Abstract: A fast-dissolving, non-caking, food grade sodium tripolyphosphate is produced by spraying into a spray dryer an aqueous sodium orthophosphate feed liquor having an Na.sub.2 O:P.sub.2 O.sub.5 ratio of from about 1.60 to about 1.67 containing potassium hydroxide or a soluble potassium salt in amounts to yield about 0.4% to 0.8% by weight potassium in the final product, heating the feed to at least 440.degree. C. to form sodium tripolyphosphate, cooling the sodium tripolyphosphate and adding to it at least about 1% by weight moisture.

Abstract: A composition of matter in particulate, crystalline form. The composition comprises a hydrate-forming phosphate selected from the group consisting of sodium tripolyphosphate, sodium pyrophosphate and trisodium phosphate, between about 0.1% by weight and about 23% by weight added water in the form of water of hydration said water containing in solution sufficient surfactant to provide the composition from about 0.20 ppm and about 125 ppm of a surfactant. Methods for preparing such compositions are also disclosed.

Abstract: Unmilled commercially available granular "natural" soda ash is reacted with orthophosphoric acid to produce a highly concentrated slurry at a molar Na:P ratio of about 1.5 in a continuous process. Problems associated with incomplete reaction are solved and the slurry can be dried to the double salt Na.sub.3 H.sub.3 (PO.sub.4).sub.2 or reacted with aqueous sodium hydroxide to produce a superior precursor for manufacturing sodium tripolyphosphate.

Abstract: A method of continuously producing ammonium polyphosphate by the reaction of urea and polyphosphoric acid, over a wide range of molecular weights, in a pre-mix slurry, under controlled temperature of about 315.degree.+/-15.degree. C. The slurry is reacted on a hot, continuously moving surface and forms ammonium polyphosphate in an ammonia atmosphere. Preferably, the pre-mixed slurry is added to a screw-type extruder, where the material comes in contact with the ammonia along the preheated moving surfaces of the extruder. The ammonium polyphosphate is scraped from the extrusion screw into a container with minimum handling. The material can be ground to a finer consistency if desired.

Abstract: A granular hydrated product consisting completely or partially of sodium triphosphate for use in detergent and cleaning compositions is made. To this end, pulverulent sodium triphosphate containing less than 22 wgt % water is compacted by initially mixing it with a quantity of water sufficient for hydrating at most 100% of the sodium triphosphate to the hexahydrate and then continuously admitting the resulting mixture to a zone inside which it is made into solid matter under pressure.

Abstract: A homogeneous, particulate mixture of alkali metal pyrophosphates and tripolyphosphates is prepared by flash drying an aqueous solution of alkali metal orthophosphates having an alkali metal to phosphorus mole ratio in the range of 5:3 to 6:3, and calcining the resultant dried particles. Preferably, the alkali metal is sodium and the orthophosphate solution is adjusted to provide a major proportion of sodium pyrphosphate and a minor proportion of sodium tripolyphosphate in the final product.

Abstract: High absorbtivity sodium tripolyphosphate is prepared by moisturizing the anhydrous orthophosphate particles produced by spray-drying an aqueous solution of sodium phosphates during the production of sodium tripolyphosphate. Following moisturization, the hydrated particles are calcined at a temperature within the range of from about 300.degree. C. to about 600.degree. C. to produce a granular sodium tripolyphosphate product having an absorbtivity value of at least about 30%.

Abstract: A process is provided for producing a powdered STPP hexahydrate particularly useful for use as a builder in a heavy duty detergent.

Abstract: A particulate hydrated trisodium phosphate composition useful in heavy duty cleansers has a core and shell structure, the shell being of trisodium phosphate hexahydrate and the core comprising trisodium orthophosphate dodecahydrate and sodium hydroxide. The composition may be made by prilling to form an intermediate body with solid shell and a liquid or solid/liquid core followed by further cooling.

Abstract: Process for production of desired phosphates from naturally occurring phosphates is characterized by treating naturally occurring phosphates under alkaline conditions to produce a material comprising trisodium and/or tripotassium orthophosphates, carbonating said material in solution, and recovering the desired phosphates from the carbonated solution. The desired phosphates include di- and mono-sodium orthophosphate; pentasodium tripolyphosphate; sodium metaphosphate and its polymers; tetrasodium pyrophosphate; ammonium phosphate; sodium ammonium hydrogen phosphate, magnesium ammonium phosphate; and the corresponding potassium salts thereof.

