Search

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).

Alkali-activated materials based on fly ash are widely developed and also produced on the present. Some of fly ashes are not suitable for production of alkali-activated materials because of their inconvenient chemical composition. Alumina-silicates are the most important components that are needed to accomplish the successful reaction. The proper content of amorphous phase of alumina-silicates and its proportion as well should be provided for the final composition of alkali-activated materials. The influence of pure aluminum oxide powder as well as raw milled natural perlite on mechanical properties and durability of alkali-activated mortars was investigated. These minerals were used as partial replacement of fly ash coming from black coal combustion. In addition, the mortars were prepared by using different alkali activators.

This study demonstrates an efficient synthesis of highly reactive silica using hydrometallurgical processing of serpentinite tailings. Proposed process uses two-stage (acid and alkaline) leaching of serpentinite tailings and precipitation of silica from the sodium metasilicate solution using hydrochloric acid. The alkaline leaching and the effect of impurities on the precipitation of amorphous silica under the conditions of maximum sol stability were examined in detail. The proposed route is technologically advantageous because the product did not contain residues of the original raw serpentinite and was characterized by high purity (99.4 wt. % SiO2), large specific area (541 m2 g–1), and consistent quality. Moreover, less sensitivity to the presence of impurities and longer gelation times, offering a longer time period for manipulating the product in the final stages of the process, were achieved. The total yield of silicon in the overall process was 90–91%.

A generalized non-porous shrinking particle model (G NSPM) of leaching has been verified and used for numerical simulations. Unlike the commonly used “simple” non-porous shrinking particle model (S NSPM), the G NSPM assumes n−th order chemical reaction, the influence of the L/S ratio and non-ideal behaviour of the leach solution. The leaching of dead-burned magnesite with hydrochloric acid has been chosen to verify the proposed mathematical model, as the (apparent) reaction order for HCl, n, may be both positive and negative, depending on the HCl concentration. Comparison of the simulations with experiments revealed that the S NSPM does not reflect the qualitatively different leaching behaviour at positive and negative n, especially at near-to-stoichiometric HCl/MgO ratio. The G NSPM reflects the effect of n not only qualitatively, but also with acceptable accuracy. At the stoichiometric HCl/MgO ratio, the mean squared error of the predictions obtained using the S NSPM was 100–130 rel. % as compared to those provided by the G NSPM. The common effect of the reaction order, n, L/S ratio (represented by the reduced HCl/MgO molar ratio, ϕ) and non-ideal behaviour of the acid were studied in detail by means of numerical simulations. The values of parameters n and ϕ can alter the leaching behaviour qualitatively. In contrast, the activity coefficient of HCl only slightly modifies the shape of the kinetic curves, but does not change the overall picture. The error of the time for complete conversion of MgO calculated using the S NSPM can reach more than 100 rel. % for n = − 0.5 and even up to several hundred rel. % for n = 1, because the conversion vs. time curves predicted by the G NSPM are markedly skewed towards longer leaching times.

Permeable reactive barriers (PRBs) are passive, in-situ remediation systems that comprise the filtration of a contaminated groundwater through a reactive media able to degrade, adsorb or precipitate the pollutants. Selection of the reactive media depends on the nature of the contamination. While the majority of PRBs are used to treat groundwater contaminated with chlorinated volatile organic compounds or acid mine drainage, the present study is focused on heavy metals, namely Fe, Al, Cu, Ni, Zn, Mn and As. To remove such inorganic pollutants, four methodologies are usually implemented: a) redox reaction, b) adsorption and ion exchange, c) biological treatment (e.g. metal sulphides precipitation) and d) pH control and precipitation. The present paper deals with the last methodology in which an adjustment of pH by alkalinity-generating materials leads to the precipitation of metal hydroxides and the decrease in the concentration of dissolved species. As the solubility of metals depends on their valence, redox-potential, concentrations and kinetics of reaction, the target pH varies with respect to the considered metal. A pH of about 10 allows the precipitation of hydroxides of divalent ions (zinc, manganese, copper, lead, nickel, cobalt and cadmium), while a pH of around 6-7 is adapted to trivalent ions (iron, aluminium and chromium). However, the accumulation of precipitates in the pore spaces can cause clogging of the pores, hence decrease the porosity and hydraulic conductivity of the reactive media and alter its efficiency. To validate this assumption, the neutralizing ability of alkaline materials such as activated zeolite and magnesite were examined in a test column by monitoring the resultant pH with attention to impact of precipitates on the hydraulic properties of reactive materials. Based on the test results, both materialls are considered suitable to remove the groundwater contaminants with high efficiency (up to 99%) and suitable grain size to encourage flow through the PRB.

This paper examines the effect of the nature of the precipitating agent (HCl, CO 2 and (NH 4 ) 2 CO 3 ) on the surface area of the resulting silica powders synthesized from serpentinite after its leaching in hydrochloric acid. In order to prepare a product consisting of small discrete particles polymerized into three-dimensional networks without using additives (surfactant and/or flocculant), synthesis from nearly-saturated sodium metasilicate solution at laboratory temperature under alkaline and acid-set conditions was applied in the present work. In highly acidic solution (e.g. at significant stochiometric excess of hydrochloric acid over the silicate) a regular density xerogel with a high specific surface area (541 m 2 g − 1 ) was achieved. Through synthesis of silica gel by acidification using CO 2 and (NH 4 ) 2 CO 3 , the values 392 m 2 g − 1 and 414 m 2 g − 1 respectively of the specific surface area were obtained. The microporosity of silica samples precipitated by HCl was confirmed by t -methods and α -methods, whereas powders prepared using CO 2 and (NH 4 ) 2 CO 3 were identified as being mesoporous. The mesopore size distribution calculated using the BJH method and DFT model shifted with rising pore diameter in the following order: silica precipitated by HCl, (NH 4 ) 2 CO 3 and CO 2 .

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.

The kinetics of the dissolution of particulate sintered polycrystalline MgO under advanced kinetic regime conditions were investigated, with special regard to the effects of concentration of H+ ions, temperature and the type of the inorganic acid used (HCl, HNO3 and H2SO4). It was observed that the rate of chemical dissolution of MgO accelerated with increase in both concentration of H+ ions (from 10−4 to 10−2 M) and temperature (from 25 to 60 °C). The overall process of MgO dissolution was found to be controlled by the surface chemical reaction of MgO with H+ ions. The presented values of the reaction order for H+ ions are consistent, allowing for experimental error, with current kinetic theories and most 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. Measured values of the reaction order for H+ ions are indicative that the rate is limited by the second protonation reaction, i.e. the slow reaction of surface hydroxide with a second proton. Though the reaction order for H+ ions seems not to be affected by the type of acid used, the apparent activation energy changes in the order HCl ≤ HNO3

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.