Calcium sulfate limitations

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

By-Products. The biomass from the fungal fermentation process is called mycellium and can be used as a supplement for animal feed since it contains digestable nutrients (25,26). The lime-sulfuric purification and recovery process results in large quantities of calcium sulfate cake, which is usually disposed of into a landfill but can find limited use in making plaster, cement, waUboard, or as an agricultural soil conditioner. The Hquid extraction purification and recovery process has the advantage of Htde soHd by-products. 

The polysulfide base material contains 50—80% of the polyfunctional mercaptan, which is a clear, amber, sympy Hquid polymer with a viscosity at 25°C of 35, 000 Pa-s(= cP), an average mol wt of 4000, a pH range of 6—8, and a ntild, characteristic mercaptan odor. Fillers are added to extend, reinforce, harden, and color the base. They may iaclude siUca, calcium sulfate, ziac oxide, ziac sulfide [1314-98-3] alumina, titanium dioxide [13463-67-7] and calcium carbonate. The high shear strength of the Hquid polymer makes the compositions difficult to mix. The addition of limited amounts of diluents improves the mix without reduciag the set-mbber characteristics unduly, eg, dibutyl phthalate [84-74-2], tricresyl phosphate [1330-78-5], and tributyl citrate [77-94-1]. 

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. 

Sulfuric acid has found limited use in boiler cleaning operations. It is not feasible for removal of hardness scales due to the formation of highly insoluble calcium sulfate. It has found some use in cases where a high-strength, low-chloride solvent is necessary. Use of sulfuric acid requires high water usage in order to rinse the boiler sufficiently. 

Although some solutions, like one consisting of water and ethyl alcohol, can have any intermediate composition between the pure components, most solutions have an upper limit to the concentration of the solute. That limit is called the solubility of the substance. For example, in a liter of solution, the maximum amount of CaS04 dissolved is 0.667 grams, which is 0.0049 moles of that solute. Therefore, the solubility of calcium sulfate may be reported either as 0.667 grams per liter or as 0.0049 M. 

Sulfate (as potassium sulfate) has long been limited to reduce addition of calcium sulfate (plastering). The practice lowers the pH and is limited to warm climatic regions where the acidity is very low (pH... 

Fig. 1. Schematic flowsheet of uranium processing (acid leach and ion exchange) operation. Numbers refer to the numbers that appear in the boxes on the flowsheet. Operations (3), (6), (9), and (11) may be done by thickening or filtration. Most often, thickeners are used, followed by filters. The pH of the leach slurry <4) is elevated to reduce its corrosive effect and to improve the ion-exchange operation on the uranium liquor subsequently separated, In tile ion exchange operation (7), resin contained in closed columns is alternately loaded with uranium and then eluted. The resin adsorbs the complex anions, such as UC fSO 4-. in which the uranium is present in the leach solution. Ammonium nitrate is nsed for elution, obtained by recycling the uranium filtrate liquor after pH adjustment. Iron adsoibed with the uranium is eluted with it. Iron separation operation (8) is needed inasmuch as the iron hydroxide slurry is heavily contaminated with calcium sulfate and coprecipitated uranium salts. Therefore, the slurry is recycled to the watering stage (3). Washed solids from 1,6). the waste barren liquor from (7), and the uranium filtrate from (11) are combined. The pH is elevated to 7.5 by adding lime slurry before the mixture is pumped to the tailings disposal area. (Rio Algom Mines Limited, Toronto)...

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). 

In distillation the water closest to the heating surface is hottest and it is there that calcium sulfate is least soluble. Thus, calcium sulfate deposits, forming an adhering film that increases the thermal resistance and decreases the heat flux. The scale is continuously deposited until the tubes are cleaned or become plugged. For scale deposition the local concentration must be at least saturated in calcium sulfate. At 100° C. this occurs in concentrated sea water at a concentration 3.1 times that of ordinary sea water. A plant has been successfully operated continuously without calcium sulfate deposition by taking only part of the available water from the sea water, so that the liquid in the evaporator is never more than 1.8 times the concentration of sea water and the wall temperature is below about 250° F. ( ). This imposes technical and economic limitations on distillation plants. Similar considerations hold for plants distilling brackish water containing calcium sulfate. 

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. 

Under these circumstances, the limit of solubility of calcium sulfate can easily be exceeded and deposition results. 

Typically, the solubility limit is approximately 1800 to 2000 ppm but it is unwise to rely on this figure, as other factors, such as high chlorides, may decrease this to perhaps 1500 to 1600 ppm or lower. Even when polyacrylates or other general dispersants are present in the cooling water, calcium sulfate can form deposits. 

The osmotic pressure of brackish water is approximately 11 psi per 1000 ppm salt, so osmotic pressure effects do not generally limit water recovery significantly. Limitations are generally due to scaling. Typical water recoveries are in the 70-90 % range, which means the brine stream leaving the system is up to 10 times more concentrated in calcium, sulfate and silica ions present in the feed. If scaling occurs, the last modules in the system must be replaced first. 

