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The kinetics of the leaching of natural magnesite with hydrochloric or nitric acid has been studied. Relatively high values of the apparent activation energy (from 46.5 to 70.0 kJ mol−1), but especially the low values of the apparent reaction order for H+ cations, $n$ n , have shown that the whole process is controlled by the surface chemical reaction. The value of $n$ n decreases continuously with increasing activity of H+ cations in the range of pH 5 to −1.4. At ${\rm{pH}}\,\lt 2.6$ pH < 2.6 there is a transition from the kinetic regime characterized by $n \cong $ n ≅ 0.5 to another one with $n$ n close to zero. [ABSTRACT FROM AUTHOR]

Leaching kinetics of natural magnesite with nitric acid was investigated, and the results of a detailed critical analysis are presented. The reaction system MgCO3-HNO3-H2O has been studied with special regard to the production of pure magnesium salts. The generalised non-porous shrinking particle model considering the n-th order liquid-solid reaction was fitted to the measured kinetic data and the values of apparent activation energy, EA, and reaction order, n, were calculated. Relatively high values of EA (43.7 and 58.5 kJ mol-1) obtained for both "low" (0.01-0.1 M) and "high" (1-6 M) concentrations of HNO3, respectively, lay within the range of values presented by other authors and indicate that the overall process is controlled by the chemical reaction in the whole range of reaction conditions considered (temperature from 70 to 100°C and concentration of HNO3 from 0.01 to 6M). This hypothesis has been confirmed by the calculated values of n, which considerably differ from 1. Until now, it has been assumed in the literature that n=1 and no attention has been paid to the actual mechanism of the intrinsic chemical reaction. In this work, the value of n=0.22 was calculated for the solutions with "low" HNO3 concentration, and n=0.05 was obtained for the concentrated HNO3 solutions. These values of n are considerably lower than the value of n=0.5, predicted by the theory. It is hypothesised that the decrease in n with increased acid concentration might be related to the formation of CO2 and its transfer from the liquid to the gaseous phase. [ABSTRACT FROM AUTHOR]

All alternatives of the non-porous shrinking particle model (NSPM) referred to in the literature were reviewed and critically compared – they differ especially in their ability to reflect the role of three crucial model parameters: the order of reaction, liquid/solid ratio and activity coefficient of the leaching reagent. A procedure to calculate an experimental error related to the methodology of laboratory leaching tests (including the effect of different ore-particle size) has been proposed. Error propagation in the process of determination of the apparent order of reaction and activation energy from experimental data was analysed in detail, to estimate the systematic error of the simulation model developed. The simulation models tested were validated using the experimental data for the reaction system MgO-HCl-H 2 O. Based on the results obtained, some recommendations for an effective practical use of the NSPM have been suggested. • Methodology of kinetic leaching tests was analysed in detail. • Reaction mechanism for the system MgO-HCl-H 2 O has been proposed. • Experimental error for the system MgO-HCl-H 2 O was estimated. • Error propagation in determining the kinetic parameters of leaching was analysed. • Practical recommendations for the non-porous shrinking particle model were proposed based on the performed simulations. [ABSTRACT FROM AUTHOR]

One of the most important environmental problems of the last decade is water (groundwater, surface water) contamination, which poses a significant ecological risk to the environment and human health. Permeable reactive barrier (PRB) is an efficient technology for remediating contaminated water on industrial and mining sites. The PRB technology is based on an emplacement of a reactive material in the subsurface designed to intercept a contaminated groundwater plume. The contaminated plume flows through the reactive material where the contaminants are transformed into environmentally acceptable species. The aim of the present study was to test experimentally caustic calcined magnesia, limestone, dolomite and blast furnace slag as potential reactive materials for the PRB technology. The effects of the chemical composition of contaminated water and water-to-reactive material weight ratio on the efficiency of heavy metals removal were investigated. The results have shown that caustic calcined magnesite (CCM-KK) was the most suitable reactive material for removal of cations Fe3+, Al3+, Cu2+, Zn2+, Ni2+, and Mn2+. This reactive material was able to increase the pH of the contaminated water to the value of about 10 and the efficiency of selected cations removal up to 100 % was observed. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: The kinetics of the reaction between particulate stibnite and mixed Na2S+NaOH solutions were studied. The effects of the concentration of the Na2S and NaOH (from 0.5% to 2.0%), temperature (from 292K to 327K) and particle size (from 40μm to 500μm) were investigated. The results are presented in terms of the shrinking (non-reacted) core model and the shrinking porous-particle model. The apparent activation energy was approximately 44kJ mol−1, and the apparent reaction order for Na2S varied from 1.4 to 1.7. The calculated values of the kinetic parameters indicate that the leaching process is controlled by both the intrinsic chemical reaction between Sb2S3 and Na2S at the liquid/solid interface and pore diffusion. [Copyright &y& Elsevier]

