Dolomite decomposition

Figure 3.2 Typical DTA trace showing the decomposition of dolomite (CaMg(C03)2). The endotherm peaked at 650°C corresponds to the decomposition of free MgC03 in magnesitic [1] dolomite. The endotherm peaked at 805°C coincides with the decomposition of dolomite CaMg(C03)2(,) = CaC03(,) + MgO( )+ C02(s). The final endotherm peaked at 920°C corresponds to the decomposition of calcite (freed from the previous dolomite decomposition) CaC03(,) = CaO(,j + C02(s).

Two Colorado oil shale samples one from the Parachute Creek Member and the other from the C-a tract, were retorted, de-charred and then subjected to temperatures between 800 K and 1100 K in order to study the mineral reactions which take place. Comparisions between these two samples include the reversible nature of ankeritic dolomite and free calcite as well as the temperatures at which significant silication takes place. Results for the C-a tract samples indicated silication appears to take place in stages and that ankeritic dolomite decomposition can be prevented by relatively low CO2 concentrations. Ankeritic dolomite and calcite decomposition rates were similar for the two samples and there was strong evidence that calcite recarbonation takes place via non-activated chemisorption of C(>2 ... 

That is, since dolomite decomposition was unaffected by the presence of C02 and calcite decomposition can be prevented by a sufficient CO2 over-pressure, Equation (4) was expected to prevail if ankerite decomposition was carried out at low temperatures (< 900 K) in a CO2 environment. Figure 4 shows the results for ankerite decomposition carried out at two different temperatures with and without CC>2 Note that the presence of CO2 completely prevents decomposition at 853 K and severely inhibits decomposition at 900 K. 

In comparing these shales, the most significant finding was that ankeritic dolomite decomposition could be prevented in the C-a sample below 930 K, with relatively low concentrations of C02 The enthalpy of reaction for Equation (4) is... 

Equation (6), dolomite decomposition," is irreversible and takes place at T >875K. Equation (7), "calcite decomposition," is reversible and can be prevented if there is a sufficient CO2 overpressure. Equation (8), "silication," is irreversible and takes place at higher temperatures (>1050K) provided that calcite decomposition is prevented. Equation (9) occurs at lower temperatures and is significant because the iron oxides that result can... 

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. 

The decomposition of MgC03 (magnesite) is an interface process [734] between 813—873 K and E = 150 kJ mole-1. In the presence of C02, E was increased to 234 kJ mole-1 but was reduced slightly on the addition of ZnO or NiO. Admixture with CaO reduced the value of E to 54 kJ mole-1. This is a surprising result since the value of E for decomposition [734,753] of the mixed carbonate (Ca, Mg)C03, dolomite, is 220 kJ mole-1, larger than the value for each constituent. The influence of PCOz and of alkali metals on MgC03 decomposition has been the subject of a DTA study [404]. 

Even higher temperatures are required for calcite dissociation. As f>co2 is increased to 760 Torr, the reaction temperature rises to 1170 K and the extent of dissociation is diminished [29]. The rate of decomposition of dolomite in vacuum [734] was intermediate between those for magnesite and calcite. Ranges of study were magnesite 810—870 K, dolomite 910— 990 K, and calcite 990—1050 K. Values of E were in the different sequence, magnesite < calcite < dolomite. Magnesite, which would decompose very rapidly at the temperature of dolomite dissociation, is, therefore, relatively stabilized, whereas the reactivity of calcite is enhanced in the mixed crystal. 

It was concluded [734] from visual inspection and chemical analysis of partially decomposed dolomite, that reaction was initiated at the outer surfaces of the crystallites and the interface established advanced thereafter into the bulk. The deceleratory a—time curves obeyed the contracting volume equation [eqn. (7), n = 3] and the values of E determined were between 206 and 232 kJ mole-1. These values of E were generally greater than those reported for other studies ( 190 kJ mole-1) which are in the range mentioned [121] for CaC03 dissociation and slightly larger than the enthalpy of that reaction. On exposure of the residue from vacuum decomposition of dolomite to C02, the gas uptake at 1070 K was... 

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. 

Powell and Searcy [1288], in a study of CaMg(C03)2 decomposition at 750—900 K by the torsion—effusion and torsion—Langmuir techniques, conclude that dolomite and C02 are in equilibrium with a glassy phase having a free energy of formation of (73 600 — 36.8T)J from 0.5 CaO + 0.5 MgO. The apparent Arrhenius parameters for the decomposition are calculated as E = 194 kJ mole-1 and activation entropy = 93 JK-1 (mole C02)-1. 

F.20 Dolomite is a mixed carbonate of calcium and magnesium. Calcium and magnesium carbonates both decompose on heating to produce the metal oxides (MgO and CaO) and carbon dioxide (C02). If 4.84 g of residue consisting of MgO and CaO remains when 9.66 g of dolomite is heated until decomposition is complete, what percentage by mass of the original sample was MgC03 ... 

Metallic magnesium is produced by either chemical or electrolytic reduction of its compounds. In chemical reduction, first magnesium oxide is obtained from the decomposition of dolomite. Then ferrosilicon, an alloy of iron and silicon, is used to reduce the MgO at about 1200°C. At this temperature, the magnesium produced is immediately vaporized and carried away. The electrolytic method uses seawater as its principal raw material magnesium hydroxide is precipitated by adding slaked lime (Ca(OH)2, see Section 14.10), the precipitate is filtered off and treated with hydrochloric acid to produce magnesium chloride, and the dried molten salt is electrolyzed. 

The compound is very unstable, and its presence may be one of the causes of the observed explosive and apparently spontaneous decomposition of calcium hypochlorite, if produced from magnesia-containing lime (derived from dolomite). 

The nucleation of these decomposition processes was studied by means of thermomicroscopy on single crystal cleavage plates of calcite, magnesite, dolomite and smithsonite (Fig. 59). The shape of the nuclei was found to be different for these carbonates, which might be also of importance for the decomposition mechanism. The partial pressure of water vapor has a pronounced effect on the decomposition of transition metal carbonates such as ZnC03 and CdC03. The evolution of C02 is probably catalyzed in the presence of water vapor and shifted to considerably... 

Uses. Production of chlorinated organic chemicals production of dyes and dye intermediates steel pickling oil well acidizing operations to dissolve subsurface dolomite or limestone formed during thermal decomposition ofPVC... 

If dolomite is the source, thermal decomposition of MgCOs at 350°C produces MgO. At this temperature, CaCOs does not decompose. The decomposition temperature for the latter is 850°C. 

K. J. D. MacKenzie and R. H. Meinhold, Thermal decomposition of dolomite (calcium magnesium carbonate) studied by Mg solid-state nuclear magnetic resonance. Ther-mochim. Acta, 1993, 230,331-337. 

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