The Effect Of Composition On The Decomposition Behaviour Of Dolomite Nuggets (original) (raw)
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Journal of Materials Science, 2005
Many workers [1–9] studied the kinetics of dolomite decomposition to study the effects of different parameters like, gas (CO2, N2 etc.) pressures, water vapor, presence of other impurities, particle size and grain size of the dolomite samples, crystallinity etc. on the decomposition kinetics of dolomite using different tools like, thermal analysis, thermo-gravimetric analyses, XRD technique etc. and different values of the activation energies for the decomposition reaction, order of reactions have been reported. It has been observed that pure dolomite decomposed in only two steps. The first stage of the thermal decomposition of dolomite resulted in the formation of Mg-calcite [(CaMg)CO3] and periclase (MgO), with the liberation of CO2. It was further observed that under CO2, dolomite decomposed directly to CaCO3, accompanied by the formation of MgO between 550 and 765∘C. Calcite decomposed to CaO between 900 and 960∘C and under air, simultaneous formation of CaCO3, CaO and MgO accompanied dolomite decomposition between 700 and 740–750∘C. At the latter temperature, the calcite began to decompose even though a significant amount of dolomite was still present and simultaneous decomposition of the two carbonates was terminated at 780∘C. Also, changes in decomposition rates of the various phases correlated with changes in the rate of weight loss determined by derivative thermo-gravimetric analysis.
The decomposition of dolomite monitored by neutron thermodiffractometry
2013
Dolomite powder from Coín (Spain) was heated in air at a constant rate of 2°C/min to 1000°C, while neutron diffraction patterns were collected every 150 s. Rietveld refinement was applied and raw intensity data were used to monitor decomposition. The full process happened in two stages: dolomite decomposition to give calcite and periclase, and calcite breakup. The first stage activation energy was 47 kcal�mol �1 from fitting to a contracting sphere model. The dolomite mean thermal expansion coefficients were (6.7 � 0.4) � 10 �6 and (2.7 � 0.2) � 10 �5 K �1 along the a and c axes, respectively. Changes in the Ca–O and Mg–O bond distances were also measured. I.
The effect of impurities on the calcination behaviour of CaCO3 nuggets
The Journal of Engineering Research, 2017
Presence of impurities like SiO 2 , Al 2 O 3 and MgCO 3 affects the calcination behaviour of Limestone in lime kilns. Some research has been carried out to study this effect. The mechanical properties like Shatter and Abrasion index are found to improve slightly with increasing content of impurities. The rate of Calcination gradually reduces with time before attaining saturation, and Interfacial chemical reaction is identified as the rate controlling step. Activation energy is calculated from the slope of the plot: 1-(1-f) 0.5 vs time, where f is the extent of Calcination. Depending on the composition of nuggets, the Activation energy varies between 155.389 and 178.112 kJ/mol. The presence of SiO 2 and Al 2 O 3 is found to raise the apparent activation energy, E, while the presence of MgCO 3 lowers the value of E. The average increase in the apparent activation energy is found to be 2.38 and 1.71 kJ/mol per unit mass % increase in SiO 2 and Al 2 O 3 respectively and the average de...
Decomposition of Dolomite Monitored by Neutron Thermodiffractometry
2002
Dolomite powder from Coín (Spain) was heated in air at a constant rate of 2°C/min to 1000°C, while neutron diffraction patterns were collected every 150 s. Rietveld refinement was applied and raw intensity data were used to monitor decomposition. The full process happened in two stages: dolomite decomposition to give calcite and periclase, and calcite breakup. The first stage activation energy was 47 kcal⅐mol ؊1 from fitting to a contracting sphere model. The dolomite mean thermal expansion coefficients were (6.7 ؎ 0.4) ؋ 10 ؊6 and (2.7 ؎ 0.2) ؋ 10 ؊5 K ؊1 along the a and c axes, respectively. Changes in the Ca-O and Mg-O bond distances were also measured.
