Calcium sulfate, scaling control

Alternatively, antisealants can be used to control calcium carbonate scale at LSI values as high as 2.0-2.5, depending on the specific antisealants. Calcium also forms scales with fluoride, sulfate, and phosphate. The LSI will not help predict these scales analysis of water quality, using the ion product and solubility constants, is required to determine the potential for scaling with calcium fluoride or calcium phosphate. Antisealants currently available can address calcium fluoride and calcium sulfate scale they do not address calcium phosphate scale (although newer antisealants will be available in the near future to address this scale). 

The controlling chemical reactions for the lime/limestone wet scrubbing SO2 removal systems have been established. In both the lime and limestone systems, the principal absorption reaction is calcium sulfite plus sulfur dioxide to form calcium bisulfite. Methods of preventing both calcium sulfite and calcium sulfate scaling are presented. 

Although the foregoing methods of scale control are particularly effective with the alkaline scales, there is evidence that calcium sulfate scale can also be controlled by codeposition with the alkaline scales. This is particularly true of the first two methods, and cases might well arise where small amounts of compounds such as sodium or magnesium bicarbonate might be added to waters containing high concentrations of calcium sulfate in order to control deposition of the latter. 

No economical control method is available for calcium sulfate other than ensuring that the concentration-temperature process path is within the solubility confines of the various crystalline modification of calcium sulfate. The limiting top temperature at which seawater mey be evaporated without calcium sulfate scale deposition is of major design significance. 

Calcium sulfate is a sparingly-soluble salt. As with barium and strontium sulfate, the potential to scale with calcium sulfate is high when the ion product exceeds 80% of the solubility constant. Antisealants or sodium softening to remove calcium can be used to control calcium sulfate scale. Calcium phosphate has become a common problem with the increase in treatment of municipal waste-water for reuse. Surface waters can also contain phosphate. Calcium phosphate compounds can also contain hydroxyl, chloride, fluoride, aluminum, and/ or iron. Several calcium phosphate compoimds have low solubility, as shown in Table 7.3. SolubiUty for calcium carbonate and barium sulfate are also shown by comparison. The potential for scaHng RO... 

Chem. Descrip. Phosphate-based high activity multicomponent formulation with acrylate dispersants and proprietary sequestrants Uses Scale control agent, dispersant for alkaline deinking systems in papermaking keeps washer and extractor surfaces clean in pulping and bleaching processes and in heal exchangers and scrubber systems Features Provides protection from scale in the presence of both temp, and pH shock effective on calcium carbonate and calcium sulfate scale Properties Liq. 

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. 

Guggenheim A process for extracting sodium nitrate from caliche, a native sodium nitrate found in Chile. The ore is leached at 40°C with water containing controlled concentrations of magnesium and calcium sulfates. Operated on a large scale in Chile. See also Shanks. 

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. 

During a study of the applicability of "spray" or "fog" evaporation to sea water desalination, it was found that this technique was particularly useful for scale deposition studies. Thus, test conditions are reproducible and heat transfer coefficients are very high, so that the effect of scale formation is readily apparent. Three novel methods for the control of scale deposits on the evaporating surfaces of a spray evaporator were explored. One involves the addition of small quantities of low molecular weight polyacrylic acid to the feed water, which prevents the formation of adherent scale. The methods are applicable under certain conditions to scales formed from sea water containing substantial amounts of calcium sulfate in addition to alkaline scale-forming substances. While spray evaporation appears to be of limited application in water desalination, the scale-control methods developed are probably applicable to other types of evaporator, particularly of the long-tube type. 

The next approach to scale control was reduction of potential calcium carbonate by acidification of makeup water. Sulfuric acid is added to reduce alkalinity and convert most of the calcium bicarbonate to calcium sulfate. Calcium sulfate is soluble up to about 1,700 parts per million in cooling waters at ordinary temperatures, whereas calcium carbonate solubility is less than 30 ppm. So by slightly acidifying makeup water we greatly reduce tendency for calcium carbonate deposition. The calcium sulfate level in concentrated cooling water is controlled by bleed-off adjustment—manually or automatically. 

Earlier the standard industrial approach to prevention of calcium carbonate scaling by addition of sulfuric acid was described. Objectives were to reduce bicarbonate alkalinity, convert calcium carbonate to calcium sulfate, and regulate sulfate concentration by bleedoff. Corrosion inhibitors were added to protect system metals. A new approach to industrial cooling system treatment does not require addition of sulfuric acid. It involves application of phosphonate seques-trants, dispersants and special corrosion inhibitors, and provides deposit control equal to that obtainable when using sulfuric acid. Availability of phosphonate sequestrants makes possible combination scale control and corrosion inhibitors that can be used without the necessity of reducing cooling water alkalinity by acid feed. 

Forced oxidation is not required for scale control with lime systems (Gogineni and Mau-rin, 1975) since sulfate scaling is controlled either by the naturally occurring seed crystals or by co-precipitation of sulfate with sulfite as described in the Basic Chemistry section. Forced oxidation may also be used to oxidize calcium sulfite when alkaline fly ash is the primary source of alkali. The large commercial FGD system at the Colstrip Power Station in Montana, which uses a mixture of fly ash and lime as sorbent, has consistently produced an efflu-... 

Chem. Descrip. Polyacrylates and polymaleic polymers Uses High performance scale and deposit control agents for boiler and cooling water systems provides effective scale control over wide range of pH, temp., and hardness ranges effective with both hard or soft feedwaters provides protection from calcium carbonate and calcium sulfate exc. dispersant for calcium phosphate, iron oxides, and other inorg. solids designed to be used in conjunction with corrosion inhibitors... 

Uses Effective scale Inhibitor for control of calcium carbonate, barium sulfate, and calcium sulfate for oil field prod. 

---