Independent variable | Unit | Symbol | Coded variable levels | ||||
−α | −1 | 0 | +1 | +α | |||
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 |