For centrifuge method, the dry material passing the 75-µm sieve, along with mineral matter, is added to the mass removed by washing to obtain total passing the 75-µm sieve.

• A. Ignore the dry material passing the 75-µm sieve.
• B. Add only the mineral matter to the washed mass.
• C. Add only the dry material to the washed mass.
• D. Add the dry material passing the 75-µm sieve, along with mineral matter, to the mass removed by washing to obtain total passing the 75-µm sieve.

Answer: (D)

In the centrifuge method, you’re aiming to recover the full amount of material that would pass the 75‑µm sieve by accounting for all fines, not just what remains after washing. Some fines exist as dry material that passes the sieve, and there’s mineral matter that also contributes to the fraction that passes. If you rely only on the washed mass, you’ll miss these portions and underestimate the total passing 75 µm. By adding both the dry material that passes the 75‑µm sieve and the mineral matter to the washed mass, you obtain the true total passing the 75‑µm sieve.