A semi-mobile flash dryer/calciner unit to manufacture pozzolana from raw clay soils — application to soil stabilisation

The use of hydraulic binders in the construction of durable housing or roads in developing countries is an efficient technique, for which there is a demand at various times and in dispersed places throughout the world. The problem of the need for these perishable products could be resolved by the local production of pozzolana from clay soils using a semi-mobile dryer/calciner plant. By means of theoretical modelling of the thermochemical phenomena involved, and different small-scale pilot tests, we have developed a flash calcination process to this end. The Malet company has designed and built an industrial 800 kg/h production unit (International patent PCT/FR98/01149-Publication No. WO98/55418). It can be dismantled, moved over land (road network) and sea, and assembled within a few days. Once installed, it can be in production in 1 h. We have turned to our advantage the various benefits of the flash technique: the construction is light and compact, there is no need for grinding materials after calcination, and short materials residence time inside the unit results in short process response time and reduced start and shutdown operations duration. We describe the plant here, giving details of establishing the mass/thermal balance, the design of the gas and materials circuits, the process control system, and the construction and packaging. The plant is largely composed of a drying cyclone, a hammer mill, a preheating cyclone, a multi-burner combustion chamber with its separator cyclone and three cooling cyclones. We then compare the performances of the hydraulic binder elaborated from the pozzolana out of the unit plus lime, with those of an ordinary Portland cement.