Calcite scales

Calcium carbonate (calcite) scale formation in hard water can be prevented by the addition of a small amount of soluble polyphosphate in a process known as threshold treatment. The polyphosphate sorbs to the face of the calcite nuclei and further growth is blocked. Polyphosphates can also inhibit the corrosion of metals by the sorption of the phosphate onto a thin calcite film that deposits onto the metal surface. When the polyphosphate is present, a protective anodic polarization results. 

Carbonate in boilers usually is present as a hard, dense, white to tan or brown calcite scale (CaC03). A tan to brown color usually indicates the presence of iron. Samples of scales and deposits normally fizz when tested with concentrated muriatic acid (hydrochloric acid, HC1) if carbonate is present, although some preliminary heating may be required. 

Application of calcite precipitation rate in predicting the utilization period of calcite scale affected wells, Philippines... 

Dulce, R.G., Nogara, J.B., Sambrano, B.G. 2000. Calcite scale deposition in production wells in Mindanao Geothermal Production Field, Philippines. PNOC-EDC Internal Report. 2000. 

Below, amorphous silica and calcite scale formation is discussed in some detail because scale formation of these minerals is the most common. Space does not permit more detailed handling of scaling and corrosion problems associated with the utilization of geothennal fluids. 

The observed formation of calcite scales in geothermal wells is consistent with calcite saturation calculations (see Fig. 14). The intensity of calcite deposition is largely determined by two factors, the temperature of the water and its salt content (Arnorsson 1978a). In relation to temperature, calcite scale formation is most troublesome around 200 C and it decreases at both higher and lower T. The cause is the temperature variation in the solubility of C02. It is at a... 

Because the decreases with temperature, heating hard water forms a calcite scale, which clogs pipes and water heaters. Find the solubility of calcite in water (a) at 10°C =... 

Measurements of friction factors and heat exchange efficiencies can indicate fouling. While all sorts of deposits can affect flow and heat transfer in an operating industrial system, biofilms are especially effective. A 165-pm thick biofilm shows 100 times the relative roughness of a calcite scale and a thermal conductivity close to that of water, that is, almost 100 times less than carbon steel [9]. 

Calcite scales can be easily dissolved with HCl. In most cases, these scales can be removed, at least temporarily, with a conventional acid treatment. The standard scale-dissolving solution is 15% HCl containing a corrosion inhibitor and an iron control agent. At high temperatures, organic acids, such as acetic acid, have application as well. 

Hardness. The hardness (qv), or related property abrasiveness, is an important filler property. Hardness is determined by comparison to materials of known hardness on the Mohs scale. On this nonlinear scale, diamond is rated 10, quartz 7, calcite 3, and talc 1. The abrasiveness of a filler is also dependent on psd and the presence of impurities, eg, ka olin clay (Mohs hardness of 3) can be quite abrasive because of the presence of quartz impurities. 

Hardness. Most limestone is soft enough to be readily scratched with a knife. Pure calcite is standardized on Mohs scale at 3 aragonite is harder, 3.5—4. Dolomitic limestone is generally harder than high calcium. Dead-burned or sintered limes are 3—4 on this scale, whereas most commercial soft-burned quicklimes are 2—3 (see Hardness). 

The choice of selected raw materials is very wide, but they must provide calcium oxide (lime), iron oxide [1309-37-1/, siHca, and aluminum oxide (alumina). Examples of the calcereous (calcium oxide) sources are calcium carbonate minerals (aragonite [14791-73-2] calcite [13397-26-7] limestone [1317-65-3] or mad), seasheUs, or shale. Examples of argillaceous (siHca and alumina) sources are clays, fly ash, mad, shale, and sand. The iron oxide commonly comes from iron ore, clays, or mill scale. Some raw matedals supply more than one ingredient, and the mixture of raw matedals is a function of their chemical composition, as deterrnined by cost and availabiHty. 

The hardness of a mineral as measured by the Mohs scale is a criterion of its resistance to crushing [Fahrenwald, Trans. Am. In.st. Min. Metall. Pet. Eng., 112, 88 (1934)]. It is a fairly good indication of the abrasive character of the mineral, a factor that determines the wear on the grinding media. Arranged in increasing order or hardness, the Mohs scale is as fohows 1, talc 2, gypsum 3, calcite 4, fluoride 5, apatite 6, feldspar 7, quartz 8, topaz 9, corundum and 10, diamond. 

The most common source is the supersaturation and subsequent scaling of minerals originating in the MU water. Insoluble calcium carbonate in the form of calcite (CaC03) resulting from the thermal decomposition of soluble calcium bicarbonate [Ca(HC03)2] is a classic example. Calcium carbonate quickly forms a white, friable deposit. In addition, the hydrolysis of excess bicarbonate increases... 

In areas of the system where the heat gradient is less severe, calcium carbonate precipitates in both crystalline and amorphous forms. It may precipitate as a calcite or aragonite sludge, but more usually an aragonite scale is produced. Aragonite is hard and adherent, depositing in FW lines and various boiler surface components such as boiler tubes. 

Catalytic devices These nonmagnetic devices use a perforated non-ferrous tube to encourage small calcite seed crystals to form and reduce the risk of bulk water scaling. They are promoted for use in hard waters under conditions where supersaturation can easily occur. 

Additionally, for previously scaled systems, it is claimed that as new calcite seed crystals flow away from scaled sites, a shift in CaC03 equilibrium occurs, resulting in a continuous descaling action. 

All deposits contain various ratios of scale and corrosion products, but often one material predominates, such as calcite or magnetite. These materials have different densities and thermal factors that influence the allowable deposit thickness or weight per unit area before cleaning becomes necessary. Practical allowances usually are between the limitations for each of these two materials. These allowances may be perhaps 50 to 100 mg/cm2 of surface area for lower pressure boilers and 25 to 50 mg/cm2 of surface area for higher pressure boilers. (For a more precise allowance, see the information below.)... 

Figure 10. SEM photographs of polished, etched thin sections of fossil Acropora palmata coral (after Edwards 1988). The scale bar in a is 10 microns, a depicts sample AFS-12, a last interglacial coral from Barbados. The crystal morphology in this well-preserved sample is indistingnishable from that of a modem sample (see Fig. 9b). The scale bar in b is 100 microns, b depicts sample PB-5B, a fossil coral collected from North Point Shelf on Barbados. The crystal morphology of this sample shows clear evidence of alteration, inclnding a large calcite crystal filling in a macroscopic pore (dark area in npper right portion of photograph).

Organic acid fluorescence. In a similar manner to trace constituents, such as Mg, Sr and P, concentrations of organic acids present in speleothem calcite are sufficient to observe variation at temporal scales of less than annual in some cases (e.g.. Baker et al. 1993, Shopov et al. 1994). Organic acids (humic and fulvic) are formed in the soil by humification, and transported to the cave void by percolating waters where they are entrapped in precipitating carbonates. Under certain circumstances, where precipitation patterns are strongly seasonal and the nature of vadose percolation is such that seasonal mixing is incomplete, bands with different luminescent intensities can be differentiated after excitation with UV radiation. In other cases, bands are not observable but secular... 

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