Abstract: A composition of matter in particulate, crystalline form. The composition comprises a hydrate-forming phosphate selected from the group consisting of sodium tripolyphosphate, sodium pyrophosphate and trisodium phosphate, between about 0.1% by weight and about 23% by weight added water in the form of water of hydration said water containing in solution sufficient surfactant to provide the composition from about 0.20 ppm and about 125 ppm of a surfactant. Methods for preparing such compositions are also disclosed.

Abstract: Solid, free-flowing, essentially nonfriable powdery detergent particulates of recrystallized sodium tripolyphosphate hexahydrate having high absorption capacity and a pore volume of at least 0.70 cm.sup.3 /g, at lest 80% of the intercrystalline porosity thereof comprising pore sizes ranging from 0.5 .mu.m to 30 .mu.m, are prepared by (i) spraying water or an aqueous medium over a first fraction of anhydrous sodium tripolyphosphate, the majority of which fraction comprises phase 1 tripolyphosphate, (ii) spraying water or an aqueous medium over a separate second fraction of anhydrous sodium tripolyphosphate, which second fraction essentially consists of phase 2 triplyphosphate, (iii) intimately admixing said first and said second sprayed fractions, (iv) suspending the resulting admixture in water or an aqueous medium, and then (v) drying the resulting suspension, e.g., by atomization.

Abstract: Alkali metal polyphosphates consisting substantially of hollow spheroidal particles with an apparent density of less than 350 g/l are made. To this end, an alkali metal orthophosphate solution is forced under a pressure of 6-40 bars into a fluidization zone; the orthophosphate solution is fluidized in the fluidization zone by injecting a finely dispersed permanent gas maintained under a pressure higher than the orthophosphate solution; and the fluidized orthophosphate solution is sprayed with the aid of a plurality of single medium nozzles arranged downstream of the fluidization zone through a flame zone peoduced inside the spray tower. Apparatus for carrying out the process.

Abstract: Alkali metal polyphosphates consisting substantially of hollow spheroidal particles with an apparent density of less than 320 g/l are made. To this end, an aqueous alkali metal orthophosphate solution and an aqueous solution of an inflating agent are simultaneously forced under a pressure of 6-40 bars into a fluidization zone; the two solutions are fluidized in the fluidization zone by injecting a finely dispersed permanent gas maintained under a pressure higher than the two solutions and the fluidized solutions are sprayed with the aid of a plurality of single medium nozzles arranged downstream of the fluidization zone through a flame zone produced inside the spray tower. Apparatus for carrying out the process.

Abstract: A process for production of alkali metal polyphosphates and/or pyriphosphates from an alkali metal orthophosphate comprises acidifying said orthophosphate with sulphuric acid, and then either(a) heating the reaction mixture to produce the desired products, and recovering said products from the reaction mixture, or(b) separating an intermediate from the reaction mixture, and heating the intermediate to form a desired product.The products are suitable for incorporation into detergents and soap powders.

Abstract: A method of continuously producing ammonium polyphosphate by the reaction of urea and polyphosphoric acid, over a wide range of molecular weights, in a pre-mix slurry, under controlled temperature of about 315.degree.+/-15.degree. C. The slurry is reacted on a hot, continuously moving surface and forms ammonium polyphosphate in an ammonia atmosphere. Preferably, the pre-mixed slurry is added to a screw-type extruder, where the material comes in contact with the ammonia along the preheated moving surfaces of the extruder. The ammonium polyphosphate is scraped from the extrusion screw into a container with minimum handling. The material can be ground to a finer consistency if desired.

Abstract: A process is described for converting wet acid into sodium tripolyphosphate by pretreating the wet acid in a pretreatment stage with a sodium source and SiO.sub.2 to precipitate fluoride values from the wet acid, solvent extracting the pretreated wet acid to remove further impurities, reacting the solvent extracted acid with a sodium source in a made up stage to form a sodium orthophosphate liquor, heating the resulting liquor in a calcining stage to produce sodium tripolyphosphate, passing gases from the calcining stage into a scrubber liquor to remove fluoride impurities and entrained sodium tripolyphosphate, recycling scrubber liquor to the make up stage to recover sodium tripolyphosphate values, and removing a bleed stream from the recycled scrubber liquor and recycling it to the pretreatment stage to remove fluoride impurities therein in the pretreatment stage.

