Calcination Of Magnesite Research Articles

Long-term mechanical performance of fly ash-incorporated reactive magnesia cements

In this research, after producing reactive magnesia directly from calcination of magnesite, reactive magnesia specimens with fly ash were prepared and cured in two different environments to investigate their long-term mechanical performance by carbonation. To investigate the effect on their microstructures of carbonation, the XRD profiles of the samples were also obtained and investigated. It is found that the strength of reactive magnesia samples is highly affected by the addition rate of reactive magnesia and the curing regime. In spite of the presence of the nesquehonite (MgCO3.3H2O) in the reactive magnesia cements having no portland cement, any type of magnesium carbonate crystal was not found for the cements with portland cement in carbon dioxide curing. The strength increase in reactive magnesia cements can be due to the incorporation of Mg2+ ions to the calcite crystals.

Flash calcination of magnesite in a one-throughput transport bed: reaction characterization and industrial justification

This study investigated flash calcination of magnesite powder in a one-throughput transport bed to clarify how the conversion and product activity as well as microstructure vary with reaction conditions including temperature, particle size, and numbers of re-calcination. Results showed that calcination of magnesite powder at 850–1000 °C is a quick reaction, allowing 99% of its containing MgCO3 to be decomposed in 1–2 s. Activity of product from the transport bed flash calcination (TBFC) is obviously higher than that from fixed bed calcination. Product activity depends on both decomposition rate and microstructure. During calcination, the surface structure of product changes from loose and porous to dense and smooth, even in a few seconds. A recently commissioned commercial TBFC plant indeed produced the light-burned magnesia according to the anticipation of this study, and it only requires a fuel consumption about 40 % lower than that of conventional reverberatory furnace.

Effects of reactive MgO on durability and microstructure of cement-based materials: Considering carbonation and pH value

Reactive MgO is a type of eco-friendly material with a lower carbon footprint, as it is produced by calcination of magnesite (MgCO3) at 750 ℃, much less than that (i.e. 1450℃) needed for calcination of (CaCO3) to produce clinker for Portland cement. Carbonation can enhance the mechanical properties of reactive MgO-based cementitious materials but will lower the pH value, leading to the neutralization of cement-based materials and further deterioration in rebar passivation. In this study, varying amounts of reactive MgO (0%, 10%, 20%, and 30%) were incorporated into mortars as cement replacements. By adjusting the duration of CO2 curing (i.e. carbonation curing) and standard curing, it was found that the curing regime involving 36 hours of carbonation curing plus 26.5 days of standard curing led to a higher pH and compressive strength of cement mortars. The pore size distribution was improved as the internal pores of the mortar were filled with carbonation products, which hindered the reaction between CO2 and alkaline hydration products inside specimens and was conducive to maintaining the alkalinity within the mortar. Therefore, the chloride ion penetration resistance and the mechanical properties of mortar were enhanced. SEM and EDS characterization results indicate that the excessive MgO did not fully participate in carbonation and continuously hydrated to generate Mg(OH)2. The expansion of Mg(OH)2 led to crack formation inside the mortar and reduced its strength, which accelerated CO2 diffusion and further reduced the pH value of reactive MgO-based cementitious materials.

Energy-saving calcination of hydromagnesite for sustainable magnesia-based cement: A new route towards MgO production

The MgO production worldwide is mainly achieved through the calcination of magnesite in the wake of higher energy requirements. This study proposes an energy-saving calcination strategy through low-temperature partially calcining hydromagnesite (4MgCO3·Mg(OH)2·4H2O). Furthermore, the feasibility of using the partially calcined hydromagnesite and slag to produce a sustainable cementitious material was investigated by various techniques. This approach requires relatively low temperature and short processing time with a potential energy saving of production and CO2 emission of fuel combustion. Moreover, it accelerates the hydration rate due to amorphous magnesium carbonate. Results indicated that the amorphous magnesium carbonate is formed during the partial calcination of hydromagnesite. It tends to transform to nesquehonite (MgCO3·3H2O) in the initial stage of hydration, which is responsible for promoting early hydration and excellent early compressive strength. Finally, the reaction mechanism of the partially calcined hydromagnesite-based cementitious materials has been established, demonstrating its enormous potential for preparing low-carbon cementitious materials.

