Dolomites reaction kinetics

Acidizing with HF finds no application in carbonates, as it forms solid calcium fluoride (CaF ) in limestone and both calcium fluoride and magnesium fluoride (MgF ) in dolomite. In any case, HF reaction in sandstones cannot be considered analogous to HCl reaction in carbonates. Whereas HF reaction in sandstones is controlled by the surface area of siliceous minerals—that is, by the surface reaction kinetics—HCl reaction in carbonates is controlled by the mass transport of acid to the mineral surfaces. In sandstones, the acid transport rate is high compared to surface reaction rates, and in carbonates, surface reaction rates are high compared to the acid transport rate. The slower rate step (acid transport or surface reaction) will control the reaction kinetics. Overall, HCl reactions in carbonates are much fester than HF reactions in sandstones. 

Carbonate Decomposition. The carbonate content of Green River oil shale is high (see Table 4). In addition, the northern portion of the Piceance Creek basin contains significant quantities of the carbonate minerals nahcoUte and dawsonite. The decomposition of these minerals is endothermic and occurs at ca 600—750°C for dolomite, 600—900°C for calcite, 350—400°C for dawsonite, and 100—120°C for nahcohte. Kinetics of these reactions have been studied (19). Carbon dioxide, a product of decomposition, dilutes the off-gases produced from retorting processes at the above decomposition temperatures. 

There have been several kinetic studies of the calcination of dolomite [29], a reaction of considerable technological importance. As in many reversible reactions, the rate of carbon dioxide release is sensitive to the prevailing pressure of this product (.Pco2) in the vicinity of the reaction interfaces. At low pressures (PCo2 < 12 Torr), reaction proceeds to completion in a single stage between 900 and 950 K... 

The decomposition of dolomite shows many points of similarity with the reactions of calcite and of other single carbonates of Group IIA metals (Sects. 3.1.1 and 3.1.2) the reaction is reversible, occurs at an interface, and both apparent kinetic parameters and reactivity are influenced by the prevailing C02 pressure. 

Thermal analysis has been widely and usefully applied in the solution of technical problems concerned with the commercial exploitation of natural dolomite including, for example, the composition of material in different deposits, the influence of impurities on calcination temperatures, etc. This approach is not, however, suitable for the reliable determination of kinetic parameters for a reversible reaction (Chap. 3, Sect. 6). 

Feldspar, among many natural substances such as termite mount-clay, saw dust, kaolinite, and dolomite, offers significant removal ability for phosphate, sulfate, and color colloids. Optimization laboratory tests of parameters such as solution pH and flow rate, resulted in a maximum efficiency for removal of phosphate (42%), sulfate (52%), and color colloids (73%), x-ray diffraction, adsorption isotherms test, and recovery studies suggest that the removal process of anions occurs via ion exchange in conjunction with surface adsorption. Furthermore, reaction rate studies indicated that the removal of these pollutants by feldspar follows first-order kinetics. Percent removal efficiencies, even under optimized conditions, will be expected to be somewhat less for industrial effluents in actual operations due to the effects of interfering substances [58]. 

Under the modest temperature and pressure conditions characteristic of the environments in which meteoric diagenesis typically takes place, many of the most important reactions are slow. This has severely constrained the study of the chemical mechanisms and kinetics involved in such fundamental processes as the aragonite to calcite transformation and dolomite formation. Information on these processes obtained under conditions not typical of the meteoric realm (e.g., elevated temperatures) are of questionable applicability to "real world" carbonate diagenesis. 

Here f is a char reactivity factor equal to one unless otherwise stated, while a represents the relative activity of HjO and C02 a typical value would be 3 [7]. Little is known about the kinetics of the reactions involving the CaO and CaS crystallites. Rates of CaS formation in calcined dolomite and limestone have been measured [8,9], but they are likely to be much slower than those possible on the crystallites. Therefore, we use ... 

Liittge A. and Metz P. (1991) Mechanism and kinetics of the reaction 1 dolomite + 2 quartz = 1 diopside + 2CO2 investigated by powder experiments. Can. Mineral. 29,... 

Observations of dolomite formation in natural systems have been used for defining additional factors that may influence the rate of dolomite formation. These include catalysis by certain clay minerals (e.g., Wanless, 1979) and production of organic by-products by bacteria (e.g., Gunatilaka et al, 1985). Mg transport to sites of dolomite precipitation can inhibit the reaction in hemi-pelagic sediments (e.g.. Baker and Burns, 1985). However, the true influence of reaction rates is largely speculative, because the kinetic factors are... 

Kinetics of Carbonate Coprecipitation Reactions and Solid Solution Formation The uptake of a cation into a carbonate has been studied by Davis et al. (1987), by Wersin et al. (1989), and by Stipp and Hochella (1991), Morse and Mackenzie (1990) have reviewed extensively the geochemistiy of dolomites and magnesian calcites. 

