Abstract: A process for the treatment of aqueous sulphuric acid solutions containing dissolved ferrous sulphate comprising forming such a solution containing from 140 to 220 grams/liter free sulphuric acid in a first zone; adjusting the pH value of the solution in this zone to one in the range 3 to 4 by the addition of finely-divided calcium carbonate thereby precipitating calcium sulphate; passing the slurry thus produced to a second zone; adding to the second zone an aqueous solution of sulphuric acid containing from 3 to 80 grams/liter of free sulphuric acid; adjusting the pH value of the mixture in the second zone to one in the range 3.5 to 6 by the addition of finely-divided calcium carbonate thereby precipitating calcium sulphate and thereafter recovering from the second zone calcium sulphate suitable for use in the manufacture of plasterboard.

Abstract: By the process of the invention a unique crystalline calcium sulfate dihydrate product is precipitated from an aqueous solution of calcium sulfate and a combination of molecularly dehydrated inorganic phosphates. Also within the scope of the invention is a wallboard manufacturing process and a gypsum wallboard which comprises the gypsum of the invention.

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: A method for removing sulfur dioxide from flue gases by simultaneous absorption and oxidation in an aqueous absorbent. An enclosed body of an aqueous liquid absorbent for the sulfur dioxide is provided and the flue gas is sparged into an upper portion of the liquid absorbent to form a first zone thereof. An oxygen-containing gas is introduced into a lower portion of the liquid absorbent to form a second zone of the liquid absorbent that is below the first zone of the liquid absorbent and which has an apparent density that is higher than that of the first zone. The oxygen-containing absorbent from the second zone is circulated to the first zone near the point of the flue gas introduction and an alkaline material reactive with the sulfur dioxide in the liquid absorbent is introduced into the body of liquid absorbent thereby fixing the sulfur dioxide in the liquid absorbent in the form of a sulfate. The sulfate is subsequently removed from the body of aqueous liquid absorbent.

Abstract: The process of treating insoluble phosphate ore in the form of calcium phosphate, such as phosphate rock or bone phosphate, at near ambient conditions with sulfur trioxide or sulfuric acid as reactants in the presence of a liquid anhydrous dispersion media of, preferably, sulfur trioxide, sulfur dioxide, or their mixtures in liquid form so that the contacting of the ore is made in the form of an anhydrous slurry, to make the phosphorous content available as a nutrient or as phosphoric acid, and subsequently separating the dispersion media from the contacted ore preceding aqueous contact or extraction of the phosphorous constituent. The presence of sulfur dioxide in the anhydrous dispersion media is desirable to minimize operational difficulties associated with the proximity of the vaporization and solidification points of sulfur trioxide.

Abstract: Fibrous calcium sulfate is produced by autoclaving a dilute aqueous suspension of gypsum, at a temperature between the minimum fiber forming temperature and about 20.degree. C. above, in the presence of a dispersing agent to provide for discrete whisker crystal development and protecting the whisker crystals from destruction or conversion to a different physical or chemical form until they have been stabilized by calcining at a temperature in excess of 500.degree. C.

Abstract: The present invention relates to an anhydrite binder and to the method of making the same from the gypsum by-product (phosphogypsum) resulting from the production of phosphoric acid by the wet method, said binder comprising particulate anhydrous CaSO.sub.4 with an insoluble CaSO.sub.4 content of at least 93%, an average particle size diameter of 5 to 30, with 15% by weight or more of the particles having a diameter smaller than 10.mu. and 20% by weight or more of the particles having a diameter larger than 20.mu., and a pore volume of less than 0.29cc/g in the case of pores having a radius less than about 6.6.mu..

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: A process for disposing of the residue of exhaust gas washers by utilizing them as a structural material in mines. The sulfur in the exhaust gases is reacted with calcium compounds to form CaSO.sub.4.2H.sub.2 O. The CaSO.sub.4.2H.sub.2 O is recrystallized to alpha-CaSO.sub.4.1/2H.sub.2 O, conveyed underground, mixed with water, implaced and allowed to harden to form barricades or other structures in the mine.

Abstract: Gypsum and magnetite which are both coarse and of good quality can simultaneously be produced by introducing calcium carbonate into an aqueous solution containing ferrous sulfate while an oxidizing gas is blown, and then carrying out a neutralizing and oxidizing operation at a pH of 5-6 and a temperature of 60-80.degree. C, and can be separately recovered by magnetic separation.