The dolomitization of CaCO3: an experimental study at 252–295°C
Geochimica et Cosmochimica Acta, 1977
The results of experiments on the hydrothermal dolomitization of calcite (between 252 and 295°C) and aragonite (at 252°C) by a 2 M CaCL-MgCl, aqueous solution are reported and discussed. Dolomitization of calcite proceeds via an intermediate high (ca. 35 mole %) magnesian calcite, whereas that of aragonite is carried out through the conversion of the reactant into a low (5.6 mole %) magnesian calcite which in turn transforms into a high (39.6 mole "i,) magnesian calcite. Both the intermediate phases and dolomite crystallize through a dissolution-precipitation reaction. The intermediate phases form under local equilibrium within a reaction zone surrounding the dissolving reactant grains. The volume of the reaction zone solution can be estimated from Sr*+ and MgZf partitioning equations. In the case of low magnesian calcite growing at the expense of aragonite at 252"C, the total volume of these zones is in the range of 2 x lo-' to 2 x lo-" I., out of 5 x 10m3 l., the volume of the bulk solution. The apparent activation energies for the initial crystallization of high magnesian calcite and dolomite are 48 and 49 kcal/mole, respectively. Calcite transforms completely into dolomite within 100 hr at 252°C. The overall reaction time is reduced to approximately 4 hr at 295°C. The transformation of aragonite to dolomite at 252°C occurs within 24 hr. The nature of the reactant dictates the relative rates of crystallization of the intermediate phases and dolomite. With calcite as reactant, dolomite growth is faster than that of magnesian calcite; this situation is reversed when aragonite is dolomitized. Coprecipitation of Sr*+ with dolomite is independent of temperature (within analytical error) between 252 and 295°C. Its partitioning, with respect to calcium, between dolomite and solution results in distribution coefficients in the range of 2.31 x lo-' to 2.78 x lo-*.
International Conference on Fluid Flow, Heat and Mass Transfer
We report on the decomposition of cylindrical dolomite (CaMg(CO 3) 2) particles through the two steps decomposition scheme with the objective of determining the thermophysical properties of the decomposition product in each step. Samples of size 24 and 32 mm were decomposed in a tube furnace at constant ambient temperature and at atmospheric pressure. By means of weighing and simultaneous temperature measurement, the decomposition behaviour is studied. Obtained decomposition behaviour are remarkably similar to that reported for pure magnesite in step 1 and pure limestone in step 2. Thermophysical properties of MgO•CaCO 3 and MgO•CaO at different temperatures were estimated by numerically solving the heat and mass transfer equations describing the process, based on the shrinking core model, using the finite difference approach. The reaction coefficient and thermal conductivity of MgO•CaCO 3 vary from 0.0040 to 0.0085 m/s and 0.79 to 0.92 W/m.K respectively and the permeability is approximately 1 x 10-12 m 2. The reaction coefficient and pore diffusivity of MgO•CaO are in the range of 0.013-0.014 m/s and 1.7 x 10-5-3.0 x 10-5 m 2 /s and the thermal conductivity is approximately 0.8 W/m.K.
Formation of calcium containing minerals in the low temperature dolomite ceramics
IOP Conference Series: Materials Science and Engineering, 2011
In order to elaborate low temperature dolomite ceramics, potentially suitable for the production of building materials, local low carbonate clay and dolomite siftings were used as a raw materials. Relationship between mechanical properties, mineral composition and firing temperature kept in the range of 600-800 o C were established. According to the obtained data it was detected that the optimal burning temperature, giving the highest crushing strength (40 MPa) was around 700-750 o C, optimal proportion of dolomite and clay expressed as ratio between CaO and Al 2 O 3-2.5 wt%. Gradual formation of C 3 A occurs during firing, yielding C 4 AH x , i.e. mainly C 4 AH 13 after hydratation of obtained composite material. The work was carried out in the frame of project "Innovative low temperature composite materials from local mineral deposits" (N o 2010/024/2DP/2.1.1.1.0/10/APIA/VIAA/152) financed by the European Regional Development Foundation (Activity 2.1.1.1.).
