Abstract :
[en] Phosphogypsum (PG), produced from phosphoric acid production, is accumulated in large stockpiles and occupies vast areas of land. The phosphate industry in Morocco produces 15 million tons of phosphogypsum waste by year (Hakkou, 2016). Those wastes contain toxic elements which may cause environmental damage during storage, especially by contact with aquatic environments (Gaudry, 2007).
The aim of this study was to propose a sustainable management of those harmful wastes. We test different mixings to trap the toxic elements. We mix some phosphogypsum waste with raw bentonite (B). Bentonite is abundant clay material in Morocco and is successfully used for decades as an adsorbent for removing toxic heavy metals from aqueous solutions (Chiban, 2012). The raw bentonite was sampled from the Tribia deposit located in Northeast of Morocco. We also test several mixings with both bentonite and lime (L). Chemical (XRF), mineralogical (XRD), thermal (ATG), geotechnical (Atterberg limits) and mechanical (Proctor and compressive strength) analyzes analysis were performed on the different mixtures.
The results display that lime and phosphogypsum significantly enhance the behavior of bentonite by increasing the compressive strength through different reactions. The optimal results are reached when 8% of PG is added to the bentonite, highlighted by an increase of 20% compared to the mixture made only by bentonite and lime at 28 days (Fig. 1). Over time, long-term modifications occur. In particular, lime as a strong base raises the pH of the clay and causes silica and alumina mobilization. Phosphogypsum decreases the time of setting. It forms then aluminates and hydrated calcium silicates which, by crystallizing, act as a binder between the grains thus causing an increase in the compressive strength.
In addition, immediate changes in geotechnical properties of bentonite were observed. Those changes are expressed by a decrease in the density of the Proctor optimum values and increase in the optimum water content with the addition of 5%, 8%, 10% and 15% of lime (Fig. 3). We suggest that the additional water is retained in the agglomerates resulting during the reaction of the lime with bentonite. The mixture evolves almost instantaneously from a plastic state to a solid, friable, non-tacky state and partially loses its sensitivity to water. The increase in unconfined compressive strength was the highest with 8% lime and 8% phosphogypsum (Fig. 2). Furthermore, the addition of phosphogypsum increases the liquid limit compared to the bentonite and lime mixture.
Disciplines :
Physical, chemical, mathematical & earth Sciences: Multidisciplinary, general & others