Phosphate-fluoride process

Modified phosphate-fluoride treatment As a result of questionable durability of the bonds made using the phosphate-fluoride process, a modified phosphate-fluoride process was introduced. In this process 0.75% sodium sulfate is added to the conversion coating solutions and this stabilizes the oxide formed. Even with this modification, the improvement over the phosphate-fluoride process is minimal. 

In the phosphate-fluoride process, the adherend is etched in 3% HF. The etched titanium is then immersed in an aqueous solution of the following composition for... 

Fluorides and dust are emitted to the air from the fertilizer plant. All aspects of phosphate rock processing and finished product handling generate dust, from grinders and pulverizers, pneumatic conveyors, and screens. The mixer/reactors and dens produce fumes that contain silicon tetrafluoride and hydrogen fluoride. A sulfuric acid plant has two principal air emissions sulfur dioxide and acid mist. If pyrite ore is roasted, there will also be particulates in air emissions that may contain heavy metals such as cadmium, mercury, and lead. 

To show that the cells were firmly attached to the surface of the hydrous metal oxide and not just loosely trapped in the gelatinous matrix, a number of arguments are invoked. Free cells of E. coli are comparatively small and cannot be centrifuged down to any significant extent in the centrifugation conditions used for collecting the immobilised cells they are not particularly robust and so any disruption process which had occurred would, therefore, have rendered them inactive and so unable to respire. Solutions of bicarbonate, phosphate, fluoride (and so on) ions which have been shown (3) to remove loosely bound proteinaceous and other materials from zirconium (IV) hydroxide were singularly ineffective in the attempted release of the immobilised cells from the matrix. 

The process variables known to influence arsenic capacity and column performance of alumina, GFH, and other adsorbents are as follows adsorbent, adsorbent particle size, flow rate, EBCT, and water quality parameters including arsenic concentration, As(III)/(V) speciation, pH, silica, phosphate, fluoride, hardness, and sulfate concentrations. Even with a complete water analysis, it is prudent to perform pilot studies with competitive adsorbents on the water to be treated because of the numerous factors that influence arsenic adsorption. Equilibrium isotherms and rapid small-scale column tests (RSSCTs) are typically run prior to the pilot study (7). 

Introduction of phosphogypsum as an additive to cement accounts for about 4 million tonnes per annum. Portland cement usually contains about 5% gypsum, which is added to control (retard) the setting time. Phosphate, fluoride, and organic impurities in the phosphogypsum are undesirable and should be reduced to acceptable levels. Material from double crystallization phosphoric acid processes is satisfactorily employed in Belgium and Japan. 

Efforts have been made in some installations to recover the fluorine evolved in phosphate-rock processing as cryolite or other marketable products. Hie quantity of fluorine evolved in such operations is very large so that steps to recover it would appear warranted. A process developed by the Tennessee Valley Authority (TVA) is claimed to accomplish this (Anon., 1957). In the TVA process, the water used to scrub nodulizing-kiln exhaust gas is maintained at a pH of S to 6 by the continuous addition of ammonia, and the resultant rich liquor is treated to precipitate impurities and yield a valuable NH4F solution. The absorber solution is recirculated to bring its fluorine content up to about 35 g/liter. Hie rich solution is then treated with sufficient ammonia to raise the pH to 9, thus precipitating iron, silica, and part of the phosphorus. The precipitate is fiitered off, and the solution is thoi used to make cryolite (by adding sodium sulfiite and alum at a pH of 6) or aluminum fluoride. 

Primary emphasis has been placed on the study of uranyl sulfate s( )lut > . -because of the superiority of the sulfate over other anions with u -pi. t u, thermal and radiation stability, absorption cross section for iiruiid>,-, ease of chemical processing, and corrosive properties. Other uiaii>l -.d.- which have either been used in reactors or studied for possililc u. c in the nitrate, phosphate, fluoride, chromate, and carbonate. It liu i" n found possilile to improve the solubility characteristics of uranyl solutions at elevated temperattircs by the addition of acids or salts of tic chosen anion. 