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.

Abstract: Anhydrous hydrogen fluoride is manufactured by dropping small particles of a metal fluoride, e.g., calcium fluoride, through a reaction zone countercurrent to a gas stream containing sulfur trioxide, sulfuric acid and water vapor.

Abstract: A process for removing impurities from an oxide of titanium-bearing material such as a titania slag by treating the material with SO.sub.3 gas and subsequently leaching with a liquid. In particular a process for upgrading an ilmenite ore by preparing a titania slag therefrom and purifying the slag such that it may be used as a synthetic rutile suitable for use in a fluidized-bed chlorination reactor to produce titanium tetrachloride which can be reoxidized to TiO.sub.2.

Abstract: A process for the manufacture of calcium sulphate alpha-hemihydrate which comprises the step of interacting an aqueous solution of calcium chloride and a source of sulphate or bisulphate ions in an aqueous system at a temperature above the calcium sulphate hemihydrate/gypsum transition temperature under the reaction conditions. The preferred reagents are the waste calcium chloride/sodium chloride effluent of the ammonia soda process and sulphuric acid (e.g. impure sulphuric acid effluent) to give hydrochloric acid as a co-product. The preferred reaction temperature is at least 20.degree. C. above the transition temperature (about 70.degree. C), e.g. at 95.degree.-100.degree. C at atmospheric pressure.

Abstract: Diluted sulfuric acid used in the production of wet process phosphoric acid is obtained by the admixing of gypsum-containing waste water with concentrated sulfuric acid and subsequently cooling in a liquid-to-liquid heat exchanger. By maintaining the concentration of the diluted acid at from about 65 to 80% H.sub.2 SO.sub.4, harmful formation of scale in the heat exchanger is prevented. The process permits the recovery of P.sub.2 O.sub.5 values from gypsum-containing waste waters and does not require expensive dilution tanks or flash coolers.

Abstract: Concentrated magnesium chloride solutions are prepared from industrial liquors or brines by debrominating the liquor with chlorine, neutralizing the debrominated liquor up to a ph value of 3-6, adding a stoichiometric excess of calcium chloride to the neutralized liquor at temperatures in the range of 30.degree.-50.degree. C to form a calcium sulfate dehydrate crystallizate, separating the crystallizate from the mother liquor, adding a sufficient amount of carnallite to the mother liquor to increase the MgCl.sub.2 content thereof to 270-330 g/l after cold decomposition of the added carnallite and concentrating the resulting solution in two or more steps in the direct current in an evaporator with crystallization characteristics up to a final concentration of 440 to 470 g/l MgCl.sub.2 whereafter the concentrated magnesium chloride solution is separated from crude crystallized carnallite and sodium chloride formed during the concentration.

Abstract: A process for the purification of phosphate containing impure gypsum to obtain therefrom calcium sulfate semihydrate in a purified form by digesting the phosphate containing gypsum with nitric acid at a temperature between 22.degree. and 100.degree. C, the temperature and nitric acid concentration being regulated such as to dissolve most of the phosphate impurities and to allow the recovery of calcium sulfate semihydrate.

Abstract: Aqueous alkaline desulfurization treating units for combustion gases generate as a product an aqueous sludge containing sulfur compounds of calcium. Many such sludge products contain more than 30 percent by weight (of the solids) of calcium sulfite hemihydrate. These waste products are in part calcined at a temperature from 250.degree.C to 650.degree.C to produce a moisture-free, cementitious product which is thereafter recombined with uncalcined aqueous sludge to produce a cementitious product which can be profitably utilized or which may be conveniently discarded as a landfill. If desired, fly ash or bottom ash produced in the combustion process can be combined with the cementitious product.

Abstract: In a method for oxidizing sulfur dioxide by contacting a sulfur dioxide-containing gas and an oxygen-containing gas with an aqueous solution containing pentavalent vanadium and divalent manganese as a catalyst, the method for regenerating and recycling the oxidation catalyst which comprises adding at least one calcium compound selected from the group consisting of calcium oxide, calcium hydroxide and calcium carbonate and an oxygen-containing gas as an oxidizing agent to at least a part of the aqueous solution catalyst used in the oxidation reaction, separating the resulting gypsum, and recycling the recovered aqueous solution catalyst for use in the oxidation of sulfur dioxide.