Abstract: A novel anhydrous sodium tripolyphosphate, well adapted for detergency applications, has an X-ray diffraction pattern essentially characterizing a phase I sodium tripolyphosphate; a viscosity, in aqueous medium, characterizing a phase II sodium tripolyphosphate; a rate of hydration characterizing a phase I sodium tripolyphosphate; and which exhibits an ROH at 1 minute of greater than about 90.degree. C.

Abstract: A novel anhydrous sodium tripolyphosphate, well adapted as an additive for foodstuffs and detergent compositions, has an Na/P ratio of essentially 1.667.+-.0.01, an apparent density of from 0.5 to 1.1, a mean agglomerate size of less than 100 microns, a phase I content ranging from 15 to 6% by weight, is devoid of insoluble fraction, and displays zero solidification or caking in a quiescent solvent medium.

Abstract: Na tripolyphosphate compositions very rapidly hexahydrated, e.g., in water, and well suited for detergency applications, are comprised of a mass of sodium tripolyphosphate particulates prehydrated with a plurality of hexahydrated sodium tripolyphosphate crystals homogeneously distributed therethrough, (i) said particulates displaying a ratio of phase I crystalline form content to the total content of the phase I plus phase II crystalline forms of at least 50%, (ii) a homogeneous distribution of phase I crystalline form throughout both said particulates and said mass, (iii) a particle size distribution such that the average Rosen-Rambler-Bennett particle diameter ranges from 130 to 250.mu., with the maximum amount by weight of particles larger than 630.mu. in diameter being about 5% and the maximum amount by weight of particles smaller than 25.mu. in diameter being about 20%, (iv) a content in water ranging from about 0.

Abstract: Unmilled commercially available granular "natural" soda ash is reacted with orthophosphoric acid to produce a highly concentrated slurry at a molar Na:P ratio of about 1.5 in a continuous process. Problems associated with incomplete reaction are solved and the slurry can be dried to the double salt Na.sub.3 H.sub.3 (PO.sub.4).sub.2 or reacted with aqueous sodium hydroxide to produce a superior precursor for manufacturing sodium tripolyphosphate.

Abstract: The concentration of Phase II sodium tripolyphosphate in the sodium tripolyphosphate produced by spray drying a feed liquor comprising sodium orthophosphate, sodium pyrophosphate or mixture thereof, is increased to at least 90% by weight of sodium tripolyphosphate product, by seeding the feed liquor with Phase II material. Phase II yield is further increased by introducing with the feed liquor, in addition to the Phase II seed material, a minor amount of sodium sulfate. Hydrolysis of the Phase II seed is minimized by concentrating the Phase II seed in feed liquor in a separate feed tank at a lower temperature than the main feed line. Dispersing the Phase II seed in water, a dilute soda ash solution or an inert organic liquid and then injecting the dispersion into the feed liquor into the spray dryer promotes good contact between the seed and feed.

Abstract: An improved process of concentrating wet process phosphoric acid by continuously contacting an acid feed containing at least about 56 weight percent of P.sub.2 O.sub.5 based on the total weight of the feed with heated gases consisting of air and gaseous fuel combustion products thereby forming a mixture of said gases containing entrained superphosphoric acid particles, continuously coalescing said mixture in a coalescence zone forming coalesced superphosphoric acid and continuously removing the coalesced superphosphoric acid from said coalescence zone.

Abstract: An improved process for the production of granular ammonium polyphosphate fertilizer of a nominal 11-56-0 grade resulting from the pressure ammoniation of phosphoric acid without the use of a preneutralizer and using a specially designed pipe reactor to effect coating and granulation of an anhydrous melt onto cascading granules of ammonium polyphosphate with polyphosphate contents thereof ranging from about 15 percent to 30 percent of the total P.sub.2 O.sub.5. The instant invention defines the operating parameters which allow for the practice of a process wherein there is essentially no degradation in the amount of P.sub.2 O.sub.5 values produced as polyphosphate in the melt as said melt is further processed in the remaining steps therein through the step of product recovery, i.e., we have discovered the parameters for operating a process wherein essentially no hydrolysis of the polyphosphate P.sub.2 O.sub.5 values in the melt occurs.

