Thermochemistry and Kinetics of the Reactions of Apatite Phosphates with Acid Solutions

Abstract

The main component of phosphate ore derive from fluorpapatite, Ca10(PO4)6F2, by a faint substitution of calcium by magnesium and sodium, phosphate ions by carbonate and fluorides by the latter and hydroxyl ions. Mineral component of calcified tissues (bone, dentine and enamel) derives from hydroxyapatie, Ca10(PO4)6(OH)2. Thermochemistry of these synthetic compounds and similar products began around the middle of last century (Gottshall, 1958). During the next 25 year period, few works have been performed in order to determine thermochemical quantities of these compounds, but a lot of work has been undertaken in the beginning of 70s on the synthesis and characterization of such products. Reactions of acid solutions on natural phosphates have also been widely studied, this is because these reactions lead to phosphoric acid which is the basic intermediate for phosphate fertilizer production. Fertilizers such as Mono-Ammonium Phosphate, (MAP), NH4(H2PO4), Di-Ammonium Phosphate (DAP), (NH4)2HPO4, are produced by the reaction of ammonia on phosphoric acid solution. The slurry is then granulated at high temperature leading to the fertilizer product. Triple Super Phosphate (TSP) results from the reaction of phosphoric acid on the phosphate ore. High purity (food grade) phosphoric acid is usually produced in two steps. The first one consists in reducing apatite by carbon in presence of silica at a temperature up to 1500°C, to produce native phosphorus which is then oxidized into P4O10. The latter reacts with water to produce H3PO4. This process is called ‘dry’ process in opposition to the ‘wet’ one in which the phosphoric acid results from the reaction of phosphate with sulfuric acid or with a mixture of the latter with diluted phosphoric acid solution. Wet process produces an acid solution containing large amounts of impurities, some of them coming from the phosphate ore and the other from reacting acid. Depending on the reaction temperature and the concentration of the acid, the wet process produces also a by-product which can be calcium sulphate dihydrate (gypsum), CaSO4,2H2O, or hemihydrate, CaSO4,0.5H2O, or anhydrous Ca-sulfate, CaSO4, (Becker, 1989). Because of the exothermicity of the reactions occurring in this process, a large amount of thermal energy is generated allowing to heat the reaction vessel at temperature up to 120°C. Due to low cost and simplicity of acid production by wet process, fertilizers are manufactured using phosphoric acid resulting from this process. This paper deals with the determination of the formation enthalpies of a series of apatites, the general formula of which is M10(PO4)6Y2 in which M could be Ca, Sr, Ba, Cd or Pb or a