Deposition, calcium sulfate

The back-pressure increase is to 7.5 inches for a similar thickness of calcium sulfate deposit... 

Calcium sulfate deposition is inhibited by the addition of sufficient soda ash. 

Scale deposits are converted to dispersed particles which can be circulated out of the wellbore. A chelating agent such as ethylenediamine tetraacetic acid can aid in dissolving calcium sulfate deposits. Hydrochloric acid following the basic treatment can also be used to dissolve calcium sulfate (167). 

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. 

Figure 8. Effect of velocity on calcium sulfate deposition...

The development of new families of high-stress tolerant, polymer-phosphonate-based chemical inhibitor programs, that now permit operators of small cooling systems to run at high cycles and alkalinity, with no sulfuric acid, has almost eliminated the risk of accidental calcium sulfate deposition in these systems. 

Also, there is a significant risk of calcium sulfate deposition occurring in a cooling system where ... 

PCA 16 is particularly effective for control of calcium sulfate deposition (and thus finds application, under different brand names and grades, as a calcium carbonate/sulfate scale inhibitor for RO systems treating brackish waters). This inhibitor is also useful for calcium phosphate control. It is stable against chlorine. 

Calcium Sulfate Deposition. The solubility of calcium sulfate is only slightly increased with increasing pH, and calcium sulfate scaling is related to the tendency of this material to form extensively stable supersaturated solutions. While chemical theory predicts that a given ionizable... 

With oil-firing, the sulfur in the fuel can be expelled in the exhaust gases. The high sulfur recycle load can lead to the formation of calcium sulfate rings at the back-end of the kiln, or of calcium sulfate deposits in the pre-heater (see section 16.4.9). 

The net heat usage is about 1,030 kcal/kg. A range of solid, liquid and gaseous fuels can be used, although they should be selected with care to avoid excessive build-ups caused by fuel ash and calcium sulfate deposits. 

A feature of rotary kilns is the formation of rings . These consist of an accumulation of material on the refractory in a part of the kiln which has the appropriate temperature for a semi-liquid phase to form. Such rings can form from ash in coal-fired kilns and from calcium sulfate deposits. Alkalis (sodium and potassium oxides), clay and lime can contribute to the build-ups, which can be troublesome. In the case of coal-firing, fine grinding of the fuel can significantly reduce the rate of build-up. 

Figs 9.9 and 9.10 can be used to obtain K, Pca and PaDc for the stability index. Similar indices may be used to predict bariiun sulfate or strontium sulfate or calcium sulfate deposition. If the product of concentration of Ca ions and SO4 ions exceeds the solubility products ( p) of CaS04, calcium sulfate would be precipitated. The solubility of calcium sulfate decreases with an increase in the temperature. CaCOj and CaS04 scaling cause major problems in plants and equipment encountering salt water. 

Obtaining maximum performance from a seawater distillation unit requires minimising the detrimental effects of scale formation. The term scale describes deposits of calcium carbonate, magnesium hydroxide, or calcium sulfate that can form ia the brine heater and the heat-recovery condensers. The carbonates and the hydroxide are conventionally called alkaline scales, and the sulfate, nonalkaline scale. The presence of bicarbonate, carbonate, and hydroxide ions, the total concentration of which is referred to as the alkalinity of the seawater, leads to the alkaline scale formation. In seawater, the bicarbonate ions decompose to carbonate and hydroxide ions, giving most of the alkalinity. 

Scaling is not always related to temperature. Calcium carbonate and calcium sulfate scaling occur on unheated surfaces when their solubiUties are exceeded in the bulk water. Metallic surfaces are ideal sites for crystal nucleation because of their rough surfaces and the low velocities adjacent to the surface. Corrosion cells on the metal surface produce areas of high pH, which promote the precipitation of many cooling water salts. Once formed, scale deposits initiate additional nucleation, and crystal growth proceeds at an accelerated rate. 

Gypsum [13397-24-5] CaSO 2H20, is a naturally occurring mineral found mainly in the western United States and eastern Canada (see Calcium COMPOUNDS, CALCIUM sulfate). The purer deposits require only minimal beneficiation to get a product pure enough for commercial appHcations. Other... 

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

Calcium sulfate [7778-18-9J, CaSO, ia mineral form is commonly called gypsum and occurs abundandy ia many areas of the wodd. In natural deposits, the main form is the dihydrate. Some anhydrite is also present ia most areas, although to a lesser extent. Mineral composition can be found ia Table 1. 

Seawater Distillation. The principal thermal processes used to recover drinking water from seawater include multistage flash distillation, multi-effect distillation, and vapor compression distillation. In these processes, seawater is heated, and the relatively pure distillate is collected. Scale deposits, usually calcium carbonate, magnesium hydroxide, or calcium sulfate, lessen efficiency of these units. Dispersants such as poly(maleic acid) (39,40) inhibit scale formation, or at least modify it to form an easily removed powder, thus maintaining cleaner, more efficient heat-transfer surfaces. 

Paints and coatings for automobiles have not been immune to damage by air polluhon. Wolff and co-workers (13) found that damage to automobile finishes was the result of scarring by calcium sulfate crystals formed when sulfuric acid in rain or dew reacted with dry deposited calcium. 

Calcium carbonate (CaCO,) calcium sulfate or gypsum (CaSOJ and iron(II) carbonate (FeCO ) are the most common types of scales formed in drilling. If hydrogen sulfide is present, then there is a possibility of iron sulfide (FeS) scale depositing. 

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. 

PCA 16 is available as Beldene 161/164 (50/35% w/w solids), Acumer 4161 (50%), and Polysperse (50%). These are low-phosphorus content materials that have found application in boiler FW formulations because of excellent sludge conditioning and particulate dispersion properties. The number 16 represents a 16 1 w/w ratio of acrylic acid and sodium hypophosphite, giving PCA 16 a MW range of 3,300 to 3,900. PCA 16 is particularly effective for the control of calcium carbonate and sulfate deposition. It is usually incorporated with other polymers in formulations and is approved for use under U.S. CFR 21, 173.310. 

Origin of sulfide sulfur of epithermal base-metal veins is thought to be same as that of Kuroko deposits because average 8 S value of base-metal vein-type deposits is - -4.7%o which is identical to that of Kuroko deposits (- -4.6%o) (Shikazono, 1987b). Namely, sulfide sulfur of base-metal veins came from igneous rocks, sulfate of trapped seawater in marine sedimentary rocks, calcium sulfate (anhydrite, gypsum) and pyrite. 8 S of sulfide sulfur of epithermal base-metal vein-type deposits can be explained by the interaction of seawater (or evolved seawater) with volcanic rocks. 

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. 

Sulfur Emissicms Sulfur present in a fuel is released as SO2, a known contributor to acid rain deposition. By adding limestone or dolomite to a fluidized bed, much of this can be captured as calcium sulfate, a dry nonhazardous solid. As limestone usually contains over 40 percent calcium, compared to only 20 percent in dolomite, it is the preferred sorbent, resulting in lower transportation costs for the raw mineral and the resulting ash product. Moreover, the high magnesium content of the dolomite makes the ash unsuitable for some building applications and so reduces its potential for utilization. Whatever sorbent is selected, for economic reasons it is usually from a source local to the FBC plant. If more than one sorbent is available, plant trials are needed to determine the one most suitable, as results from laboratory-scale reactivity assessments are unreliable. 

Anhydrite A mineral composed of calcium sulfate. It is a common hydrogenous mineral deposited in hydrothermal systems. 

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