Abstract: In the manufacture of sodium tripolyphosphate, the make up liquor containing the requisite sodium and phosphorus values, is treated with from about 0.01 to 0.1 weight percent of hydrogen peroxide before the liquor is heated to form sodium tripolyphosphate. The hydrogen peroxide treatment prevents dark oily impurities from forming on the surface of aqueous solutions of the sodium tripolyphosphate thus treated.

Abstract: The present invention relates to a method for producing lightweight granules of sodium tripolyphosphate from a powderous sodium-phosphate mixture. The material is first processed in the schugi-mixer, while adding a given quantity of water thereto, whereafter the moist mixture is fed to a rotating calcining furnace, to which the given, remaining quantity of water is supplied to the material prior to calcination.

Abstract: A one step spray drying process for making high assay sodium tripolyphosphate having a high concentration of form I material by spray drying an aqueous sodium orthophosphate feed liquor having an Na.sub.2 O:P.sub.2 O.sub.5 ratio of 1.60 to 1.70 at a temperature between 250.degree. and 420.degree. C. for up to one minute to produce sodium tripolyphosphate and cooling the product to 150.degree. C. or below in twenty minutes or less.

Abstract: The disclosure relates to a process for making alkali metal polyphosphates, especially alkali metal triphosphates. To this end, the disclosure provides for an alkali metal phosphate solution or suspension to be forced through at least one nozzle and for them to be sprayed inside a spray tower through a flame zone produced by a one or more burners arranged in annular fashion. More particularly, the disclosure provides for fine particulate alkali metal polyphosphates, especially alkali metal triphosphates, to be additionally introduced into the spray tower from above and for them to be regularly distributed across the spray region(s) of the nozzle(s) with formation of agglomerated products.The disclosure also relates to an apparatus for carrying out the process.

Abstract: The disclosure provides activated ammonium polyphosphate undergoing decomposition at an increased rate at temperatures within the range 300.degree. to 600.degree. C., the ammonium polyphosphate having been obtained by reacting it with melamine and/or dicyandiamide or their derivatives at 100.degree. to 200.degree. C.

Abstract: The disclosure relates to a process for making linear, substantially water-insoluble ammonium polyphosphates of the general formula (NH.sub.4 PO.sub.3).sub.10-1000 by heating and continuously mixing phosphorus pentoxide and ammonium orthophosphate in the presence of ammonia and tempering the resulting reaction product over a period of several hours at 200.degree. to 400.degree. C. in the presence of ammonia. To this end, the phosphorus pentoxide and ammonium orthophosphate are subjected at 50.degree. to 150.degree. C.

Abstract: The disclosure relates to sodium tripolyphosphate containing up to 95 weight % phase-I, 0.1 to 22 weight % water of crystallization and 0.01 to 2.0 weight %, calculated as metal, of a compound of the metals selected from calcium, magnesium, zinc or strontium, the compound yielding ions in an aqueous medium. The disclosure also relates to a process for making the tripolyphosphate, and to its uses.

Abstract: The invention relates to a process for making sodium tripolyphosphate from wet-processed phosphoric acids which contain organic impurities, are neutralized and concentrated to give an orthophosphate solution or suspension which is sprayed in a spray tower in contact with one or more oxidants. More particularly, sodium tripolyphosphate, which is suitable for use in detergent slurries and causes the temperature determined after 1 and 5 minutes, respectively, in the hydration test (ROH) to be increased by at least 7.degree. C., is made; to this end, the wet-processed phosphoric acids are first admixed with the oxidants and then concentrated.

Abstract: Disclosed is a process for recovering phosphate (P.sub.2 O.sub.

Abstract: A method of separating magnesium from wet process superphosphoric acid by filtration characterized by no or minimizing agitation during the crystallization of the magnesium in order to form readily filterable agglomerates of MgH.sub.2 P.sub.2 O.sub.7.

Abstract: The invention relates to a process for making long chain ammonium polyphosphate consisting essentially of crystalline modification II. To this end, a mixture of phosphorus pentoxide and ammonium orthophosphate in a molar ratio of 1:0.9 to 1:1.1 is reacted with 0.1 to 15 mol % of melamine or its condensation products selected from melam, melem or mellon, calculated as melamine, at a temperature within the range 50.degree. to 150.degree. C. in the presence of an excess of ammonia and the reaction product is heat-treated at temperatures within the range 200.degree. to 400.degree. C.

