Production of Lightweight Aggregates from Phosphate Washing Plant Sludge

In this study, we assessed the potentialities of using the phosphate washing by-products as row materials for the production of lightweight aggregates. Sludge samples from different washing plants have been physic-chemically characterized. Slurries were dried, milled and shaped into spherical pellets. These raw aggregates were sintered for 5 min in the laboratory kiln at different temperatures between 1,120 and 1,180 °C. The bloating potential of the lightweight aggregates as well as the effect of the firing temperatures on their properties (expansion, apparent density, water absorption and compressive strength) were assessed. The mineralogical data shows that the collected sludge samples are mainly composed of carbonates, francolite, clinoptilolite, quartz and feldspars; smectite, palygorskite and sepiolite are also present as clayey fractions. Chemically, the samples exhibited significant amount of SiO2, CaO, and P2O5. When sintered, the aggregates expand; this expansion results from the concomitant reaction of gas release and viscous phase formation. The produced lightweight aggregates revealed acceptable technological properties with apparent density values often lower than 0.9 g/cm3. Moreover, good expansion (60 % in volume) and water absorption (close to 10 %) were obtained. These results are encouraging and allow considering the studied sludge as promising material for the production of lightweight aggregates.