Abstract: A sulfur dioxide-containing exhaust gas is scrubbed by contact with an aqueous solution containing an alkali salt of an organic acid to dissolve sulfur dioxide in the aqueous solution in the form of alkali sulfite. The alkali sulfite in the solution is converted by oxidation into alkali sulfate, which is contacted for reaction with a calcium compound to form calcium sulfate. The calcium sulfate so formed is separated by filtration from the aqueous solution for recovery while the filtrate is recycled for use as the aqueous solution for scrubbing the exhaust gas. The concentration of alkali sulfite in the scrubbing aqueous solution is maintained at a level of less than 1% to ensure efficient removal of sulfur dioxide from the exhaust gas in the form of calcium sulfate.

Abstract: This process provides for the oxidation of calcium sulfite produced from wet scrubbing sulfur oxides to gypsum in an aqueous medium containing magnesium sulfate. The process renders scrubber sludge filterable for further manufacturing use or disposal without use of acidic streams.

Abstract: Magnesia of very high purity is produced industrially from a magnesite waste material wherein impurities are present in such amounts that the weight ratio of the precipitable hydroxide ions, calculated as the sum of Fe.sub.2 O.sub.3 + Al.sub.2 O.sub.3 + Cr.sub.2 O.sub.3 + Mn.sub.3 O.sub.4 to the sum of SiO.sub.2 + TiO.sub.2 + B.sub.2 O.sub.3, is at least 1 : 3. The starting material is dissolved in hydrochloric acid of 15-32% concentration, the impurities are precipitated by adjusting the pH of the acidic solution to 4 - 9, and the purified magnesium chloride solution is thermally decomposed into magnesia and hydrochloric acid gas, the latter being recycled to form the hydrochloric acid of the stated concentration for dissolving the starting material.

Abstract: Gases containing SO.sub.2 from a furnace are contacted in the scrubbing zone of a scrubber with an aqueous scrubbing medium containing solid alkaline earth metal carbonate, seed crystals of alkaline earth metal sulfate and dissolved alkaline earth metal carbonate and sulfate and having a pH of from 6.0 to 6.5. The SO.sub.2 reacts to form sulfites and bisulfites. The scrubbing medium is then withdrawn from the scrubbing zone and discharged to a reaction tank. Air is blown through the scrubbing medium in the reaction tank and then up into and through the scrubber and scrubbing zone. The oxygen reacts with sulfite to form sulfate in both the reaction tank and the scrubbing zone. The amount of oxygen is controlled so that the pH in the scrubber will not drop below about 4.5 and so as to oxidize not more than about 30 to 50% of the sulfite.

Abstract: A sulfur dioxide containing gas stream is passed through a gas scrubber through which is circulated a scrubbing medium, preferably slurried, containing a reactant which reacts with the sulfur dioxide to produce sulfites and/or sulfates, a portion of which precipitate. At least a portion of the precipitated sulfites and/or sulfates are removed from the scrubbing medium which is rejuvenated with fresh reactant to maintain substantially steady state conditions in the scrubber. To minimize scaling in the scrubbing system, seed crystals of the sulfites and/or sulfates are introduced into the scrubbing medium, the seed crystals being in number and size so as to control the level of supersaturation of the scrubbing medium with respect to the sulfites and/or sulfates.

Abstract: Process for through desulfurization of exhaust gas by treating the gas through the steps of absorption, double decomposition, oxidation, and alkali sulfate conversion.

Abstract: Sulfur dioxide can be removed from a combustion exhaust gas and converted to gypsum by first scrubbing a sulfur dioxide-containing combustion exhaust gas with an aqueous alkali sulfite solution to form an alkali bisulfite-containing aqueous solution and then treating, within a closed system, the thus formed aqueous bisulfite solution with a calcium compound. The efficiency of the sulfur dioxide removal can be increased greatly by adding a portion of alkali sulfite aqueous solution circulating in the system to at least a portion of filtrate from the gypsum separation process to precipitate metallic ionic impurities contained in the filtrate in the form of salts and separate these salts by filtration, and then recirculating the filtrate from the salt separation process to the absorption system.

Abstract: Sulphur dioxide is removed from a stack gas by bringing said gas containing SO.sub.2 into contact with an absorbing solution prepared by adding an hydroxide of a metal from either Groups II, III and VIII of the periodic Table to an aqueous solution of a halide of said metal, wherein the hydroxide is present in an amount of from 0.3 to 0.8 weight percent.

