Crystal size Crushing

Crystal Size Measurement. The size distribution of the sieve crystals used also was measured in some auxiliary experiments. This was done by first suspending the sieve sample (crystal powder or crushed pellets) in a sodium hydroxide solution (to help disperse the easily-formed crystal agglomerates) and then examining the solution with a calibrated optical microscope at a magnification of 5000. About 250 particles picked at random on a slide were measured for each sample. The particles, which have a shape somewhere between that of a sphere and a cube, were treated as cubes with the length of a side equal to 2a microns. 

Quality. The material on the market often has coarse grains of 3 5 mm in size. The difference in the crystal size does not seem to have much influence upon the duration of the smoking. But it is better to use it after crushing it to pass I6 mesh to avoid delay in the initiation of the smoke composition. 

Three zeolites ZSM-5 with different Si/Al ratios and crystal sizes L (table 1) were modified by ion exchange with 0.005 M metal acetate solutions (Co, Ni, Pd, Cu, Ag, Zn) for 12h at 300 K. The zeolites were W2tshed and dried at 393K for Ih. After calcination at 823K in air for 6h, the powders were pressed into pellets (1900 bar) without binder. The pellets were crushed and size fractions of 0.3 to 0.5 mm were obtained by sieving and used as catalysts. 

Reactant particle size distributions Kinetic characteristics of some reactions of solids depend sensitively on reactant particle sizes (29). Ideally, reactants to be used in kinetic studies should be composed of crystallites of identical (known) sizes and shapes, to which the geometry of interface advance can be related quantitatively. This is not, however, always (or easily) achieved in experimental studies, and most powder samples contain particles of disparate sizes for example, sometimes crystals are mixed with fine powder. The kinetic model giving the best apparent fit to the data then may not accurately represent the reaction. Dependencies of rate on particle size are only rarely investigated. The state of subdivision of a solid reactant is most frequently described in literature reports only by qualitative terms, such as single crystals or crushed powder. 

A chemistry student is investigating how particle size affects the rate of dissolving. In her experiment, she adds a sugar cube, sugar crystals, or crushed sugar to each of three beakers of water, stirs the mixtures for 10 seconds, and records how long it takes the sugar to dissolve in each beaker. 

Samples of quartz crystals were crushed into micron size particles using vibro-set with W-carbid. They did not indicate the condition of the pounding of the crystals although it is well known that crushing of the optically active quartz particularly with vibro-mil can cause twinnings formation, amorphisation of quartz, and its hydrophilisation... 

At the alumina plant, the bauxite ore is further crushed to the correct particle size for efficient extraction of the alumina through digestion by hot sodium hydroxide liquor. After removal of "red mud" (the insoluble part of the bauxite) and fine solids from the process liquor, aluminum trihydrate crystals are precipitated and calcined in rotary kilns or fluidized bed calciners to produce alumina (AljOj). Some alumina processes include a liquor purification step. 

Heavy-Media Separation. Heavy-media separation, depicted in Figure 3, can only be used for relatively rich sylvinite ores that consist of large crystals of KC1 and NaCl, such as those mined in Saskatchewan (6,20). Crystals of the two salts in the Saskatchewan deposits are 6—9 mm in diameter. Mine ore that is crushed sufficiently to pass through 6-9-mm screens in this size range results in a mixture consisting of discrete grains of each salt. 

This precipitate is not suitable for a pigment until it is filtered, dried, crushed, heated to a high temperature, and quenched in cold water. The second heating in a muffle furnace at 725°C produces crystals of the right optical size. 

In diagnostic tests crushing of the particles will not always be conclusive. Egg-shell catalysts or other types, zeolites and washcoated monoliths are exceptions. In washcoated monoliths the layer thickness is generally already quite low (<50 (im) and crushing will not yield smaller sizes. Cracking catalysts consist often of zeo-litic crystals of /im dimensions and a binder yielding particles of about 30 /im. If diffusion limitations exist in the zeolitic crystals, crushing will not eliminate these. 

Evaluating the catalytic shape-selectivities of these materials by use of the disproportionation of ethylbenzene (23.24) at 523 K at a conversion of 2% in differential reactor mode, it was observed that larger crystals of sample A gave 85% para-diethylbenzene and 15% meta-diethylbenzene. The smaller crystals of sample B with the smoother aluminium gradient yielded 96% para-diethylbenzene and only 4% meta-isomer. In a second series, samples of crushed large crystals with mean sizes of 1-10 (m were examined. No increase in activity was observed as is expected when the reaction is controlled by the diffusion limitation of molecules in the large crystals. However, this treatment created larger non-selective external surface area and hence a smaller selectivity of 87% para-diethylbenzene for sample B was recorded. 

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