Evaluation of the technical properties of reactive-MgO cements produced by solar calcination of magnesite in a fluidized bed reactor

Magnesium oxide (MgO)-based cements display very interesting technical properties and environmentally-friendly features. The novel idea investigated in this study is to synthesize MgO cements, using as raw material natural magnesite calcined in fluidized bed heated by concentrated solar energy. Calcination was performed in a lab-scale system equipped with a concentrated solar simulator, operated under different process conditions. The most reactive MgO was mixed with 3% by weight of MgCO3 (nucleation agent) and four different solutions containing magnesium acetate or chloride. The binders were hydrated in air or 20% CO2 atmosphere (accelerated carbonation conditions) until 28 days. X-ray diffraction, differential-thermal and mercury intrusion porosimetry analyses, and compressive mechanical strength tests, were performed on the hydrated systems. Solar calcination produced a highly reactive MgO. The performance of the cement pastes improved at higher curing times, and when using magnesium acetate as hydration agent, as also witnessed by the application of a kinetic model. Accelerated carbonation conditions further enhanced the mechanical properties of the cements thanks to the formation of nesquehonite, allowing to reach a mechanical strength comparable to that of ordinary Portland cements class 32.5. The achieved outcomes encourage the production of low-CO2 magnesite cements from solar calcined magnesite, boosting the green aspect of the entire process.

Experimental study on the kinetics of magnesium carbonate calcination under elevated heating rates

This paper reports an experimental investigation into the kinetics of magnesium carbonate calcination, conducted using a broad spectrum of heating rates (0.1°C/s to 179.9°C/s) in an air environment for a thin layer bed with Biot number < 0.1. These heating rates surpass those typically employed in thermogravimetric analysers (< 1°C/s) by many orders of magnitude. It was found that higher heating rates led to lower activation energies, as demonstrated by a 24.5% reduction in activation energy, E, from 220.8 kJmol−1 to 166.6 kJmol−1, for an increase in the heating rate from 0.1°C/s to 102.9°C/s. Furthermore, a higher heating rate led to a non-linear reduction in the residence time required to achieve the maximum conversion fraction. For instance, the time required for magnesite calcination to attain a conversion fraction from X=0.1 to X=0.84 decreased by a factor of eight, from 1920 s to 240 s, for only a three-fold increase in the heating rate, from 59.2°C/s to 179.9°C/s. These increases were associated with a higher specific surface area, highlighting the strong influence of the heating rate on the kinetics of magnesium carbonate calcination.It was also found that the highest specific surface area of cases examined for magnesite occurs for a heating rate of 59.2°C/s and a temperature less than 950°C, which avoids significant sintering. Increasing the temperature beyond 950°C reduces the specific surface area by causing sintering.

Recovery of ultra-high purity reactive magnesia from reject brine and its comparison with commercial magnesia

CO2 emissions and energy consumption associated with ordinary Portland cement production have increased the urgency for the development of alternative construction materials. Reactive magnesia cement (RMC) has been widely studied in the recent years due to the lower calcination temperature used during its production and its ability to permanently sequester CO2. In addition to the calcination of magnesite, reactive MgO, the main component of RMC, can also be synthesized from reject brine, which is rich in Mg2+. However, the presence of other components such as calcium in reject brine leads to the precipitation of other mineral phases together with Mg(OH)2, thereby reducing the purity of the final product. With the goal of improving the purity and yield of MgO synthesized at the end of this process, this study proposed a selective precipitation approach under a controlled pH. An optimum condition was determined for the synthesis of magnesium oxalate dihydrate with an ultra-high purity of 99.6 % and a high magnesium recovery rate of 94.1 %. Reactive MgO with a specific surface area of 30.2 m2/g was obtained after the calcination of the synthesized magnesium oxalate dihydrate at 700 °C for 2 h, which was higher than some of the commercially available MgO powders.

