Calcium phosphate scale deposition

Calcium phosphate is equally as troublesome as calcium carbonate by forming scales that reduce heat-transfer efficiencies and sludges that foul the system. In addition, calcium phosphate scale is intractable and once formed, deposits can be extremely difficult to remove from dirty cooling systems, so prevention is vital. 

Calcium ions are fixed into the biofilm by the attraction of carboxy-late functional groups on the polysaccharides. In fact, divalent cations, such as calcium and magnesium, are integral in the formation of gels in some extracellular polysaccharides. A familiar biofilm-induced mineral deposit is the calcium phosphate scale that the dental hygienist removes from teeth. When biofilms grow on tooth surfaces, they are referred to as plaques. If these plaques are not continually removed, they will accumulate calcium salts, mainly calcium phosphate, and form tartar (scale). 

In addition to the requirement to conform to steam purity needs, there are concerns that the boiler water not corrode the boiler tubes nor produce deposits, known as scale, on these tubes. Three important components of boiler tube scale are iron oxides, copper oxides, and calcium salts, particularly calcium carbonate [471-34-1]. Calcium carbonate in the feedwater tends to produce a hard, tenacious deposit. Sodium phosphate is often added to the water of recirculating boilers to change the precipitate from calcium carbonate to calcium phosphate (see also Water, industrial water treatment). 

Phospha.te Treatment. Calcium phosphate is virtually insoluble in boiler water. Even small levels of phosphate can be maintained to ensure the precipitation of calcium phosphate in the bulk boiler water, away from heating surfaces. Therefore, the introduction of phosphate treatment eliminates the formation of calcium carbonate scale on tube surfaces. When calcium phosphate is formed in boiler water of sufficient alkalinity, a particle with a relatively nonadherent surface charge is produced. This does not prevent the development of deposit accumulations over time, but the deposits can be controlled reasonably well by blowdown. 

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. 

Scale formation Controlled scale deposition by the Langelier approach or by the proper use of polyphosphates or silicates is a useful method of corrosion control, but uncontrolled scale deposition is a disadvantage as it will screen the metal surfaces from contact with the inhibitor, lead to loss of inhibitor by its incorporation into the scale and also reduce heat transfer in cooling systems. Apart from scale formation arising from constituents naturally present in waters, scaling can also occur by reaction of inhibitors with these constituents. Notable examples are the deposition of excess amounts of phosphates and silicates by reaction with calcium ions. The problem can be largely overcome by suitable pH control and also by the additional use of scale-controlling chemicals. 

In particular, where polyphosphate is added either to the MU waterline (say, as a stabilizer against the risks of after-precipitation) or to the FW line or FW tank (as a precipitating treatment for residual hardness), there is some risk of FW line phosphate deposits developing. Such deposits are likely to be primarily composed of hard, intractable calcium phosphate [tricalcium phosphate Ca3(P04)2] scale, but they may include magnesium phosphate [Mg3(P04)2] and other insoluble phosphates and hydroxides. The risk of precipitation and subsequent deposition is increased where the pH is below 8.3, if the FW line is particularly long, or when the FW temperature is high. 

Scales from individual salts vary in their heat-transfer insulating effects, but most adherent deposits found on heat-transfer surfaces consist of one or two predominant salts (such as calcium carbonate or calcium phosphate) intimately combined with a variety of lesser amounts and types of scales, together with some corrosion debris and other fouling matter. 

PCA 16 is particularly effective for control of calcium sulfate deposition (and thus finds application, under different brand names and grades, as a calcium carbonate/sulfate scale inhibitor for RO systems treating brackish waters). This inhibitor is also useful for calcium phosphate control. It is stable against chlorine. 

The development of bisphosphonates for clinical purposes began with the discovery that inorganic pyrophosphate is present in blood and urine and inhibits the precipitation of calcium and phosphate (1). Derivatives of pyrophosphate had been widely used for industrial purposes, because they inhibit the precipitation of calcium carbonate. Their principal use was as antiscaling additives in washing powders, water, and oil brines, to prevent deposition of calcium carbonate scale. It was then found that pyrophosphate binds strongly to calcium phosphate, prevents both the formation and dissolution of calcium phosphate crystals, and inhibits calcification in vitro. The bisphosphonates are used to treat bone diseases characterized by increased osteoclastic bone resorption (2). Long-term administration of low doses of oral bisphosphonates is considered to be valuable in patients with postmenopausal osteoporosis (3,4). 

Acumen [Rohm Haas] PolytK rylic acids or their sodium salts dispersant, thickener, anti-scale deposition agent for NR, SR latexes, for water treatment calcium phosphate stabilizer. 

Sodium, potassium and calcium salts of mono and polyphosphoric acids Inhibition of scale deposits 50 mmol/m corresponding to 4.7 mg/1 calculated as phosphate (P0 ) 50 mmol/m corresponding to 4.7 mg/1 calculated as phosphate (PO -) Maximum value for potassium 300 mmol/m corresponding to 12 mg/1 in treated drinking water ... 

There are several types of scales that deposit on the hot surfaces in a boiler. The most common is calcium carbonate, which is formed from the hardness of water. Scale is also formed from the deposition of insoluble salts such as calcium sulfate, calcium phosphate, or insoluble silicates. The relation used to calculate the influence of scale on the temperature drop across a boiler tube is... 

SEM micrographs of the electrochemically deposited calcium phosphate coating (a) microporous structure, (b) nanometre-scale crystal grains (Hu etaL, 2007). 

Uses Corrosion inhibitor and deposit control agent in cooling water treatment preventing the development and accumulation of calcium carbonate, sulfate and phosphate scales... 

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