Abstract: The purification of the raw phosphogypsum issued from the fabrication of phosphoric acid by reaction between phosphate rock and sulphuric acid consists of removing the extreme grain size fractions therefrom.This method has been developed after it has been found that a large part of the undesirable impurities are concentrated in the extreme grain fractions which constitute only a small percentage of the phosphogypsum. The impurities thus removed are P.sub.2 O.sub.5, F, SiO.sub.2, C, F.sub.2 O.sub.3 and Al.sub.2 O.sub.3.Purification efficiencies can be obtained of the order of 70 % for the total P.sub.2 O.sub.5 and fluorine, 60 % for silica, and 80 % for organic carbon, while water-soluble P.sub.2 O.sub.5 is substantially completely removed.

Abstract: In the manufacture of phosphoric acid by treatment of phosphate ores with excess phosphoric acid and precipitation of calcium ions as calcium sulfate, the purity and quality of the byproduct calcium sulfate is substantially improved by subjecting the monocalcium phosphate extract of the ore before precipitation of calcium sulfate to one or more preliminary treatments, whereby contaminent ions are substantially and preferentially removed from the extract while the calcium remains substantially in solution. These preliminary treatments may include separation of fluoride by reaction with silicon compounds, separation of radium by reaction with barium compounds in the presence of sulfate ions, and separation of strontium as strontium sulfate.

Abstract: A continuous process for extraction of niobium, rare earths and thorium from niobium ore concentrates which includes digesting the ore with a hot solution containing 13 to 16 moles of sulphuric acid per liter, diluting the solution to a concentration of 10 to 13 moles of sulphuric acid per liter, separating the insolubles from the solution which includes alkaline earth sulphates and the sulphates of thorium and rare earths that are present, reducing titanium in solution to the trivalent state and diluting the solution to a concentration of 5 to 7 moles of sulphuric acid per liter, separating the precipitated niobium oxide and sulphates of thorium and rare earths, and then concentrating the resulting solution to the level desired for recycle to the digestion stage.

Abstract: Method and apparatus for removing sulfur dioxide from stack gases. The stack gases are scrubbed with an aqueous wash liquid containing one or more alkali metal compounds which react to form the bisulfite, sulfite and sulfate salts of the alkali metal. The spent absorbent liquid effluent is then treated with calcium compounds in at least two separate reaction stages, the conditions of ion concentration and pH of the first stage being maintained to favor the precipitation of calcium sulfate in the presence of SO.sub.3.sup.= ions. In the succeeding reaction stages the precipitation of the desired amount of SO.sub.3.sup.= ions is carried out to form calcium sulfite. The resulting compounds of calcium with sulfite and sulfate (which may in part be in the form of mixed crystals) are removed and the regenerated liquid is recycled.

Abstract: A process, in which sulfur dioxide gas contained in waste gas is removed therefrom and collected as gypsum. In said process, the waste gas is subjected to a flow of detergent solution of basic aluminium sulfate so that the sulfur dioxide is absorbed into the solution. This solution, now containing the sulfur dioxide, is thereafter subjected to oxidation, resulting in a decrease in the basicity thereof. The solution is then neutralized by calcium carbonate or calcium hydroxide, whereby gypsum is precipitated in the solution and collected therefrom and whereby the crude content of the solution is recovered so that it may be used again as a detergent for the waste gas.

Abstract: A method and apparatus for manufacture of wet process phosphoric acid wherein phosphate rock and sulfuric acid are separately added to a combination reactor and cooler unit at a rate of addition such that the increase in calcium content and sulfate content in the slurry contained in the unit are small. Circulation of the slurry is provided and the rate at which sulfuric acid and phosphate rock are added is small compared to the rate of circulation of the reactor slurry.

Abstract: A method for effectively removing sulfur dioxide in the form of calcium sulfite from a combustion exhaust gas, comprising the steps of (1) feeding a portion of the combustion exhaust gas to a dilute sulfuric acid solution to produce sulfurous acid in the solution and oxidizing the resultant sulfurous acid with use of air or oxygen in the presence of an oxidizing catalyst to produce sulfuric acid; (2) feeding a residue of the combustion exhaust gas and the gas exhausted from the step (1) to an alkali sulfite aqeuous solution to produce an alkali bisulfite aqueous solution; (3) mixing a major portion of the alkali bisulfite aqueous solution produced in the step (2) with slaked lime or limestone for reaction to give a calcium sulfite-containing reaction solution; (4) separating calcium sulfite from the reaction solution obtained in the step (3); and (5) circulating the filtrate of the step (4) to the step (2) and mixing a minor portion of the alkali bisulfite aqueous solution produced in the step (2) with sulfur