecting and foaming reagents is applied to recover the barite content of such muds thus producing a marketable barite concentrate. The flotation should preferably be carried out at pH 8 to 9 with CaO, Ca(OH).sub.2 or KOH as regulating reagents and with the optimal solids content in the flotation pulp (preferably 100 g/l). The solids content in the flotation pulp can easily be adjusted by diluting with process water.

Abstract: Essentially pure strontium carbonate is recovered from low to medium grade strontium sulfate containing ores by first treating the ore with a hydrochloric acid solution to remove extraneous materials including calcium, magnesium, barium, and iron. The remaining strontium sulfate containing residue is treated with ammonium carbonate to produce insoluble strontium carbonate and soluble ammonium sulfate. The strontium carbonate then is converted to soluble strontium chloride through treatment with a second, stronger hydrochloric acid solution. Any remaining extraneous barium or iron is removed from the second acid solution before the strontium chloride is reconverted to insoluble strontium carbonate and precipitated out of solution for recovery.

Abstract: Disclosed is a method of purifying by-product gypsum by making it into a sludge and washing the sludge in a single, multi-stage filtering washer. The washing liquid is recovered from the filtrate from the first stage of the washer and subject to neutralization in a two-stage neutralizing unit. The sludges separated in the neutralizing unit are dried and the excessive washing liquid from the second stage of the neutralizing unit is further processed in an evaporation stage.

Abstract: The present invention relates to the discovery of a method by which elemental sulfur may be delivered to a reaction zone, for gas-solids reaction, in the form of a concentrated, coherent, diffusion-resistant gas thereby permitting gas-solid reactions of greatly enhanced reaction rates. It has been found that sulfur, which is first vaporized at approximately 1 atmosphere pressure, and then super heated to at least 1270.degree. F. while in transit through a confined space, such as a transfer line, will debouch from said confined space into a reaction zone as a coherent gas which resist diffusion throughout the free space of the reaction zone. This coherent form of sulfur gas exhibits a dark reddish-violet color. The coherent form of sulfur gas, since it resists diffusion throughout the free space of the reaction zone, flows into concentrated contact with, and remains concentrated at, the solids surface, thereby producing a gas-solids reaction of greatly enhanced reaction rate.

Abstract: A process for producing dry flue gas gypsum substantially free of hemihydrate and anhydrous calcium sulfate comprises drying wet flue gas gypsum, typically having a free water content of about 4-12% by weight, in direct contact with a partial stream withdrawn from a primary stream of a dry dedusted flue gas. Flue gas gypsum produced in a wet desulfuration stage is fed to contact the partial stream and is separated from this partial stream and removed as dried flue gas gypsum. Remaining superfine flue gas gypsum is then fed to the intake side, or to the delivery side of a booster blower arranged upstream of the flue gas desulfuration device along with the partial flue gas stream discharged from the drying stage, and the superfine particulate flue gas gypsum is conducted from the booster blower into the flue gas desulfurizing device to serve as seed crystals for additional production of gypsum therein.

Abstract: A process for reducing radioactive contamination of phosphogypsum. Phosphogypsum containing radioactive material is calcined to form hemihydrate crystals carrying the radioactive contaminants, and a portion of the crystals is converted to substantially radiation-free gypsum crystals which are readily separated from the hemihydrate crystal relics containing substantially all of the radioactive contamination.

Abstract: A process for making an industrially usable gypsum from fluoroanhydrite in which fluoroanhydrite is contacted with a fluid reaction medium slurry containing soluble sulfate ions and a substantial proportion of small gypsum seed crystals to rapidly hydrate a large proportion of the fluoroanhydrite to a coarser, substantially pure gypsum. The slurry is clarified to separate coarse purified gypsum reaction product, and the reaction medium is reconstituted and recycled containing gypsum crystals less than the desired reaction product size and constituting about 50-90% of the total solids of the reconstituted fluid reaction medium.

