Stratification is the process by which vibratory motion causes particles in a material bed to rearrange by size, with fine particles migrating downward toward the screen surface and coarse particles rising to the top. This natural size sorting is essential for efficient screening because only particles in contact with the screen cloth surface have the opportunity to pass through the openings. Without adequate stratification, fine particles remain trapped above coarse material and are carried to the discharge as oversize, reducing screening efficiency.
Stratification occurs because vibration creates momentary gaps between larger particles. Gravity pulls smaller particles downward through these gaps, while the upward movement of the vibration cycle tends to lift coarser, heavier particles. The result is a layered bed where the finest particles concentrate at the screen surface and the coarsest particles form the top layer. The effectiveness of stratification depends on amplitude, frequency, bed depth, feed rate, and the particle size distribution of the material. On ScreenerKing, Sweco, Kason, and Cleveland Vibratory separators, operators optimize stratification by adjusting vibration parameters and controlling feed rates.
Factors Affecting Stratification Quality
| Factor | Effect on Stratification | Optimization Approach |
|---|---|---|
| Bed depth | Excessive depth prevents fines from reaching screen surface | Keep discharge-end depth at 2-4x largest particle size |
| Amplitude | Higher amplitude increases particle separation and gap formation | Increase amplitude for deep beds; reduce for thin beds |
| Frequency | Higher frequency creates more opportunities for fines to migrate | Use higher frequency for fine materials |
| Feed rate | Excessive feed rate creates deep beds that resist stratification | Reduce feed rate or increase screen area |
| Particle shape | Spherical particles stratify easily; flat/elongated particles resist | Adjust vibration intensity for irregular particles |
| Moisture content | Moisture causes particles to stick together, resisting separation | Dry material before screening when possible |
Why This Matters in Vibratory Screening
- Screening efficiency — Stratification is the first step in separation. A particle cannot pass through the screen if it never reaches the screen surface. Poor stratification is the leading cause of low screening efficiency on overloaded machines.
- Capacity optimization — Understanding stratification helps operators find the optimal feed rate: high enough to maximize throughput but low enough to maintain adequate stratification and separation quality.
- Multi-deck performance — On multi-deck separators, stratification on the top deck determines how much material reaches the lower decks. Poor top-deck stratification starves lower decks of material.
- Vibration tuning — Amplitude and frequency adjustments directly affect stratification speed. Heavier, coarser materials need higher amplitude for effective stratification; lighter, finer materials respond better to higher frequency.
Related Glossary Terms
- Particle Size Distribution — Determines the volume of fines that must stratify
- Amplitude — Controls the intensity of particle bed disruption
- Frequency — Determines how many stratification cycles occur per second
- Screening Efficiency — Directly dependent on stratification quality
- Near-Size Particles — The hardest fraction to stratify and separate
Stratification FAQs
What is stratification in vibratory screening?
Stratification is the rearrangement of particles by size within the material bed. Vibration causes fine particles to migrate downward through gaps between larger particles, settling near the screen surface where they can pass through the openings. Coarse particles rise to the top. This natural size sorting is essential because only particles touching the screen surface can be separated.
How does bed depth affect stratification?
Excessive bed depth inhibits stratification because fine particles buried deep in the material layer cannot reach the screen surface before being carried to the discharge. Bed depth at the discharge end should not exceed 2 to 4 times the size of the largest particle. Reducing feed rate or increasing screen area ensures proper stratification.
Optimize Your Screening Performance
ScreenerKing engineers can help size your separator and tune vibration parameters for optimal stratification and throughput.
