Contamination-sensitive milling: In pebble mills used for paints, pigments, and cosmetics (where iron contamination must be avoided), what are the pebbles typically made of?

Introduction / Context:
When grinding color-sensitive or purity-critical products, iron contamination from grinding media must be minimized. Pebble mills use nonmetallic media to prevent discoloration and unwanted catalytic effects in paints, pigments, and cosmetics.

Given Data / Assumptions:

• Tumbling (ball/pebble) mill configuration.
• Product sensitive to metallic contamination.
• Desire for chemically inert, hard, and wear-resistant media.

Concept / Approach:
Flint pebbles and porcelain (alumina-based ceramics) are traditional nonmetallic media. They are hard, relatively dense, and chemically inert, reducing contamination while still providing effective grinding. Stainless steel or steel shot is avoided when iron-free product is required.

Step-by-Step Solution:
Define requirement: nonmetallic, inert media → ceramic or natural stone.Compare options: bronze/stainless/concrete introduce metals or impurities.Select flint or porcelain as the standard choice.

Verification / Alternative check:
Materials handbooks list flint pebbles and high-alumina porcelain balls for contamination-sensitive grinding applications.

Why Other Options Are Wrong:

• Bronze/stainless/mild steel: risk metallic contamination.
• Concrete: not wear-resistant or clean enough for high-purity pigments.

Common Pitfalls:
Using mixed media that introduces tramp metals; always align media choice with purity specifications.

Final Answer:
Flint or porcelain