Scale calcium carbonate

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. 

Theoretically, controUed deposition of calcium carbonate scale can provide a film thick enough to protect, yet thin enough to allow adequate heat transfer. However, low temperature areas do not permit the development of sufficient scale for corrosion protection, and excessive scale forms in high temperature areas and interferes with heat transfer. Therefore, this approach is not used for industrial cooling systems. ControUed calcium carbonate deposition has been used successhiUy in some waterworks distribution systems where substantial temperature increases are not encountered. 

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

Calcium—In general, calcium (as CaCOs) below 800 ppm should not result in calcium sulfate scale. In arid climates, however, the critical level may be much lower. For calcium carbonate scaling tendencies, calculate the Langelier Saturation Index or the Ryznar Stability Index. 

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. 

Lucey examined a number of examples of pitting of copper pipes and tanks from hard water districts, and found that there was no more calcium carbonate scale deposited around the pits than on other parts of the metal surface. There was, however, a large amount of CaCOs in the mound... 

With insufficient carbon dioxide of type 3 (and none of type 4) the water will be supersaturated with calcium carbonate and a slight increase in pH (at the local cathodes) will tend to cause its precipitation. If the deposit is continuous and adherent the metal surface may become isolated from the water and hence protected from corrosion. If type 4 carbon dioxide is present there can be no deposition of calcium carbonate and old deposits will be dissolved there cannot therefore be any protection by calcium carbonate scale. 

The deposition of 0.010 in of calcium carbonate scale results in an increase in back-pressure of 6 inches... 

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. 

Provision of pretreatment The initial fill volume and MU supply is almost always pretreated in some manner. Because of the large volume of water in these systems, even low-hardness waters can produce sufficient quantities of calcium carbonate scale to severely impede heat transfer thus, for MTHW pretreatment, the use of ion-exchange softeners is the norm. For HTHW, some form of demineralization such as reverse osmosis (RO) or deionization by cation-anion exchange is typically preferred. 

Consequently, a loss of free carbon dioxide in the water, because of either a rise in temperature (as occurs in a FW heater or boiler) or an increase in pH (all boilers operate at an alkaline pH) leads to a change of equilibrium and the resultant rapid and troublesome precipitation of insoluble calcium carbonate scale on heat transfer surfaces. The reaction is as shown here ... 

Maleate chemistry has proved to be an enduring mainstay of many water treatment formulations, primarily as non-phosphate-containing calcium carbonate scale inhibitors. For most water treatment applications, polymaleic acid and its derivatives offer a good alternative to phosphonate chemistries, when required. 

If hardness breakthrough occurs and goes undetected for any length of time, the treatment reserve is swamped and quickly becomes depleted. This loss of treatment is serious because calcium carbonate scaling can result. The reduction in alkalinity also can permit silicate scaling to occur and prevent adequate maintenance of the magnetite film, which protects the waterside metal surfaces from corrosion. 

D. W. Fong, C. F. Marth, and R. V. Davis. Sulfobetaine-containing polymers and their utility as calcium carbonate scale inhibitors. Patent US6225430, 2001. 

Ryzner, J.W., A new index for determining amount of calcium carbonate scale formed by water, J. Am. Water Works Assoc., 36, 472-486, 1944. 

Dalas E (2001) The effect of ultrasonic field on calcium carbonate scale formation. J Cryst Growth 222(l) 287-292... 

The most common type of troublesome scale is that of amorphous silica and calcium carbonate. Scales of various metallic sulphides is the rule rather than the exception. By far the most abundant sulphide scale consists of iron sulphides. They include pyrite, marcasite, and pyr-rhotite (Kristmannsdottir 1989), but sulphide scale of other metals have also been observed, such as Cu, Pb, and Zn (White et al. 1963 Gallup 1989 Gallup et al. 1995 Hardardottir et al. 2001 Reyes et al. 2002). Sulphide scales are often poorly crystalline and they may be amorphous to X-rays. Moreover, the sulphidebearing scales are known to be enriched in various elements such as Ag, As, Au, Cd, and Mn. Reyes et al. (2002) observed that scales at Rotokawa, New Zealand, also contained elevated concentrations of Hg, Sb, and Se, which were incorporated in pyrite. The quantity of sulphide scale formation is generally very limited and may in fact be beneficial rather than troublesome as the scale forms a stable protective... 

