Calcined Dolomite Research Articles

Reduction of undesirable element leaching from fly ash by adding hydroxylated calcined dolomite

Fly ash always contains many toxic elements which can be released into environment, thereby easily leading to environmental contaminations. In order to dispose fly ash safely, related strategies are needed. In this investigation, two kinds of hydroxylated calcined dolomites (HCD60 and HCD100) were used as the additives and compared with lime on the leachabilities of anionic species from fly ash. Both additives were found effective in reducing the leaching concentrations of these elements, which was better than that of only lime addition. Mg(OH)2 and MgO were believed to play important roles in the hydration reaction of fly ash. In the presence of Mg(OH)2 and MgO, there were more hydration products including calcium silicate hydrate, ettringite, hydrocalumite and other Layered double hydroxides (LDHs) generated which were effective candidates for anion removal. Thus, the final leaching results were controlled by these newly formed phases through adsorption, incorporation or encapsulation. On the other hand, compared with Mg(OH)2, MgO can promote the formation of hydration products in a larger extent because of the hydration process of MgO into Mg(OH)2. There was no systematic trend in the promotion of fly ash hydration by Mg(OH)2 or MgO because it had a close relationship with the properties of original fly ash. Objectively, hydroxylated calcined dolomites can be promising candidate additives for reduction of toxic elements leaching from fly ash.

A systematic study on solid-state synthesis of monticellite (CaMgSiO4) based ceramic powders obtained from boron derivative waste

Solid-state synthesis of monticellite based ceramic powders from boron derivative waste in an eco-friendly route and the investigation of phase transformation mechanisms were studied. The heat-treatment of boron derivative waste, mainly composed of dolomite, calcite and quartz, was systematically carried out at 650 °C and 800 °C for various dwell times between 1 min and 4 h. The heat-treatment temperatures were selected considering TG-DTA curves of waste and ΔG – T diagrams obtained using FactSage Thermochemical Software. XRF, XRD, FTIR, TG-DTA and SEM analysis, particle size measurement and crystallite size determination carried out extensively. The obtained results showed that monticellite based ceramic powder synthesized at 800 °C for 4 h was composed of monticellite, akermanite, diopside, calcium magnesium borate and zeolite. The calcination of dolomite was completed both at 650 °C for 1 h and up to 800 °C, and calcite was decomposed at 800 °C for 30 min. Both diopside and monticellite were firstly detected at 650 °C for 30 min and at 800 °C for 1 min. Also, akermanite was presented at 800 °C for 1 min. The presented method offers the lowest temperature (800 °C) in literature for synthesis of monticellite and akermanite which reduces the energy consumption during heat-treatment.

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, ...

Alternative approach to the organization of hardening of dolomite binding materials

The results of experimental studies to determine the possibility of organizing the carbonate hardening of the dolomitic binder resulting from calcination of dolomite are presented. The optimum temperature of calcination of this type of raw material for obtaining a dolomitic binder with a high reactivity is established. The physicomechanical characteristics of the test samples that have been hardening in the medium ofhigh carbon dioxide concentration are determined. A forecast regarding the possibility of applying the results obtained in the production of building materials and carbonization hardening productsis made.

Solids mixing in a shallow cross-flow bubbling fluidized bed

Gas-solid, gas-catalytic and physical applications in a fluidized bed require a strict control of the solids residence time and a limited back-mixing for a required conversion. Since conversion proceeds with residence time, this residence time and its distribution (RTD) are essential design parameters in fluidized bed modeling. The experiments of the present research investigate the use of a shallow cross-flow bubbling fluidized bed as reactor. A tracer stimulus response technique was used to determine the RTD, in a fluidized bed with and without internals.Experimental results were compared with fittings from several models. Although a cascade of perfectly mixed reactors or a plug flow with dispersion model can be applied, the latter is preferred, and the dispersion parameter, expressed as Peclet number, exceeds ∼25.The results are moreover used in a test case design of hexane devolatilization from rice bran cake. With the RTD model of the horizontal fluidized bed, and with batch kinetic experiments, the size of the desolventizer can be designed on the basis of the required residence time. The superficial fluidization velocity applied is normally 3–4 times Umf. A similar RTD-approach can be used for alternative physical processes (e.g. drying) and for chemical reactions (e.g. calcination of dolomite) provided kinetic data from batch experiments are known.

