Scale control, calcium carbonate

But the major reduction of heat is accomplished by evaporation. As we evaporate and concentrate cooling water, dissolved minerals soon would reach levels at which scale deposits would form, in the absence of corrective measures. Calcium carbonate scale control is a major concern. 

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. 

Fig. 12. Calcium carbonate scaling of a surface condenser due to poor pH control.

The raw water silica is 22mg/l as Si02, and therefore becomes a major constituent of the treated water. Silica scale must now be avoided by raising the boiler water pH and letting silica rather than the TDS control the necessary blowdown. Silica scale not only has a tenth of the heat conductivity of calcium carbonate scale but it is glassy, adherent, and extremely resistant to boilercleaning chemicals. 

Of course, this argument is perfectly true where it can be positively demonstrated that MU water requirements really are very low. Once again however, if this is not the case, then—most treatment programs are designed primarily for corrosion control and do not compensate for undue hardness entering the boiler—calcium carbonate scale can and does develop over time. This process takes place even where the MU water is relatively soft, and results in the formation of insulating boiler tube deposits or boiler vessel sludge. 

Scale control is complex the particular procedure depends on the composition of the feed water. Fortunately, calcium carbonate scale, by far the most common problem, is easily controlled by acidifying the feed or by using an ion exchange water softener to exchange calcium for sodium. Alternatively, an antisealant chemical such as sodium hexametaphosphate can be added. Antisealants interfere with the precipitation of the insoluble salt and maintain the salt in solution even when the solubility limit is exceeded. Polymeric antisealants may also be used, sometimes in combination with a dispersant to break up any floes that occur. 

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). 

Moreover, although calcium carbonate scales can be successfully removed at production wells by acidizing or by using inhibitors, no long-term treatment exists to control silicate-containing precipitation. This is probably one reason that sodium orthosilicate is not frequently used in chemical flooding. Because of the plugging function, sodium silicate is mixed with calcium chloride alternately to improve sweep efficiency. 

Sodium hexametaphosphate is one of a series of chain polymers termed polyphosphates, and threshold treatment using various members of the series has found extensive use in controlling calcium carbonate scale formation. Veale [1984] states that typically 5 mg/l of polyphosphate will inhibit the precipitation of 500 mg/l of CaCOy To control scale in sea water evaporation an additive mixture could include sodium tripolyphosphate, lignin sulphonates and an antifoaming agent. 

A high proportion of hydrated lime is slurried in water and used as a milk or putty. For more details, see chapter 23. Particular attention is drawn to the need for precautions to be taken to control the formation of calcium carbonate scale when pumping milk of lime (sections 23.1.3 and 23.1.4). 

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. 

Acid addition is determined by the LSI or SDSI of RO reject water. To control calcium carbonate scaling by acid addition alone, the LSI or SDSI in the concentrate stream must be negative as indicated in Table 2.6. When an anti-sealant (A/S) is used, the LSI... 

For RO applications, a positive LSI or SDSI indicates that the influent water has a tendency to form calcium carbonate scale. In these cases, pre-treatment in the form of softening (either with lime or ion exchange), or via the use of antisealants and/or acid is required. Note that most membrane manufactures recommend an LSI of +1.8 or lower in the concentrate with antisealant feed to control scaling. 

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... 

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. 

A big concern in swimming pools is prevention of etching and scaling (ie, precipitation of CaCO ) which can be controlled by maintenance of a proper degree of saturation with respect to calcium carbonate. The calcium carbonate dissolution-precipitation equiUbrium is represented by ... 

Most chemical treatment programs have not in the past specifically focused on controlling silica levels in cooling water, and as a consequence almost all analyses of scales and deposits taken from the waterside of cooling systems, especially from heat exchangers, have shown the presence of small percentages of silica. Research over the last five years or so has led to the introduction of silica-specific deposit control polymers and has also led, with some success, to the reevaluation and promotion of some established calcium carbonate polymer products for effective silica control. 

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