Experience in many parts of the world has shown that calcium carbonate scale formation is only a problem in producing wells, when... 

Evanoff, J., Yeager, V. Spielman, P. 1995. Stimulation and damage removal of calcium carbonate scaling in geothermal wells A case study. In Proceedings World Geothermal Congress, Florence, 18-31 May, 2481-2486. 

In the. first run, acid feed rate was gradually reduced without immediate evidence of scale formation and then was cut off entirely. Heat transfer coefficients started to drop immediately and at the end of the week the tubes were coated with calcium carbonate scale. In this first run, the pH entering the evaporator was 8.3 and leaving it was 8.4. Langelier, Caldwell, and Lawrence have measured the equilibrium pH above which a sea water concentrate is supersaturated with respect to calcium carbonate and magnesium hydroxide (4). Under the conditions of this test, the following conditions were encountered ... 

It was evident that, under conditions of this test, calcium carbonate scale would form if the equilibrium pH were exceeded by 1.5 pH units. 

NOTE Calcium carbonate is in fact sparingly soluble (typically the solubility ofCaCOi is to the extent of 15 to 20 ppm depending on temperature and other factors). If a decrease in cooling water pH occurs and there is a resultant increase in CO2. in excess of the minimum necessary to establish equilibrium, this can, under certain conditions, resolublize some or all of the calcium carbonate scale. 

However, it was polymaleic acid (PMA) and its derivatives, first developed in the late 1960s and early 1970s, that offered a serious challenge to the new supremacy of phosphonates. PMA proved to be a particularly effective DCA for calcium carbonate scale under high-stress conditions, and maleic acid chemistry remains a third basic organic polymer constituent of many modern cooling water treatment formulations. 

AEC is a non-phosphonate calcium carbonate scale inhibitor, designed to be used under alkaline cooling water conditions (pH 7.8 to 9.0). It is halogen-stable, and is presumably positioned against PBTC, with the absence of phosphorus being promoted as an environmental benefit. 

In general, SS/MA and the other calcium phosphate inhibitors are not as good at inhibiting calcium carbonate scale as PMA or HEDP or as good at inhibiting calcium sulfate scale as PMA and 2000 MW PAA. 

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. 

If LSI is greater than 0, the water has the tendency to form calcium carbonate scale. If the LSI is equal to 0, the water is in chemical balance. If the LSI is less than 0, the water is corrosive (refer to Table 3.6). 

For RO applications, a positive LSI or SDSI indicates that the influent water has a tendency to form calcium carbonate scale. In these cases, pretreatment in the form of softening (either with lime or ion exchange), or via the use of antisealants and/or acid is required. 

Adsorption of color onto an RO membrane is favored when the compounds are hydrophobic or positively charged. As with other organics, a high pH (>9) helps to minimize fouling with color, but causes other concerns, including calcium carbonate scaling. 

Calcium carbonate scaling is perhaps the most common type of problem, with the possible exception of microbial fouling, that RO membranes experience. Fortunately, it is fairly easy to detect and handle. Basically, if the ion product (IP) of calcium carbonate in the RO reject is greater than the solubility constant (Ksp) under reject conditions, then calcium carbonate scale will form. If IP < Ksp/ scaling in unlikely. The ion product at any degree of saturation is defined as ... 

A positive LSI means that scaling is favored a negative LSI means that corrosion is favored. It is desirable to keep the LSI near zero (or below) in the RO concentrate to minimize calcium carbonate scaling. This is usually accomplished by feeding acid to lower the pH or feeding an antisealant (see Chapter 8.2.3). Care must be given if sulfuric acid is used to adjust the pH, as this may exacerbate sulfate-based scales, such as calcium sulfate, barium sulfate, and strontium sulfate. 

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