REACTION KINETICS AND MASS TRANSFER STUDIES IN SELECTIVE LEACHING OF LOW-GRADE CALCAREOUS PHOSPHATE ROCK

Abstract

In the present thesis, the dilute solutions of succinic acid have been used to investigate the selective leaching of calcareous material in low-grade phosphate rock. The effective parameters on the dissolution rate are reaction temperature, particle size, acid concentration and liquid-solid ratio. The results indicate that succinic acid can be used to selectively dissolve the calcareous material in low-grade phosphate rock as it improves the P 2 O 5 content of the rock and makes it viable as a feed to an acidulation plant. Using the known size particles of the sample, acid concentration and liquid-solid ratio, the influence of different reaction temperatures has been studied in order to elucidate the leaching kinetics of calcareous material in the rock. The results show that the leaching rate of calcareous material increases with increasing temperature. A kinetic model has been suggested to describe the selective leaching process of calcareous material analyzing the kinetic data. The selective leaching curves have been evaluated in order to check the validity of shrinking core models for liquid-solid systems. The experimental data have been tested by graphical and statistical methods and it is found that the leaching of calcareous material in the rock is controlled by the chemical reaction i.e. 1  ( 1  x ) 1 / 3  1 . 47  10 6 e  64 . 92 / RT t . The apparent activation energy of the dissolution process has been found to be 64.92 kJ mol -1 over the reaction temperature range from 313 to 353 K. Such a value of activation energy indicates that the process is a chemically controlled reaction and agrees with the values obtained in the similar research of fluid- solid reaction systems. The agreement between the experimental conversion and the 17values calculated from the suggested empirical equation has been tested, and which is found to be very good. For a comparison study, lactic acid has also been used to study the selective leaching kinetics of calcareous phosphate rock. The effect of acid concentration, liquid/solid ratio, particle size and temperature has been studied in order to expound the leaching kinetics of calcareous material in the rock. It has been found that the leaching rate of calcareous material increases with increasing the acid concentration, liquid/solid ratio and temperature and decreasing particle size. A semi-empirical model has been suggested to illustrate the selective dissolution of calcareous material analyzing the experimental data. The selective leaching curves have been evaluated to test the validity of kinetic models for liquid-solid systems. The kinetic data are analyzed by graphical and statistical methods and it has been found that the dissolution of calcareous material in the rock is controlled by 1  ( 1   ) 1 / 3  19 . 1 C 1 . 753 ( L / S ) 1 . 627 D  0 . 737 e  42954 chemical . 62 / RT reaction i.e. t . The analysis of the obtained results reveals that the applicability of the suggested model is good and it can work within a certain range for the choice of adjustable parameter values depending on the degree of selective leaching. The results show that the controlling step of the overall process of the heterogeneous reaction is a chemical change. According to the analyzed results, it is recommended that the parameter values for the optimum selective leaching rate are C = 8% v/v, L/S = 7 cm 3 g -1 , T = 318 K, SS = 350 min -1 and D = 0.1255 mm.