Phosphoric impurity removal

The removal of impurities from the molten metal during the course of the oxygen blow is illustrated in Fig. 27.4. Silicon is removed rapidly at the start of the blow (27.1), carbon is removed progressively throughout the blow (27.2), with phosphorous being removed towards the end of the blow (27.3). Manganese, and iron are also oxidised (27.4-27.6). 

Sr Bone Ignite, dissolve with HCI °Y extracted into ethylhexyl phosphoric acid impurities removed by amine extraction /i-Counter... 

In order to make an efficient Y202 Eu ", it is necessary to start with weU-purifted yttrium and europium oxides or a weU-purifted coprecipitated oxide. Very small amounts of impurity ions, particularly other rare-earth ions, decrease the efficiency of this phosphor. Ce " is one of the most troublesome ions because it competes for the uv absorption and should be present at no more than about one part per million. Once purified, if not already coprecipitated, the oxides are dissolved in hydrochloric or nitric acid and then precipitated with oxaflc acid. This precipitate is then calcined, and fired at around 800°C to decompose the oxalate and form the oxide. EinaHy the oxide is fired usually in air at temperatures of 1500—1550°C in order to produce a good crystal stmcture and an efficient phosphor. This phosphor does not need to be further processed but may be milled for particle size control and/or screened to remove agglomerates which later show up as dark specks in the coating. 

Elemental phosphoms from the electrothermal process is a distilled product of high purity and yields phosphoric acid pure enough for most industrial uses without any further treatment. The main impurity is ca 20—100 ppm arsenic present in the phosphoms as the element and in the phosphoric acid as arsenious acid. To remove the arsenic, the phosphoric acid destined for food, pharmaceutical, and some industrial-grade appHcations is treated with excess hydrogen sulfide, filtered, and blown with air to strip out excess H2S. This treatment generally reduces the arsenic content of the phosphoric acid to less than 0.5 ppm. The small amount of filter cake is disposed of in approved chemical landfills. 

Solvent extraction—purification of wet-process phosphoric acid is based on preferential extraction of H PO by an organic solvent vs the cationic impurities present in the acid. Because selectivity of acid over anionic impurities is usually not sufficient, precipitation or evaporation steps are included in the purification process for removal. Cmde wet-process acid is typically concentrated and clarified prior to extraction to remove post-precipitated sludge and improve partition of the acid into the solvent. Concentration also partially eliminates fluoride by evaporation of HF and/or SiF. Chemical precipitation of sulfate (as Ba or Ca salts), fluorosiUcates (as Na salt), and arsenic (as sulfides) may also be used as a prepurification step preceding solvent extraction. 

A second pressure on elemental production was the development of processes which remove impurities from phosphoric acid made by the wet process, to generate acid of equivalent purity to that obtained by the electric furnace route. Two such plants were brought on stream one at Aurora, North Carolina in 1990 by a joint venture of Albright Wilson, Texasgulf, and Olin, and another at Geismar, Louisiana, in 1991 by Rhc ne-Poulenc. These units have reported capacities of 47,600 and 31,700 t/yr elemental phosphoms equivalent, respectively (14). 

Chemical Processing. Activated carbon consumption in a variety of chemical processing appHcations is about 8% of the total (74). The activated carbon removes impurities to achieve high quaHty. For example, organic contaminants are removed from solution in the production of alum, soda ash, and potassium hydroxide (82). Other apphcations include the manufacture of dyestuffs, glycols, amines, organic acids, urea, hydrochloric acid, and phosphoric acid (83). 

Typical applications in the chemical field (Beaver, op. cit.) include detarring of manufactured gas, removal of acid mist and impurities in contact sulfuric acid plants, recovery of phosphoric acid mists, removal of dusts in gases from roasters, sintering machines, calciners, cement and lime Idlns, blast furnaces, carbon-black furnaces, regenerators on fluid-catalyst units, chemical-recoveiy furnaces in soda and sulfate pulp mills, and gypsum kettles. Figure 17-74 shows a vertical-flow steel-plate-type precipitator similar to a type used for catalyst-dust collection in certain fluid-catalyst plants. 

In a typical process, potassium permanganate is used to treat the cracked liquor exiting the depolymerization plant without any pH adjustment. The liquor is usually acidic because it contains some of the phosphoric acid depolymerization catalyst. The KM11O4 treatment is followed by treatment of the CL aqueous solution with carbon followed by filtration. Next the filtered 20-30% CL aqueous solution is concentrated to 70% and the pH is adjusted to 9-10 by addition of sodium hydroxide. The caprolactam alkaline concentrate is treated widi KMn04 followed by distillation under reduced pressure to remove water and low-boiling impurities. 

PHOSPHATATION A clarification process where phosphoric acid or a soluble phosphate is used with lime and heat. The impurities are removed by flocculation, flotation, and surface scraping. 

The host lattices are frequently prepared from ZnS, Zn xCdxS2, alkaline earth carbonates, MHP04 (M = Ca, Sr, or Ba), and MNH4P04 H20 (M = Cd or Mn).8 The general (industrial method) of preparation involves precipitation of the phosphor itself, or an intermediate, from solutions of pure cationic and anionic precursors. The purity of all materials used is vital, and impurities must not be present at levels great than 1.0 ppm. Impurities, which are detrimental to phosphor quality and are usually referred to as killers , 4 may include M2+ (M = Fe, Co, Ni, Cu, Pd, and Pt), M3+ (M = Ti, V, Cr, Fe, Ru, Zr, Nb, Mo, Rh, Hf, Ta, and W), and M4+ (M = Os, Ir, and Re). Such cations are usually removed by precipitation or sequestration by selective chelation on solutions before processing.8... 

Guillini A process for making gypsum from the waste product from the Wet Process for making phosphoric acid. The waste is heated with water in an autoclave this removes impurities and converts the calcium sulfate dihydrate to the hemi-hydrate. 

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