---
title: "Geldart classification"
description: "The Geldart classification (Derek Geldart, 1973) groups powders by particle size and density into four classes that predict fluidisation, bridging and discharge behaviour. It is the most widely used powder-behaviour map in industrial bulk-solids handling."
canonical_url: "https://sylio.co/glossary/geldart-classification"
last_updated: "2026-06-28T02:29:32.850Z"
---

The **Geldart classification** (Derek Geldart, 1973) groups powders by particle size and density into four classes that predict fluidisation, bridging and discharge behaviour. It is the most widely used powder-behaviour map in industrial bulk-solids handling.

## The four classes

<table>
<thead>
  <tr>
    <th>
      Class
    </th>
    
    <th>
      Particle size / density
    </th>
    
    <th>
      Behaviour
    </th>
    
    <th>
      Example materials
    </th>
  </tr>
</thead>

<tbody>
  <tr>
    <td>
      <strong>
        A
      </strong>
    </td>
    
    <td>
      Small (30–100 µm), low density
    </td>
    
    <td>
      Fluidises well; expands before bubbling
    </td>
    
    <td>
      Cracking catalyst, alumina fines
    </td>
  </tr>
  
  <tr>
    <td>
      <strong>
        B
      </strong>
    </td>
    
    <td>
      Medium (100–500 µm), medium density
    </td>
    
    <td>
      Bubbles immediately on fluidisation
    </td>
    
    <td>
      Sand, salt, larger cement particles
    </td>
  </tr>
  
  <tr>
    <td>
      <strong>
        C
      </strong>
    </td>
    
    <td>
      Very fine (< 30 µm), cohesive
    </td>
    
    <td>
      Hard to fluidise; channels; cohesive arching
    </td>
    
    <td>
      Cement, fly ash, flour, talc
    </td>
  </tr>
  
  <tr>
    <td>
      <strong>
        D
      </strong>
    </td>
    
    <td>
      Large (> 500 µm), dense
    </td>
    
    <td>
      Spouts rather than fluidises
    </td>
    
    <td>
      Coal, gravel, grain
    </td>
  </tr>
</tbody>
</table>

## Why it matters for hopper design

- **Class C powders** are the most prone to [bridging](/glossary/bridging) and [rat-holing](/glossary/rat-holing). [Sonic horns](/glossary/sonic-horn), [air cannons](/glossary/air-cannon-air-blaster) and aeration are routinely needed
- **Class A powders** flow well from properly-designed hoppers; problems usually trace to wet incoming material
- **Class B powders** are predictable and well-suited to standard hopper geometry
- **Class D powders** rarely bridge but are abrasive and shock-loading the hopper

## Acoustic-cleaning fit

[Sonic horns](/glossary/sonic-horn) are most often deployed on Class C powders — fly ash, cement, lime, fine carbon black, food powders — because that is where cohesive flow problems concentrate.

## Related terms

- [Hopper](/glossary/hopper)
- [Bridging](/glossary/bridging)
- [Rat-holing](/glossary/rat-holing)
- [Fly-ash hopper](/glossary/fly-ash-hopper)