In industrial comminution: Balls used in ball mills are commonly made from hard, wear-resistant materials. Which of the following materials is NOT used for manufacturing mill balls for routine grinding service?

Introduction / Context:
Ball mills rely on hard, wear-resistant grinding media to crush and grind ores, pigments, and ceramic raw materials. The choice of ball material affects contamination, wear rate, energy efficiency, and product purity.

Given Data / Assumptions:

• The question concerns standard industrial duty (not specialty laboratory use).
• Typical ball materials include forged/cast steels, high-Cr white irons, and sometimes ceramics or flint pebbles for low contamination.
• Media must withstand high impact and abrasion without deforming excessively.

Concept / Approach:

Grinding media must be hard, tough, and dimensionally stable. Ductile or very soft metals deform, smear, and shed, causing contamination and poor grinding. Toxicity and low melting point can also disqualify a material for routine milling.

Step-by-Step Solution:

Verification / Alternative check:

Handbooks and supplier catalogs list forged/cast steel and cast irons; lead appears only as a milling additive or as a component in lead-lined mills for specialty corrosion service, not as grinding media.

Why Other Options Are Wrong:

Alloy steel / Forged steel / Cast iron: All are conventional grinding media materials with proven wear resistance and service life.

Common Pitfalls:

Confusing “lining” or process additives with grinding media; assuming any metal sphere can serve as a mill ball.

Final Answer:

Lead