Calcium sulfate , solubility

The chemical composition of the cooling water makeup supply used in the plant determines the choice of the cycles of concentration. Some of the important constituents that must be controlled in the tower are calcium, magnesium, silica, carbonate, bicarbonate and sulfate ions. Alkalinity levels are regulated by the addition of acid or alkali to achieve the desired pH. When adding H2S04 (sulfuric acid) for pH control, it should be assured that calcium sulfate solubility limits are not exceeded (see Chapter 8). 

Calcium Sulfate Solubility in Dynamic Cooling Tower Systems. . . Zero Blowdown... 

Sulfates in surface waters are usually present at lower levels, typically 20 to 60 ppm, but the level can rise to several hundred ppm in subsurface waters. If high alkalinity makeup waters also contain high sulfates, the use of sulfuric acid dosing as a treatment to reduce the alkalinity can be ruled out because when the cooling water is cycled up, the total sulfate content can easily exceed the solubility limit of calcium sulfate (solubility is dependent on temperature but is in the range 1800 to 2000 ppm), and scaling readily occurs. 

Skillman, H. L, James P. MacDonald, Jr., and Henry A. Stiff, Jr. "A Simple, Accurate, Fast Method for Calculating Calcium Sulfate Solubility in Oil Field Brines, " Spring Meeting of the Southwest District, Division of Production, American Petroleum Institute, March, 1969, Lubbock, Texas. 

Li, Z., and Demopoulos, G. P. (2002). Calcium Sulfate Solubility in Concentrated Aqueous Chloride Solutions. Chloride Metal. 2, 561 -574. 

Extraction of Nonmetallic Inorganic Compounds. Phosphoric acid is usually formed from phosphate rock by treatment with sulfuric acid, which forms sparingly soluble calcium sulfate from which the phosphoric acid is readily separated. However, in special circumstances it may be necessary to use hydrochloric acid ... 

On archaeological glass objects, layers of reaction products are formed and the main constituents of these cmsts are the less-soluble compounds such as siHca and calcium carbonate, which becomes calcium sulfate. 

This PAG contains 1—2% sulfate as soluble calcium sulfate. Sulfate has been found to make PAG products unstable precipitate forms in less than one week at 50°G. Sulfate, however, has also been seen to increase PAG activity in water clarification and is thus intentionally added in one preparation (24). Precipitated calcium sulfate creates a sludge disposal problem. Typical Al content as AI2O2 of PAG products made from alum is 6 —8%. 

Tofu. Tofu is prepared by adding a coagulant such as calcium sulfate to soymilk to precipitate the protein and oil into a gelatinous curd. The curd is then separated from the soluble portion (whey), pressed, and washed to yield a market-ready product. Tofu, a traditional food in Japan (90), was populari2ed in the United States in the late 1970s and is available in many U.S. supermarkets. 

Lime-Sulfuric. Recovery of citric acid by calcium salt precipitation is shown in Figure 3. Although the chemistry is straightforward, the engineering principles, separation techniques, and unit operations employed result in a complex commercial process. The fermentation broth, which has been separated from the insoluble biomass, is treated with a calcium hydroxide (lime) slurry to precipitate calcium citrate. After sufficient reaction time, the calcium citrate slurry is filtered and the filter cake washed free of soluble impurities. The clean calcium citrate cake is reslurried and acidified with sulfuric acid, converting the calcium citrate to soluble citric acid and insoluble calcium sulfate. Both the calcium citrate and calcium sulfate reactions are generally performed in agitated reaction vessels made of 316 stainless steel and filtered on commercially available filtration equipment. 

More recendy, the molten caustic leaching (MCL) process developed by TRW, Inc. has received attention (28,31,32). This process is illustrated in Eigure 6. A coal is fed to a rotary kiln to convert both the mineral matter and the sulfur into water- or acid-soluble compounds. The coal cake discharged from the kiln is washed first with water and then with dilute sulfuric acid solution countercurrendy. The efduent is treated with lime to precipitate out calcium sulfate, iron hydroxide, and sodium—iron hydroxy sulfate. The MCL process can typically produce ultraclean coal having 0.4 to 0.7% sulfur, 0.1 to 0.65% ash, and 25.5 to 14.8 MJ/kg (6100—3500 kcal/kg) from a high sulfur, ie, 4 wt % sulfur and ca 11 wt % ash, coal. The moisture content of the product coal varies from 10 to 50%. 

Compare the solubilities in water of calcium carbonate, calcium sulfite, calcium sulfate, magnesium sulfate, and dolomite. 

The degree of concentration that can be achieved by RO may be limited by the precipitation of soluble salts and the resultant scaling of membranes. The most troublesome precipitate is calcium sulfate. The addition of polyphosphates to the influent will inhibit calcium sulfate scale formation, however, and precipitation of many of the other salts, such as calcium carbonate, can be prevented by pretreating the feed either with acid or zeolite softeners, depending on the membrane material. 

Calcium, Magnesium and/or Sodium-Cation Exchanger (Insoluble) + Sulfuric Acid (Soluble) = Hydrogen Cation Exchanger (Insoluble) + Calcium, Magnesium and/or Sodium Sulfates (Soluble). 

