Table 1. Levels for each factor in the experimental design.

Factor

Axial point
(-α)

Low level
(-1)

Central level (0)

High level
(+1)

Axial point
(+α)

 Time of exposure (min)

27

40

60

80

93

 Acid concentration
 (% vv)

0.50

0.75

1.13

1.50

1.75

 Solid:liquid ratio
 (g:ml)

1:3.3

1:3.0

1:2.5

1:2.0

1:1.7