Strelyuk described a thin-layer chromatographic method for the qualitative detection of dipyridamole [62], Data were presented on the use of various reagents for the detection by solution color change and precipitate formation in the thin-layer chromatography of dipyridamole. Thin-layer chromatography was on silica gel-calcium sulfate dihydrate, with methanol aqueous ammonia or benzene-dioxane being used as the mobile phase with detection at 270-330 nm. The Rvalues were 0.7-0.75 and 0.5-0.55 for the two solvent systems, and the detection limit was... 

Calcium Content Dissolve about 1.5 g of sample, accurately weighed, in 100 mL of water containing 2 mL of 2.7 A hydrochloric acid. While stirring, preferably with a magnetic stirrer, add about 30 mL of 0.05 M disodium EDTA from a 50-mL buret, then add 15 mL of 1 A sodium hydroxide and 300 mg of hydroxy naphthol blue indicator, and continue the titration to a blue endpoint. Each milliliter of 0.05 M disodium EDTA is equivalent to 2.004 mg of calcium (Ca). Halides Determine as directed in the Chloride Limit Test under Chloride and Sulfate Limit Tests, Appendix IIIB. A 1.2-g sample shows no more turbidity than 0.7 mL of 0.020 A hydrochloric acid. 

Oxalate Transfer 1 g of sample into a 125-mL separator, dissolve in 10 mL of water, add 2 mL of hydrochloric acid, and extract successively with one 50-mL portion and one 20-mL portion of ether. Transfer the combined ether extracts to a 150-mL beaker, add 10 mL of water, and remove the ether by evaporation on a steam bath. Add 1 drop of glacial acetic acid and 1 mLof a 1 20 calcium acetate solution to the residual aqueous solution. No turbidity develops within 5 min. Sulfate Dissolve 100 mg of sample in 2.7 N hydrochloric acid, and dilute to 30 to 40 mL with water. Proceed as directed in the Sulfate Limit Test under Chloride and Sulfate Limit Tests, Appendix IIIB, beginning with the addition of 3 mL of barium chloride TS. Any turbidity produced does not exceed that shown in a control containing 300 pig of sulfate (S04). 

Although this method has had limited application, it represents the most convenient synthesis for the important diphenylketene. This consists in converting benzil monohydrazone to the diazo compound by the action of mercuric oxide suspended in benzene. The presence of anhydrous calcium sulfate is needed to remove the water formed in the oxidation. The benzene solution is then dropped slowly into a distilling flask maintained at 100-110°, whereby the benzene distils and the diazo... 

In the case of a diffusion-limited reaction, hydrodynamic factors, such as turbulent flow, may influence the dissolution process. Humidity fluctuations also may alter the solute concentrations in the moisture film at the rock surface, and they may result in the dissolution and recrystallization, not only of the carbonates, but also of the secondary minerals, such as calcium sulfate. Wetting and drying cycles may lead to measureable changes in fluid composition at the stone surface. 

The presence of sodium sulfate and sodium chloride is principally the result of secondary absorption reactions. Sodium sulfate is formed by the oxidation of sodium sulfite via reaction with oxygen absorbed from the flue gas. Oxidation also occurs in other parts of the system where process solutions are exposed to air however, the amount of oxidation is small relative to the oxidation which occurs in the absorber. At steady state, the sulfate must leave the system either as calcium sulfate or as a purge of sodium sulfate at the rate at which it is being formed in the system. Although a practical limit for the level of oxidation that can be tolerated by the limestone dual alkali system has not yet been established, it appears that oxidation rates equivalent to 15 to 20% of the S02 removed might be accommodated without intentional purges of sodium sulfate. 

Simultaneously with the above reaction, a limited amount of calcium sulfate will also be precipitated ... 

As the concentration of sulfate increases relative to sulfite, the amount of sulfate precipitation increases. Thus, as the rate of oxidation increases, the ratio of sulfate to sulfite in solution will increase until the rate of calcium sulfate precipitation is sufficient to keep up with the rate of sulfate formation by oxidation. This self-adjustment by the system may, however, be limited by the need to maintain a high active sodium concentration which will limit sulfate concentrations (and consequently the sulfate/ sulfite ratio) simply by solution saturation considerations. Furthermore, the sulfate/sulfite ratio may also be limited by the need to ensure high limestone utilizations and good solids properties. 

Reverse osmosis is a cross-flow membrane separation process which separates a feed stream into a product stream and a reject stream. The recovery of a reverse osmosis plant is defined as a percentage of feedwater that is recovered as product water. As all of the feedwater must be pretreated and pressurized, it is economically prudent to maximize the recovery in order to minimize power consumption and the size of the pretreatment equipment. Since most of the salts remain in the reject stream, the concentration of salts increases in that stream with increased recovery. For instance, at 50% recovery, the salt concentration in the reject is about double that of the feed and at 90% recovery, the salt concentration in the reject is nearly 10 times that of the feed. In cases of sparingly soluble salts, such as calcium sulfate, the solubility limits may be exceeded at a high recovery. This could result in precipitation of the salt on the membrane surface resulting in decreased flux and/or increased salt passage. In addition, an increase in recovery will increase the average salt concentration in the feed/reject stream and this produces a product water with increased salt content. Consequently, the recovery of a reverse osmosis plant is established after careful consideration of the desired product quality, the solubility limits of the feed constituents, feedwater availability and reject disposal requirements. 

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