IMPROVED MILBOND-TX® UNDERSTANDING FEED-GRADE CALCIUM PHOSPHATE SOURCES
“A BRIEF GENERAL OVERVIEW ABOUT THEIR PRODUCTION”
In the last issue of Milwhite’s Journal the topic selected for discussion addressed and clarified a major misconception that exists about the feed-grade phosphates, mono- di- and tricalcium phosphate. The reason this topic was selected was because even though each year animal feed-grade phosphates account for only about 5% of the world’s total phosphate consumption, monocalcium phosphate (MCP) and dicalcium phosphate (DCP) accounted for over 90% of the world’s feed-grade phosphate usage. Also, there seems to be a void of meaningful discussions regarding the basic chemistry of these phosphate sources as well as the manufacturing processes involved in their production. During the past few decades, meaningful discussions on this topic have been very limited in college and university classrooms as well as in the many nutrition conferences and symposia sponsored each year within the animal feed industry. Therefore, along with the information presented in the previous issue of Milwhite’s Journal, this issue presents its readers with a brief general overview of the manufacturing process involved in production of these high quality feed-grade phosphorus sources in order to promote a better appreciation for their use by the world-wide animal feed industry.
MONOCALCIUM AND DICALCIUM PHOSPHATE
As pointed out and clearly explained in the previous issue of Milwhite’s Journal, the prefixes “mono” and “di” in front of calcium phosphate are often erroneously interpreted to mean one and two atoms of calcium in the products commonly referred to in the animal feed industry as MONOCAL (MCP) and DICAL (DCP). The reader should refer to the previous issue of this journal for a complete explanation and understanding of the definition of MCP and DCP. In reactions I and II below, the mineral involved in the production of the phosphate salt is calcium from calcium carbonate (limestone) and depending on the final product will displace either one or two hydrogens from phosphoric acid. When dibasic calcium phosphate is formed, in reaction I, two hydrogens are displaced from a molecule of H3PO4 and the phosphate species has a valence of “─ 2” (i.e., HPO4=). In reaction II only one hydrogen is displaced from each of two phosphoric acid molecules in the formation of monobasic calcium phosphate and the phosphate species has a valence of “─ 1” (i.e., H2PO4─ ).
During the manufacture of feed-grade calcium phosphates, the limestone (CaCO3) and phosphoric acid (H3PO4) react together under carefully controlled industrial conditions. When these two ingredients are mixed together, a chemical equilibrium is reached and the final product is a mixture containing monobasic calcium phosphate (MCP) and dibasic calcium phosphate (DCP). When the reaction is complete the final mixture in the reaction vessel is removed, dried, ground and screened in order to assure a specific particle size for the feed industry.
It is sometimes confusing when the phosphoric acid used in the production of feed-grade phosphate sources is often referred to by manufacturers as “orthophosphoric acid”. When this is done it is simply another way of referring to phosphoric acid. They are exactly the same compound with the same chemical formula. The “ortho” is simply referring to the fact that none of the hydrogens have been displaced (removed) from phosphoric acid (H3PO4). When the hydrogens are displaced to form phosphate compounds such as the salts of phosphoric acid there can be many phosphate salts produced and these may also be referred to as orthophosphates (e.g., sodium phosphates or sodium orthophosphates, potassium phosphates or potassium orthophosphates, calcium phosphates or calcium orthophosphates, etc.).
The quantity of each form of phosphorus (i.e., MCP and DCP) present in the final mixture can be predetermined by the manufacturer by controlling the amount of limestone used to react with phosphoric acid. This allows for the production of MCP and DCP which is familiar to the animal feed industry. The more limestone used in the reaction the more DCP will be formed and decreasing the amount of limestone will result in more of the MCP form. Normally, companies that manufacture feed-grade phosphates have high quality control standards and the guaranteed phosphorus and calcium contents of their products are very consistent from one batch to the next.