Kinetics of the reaction between particulate stibnite and mixed Na2S + NaOH solutions were studied. The effects of concentrations of Na2S and NaOH, temperature, particle size and liquid-to-solid ratio were investigated. It was observed that the rate of leaching of stibnite: a) increased with an increase in both Na2S and NaOH concentration (from 0.5 wt. % to 2.0 wt. %), and temperature (from 292 K to 327 K); b) reached its maximum at Na2S:NaOH molar ratio equal to 1:2; c) decreased with an increase in particle size (from 0.04 mm to 0.5 mm) and L/S ratio (from 10 to 100). The results are presented in terms of the shrinking-particle model. Calculated values of the kinetic parameters indicate that the leaching process is controlled by the chemical reaction between Sb2S3 and Na2S at the liquid/solid interface. Apparent activation energy is approximately 44 kJ mol-1 and the apparent reaction order for Na2S varies from 1.4 to 1.7. [ABSTRACT FROM AUTHOR]

Abstract: 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

Abstract: Kinetics of the reaction between particulate sintered polycrystalline MgO and dilute hydrochloric acid were studied. The effects of process parameters, i.e., concentrations of and 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 to ) and temperature (from 25 to ); (b) decreased with increase in particle size (from 63 to ); (c) was not affected by 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 and the reaction order for ions from 0.41 to 0.76 were obtained. The presented values of the reaction order for 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 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. [Copyright &y& Elsevier]

Abstract: 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 58kJmol−1. Acid concentration (from 0.5M to 5.1M) 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 Mg2+:H+ molar ratio. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

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). [ABSTRACT FROM AUTHOR]

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 q ualitatively 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. • G eneralized non-porous shrinking particle model (SPM) of leaching was verified. • Verified model reflects reality significantly better than the commonly used SPM. • Simulation model reveals the influence of the reaction order and L/S ratio. • The model shows the effect of non-ideal behaviour of the leach solution. [ABSTRACT FROM AUTHOR]

A generalized non-porous shrinking particle model (NSPM) of leaching is proposed, which - unlike commonly used "simplified" mathematical models - takes into account the influence of the L/S ratio (by means of the so-called reduced A/B molar ratio , ϕ , which indicates how many times the leaching agent (A)-to-valuable substance (B) molar ratio actually used exceeds its stoichiometric value) and non-ideal behaviour of concentrated lixiviants. Moreover, n-th order chemical reaction with power-law kinetics is considered, which makes determination of the rate-determining step of the overall process more reliable. Correlation between the proposed model and experiment was tested using the hydrochloric acid leaching of dead-burned magnesite, which provides the opportunity to experimentally verify how the mathematical model fits the leaching data for both the positive and negative reaction order. Calculated values of the reaction order for HCl, n , different from unity (n = 0.28 and n = − 0.23 obtained for low (0.20–1.03 M) and high HCl concentrations (2.06–3.83 M), respectively), together with relatively high values of the activation energy (57.5 J mol−1 and 56.6 J mol−1) indicate that the leaching rate is controlled by the intrinsic chemical reaction under the conditions considered in the present work. Simulations made using the calculated values of the parameters have shown that the proposed generalized NSPM model clearly reflects two most important experimental facts: (a) L/S ratio affects the leaching process; and (b) qualitatively different leaching behaviour has been observed for the positive and negative reaction order. Unlike the simplified NSPM model, the generalized model describes the leaching process quantitatively in the whole range of the parameter ϕ , for both n > 0 and n < 0. The simplified NSPM (which does not take into account the L/S ratio) describes the leaching process with sufficient accuracy when ϕ ≥ 10, but at ϕ < 10 the error grows exponentially with decreasing ϕ and can reach up to 11–17% rel. under the stoichiometric conditions (i.e., at ϕ = 1), in dependence on the value of n. It is assumed that the proposed generalized NSPM model provides a useful tool for planning kinetic experiments, identifying the kinetic model parameters and simulation of the leaching process even under stoichiometric conditions. • Reflects reality better than the commonly used simplified form, especially at low L/S ratio. • Makes determination of control mechanism more reliable. • Can be used for planning kinetic experiments, identifying model parameters and simulation. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

This paper examines the effect of the nature of the precipitating agent (HCl, CO2 and (NH4)2CO3) 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 (541m²g-1) was achieved. Through synthesis of silica gel by acidification using CO2 and (NH4)2CO3, the values 392m²g-1 and 414m²g-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 CO2 and (NH4)2CO3 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, (NH4)2CO3 and CO2. [ABSTRACT FROM AUTHOR]