Air-elutriation method

BS 3406, (1963) reconfirmed (1983), British Standard Methods for Determining Particle Size Distribution, Part 3 Air Elutriation Methods, 266 Stein, F. and Com, M. (1976), Powder TechnoL, 13, 133-141,266 Crandall, W.A. (1964), Development of standards for determining the properties of fine particulate material. Winter Annual Meeting ASME,... 

Sedimentation methods are also useful in determining particle size distribution. Webb (28) describes the techniques employed in liquid sedimentation and gives data comparing the results with those obtained in mechanical screening. Sedimentation methods are particularly useful in measuring sub-sieve sizes. Air-elutriation methods (Roller, 29) are also useful, especially when used in connection with microscopic examination (Wiley, Deloney, and Denton, 30 Matheson, 31). 

Table 5.1. Comparative data on elutriator performance reported in British Standards Specification Number 3406, Part 3, 1963, Air Elutriation Methods . (Data for a silica powder.)...

This part covers air elutriation methods, such as the Gonell, the Roller (modified ASTM), and the miniature elutriation methods. As these are rarely, if ever, used for particle size distribution determination nowadays, it is likely that this Standard will be withdrawn. 

Gravitational Methods GONELL AIR ELUTRIATOR. This is the prototype of all analytical separators with laminar air flow. It consists of a cylindrical brass tube (or a series of tubes) with a conical base. An air inlet is provided in this base on the axis of the tube. The sample of powder is placed in the inlet cone, and air is blown thru the largest tube until separation is deemed complete, or for specified periods of time. The residue is removed, weighed, and transferred to a smaller diameter tube, and the test is repeated. The tube should have polished internal surfaces and should be periodically tapped or vibrated to disturb settled powder... 

The frozen-drop and the wax methods eliminate the tedious and time-consuming operations of microscopic counting of the larger droplets, and sampling errors are less likely because all of a large increment of spray can be collected and handled by employing screens and gravimetric methods of analysis. The chief limitation is that screens are not available to obtain data on particles smaller than 75 microns. Some form of microscopic count or air elutriation procedure is necessary for the smaller droplets. 

Sizing by elutriation is the reverse of the technique used for sedimentation. While any fluid may be used for elutriating particles, air has been found most satisfactory thus far. The basis of elutriation methods is... 

Experimental Results—Gottschalk and Wartman (loc cit) investigated the properties of a number of magnetite powders. Magnetites from four sources were purified and sized, each size-fraction then being analyzed for FesC>4. Only size-fractions below 100-U. S. mesh were studied. Sizes below 350-mesh were air elutriated by the Roller method. 

These methods fall into two general classifications those using a liquid sedimentation technique, and those employing an air elutriation system. 

Lights Removal Air classification methods are used to remove light contaminants such as dust, film and foam fi agments, and paper glass powder in a recycling operation in the absence of water [1, 10]. Two common types of air classifier systems, cyclone separators and multiaspirators, are shown in Figures 14.16 and 14.17. Other types of air classifiers are air knives, elutriators, zig-zag classifiers, and air tables. Air classifiers are rather simple equipment where control is often more art than science, and the equipment must be tuned for each stream of material. Separations of materials are based on differences in terminal velocities in an airstream and are highly dependent on particle size and shape. 

Various sedimentation and elutriation methods are available but these are of value only after standardization for a given type of powder or gel with constant aggregate density. The same applies to measurement of permeability to a stream of air or liquid. The external surface area of porous aggregates, which is an indirect measure of aggregate, size, can be measured by ambient pressure permeametric measurements. Gas is forced through a bed of powder and the flow rate and pressure drop are measured. The apparatus that is used for the flow method of measuring surface area by nitrogen adsorption (Quantasorb from Quantochrome Corporation) can be adapted to this measurement. 

Of the various mechanical properties of a formed catalyst containing zeolite, attrition resistance is probably the most critical. This is particularly the case for FCC catalysts because of the impact on the addihon rate of fresh catalyst, particulate emissions of fines and overall catalyst flow in the reactor and regenerator. Most attrition methods are a relative determination by means of air jet attrition with samples in the 10 to 180 xm size range. For example the ASTM D5757 method attrites a humidified sample of powder with three high velocity jets of humidified air. The fines are continuously removed from the attrition zone by elucidation into a fines collection assembly. The relative attrition index is calculated from the elutriated fines removed at a specific time interval. 

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