Revolving Riffle for Subdividing Ground Magnesium Chips James R. Montgomery
Mallinckrodf Chemical Works, P . 0.Box 5439, Sf. Louis, Ma. 63160
GROW MAGNU~IUM cmps are used in a thermite reaction in which uranium(1V) fluoride is reduced to uranium metal. The quality of the uranium metal is dependent on the particle size distribution and the chemical purity of the magnesium chips. A sample of the magnesium chips is subdivided into portions of 25 to 200 grams for use in conducting various laboratory tests. It was discovered that a Jones rime used for this purpose resulted in an imprecise and biased sieve analysis (see Table I, Jones rime). The source of this error was attributed to the manner in which the sample was introduced into the rime. In order to eliminate this error, a 15-inch revolving riffle was developed (see Figure 1). This device is a combination of a riffle (I) and a rotary sampler (2, 3) and is suitable for samples of 200 grams or larger. A 6-inch diameter rime has been built to accommodate samples of less than 500 grams (see Figures 2 and 3). Detailed drawings may be obtained from the author. EXPERIMEN?AL
The 15-inch revolving riffle was evaluated hy comparing its performance in splitting a sample of ground magnesium with that of a Jones riffle. The particle size distribution of a 1.5-kilogram sample of ground magnesium was determined by sieving the entire sample and computing the weight per cent retained on each sieve. The sieve fractions were recombined.and mixed to regenerate the sample. The known weight per cent thus determined will be used for statistical evaluation. The 15-inch revolving rime was used to reduce the recombined 1.5-kilogram sample to one sixteenth its original weight. The sample was passed through the rime four times, and the outside pan fraction was used each time for the succeeding pass. The fourth outside pan fraction was sieved and the weight per cent retained on each sieve was computed. All portions of the original sample were recombined, rimed, and sieved in the above manner nine more times. The differences between the known composition and the riffled composition of each sieve fraction were used to evaluate the rime for hias. The absolute precision and any ohserved biases are shown in Table I for the 15-inch revolving ’ rime. , All portions of the original sample were again recombined and were reduced with a Jones riffle to one sixteenth the original weight. This was accomplished by passing the sample through the rime four times, the left pan fraction being the sample for successive passes. The fourth left ’ pan fraction was sieved and the weight per cent particle size distribution was calculated. The sample was recombined, rimed, and sieved nine more times. The differences between the known cornposition and the rimed composition of each sieve fraction were again used to evaluate the riffle for bias.
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Figure 1. A 15-inch revolving rime The absolute precision and any observed biases are shown for Analyst A in Table I. The foregoing experiment was repeated by a second Analyst (see Table I, Analyst B). The 6-inch revolving rime was used to determine the minimum acceptable sample size for ground magnesium chips. Ten 200-gram samples of ground magnesium were riffled five times and the outside pan fraction was saved after each split. A sieve analysis was performed on each of the saved
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Table I. Absolute Precision at the 95% Confidence Level of Individual Weight Per Cent Results of Sieve Fractions U. S. Std sieve size -10
(1) E. G. Boemer, “A Device for Sampling Grain, Seeds, and Other
Material,” United States Department of Agriculture, Bulletin 287 (1915). (2) R. W. M. Howes and L. D. Muller, “A Small Rotary Sampler and Preliminary Study of Its Use,” AERE R-3051, Hanvell, Berkshire, England, 19M). (3) .I. H. Pownaii, “The Construction and Testing of a Large Laboratory Rotary Sampling Machine,” AERE R-2861, Harwell, Berkshin!, England, 1959.
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15“
Revolving
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10.8 10.6
40
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Jones riffle
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Analyst I
riffle
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-50 10.03 10.1 3~0.4 Pan 10.01 10.06 +0.05* (bias) Because of significant bias, no precision was determined and the observed average difference is shown.
VOL 40, NO. 8, JULY 1968
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Table Il. Absolute Precision at the 95% ConffdenceLevel of Individual Weight Per Cent Resulb of Sieve Fractions Sample size in grams U. S. Std sieve size 2M) 100 50 25 12.5 10.7 A0.8 11.7 +1.7# (bias) 11.8 + 8 10.6 +0.7 10.4 +1.2 +1.4 -8 12 -12 16 10.3 zt0.4 10.7 A0.7 A1.2 -16 20 10.2 10.7 zt0.6 -0.4. (bias) 11.0 -20 30 10.3 +0.3 10.5 -0.7a (bias) +1.4-(bias) -30 50 10.3 *0.3 zt0.4 +0.5 10.8 Pan 1 0 .1 270.1 +0.2 - 0 . 9 (bias) ...
+ + + + +
Because of significantbias, no precision +vas ucrsLrrdnedand the observed average difference is shown. Trace of material splits and the a b s o u c pr~rsiuim ar LLLC r ~ j bCULUIUCIILC level are shown in Table 11. The bias resulting from too small a sample is also indicated. RESULTS AND DISCUSSION
Table I shows that the 15-inch riffle is capable of splitting a sample of ground magnesium in a precise and unbiased manner. Table I also shows that the results obtained using- a Jones rime for the same split may be biased and imprecise. The minimum acceptable a m p l e si7c for ground magnesium, split with a 6-inch revolving r~mc,is 50 grams. Table I1 shows that, as the sample size is decreased, the results are less precise and that samples smaller than 50 grams are biased. The advantages of the revolving riffle are an increased number of cuts and easier sample introduction. The revolving riffle cuts a 1.5-kilogram sample into lop to IO6 portions of about 0.10 gram each, whereas a similar Jones riffle cuts the sample into 18 portions of 80 grams each. The total number of cuts generated by the revolving riffle is computed by multiplying the number of slots by the revolutions per minute and the feed time. The result of this increased number of cuts is less sensitivity to sample heterogeneity. The revolving riffle was fed from a vibratory .feeder, a hopper, and a folded sheet of glassine paper, with good results. On the other hand, it was difficult, if not impossible, to load the dumping scoop of a Jones riffle sufficientlyevenly to avoid biasing the results. The 15-inch riffle used for 2% years in this laboratory has proved to be a suitable device for subdividing heterogeneous ground magnesium. This riffle has also been used to split mixtures of greatly dissimilar materials such as blends of ground magnesium and uraniumflv) fluoride. Other materials that could be rimed include grains, soap powder, pulverized coal, and pelletized feeds.
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ACKNOWLEDGMENT
The author acknowledges the suggestion of Harold W. Thieman, which aided in the refinement and construction of this device.
RECEIVEDfor review January 22, 1968. Accepted April 18, 1968. Work supported by the United States Atomic Energy Commission under Contract No. W-14-108-Eng-8.
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ANALYTICAL CHEMISTRY
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