Abstract: The present invention relates to a wet process for the manufacture of phosphoric acid, which process essentially comprises a digestion step in which phosphate rock is digested by means of phosphoric acid and sulfuric acid to form a slurry containing highly concentrated phosphoric acid together with calcium sulfate hemihydrate, and a separation step in which the slurry is filtered for separation of calcium sulfate hemihydrate from the highly concentrated phosphoric acid (in which the concentration of P205 in the product acid is higher than 40% by weight, based on the weight of the product acid) and which is characterized by supplying active silica at a ratio of 0.2-5% by weight, based on the weight of phosphate rock, in order that active silica may be present in the above mentioned digestion step.

Abstract: In an industrial process of preparing high-purity magnesia from an impure magnesium-containing starting material, wherein the starting material is dissolved in HCl and the resultant acidic solution is subjected to a multistep treatment for precipitating the impurities out of the solution, the precipitate is separated from the residual purified magnesium chloride solution, and the magnesium chloride is thermally decomposed to obtain magnesia, the purified magnesium chloride solution is first concentrated and sulfate ions are then added thereto to precipitate calcium ions from the concentrated solution as calcium sulfate.

Abstract: Process for the recovery of pigment-grade iron oxide and technical hydrochloric acid of predetermined molarity, from iron chloride solutions such as pickling solutions containing hydrochloric acid. The iron chloride solution is concentrated until the molarity of the chloride ion therein is the same as the molarity of the hydrochloric acid which it is desired to produce. Concentrated sulfuric acid in slight stoichiometric excess relative to the iron, is then added to the iron chloride solution, which solution is thereafter evaporated to dryness. The distillation from this evaporation is hydrochloric acid of the predetermined molarity, and the dry residue is iron sulfate. The iron sulfate is calcined to drive off sulfur dioxide and sulfur trioxide and to leave iron oxide which may contain sulfates; and to remove these latter, the iron oxide is leached with dilute hydrochloric acid and is thereafter washed with water. The washed iron oxide is dried and micropulverized to produce a red iron oxide pigment.

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 making phosphoric acid and gypsum through a precipitated phosphate intermediary which comprises mixing a precipitated dicalcium phosphate with a highly dispersed reaction admixture of sulfuric acid and phosphoric acid, at a temperature between about 25.degree. and 80.degree. C; and withdrawing and separating phosphoric acid containing up to about 45% or more P.sub.2 O.sub.5 and gypsum containing generally less than 0.25% P.sub.2 O.sub.5. The resultant gypsum is highly filterable and very low in occluded phosphate value. Preferably, the process involves first obtaining a weak phosphoric acid solution from and by leaching as-mined phosphate mineral containing material to obtain the precipitated dicalcium phosphate; and then converting the precipitated phosphate by the process to the strong phosphoric acid solution.

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.

Abstract: Alkali metal or alkaline earth metal carbonates such as calcium carbonate and magnesium carbonate found in dolomite or limestone are employed for removal of sulfur dioxide from combustion exhaust gases. The sulfated carbonates are regenerated to oxides through use of a solid-solid reaction, particularly calcium sulfide with calcium sulfate to form calcium oxide and sulfur dioxide gas. The regeneration is performed by contacting the sulfated material with a reductant gas such as hydrogen within an inert diluent to produce calcium sulfide in mixture with the sulfate under process conditions selected to permit the sulfide-sulfate, solid-state reaction to occur.

Abstract: In the electrolysis of an alkaline earth metal-containing alkali metal brine wherein the alkaline earth metals contained in the brine are removed, the brine is subjected to electrolysis, and the residual brine is concentrated and recycled for further electrolysis along with make-up fresh brine, the improvement which comprises contacting the alkaline earth metal-containing brine with a weakly acidic cation exchanger in the Na+ form, the exchanger comprising units of at least one of acrylic acid and methacrylic acid, whereby the alkaline earth metals are adsorbed on the cation exchanger, and periodically regenerating the cation exchanger. The cation exchanger is regenerated by contact with a hydrochloric acid solution of about 5 to 10% concentration, the regenerating solution effluent from the column being combined with the redissoluted sodium sulphate which is precipitated during concentration of the cell liquor thereby to precipitate any sulphate contained in the brine as the alkaline earth metal sulphate.

