Preventing Scale Formation

One of the primary causes of scale formation is mixing two or more waters that are incompatible. The separate waters may be stable, but react to form a precipitate when mixed. 

Mixing water produced from an oil well with water from a lake, river, or source well must be checked. Likewise, mixing a proposed injection water with the natural formation water must 

The tendency for waters to form a precipitate when mixed can be evaluated by calculation, as discussed later, or by mixing the waters in a clear bottle, then setting aside for several days for 

Water may be modified so that a precipitate of scale will not be formed by  

In practice, none of these methods find much use - primarily because of cost. 

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In addition to the formation of scale or corrosion of metal within boilers, auxiliary equipment is also susceptible to similar damage. Attempts to prevent scale formation within a boiler can lead to makeup line deposits if the treatment chemicals are improperly ehosen. Thus, the addition of normal phosphates to an unsoftened feed water ean eause a dangerous eondition by elogging the makeup line with preeipitated calcium phosphate. Deposits in the form of calcium or magnesium stearate deposits, otherwise known as "bathtub ring" can be readily seen, and are caused by the eombination of ealcium or magnesium with negative ions of soap stearates. 

Despite the benefits of blow-down, however, chemical, electrostatic, or electronic treatment of the water is often required to prevent scale formation, corrosion, or biological growth. When treatment is required, or anticipated to be required, the services of a reliable water treatment company should be obtained. ... 

Low-volume waste sources include water treatment processes that prevent scale formation such as clarification, filtration, lime/lime soda softening, ion exchange, reverse osmosis, and evaporation. Also included are drains and spills from floor and yard drains and laboratory streams. 

EDTA, DTPA, DTPMP, and EDTM Pare all strong chelants across all metals, which is why they are so widely used. Phosphonates can also prevent scale formation by inhibiting the crystal growth of calcium carbonate, of which none of the amino-carboxylates and polysuccinates are capable. This strength can also be a weakness, in that they can be less selective for transition metal ions in the presence of hardness... 

Chelants can also aid stain removal by removing metal ions from stains to help lift the stain from the cloth and reduce the color of the stain. Additionally, chelants in combination with builders such as zeolite and phosphate bind with hardness ions to keep these in solution and prevent cloth graying, and in the case of phosphonates, also to prevent scale formation in the washing machine, which can lead to blockages and reduced machine performance. 

Phosphonates are key components for preventing scale formation in washing machines and are also cheaper than EDDS. Therefore, to give formulators more flexibility, 50 50 EDDS and phosphonates blends were investigated in the same two tests. Phosphonates are likely to be restricted to 0.5 wt% phosphorus, so combinations will be needed where higher amounts of chelant are required. 

Direct treatment of boiler water, usually referred to as internal treatment, is used (1) to prevent scale formations caused by hardness constituents, and (2) to provide pH control to prevent corrosion. Treatment that is incorrect or inadequate in either respect can lead to tube failures and result in cosily unscheduled oulages. The permissible limits on contaminants entering the boiler and also on treatment chemicals that can be added to the boiler decrease with rising boiler pressures. 

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. 

The next series of tests was made to prove out the seeding method of scale prevention. The first test was made under the same conditions as for the acid trials. Once the mechanical problems of recycling the seeds in the pilot plant were solved, it was found possible to prevent scale formation completely, if the evaporating liquid contained 0.5% calcium carbonate solids. The solids were made initially by slowly adding soda ash to sea water. 

As mentioned in Chapter 10 on water softening, as long as the concentrations of CaCOs and Mg(OH)2 exceed their solubilities, the solids may continue to precipitate. This condition can cause scale to form, a solid that deposits due to precipitation of ions in solution. To prevent scale formation, the water must be stabilized. A water is said to be stable when it neither dissolves nor deposits precipitates. If the pH is high, stabilization may be accomplished using one of several acids or using CO2, a process called recarbomtion. If the pH is low, stabilization may be accomplished using lime or some other bases. 

Ion exchange is becoming used more extensively in water and wastewater treatment. Ion exchange is primarily used for water softening (Ca + and Mg +) and for water demineralization. For water softening, Ca + and Mg + are replaced with Na to prevent scale formation. For complete water demineralization, all cations and anions are replaced with H+ and OH , respectively. This approach is also used in wastewater treatment. It is important to note that not all dissolved ions are removed equally and/or completely. Ions that are low on the selectivity preference order (described in Section 8.5) may not be completely removed. 

A number of papers have been published on the merits and application of ozone in industrial cooling water systems. Claims made in a patent [Humphrey et al 1979] are that ozone in concentrations of 2 - 20 mg/l prevents scale formation and removes existing scale, passivates metal surfaces to inhibit further corrosion and disinfects the water to prevent algae growth. Edwards [1983, 1987] and Edwards and Sellers [1991] have published valuable reviews of ozone technology and cooling water. As well as biocidal activity the benefits of using ozone include ... 

Sand blasting and high-pressure cleaning can be used to ronove deposits already formed. Few mechanical methods to prevent scale formation exists, other than opening up the system to bleed out the dissolved salts to prevent accumulation above the solubility of the salt. 

Methods are also available that will effectively prevent formation of scale. Through the addition of chelants (EDTA, DTPA, etc.) it is technically possible to prevent scale formation. However, this is not a commercially viable process as chelants only operate on a stoichiometric basis, and the metal ion concentration is far too high for such an approach to be cost-effective. 

THRESHOLD TREATMENT - Chemical treatment, used to prevent scale formation, which acts to hold hardness in solution at the threshold of precipitation. 

Recently, a class of alkaline salts called sequestrants has come into general use for water treatment to prevent scale formation and for periodic removal of both water scale and corrosion products. The most useful examples are derived from an organic acid called ethylenediaminetetraacetic acid (EDTA). The sodium salt dissolves water hardness scale, while the ammonium salt is now being used to remove iron oxides and copper from high pressure steam generators. A related compound is nitrilotriacetic acid (NTA). The sodium salt is used in boiler water treatment. 

In cooling systems operating at a low pH (6.4/7), the total SiO concentration is kept down to 150 mg-r to prevent scale formation. 

Gantrez AN resins have been suggested as a replacement for sodium tripolyphosphate and other phosphates commonly used as detergent builders.The copolymers also function very well as stabilizer additives for heavy-duty liquid detergent formulations and as deflocculants for hard water detergents. As polymeric additives, these resins prevent scale formation or precipitation of alkaline hardness salts from saline waters. If such uses could be fully developed, a substantial market for Gantrez resins could be realized. However, lack of biodegradability and economic considerations would appear to preclude this future. 

The presence of sulfide or dithionate also prevents scale formation during polymerization. Thus, in the system dioctyl peroxydicarbonate-sodium sulfide [172], vinyl chloride with or without vinyl comonomers was polymerized in the presence of 0.1-1000 ppm (based on monomers) inorganic sulfides according to the recipe presented in Table 12 to give PVC with good heat stability, with no scale formation, compared with 450 g m for a similar run without Na2S. ... 

Uses Chelating agent, dispersant for metal ions for use in scouring and soaping synthetics prevents scale formation on equip. 

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