Dicalcium phosphate

Dicalcium phosphate is a common constituent of nitrophosphate fertilizers and of compound fertilizers formed by ammoniation of siperphosphates. There is a relatively small but substantial production of straight dicaldum phosphate in Europe, diich is based on use of byproduct hydrochloric add. The process consists of dissolving phosphate rock in hydrochloric add and then precipitating dicalcium phosphate by stepwise addition 

Various other methods for producing dicalcium phosphate are known, but none are known to be used commercially for fertilizer production. Direct neutralization of pure or defluorinated phosphoric acid with lime or limestone is used to produce feed- or food-grade dicalcium phosphate. 

Monomagnesium, dimagnesium, and trim nesium phosphates are known to be effective fertilizers, but there is no known commercial production of these materials for fertilizer use. No doiibt small percentages of these compounds are formed in processing phosphate rock containing magnesium. 

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Minerals. Supplementation of macrominerals to mminants is sometimes necessary. Calcium and phosphoms are the minerals most often supplemented in mminant diets. One or both may be deficient, and the level of one affects the utilization of the other. Limestone, 36% calcium, is commonly used as a source of supplemental calcium. Dolomite, 22% calcium oyster sheUs, 35% calcium and gypsum, 29% calcium, are sources of calcium. Bone meal, 29% calcium, 14% phosphoms dicalcium phosphate, 25—28% calcium, 18—21% phosphoms and defluorinated rock phosphate, 32% calcium, 18% phosphoms, are sources of both calcium and phosphoms. Diammonium phosphate, 25% phosphoms phosphoric acid, 32% phosphoms sodium phosphate, 22% phosphoms and sodium tripolyphosphate, 31% phosphoms, are additional sources of phosphoms (5). 

In internal or bulk settiag, which is normally carried out at room temperature, the calcium is released uader coatroUed coaditioas from within the system. This method led to the developmeat of stmctured fmits, stmctured pet foods, and a host of cold prepared desserts. Calcium sulfate (usually as the dihydrate) and dicalcium phosphate (calcium hydrogen orthophosphate) are the sources of calcium most commonly used. 

Mineral Feed. Mineral feed supplements for domestic animals and fowl usually contain a pure form of pulverized limestone. In fact, some state laws require the supplement to be at least 35% available calcium. Other sources of calcium are bone meal and dicalcium phosphate. Use as mineral feed has been a steadily growing market for limestone. The material is ground to 90% minus 0.15 mm (100 mesh) or 80% minus 0.9074 mm (200 mesh), is low in silica, and has strict tolerances on arsenic and fluorine (see Feeds and feed additives). 

Miscellaneous. Both whiting and hydrated lime are used as diluents and carriers of pesticides, such as lime—sulfur sprays, Bordeaux, calcium arsenate, etc. The most widely used bleach and sterilizer, high test calcium hypochlorite, is made by interacting lime and chlorine (see Bleaching AGENTS). Calcium and magnesium salts, such as dicalcium phosphate, magnesium chloride, lithium salts, etc, are made directly from calcific and dolomitic lime and limestone. 

Both monocalcium phosphate and dicalcium phosphate dissolve incongmently in water, disproportionating to more basic calcium phosphate and phosphoric acid. The extent of these reactions varies with the temperature and the amount of water. If water is added gradually to anhydrous monocalcium phosphate, equiUbrium conditions first correspond to a mixture of the anhydrous salt and its monohydrate. After conversion to the monohydrate, further reaction affords dicalcium phosphate plus free phosphoric acid. Dicalcium phosphate decomposes in aqueous solution to the more basic hydroxyapatite and phosphoric acid via intermediate octacalcium phosphate. The compHcated stepwise conversion of the acidic mono- and dicalcium phosphates to hydroxyapatite is summarized in equations 6—9. The kinetics are quite complex. 

Because monocalcium phosphate is incongmently soluble, it is typically contaminated with various amounts (6—10%) of dicalcium phosphate and free phosphoric acid resulting from in-process disproportionation of the monocalcium salt. Free phosphoric acid may render the product hygroscopic, and absorbed water plus acid catalyzes further decomposition to additional free acid and dicalcium phosphate. For this reason, industrial monocalcium phosphate may contain some dicalcium phosphate resulting from excess lime addition and then aged to ensure the removal of residual free phosphoric acid. 

Crystalline CaHPO 2H20 loses both water molecules in a single step at moderately elevated temperature or upon storage to yield the anhydrous salt. The presence of free moisture accelerates this dehydration, which results in anhydrous dicalcium phosphate, often as a hard mass. Addition of a few percent of tetrasodium pyrophosphate or trimagnesium phosphate, Mg2(P0 2> stabilizes the dihydrate. The mechanism, however, is not well understood. Nonetheless, these materials are used widely to stabilize CaHPO 2H20, particulady for toothpaste appHcations. 

