Mylène Krebs, Jean-Michel Charmet, and Damien Chudeau, Fives FCB, discuss the key factors influencing the calcination process and present insights from a calcination test centre in Northern France.
The calcination of clay is a transformative process, unlocking its potential as a reactive supplementary cementitious material (SCM). At Fives FCB, the Cement and Minerals division of Fives Group, a dedicated calcination pilot has been operational since 2021. Equipped with advanced technologies, the facility enables precise parameter optimisation for a wide range of clays. This article examines the major calcination parameters, the performance of two distinct calcination technologies, and considerations for assessing clay suitability.
Calcination equipment and methods
The Fives FCB test centre houses two calcination technologies:
- Flash calciner: Featuring the four independent process steps (i.e. heat generation, calcination, colour control, and final cooling) as in Fives' industrial plants. This technology ensures the best overall performance and versatility across a wide range of clay qualities. It can be operated up to 1300°C, extending the use of flash calcination to other types of materials.
- Rotary kiln: A 4 m long rotary kiln providing residence time of 30 min., suitable for producing both calcined clay and clinker. To date, dozens of materials have been tested, resulting in a robust database of 150 samples.
Key parameters influencing calcination
Optimising the calcination process requires attention to three principal factors:
- Temperature: Insufficient heating results in incomplete dehydroxylation, reducing reactivity. Excessive heating induces sintering and crystallisation reactions, creating unreactive phases.
- Particle size distribution: The proper grinding sequence has a strong influence on process efficiency. In flash process, preliminary grinding to a d50 in a range of 30 µm to 40 µm often strikes a balance, minimising operational issues like clogging and reducing the need for subsequent grinding.
- Residence time: Residence time varies significantly between technologies. Flash calciners enable short residence times (sec.), while rotary kilns require prolonged durations (30 min.). Optimal residence time correlates with particle size and ensures uniform calcination.
Experimental design for flash calcination parameter optimisation
A systematic survey of flash calcination parameters was conducted using the Taguchi method (Figure 2), a robust statistical approach to experimental design. The study evaluated the influence of the three key factors – temperature, residence time, and particle fineness – each tested at three distinct levels. This factorial design resulted in a total of 18 tests, carefully structured to assess the interactions and individual effects of these parameters. The reactivity of the calcined clay was measured using the modified Chapelle Test, following the NF P18-513 standard. This methodology ensures reliable and reproducible insights into the optimisation of calcination conditions for maximising clay reactivity.
Enjoyed what you've read so far? Read the full article and the rest of the March issue of World Cement by registering today for free!