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The proposed variable values by the Box–Behnken Design-expert (6.0.11) software.
| Independent Variables | Symbols | Coded and Actual Levels | ||
|---|---|---|---|---|
| Low (−1) | Middle (0) | High (+1) | ||
| Concentration of NaOH (wt.%) | X1 | 3 | 6 | 9 |
| Soaking duration (hours) | X2 | 3 | 6 | 9 |
| Drying duration (hours) | X3 | 24 | 48 | 72 |
