Abstract:
The development of an appropriate solid-state kinetic model which represents the leaching process of phlogopite was investigated. Phlogopite samples were leached with nitric acid solutions of different concentrations, at different temperatures and for different reaction times. Leach liquors were analysed by ICP-OES for concentration, while the raw phlogopite and the acid-leached solid residues were analysed by XRF, XRD, ATR-FTIR, BET, TGA-DTG and SEM-EDS for characterisation to support the reaction rate model selection.
It was found that the reaction was diffusion-controlled and the model which represents onedimensional diffusion through a flat plate (model D1) most accurately predicts the leaching behaviour. The observed activation energies and preexponential constants varied with initial acid concentration. The observed activation energies decreased from 98.8 – 88.9 kJ mol-1 as the initial acid concentration increased from 2 – 4 M, while the observed preexponential constants decreased from 3.30 x 10+12 – 2.30 x 10+11 min-1.
Additional experiments were conducted at different temperatures, using different initial acid concentrations and over different reaction times to test the model. The experimental data points obtained (“testing data”) were in agreement with the predicted values. Analyses of the solid residues also revealed complementary results with respect to the leaching model selection. The raw phlogopite was found to be highly crystalline (XRD). Therefore, the absence of defects in the lattice means that the motion of H+ ions permeating into the lattice is restricted (Ropp, 2003; Schmalzried, 1995). This confirms that the leaching is internal diffusion-controlled since the mobility of constituents into the system is the controlling factor, and since the phlogopite particles are plate-like (SEM-EDS, BET) in shape, the use of the D1 model for one-dimensional diffusion through a flat plate is the recommended model to represent the leaching process. Furthermore, results obtained from the different analytical techniques were supportive of each other.
It was also found that the amount of acid consumed is inequivalent to the amount theoretically required. Using the theoretically required acid concentration (2.45 M) results in incomplete conversion (< 80 % according to Kgokong (2017)). When initial acid concentrations between 2.4 – 2.6 M were used, only 88 – 91 % conversion was obtained after 6 hours of leaching at 65 °C, leaving behind excess H+ in solution. If fertiliser is the desired end product, it would be favourable to minimise the H+ concentration of the leach liquor. Therefore, the leaching process should be optimised so that the acidity of the leach liquor is minimised while obtaining complete leaching of all cations from the phlogopite particles into solution. Furthermore, since the SiO2 by-product is highly porous (surface area of 517 m2 g-1), its application in industrial adsorbents, catalysts, polymers, pigments, cement, etc. should be further explored.