Abstract: The invention relates to a process for converting water-soluble, condensed phosphates of the following general formula (I)M.sub.2 O(MPO.sub.3).sub.n (I)in which M stands for sodium, potassium or ammonium and n stands for a number between 4 and about 100, to granulate, wherein the pulverulent, water-soluble, condensed phosphate is thoroughly agitated and granulated in the presence of water and of a phosphate salt other than the condensed phosphate. More specifically, the condensed phosphate of general formula (I) or a mixture of the phosphate with an anhydrous or partially hydrated alkali metal phosphate is thoroughly agitated and granulated in the presence of water and an ammonium polyphosphate of the following general formula (II)H.sub.(n-m)+2 (NH.sub.4).sub.m P.sub.n O.sub.3n+1 (II)in which n stands for an integral average value of 3 to 1000, m stands for a whole number of up to n+2, and the m/n-ratio is between about 1.0 and 1.67.

Abstract: The invention relates to a process for making granulated, partially or completely hydrated alkali metal diphosphates or triphosphates. To this end, the invention provides for alkali metal diphosphates or triphosphates which consist to an extent of at least 20% of particles with a size of less than 0.045 mm and are hydrated to an extent of less than 90%, based on the phosphates' theoretical content of water of hydration, to be intensively mixed and granulated in the presence of water and an ammonium polyphosphate, this latter being used in a proportion of about 0.5 to 50 weight %, based on the quantity of water used.

Abstract: The disclosure relates to a process for making granular sodium tripolyphosphate of high abrasion resistance. To this end, the disclosure provides for sodium orthophosphate to be subjected to a one-stage spray-drying operation inside a heated spray tower by means of a nozzle with condensation of the orthophosphate. More particularly the orthophosphate is sprayed jointly with a solution of a high molecular weight phosphate of sodium, potassium or ammonium.

Abstract: The present invention relates to a process for the oxidation of alkali metal phosphites to alkali metal phosphates at temperatures of between 400.degree. and 500.degree. C. To this end, alkali metal phosphites are employed in the form of an aqueous solution or suspension and, if necessary, the alkali metal : P atomic ratio in this feed material is adjusted to a ratio which at least corresponds to the atomic ratio desired for the final product.More particularly, the solution or suspension is spray-dried in a tower, in a single operational stage, by spraying in a flame zone. Starting material free from alkali metal phosphate and with an alkali metal : P atomic ratio equal to or greater than 3.5 is used for making a trialkali metal phosphate which is obtained in admixture with an alkali metal hydroxide. Starting material which contains from 25 to 45% by weight of alkali metal phosphate, calculated as phosphorus, based on their total phosphorus content, is employed for making alkali metal polyphosphates.

Abstract: This invention is concerned with compositions of granular sodium tripolyphosphate (STPP) and their manufacture, which are suitable for use in the manufacture of automatic dishwasher detergents. The granular STPP compositions contain an overall 30 to 40% by weight of the STPP Phase I modification and no more than 5% of the granules composing the composition shall be of both +50 mesh size and under 25% Phase I concentration. Particle size of the total composition is 10% maximum of both +20 mesh and -100 mesh fractions. Apparent density is 0.7 to 0.9 grams per cubic centimeter. Such compositions are made by using only STPP materials in the 30 to 40% range or by mixing -50 mesh fractions of lower Phase I material with coarser fractions of higher Phase I material so that Phase I content of the total mixture is 30 to 40%.

Abstract: A photosensitive material in the form of a silver and chlorine-containing polyphosphate which exhibits permanent visible darkening on exposure to ultraviolet or short wavelength visible light, is described.

Abstract: The invention relates to a process for making a finely dispersed, sparingly soluble salt of an oxyacid of phosphorus, having a maximum particle size of 20 .mu.m, by reacting a compound of a divalent metal with the oxyacid of phosphorus or an alkali metal or ammonium salt thereof. More particularly, finely dispersed salt consisting to an extent of at least 90% of particles with a size between 0.05 and 7 .mu.m is made. To this end, an at least 5% by weight aqueous solution or suspension of a compound of a divalent metal selected from the group Mg, Ca, Sr, Ba, Mn, Zn, Cu, Cd, Pb, Sn, Co and Ni is reacted, while maintaining a pH range of between 3 and 9 and while stirring, with an at least 2 mol % solution of at least one trivalent metal selected from the group Al, Fe and Cr in the oxyacid of phosphorus and, optionally, with an aqueous solution of an alkali metal or ammonium salt of the oxyacid of phosphorus, and the precipitated salt is separated off and dried.

