Deposition, calcium sulfite

Calcium Sulfite Deposition. Calcium sulfite (CaS03 %H20) is formed in the scrubber under those conditions that favor sulfite formation. These conditions are apparent when one considers the sulfite-bisulfite equilibrium and compares the relative solubilities of the corresponding calcium salts. As seen in Figure 2, extremely soluble bisulfite in solution changes to relatively insoluble sulfite when the solution pH shifts from 4 to 10. When SO2 is absorbed, the scrubber solution is usually between pH 4 and 6 and, therefore, the predominant species is... 

NOTE A white scum of calcium sulfite may occur on films processed in high-sulfite, low-alkalinity developers. This scum is soluble in acid stop baths and in fresh acid fixing baths, especially if the film is well agitated. It is slowly soluble in water and may also be wiped or sponged off wet film, although light deposits may not be noticed until the film is dry. Kodak SB-5 Non-swelling Acid Rinse Bath is recommended for its removal. 

Sulfates are widely known in the environment and a few such as those of calcium, strontium, barium, and lead are insoluble. The sulfates of magnesium and the alkali metals are soluble and so are typically found in brines and salt deposits. Sulfites are rarer because of the ease of oxidation, but calcium sulfite, perhaps as the hemihydrate, can be detected on the surface of building stone in urban atmospheres containing traces of sulfur dioxide (Gobbi et al, 1998). 

There was a slow buildup of a layer of scale on the walls of the first reactor. Insufficient agitation may have contributed to the deposition of this scale which was primarily made up of calcium sulfite/sulfate solids. The formation of this scale was also evident in the overflow pipe connecting the first and second reactors. This pipe had to be replaced in early March as the 6-inch line had been reduced to a 3-1/2 inch line due to scale buildup. This overflow line was rather long—6 feet—and had an angle of incline of only 3° because of limitations in the existing plant layout. A greater angle of incline may have reduced this scale buildup. 

Another type of solids deposition in the form of round/oval beads was also observed in the first reactor. These beads—1/8 to 3/8 inch in size—were also made up of calcium sulfite/sulfate which built up in layers around a seed particle much like the growth of pearls. Scale buildup in the other reactors was minimal. Based on the time of operation at Scholz, it appears that a semiannual cleaning operation as part of a regular maintenance program might be adequate to control reactor scaling. 

Because of the marked difl[erence in the water solubility of CaS04 and CaSOs (0.2016 g/100 ml and 0.0045 g/100 ml), the latter is formed by treatment of the sulfate with alkali sulfite. However, this conversion is not directly observable, but if a solution of sodium selenosulfate is used as a SOs donor, the slow deposition of red elemental selenium indicates the formation of calcium sulfite ... 

If alkali selenosulfate is employed instead of alkali sulfite, the formation of calcium sulfite is accompanied by the deposition of red selenium ... 

Sulfates and Sulfites. Calcium sulfate [7778-18-9] occurs in large deposits as CaSO and as gypsum, CaSO -2H20. The dihydrate is a... 

Howden An early flue-gas desulfurization process using a lime or chalk slurry in wooden grid-packed towers. The calcium sulfate/sulfite waste product was intended for use in cement manufacture, but this was never commercialized. The key to the process was the use of a large excess of calcium sulfate in suspension in the scrubbing circuit, which minimized the deposition of scale on the equipment. The process was developed by Imperial Chemical Industries and James Howden Company in the 1930s and operated for several years at power stations at Fulham, London, and Tir John, South Wales, being finally abandoned during World War II. British Patents 420,539 433,039. 

When hard water is used for mixing developers, milkiness is sometimes produced. This is caused by the action of alkali and sulfite on the lime salts in the water. If the lime is excessive, a precipitate of calcium carbonate and sulfite may be deposited on film. 

Sodium chlorate is readily soluble in water (790 g/1 at 0°C) and is therefore applied as an aqueous solution. It is a strong oxidant. It reacts with organic materials in the presence of sunlight, creating a serious fire hazard when spray splashed on clothing forms deposits. Residual compound on wilted weeds has a tendency to self-ignite. To reduce the fire hazard, calcium chloride, sodium sulfite or trisodium phosphate is mixed with the sodium chlorate. 

Inorganic cathodic Cl are able to restrict corrosion rate of metals via diminishing oxygen concentration in the corrosive medium (sodium sulfite, hydrazine), formation of screening insoluble residues (calcium bicarbonate, zinc sulfate and chloride for steel) on cathodic sites of the metal or by raising overstrain of the cathodic process as a result of reduction of metal cations and their deposition on the metal surface that is being protected (salts of heavy metals, such as antimony, bismuth, arsenic). 

Forced oxidation (in limestone systems) can increase oxidation to calcium sulfate to well over 95%. In fact, one supplier of forced oxidation systems guarantees 99.5-t% convosion to calcium sulfate and considers operation at 95% a result of process chemistry imbalances (Klingspor, 1993). The calcium sulfate so formed is precipitated in the absorber sump/reac-tion tank as gypsum, provided sufficient time and seed crystals are available. Hus reduces the amount of dissolved sulfite returned to the absorber, and minimizes the possibili of sulfite oxidation and sulfate scale deposition on equipment surfaces. 

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