Recycling MgO from the waste magnesium oxychloride cement (MOC): Properties, CO2 footprint and reuse in MOC

Recycled magnesia was prepared via using magnesium oxychloride cement (MOC)-based waste as the raw material instead of magnesite through calcination under the temperatures from 600 °C to 1000 °C. The effects of calcination temperature on the components, microstructures, and properties of recycled MgO, and its influence on the performance of MOC containing the recycled MgO were investigated. Results indicated that the reactivity of recycled MgO depended on the calcination conditions, the active MgO content approached 80 wt% through the calcination of MOC-based waste in the temperature range of 700 °C–900 °C. The MOC paste was prepared by using 30 wt% MgO recycled under 800 °C exhibited higher mechanical performance, denser microstructure and more stable volume. The carbon emissions of recycled MgO are 0.49 t/t of magnesia, which is only 31.2% of that emitted by the calcination of magnesite via conventional dry route. This study provides an efficient approach to reducing the consumption of non-renewable natural resources and the generation of construction waste to realize cleaner and sustainable production of reactive MgO.

The natural magnesite efficacy on arsenic extraction from water and alkaline influence on metal release in water

Arsenic (As) removal studies were carried out through batch experiments to investigate the performance of the locally available calcined magnesite mineral rocks from Tanzania. Natural water from a stream source in Tanzania and the prepared synthetic water at the laboratory were used for the studies. Parameters such as initial As concentration, calcined magnesite dosage, contact time and pH were evaluated for As removal using an overhead rea×2 shaker. Arsenic concentration was reduced from 5.3 to 1.1 mg/L As(V) at 180 min when 0.5 g/L calcined magnesite was applied to a synthetic water sample, whereas the concentration of 117 μg/L As(V) and 5.2 μg/L As(III) was reduced to below 0.1 μg/L in natural water. An increase in calcined magnesite dosage resulted in increased As removal up to below 0.01 mg/L. The calcined magnesite raised the pH of the water sample from 6.8 to 10 when the applied dosage increased between 0.002 g/L and 0.05 g/L. The pH was constant at around 10 even when the amount of 0.05 g/L was added 2000 times. Despite the high pH, the amount of magnesium released in water was low. The calcination of magnesite at 500 °C increased surface area by 4 times as compared to the natural magnesite and X-ray diffraction showed presence of MgCO3 phase as the dominant phase at this temperature. The reaction kinetics of As removal on 0.5 g/L calcined magnesite fitted with the pseudo-second-order (R2 = 0.96). Reaction isotherm was strongly fitted with Freundlich isotherm (R2 = 0.98). Linear regression and artificial intelligence neural network showed the As removal was influenced by both contact time and pH. Arsenic can be removed from As water using calcined magnesite and will be suitable for water treatment around gold mining areas.

Struvite precipitation in wastewater treatment plants anaerobic digestion supernatants using a magnesium oxide by-product

Struvite precipitation is a well-known technology to recover and upcycle phosphorus from municipal wastewater as a slow-release fertiliser. However, the economic and environmental costs of struvite precipitation are constrained by using technical-grade reagents as a magnesium source. This research evaluates the feasibility of using a low-grade magnesium oxide (LG-MgO) by-product from the calcination of magnesite as a magnesium source to precipitate struvite from anaerobic digestion supernatants in wastewater treatment plants. Three distinct LG-MgOs were used in this research to capture the inherent variability of this by-product. The MgO content of the LG-MgOs varied from 42 % to 56 %, which governed the reactivity of the by-product. Experimental results showed that dosing LG-MgO at P:Mg molar ratio close to stoichiometry (i.e. 1:1 and 1:2) favoured struvite precipitation, whereas higher molar ratios (i.e. 1:4, 1:6 and 1:8) favoured calcium phosphate precipitation due to the higher calcium concentration and pH. At a P:Mg molar ratio of 1:1 and 1:2, the percentage of phosphate precipitated was 53–72 % and 89–97 %, respectively, depending on the LG-MgO reactivity. A final experiment was performed to examine the composition and morphology of the precipitate obtained under the most favourable conditions, which showed that (i) struvite was the mineral phase with the highest peaks intensity and (ii) struvite was present in two different shapes: hopper and polyhedral. Overall, this research has demonstrated that LG-MgO is an efficient source of magnesium for struvite precipitation, which fits the circular economy principles by valorising an industrial by-product, reducing the pressure on natural resources, and developing a more sustainable technology for phosphorus recovery.