Thermal Depolymerization of Biomass with Emphasis on Gasifier Design and Best Method for Catalytic Hot Gas Conditioning

This paper reviews ways that biomass can be converted by thermal depolymerization to make synthetic gas, i.e. syngas. Biomass, being carbon neutral, is considered as a form of solar energy stored during the growing season by photosynthesis. An effective biomass is one with low moisture and ash content, high lignin content, high calorific value, and small particle size. Woody biomass with low ash content (<1%), nut shells with high lignin content (30 to 40%), and municipal solid waste with synthetic polymers are effective at creating value-added synthetic gases. An allothermal downdraft gasifier produces a low tar syngas (99.9% tar conversion) at 850 oC and provides a simple and low-cost process. Integrated gasification combined cycle (IGCC) improves thermodynamic efficiency. To avoid thermal loss, a hot gas filtration system uses trona sorption material for sulfur and halogen compounds. Secondary systems can use multiple cyclones followed by reactors employing calcined dolomite, olivine, and others for adsorption or reaction with residual sulfur, ammonia, metals, and halogens. Reforming of residual tar to syngas can take place within chambers with ceramic tubes doped with nano-nickel particles. Syngas can then be used in boilers, gas turbines for production of electricity or production of chemicals by Fischer-Tropsch conversion.

Stabilization of bio-oil over a low cost dolomite catalyst

A low cost alkaline catalyst of dolomite (CaMg(CO3)2) was used to stabilize acacia sawdust bio-oil mixed with methanol. The upgrading efficiency was evaluated in terms of the total acid number (TAN) and viscosity. A change in the dolomite calcination temperature from 700 to 900 °C led to a significant change in the TAN and viscosity of the methanol-added bio-oil. Dolomite activated at higher temperatures had larger amounts of active CaO and MgO species due to the enhanced decarboxylation of calcium and magnesium carbonates. An increase in the dolomite content (1-5 wt%) decreased the TAN value of bio-oil remarkably. A thermal aging test of the methanol-added bio-oil upgraded using dolomite (calcined at 900 °C) at 50 °C for 24 h was carried out by storing the bio-oil at 80 °C for one week. Although the TAN value increased after the aging process, it was still lower than the TAN of raw bio-oil. In addition, increasing the methanol content (10-30 wt%) decreased the TAN and viscosity of the bio-oil significantly.

Performance of Dolomite Calcination in a Bench-Scale Rotary Kiln

Dolomite calcination is one of process steps to prepare calcined dolomite for raw materials in magnesium production. Calcination of dolomite involves heating the raw material at sufficient temperature in order to release the carbon dioxide from its carbonate minerals. This process is commonly conducted in a rotary kiln. There have been a number of calcination studies in a laboratory scale, but the study of dolomite calcination in a larger scale is very scarce. This research is aimed to study the performance of dolomite calcination in a bench-scale rotary kiln with 500 gram of feed. The effect of various parameters, including temperatures, feed rate, rotating frequency, and particle size were examined. The temperature of rotary kiln was varied between 700 and 1000 °C, while the particle size of dolomite was varied between 0.149 – 0.297 mm and up to 10 – 15 mm. The temperature distribution inside the rotary kiln was also measured. It is obtained that a conversion of 92% was attained at operation temperature of 1000 °C, which is at a higher temperature compared to the laboratory scale, where a conversion of 100% was obtained at 900 °C. This imply that the effect of heat transfer also plays important role in the calcination of dolomite especially at a larger scale.

Energy optimisation of vertical shaft kiln operation in the process of dolomite calcination

The essential part of the refractory materials production on a basis of sintered dolomite as raw material is the process of dolomite calcination. The technology process usually takes place in shaft or rotary kilns, where the dolomite stone, CaMg(CO3)2, is subjected to a high temperature heat treatment. The calcination of the dolomite is highly endothermic reaction, requiring significant amount of thermal energy to produce sintered dolomite (CaO, MgO), generating a large flow of hot gases at the furnace outlet. The objective of this work was to assess the possibilities of utilization of waste heat of exhaust gases from a shaft kiln in order to improve the overall energy efficiency of the technology process. Several different options were analyzed: (a) preheating of a raw material, (b) preheating of heavy fuel oil, (c) preheating of combustion air, (d) preheating of combustion air and raw material with flue gas, and (e) preheating of air for combustion and for drying of a raw material. Option (e) was selected as the most attractive and therefore it was analyzed in more details, showing significant annual energy savings and relatively short simple payback period on the investment.