Abstract: The level of radioactivity in gypsum produced from phosphate rock can be reduced by adding to the slurry of acid and phosphate rock in the conventional process for producing phosphoric acid a combination of concentrated nitric acid or hydrochloric acid and an ammonium salt, especially ammonium nitrate or ammonium phosphate, before the precipitation of the calcium sulfate.

Abstract: A process of two crystallization stages for reducing radioactive contamination of phosphogypsum is disclosed. In the process anhydrite crystals are obtained through dehydration of the radiation containing phosphogypsum in strong sulfuric acid; a portion of the anhydrite crystals containing the radioactive contamination is converted to substantially radiation free gypsum by crystallizing out on a large solids concentration of radiation free gypsum seed crystals; and coarse radiation free gypsum crystals are separated from small anhydrite crystal relics containing substantially all of the radioactive contamination.

Abstract: A process for preparing calcium sulphate anhydrite from calcium sulphate dihydrate so that the calcium sulphate anhydrite crystals will have a predetermined average particle size. The process is conducted by recrystallizing the calcium sulphate dihydrate at moderate temperatures in the presence of sulphuric acid having a concentration of 40-65% wt. calculated relative to the quantity of liquid phase in the recrystallization mixture. The particle size of the final product is controlled by varying the temperature and residence time of the recrystallization mixture in recrystallization zone and by varying the temperature in an inverse relationship to the acid concentration.Calcium sulphate anhydrite crystals having an average particle size between about 0.5 and about 3 micron are produced by maintaining the temperature between about 40.degree. C. and 90.degree. C. and the residence time from about 5 minutes to less than about 30 minutes.

Abstract: A process for the whitening of phosphogypsum comprising treating a phosphogypsum in aqueous suspension with an amount of ozone equal to at least 150 parts by weight per million parts by weight of phosphogypsum calculated as Ca SO.sub.4.2H.sub.2 O, the pH of the aqueous suspension being lower than or equal to 3, to yield a whiteness grade of phosphogypsum higher than 75, according to French Standard NF Q 03-038.

Abstract: Disclosed is a process for refining a magnesium and nickel containing ore, comprising the steps of grinding the ore; preparing a slurry of the ground ore in sulfuric acid containing less than ten percent of water; adding water to the acid-ore slurry in an amount which is effective to initiate a sulfation reaction; allowing the sulfation reaction to continue, utilizing the heat of the sulfation reaction to heat the acid-ore slurry, whereby water-soluble metal sulfates and insoluble residue are formed; leaching the sulfation product with water to extract the water-soluble metal sulfates in solution; separating the metal sulfates solution from the insoluble residue; precipitating from the solution metals other than Mg as hydroxides; separating the solution from the metal hydroxide precipitate; concentrating the solution sufficiently to precipitate CaSO.sub.4 ; separating the concentrated solution from the precipitate; recovering dehydrated MgSO.sub.

Abstract: A method of producing a complex mineral fertilizer is disclosed, comprising the steps of decomposing polyhalite at a temperature not exceeding 110.degree. C. in the presence of dilute nitric acid, neutralizing the obtained suspension with a substance selected from the group consisting of calcium carbonate, calcium hydroxide and calcium oxide, and separating the resultant product.

Abstract: A process for reducing the radioactive contamination in waste product gypsum in which waste product gypsum is reacted with a dilute sulfuric acid containing barium sulfate to form an acid slurry at an elevated temperature, the slurry is preferably cooled, the acid component is separated from the solid, and the resulting solid is separated into a fine fraction and a coarse fraction. The fine fraction predominates in barium sulfate and radioactive contamination. The coarse fraction predominates in a purified gypsum product of reduced radioactive contamination.

Abstract: In the process for the production of chlorine and alkali metal hydroxide by electrolysis according to the amalgam process using calcium- and/or sulphate containing crude salt, the calcium and/or sulphate contents introduced into the brine circuit by the crude salt are removed from the brine by precipitation of the double salt Na.sub.2 SO.sub.4 . CaSO.sub.4. Small particles of glauberite may be introduced to the brine to initiate and accellerate precipitation.