Heated blends of phosphate waste: Microstructure characterization, effects of processing factors and use as a phosphorus source for alfalfa growth.

Microstructure of expandable lightweight aggregates (LWAs), which was composed of phosphate waste (PW), cement kiln dust (CKD) and raw clay (RC) was investigated, and the effects of processing factors (temperature, waste content, soaking time) on their physical properties were quantified by using response surface methodology (RSM). The potential use of LWAs as a phosphorus source was assessed through the use of seeds of alfalfa. It was found that the main minerals of the waste, namely carbonates and fluorapatite, were involved in the formation of labradorite/anorthite and melt respectively. Stability of mullite- the main constituent of CKD- was sensitive to the melt content. The assemblage of the identified phases was discussed based on the CaO-SiO2-Al2O3 phase diagram. The results of RSM showed that the change of compressive strength, firing shrinkage and water absorption of LWAs versus processing factors was well described with a polynomial model and the weights of the effects of the factors increased in the following order: sintering temperature > waste content (in the case of PW-RC) > soaking time. On the other hand, it was found that due to the release of phosphorus by soil-embedded pellets, the growth of alfalfa plants improved, and the rate enhanced in this order: PW-RC > PW-CKD > PW-CKD-RC. The absorbed quantity of phosphorus (0.12%) was still lower than the common uptake amount.

Phosphate sludge: thermal transformation and use as lightweight aggregate material.

Phosphate sludge generated from beneficiation plants of Moroccan phosphate rocks was investigated at 900-1200 °C by X-ray diffraction, scanning electron microscopy, thermal analysis and Fourier-transform infrared spectroscopy. Mixtures of the phosphate sludge and a swelling clay (up to 30 wt.%) were investigated and their properties (shrinkage, density, water absorption and compressive strength) were measured as a function of temperature and clay addition. The results showed that gehlenite neoformed from lime of decomposed carbonates and breakdown products of clay minerals and that fluorapatite (original mineral) resisted heating until fusion. The measured properties were mainly controlled by temperature, and the effect of clay addition was less regular, except for water absorption. Considering the mixtures densities (1.44-3.02 g/cm(3)), lightweight agglomerates can be produced at 900 or 1100 °C, but their compressive strengths were relatively low (2-4.5 MPa). Based on SiO2-Al2O3-Fluxes diagram and taking account of the chemical composition of the materials used, the production of expanded aggregates requires clay additions as high as 80 wt.%.