Simple pyrolysis experiments for the preliminary assessment of biomass feedstocks and low-cost tar cracking catalysts for downdraft gasification applications

The pyrolysis behaviour of beech wood, two rice husk variants from Brazil (BRH) and Thailand (TRH) and a solid waste water treatment residue from textile manufacture (TIR) were investigated using a lab-scale, 2-stage fixed-bed reactor at 773 K. Char yields increased and volatile yields decreased with increasing ash content. The TRH released 40% less tar than the BRH which was attributed to the substantially higher potassium content of the Thai species. The combustion reactivity of the TRH char in air at 773 K was similar to the BW char and almost double the reactivity of the BRH and TIR chars. The BW and TRH chars had a greater volume of macropores indicating that char combustion occurs predominantly through the growth and extension of the macroporous pore network. A different trend was observed for the char gasification reactivity with CO2 at 1173 K. The Ca and Mg content of the chars were found to have a more important catalytic role in the char gasification reactions with CO2.The effect of exposing volatile products from beech wood pyrolysis to elevated temperatures (973–1173 K) and sand beds containing calcined limestone or dolomite in a simulated downdraft gasification environment was also investigated. Tar yields decreased after exposure to elevated temperature and calcined limestone or dolomite. Tar cracking favoured the production of CO. CO yields were between 22 and 23 wt% at 1173 K. Calcined dolomite was slightly more effective at cracking tar than calcined limestone, eliminating 98 wt% of the tar at 1173 K.

Upgrading bio-oil produced from the catalytic pyrolysis of sugarcane (Saccharum officinarum L) straw using calcined dolomite

In this study, the catalytic pyrolysis of sugarcane straw (SCS) into bio-oil and chemicals using calcined dolomite was applied for upgraded bio-oil production. Experiments were performed in a custom-built SS316 tube reactor, and the effects of the pyrolysis parameters, including the different dolomite calcination conditions, temperature (400-600°C), biomass feed rate (0.3–1.2kgh−1), sweeping gas flow rate (80–200cm3min−1) and average size distribution (250–1000µm), were systematically investigated. The results showed that the SCS catalytic pyrolysis process obtained liquid yields of 36.15wt%, gas yields of 52.09wt% and solid yields of 11.76wt% when using a pyrolysis temperature, biomass feed rate, nitrogen sweep gas, and average biomass size of 450°C, 0.6kgh−1, 80cm3min−1 and 500µm, respectively, with 10wt% calcined dolomite. The calcined dolomite influenced the bio-oil components from the carbonylation and the cracking of volatile vapor and resulted in an upgraded bio-oil with a lower oxygen content, higher gross calorific value and decreased acid corrosion.

The influence of silica nanoparticles addition on the physical, mechanical, thermo-mechanical as well as microstructure of Mag-Dol refractory composites

The high hydration potential of CaO and MgO phases restricted the application of Mag-Dol refractory composites. In this study, the impact of nano-silica (SiO2) addition on the physical, mechanical, thermo-mechanical as well as microstructure of Mag-Dol refractory composites is investigated. Mag-Dol compositions were prepared by using calcined dolomite and magnesite particles (micron, 0–1, 1–3, 3–5, and 5–8mm), liquid resin, and 0, 0.5, 1, 1.5, 2, and 2.5wt% nano SiO2 as additives. Specimens were heated up to 1650°C for the 3h soaking period. Fired specimens were characterized by physical (apparent porosity, bulk density, and hydration resistance), mechanical (cold crushing strength), and thermo-mechanical (flexural strength at 1200°C) measurements. XRD and SEM/EDS analysis were done to study phases and microstructure analysis of the fired samples, respectively. Results showed that by adding up to 2.5wt% nano-SiO2, due to the formation of CaO·MgO·2SiO2 (Diopside), 2CaO·MgO·2SiO2 (Akermanite), and CaO·MgO·SiO2 (Monticellite) phases, physical and mechanical properties were enhanced. But the highest flexural strength value is achieved for 1wt% nano-SiO2 containing sample.