Write the equation for the dissolving of calcium sulfate, CaSOt, and the solubility product expression. 

Calculate the solubility, in moles per liter, of calcium sulfate in water, using the solubility product given in Table 10-11. 

When two solutions are mixed, a precipitate may form. For example, suppose solutions of calcium chloride, CaCl2, and sodium sulfate, Na2S04, are mixed. The mixture contains both calcium ions, Ca+1, and sulfate ions, S04-2, so solid calcium sulfate may form. The solubility product permits us to predict with confidence whether it will or not. 

The precipitation of anhydrite (anhydrous calcium sulfate, CaS04) may also occur. Under ambient temperatures, anhydrite is much more soluble than calcium carbonate, but because calcium sulfate, in common with other calcium salts such as calcium phosphate (also known as tricalcium phosphate [Ca3(P04)2]), has an inverse-temperature solubility, it deposits more rapidly on the hottest heat transfer surfaces. 

Most salts absorb heat when they go into solution, and their solubility increases with a rise in temperature however, calcium carbonate (CaC03), in common with several other anhydrous salts such as calcium sulfate (CaS04) and calcium phosphate [Ca3(P04)2], has an inverse temperature solubility and thus readily precipitates to form deposits in hot water areas (FW tanks, FW lines, and boiler heat exchange surfaces). 

Sulfates in surface MU water sources usually are present at lower concentrations (typically 20-60 ppm) but this level may rise to several hundred ppm in subsurface waters. The maximum solubility of calcium sulfate is dependent on temperature but is in the range of 1,800 to 2,000 ppm in cold water. This rate is significantly less in hot BW where boiler deposits occur, the sulfate scale normally is present as anhydrite (CaS04). Sulfate scales are hard and very difficult to remove, so treatment programs employed must be carefully controlled to avoid risks of scaling. 

NOTE The calcium carbonate limit that RO system designers typically require is +1.6 to 1.8 LSI in the concentrate or reject water, and the calcium sulfate design limit typically calls for a maximum reject water saturation ratio of 1.6 to 1.8 times solubility product. 

Calcium salts have a low solubility in water but this is only true for dodecyl and higher sulfates. Pure octyl and decyl calcium sulfates are soluble substances. 

C16-0105. Write the equilibrium reaction and equilibrium constant expression for each of the following processes (a) Trimethylamine, (CH3)3 N, a weak base, is added to water, (b) Hydrofluoric acid, HF, a weak acid, is added to water, (c) Solid calcium sulfate, CaSOq, a sparingly soluble salt, is added to water. 

Gypsum is a relatively soft rock made of calcium sulfate. Rainwater percolates through g q)sum, dissolves some of the rock, and eventually becomes saturated with Ca ions and SOq ions. A geochemist takes a sample of groundwater from a cave and finds that it contains 8.4 X 10 M SO4 and 5.8 X 10 M Ca. (The ratio is not 1 1 because other sulfate rock contributes some of the SOq ions to the solution.) Use these data to determine the solubility product of calcium sulfate. 

As examples of some water-soluble salts, mention may be made of potassium chloride, copper sulfate, and sodium vanadate. As examples of some water-insoluble salts, mention may be made of some typical ones such as lead chloride, silver chloride, lead sulfate, and calcium sulfate. The solubilities of most salts increases with increasing temperature. Some salts possess solubilities that vary very little with temperature or even decline. An interesting example is provided by ferrous sulfate, the water solubility of which increases as temperature is raised from room temperature, remains fairly constant between 57 and 67 °C, and decreases at higher temperatures to below 12 g l-1 at 120 °C. Table 5.2 presents the different types of dissolution reactions in aqueous solutions, and Table 5.3 in an indicative way presents the wide and varied types of raw materials that different leaching systems treat. It will be relevant to have a look at Table 5.4 which captures some of the essential and desirable features for a successful leaching system. 

The action of carbonic acid on limestone produces a calcium bicarbonate solution that is exceedingly soluble in water. (For comparison, at 20°C the solubility of calcium carbonate in water is only 0.0145 g per liter while the solubility of calcium bicarbonate is 166 g per literJ ) Magnesium ions from dolomite are also released into aqueous solution according to the same mechanism. The weathering of gypsum, calcium sulfate, also releases calcium ions into natural water supplies. 

The product, Ca(H2P04)2, is more soluble than the phosphate. Sulfuric acid is produced in the largest quantity of any compound, with production that approaches 100 billion pounds annually. Approximately two-thirds of this amount is used in the production of fertilizers. The mixture containing calcium dihydrogen phosphate and calcium sulfate (gypsum) is known as superphosphate of lime, and it contains a higher percent of phosphorus than does calcium phosphate. 

Values from Refs. (272,646) are derived from measurements of the solubility of calcium sulfate in solutions of the respective sugars (at 298K, 0.2stability constants from solubility measurements has been reviewed - see Ref. (225). 

The fluoride portion is either removed as gaseous silicon tetrafluoride, if silica is added, or as the sparingly soluble sodium hexafluorosilicate, which remains with the calcium sulfate. 

Dundon ML, Mack E. The solubility and surface free energy of calcium sulfate. J Am Chem Soc 1923 45(ll) 2479-2485. 

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