Évaluation de la stabilité chimique des polymères superabsorbants et leur potentiel de séquestration en métaux dans les eaux des pulpes de résidus

In recent years, many researches have been performed to develop new techniques for controlling the management of mine waste. Most of these methods allowing to store mine wastes in a safe and economical manner. However, the management of mine waste is one of the most challenging environmental issues. This is due to the significant quantities of wastes, particularly waste rock (rock fragments of varying granulometry) and tailings from concentrators. The conventional method of managing tailings is to store them in tailings storage facilities (TSF). These require the construction of large and often very expensive containment dams, which generate some vulnerabilities of instability. The problem of tailings disposal on the surface is mainly related to two factors: the large quantities of water rejected and the mineralogical properties of the ore. As well, acid mine drainage (AMD) is one of the most critical environmental issues confronting the mining industry. It occurs when sulphide minerals oxidize in the presence of water and oxygen. The result is the generation of acidity and the solubility of metals in varying concentrations. The TSF therefore become complex to maintain and to control over the long term. A new alternative technique in the process of tailings pulp management has been proposed. This technique consists of using superabsorbent polymers (SAPs) in the process of pulp densification. In order to reduce the risk of physical and chemical instabilities of tailings and provide optimal water management in TSF. Using polymers reduce the water content of pulps, due to their high-water absorption capacity and the possibility of metal sequestration. The purpose of this project is to evaluate experimentally the long-term physico-chemical stability of SAPs for their use in the dewatering of mine tailings pulps for surface storage. To this end, some experimental studies have been performed using recycled PA-Na with the objective to evaluate their behaviour in supernatant water. Three supernatant waters (mine A, mine B and mine C) with different chemical compositions were tested. These waters are mainly loaded with metal cations and sulphates with a slightly basic pH. The results obtained showed the efficiency of PSAs in term of the water retention and sequestration of heavy metal ions. Sequestration potentials vary from one water to another according to their chemical composition. Thus the swelling rates are 46.8 g/g for Mine A, 33.3 g/g for Mine B and 215.4 g/g for Mine C, respectively, and the sequestration potential of Cu is 31.9% in Mine A (CM), 28.6% in Mine B (WW) and 51.2% in Mine C (LR). This variability in sequestration is mainly related to the geochemistry of the water. Thus, the presence of certain cations such as calcium and potassium lead to competition between metal cations and decrease the absorption of SAPs. Moreover, the behaviour of PA-Na was evaluated in saline synthetic solutions to determine their sequestration potential in relation to each metal ion, and to optimise the factors influencing the sequestration capacity of SAPs. The results obtained showed that the PSA sequestration process supports the Freundlich adsorption model. The main factors affecting sequestration are concentration of the element, pH of the medium and of the sites available in the polymer. Thus, the sequestration selectivity depends on the atomic valence, ion radius and type of SAP. The influence of climatic conditions on recycled PA-Na was also evaluated to assess the possibility of their use in the tailings thickening process. As well as their long-term stability when exposed to climatic conditions in the TSF. The results obtained show the effectiveness of SAPs in sequestering supernatant water without releasing it under the effect of rainwater or in the case of their direct and indirect exposure to the freeze-thaw cycle. However, UV radiation caused a damage to the cross-linking of SAPs, which implies their total dissociation in water. UV radiation can destroy the polymer.