858 THE COLLIERY GUARDIAN. April 23, 1915. _______________________________________ __________________________________________________________________________________________________________________________ cases, and “ practically at right angles ” was perhaps the safest way of leaving it. Mr. Noah T. Williams (bon. secretary) said that if they set a prop absolutely at right angles in an inclined mine, any tendency to drive the prop down would make it loose; if it was set a little to the rise, a slightly ______ longer prop would probably be necessary, but it would be safer. Personally, he advocated the latter method. It was easier to extract coal if working to the rise than if working to the dip, the faces in the former method being cut in a stepped fashion with the object of con- trolling the rise. When working uphill the fissures were more or less open, and when working downhill they were closer. Mr. Miller said he was not of the opinion that props should be set exactly at right angles, but he had some set in that position for the purpose of testing. They A NEW TYPE OF STONE DUST GRINDING PLANT. A machine of the ball mill type, arranged for continuous feed and discharge; a fine buoyant dust is produced. The drawing and photograph illustrate a new type of stone dust grinding plant which has recently been designed by Messrs. George Fletcher and Company Limited, of Derby. This plant, which is an adaptation of a well known type of mill, used rather extensively in mining processes abroad, and also in this country, has been modified to suit the requirements of stone dust grinding for collieries. It will be gathered from the illustrations that the machines are of the ball mill type, arranged for con- Fig. 1.—Cross-section through Ball Mill. and delivery shoot. The driving gear comprises fast and loose belt pulleys, and massive cast iron machine- moulded gearing with all shafts, bearings, and founda- tion bolts. The feeding arrangement consists of a large hopper on the side of the machine, through which the pieces of shale or bind up to 6 in. cube are fed. The grinding plates are perforated over a portion of their surface so, as to allow the partly-ground material to fall through on to the screen plates fixed all around the outside of the grinding plates, and these in turn only pass the finer portion to the outer screen of fine woven wire mesh sieve. The finished product passes through the sieve and falls into the hoppered casing, from which point it is conveyed away as required. In both cases the rejected portions of the material are returned automatically to the interior of the mill, where they again come into the grinding area and undergo further reduction, the conditions of grinding being con- tinuous. and automatic. The grinding balls are of specially hard hammered steel, and the grinding, screen- ing, and fine mesh plates are all interchangeable for the purpose of convenient repairs and renewals. From tests made in the grinding of average shale or bind from colliery refuse, mills with 30 mesh sieves have given the following fineness of output :—70 per cent, pass 100 mesh, 15 per cent, pass 200 mesh, 15 per cent, pass 30 mesh. From these figures, it will be gathered that the greater proportion of the output is fine, and, therefore, buoyant dust, which gradually settles after scattering, the larger particles being in correct amount to break up the angle of repose of fine coal dust when scattering is being done, and thereby preparing the surface for the finer particl'-s to settle upon in a stable manner, completely covering or mixing the inflammable or dangerous coal dust, and rendering it harmless. We understand these machines may be fitted with meshes to suit special conditions, and without any increase in initial cost. The following is a table of particulars relative to the two sizes of these machines :— Number Approximate Size RPJI of output in B.H.P. of pulley machines, cwt. per hour. pullev. 2 13—14 ... 5—6 ... 30 by 6 ... 195 4 ... 20—25 ... 12—15 ... 48 by 8 ... 170 ____________________________ The Russian Coal Trade.—According to the South Russian Mining Congress, the quantity of coal produced in European Russia in the year 1914 amounted to 2,060.340,000 poods (1 pood = 36-113 lb.) This is 5,840,000 poods, or 0-3 per cent, less than in 1913. The following table shows that the decline was caused almost entirely by the military opera- tions in Poland. The early half of the year, including July, showed a substantial increase in all parts of Russia, particu- larly in the Donetz Basin and the Urals; but this was more than neutralised, however, by the decline referred to in the Dombroff Basin :— 1913. 1914. Poods. Poods. Donetz basin......... 1,543,790,000 ... 1,713.100,000 Dombroff basin...... 426,310,000 ... 2.-6.000,(100 Ural ............ 73,460,000 ... 