Tricalcium phosphate, Ca2(P0 2> is formed under high temperatures and is unstable toward reaction with moisture below 100°C. The high temperature mineral whidockite [64418-26-4] although often described as P-tricalcium phosphate, is not pure. Whidockite contains small amounts of iron and magnesium. Commercial tricalcium phosphate prepared by the reaction of phosphoric acid and a hydrated lime slurry consists of amorphous or poody crystalline basic calcium phosphates close to the hydroxyapatite composition and has a Ca/P ratio of approximately 3 2. Because this mole ratio can vary widely (1.3—2.0), free lime, calcium hydroxide, and dicalcium phosphate may be present in variable proportion. The highly insoluble basic calcium phosphates precipitate as fine particles, mosdy less than a few micrometers in diameter. The surface area of precipitated hydroxyapatite is approximately... 

In the human market, oral and parenteral dosage forms are prepared from the crystal. However, because of the extremely high potency, more dilute (0.1—10%) forms are avabable. These include dilutions with mannitol, triturations on dicalcium phosphate or resins, and spray-dried forms. Prices for these forms are driven by that of the crystal, which in early 1996 was ca 9.50/gram (95). Prices for the vitamin have risen during the first half of the 1990s. However, Htde growth in price beyond inflation is anticipated. 

Dicalcium Phosphate Dihydrate (DPD). Dicalcium phosphate cHhydrate is completely nonreactive at room temperature. At 65—71°C and in the presence of water, it dehydrates and decomposes into hydroxyapatite and acidic monocalcium phosphate, or a free phosphoric acid (18). It is used to some extent in cake mixes in combination with faster acting acid. Its primary function is to provide acidity late in the baking cycle and thus produce a neutral and palatable product. DPD has an NV of 33. It provides sufficient acidity only in products requiring long baking times. 

Polymeric Calcium Phosphate Cements. Aqueous solutions of polymers such as poly(acryHc acid), poly(vinyl alcohol), gelatin, etc, and/or autopolymerizable monomer systems, eg, 2-hydroxyethyl methacrylate, glycerol dimethacrylate, calcium dimethacrylate, etc, have been used as Hquid vehicles (41,42,76) for the self-setting calcium phosphate cement derived from tetracalcium phosphate and dicalcium phosphate [7757-93-9J. 

An abrasive is usually chemically inert, neither interacting with other dentifrice ingredients nor dissolving in the paste or the mouth. Substances used as dentifrice abrasives include amorphous hydrated silica, dicalcium phosphate dihydrate [7789-77-7] anhydrous dicalcium phosphate [7757-93-9] insoluble sodium metaphosphate [10361-03-2], calcium pyrophosphate [35405-51-7], a-alumina trihydrate, and calcium carbonate [471-34-1]. These materials are usually synthesized to specifications for purity, particle size, and other characteristics naturally occurring minerals are used infrequently. Sodium bicarbonate [144-55-8] and sodium chloride [7647-14-5] have also been employed as dentifrice abrasives. 

FIG. 20-70 The influence of moisture as a percentage of sample saturation S on granule deformabihty. Here, deformation strain (AL/L) is measured as a function of applied stress, with the peak stress and strain denoted by tensile strength and critical strain (AL/L) of the material. Dicalcium phosphate with a 15 wt % binding solution of PVP/PVA Kolhdon VAG4. [Holm et al., Powder Tech., 43, 213 (1.9S.5J,] With land permission from Elsevier Science SA, Lausanne, Switzerland. 

Finally, the reason for not forcing dicalcium phosphate (a very valuable formulation aid in direct compression) beyond 50°C is that at higher temperatures (actually [57] above 70° C) it converts to the anhydrate, a conversion that is, curiously enough, catalyzed by water. In other words, the dihydrate will be autocatalytic in this respect at elevated temperatures, and it should not be ruled out based on high-temperature findings. 

Fig. 11 Effect of amylose on the hardness and disintegration of dicalcium phosphate tablets. (From Ref. 62.)...

In a later study [133] the effect of disintegrants on hydrochlorothiazide dissolution from both soluble (anhydrous lactose) and insoluble" (dicalcium phosphate) fillers was compared for different lubricant levels and tamping forces (instrumented Zanasi LZ-64 machine). Statistical analysis of this multivariable study revealed all main factors and their interactions to... 

Fig. 19 Effect of disintegrants on hydrochlorothiazide dissolution from hard gelatin capsules (filler, dicalcium phosphate lubricant, 1% magnesium stearate). (From Ref. 132.)...

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