Influence de la chimie de l'eau sur la performance des polymères superabsorbants utilisés pour l'épaississement des pulpes de résidus miniers

Mining in Canada produces tonnes of solid waste, including concentrator wastes commonly referred to as tailings. These are commonly deposited in tailings impoundments surrounded by dikes. Conventional tailings consist in pulps with solids contents varying between 25 and 45% which can be the cause of the failure of the dikes. These tailings must be managed properly to avoid natural disasters. For this reason, alternative tailings management methods are used. Tailings densification is considered to date as the best management method which can contribute to the stabilization of dikes and thus avoid potential dike failure. However, sometimes these techniques fail to achieve the targeted solids contents. For this reason, this project is studying the possibility of using superabsorbent polymers (SAP) to densify the tailings. SAPs are materials capable of absorbing and storing a large amount of water within them for very long periods of time. The absorption capacity of tailings water by SAP will depend on various physico-chemical parameters of the water such as temperature, pH, chemical composition, concentrations of chemical elements, and presence of salts and/or cations. The general objective of this project was then to evaluate the influence of the chemistry of the tailings water on the absorbency of the SAPs used for the densification of mine wastes. Other factors were also evaluated such as the effects of the amount of solids present (initial solids content), the temperature of the medium, the pH, the SAP dosage on the absorbency of the SAP. More specifically, the objective was to isolate the influence of the different chemical elements that could be found in mining water on the absorption rate of SAP. For this, synthetic supernatant waters of sulphates and chlorides were prepared and used to perform absorption tests for PSA in these solutions and to understand the absorption kinetics, the effect of varying the concentration and the pH. Subsequently, an inert material (an artificial silica powder called SIL-CO-SIL®), whose particle size approximates that of mine tailings, was mixed with these aqueous solutions to simulate tailings with controlled physical and chemical properties. SAP absorption tests were then carried out on these mixtures to determine the final solids content reached when the chemical element concentrations, the initial solids content, the SAP dosage, the residence time and the temperature were varied. Finally, absorption tests were carried out on the supernatant water of tailings and on pulps of real tailings for comparison purposes of the absorption rates and final solids contents achieved with the synthetic solutions and the SIL -CO-SIL® - solutions mixtures, respectively. Deionized water (reference) as well as nine synthetic solutions of chlorides and sulphates were tested at concentrations varying between 500 and 5000 mg/l, namely: NaCl, KCl, MgCl2, FeCl3, K2SO4, MgSO4, FeCl3, Fe2(SO4) 3, Al2(SO4)3). Mono-, bi- and trivalent cations were targeted. Acid solutions of HCl (at pH 1.39; 2.31 and 4.34) and basic solutions of NaOH (at pH 8.87; 10.6 and 11.71) were also used to study the pH effect. Two types of SAP (a finer, SAP 1, and a coarser, SAP2) were tested. Tailings supernatant water from the LaRonde, Canadian Malartic and Westwood mines were also used. Regarding the absorption tests in solutions without solids, the direct addition mode of SAP was used, i.e., the introduction of the SAP directly into the different solutions. After a targeted contact time, the SAP swollen after absorption of the solutions are filtered to determine their absorption rate (= mass of liquid absorbed / dry mass of SAP). Preliminary tests have shown that the (coarser) SAP2 performs better in terms of absorption rate in deionized water. For this reason, the rest of the tests were carried out with the SAP 2. For absorption tests in pulps based on SIL-CO-SIL® and in real tailings, the indirect addition mode of SAP was used. This method consisted in using geotextile bags in which the SAP was placed at the desired dosage. The bags were immersed in the pulps. Once the target residence time reached, the bags are hung to allow free water to drain over the densified tailings. The gravimetric water content of the tailings is then determined, which makes it possible to calculate the absorption rate and the final solids content of the densified tailings. The results show that the presence of ions in the different solutions contributes to a drastic decrease in the absorption rate of SAP compared to the results obtained in deionized water. For a given concentration, this decrease depends on the type of solution, the metal valence of the cations and the pH. For example, the absorption rates obtained in the solutions for a concentration of 2500 mg/l and at 24 hours of residence are: 58 g/g for KCl (monovalent cation; pH = 5.8) and 6 g/g for MgCl2 (divalent cation; pH = 6.60); 40 g/g for K2SO4, (monovalent cation; pH = 5.80) and 20 g/g for Al2 (SO4) 3) (trivalent cation; pH = 2.8). These results highlight the influence of the metallic valence on the absorbency of SAPs. For a given solution, it was observed that the absorption rate decreases when the concentration of chemical elements increases. To study the absorption kinetics both for deionized water and for the nine solutions studied, tests were carried out up to a residence time of 72 hours. The absorption rate remains almost stable after 24 contact time. The study of the influence of pH, based on all solutions prepared in the laboratory and the mine tailings pulp supernatant waters, shows that the closer the pH of the solutions approaches the circumneutral pH , the greater the absorption rate. In the SIL-CO-SIL®- deionized water mixtures and SIL-CO-SIL®- synthetic solutions mixtures, it has been observed that the final solids contents that can be reached after absorption of the solution were influenced by several parameters, including the type of solution, concentration in chemical elements, dosage in SAP and initial solids content of the mixture. As the SAP dosage increased, the final solids content increased of a given solution and concentration but decreased when the concentration of chemical elements was increased. The maximum final solids content around 84% was reached in the SIL-CO-SIL® - deionized water mixture considered as control for an initial solids content of 50% and a dosage of 5 kg of SAP per ton of pulp. For mixtures of SIL-CO-SIL® and solutions of monovalent cationic sulphates and chlorides which are more favourable to absorption by SAPs, final solids contents of about 70% were achieved for an initial solids content of 50% and a dosage of 5 kg of SAP/t of pulp. The various results obtained on the supernatant water and the mine tailings showed that the tailings from the Westwood mine (pH = 7.69) can be better densified by SAP 2 compared to those from the LaRonde (pH = 4.32) and Canadian Malartic (pH = 8.73) mines. Indeed, with a dosage of 5 kg of SAP/t of pulp, it was possible to densify the Westwood tailings up to a solids content of 68% compared to 62% for the LaRonde tailings and 66% for the Canadian Malartic tailings. These results confirmed that the presence of various metals in the tailings as well as their pH influence the absorbency of SAPs, but depending on the SAP dosage used, it was possible to densify them up to a target solids content. It should be noted, however, that the various tests conducted in this study remain at the laboratory scale. It is strongly recommended that this work be continued on a larger scale in order to assess the performance of SAPs under field conditions.