Reaction I (In this reaction Dibasic Calcium Phosphate (DCP) is formed)
CaCO3 (Limestone) + H3PO4 → CaHPO4 + 2H2O + CO2
Reaction II (In this reaction Monobasic Calcium Phosphate (MCP) is formed)
CaCO3 (Limestone) + 2H3PO4 → Ca(H2PO4 )2 ∙ H2O + CO2
In reaction I, the end product, CaHPO4 contains Ca bound to the P in the dibasic (HPO4= ) form and in reaction II the end product is Ca(H2PO4)2 with one water of hydration and Ca is binding to two P in the monobasic (H2PO4─ ) form.
Note: In each reaction above the phosphate compound that was produced contains only one calcium atom. Although the manufacturers of these feed-grade phosphorus products control the production variables in the reaction vessel, due to reaction kinetics there is not complete conversion to only one product (i.e., all MCP or all DCP). Therefore, because of this, in the production of MCP there exists some DCP in the final mixture. Similarly, in the production of DCP there exists some MCP.
The actual composition of the final feed-grade phosphate source (i.e., the total amount of MCP and DCP) is influenced by variables such as the ratio of limestone to phosphoric acid, concentration of phosphoric acid, temperature and the purity of the raw materials. However, the total concentration of phosphorus in the final feed-grade phosphate product will always be determined by the initial concentration of H3PO4 used in the manufacturing process.
As discussed previously, when purchasing MCP or DCP the buyer knows the minimum amount of phosphorus in the product, but not the exact form of phosphorus (i.e., (MCP or DCP). This is because the exact amount of each form in which the phosphorus exists is subject to some variability dependent on the degree of quality control implemented by the manufacturer. For instance, in a feed-grade phosphorus product such as DCP containing a guaranteed minimum phosphorus concentration of 18.5%, it is possible for the product to contain 20 to 50% of its phosphorus in the monobasic (H2PO4─ ) form and 80 to 50% of its phosphorus in the dibasic (HPO4= ) form. Also, for a product that is guaranteed to contain 21% phosphorus, such as MCP, there may be from 60 to 90% of the phosphorus in the H2PO4─ form and 40 to 10% in the HPO4= form. In either case, regardless of what form of phosphorus predominates in the final mixture of the feed-grade product, the total percentage of the two forms (MCP and DCP) must always equal 100%.
A molecule of tricalcium phosphate (TCP) is totally different from MCP and DCP because it has all three hydrogens displaced from phosphoric acid [i.e., Ca3(PO4)2 ]. This feed-grade product, which is considered a calcium orthophosphate, should actually be referred to as tribasic calcium phosphate. However, as with MCP and DCP, it is not common for someone to use the word “basic” when discussing TCP even though it is more “chemically” correct to do so. Another name which is commonly used when referring to TCP is “defluorinated phosphate” and this product must contain a phosphorus to fluorine ratio of, at least, 100:1 in order to be legally classified as being “defluorinated”. A majority of the TCP produced in the United States as well as other countries is used in the diet of broilers because of several additional benefits associated with its use in the diet other than its content of phosphorus and calcium. These “bonus” benefits which are associated with TCP will be the topic of discussion in the next issue of Milwhite’s Journal.
The majority of the TCP available to the animal feed industry is manufactured by mixing raw rock phosphate with phosphoric acid and soda ash (sodium carbonate, Na2CO3). This mixture is heated in a kiln at a very high temperature (~ 1300 to 1500 ºC). Raw rock phosphate is mined directly from the earth and may contain 13 to 14% phosphorus and 3.5 to 4.0% fluorine. Since raw rock phosphate contains such high concentrations of fluorine it is not suitable for direct use by the animal feed industry. The fluorine is driven off by the high temperature and the resulting product that contains an acceptable fluorine concentration is cooled, ground, screened and bagged. Normally, TCP contains approximately 17% phosphorus and because sodium carbonate is used in the manufacturing process TCP also contains sodium that nutritionists consider when they are formulating feeds.
The information presented in this issue of Milwhite’s Journal was compiled by Dr. Orlando Osuna, Director of Health Science at Milwhite, Inc. and Dr. Richard Miles, Professor Emeritus, University of Florida, Gainesville, FL, USA.