Carbon-12 scale

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. 

Scale Inhibitors. When scaling conditions exist, scale inhibitors can be used to control the scaling tendencies, and keep metal surfaces free of scale deposits. Scale inhibitors are chemicals that interrupt and deform the normal crystalline growth pattern of carbonate scales. The three most commonly used classes of scale-inhibiting chemicals used in drilling fluid are [191,197] ... 

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. 

Carbon dioxide affects the acidity of the water and, as already noted, influences the formation of protective carbonate scales. 

Those waters in which the carbon dioxide content is in excess of that required as bicarbonate ion to balance the bases present are among the most aggressive of the fresh waters. Hard waters usually, though not invariably, deposit a carbonate scale and are generally not appreciably corrosive to cast iron, corrosion rates of less than 0-02 mm/y being frequently encountered. Water-softening processes do not increase the corrosivity of the water provided that the process does not result in the development of an excess of dissolved carbon dioxide. 

Solders are anodic to copper, but soldered joints in copper pipes are widely used without trouble for cold supply waters possibly corrosion is restricted by the deposition of cathodic carbonate scales and the formation of insoluble lead compounds. Hot supply waters tend to be more aggressive and, where these are involved, it is wise to tin any copper which has a soldered joint. Electrolytes of high conductivity such as sea-water will also attack soldered joints in copper. 

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

Yet a further problem concerning excessive water loss is the increased risk of carbonate scale deposition. It is the usual case to propose that, because heating systems are closed loops with minimal losses, many operators believe that they do not require sophisticated chemical treatment programs, injection-feed methods, or monitoring and control processes. To further this view comes the added philosophy that, irrespective of hardness content, the MU water supply to these systems does not require any pretreatment such as ion-exchange softening. 

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. 

Hardness breakthrough with ion-exchange (base exchange, BX) softening NOTE Caused by Fe/Mn fouling, resin breakdown/loss, or inadequate regeneration. Increased risk of carbonate scale or phosphate sludge Loss of alkalinity and hence an increased silica deposition risk... 

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 ... 

Where a softener has not been provided, a higher volume of sludge can be expected in the BW. Typically, the calcium precipitates either as insoluble carbonate scale or as phosphate sludge except where a chelant-based chemical treatment program is employed (which is unlikely in smaller boiler plants) or when an all-polymer program is employed. 

Under hot BW conditions this reaction is reversible, leading to a serious risk of carbonate scale depositing on heat transfer surfaces. Consequently, many large water utilities and industries around the world continue to use the old established, but effective lime (calcium hydroxide) and soda ash (sodium carbonate) processes to soften water by precipitating out insoluble hardness salts. 

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. 

Carbon/ scale Batch Conversion (%) TOF (h ) Selectivity to glycolic acid (%)... 

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

G. M. Graham, S. J. Dyer, and P. Shone. Potential application of amine methylene phosphonate based inhibitor species in HP/HT (high pres-sure/high temperature) environments for improved carbonate scale... 

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. 

Where open-loop systems are unavoidable, mine water should be circulated through heat pumps and exchangers in such a way as to minimize contact with atmospheric Oz, which promotes oxidation of Fe2+ to poorly soluble Fe3+. Also consideration should be given to pressurized systems, in which down-mine pressures are maintained as far as possible, to hinder degassing of, for example, C02. Such degassing may elevate pH and promote precipitation of Fe-oxyhydroxides or carbonate scales. 

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