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A laboratory method was designed and verified that allows for the testing of alkaline, magnesite-based reactive materials for permeable reactive barriers (PRBs) to remove heavy metals from contaminated groundwater. It was found that caustic calcined magnesia (CCM) with high reactivity and low solubility to remove Cu2+, Zn2+, Ni2+, and Mn2+ cations from mixed aqueous solutions can be prepared by calcination at a suitable temperature and residence time. Regarding the solubility of both the reactive material itself and the precipitates formed, the CCM should contain just a limited content of lime. One way is the calcination of a ferroan magnesite at temperatures above 1000 °C. However, the decrease in pH is accompanied by lower efficiency, attributed to the solid-phase reactions of free lime. A different way is the calcination of magnesite under the conditions when CaCO3 is not thermally decomposed. The virtually complete removal of the heavy metals from the model solution was achieved using the CCM characterised by the fraction of carbonates decomposed of approximately 80% and with the highest specific surface area. CCM calcined at higher temperatures could also be used, but this would be associated with higher consumption of crude magnesite. Under the conditions considered in the present work, the product obtained by the calcination at 750 °C for 3 h appeared to be optimal. The full heavy metal removal was observed in this case using less magnesite, and, moreover, at a lower temperature (resulting, therefore, in a lower consumption of energy for the calcination and material handling).

Slovakia holds 4th to 5th position in the world with respect to the supplies and mining output of magnesite. The most important product of natural magnesite processing is magnesia. While dead-burnt magnesia is used for producing basic refractory materials for metallurgy and cement industry, caustic calcined magnesia (CCM) founds new applications in industries, agriculture and environment protection. Use of CCM, due to its alkaline nature, as a reactive material for remediation of wastewaters, can provide all advantages of methods based on neutralization (pH adjustment) by lime. In this study, CCM was experimentally tested as a reactive material for the removal of cations Cu2+, Zn2+ and Ni2+ from wastewaters. The effects of chemical composition of model wastewaters tested and water-to-CCM weight ratio were investigated. The results have shown that CCM is a suitable material for the removal of heavy metals, especially copper, zinc and nickel, from acid wastewaters. The efficiency of cation removal up to 100% has been observed.

The overall process for producing pure magnesium compounds from serpentinite usually starts with acid leaching. However, if serpentinite is calcined prior to leaching, not only faster magnesium dissolution is achieved, but use of thermally activated serpentine also significantly reduces the problems relating to corrosion of the leaching equipment, because less aggressive leaching agents can be used and/or lower leaching temperatures and pressures can be applied. This paper compares how calcination influences the dissolution behaviour of magnesium in solutions of hydrochloric acid, acetic acid and ammonium chloride. Fine-grained serpentinite, characterized by the extent of serpentine decomposition between 85 and 95%, displayed the highest reactivity. The initial magnesium dissolution rate of calcined serpentinite was up to 30-, 125- and 165-times higher as compared to that of uncalcined serpentinite in solutions of hydrochloric acid, acetic acid and ammonium chloride, respectively, under identical reaction conditions.

In the production of pure magnesium compounds from serpentinite, acid leaching is usually the first stage of the overall process. However, faster magnesium dissolution can be achieved and the size of a potential leaching reactor can be reduced if serpentinite is calcined prior to leaching. Moreover, use of calcined serpentinite can reduce problems relating to corrosion of the reactor (lower leaching temperatures and pressures can be applied) and foam formation (chemically bonded water, which forms bubbles when released in the reactor, can be removed by calcining). This paper examines how calcination temperature and time influence the amount of magnesium dissolved during the initial period of leaching of calcined serpentine in hydrochloric acid. Fine-grained serpentinite calcined between 640 °C and 700 °C displayed the highest reactivity. The fraction of magnesium dissolved was up to 30-times higher as compared to leaching of uncalcined serpentine under identical reaction conditions.

Kinetics of the reaction between particulate sintered polycrystalline MgO and dilute hydrochloric acid were studied. The effects of process parameters, i.e., concentrations of H+ and Mg2+ ions, temperature and particle size were investigated. It was observed that the rate of chemical dissolution of MgO: (a) increased with increase in both HCl concentration (from 10-4 to 10-2M) and temperature (from 25 to 60∘C); (b) decreased with increase in particle size (from 63 to 355μm); (c) was not affected by Mg2+ ion concentration. Thorough comparison of our results with those published by other authors revealed that the “initial” stage of MgO dissolution has never been observed under the conditions of the experiments in the present work. It was concluded that the initial stage of MgO dissolution was probably too short to be detected and just the “advanced” kinetic regime was investigated. Values of the kinetic parameters were calculated, and apparent activation energy between 93 and 101kJmol-1 and the reaction order for H+ ions from 0.41 to 0.76 were obtained. The presented values of the reaction order for H+ ions are consistent, allowing for experimental error, with current kinetic theories and most of other experiments. However, the rate-controlling step could not be assigned unambiguously, because the same value of the reaction order for H+ ions can be predicted by various theories for different rate-determining steps. The broad variety of likely rate-determining steps indicates some intrinsic diversity in current kinetic models.

Kinetics of the dissolution of particulate polycrystalline MgO in excess of hydrochloric acid were investigated, with special regard to the effect of activity of H + ions, temperature and particle size. The results are presented in terms of the shrinking particle model. The overall process is controlled by the chemical reaction of MgO with H + ions at the liquid–solid interface. The apparent activation energy was 58 kJ mol −1 . Acid concentration (from 0.5 M to 5.1 M) was surprisingly found to have a decelerating effect on the dissolution rate of MgO, resulting in negative values of reaction order for H + ions (from −0.16 to −0.18). We conclude that the regime studied here, characterised by decrease in the dissolution rate of MgO with an increase in H + activity, occurs under far from mass-transfer control conditions in the solutions with low Mg 2+ :H + molar ratio.

Kinetics of the reaction between dead-burned magnesite and hydrochloric acid were investigated with special regard to the rate of chemical dissolution of MgO. The effect of process parameters viz. temperature, activity of H + ions, and particle size and composition of the solid was investigated. The dissolution of MgO was strongly affected by temperature (from 45 to 75 °C) and particle size (from 63 to 355 μm), while the effect of composition of the solid was weak. The results are presented in terms of the shrinking particle model. The dissolution of MgO is controlled by the chemical reaction of MgO with H + ions at the liquid–solid interface. The apparent activation energy was 58–64 kJ mol −1 . Acid concentration (from 1.0 to 5.3 M) was surprisingly found to have a decelerating effect on the dissolution rate of MgO, resulting in negative values of reaction order for H + ions (from −0.1 to −0.2). We conclude that the leaching behaviour studied here corresponds to the regime referred to in the literature as the “initial stage of dissolution of MgO”. This regime, during which the dissolution rate increases rapidly despite a decrease in H + concentration, is characterised by far from mass-transfer control conditions, which was exactly the situation analysed in the present work.

The leaching of calcined magnesite using ammonium chloride has been tested in a pH-stat to ascertain the effect of process parameters viz. temperature, concentration of NH 4 Cl, pH, and particle size and reactivity of the solid. The main purpose of the approach adopted was to keep the lixiviant composition constant during individual measurements. A simple mathematical model has been used to describe the dissolution of magnesium during leaching and to analyse the kinetic data. The apparent activation energy of leaching was found to depend on the particle size of the solid — the values 57.8 and 48.5 kJ mol −1 were obtained for the leaching of particles of −100+90 and −180+160 μm, respectively. It was concluded that the leaching process is controlled by the chemical reaction of MgO with H + ions at the liquid–solid interface and by pore diffusion.