Level | Oxalic acid concentration (A) | Solid-liquid ratio (B) | Ultrasonic duration/min (C) | Blank (D) |
1 | 1% | 1:8 | 10 | 1 |
2 | 2% | 1:10 | 20 | 2 |
3 | 3% | 1:12 | 30 | 3 |