Abstract: A process and apparatus for condensing solid materials such as phosphates which tend to sinter and adhere when heated in a reactor wherein the reactants are introduced into a trough-like reactor provided with at least one set of rotatable agitating paddles. Heat is supplied to the reactor by passing a heating medium, such as a molten salt or a molten metal, through one or more heating jackets on the exterior of the reactor housing. The agitating paddles press the solid material in a thin layer against the wall of the reactor housing, and the material in the layer absorbs heat from the heating medium and sinters to a hard layer. One paddle of each set of paddles is provided with a stripping knife which moves along a path closely adjacent to the exterior wall of the reaction chamber to strip sintered material from the wall of the reactor housing. Heat is also transferred to the interior of the reactor housing by circulating the peeled material through the interior region of the reactor housing.

Abstract: Crude wet-process phosphoric acid prepared by mineral acid acidulation of uncalcined phosphate rock and containing contaminating amounts of organic impurities is purified and decolorized into an essentially colorless food-grade phosphoric acid, by (i) preliminarily decolorizing such crude acid, (ii) liquid-liquid extracting said predecolorized acid with an essentially water-insoluble organic extractant, (iii) back-extracting the extracted phosphoric acid with an aqueous phase, (iv) concentrating the back-extracted aqueous phosphoric acid solution at elevated, coloration-developing temperatures, and (v) then subjecting the concentrated solution which results to a final decolorization, to afford a purified, essentially colorless phosphoric acid solution.

Abstract: Screen undersize anhydrous sodium tripolyphosphate particles, containing at least 2 weight percent of an alkali metal pyrophosphate, preferably sodium pyrophosphate, are moisturized with from about 145% to about 185% by weight of the theoretical amount of water to convert the anhydrous sodium tripolyphosphate into the hexahydrate form. The material is agitated during moisturization to keep the temperature below about 80.degree. C., and the moisturized material is dried at a temperature below 80.degree. C. to remove most of the free moisture. The product recovered from the drying step is a granular sodium tripolyphosphate hexahydrate product having a bulk density within the range of 0.59 g/cc to 0.64 g/cc and a friability of no higher than 30%.

Abstract: Stable aqueous solutions of sodium polyphosphate useful in granulating dry solid cleaning composition components are formed by direct reaction of polyphosphoric acid with sodium carbonate generally at a temperature of about 50.degree. C. The reactants are used in an Na:P ratio of 1.2:1 to 1.5:1, and the product solution contains about 32 to about 37% w/w P.sub.2 O.sub.5 of which a high proportion is present as polyphosphate and has a pH of about 5 to about 6.5.

Abstract: Dense rotary dried sodium tripolyphosphate (STPP) particles having a bulk density of at least 0.9 g/cc and a particle size distribution of 0 to 85% by weight +20 mesh, 15 to 100% by weight -20 +100 mesh and 0 to 20% by weight -100 mesh are moisturized by adding a sufficient amount of water to produce a material containing on the average between about 10% and about 30% by weight moisture. The moisturized material is calcined to a temperature within the range of from about 300.degree. to about 600.degree. C., to produce a granular STPP product having low friability, high absorptivity and a bulk density within the range of from about 0.65 to about 0.87 g/cc.

Abstract: Disclosed is a process for producing sodium tripolyphosphate (STPP) with reduced amounts of silicon and magnesium impurities from wet process phosphoric acid.

Abstract: Screen undersize sodium tripolyphosphate (STPP) particles are compacted, milled and moisturized by adding a sufficient amount of water thereon to produce a material containing on the average between about 15% and about 35% by weight moisture. The moisturized material is calcined to a temperature within the range of from about 300.degree. to about 600.degree. C., to produce a granular STPP product having a low friability, high absorptivity, and a bulk density within the range of from about 0.65 to about 0.87 g/cc.

Abstract: A method for the treatment of phosphoric acid containing one or more impurities comprising the ions of at least one of the metals from the group magnesium, iron and aluminium, including the step of heating the acid under such conditions of temperature, pressure, time and concentration as to cause at least some of these impurities, (and in the case where magnesium is the major constituent among such impurities, more than 60% (mass/mass) of such magnesium) to precipitate as a pyrophosphate of the relevant metal ion.