Caving-in has different mechanism compared with general mining subsidence. We have studied theoretically on the mechanism of caving-in at 1st report.
In this paper, the mechanism of caving-in was studied experimentally and compared with theoretical analysis. Futhermore, the effect of underground water level was studied theoretically.
The results obtained are as follows.
1) When cohesion is over 1kg/cm², the effect of internal friction angle is a little, but the effect of unit weight is little. When cohesion is under 1kg/cm², the effect of internal friction angleand unit weight are serious. Especially when cohesion is under 0.5kg/cm² and internal friction angle is under 40°, the effect of unit weight is more serious.
2) The ratio of the limitting height of fall to width of mining cavity is about 0.2-0.6.
3) When the underground water level has rised up after a long rain or a heavy rain or at the moment when the water in mining cavity is drawn out, caving-in comes out very often and easily.

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The test results obtained, using several kinds of DC power sources, test apparatuses for intrinsic safety and power circuits, may be summarized as follows;
(1) When batteries are used as power source, the minimum current to ignite methane becomes smaller if high capacity batteries were used and terminal voltage were kept constant. The minimum ignition current with an alkaline battery is smaller than that with a lead-acid storage battery.
(2) In the case of a rectifier type power source, the minimum ignition current is obtained with a silicon rectifier of 6 A to 12 A and a smoothing condenser of 100 to 200μEF.
(3) When an alkaline battery of 15 AH and a lead-acid storage battery of 75 AH are used as the power source of constant voltage charging, minimum ignition currents in both cases are the same.
(4) If condensers of 10 to 100 pF were connected in parallel to each part of the test circuits, the minimum ignition current sometimes falls by 20% at maximum.
(5) Comparing ignition currents of DC power sources, a rectifier type power source and a lead-acid storage battery give the minimum value when voltage is under and over 30 V respectively.
(6) Replacing the electrode of an English type test apparatus with a grooved cadmium electrode of a German type, the result is almost same as or somewhat better than that obtained with the ordinary English type
(7) It is concluded from the test results that the safety limit of E2x I is 200 in the case of resistive load used in intrinsic safe equipment for coal mines.

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A periodic inspection of a vertical shaft winding rope with the concentric construction equipped in a Koepe system was carried out by the electromagnetic method throughout its lifetime.
Various features of degrade of the rope resulted from repeated haulage, which may also be a characteristic of particular portion of the rope owing to the winding system, were explained.
Applying appropriate curing at every incident recognized on the inspection chart, approximately 250, 000 windings at the ultimate, which was far beyond the life expectancy, was attained consisting with the thorough safety, while the last three ropes in the same shaft had been renewed conventionally at 170, 000 windings or less
The rope tension, recorded in the vicinity of the head sheave through the rope travel, including various sequence such as an emergency braking, was also discussed standing on the intrinsic safety, referring to the electromagnetic inspection chart.

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The authors have investigated the difference in specific gravity, magnetic separability and floatability of micas containedi n the samples which are made from various kind and weathering degree of granitic rocks through attrition, grinding and sieving. The reaults obtained are summarized as follows;
(1) As weathering proceeds, biotite changes into chrolite, hydrobiotite, vermiculite, kaoline, gibbsite etc., remainning its appearance, and same time its specific gravity decreases. So it becomes difficult to separate mica from quartz and feldspar by “Sink and Float Separation”.
(2) As biotite is weathered, the density of magnetic flux necessary for attraction of it becomes higher. Muscovite is less affected by weathering and it needs higher density of magnetic flux than biotite
(3) Since there are many factors which affect the result of mica flotation, the degree of weathering could not be related strictly to the difficulty of flotation. However, it was found that the finer and thinner particles of mica are, the easier it is to float, and, comparing mica in froth with that in tailing, the latter has been altered than the former.
(4) It was easy to perform the mica flotation rather satisfactorily on the samples made from the so called “Masa-tsuchi” of muscovite-granite and biotite-granite, but was not so easy on all weathered products of granodiorite.

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Various recovery processes of scandium from the waste residue of leached tungsten ore were examined.
After treatment of hydrochloric acid and ammonium thiocyanate, about 95% of scandium could be extracted into the diethyl ether. In the solvent extraction using tributyl phosphate, scandium was extracted from strong hydrochloric acid solution, then it was stripped into the aqueous solution with the recovery of 99.9%.
Sufficient extractions were not obtained with JM-T, because of the high impurity levels of products.
Impurities of the produced scandium compounds were eliminated by reprecipitation using ammonium tartaric acid.
Scandium oxide of 99.9% purity could be obtained from the produced scandium oxide by heating it to 800°C in a stream of hydrofluoric acid gas.

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