Calcination effect

For microporous compounds with special compositions, calcination effects are even more severe. As compared with zeolites, these compounds have lower thermal stability. Strictly speaking, most of them are nonporous since removal of the occluded guest molecules by calcination usually results in collapse. This is due to strong H-bonds with the framework, coordination bonds, and sometimes the templating molecule is shared with the inorganic polyhedra. Relevant examples of low-stability microporous compounds with interesting structural features are zeolitic open-framework phosphates made of Ga [178], In [179], Zn [180], Fe [181],... 

Lim, H.H. Gilkes, R.J. Beneficiation of apatite rock phosphates by calcination effects on chemical properties and fertilizer effectiveness. Aust. J. Soil Res. 2001, 39 (2), 397 02. 

Calcination is important in these circumstances. Crystallized salt redissolves when the dehydrated catalyst is exposed to moist environments and subsequent process drying may violate optimum conditions. Calcination converts the salt to an oxide or metal and essentially freezes the distribution. Other calcination effects, such as solid state reaction, also take place. 

For identical calcining conditions, different limestones may produce quicklimes with measurably different reactivities. While the differences in reactivity may not be significant in most operations, they could affect the suitability of the quicklimes for certain applications (e.g. aerated concrete). There may be several causes of such differences, e.g. rate of calcination, effects of impurities and the microstructure of the lime, (e.g. lime produced from oolitic and coarsely crystalline limestones is more prone to sinter and become less reactive [14.2]). 

Araujo LRR, Schmal M. The calcination effects on Pt/HZSM-5 catalysts in the aromatization of propane. Appl Catal A. 2000 203(2) 275. 

It is possible to modify the quality of the coke by calcination at high temperatures (1200-1400°C) this has the effect of reducing the volatile material and to increase the density. 

It is less well known, but certainly no less important, that even with carbon dioxide as a drying agent, the supercritical drying conditions can also affect the properties of a product. Eor example, in the preparation of titania aerogels, temperature, pressure, the use of either Hquid or supercritical CO2, and the drying duration have all been shown to affect the surface area, pore volume, and pore size distributions of both the as-dried and calcined materials (34,35). The specific effect of using either Hquid or supercritical CO2 is shown in Eigure 3 as an iHustration (36). 

Fig. 3. Effect of using either liquid or supercritical carbon dioxide on the textural properties of titania aerogels calcined at the temperatures shown. (—), dried with Hquid carbon dioxide at 6 MPa and 283 K (-------), dried with supercritical carbon dioxide at 30 MPa and 323 K. Reproduced from Ref. 36.

Fig. 7. The effect of preparation on the pore size distribution (a), titanium dispersion (b), and the activity for epoxidation of cyclohexene (c) of titania—siUca containing 10 wt % titania and calcined in air at 673 K. Sample A, low-temperature aerogel Sample B, high-temperature aerogel Sample C, aerogel.

The chemical and physical properties of limestone vary tremendously, owing to the nature and quantity of impurities present and the texture, ie, crystallinity and density. These same factors also exert a marked effect on the properties of the limes derived from the diverse stone types. In addition, calcination and hydration practices can profoundly influence the properties of lime. 

Stability. AH calcitic and dolomitic limestones are extremely stable compounds, decomposing only in fairly concentrated strong acids or at calcining temperatures of 898°C for high calcium and about 725°C for dolomitic stones at 101.3 kPa (1 atm). A very mild destabilizing effect is caused by C02-saturated water, as described in the preceding section on solubihty. Aragonite, however, is not as stable as calcite. In sustained contact with moisture,... 

Iron Browns. Iron browns are often prepared by blending red, yellow, and black synthetic iron oxides to the desired shade. The most effective mixing can be achieved by blending iron oxide pastes, rather than dry powders. After mixing, the paste has to be dried at temperatures around 100°C, as higher temperatures might result in the decomposition of the temperature-sensitive iron yellows and blacks. Iron browns can also be prepared directiy by heating hydrated ferric oxides in the presence of phosphoric acid, or alkaU phosphates, under atmospheric or increased pressure. The products of precipitation processes, ie, the yellows, blacks, and browns, can also be calcined to reds and browns. 

Mineral fillers are used for light-colored compounds. Talc has a small particle size and is a semireinforcing filler. It reduces air permeabihty and has htde effect on cure systems. Calcined clay is used for halobutyl stoppers in pharmaceutical appHcations. Nonreinforcing fillers, such as calcium carbonate and titanium dioxide, have large particle sizes and are added to reduce cost and viscosity. Hydrated siUcas give dry, stiff compounds, and their acidity reduces cure rate hence, their content should be minimized. 

Irregular-shaped particles exhibit greater surface area than regular-shapea cubes and spheres, the amount of this increase being possibly 25 percent. The effect of particle size and size distribution on effective surface, in a shaft employed for calcination of limestone, is shown in... 

The processing of diatomite is unique, since its particle-size control is effected by calcination treatments and air classification. 

Since theoretical calcination of effectiveness is based on a hardly realistic model of a system of equal-sized cylindrical pores and a shalq assumption for the tortuosity factor, in some industrially important cases the effectiveness has been measured directly. For ammonia synthesis by Dyson and Simon (Ind. Eng. Chem. Fundam., 7, 605 [1968]) and for SO9 oxidation by Kadlec et aJ. Coll. Czech. Chem. Commun., 33, 2388, 2526 [1968]). 

At elevated pressure, the partial pressure of carbon dioxide inhibits calcination, and siilfur dioxide is captured by displacement of the carbonate radical. The overall effect is similar except, as no free hme is formed, the resulting sorbent ash is less alkahne, consisting solely of CaS04 and CaC03. 

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