Scale saturation index

KEYWORDS smectite, scale, saturation index, deposits, brine... 

Calcium Carbonate Protective Scale. The LangeHer saturation index (LSI) is a useful tool for predicting the tendency of a water to deposit or dissolve calcium carbonate. Work pubHshed in 1936 deals with the conditions at which a water is in equiHbrium with calcium carbonate. An equation developed by LangeHer makes it possible to predict the tendency of calcium carbonate either to precipitate or to dissolve under varying conditions. The equation expresses the relationship of pH, calcium, total alkalinity, dissolved soHds, and temperature as they relate to the solubiHty of calcium carbonate in waters with a pH of 6.5—9.5 ... 

The LSI measures only the directional tendency or driving force for calcium carbonate to precipitate or dissolve. It caimot be used as a quantitative measure. Two different waters, one of low hardness (corrosive) and the other of high hardness (scale-forrning), can have the same saturation index. 

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. 

Langelier Saturation Index—Ideally, maintain between -0.5 and +0.5 A negative LSI indicates corrosion tendencies. A positive LSI indicates CaCOs scaling tendencies. 

LSI (Langelier Saturation Index) an indication of the corrosive (negative) or scale-forming (positive) tendencies of the water. Hardness the total dissolved calcium and magnesium salts in water. Compounds of these two elements are responsible for most scale deposits. Units are mg/l as CaCOs. 

If the Saturation Index is 0, water is said to be in chemical balance. If the Saturation Index is positive, scale-forming tendencies are indicated. Finally, if the Saturation Index is negative, corrosive tendencies are indicated. 

When the number of concentrations of the circulating water is in the order of 3-7, some of the salts dissolved can exceed their solubility limits and precipitate, causing scale formation in pipes and coolers. The purpose of the treatment of the cooling water is to avoid scale formation. This is achieved by the injection of sulfuric acid to convert Ca and Mg carbonates (carbonate hardness) into more soluble sulfates. The amount of acid used must be limited to maintain some residual alkalinity in the system. If the system pH is reduced to far below 7.0, it would result in an accelerated corrosion within the system. As stated earlier, scale formation and/or corrosion tendency is defined by the Saturation Index (Langelier Index) and Stability Index (Ryznar equation). 

If the Saturation Index is positive (which implies that the Stability Index is less than 6.5), then the water has scale tendency and the addition of sulfuric acid in appropriate quantities would be required to prevent scaling formation. The following example illustrates the estimation of the required amount of acid. 

Over the years, the use and application of several different saturation indices that aid the prediction of scaling and corrosion potential in different waters have evolved to become an integral part of the business of treating and managing cooling water systems. The two most popular are the Lange-lier Saturation Index (LSI) and the Ryznar Stability Index (SI), which were both introduced more than 50 years ago. 

Other indices proposed that have tried to overcome the basic limitations of LSI include the Stiff and Davies Saturation Index, Larson and Buswell Index, Puckorius (Practical) Scale Index, Oddo-Tomson Index, and Larson-Skold Corrosivity Index, some of which are discussed briefly here. 

The Langelier Saturation Index (LSI) is a method for quantifying the scaling or corrosion tendency of water. It was originally applied to cooling water. The LSI is based on the pH and temperature of the water in question as well as the concentrations of TDS, calcium hardness, and alkalinity. 

Langelier Saturation Index (LSI) is used to determine the scaling potential of calcium carbonate. (Note that LSI is used up to about 4,000 ppm TDS higher concentrations rely on the Stiff-Davis Saturation Index.) The LSI is calculated using the following formulas... 

Table 15.2 lists the saturation indexes for the untreated feed water, feed water with 10.2 ppm antisealant, and 4.2 ppm of antisealant plus 3.4 ppm sulfuric acid for pH reduction form 8.1 to 7.5. As the untreated water shows, the major species of concern are the calcium carbonate, barium sulfate, and calcium phosphate. The antisealant does a good job with all but the calcium phosphate. To address this potential scale, acid must be added. This reduces the antisealant demand by 60%. 

I I Water treatment engineers use the Langelier saturation index (LI) and Ryznar stability index (Rl) [7,2] to evaluate the scaling and corroding tendencies of water. Here is a graphical method for determining these pa-... 

The potential for well-bore scale during these CO2 treatments is determined by first predicting how much carbonate mineral dissolution will take place. After a calcite saturation index has been calculated for a water analysis for both pre- and post-C02 treatment, then Fig. 7 can be used to determine how much calcium (and therefore bicarbonate) will have been added to treated waters from calcite dissolution. These calcium and bicarbonate values can be added to the original formation water, which is then used to calculate a new calcite saturation index for the new water. 

Fig. 12. Modelling of Cole Creek water to predict well-bore scale. Upper graph shows modelled calcite saturation index (ST iJ for various modelling conditions. Lower graph shows anticipated calcium concentrations. Solid black is original, pre-C02 calcium concentration. Open circles are predicted calcium concentrations. Diamond is actual post-C02 concentration observed 4 months prior to intense well-bore scale precipitation in this well.

In aqueous systems the dilemma often facing the water chemist is whether or not a particular industrial water is likely to be scale forming or corrosive. The reason for this diffiiculty may be attributed to the presence of CaCO (see Chapter 8). A simple test that may be applied to a water to provide a qualitative assessment of the problem is to add powdered CaCO to the water. If the water is supersaturated in respect of CaCO the addition of the solid particles will cause precipitation from the solution. Under these conditions there is a reduction in the pH of the water. It may be said that the water has a positive saturation index. If however the water is not saturated in respect of CaCO (such waters are corrosive), some of the added solid CaCO will enter the solution thereby increasing the hardness, alkalinity and pH. Water displaying these properties may be said to display a negative saturation index. 

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