A method of preparing particulate calcium phosphate animal feed minerals and useful carbon dioxide by coreacting calcareous proteinacious by-products recovered from poultry hatcheries with phosphoric acid. Aqueous hatchery by-product containing protein and calcium in a weight ratio between 0.25 and 1.25 is comminuted until the contained dry matter particles exhibit diameters less than 1 millimeter and are reactive with phosphoric acid. The calcium in the comminuted hatchery by-product is coreacted with phosphoric acid using between one and two mols of P per mol of Ca until carbon dioxide formation ceases and the reacted aqueous hatchery by-product exhibits a pH between 2 and 4. Dry recycled calcium phosphate is admixed with the aqueous hatchery by-product to form damp solids containing between 7 and 18 percent water. The damp solids are rolled by means of a granulator until spheroid granules containing calcium phosphate homogeneously mixed with protein are formed. The calcium phosphate granules are dried by commercial means until the protein hardens and forms a matrix bonding the calcium phosphate into attrition resistant granules of animal feed mineral containing between 5 and 20 percent protein, between 12 and 18 percent calcium, and between 15 and 24 percent phosphorus. The method allows the use of economical wet process phosphoric acid containing low fluoride contents. The chemical intermediate carbon dioxide may be effectively used to produce animal feed mineral potassium carbonate by recovering the carbon dioxide by reacting with aqueous potassium hydroxide.