Excessive grinding speeds primarily affect particle size distribution in terms of range and uniformity:
Increased Risk of Bimodal Distribution
High-speed impacts cause brittle particles (e.g., ceramic powders) to rapidly break into ultrafine particles (<1μm), while ductile particles (e.g., polymers) form residual coarse particles (>50μm) due to insufficient stress release. This creates abnormal bimodal distribution where ultrafine and unbroken coarse particles coexist.
Agglomeration Effects Intensify
When speed exceeds the critical threshold (typically 80% of media centrifugal speed), grinding media centrifugally adhere to walls, reducing effective collisions. However, instantaneous friction-induced heat (localized 60-80°C) causes fine particle surface melting, forming secondary agglomerates (5-20μm). In excessive grinding , this increases deviation between nominal and actual dispersed particle sizes.
Widened Distribution Span
Typically manifests as D90/D10 ratio increasing from an optimized 2-3x to 4-5x. For example, alumina powder ground at 5000 rpm showed D50 reduced from 10μm to 3μm, but with D10=0.8μm and D90=15μm – significantly broader than the normal process result of D10=2μm/D90=6μm.
Case Study:
When zirconia beads grind silica powder at >75% critical speed, coarse particle retention reaches 10-30%. Optimized speed (60-70% critical speed) reduces retention to <5%.
Recommendations:
Real-time monitoring with laser particle analyzers
Dynamic speed adjustment using energy density formula E=0.5m(v²), where kinetic energy is proportional to speed squared
Match material fracture thresholds (typically 0.1-1J/g)
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