Experimental investigation of the breakdown of dolomite in rock cores at 100 MPa, 650-750 C
American Mineralogist, 2007
The kinetics of the breakdown reaction dolomite = periclase + calcite + CO 2 were investigated using cores of dolomitic marble. Two samples of Reed Dolomite from southwestern Nevada were cut into cylinders approximately 4×6 mm in size. The cores were sealed in gold capsules with isotopically enriched water (H 2 18 O or HD 18 O 0.5 16 O 0.5). The samples were heated in a cold-seal hydrothermal apparatus to 650-750°C at 100 MPa for durations ranging from 2-59 days. The cores were then sectioned and examined by EMP, XRD, and SIMS techniques. All experiments show some amount of reaction regardless of duration or temperature. Reaction products occur mainly along grain boundaries, fractures within grains, and along sample edges. Ion images and isotope-ratio analysis indicate that reaction products exchanged with infiltrating fluids. Reaction rates were calculated from measured extents of reaction, which were determined from modal analysis from optical point counting, automated EMP modal counts, and by CO 2 yield. At 700°C, a range of reaction rates from 6.6×10 −12 to 2.2×10 −10 mols/cm 2 /s was determined. The extent of reaction was found to have a log-linear relationship with the square root of time, suggesting a diffusion-controlled rate. Activation energies were 106.5 kJ/mol for coarse-grained samples and 202.0 kJ/mol for fine-grained samples. Initial reaction occurs relatively fast near the surface of dolomite grains, but continued diffusion through the reaction products ultimately controls the rate of dolomite breakdown.
Geology, Geophysics & Environment, 2015
In this work, characterization of dolomite powder was carried out in order to specify possible industrial applications. After the technological use of dolomite aggregates, the remaining fine powder becomes a waste. Raw and calcined powder samples were subject to mineralogical, textural and chemical studies involving leaching tests. The results of the calcination process indicate that the carbonate minerals present in the material sample undergo complete decomposition to form oxides. After the calcination, the material is practically non-porous, and its surface area is more than five times lower than that of the raw material. However, due to the high content of calcia in the calcined sample (CaO > 45% wt.), the material cannot be used as an additive in cement. The leaching tests showed that the concentration of metals released from the dolomite powder is low enough to classify the material as hazardous waste according to the TCLP test. Moreover, the concentration of metals that can get into the environment does not exceed permissible values as set by Polish law. Thus, it is recommended and justified to carry out detailed tests for the purpose of environmental protection; i.e. wet flue gas desulfurization, heavy metals absorption, and CO 2 capture.
Chemical Engineering Science, 1987
Measurements have been made of the rates of calcination of limestone particles (diam. 0.4-2.0 mm) in a fluid&d bed, electrically heated to a well-defined temperature. Experiments were conducted at atmospheric pressure and also at pressures of 3,6,12 and 18 bar, for bed temperatures varying from 1073 to 1248 K. The fluidising gases were air, or occasionally nitrogen, containing up to 20 vol. % CO1. The results indicate that under these conditions the rate of calcination of such limestone particles is controlled by chemical reaction at a sharp interface between CaCOJ and CaO. The temperature of this reaction zone is only a few degrees (< 15 K) below that of the fluidised bed. The rate of calcination is found to be of the form: Il(p&, -p&o, -Py,) kmol/m' s, where P&J, and p&, are, respectively, the partial ressures of COa for equilibrium at the temperature of the reactron interface and in the fluidising gas, [is the rate constant associated with the reverSe carbonation reaction (CO2 + CaO --+ CaCO,), P is the total pressure, and y, is a constant, which depends on the temperature of the bed. Values of k were measured. They appear to be independent of temperature, indicating that carbonation proceeds without an associated activation energy. It is hard to explain the appearance of y1 (an effective mole fraction for COz) in the above rate expression. The calcination of one dolomite has been briefly studied and calcination times etc. measured. In general, this a@pears to be a two-stage process, with the calcination of the MgCOl component being ir&nsitive to pressure, unlike the CaCO, . The value of E for CO1 + MgO -MgCOli is similar to that for the calcium case.