Life cycle assessment of brucite and synthetic MgO produced from reject brine using different alkalis

A cradle-to-gate life cycle assessment (LCA) approach is carried out to determine the environmental impact of brucite (Mg(OH)2) and synthetic magnesium oxide (MgO) recovered from reject brine in the United Arab Emirates (UAE) using SimaPro 9.1 software. The environmental impacts of synthetic Mg(OH)2 and MgO production were quantified using 18 ReCiPe midpoint indicators. Four different alkali sources, calcium oxide (CaO), sodium hydroxide (NaOH), ammonium hydroxide (NH4OH), and ethanolamine (C2H7NO), which can be utilized for the production of synthetic MgO, are considered. The results demonstrate that CaO was the most environmentally friendly alkali that can be used to produce synthetic Mg(OH)2 with the lowest environmental impact for all 18 ReCiPe midpoint indicators. The results for brucite and synthetic MgO production in the UAE were scaled up from laboratory experiments for LCA analysis and compared to the results of commercial MgO produced from the calcination of magnesite (MgCO3). The carbon dioxide equivalent (CO2 eq.) emissions for synthetic MgO generated from reject brine using CaO are 6.7–25.2% higher than that from commercial MgO. However, synthetic brucite recovered from reject brine using CaO produces 28.8–37.1% less CO2 eq. emissions compared to commercial MgO, making it a promising magnesium-based binder that can be adopted in the UAE.

Reactive, Sparingly Soluble Calcined Magnesia, Tailor-Made as the Reactive Material for Heavy Metal Removal from Contaminated Groundwater Using Permeable Reactive Barrier

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).

Non‐isothermal kinetic study of high‐grade magnesite thermal decomposition and morphological evolution of MgO

AbstractThermal decomposition was the prerequisite and basis for the utilization of magnesite resources. However, the calcination of magnesite was usually accompanied by high energy consumption, which not only caused serious waste of magnesite resources, but also restricted its high value‐added utilization. Therefore, calcination conditions were the key to controlling thermal decomposition process of magnesite. The kinetics of high‐grade magnesite thermal decomposition was elaborated by non‐isothermal thermogravimetric analysis, and meanwhile the effect of heating rate on the magnesite thermal decomposition reaction and morphology of MgO particles were characterized. Both Doyle and Gorbatchev approximate functions were used to simulate the magnesite thermal decomposition process, where the experimental data (correlation coefficient) fitted by the latter could obtain more acceptable kinetic parameters of the magnesite thermal decomposition. The good linear relationship between the activation energy and the pre‐finger factor allowed for a kinetic compensation of the thermal decomposition of magnesite. Furthermore, higher heating rate induced the formation of terraced grains, grain network group, cubic grains, and spherical grains for the samples sintered at 1200°C. The heating rate largely affected the magnesite particle morphology, grain size distribution and activity, and also provided important technical indicators in the actual production of magnesia refractory raw materials.

