Calcining variables

Aluminium fluoride (anhydrous) [7784-18-4] M 84.0, m 250°. Technical material may contain up to 15% alumina, with minor impurities such as aluminium sulfate, cryolite, silica and iron oxide. Reagent grade AIF3 (hydrated) contains only traces of impurities but its water content is very variable (may be up to 40%). It can be dried by calcining at 600-800° in a stream of dry air (some hydrolysis occurs), followed by vacuum distn at low pressure in a graphite system, heated to approximately 925° (condenser at 900°) [Henry and Dreisbach J Am Chem Soc 81 5274 1959]. 

Table 21 reports the ash content and ash composition (determined by inductively coupled plasma-atomic emission spectroscopy, ICP-AES) for all of the calcined cokes used to fabricate the test graphites. It can be seen that the amount of ash and its make-up are variable, but are within the range observed for petroleum-based calcined cokes. Although the ash contents in all of the calcined cokes appear rather high, these materials may still be acceptable because many of the metallic species are driven off during graphitization. This aspect is addressed in the next section. 

Two synthesis variables seemed to have received most attention in the work reviewed here, the cation composition and the nature and source of the aluminosilicate reactant. Extensive use of mixed bases of the alkali, alkaline earth, and organic cations have been reported as well as a wide variety of reactant aluminosilicates including solutions, hydrogels, glasses, kaolin (raw and calcined), and naturally occurring zeolites. 

Ambs and Flank correctly observed that variables can be introduced into the calcination of ammonium Y so that a variable series of products can be obtained 33). However, there is no doubt that the normal hydrogen zeolite can be obtained from the ammonium form by carefully controlled calcination. In addition, carefully controlled calcination of the acid yields the dehydroxylated form. The ultrastable form, which can be prepared by a number of procedures described below, differs drastically in stability and composition from the other two forms. That it may contain some sites similar to, or perhaps identical with, sites in the hydrogen and dehydroxylated forms cannot be refuted. Unquestionably, however, the ultrastable form differs significantly from the other two forms. 

Side rite. FeCO-,. corresponding to 48.2 X P e. 51.86/ CO , sp gr 3.83 3.88. white to greenish-gray and brown, contains variable amounts of calcium, magnesium, and manganese, varies from dense, line-grained and compact to crystalline, sometimes referred to as spathic iron ore. or black-band ore. The carbonate ores are calcined... 

The performance of a catalyst is well known to be sensitive to its preparation procedure. For this reason, ideally an oxide-supported metal catalyst should be subjected to a number of characterization procedures. These may include measurements of the metal loading within the overall catalyst (usually expressed in wt%), the degree of metal dispersion (the proportion of metal atoms in the particle surfaces), the mean value and the distribution of metal particle diameters, and qualitative assessments of morphology including the particle shapes and evidence for crystallinity. These properties in turn can depend on experimental variables used in the preparation, such as the choice and amounts of originating metal salts, prereduction, calcination or oxygen treatments, and the temperature and duration of hydrogen reduction procedures. 

Natural umbers contain somewhat variable amounts of hygroscopic water (up to 20% or more), and the loss on ignition usually varies from 7 to 11%. The dried earth contains much less hygroscopic moisture (about 6%),but the whob of the combined water, so that the loss on Calcination is the same as with the natural earths. The burnt earth contains little hygroscopic water (usually less than 5%) and give a loss of 3-4% on calcination. In general it may be said that when an umber loses less than 5% on calcination, it is burnt —this independently of the hygroscopic water, which may be considerable even in burnt umber if this has been stored in a moist place. 

The appearance of the hydroxyl bands at 3650 and 3550 cm-1 upon heating the ammonium form accompanies the decrease and disappearance of the NH-stretching bands as ammonia is evolved. The rate of decomposition of the ammonium ions appears to be influenced by the calcination conditions. Ward observed that most of the ammonium ions decomposed between 200° and 350°C, and at 420° only discreet hydroxyl bands were present (148). With extensively exchanged samples (>90% of the exchange sites occupied by ammonium ions), the 3550-cm I band was more intense than that at 3650 cm-1, in contrast to the intensity relationship observed at lower ammonium-exchange levels. Angell and Schaffer also noted the variability of the relative intensities of the two bands with different extents of ammonium ion exchange. 

It is difficult to predict a priori which preparative method will produce the most active and selective catalyst or which preparative method will affect which, if any, of the previously mentioned properties. A great number of recipes have appeared in the patent literature, but any detailed description of the methods which yield the most active and selective catalyst, at least from a commercial viewpoint, remains proprietary. Of course, this makes it very difficult to make comparisons between experimental catalysts and commercial catalysts. Nevertheless, a number of general chemical variables have been identified as important in attempting to produce a specific catalyst. For example, for molybdate catalysts prepared by precipitation, these variables include the temperature of the precipitation, the concentration of the reagents, the aging of the precipitate, and the temperature of the calcination (6J). For supported catalysts, the nature of the support also becomes an important variable in determining the final catalytic activity and selectivity. 

Calcination The rate of heat schedule as well as the final temperature employed are important variables. A high enough temperature must be employed to ensure decomposition of the salts used in the preparation. At sufficiently high temperature, some solid-state reactions are engendered. Further, differences can be obtained whether using forced... 

Catalyst preparation and activation conditions are included as parameters of importance in the optimisation algorithm. The preparation and activation procedures are very relevant aspects since minor variations in such conditions would cause major changes to the final phase of the solid and, consequently, to its catalytic properties. Typical preparation variables are promoter precursors, type of impregnation, calcination atmosphere, time and temperature, time and temperature for metal reduction and so forth. 

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