88,600,0(10 Moscow ............... 18,340,000 ... 18,560,000 Caucasus ............. 4,280,000 ... 4,080,000 The Karkoff Coal and Iron Bourse Bulletin says that the quantity of coal produced in January last in the Donetz Basin was 135,000,000 poods; in February, 121,000,000 poods. The quantity estimated for March was 86,000,0000 poods; and for April, 92,500,000 poods. The quantities were working downhill at the time, and the prop gradually went rather -more downhill at the head, perhaps an inch in the 4 ft. 6 in. length of the prop. The prop was allowed to stand longer than it would have done in the ordinary course of things, and they found that it not only came back to right angles, but came back an inch less than right angles. When working uphill the prop gradually went down slightly at the head when set at right angles. Another point was that it was much more difficult for a man to set a' prop at right angles than a bit short of it, because he was liable to leave it not quite as tight as it should be. On the other hand, if he left it slightly less than at right angles, there was not very much fear of it coming out; he could thump at it, and make a good, job of it. By that method the props were, on the "whole, better set than if set at right angles. Mr. Sydney A. Smith said there was no doubt that the popular idea was that props should be set at a little less than right angles, but, in his opinion, it was entirely wrong. He had had considerable experience in steep mines, and he had come to the conclusion that the props, should be set exactly at right angles, the only exception being where there was a loose stone which might pos- sibly slip down. The prop itself if set at right angles was very much stronger, and was not liable to split. The Chairman remarked that sufficient points had been raised in the discussion to form material for another contribution from Mr. Siddall, accompanied, as Mr. Drummond Paton had suggested, by a series of diagrams. Personally, he was not inclined to agree with Mr. Siddall in toto. As other speakers had observed, circumstances altered cases, and they had to distinguish between a prop and a sprag. A great deal also depended on the man who set the prop, and on the character of the props used. In his final reply, Mr. Siddall pointed out that if a prop was not set. absolutely square and at right angles, there must be a space between the roof and the pack, and that would reduce the value of the prop. There was only a support for part of it, and the other part was no good at all. It was true he had taken rather a strong line of argument, but it had created a good deal of dis- cussion, and in matters of that kind discussion was of the greatest value. He was really only speaking generally in order to evoke expressions of opinion. Fig. 2.—Ball Mill under Erection. tinuous feed and discharge, and constructed on the most approved lines, of strong, rigid design, and first-class workmanship. The machines -are made in two sizes for the produc- tion of buoyant stone dust-for dusting mines, and they comprise self-contained crushing, grinding, and screen- ing plant, which automatically delivers the product of the necessary fineness, and delivers it into bags or tubs for transport. The mills include a charge of hard steel balls, graded for the rapid reduction of the material to be ground. The mill body is mounted on cast :ron centres, keyed to the main shaft. The body is lined on sides and periphery, with hard grinding plates stepped and perforated. Two concentric rings of screen plates are fitted, the outer screen being of specially woven gauge, and of a size to deliver the required degree of fineness. The whole body of the mill is totally enclosed in a mild steel casing, with removable inspection doors required for the railways, and for the factories producing Government goods make about 100,000,000 poods -per month. Notwithstanding these -requirements, -it has been impossible to despatch even the reduced quantities extracted. Thus, in January the quantities despatched from the Donetz mines amounted to 117,000,000 poods; in February to 93,000,000 poods only; and in the first half of March to 36,000,000 poods. The difficulty arises chiefly from the want of wagons. The price of coal is firm, and bending upwards. The Russian Custom House statement just issued gives the quantity of coal imported into Russia -in 1914 as 293,350,000 poods, value 46,157,000 r., against 468,431,000 poods, value 75,343,000 r. in 1913. The corresponding figures -for coke were 32,854,000 poods, value 6,192,000r., and 59,391,000 poods, value 11,342,000 r. in 1914 and 1913 respectively. The great decline, of course, took place in the last five months of the year. There was a great increase in the importation of coal in the first seven months, whilst the coke figures were about as in 1913.