Polyphosphates have long been used as an alternative form of hard water treatment. Polyphosphate is a soluble mineral compound which acts as a sequestering agent when added to water. Sequester- ing is simply the ability or tendency to tie up and hold in solution other minerals such as scale caus- ing calcium and magnesium.
The polyphosphates form bonds with scale causing minerals which make them more water soluble. Therefore, the hard minerals act soft and remain in solution, as opposed to depositing scale. Poly- phosphates also change the morphology (shape) of the scale causing molecules from a cube-like form to a round-like form which does not allow it to cling and prevents it building up on itself.
The quality of the polyphosphate will determine the ability for it to hold scale causing minerals in solution. Higher quality phosphates such as hexametphosphate and Siliphos perform better than orthophosphate which is a lower form of polyphosphate. The higher forms will alter the shape of the scale causing molecules more than orthophosphate will, thus providing better scale prevention. Also as important polyphosphates, when added to water, will form a protective thin glass-like lining on the inner walls of pipes water fed equipment. This protects the pipes or water fed equipment from corrosion due to low pH, high temperature, or dissolved gases in water.
Water containing a high concentration of calcium that is in a condition to precipitate, treated with only 0.5 mg/L of polyphosphate, can be kept from depositing CaCO3 scale. Polyphosphates can also hold iron and manganese in solution (sequestration) in a water environment where they would oth- erwise precipitate, e.g., in the presence of oxygen or chlorine at a high pH.
Polyphosphates also inhibit scale build-up by distorting the used calcite crystal form when CaCO3 precipitates. The scale structure is weak and not as capable to build upon itself. Because polyphos- phates are strongly charged, they adsorb on silt particles and help to keep them from settling be- cause the individual particles repel one another.
Blended phosphates comprise various mixtures of chain polyphosphates and orthophosphate chemi- cals. The blended of formulations provide both the corrosive-inhibiting properties of the orthophos- phate and the sequestering (complexing) ability of the polyphosphate.
Corrosion control phosphates typically utilize a blend of about 60 percent poly- and 40 percent or- thophosphate. Phosphate films are not permanent. They will dissolve and wash away over a period of time. To combat this, it is necessary to feed phosphates into the water more or less continuously.
Initially, phosphate inhibitors must be dosed at rates up to 2 times that required for normal mainte- nance needs. This is because a passivating film must be built up in order to be effective. Once the film is in place, doses of about 0.5 to one mg/L are effective to maintain uniform passivating film plus the amounts needed to accommodate iron, manganese, hardness, and other constituents in the water. For best results, generally feed corrosion control phosphate blends at a ratio of about one to three mg/L and maintain pH at 7.4 to 7.8.
For sequestering, blends of about 80 percent poly- and 20 orthophosphates are applied.
Feed rates are about one mg/L for each mg/L of iron and manganese, plus one mg/L for each ten grains per gallon of total hardness. For sequestering magnesium hardness, potassium-based phos- phate compounds are more effective than sodium based phosphates.
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