---
title: "Sulphur / chloride / alkali cycles"
description: "The sulphur, chloride and alkali cycles describe how volatile species evaporate from the rotary-kiln burning zone, rise with the gas flow, condense in the cooler preheater above, return to the kiln in the descending raw meal, and recirculate. Each cycle has its own behaviour and operational consequences."
canonical_url: "https://sylio.co/glossary/sulphur-cycle-chloride-cycle-alkali-cycle"
last_updated: "2026-06-28T02:29:27.745Z"
---

The **sulphur**, **chloride** and **alkali cycles** describe how volatile species evaporate from the [rotary-kiln](/glossary/rotary-kiln) burning zone, rise with the gas flow, condense in the cooler [preheater](/glossary/preheater-tower) above, return to the kiln in the descending raw meal, and recirculate. Each cycle has its own behaviour and operational consequences.

## The three cycles

<table>
<thead>
  <tr>
    <th>
      Cycle
    </th>
    
    <th>
      Volatile species
    </th>
    
    <th>
      Condensation window
    </th>
    
    <th>
      Operational consequence
    </th>
  </tr>
</thead>

<tbody>
  <tr>
    <td>
      <strong>
        Sulphur cycle
      </strong>
    </td>
    
    <td>
      SO₂, SO₃, alkali sulphates
    </td>
    
    <td>
      800–1,000 °C
    </td>
    
    <td>
      Sticky alkali-sulphate coatings in preheater stages 4–5
    </td>
  </tr>
  
  <tr>
    <td>
      <strong>
        Chloride cycle
      </strong>
    </td>
    
    <td>
      KCl, NaCl
    </td>
    
    <td>
      700–900 °C
    </td>
    
    <td>
      Aggressive sticky coatings; primary driver of <a href="/glossary/kiln-inlet-ring-snowman">
        kiln-inlet snowmen
      </a>
    </td>
  </tr>
  
  <tr>
    <td>
      <strong>
        Alkali cycle
      </strong>
    </td>
    
    <td>
      K₂O, Na₂O
    </td>
    
    <td>
      wide
    </td>
    
    <td>
      Sets cement chemistry; affects strength development
    </td>
  </tr>
</tbody>
</table>

## Why the cycles matter operationally

All three cycles concentrate volatiles in the gas-phase recirculation loop unless something extracts them. Conventional cement raw materials and fossil fuels carry modest loadings; [alternative fuels](/glossary/alternative-fuel) — especially [RDF, SRF and TDF](/glossary/rdf-srf-tdf) — add substantially more chlorine, sulphur and sometimes alkali.

When a cycle saturates:

- **Chloride cycle** — heavy [kiln-inlet build-up](/glossary/kiln-inlet-ring-snowman); kiln stop unavoidable
- **Sulphur cycle** — preheater coatings; cyclone pluggage
- **Alkali cycle** — clinker quality issues; cement performance drift

## Cycle management

- **Chloride bypass** — extracts a slipstream of gas from the kiln inlet to remove chlorine
- **Raw-material substitution** — selecting lower-Cl/-S/-alkali raw materials
- **Fuel blending** — controlling AFR chlorine and sulphur content
- **Sonic horns and air cannons** on the preheater and kiln inlet to keep accumulating coatings under control

## Related terms

- [Preheater tower](/glossary/preheater-tower)
- [Kiln-inlet ring / snowman](/glossary/kiln-inlet-ring-snowman)
- [Alternative fuel](/glossary/alternative-fuel)
- [Chloride bypass](/glossary/chloride-bypass)