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Boiler deposits are controlled and prevented chemically through one of two approaches, a precipitating program or a solubilizing program. Precipitating programs result in the formation (or precipitation) of sludge instead of scale. The sludge is relatively non-adherent and can be removed from the boiler through bottom blowdown. Phosphate-based treatments are the most common precipitating programs. With this approach, desirable hydroxyapatite and serpentine sludge are formed and removed. Phosphate dosage levels must be carefully monitored and controlled. If not, scales such as calcium silicate, magnesium hydroxide and magnesium phosphate may form instead. Dispersants are sometimes added to the program to condition the sludge and prevent excessive sludge build-up. Solubilizing programmes The other approach, the solubilizing chemical treatment, is not subject to most of the problems associated with precipitating chemical treatment. In a solubilizing program, hardness ions are kept in a soluble form rather than being precipitated to form sludge. Solubilizing programs can be divided into two types. One type utilizes chemicals which react on a one-to-one basis with impurities to keep them solubilized. In other words, one part of chemical is necessary to react with each part of hardness to keep it solubilized. This type of programme relies on precise proportions and is called stoichiometric treatment. Chelants such ad EDTA and NTA are the two most common chemicals used in these types of programs. Chelants are chemicals that tie up dissolved impurities and prevent them from precipitating. For example, calcium ions in the feedwater are bound by the chelant, preventing their reaction with carbonate, sulphate, or silicate ions to form scale. A major disadvantage in the use of chelants is that they require very careful control to be effective. A low residual of free chelant must be maintained in the boiler for two reasons:
On the other hand, keeping this low residual chelant level, makes such programmes highly sensitive to upsets in the feedwater quality. Excessive hardness levels entering the boiler will consume the free chelant residual resulting in the excess hardness precipitating as scale. The other type of solubilizing approach employs polymers, dispersants, and
organic sludge conditioners to prevent scale formation.
These chemicals function by absorbing onto the surface of precipitates, effectively blocking their growth into larger particles, and simultaneously keeping the particles relatively soft and non-adherent. Additionally, these chemicals serve to disperse any scale fragments, minimizing their build-up. Since one polymer molecule can react with more than one molecule of scale, these types of programs are referred to as substoichiometric. Because the chemicals function by distorting the scale structure, precise control in these programmes is not as critical as with the chelant programs. However, gross overfeed should be avoided as this can cause particles to bind and settle out on metal surfaces. |
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