Abstract: In the reaction of phosphate-containing fluorspar with sulphuric acid to produce gaseous hydrofluoric acid, withdrawing the hydrofluoric acid and condensing it, the improvement which comprises adding to the fluorspar a reactive iron compound in an amount approximately equivalent to the phosphate content whereby the phosphate content of the condensed hydrofluoric acid is markedly diminished. The reactive iron compound can be contained in another fluorspar which is blended in the first fluorspar in the requisite amount and/or it can be an iron oxide, fluoride or sulphate, or a hydrated or hydratable iron compound added to the phosphate-containing fluorspar in about 0.2 to 5% by weight calculated as Fe.sub.2 O.sub.3 content and based on the fluorspar.

Abstract: A process of producing calcium aluminate monosulfate hydrate by reacting a mixture of a lime component, an alumina component, a calcium sulfate component, and water at a temperature of from about 100.degree. C. to about 200.degree. C.By the process of this invention, the aimed product having a high purity is obtained at a high yield without need of high temperature for the reaction. The product obtained is useful as building materials.

Abstract: In a process for preparing phosphoric acid by contact of sulphuric acid and phosphate rock with filtration of the gypsum slurry and recycle of the rest for contact with fresh rock, a fraction of the recycle slurry is treated with sulphuric acid to convert at least some of the gypsum to calcium sulphate hemihydrate and the slurry comprising hemihydrate is returned to contact the mixture of phosphate rock, phosphoric acid and recycle gypsum slurry. The process gives an easily filtered gypsum slurry with low phosphate losses in the gypsum filter cake.

Abstract: In a process for the recovery of metal chlorides from jarosite in sludges which result from the sulphuric acid processing of ores, the sludge is treated with an aqueous solution of an alkaline earth metal chloride. This solution contains free hydrochloric acid and is in excess stoichiometric ratio to the sulphate content of the sludge. The insoluble materials are separated from the metallic chloride solution by filtering, and are stripped of metallic chlorides by washing and further filtering. The metallic chloride solutions are then collected. The alkaline earth chloride is preferably calcium chloride. The process is particularly, but not exclusively, applicable to recovering metals from jarosite-type sludge resulting from the processing of zinc ore.

Abstract: A process for producing .alpha.-type hemihydrate calcium sulfate by oxidizing calcium sulfite with an oxidizing agent in a suspension containing at least one anionic, nonionic or amphoteric surface active agent.

Abstract: A process for preparing concentrated solutions of hydroxylammonium nitrate (HAN) or hydroxylammonium perchlorate (HAP) which comprise reacting hydroxylammonium sulfate with either barium nitrate or barium perchlorate, in such a way that the dissolved barium salt is reacted with the dissolving hydroxylammonium sulfate while agitating the system so as to keep the slurried barium salt from directly contacting the hydroxylammonium sulfate. The rate of addition of the hydroxylammonium sulfate is slower than the rate of solution of the barium salt. An excess of barium nitrate or barium perchlorate ions is needed in the solution to which the hydroxylammonium sulfate is added until the reaction is terminated. The concentration of the HAN or HAP in the final solution can be increased still further if a solution of HAN or HAP is respectively used in forming the barium salt slurry.

Abstract: In a process for the reaction of phosphate rock and sulfuric acid in the presence of potassium ion to form a mixture of phosphoric acid and a precipitated gypsum wherein the presence of the potassium ion causes precipitation of fluorides from the phosphate rock as potassium silicofluoride as a coprecipitate with the gypsum, and the phosphoric acid is separated from the gypsum slurry, the potassium silicofluoride is separated from the gypsum by extraction with phosphoric acid which may contain from 0-25 weight percent of sulfuric acid based on the amount of gypsum to be extracted or such phosphoric acid which may contain some fluorine. This extraction procedure provides a substantially fluoride-free gypsum product and an extraction solution containing recoverable potassium silicofluoride.

Abstract: A filler for a mica-free joint compound, comprising by-product gypsum having a mean particle size between about 11 and about 12 microns, a percentage by weight of particles smaller than 1 micron of between about 5 and about 6.5, and a packed dry bulk volume no greater than about 93 cc. per 100 grams.

