Air elutriators

Air elutriators are especially useful for powders that are, in praetice, subject to grading by airflow e.g. fine dust, which contains particles of different densities with settling velocities, which are not uniquely related to physical dimensions. The major problems encountered in air elutriation are the difficulty in break up agglomerates and preventing particles from sticking to the walls of the elutriator tubes. The three main types of air elutriator are the up-blast, the down-blast and the circulating. 

The circulating type of elutriator, as developed by Roller is similar to the up-blast but the sample of powder is caused to circulate in a U-tube at the base of the elutriator tube. The elutriator has four chambers, to each of which is attached a paper extraction thimble collector for the fines. Ancillary equipment, including mechanical rappers, is included and the instrument is contained in a sound proof cabinet. 

In the miniature elutriator [20] a high velocity jet blows downwards into a thimble at the base of the elutriator tube. The tube is much smaller than that of other elutriators being 14 in long and 1 in diameter. 

The Sepor Haultain Infrasizer [21] consists of six elutriating tubes in series, air-flow entry being into the smallest. Air enters through a conical seating supporting a golf ball, which, by rotation and impact, breaks down agglomerates. 

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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... 

Although screens of 150 mesh and finer are made, they are fragile and slow, so that it is often preferable to employ air elutriation to... 

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. 

Unpublished data. Atmospheric samples taken during the summer of 1936 after the dust clouds from the Dust Bowl reached Washington, D. C., showed concentrations as high as 250 mg per cu cm at breathing level. Settled dust samples from various localities in Kansas and Nebraska were furnished the author by Dr. James Leake of the U. S. Public Health Service. The samples were air-elutriated and fractions submitted for both petrographic and chemical analysis. 

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. 

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,... 

Two instruments have been developed for on-line measurement of flowing powders coarser than 100 pm in size [23-26]. In the first instrument a side stream of solid particles from a process line is fed into an air elutriator that separates it into an oversize and undersize stream. The particle flow rate into the elutriator is measured and the cut size for the elutriator adjusted so that the flow of oversize particles out equals 50% of the inlet flow. The elutriator cut size is then equal to the average size of the powder. In the second instrument the flow rate is varied and the signal ratio of the two... 

BANCO CENTRIFUGAL CLASSIFIER. This is a proprietary instrument that is essentially a centrifugal air elutriator. Air and dispersed powder samples are drawn thru the cavity of a rotating hollow disc in a radially inward direction against centrifugal forces. The powder particles are thus divided into under- and oversize fractions, collected, and weighed. Separation into different size-fractions is made by altering the air velocity. About 20g of sample are required for analysis, and 8 size determinations can be made in 2 hours (Ref. 10)... 

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

The rapid development of HPLC columns for example was due to three major technical achievements the ability to manufacture micro-particulate silicas, the invention of air elutriation technique as sizing technology and progress in the slurry technique for packing HPLC columns. However, it took more than ten years to build up sufficient know-how to produce stable, robust and reproducible HPLC columns that satisfied the continuously increasing demands of the chemical and pharmaceutical industry (Fig. 1.2). 

Special processes have been developed to manufacture spherical packing materials with a given particle size distribution. The size distribution is narrowed by a size classification process, e.g. by air elutriation. Typically, the average particle size of a packing for an analytical column lies between 3 and 5 xm, the dp of a packing for preparative columns ranges from 10 to 50 xm. 

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). 

In earlier studies of friction with sliding of a powder layer, the particles that were used were relatively large (diameter 0.1-2.5 mm) and irregular in shape. The particles were separated into fractions by screening. In later work [29], the objects of investigation were spherical glass particles with diameters less than 100 ium, preseparated into monodisperse fractions by air elutriation. Steel sur-... 

Infrasizer. An air elutriator for the fractionation of powders into seven grades within the size range 100 mesh to 7pm. It was designed by H. E. T. Haultain (Trans. Canadian Inst. Min. Met, 40,229,1937 Mine Quarry Engng., 13, 316,1947). 

Pearson Air Elutriator. A down-blast type of ELUTRIATOR (q.v.) designed by J. C. Pearson (US Bur. Stand. Tech. Paper No. 48,1915) and used for determining the fineness of portland cement. 

Petersen Air Elutriator. An up-blast elutriator (q.v.) that has found some use in determining the fineness of Portland cement. 

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.)...

Figure 5.4. The Analysette 8 is an air elutriator with a short fractionation column that was described by Leschonski and Rumpf. a) Side view of the structure of the Analysette 8. b) Comparative performance data for three types of elutriators.

It is common practice in the air elutriation of fine powders to add what is known as a dea lomerating j ent or a flow agent to improve the flow characteristics of the powder. A frequendy used flow agent is colloidal silica. The exact mechanisms by which the improvement of flow is achieved by the addition of these very fine powders is not fully under stood [10] In their discussion of the performance of the elutriator shown in Figure 5.4, Leschonski and Rumpf state that the equipment should not be used to fractionate powders smaller than 10 microns [10]. 

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. 

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