CO2 capture performance of Ca-Mg acetates at realistic Calcium Looping conditions

The Calcium Looping (CaL) process, based on the cyclic carbonation/calcination of CaO, has emerged in the last years as a potentially low cost technique for CO2 capture at reduced energy penalty. In the present work, natural limestone and dolomite have been pretreated with diluted acetic acid to obtain Ca and Ca-Mg mixed acetates, whose CO2 capture performance has been tested at CaL conditions that necessarily imply sorbent regeneration under high CO2 partial pressure. The CaL multicycle capture performance of these sorbents has been compared with that of CaO directly derived from limestone and dolomite calcination. Results show that acetic acid pretreatment of limestone does not lead to an improvement of its capture capacity, although it allows for a higher calcination efficiency to regenerate CaO at reduced temperatures (∼900°C) as compared to natural limestone (>∼930°C). On the other hand, if a recarbonation stage is introduced before calcination to reactivate the sorbent, a significantly higher residual capture capacity is obtained for the Ca-Mg mixed acetate derived from dolomite as compared to either natural dolomite or limestone. The main reason for this behavior is the enhancement of carbonation in the solid-state diffusion controlled phase. It is argued that the presence of inert MgO grains in the mixed acetate with reduced segregation notably promotes solid state diffusion of ions across the porous structure created after recarbonation.

Influence of Ball Milling on CaO Crystal Growth During Limestone and Dolomite Calcination: Effect on CO2 Capture at Calcium Looping Conditions

The multicycle CO2 capture performance of CaO derived from the calcination of ball-milled limestone and dolomite have been tested under high temperature and high CO2 concentration environment for the first time. Here it is shown that the CO2 capture capacity of CaO is inversely related to the milling power applied to the starting mineral and the size of nascent CaO nanocrystals. In situ X-ray diffraction analysis used to follow the average crystallite size of CaCO3 and CaO during the calcination process as a function of temperature demonstrates that crystal growth is notably enhanced in a CO2-rich atmosphere for milled sorbents. Contrary to early reports suggesting improved reactivity towards carbonation of CaO from milled sorbents, promoted agglomeration, and crystal growth under these more “realistic” conditions lead to a severe deterioration of both capture capacity and recyclability, as observed from the multicyclic carbonation/calcination experiments. Yet the negative effect of milling is less pronou...

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.

Morphology modification in freshly Precipitated Calcium Carbonate particles using surfactant-polymer template

Morphology study of synthesized Precipitated Calcium Carbonate (PCC) is done using calcined dolomite (CaO.MgO). These PCC nano-composites were fabricated at ambient temperature using surfactant-polymer template: cationic surfactant cetyltrimethylammonium bromide (CTAB) with different polymers viz., polyvinyl alcohol (PVA), polyvinyl acetate (PVAc) and polyethylene glycol (PEG). In this work, we report the systematic characterization of the modified crystals using these templates by spectral, diffraction, scattering and microscopy techniques. Results obtained provide a reasonable explanation to the influence of the polarity of the polymer on to the templated nucleation where the inclusion of such molecules within the crystal lattice of the host growing crystal influences the overall crystal morphology. Scattering measurements further confirms and compliments the structural changes occupying in calcium carbonate particles with our microscopic findings.

Interfacial effects of MgO in hydroxylated calcined dolomite on the co-precipitation of borates with hydroxyapatite

