Independent variable

Unit

Symbol

Coded variable levels

−α

−1

0

+1

Leaching temperature

K

X1

308

323

338

353

368

Acid concentration

M

X2

0.25

1.5

2.75

4.0

5.25

Solid/liquid ratio

min

X3

0.01

0.02

0.03

0.04

0.05

Stirring rate

g/ml

X4

75

230

385

540

695

Leaching time

rpm

X5

30

60

90

120

150