The product chemical composition analysis allows assessment of the dolomite catalyst selectivity to be made and is used to compute the heat of reaction and thereby provide a measure of the endotherm. The sink of heat of n-heptane is associated with hydrocarbon conversion to the equilibrium composition of the products where any variance could also be attributed to kinetics and differences in reaction products [13j. 

All the above problems can be applied to the upper part of the CFB gasifier. With an accurate fluid dynamic model the axial profiles of biomass, char, calcined dolomite and silica sand could be calculated, but the lack of accurate kinetic information on the effect of the concentration of these solids on the rate of all reactions involved avoids to calculate the axial profiles of the gaseous confounds (H2, CO, CO2... and tars). So, it... 

Simple salt reactants (131 entries). Articles concerned with decompositions of simple salts were often concerned with kinetic characteristics, many used non-isothermal data, and stoichiometric information was provided for some of these chemical changes. Several of these studies were concerned with determining trends of behaviour through comparisons between related salts. Detailed descriptions of the chemical steps and identifications of the rate-controlling processes in the mechanisms were less frequently provided. A small proportion of the papers was concerned with previously well-studied reactions such as the dissociations of carbonates (13 entries), including the effects of procedural variables on the decompositions of CaCOj (4 entries) and of dolomite (5 entries). 

The stability relationships between calcite, dolomite and magnesite depend on the temperature and activity ratio of Mg " /Ca " (Fig. 5d). Lower Mg/Ca activity ratios are required to induce the dolomitization of calcite and to stabilize magnesite at the expense of dolomite (Fig. 5d) (Usdowski, 1994). Formation waters from the Norwegian North Sea reservoirs have an average log(an g -/ cz- ) - TO to 0.0 and thus fall within the stability field of dolomite. Nevertheless, both calcite and dolomite are common cements in these rocks, indicating that dolomitization is a kinetically controlled reaction. Further evidence of this is revealed from Recent sediments, such as the Fraser River delta in Canada (Simpson Hutcheon, 1995) (log (aMg2+/aca=+) -2.2 to h-1.0), where the pore waters are saturated with respect to dolomite, but it is calcite rather than dolomite that precipitates. Calcite rather than dolomite forms below the deep>-sea floor, yet the pore waters plot at shallow, near sea bottom temperatures in the stability field of dolomite and shift with an increase in depth towards the stability field of calcite (Fig. 5d). This shift is due to a diffusion-controlled, downhole decrease in Mg/Ca activity ratio caused by the incorporation of Mg in Mg-silicate that results from the alteration of volcanic material, a process which is coupled with the release of calcium (McDuff Gieskes, 1976). 

Many natural aquatic systems have a chemical composition close to saturation with respect to calcite or even dolomite. This is the case, for instance, for seawater, which is usually slightly oversaturated in the upper part of the water column and slightly undersaturated at greater depths. Under these conditions, the rates of both precipitation and dissolution contribute significantly to the overall rate of reaction. Even though the reaction paths may be very complex, there is a very direct and important link between the kinetic rate constants, according to which the rates of forward and reverse microscopic processes are equal for every elementary reaction. The fundamental aspect of this principle forms the essential aspect of the theory of irreversible thermodynamics (Frigogine, 1967). 

Kinetics of the Reaction of Half-Calcined Dolomite with Sulfur Dioxide... 

Kinetics of the reaction of sulfur dioxide with half-calcined dolomite have been studied using gravimetric techniques. The reaction rate depends significantly on the presence of water in the reactant gas mixture. With water, the reaction is first order with respect to the sulfur dioxide concentration. Without water, the reaction rate is slower, and the reaction is 0.76 order with respect to sulfur dioxide concentration. This suggests that the rate-determining step differs depending on whether or not water is present. The reaction has an apparent activation energy of 7.3 kcal/mole with water present in the reactant gas. 

Further work is needed to understand the role of water in the sulfation mechanism more fully and to extend the kinetic studies to the reduction and regeneration reactions outlined above. The potential advantages of a process using dolomite in a closed cycle for sulfur dioxide control are sufficiently great to warrant continued effort. 

Norris TH (1950) The kinetics of isotopic exchange reactions. J Phys Colloid Chem 54 777-783 Northrop DA, Clayton RN (1966) Oxygen isotope fractionations in systems containing dolomite. J Geol 74 174-196... 

The COt Acceptor Gasification Process is discussed in light of the required properties of the CaO acceptor. Equilibrium data for reactions involving the CO% and sulfur acceptance and for sulfur rejection jit the process requirements. The kinetics of the reactions are also sufficiently rapid. Phase equilibrium data in the binary systems CaO-Ca(OH)t and Ca(OH)jr-CaCOs show the presence of low melting eutectics, which establish operability limits for the process. Data were obtained in a continuous unit which duplicates process conditions which show adequate acceptor life. Physical strength of many acceptors is adequate, and life is limited by chemical deactivation. Contrary to earlier findings both limestones and dolomites are equally usable in the process. Melts in the Ca(OH)2-CaC03 system are used to reactivate spent acceptors. 

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