Abstract: A process for reducing formation of calcium sulphate scale during the washing of calcium sulphate hemihydrate crystals by washing the crystals with a liquor containing certain surface-active agents in sufficient amount to give a concentration of from 0.0002 to 1% by weight of surface-active agent in the wash liquors.

Abstract: In a method of treating clay-containing phosphate rock comprising at least mining the phosphate rock, reducing the phosphate rock to a predetermined size range, adding water to the phosphate rock, and storing the admixture containing at least the phosphate rock and water; the improvement characterized by adding to the admixture an amount effective to control problems associated with the swelling of residual clays of a chemical treatment selected from the class consisting of aliphatic hydroxy acids containing 2-6 carbon atoms, inclusive, of which citric acid and tartaric acid are members; the aliphatic hydroxy acid salts, and effective blends of the aliphatic hydroxy acid or the aliphatic hydroxy acid salts with a strong caustic, or base. The chemical treatment prevents a build up of viscosity such that the admixture remains pumpable even with a concentration of solids in excess of 66 percent by weight.

Abstract: Calcium sulphate produced by reacting crude phosphate with sulphuric acid is washed with aqueous solutions of condensed and complex aluminium fluorides of the general formulae MeAlF.sub.4, Me.sub.2 AlF.sub.5 and Me.sub.5 Al.sub.3 F.sub.14 as well as AlF.sub.3.nH.sub.2 O, in which Me represents an alkali metal, ammonium or thallium-I ion and n is a number between 1 and 9.

Abstract: A method for removing hydrogen sulfide from a hydrogen sulfide entrained gas comprises the steps of bringing the hydrogen sulfide entrained gas into contact with an alkaline absorption solution with a pH of 10 to 14 to cause hydrogen sulfide in the gas to be absorbed into an alkaline absorption solution, subjecting the resultant absorption solution to a DC electrolytic treatment whereby an alkaline solution is separated into a cathode chamber and sulfur and sulfuric acid are separated into an anode chamber, and circulating as the alkaline absorption solution the alkaline solution produced within the cathode chamber. This constitutes a closed system, thereby completely eliminating a cause for public nuisance.

Abstract: Pyrrolidone-5,5-diphosphonic acids having the formula ##STR1## wherein R is a member selected from the group consisting of hydrogen and alkyl having from 1 to 6 carbon atoms; as well as their water-soluble salts. The pyrrolidone-5,5-diphosphonic acids are excellent sequestering agents especially for alkaline earth metal ions. They are stabilizers for percompounds and are useful in the delaying of the setting times for gypsum. In addition, the compounds are useful as cosmetic preparations such as toothpastes and mouthwashes where they prevent formation of tartar and plaque and are useful in therapy in the treatment of diseases related to the abnormal deposition or dissolution of difficultly soluble calcium salts in the animal body.

Abstract: Sulfur dioxide is removed from stack gases by absorption in an aqueous medium, the preferred embodiment containing a chemical oxidant which generates hydrogen peroxide on a demand that equals the rate sulfur dioxide is absorbed into the medium whereby the absorbed sulfur dioxide is converted to sulfate form. The converted sulfur dioxide is removed in the form of an alkaline earth metal sulfate precipitate and spent oxidant is regenerated for reuse. In the other illustrated embodiments, the aqueous medium is maintained acidic from the start-up of the system. Barium peroxide is the preferred oxidant and acts both as oxidant and precipitant. Alternately, the oxidant and precipitant may be separate compounds.

Abstract: A process is described for producing high purity phosphoric acid from phosphate rock including low grade phosphate rock material such as Idaho phosphate ores having P.sub.2 O.sub.5 contents as low as 20 percent. Finely divided phosphate rock, sulfuric acid, phosphoric acid and water are charged to a mixing zone in proportions such that the pH of the resultant slurry is in the range 0.8 to 1.2 and the calcium ion content of the liquid phase of the slurry is in the range 3 to 4 percent by weight. Sulfuric acid is charged to the mixing zone in a quantity sufficient to convert the nonphosphate calcium compounds in the rock to calcium sulfate plus the quantity required to convert the contained calcium phosphate to monocalcium phosphate, phosphoric acidis charged to the mixing zone in quantities such that it is, on a molar basis, from about 1.2 to about 1.4 times the P.sub.2 O.sub.

