108 U. S. BUREAU OF MIJJES NOTES. [J. F. I.

Many industrial dust respirators, and many fabrics and filtering materials were tested. Reference to the patent literature discloses numerous types of dust respirators, mostly based on the filter principle. The present study included only representative filter- type respirators, purchased on the market.

Filtering efficiencies of the respirators were determined by passing air containing either tobacco smoke or suspended silica dust (particles about I micron or 0.00004 inch in diameter) through the respirator at rates of 32, 64, or 85 litres per minute. A small stream of the air that escaped from the respirator was viewed in a beam of light in a dark box. An equal stream of the unfiltered air was viewed alongside the first stream, and the unfil- tered stream was diluted with measured portions of pure air until the two streams reflected light of equal intensity, in this way a measure of the filtering efficiency of the respirators was obtained. Many filter fabrics were similarly tested by clamping the fabric in a holder which exposed filter areas of IOO square centimetres. The number of plies of filter was varied, and resistance to flow of air was determined both before and after smoke and dust were deposited. Further information on the results of the tests will be found in Serial 2745.

FLOTATION OF LIMESTONE FROM SILICEOUS GANGUE.

By Oscar Lee.

FLOTATIOX is generally regarded as a process that is only applicable to metallic minerals. However, its scope is now known to be greater than this, because some of the non-metallic minerals are floatable, such as graphite, sulphur, and fluorite. Recent observations show that limestone may be added to the list of floatable non-metallic minerals. Such work is being done on a laboratory scale at the Southern Experiment Station (Tusca- loosa, Ala.) of the Bureau of Mines, Department of Commerce, where experimental magnetic concentration of the low-grade iron ores of the Birmingham district is in progress. Certain experi- ments have demonstrated that the limestone may be floated from the magnetic log-washer tailing, and then incorporated in the iron concentrate.

If the loss of lime could be prevented, some ores though low in iron and high in insoluble contain enough lime to make their concentrates self-fluxing. Since this loss in the magnetic log-

July, 1926.1 U. S. BUREAU OF MINES NOTES. Iog

washer cannot be avoided, the lime must be recovered from the tailing by some method and added to the magnetic iron con- centrate before sintering. Then the resulting concentrate will be self-fluxing.

While seeking a suitable means of treating the tailing for recovery of lime, flotation was tried. It was at once evident that this method had possibilities. During the early part of the investigation, soap solution made from a common soap was used as a flotation reagent. The froth obtained was a fairly good lime concentrate. This result was an important clue, and led to tests with the use of such reagents as oleic acid and sodium oleate. I,ater, cresol, creosote, coal-tar and other similar flotation rea- gents were tested in combination with the oleic acid and soap solutions. As a result, oleic acid and cresol used together were found to be satisfactory.

The tailing from the magnetic log-washer is in excellent physi- cal condition for flotation. The iron ore had been ground in a ball mill to IOO mesh for magnetic concentration. Hence the bond between the lime carbonate and the other constituents of the tail- ing was well broken.

Fair results may be obtained by flotation in one stage. How- ever, the work is more efficient when the roughing-and-cleaning system is followed; the tailin, u from the cleaner is returned to the rougher. Typical results with this system are presented in Serial 2744.

The Principles of the Systematic Application of Geophysical Methods of Prospecting to Mining and Subterranean Engineer- ing. RICHARD AMBRONN. (?‘errestvial; dgng., March, 1926.)-- Until recently the geologist or mining engineer had to be content with what he could find on the surface or at a short distance beneath it. Meanwhile the valuable deposits near the surface have been worked until search needs to be made for others to replace them as exhaustion approaches. The deeper the latter are the more expensive it is to open them and in consequence the more important it is to be sure of them before operations are begun. Thus during the past decades attempts have been made to detect the presence and to determine the structure of economically valuable strata through some action which they exert at a distance. Fortunately some deposits do manifest their presence by effects which they produce. Thus in Sweden toward 1850 magnetic measurements were employed to locate magnetic iron ore. Good results were sometimes secured but this was not always the case. These early attempts were handi-