Study of using dolomite ores as raw materials to produce magnesium phosphate cement

Although magnesium phosphate cement (MPC) is a type of chemically bonded ceramic with many vital properties, the practical application is limited by the high cost of raw materials. Dead burned MgO generally obtained from calcination of magnesite is used in MPC production. Dolomite as another abundant magnesium resource is widely distributed and has larger deposits than magnesite. This paper aims to replace magnesite with dolomite as raw materials to reduce the cost in MPC production. Quartz powders were also introduced in the calcination process to chemically combine with CaO components in dolomite. The effect of calcination temperature and dolomite-to-quartz mass ratio on the properties of calcined raw material were investigated. After calcination, the influence of dolomite-quartz powders under different calcination conditions on the performance of MPC pastes was assessed by observing the setting time, compressive strength, phase development and microstructure of hydration products as well as soundness. The calcination model of fine dolomite particles and coarse quartz particles was proposed to guarantee less free-CaO and more MgO after calcination. When the setting time of MPC prepared by the sample T1250-R1.5 was 16 min, the compressive strength of the pastes could reach 22 MPa at curing age of 3 h and 63 MPa at 7 d, with sufficient soundness and safety by preliminary judgment. The relatively lower calcination temperature at 1250 °C compared to conventionally prepared dead burnt magnesite would significantly reduce energy consumption and operating costs in production of magnesium bearing raw material for MPC.

Demonstration of a kW-scale solid oxide fuel cell-calciner for power generation and production of calcined materials

Carbonate looping (CaL) has been shown to be less energy-intensive when compared to mature carbon capture technologies. Further reduction in the efficiency penalties can be achieved by employing a more efficient source of heat for the calcination process, instead of oxy-fuel combustion. In this study, a kW-scale solid oxide fuel cell (SOFC)-integrated calciner was designed and developed to evaluate the technical feasibility of simultaneously generating power and driving the calcination process using the high-grade heat of the anode off-gas. Such a system can be integrated with CaL systems, or employed as a negative-emission technology, where the calcines are used to capture CO2 from the atmosphere. The demonstration unit consisted of a planar SOFC stack, operating at 750 °C, and a combined afterburner/calciner to combust hydrogen slip from the anode off-gas, and thermally decompose magnesite, dolomite, and limestone. The demonstrator generated up to 2 kWel,DC power, achieved a temperature in the range of 530–550 °C at the inlet of the afterburner, and up to 678 °C in the calciner, which was sufficient to demonstrate full calcination of magnesite, and partial calcination of dolomite. However, in order to achieve the temperature required for calcination of limestone, further scale-up and heat integration are needed. These results confirmed technical feasibility of the SOFC-calciner concept for production of calcined materials either for the market or for direct air capture (DAC).

A comparison of carbon footprints of magnesium oxide and magnesium hydroxide produced from conventional processes

In this study, modelling the carbon footprints of magnesium oxide and magnesium hydroxide (>99% purity) production based on technologies treating bischofite brines (e.g. Aman process) and serpentinite ores (e.g. Magnifin process) was performed. The two technologies have been utilised by many producers around the world to deliver specialty magnesium products. Using theoretical values of heat of reaction obtained from HSC (H-enthalpy, S-entropy and Cp-heat capacity) software simulations and the practical thermal efficiency of roasting and pyrohydrolysis equipment, greenhouse gas (GHG) emissions of 2.7–5.6 kg CO2eq/kg MgO and 1.6–3.3 kg CO2eq/kg Mg(OH)2 were estimated for the process treating a bischofite brine. The corresponding figures calculated for the process recovering magnesium values from a serpentinite ore were determined as 3.8–7.5 kg CO2eq/kg MgO and 2.6–5.2 kg CO2eq/kg Mg(OH)2. They are somewhat comparable to MgO's carbon footprint of 3.1–4.5 kg CO2eq/kg MgO from Chinese producers using one-stage magnesite calcination to produce caustic calcined magnesia (∼92% purity). From a carbon footprint perspective, it is apparent that the brine process provides the lowest environmental burdens compared to the serpentinite and magnesite routes.

Comparison of the Environmental Impacts of Reactive Magnesia and Calcined Dolomite and Their Performance under Different Curing Conditions

AbstractThis study compared two binder systems composed of reactive magnesite cement (RMC) and calcined dolomite (D800), which were produced via the calcination of magnesite and dolomite at 800°C, ...