Abstract: A process is described for the formation of fibrous insoluble calcium sulfate anhydrite, the fibers thereof having specified dimensions. A dilute aqueous suspension of gypsum and, if desired, a crystal habit modifier suitable for the formation of fibrous calcium sulfate hemihydrate, is first formed. The gypsum in this suspension is then converted to the fibrous hemihydrate by reaction in the presence of saturated steam at 120.degree. C to 155.degree. C for 0.5 to 2.5 hours, thereby forming fibrous calcium sulfate hemihydrate of the desired dimensions. The fibrous hemihydrate is thereafter calcined at 500.degree. C to 750.degree. C for 0.5 to 3.0 hours to convert the fibrous hemihydrate to fibrous insoluble calcium sulfate anhydrite having substantially the same crystalline shape and dimensions.

Abstract: A process for purifying waste gypsum from the wet phosphoric acid process to a maximum fluorine content of 0.25 % by weight based on the anhydride, which comprises adding active silicic acid in two portions during the course of the phosphoric acid process, wherein at least half the total amount of silicic acid added is added during the decomposition of the crude phosphate and the remainder is added after the end of the decomposition but before the waste gypsum is filtered off, and the waste gypsum, after it has been filtered, is made into a suspension with water, separated from the liquid phase and dried.

Abstract: This invention relates to a process for hydrometallurgical treatment of concentrates of sulphides of copper, nickel, cobalt, lead and iron and copper-containing mattes. A first embodiment of the process includes subjecting a finely ground slurry of a sulphide concentrate to an oxidation leach at elevated temperature and pressure in the presence of an oxygen-bearing gas to preferentially leach cobalt and nickel and to convert galena to lead sulphate. The reaction mixture is heated to a further elevated temperature, in the absence of oxygen, to convert a portion of chalcopyrite to insoluble simple copper sulphides and ferrous sulphate solution and to decompose complex insoluble ferric compounds. The resulting mixture is subjected to differential flotation whereby a lead concentrate and a copper concentrate are obtained.

Abstract: The process for the production of synthetic anhydrite of uniform quality from the residue resulting from the production of hydrofluoric acid and containing calcium sulfate and acid, comprising suspending said residue in water with vigorous agitation, neutralizing said residue, grinding the suspension and separating water from the suspension to leave synthetic anhydrite containing less than about 50% water by weight. The product is thereafter dried after the optional addition of fillers and/or additives which accelerate subsequent hydration.

Abstract: Waste gas containing sulfur oxide (SO.sub.x) and nitrogen oxide (NO.sub.x) is mixed with an ozone-containing gas to have the value x of NO.sub.x fall in the range of 1.2 to 2.5 and thereafter brought into contact with an aqueous solution containing sulfuric acid, nitric acid and an iron compound to be deprived of SO.sub.x and NO.sub.x. To the solution with which the waste gas has been brought into contact, air or oxygen is added to effect oxidation of the part of said oxides still remaining therein in an unoxidized state and regenerate the activity of the iron compound functioning as a catalyst. Part of the solution to which the addition of air or oxygen has been made is recycled as the solution for the purpose of contact with waste gas.

Abstract: In the production of hydrofluoric acid and calcium sulfate by subjecting sulfuric acid and fluorspar to a partial preliminary reaction in a preliminary reaction zone and the reaction is completed in an after-reaction zone at a temperature from about 100.degree. to 500.degree. C, the improvement which comprises effecting the preliminary reaction by circulating preliminary reaction product through said preliminary reaction zone, adding to the circulating preliminary reaction product at spaced locations in the preliminary reaction zone sulfuric acid preheated to a temperature of about 80.degree. to 200.degree. C and fluorspar preheated to a temperature of about 400.degree. to 800.degree. C, removing from said preliminary reaction zone gaseous hydrofluoric acid and a substantially dry mixture of calcium sulfate, unreacted sulfuric acid and fluorspar, and passing said dry mixture of calcium sulfate, unreacted sulfuric acid and fluorspar to said after-reaction zone.

Abstract: The present invention relates to a process for wet-treating an exhaust gas which comprises simultaneously removing a nitrogen oxide (NO.sub.x) and a sulfur oxide (SO.sub.x) from an exhaust gas containing these oxides by treating exhaust gas containing oxides of nitrogen and sulfur with a lime or limestone slurry containing alkali metal or alkaline earth metal iodide.