76 Journal of Applied Solution Chemistry and Modeling, 2016, 5, 76-81 E-ISSN: 1929-5030/16 © 2016 Lifescience Global The Ternary System H 3 PO 4 – Ca(OH) 2 –H 2 O Isotherms at 15 and 45 °C K. Labgairi * , A. Jourani and M. Kaddami Laboratory “Physical Chemistry of Processes and Materials”, Faculty of Sciences and Technology, University Hassan 1, BP 577, 2600 Settat, Morocco Abstract: The solid–liquid phase equilibria of the system H3PO4– Ca(OH)2–H2O were studied under air pressure using a synthetic method based on conductivity measurements. Two isotherms at 15 and 45 °C were completely defined. The stable solid phases which appear in this range of temperature are hydroxyapatite (Ca10(PO4)6(OH)2), calcium hydrogen phosphate dihydrate (CaHPO4.2H2O), calcium hydrogen phosphate anhydrous (CaHPO4) and calcium dihydrogen phosphate monohydrate (Ca(H2PO4)2.H2O). These solid phases were characterized using X ray diffraction. Keywords: Solid-liquid phase equilibria, ternary system H 3 PO 4 – Ca(OH) 2 –H 2 O, calcium phosphate, isotherm. 1. INTRODUCTION The operation of purification is a major component in the process of extraction of the sugar from the beet or from the sugar cane. Phosphatation which consists of adding the phosphoric acid or a soluble phosphate to the raw juice of cane establishes a precious complement in the operation of clarification. Indeed, the precipitate of calcium phosphate formed by reaction of the phosphoric acid or a soluble phosphate with milk of lime, has the role to adsorb impurities, in particular the coloring impurities present in the raw juice and to agglomerate the floculat formed. However, the phases of calcium phosphates which precipitate remain little mastered. With the aim of mastering the mechanisms of these reactions, and following the nature of calcium phosphates and of sugar-calcium compound, we began the study of the quaternary system: water - Phosphoric acid - lime - sugar. To reach this objective, we approached, in the laboratory, the study of the phase equilibria of the various sub-systems of this quaternary. In a preceding work we established the isotherm 30 °C of the system H 3 PO 4 –Ca(OH) 2 –H 2 O [1]. In the present work, we established isotherms 15 and 45 °C of this system, by basing us on the conductivity measurement. These two temperatures were chosen because they frame the ambient temperature in a sugar refinery which is 30 °C and which corresponds to the temperature of preliming. The characterization of the stable solid phases of these isotherms was driven by the method of the sets and the rests and by the X ray diffraction. *Address correspondence to this author at the Laboratory “Physical Chemistry of Processes and Materials”, Faculty of Sciences and Technology, University Hassan 1, BP 577, 2600 Settat, Morocco; Tel: (212) 0528930812; E-mail: labgairi@yahoo.fr 2. BIBLIOGRAPHIC STUDY Because of the importance of calcium phosphates in diverse domains, several searches are made on the solubilization, the precipitation and the synthesis of these products for the temperature 25 °C. In this context, we can quote the works of K. Cameron and A. Seidll [2], Frank K. Cameron and Jannes M. Bell [3] and A. Jourani [4]. On the other hand, few works which handled the subject by the use of the phase diagrams, we quote the work of Bassett (1958), quoted by Roger I. Martin [5] who published a diagram of system H 3 PO 4 - Ca(OH) 2 -H 2 O at the temperature 25°C, so Van Wazer who compiled this diagram in some temperature [6]. Another study concerns the establishment of the invariant points in the same system was more made recently (1977), by Roger I. Martin and Brown [5]. This study was dedicated to the establishment of the invariant metastable points containing combinations of solids Ca(H 2 PO 4 ) 2 , Ca(H 2 PO 4 ) 2 .H 2 O, CaHPO 4 .2H 2 O and CaHPO 4 . 3. EXPERIMENTAL METHODS The curves of solubility are determined by the synthetic method based on conductivity measurements [7, 8]. It consists of adding step by step a solvent to a saline mixture of known composition. Conductivity of the solution is measured at thermodynamic equilibrium and the evolution, according to the quantity of the added liquid, is represented. The obtained curves present angular points and plateaux that correspond to phase transformations. The analysis of series of mixtures permits to identify the liquids curves and to determine the compositions of the isothermal invariant solutions and solid phases (anhydrous or hydrated). Calcium phosphates were identified by using the method of wet residues [9, 10].