An efficient SO2-adsorbent from calcination of natural magnesite

Abstract MgO-based adsorbents were prepared by controlling the temperature of magnesite calcination, and the adsorption mechanism of SO2 and these adsorbents were investigated. The obtained adsorbents were characterized by a combination of characterization techniques, including X-ray diffraction (XRD), transmission electron microscopy (TEM), N2 adsorption/desorption analysis and Hammett indicator method. XRD patterns and TEM images revealed that a complete decarbonation of MgCO3 occurred at ca. 650 °C, and the crystallinity and crystal size of MgO increased with the increase of calcination temperature. The maximum specific surface area (SBET = 69.6 m2/g) was obtained for the sample calcined at 650 °C (M-650), which also had the maximum value of base site density as revealed by the Hammett indicator tests. The adsorption capacity of adsorbents towards SO2 increased with the increase of the base site density and M-650 with the highest base site density displayed the maximum adsorption capacity. The adsorption of SO2 onto MgO was mainly chemisorption, resulting in the formation of magnesium sulfite and magnesium sulfate on the surface of MgO-based adsorbents, as indicated by X-ray Photoelectron Spectroscopy (XPS) and Fourier Transform Infrared (FTIR) spectra. The chemically bound SO2 could be desorbed at ca. 435 °C as revealed by thermal analysis under N2. This study suggests that calcination of naturally occurring magnesite is highly amenable to obtain efficient SO2 adsorbents.

Synthesis of Carbon Monoxide from Hydrogen and Magnesite/Dolomite

AbstractThe calcination of magnesite and dolomite is a state of the art technology step in minerals processing. The concomitantly produced carbon dioxide (CO2) is released into the flue gas, with all its negative effects on the global carbon balance. Reductive calcination of magnesite and dolomite to carbon monoxide (CO) and methane would open a window to reuse CO2 and produce value‐added carbonaceous products. The conversion of more than 70 % of the CO2 emitted during reductive calcination of the magnesite content of a mixed magnesite/dolomite (1:1 mol/mol) to CO at moderate temperature is feasible without admixture of catalyst.

Comparative life cycle assessment of reactive MgO and Portland cement production

Reactive magnesia (MgO) cements are proposed as a potentially sustainable binder due to their lower production temperatures (∼800 vs. 1450 °C) than Portland cement (PC) and ability to fully carbonate and gain strength during setting. Reactive MgO is mainly produced via the calcination of magnesite. Environmental implications of reactive MgO production must be analyzed before any final conclusions can be made regarding their contribution to the sustainability of the cement industry. This study evaluates the environmental impacts of reactive MgO production and provides a comparison with PC production using a life-cycle assessment (LCA) approach. The advantages of MgO production with respect to radiation, ozone layer, eco-toxicity, acidification/eutrophication, minerals and fossil fuels outweigh the disadvantages when compared to PC production. MgO has a lower impact on the overall ecosystem quality and resources than PC, but poses a larger damage to human health due to the high coal usage by most plants. The decomposition of magnesite releases a higher amount of CO2 than limestone (∼1.1 vs. 0.78–0.83 t/t), creating a higher climate change score for MgO despite its lower production temperatures. However, when the carbonation capability of MgO cements is considered, their net CO2 emissions are ∼73% lower than PC. The introduction of a strength ratio modification to MgO production results in ∼13% and ∼32% lower CO2 intensity and damage impact on human health when compared to PC production, respectively. The influence of energy type and amount on different impact and damage categories associated with MgO production was revealed by scenario modelling. The quantity of raw materials had a negligible effect on the overall environmental impact of reactive MgO production, whereas increasing the emissions led to an increase in climate change. Energy was identified as the key parameter with the highest influence on the environmental burdens of reactive MgO production. Results indicate that the use of alternative fuels could further improve the overall sustainability of reactive MgO cement production.