Phosphate sludge-based ceramics: Microstructure and effects of processing factors

https://doi.org/10.1016/j.jobe.2017.04.002Get rights and content

Highlights

  • Thermal transformations of phosphates sludge – clay (up to 20 wt%) were investigated.

  • The physical ceramic properties were correlated to the processing factors.

  • The influence of the factors on the microstructure and the measured properties were evaluated.

  • The optimum condition for the elaboration of sludge-based ceramics was determined.

Abstract

The potential use of clay amended phosphate sludge in lightweight aggregates manufacturing was assessed. For this purpose, the microstructure of heated blends was examined with X-ray diffraction, thermal analysis and scanning electron microscope and impedance spectroscopy, and their physical properties were followed as a function of heating temperature, soaking time and clay content. For the latter study, the full factorial design was used. The results showed that phase neoformation involved the breakdowns of the clay minerals and lime of carbonates. Moreover, the sintering process was significant as the clay content exceeded 5 wt% and 900<T<1000 °C. In such a condition, the sintering was chiefly controlled by melt. The results of the full factorial design showed that the increase of one of the processing factors affected positively water absorption, density and compressive strength. These effects were mainly linked to the conjugate formations of calc-aluminosilicate and viscous phase. The results also pointed out that the effects of the interactions between the factors on the measured properties were almost negligible. By using the full factorial design, the optimum values of the physical properties were evaluated.

Introduction

The intense mining activity related to natural phosphate beneficiation generates large amounts of sludge [1], [2], [3]. In Morocco for instance, 28.1 million metric tons of phosphate sludge were deposited of during 2010 [4]. As reported elsewhere, phosphate sludge was composed of fluorapatite, quartz, carbonates and montmorillonite [5]. Because of the presence of the three latter minerals, such a waste could be appropriate for manufacturing ceramic products. However, because of the low contents of alumina and silica [6], phosphate sludge should be adequately amended in order to be used as a prime material for ceramics.

Clays, as mixtures of clay minerals and ancillary minerals, are suitable raw materials for traditional ceramic manufacturing [7], [8]. In addition to their good plasticity, clays are considered as sources of alumina, silicate and alkalis. These oxides, among others, react at high temperature and may lead to vitreous and refractory phases, which presence has a positive impact on ceramic properties. For these reasons, clays appear to be appropriate amendment materials for ceramics composed of solid wastes (blast-furnace slag, brick grog, glass, construction waste and phosphate sludge). The recycling of the solid wastes contribute to the control of waste dumping and the preservation of natural ceramic bodies, and may reduce the cost of ceramic products [9]. In this respect, it was pointed out that phosphate sludge was suitable for lightweight aggregates manufacturing [4], and fertilizing pellets [10].

The aim of this study was to evaluate the effects of some processing factors (firing cycle parameters and clay addition) on heated clay amended phosphate sludge using full factorial design. For this goal, the microstructure was investigated and the change of the main ceramic properties was followed.

Section snippets

Materials

The used phosphate sludge (PS) and clay (SC) were from the phosphate beneficiation plants and a pit located nearby Youssoufia (Gantour, Morocco). The mineralogical and chemical compositions data (Table 1), determined by (ICP-AES) (PerkinElmer Optima 3100 RL ICP–AES) and the RIR method by using the X'Pert HighScore software respectively, showed that PS is composed of fluorapatite, quartz, carbonates (calcite and dolomite) and montmorillonite, and SC consists of quartz, dolomite and

Microstructural investigation

Due to heating, carbonates and montmorillonite vanished. Fluorapatite and quartz persisted and gehlenite (Ca2Al2SiO7) neoformed (Fig. 1a). The relative amounts of the identified minerals in the heated samples are shown in Fig. 2. Globally, the amounts of quartz and fluorapatite remained almost constant up to about 1000 °C. However, they substantially decreased with further heating. The quantity of gehlenite, which formed nearby 884 °C as carbonates decomposed (Fig. 3), increased with increasing

Conclusion

The results of this study, which addressed the effects of processing factors on phosphate sludge-based ceramics by using full factorial design, showed that: i) Heated clay-amended phosphate sludge blends were the subject of formation of melt and gehlenite (Ca2Al2SiO7). The amounts of these phases increased with increasing temperature and/or clay content. ii) Fluorapatite partially decomposed at high temperature, and contributed to the melt formation. iii) The sintering process was mainly

Acknowledgments and conflict of interest statement

The authors declare that they have no conflict of interest.

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