(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(j)
Figure 6: Response surface three-dimensional plots of the interaction of every two variables: (a) percentage of HCl (X1) and temperature (X2); (b) percentage of HCl (X1) and stirring speed (X3); (c) percentage of HCl (X1) and S/L ratio (X4); (d) percentage of HCl (X1) and time (X5); (e) temperature (X2) and stirring speed (X3); (f) temperature (X2) and S/L ratio (X4); (g) temperature (X2) and time (X5); (h) stirring speed (X3) and S/L ratio (X4); (i) stirring speed (X3) and time (X5); (j) S/L ratio (X4) and time (X5).