Alternative Processes. Because of the large quantity of phosphate rock reserves available worldwide, recovery of the fluoride values from this raw material source has frequently been studied. Strategies involve recovering the fluoride from wet-process phosphoric acid plants as fluosiUcic acid [16961-83-4] H2SiFg, and then processing this acid to form hydrogen fluoride. 

The majority of the fluorine ia the earth s cmst is present in the form of the phosphoms fluoride fluoroapatite [1306-05 ] Ca (P0 2F- Phosphate rock deposits contain an average concentration of 3.5 wt % fluorine. During phosphate processing these fluorine values are partially recovered as by-product fluorosihcic acid. The amount of fluorosiUcic acid recovered has grown steadily, in part because of environmental requirements (see Phosphoric acid and THE phosphates). 

The purified acid is recovered from the loaded organic stream by contacting with water in another countercurrent extraction step. In place of water, an aqueous alkafl can be used to recover a purified phosphate salt solution. A small portion of the purified acid is typically used in a backwashing operation to contact the loaded organic phase and to improve the purity of the extract phase prior to recovery of the purified acid. Depending on the miscibility of the solvent with the acid, the purified acid and the raffinate may be stripped of residual solvent which is recycled to the extraction loop. The purified acid can be treated for removal of residual organic impurities, stripped of fluoride to low (10 ppm) levels, and concentrated to the desired P2 s Many variations of this basic scheme have been developed to improve the extraction of phosphate and rejection of impurities to the raffinate stream, and numerous patents have been granted on solvent extraction processes. 

Aluminum fluoride is also made by the reaction of fluosiUcic acid [16961 -83-4] H2SiFg, a by-product from phosphoric acid production (see Phosphoric ACID AND THE PHOSPHATES), and aluminum hydroxide from the Bayer process. 

This input to design refers to the long-term stability of the raw material sources for the plant. It is only of importance where the raw materials can or do contain impurities which can have profound effects on the corrosivity of the process. Just as the design should cater not only for the norm of operation but for the extremes, so it is pertinent to question the assumptions made about raw material purity. Crude oil (where HjS, mercaptan sulphur and napthenic acid contents determine the corrosivity of the distillation process) and phosphate rock (chloride, silica and fluoride determine the corrosivity of phosphoric acid) are very pertinent examples. Thus, crude-oil units intended to process low-sulphur crudes , and therefore designed on a basis of carbon-steel equipment, experience serious corrosion problems when only higher sulphur crudes are economically available and must be processed. 

Phosphate solutions containing fluorides are used for processing steel, zinc and aluminium when assembled together, but chromate solutions are generally preferred when aluminium is treated alone. The increasing use of cathodic electrophoretic painting on steel, however, has led to a reassessment of the basic processes and formulations that might be most effective. 

Acid treatments The principal acid processes were developed in the USA under the name Alodine, and are marketed in the UK as Alocrom and under other names. The original solutions were based on acid solutions containing phosphate, chromate and fluoride ions. Immersion for up to 5 min in the cold or warm solution leads to the deposition of a greenish film containing the phosphates of chromium and aluminium, and possibly some hexavalent chromate. The more recent Alocrom 1 200 process uses an acid solution containing chromate, fluoride and nitrate. Room-temperature immersion for 15 s to 3 min deposits golden-brown coatings which contain chromate as a major constituent. 

Of the principal minerals of Ca listed in Table 1, the most important ores are the various deposits of CaCOj, especially limestones, which occur as immense sedimentary beds over extensive parts of the earth s surface. Extraction of Ca from CaCOj is a simple and relatively inexpensive process. Although the other Ca-bearing minerals are rarely considered as potential Ca sources, they are widely distributed and extensively mined fluorite and apatite for their fluoride and phosphate content, gypsum and anhydrite for their use in construction. 

Modem production of elemental phosphoras uses a technique similar to the metallurgical processes described in Chapter 20. Apatite is mixed with silica and coke and then heated strongly in the absence of oxygen. Under these conditions, coke reduces phosphate to elemental phosphoms, the silica forms liquid calcium silicate, and the fluoride ions in apatite dissolve In the liquid calcium silicate. The reactions are not fully understood, but the stoichiometry for the calcium phosphate part of apatite is as follows ... 

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