Calcium phosphate, 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). 

Antisealants are usually fed alone for most applications. Acid feed is sometime used in conjunction with an antisealant to control LSI for calcium carbonate scale and to control calcium phosphate and calcium fluoride scales. Antisealants currently on the market are not generally effective at controlling calcium phosphate scale and have difficulty controlling calcium carbonate scale when the LSI is greater than about +2, depending... 

Phospha.te Treatment. Calcium phosphate is virtually insoluble in boiler water. Even small levels of phosphate can be maintained to ensure the precipitation of calcium phosphate in the bulk boiler water, away from heating surfaces. Therefore, the introduction of phosphate treatment eliminates the formation of calcium carbonate scale on tube surfaces. When calcium phosphate is formed in boiler water of sufficient alkalinity, a particle with a relatively nonadherent surface charge is produced. This does not prevent the development of deposit accumulations over time, but the deposits can be controlled reasonably well by blowdown. 

The most commonly used scale inhibitors are low molecular weight acrylate polymers and organophosphoms compounds (phosphonates). Both classes of materials function as threshold inhibitors however, the polymeric materials are more effective dispersants. Selection of a scale control agent depends on the precipitating species and its degree of supersaturation. The most effective scale control programs use both a precipitation inhibitor and a dispersant. In some cases this can be achieved with a single component (eg, polymers used to inhibit calcium phosphate at near neutral pH). 

Scale formation Controlled scale deposition by the Langelier approach or by the proper use of polyphosphates or silicates is a useful method of corrosion control, but uncontrolled scale deposition is a disadvantage as it will screen the metal surfaces from contact with the inhibitor, lead to loss of inhibitor by its incorporation into the scale and also reduce heat transfer in cooling systems. Apart from scale formation arising from constituents naturally present in waters, scaling can also occur by reaction of inhibitors with these constituents. Notable examples are the deposition of excess amounts of phosphates and silicates by reaction with calcium ions. The problem can be largely overcome by suitable pH control and also by the additional use of scale-controlling chemicals. 

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 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 contain hydroxyl, chloride, fluoride, aluminum, and/ or iron. Several calcium phosphate compounds have low solubility, as shown in Table 7.2. Solubility for calcium carbonate and barium sulfate are also shown by comparison. The potential for scaling RO membranes with the calcium phosphate compounds listed in Table 7.2 is high and will occur when the ion product exceeds the solubility constant. This can occur at orthophosphate concentrations as low as 0.5 ppm. Sodium softening or antisealants together with low pH help to control phosphate-based scaling. 

Fig. 17 Schematic representation of ion-channel controlled precipitation of calcium phosphate in block copolymer vesicles (middle drawing), and TEM micrographs right drawing) of phosphate-loaded PMOXA-PDMS-PMOXA triblock copolymer giant vesicles after 1 and 24 h of incubation with CaCl2 solution in the presence of the ionophore scale bar. 500 nm [163]. Reproduced with permission of The Royal Society of Chemistry...

In the case of calcium carbonate scale, indices are typically calculated at the highest expected temperature and highest expected pH—the conditions under which calcium carbonate is least soluble. In the case of silica, the opposite conditions are used. Amorphous silica has its lowest solubility at the lowest temperature and lowest pH encountered. Indices calculated under these conditions would be acceptable in many cases. Unfortunately, cooling systems are not static. The foulants silica and tricalcium phosphate are used as examples to demonstrate the use of operating range profiles in developing an in-depth evaluation of scale potential and the impact of loss of control. 

Uses Corrosion inhibitor and deposit control agent in cooling water treatment preventing the development and accumulation of calcium carbonate, sulfate and phosphate scales... 

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. 

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 Corrosion inhibitor designed to control deposition of calcium scales in once-through and recirculating cooling water systems controls precipitation of calcium phosphate dispersant for silt and metal oxides... 

Calcium, magnesium, potassium, phosphates (phosphorus), silica, and carbon dioxide may be present in water, and their presence generally does not necessitate control. Calcium and magnesium are contributors to the hardness of water. The control of their levels in water may be required mainly as a preventive measure for scale formation especially on heated surfaces or for taste appeal of the water. 

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