Abstract Hydroxylated calcined dolomite (HCD) is used as a mineralizer to immobilize borates by co-precipitation with hydroxyapatite (HAp). HCD sometimes includes appreciable quantities of MgO depending on the degree of hydroxylation of the calcined dolomite. In this work, the interfacial effects of MgO in HCD on the removal kinetics of borates were examined using Ca(OH)2 and HCDs with different degrees of hydroxylation. As the fraction of MgO phases within the HCD increased, the borate immobilization rate stagnated because the formation of hydroxyapatite (HAp) was inhibited. This effect arose from the interaction of boric acid (H3BO3) with the MgO in HCD, followed by hydroxylation to cover the Ca(OH)2 surfaces with newly formed Mg(OH)2. In the presence of borate, the unstable intermediate [MgB(OH)4]+ complex was formed through ligand-promoted dissolution of MgO in the HCD. 11B NMR spectra revealed that the stagnation in immobilization of tetragonal boron ([4]B) species was observed for up to 45 min, accompanied by the suppression of HAp formation. TEM images of the HCD suggested that Ca(OH)2 was partially associated with MgO and Mg(OH)2 phases within the HCD and hydroxylation of MgO led to a thin covering of the surface of Ca(OH)2 with the newly formed gel phases comprising Mg(OH)2/H3BO3 that hinders the dissolution of Ca(OH)2. However, when MgO was completely hydroxylated in the HCD this delay was not observed. This was supported by zeta potential measurements on the solid residues during co-precipitation. The results suggest that the degree of hydroxylation of the calcined dolomite in HCD markedly influences the co-precipitation kinetics of borates in industrial water treatment.

Cracking of Model Tar Species from the Gasification of Municipal Solid Waste Using Commercial and Waste-Derived Catalysts

The cracking of model tar species from municipal solid waste gasification using dried sewage sludge char (DSS char) and bottom ash catalyst (BAC) was investigated, and the catalytic performance was compared to that of well-studied calcined dolomite, NiO/γ-Al2O3, and non-catalytic thermal cracking. The effects of the temperature, internal structure, chemical composition, and functional groups on the performance of tar cracking were characterized. The results showed that, when toluene was selected as the model tar species, conversion ratios for all catalysts were over 94% at 950 °C. The cracking efficiency was ordered as NiO/γ-Al2O3 > calcined dolomite > DSS char > BAC > thermal cracking. When the temperature increased from 750 to 850 °C, the conversion ratios for DSS char and BAC increased from 68.8 and 40.1% to 81.5 and 63.2%, respectively. H2 and coke were the major products of toluene cracking, and catalysts promoted the yield of hydrogen. The lower heating value of the product gas followed the same rul...

Ultrasound assisted biodiesel production in presence of dolomite catalyst

Ultrasound (US) assisted transesterification of canola oil in presence of heterogeneous catalysts calcined dolomite and CaO was investigated in comparison to each other. An US generator (200W, 20kHz) equipped with an horn type probe (19mm) was used to study the effect of catalyst amount (3–7wt.% of oil), methanol/oil molar ratio (4/1–15/1), ultrasound power (30–50W), temperature (25–60°C) and time (60–120min) on US assisted biodiesel synthesis. Biodiesel yield reached over 97.4% for calcined dolomite at the end of 90min and 95.5% for CaO at the end of 75min. According to the results, US improved the transesterification reaction by reducing necessary time for high biodiesel yield, using calcined dolomite as well CaO as heterogeneous catalyst.

OPERATING SPECIFICATIONS OF CATALYTIC CLEANING OF GAS FROM BIOMASS GASIFICATION

The paper focuses on the theoretical description of the cleaning of syngas from biomass and waste gasification using catalytic methods, and on the verification of the theory through experiments. The main obstruction to using syngas from fluid gasification of organic matter is the presence of various high-boiling point hydrocarbons (i.e., tar) in the gas. The elimination of tar from the gas is a key factor in subsequent use of the gas in other technologies for cogeneration of electrical energy and heat. The application of a natural or artificial catalyst for catalytic destruction of tar is one of the methods of secondary elimination of tar from syngas. In our experiments, we used a natural catalyst (dolomite or calcium magnesium carbonate) from Horní Lánov with great mechanical and catalytic properties, suitable for our purposes. The advantages of natural catalysts in contrast to artificial catalysts include their availability, low purchase prices and higher resilience to the so-called catalyst poison. Natural calcium catalysts may also capture undesired compounds of sulphure and chlorine. Our paper presents a theoretical description and analysis of catalytic destruction of tar into combustible gas components, and of the impact of dolomite calcination on its efficiency. The efficiency of the technology is verified in laboratories. The facility used for verification was a 150 kW pilot gasification unit with a laboratory catalytic filter. The efficiency of tar elimination reached 99.5%, the tar concentration complied with limits for use of the gas in combustion engines, and the tar content reached approximately 35 mg/m<sub>n</sub><sup>3</sup>. The results of the measurements conducted in laboratories helped us design a pilot technology for catalytic gas cleaning.