As shown in Fig. 7, the correlation coefficients (R2) of M1, M2 and
M3 are 0.9803, 0.9952 and 0.9680, respectively, implying that the
M1 and M3 curves do not generate a straight line. Moreover, the
intercept of the regression curves of M1, M2 and M3 is 0.15, 0.05
and 0.23, respectively. The regression curve of M2 with highest
regression coefficient can be viewed as a straight line through the
origin, and thus the leaching process is controlled by chemical reaction
control model.