July 15, 1932
1.NDUSTRIAL AND E N G I N E E R I N G C H E M I S T R Y
In general, it was apparent from the experiments that the amount of sodium in sample B represented the lowest possible percentage that could be estimated by this procedure, and that even in this case the results were not very satisfactory. It is recommended, therefore] that this method be applied only to the determination of sodium in metallic aluminum when the percentage of this impurity is greater than 0.01 per cent.
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When this is the case, the above simple procedure is capable of yielding good results.
LITERATURE CITED (1) Caley J Am Chem. Soc., 54,432-37 (1932). (2) Caley’a;d Fo;lk, IMd., 51, 1664-,4 (1929). (3) Caley and Sickman, Ibid., 52,4247-51 (1930). February ST, 1932.
Construction of Accurate Air Separator PAULS. ROLLER, U. S. Bureau of Mines, New Brunswick, N. J. HE author has previously described (1) a new type o€ air separator holding a charge of 600 cc. of powder and capable of effecting a particle-size separation into successive fractions beginning with 0-3 or 0-5 microns. Operations with this apparatus were a t the time confined chiefly to Portland cement and pulverized anhydrite. Recently there has been occasion to fractionate a gypsum powder that had been ground very fine in a pebble mill. I n making a 0-5 micron cut on this material the rate of separation was found to be unusually low compared to the corresponding anhydrite fraction, even when allowing for the smaller rate of air flow due to the lower density of gypsum (2.32 as against 2.98). With the anhydrite the rate a t which 0-5 micron particles collected in the felt filter bag, averaged over the first hour and a half, was 27 grams per hour; with the gypsum, however, the rate was only 3.3 grams per hour. The difference was attributed to suspension and adherence of the soft fine gypsum particles in the lower conical portion of the 60.8-cm. (24-inch) settling chamber used. To counteract this condition, it was decided to tap the conical portion automatically. When this was done the 0-5 micron gypsum fraction separated out much more rapidly; the rate, averaged over the first hour and a half, being 16 grams per hour as against 3.3 grams per hour without t a p p i n e i . e., the increase in rate was fivefold. Besides greatly augmenting the rate of separation, the automatic tapper also increases the homogeneity of a given fraction. To understand how this comes about, it is recalled that in a given fractionation the particles that are blown over progressively increase in size with time. At the end point, corresponding to a definite rate of separation, which depends upon air flow and particle size and may be as ligh as 10 or 12 grams per hour, the particle sizes are in the boundary between the given fraction and the next succeeding fraction. If the fractionation is prolonged beyond the end point, the particle sizes will increase to a maximum value 1.41 times the theoretical maximum given by Stokes’ law. This is because the maximum velocity a t the center of the vertical chamber is by Poiseuille’s law twice the mean velocity. Automatic tapping of the settling chamber renders the succession of particle sizes blown over more uniform, so that at the end point the number of particles which are greater or smaller than the theoretical maximum in size are reduced to a minimum. This result is particularly important at the higher rates of air flow. Because of the pronounced increase in rate of separation and greater homogeneity of the fractions, not only for gypsum but also as observed for all other powders, the automatic tapper is now used as a standard adjunct to the air separator previously described (1). The construction and method of mounting the automatic tapper, I, are shown in Figure 1. The oscillations of the U-tube, C, about bearing B impart the required motion to the tapper. The latter consists of three leaves, clipped to-
gether, of 0.8-mm. (l/az-inch) spring steel about 4.5 cm. (1.75 inches) wide and 30.5 cm. (12 inches) high, At the upper end is a wooden hammer head weighted with lead so as to give an optimum blow. At the lower end the tapper is clamped in cantilever fashion to an L-shaped bracket which moves along a slotted horizontal plate. The latter is rigidly attached to the U-bend by means of uprights. The
FIGURE 1. AIR SEPARATOR,VERTICALSECTION tapper is brought into position for maximum impact by moving it along the two slots in the horizontal plate, and is fastened in place on the horizontal plate by means of washers and wing nuts that screw onto bracket studs projecting through the slots.
LITERATURE CITED (1) Roller, IND.ENG.CHEM.,Anal. Ed., 3, 212 (1931). RXICEIVED February 23, 1932. Published by permission of the Director, U. 9. Bureau of Mines. (Not subject t o copyright.)
