Magnesium carbonate, decomposition

Discernible endothermal effect for magnesium carbonate decomposition is evident even at a dolomite content of 0.3%. The activation energy of limestone decomposition has a strong bearing on the clinker formation. The de-carbonation of limestone varies, depending on the type, particle size, and impurities. DTA has been used to test the effect of fineness on the disassociation of limestones from different sources. It has been concluded that the finer the grain sizes and more the impurities, the lower is the activation energy. 

Fig. 15. Basic magnesium carbonate decomposition-pressed at 900°C in vacuum showing relic structures, well developed crystallites, and porosity.

Fig. 16. Basic magnesium carbonate decomposition-pressed at 1000°C and refired to 1500°C showing pore morphology typical of reheated material.

Fig. 18. Basic magnesium carbonate decomposition-sintered at 1200°C in vacuum showing grain boundary erosion from polishing.

Pyrotechnic mixtures may also contain additional components that are added to modify the bum rate, enhance the pyrotechnic effect, or serve as a binder to maintain the homogeneity of the blended mixture and provide mechanical strength when the composition is pressed or consoHdated into a tube or other container. These additional components may also function as oxidizers or fuels in the composition, and it can be anticipated that the heat output, bum rate, and ignition sensitivity may all be affected by the addition of another component to a pyrotechnic composition. An example of an additional component is the use of a catalyst, such as iron oxide, to enhance the decomposition rate of ammonium perchlorate. Diatomaceous earth or coarse sawdust may be used to slow up the bum rate of a composition, or magnesium carbonate (an acid neutralizer) may be added to help stabilize mixtures that contain an acid-sensitive component such as potassium chlorate. Binders include such materials as dextrin (partially hydrolyzed starch), various gums, and assorted polymers such as poly(vinyl alcohol), epoxies, and polyesters. Polybutadiene mbber binders are widely used as fuels and binders in the soHd propellant industry. The production of colored flames is enhanced by the presence of chlorine atoms in the pyrotechnic flame, so chlorine donors such as poly(vinyl chloride) or chlorinated mbber are often added to color-producing compositions, where they also serve as fuels. 

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 ... 

The same may be observed with magnesium carbonate in cellulose [70] (Figure 26). The chemiluminescence intensity at a given temperature increases with pH of the sample almost linearly. As it is evidenced by DSC, the sample with pH 7.2 is the least stable. Figure 26 is also a demonstration of the much higher sensitivity of the chemiluminescence method when compared with DSC. DSC exotherms, which accompany the final stages of the cellulose decomposition... 

The endothermic decomposition of solid magnesium carbonate produces solid magnesium oxide and carhon dioxide gas. For each mole of magnesium carbonate that decomposes, 117.3 kJ of energy is absorbed. As for an exothermic reaction, there are three different ways to represent the enthalpy change of an endothermic reaction. 

Figure 5.5 shows how the decomposition of solid magnesium carbonate can be represented graphically. 

The thermochemical equation for the decomposition of magnesium carbonate, shown above, indicates that 117.3 kJ of energy is absorbed when one mole, or 84.32 g, of magnesium carbonate decomposes. The decomposition of two moles of magnesium carbonate absorbs twice as much energy, or 234.6 kJ. 

Direct experimental determinations of these quantities do not exist. The nearest approach seems to be in some observations made by Nicolson (26) in his work on surface tension. He found that when he made magnesium oxide particles by burning magnesium in air, their lattice constants were the same as those of the bulk material. When the crystals were made by the decomposition of magnesium carbonate in vacuo, the expected change in lattice parameter took place due to the surface tension. These negative results obtained in the first method of preparation were attributed to the presence of gases adsorbed from the air. 

Recently, the influence of the preparation method of various MgO samples on their catalytic activity in the MPV reaction of cyclohexanone with 2-propanol has been reported 202). The oxides were prepared by various synthetic procedures including calcination of commercially available magnesium hydroxide and magnesium carbonate calcination of magnesium hydroxides obtained from magnesium nitrate and magnesium sulfate sol-gel synthesis and precipitation by decomposition of urea. It was concluded that the efficiency of the catalytic hydrogen transfer process was directly related to the number of basic sites in the solid. Thus, the MgO (MgO-2 sample in Table IV) prepared by hydration and subsequent calcination of a MgO sample that had been obtained from commercially available Mg(OH)2 was the most basic and the most active for the MPV process, and the MgO samples with similar populations of basic sites exhibited similar activities (Table IV). 

Materials that decompose at elevated temperatures with the absorption of heat (endothermic decomposition) can work well as rate retardants. Calcium and magnesium carbonate, and sodium bicarbonate, are sometimes added to a mixture for this purpose. 

K. J. D. MacKenzie and R. H. Meinhold, A Mg magic-angle spinning nuclear magnetic resonance study of the thermal decomposition of magnesium carbonate. /. Mater. Sci. Lett, 1993,12,1696-1698. 

There is other behavior that is different. Most noticeable is a much lower carbon dioxide generation rate in the lean shale. Only very long exposures at high temperatures resulted in calcium carbonate decomposition. In most of the runs only 65% of the magnesium carbonate decom-... 

Hydroxides, hydroxy carbonates, and hydrates of aluminum, calcium, and magnesium that potentially meet these requirements are shown in Table 7.1, together with relevant thermal properties and gaseous products evolved on decomposition. However, of those in commercial use, aluminum hydroxide makes up about 90% of the market by tonnage, with magnesium hydroxide and basic magnesium carbonate products being used in niche applications. 

A classic example of a solid—fluid ceramic powder synthesis reaction is that of calcination and dehydration of natural or synthetic raw materials. Calcination reactions are common for the production of many oxides from carbonates, hydrates, sulfates, nitrates, acetates, oxalates, citrates, and so forth. In general, the reactions produce an oxide and a volatile gaseous reaction product, such as CO2, SOg, or HgO. The most extensively studied reactions of this type are the decompositions of magnesium hydroxide, magnesium carbonate, and calcium carbonate. Depending on the particular conditions of time, temperature, ambient pressure of CO2, relative humidity, particle size, and so on, the process may be controlled by a surface reaction, gas diffusion to the reacting... 

MgC03+K2C03-f CO2+5H2O, or treated with magnesium hydroxide below 20 . The soln. of potassium carbonate is filtered ofi the magnesium carbonate and carbon dioxide are used for the treatment of more potassium chloride. The decomposition proceeds better under half an atm. press., and at a temp, over 115 , since the magnesium carbonate is then in a form which filters easily, and the formation of bicarbonates is avoided. 

Magnesium carbonate is decomposed to oxide and CO2 from 400 °C upwards, and the decomposition is rapid at 600 C. However, the product is highly porous and... 

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