International Journal of Materials Engineering 2014, 4(6): 185-191 DOI: 10.5923/j.ijme.20140406.01 Purification of an Industrial Fertilizer (Diammonium Phosphate) Using Design of Experiments Manel Gargouri 1,* , Chaker Chtara 2 , Patrick Sharrock 3 , Ange Nzihou 4 , Hafed El Feki 1 1 Laboratory of Materials Sciences and Environment, Science University of Sfax, University of Sfax, Sfax, Tunisia 2 Chemical group of Tunisia, Gabes 3 LERISM, IUT P. Sabatier, Castres, France 4 RAPSODEE, Ecole des Mines d’Albi-Carmaux, campus Jarlard, Albi France Abstract A batch crystallizer in a pilot setup was utilized to produce di-amonium phosphate (DAP) crystals for purification studies by desing of experiments using a fractional experimental design 2 5-2 . After purification, the selling price of DAP is increased five fold, which is the incentive for this optimization. Purified DAP is also used in the pharmaceutical and cosmetic industries. The physicochemical comparison of industrial DAP and recrystallized product, through spectroscopic analyses and chemical analyses, showed that purified DAP was made. Keywords Diammonium phosphate, Crystallization to pilot scale, Fractional experimental design 2 5-2 1. Introduction Di-ammonium phosphate (DAP) is produced by simple reaction (neutralization) between ammonia and phosphoric acid resulting in the formation of the di-basic salt. When the mole ratio of nitrogen from ammonia and phosphorus from phosphoric acid is 2.0, di-ammonium phosphate (NH 4 ) 2 HPO 4 ) is formed. The reaction is exothermic, so the DAP is produced at an elevated and relatively constant temperature of 110℃, yielding a dry solid. The equations for the neutralization reaction between ammonia and phosphoric acid are as follows: NH 3 (g) + NH 4 H 2 PO 4 (aq) → (NH 4 ) 2 HPO 4 (s) + H 2 O (g) Previous laboratory scale results on purification of industrial DAP were reported earlier [1]. The satisfactory results obtained prompted us to continue this work on a larger scale. To better understand the influence of thermodynamic parameters (temperature and time) on the purification of DAP, design of experiments (DOE) allows finding the important factors influencing recrystallization. In this work, the purification of industrial DAP to pilot scale was conducted in batch crystallization process to improve the product quality, specifically the stoichiometric purity (defined by the molar ratio MR) and the heavy metal contents. During this process the solubility of DAP was evaluated in two types of solvent (water, water-alcochol). DOE is very important in chemometrics, because chemical * Corresponding author: gargouri1412@yahoo.fr (Manel Gargouri) Published online at http://journal.sapub.org/ijme Copyright © 2014 Scientific & Academic Publishing. All Rights Reserved experiments take time to be performed to aquire knowledge to optimize a process or system. The fractional experimental design 2 5-2 was a comprehensive 2 5 design, including the five following factors: 1 - Temperature, 2 - Duration, 3 - stirring speed, 4 - type solvent and 5 - addition of charcoal. We present here the physicochemical characterization of DAP fertilizer upstream and downstream of the purification process, aiming at determining the optimum performance of recrystallization. 2. Experimental Part 2.1. Crystallization of Industrial DAP The experimental crystallization of DAP was performed in a batch reactor (Fig.1) which was equipped with a magnetic stirrer. The suspensions were stirred by a propeller at different stirring speeds, ranging from 400 to 600 rpm. Temperature control in the crystallizer was achieved through recycling the water from a thermostated basin. The uncertainty of temperature control was 0.05 K. In a typical run, a hot and filtered saturated aqueous solution of DAP was put into the crystallizer. The crystallizer was kept 10 K higher than that of the saturated DAP solution in order to ensure that no crystals existed in the solution prior to crystallization. After 1h, the temperature of the crystallizer was reduced to the saturation temperature of DAP solution. After a while, a small amount of DAP crystal seeds was poured into the crystallizer, and the batch cooling crystallization experiment was started. The supersaturation was obtained by slowly cooling. The solid product collected was dried at 330 K under vacuum [2-4].