748 ____________________________________________________________________________________________________________________ THE COLLIERY GUARDIAN. April 11, 1913. is radically wrong. Where the moisture percentage is not over 5, the small dust under 1 mm. cube (which is the cause of all the slurry trouble) can be just as efficiently treated by dry-percussive screening as by any other method. It is many years since, by numerous trials and experiments, it was proved by coal-washing experts, and the Humboldt Company in particular, that small coal from 0’5 mm. cube downwards could not be enriched in washers. If the dust together with the grainy coal goes into the washer, the formation Of sludge is, of course, unavoidable, and the fine sludge, on account of its larger surface, can take up an immense quantity of water. A great number of mines which produce such coal do not succeed at all in obtaining coke from washed coal containing more than 11 per cent, of moisture, owing to the ovens being too slow in reaching the required temperature. Manufacturers of coal plants have been trying for more than 30 years entirely to eliminate the dust which could not be washed and to add it to the washed coal only after the coal had been dried to such a degree that no water dropped out of it, and several experts hoped to succeed by employing an air-current. This method, however, only partly solved the problem, as, despite all endeavours, part of the dust remained in the grainy coal. If, however, the air-current was employed of such a strength as to absolutely free the coal of the dust, then the loss in coal through the grains being simultaneously torn away proved to be most detrimental. The Humboldt Company succeeded about 10 years ago in inventing a percussion screen with a 1 mm. mesh, which entirely serves the purpose if the moisture percentage of the .crude coal does not exceed 4|; if this figure is exceeded, the wire-cloth meshes must be increased by T to f millimetre. If the writer’s assertions are proved to be correct, the ash contents of a mixture of washed fine coal and dust can- not exceed the ash contents of washed fine coal mixed with the whole sludge. But, unfortunately, the ash contents of the dirt sometimes prohibits adding the whole quantity to the washed coal, if the desired percentage of ash in the briquetting or coking coal is to be obtained. There is no objection to adding the whole coaldust to the coal if the demand for coking coal is sufficient and this coal is produced from coal of 0 to 10 or 0 to 15 millimetres, because the proportion of the washed coal and dust is as 2 to 1 or as 3 to 1. The author here gave a description of the Humboldt- Weissmann percussion screens, the advantages of which, he said, were as follow:—(1) Continuous feed on to large screen surfaces; (2) prevention of clogging by vibration; (3) large output, with minimum power consumption; and (4) small initial and upkeep cost. The advantages offered by a washery equipped with percussion screens are:—(1) The prevention of the formation of sludge to the widest extent possible; (2) improved classification of the water, without sludge basins and spitzkastans; and (3) yield of dry fine coal. The power consumption of one battery of percus- sive screens, comprising 18 screens, amounts to only 1-horse power, whereas 4-horse power are required for driving the distributing screw. The wear on the moving parts is reduced to a minimum. Percussion wheels supplied eight years ago are found to be still in good order, and there is no indication as to the time when they will be worn out. The cost of one pair of spare wheels amounts to about £1 4s. The cost of one phosphor-bronze wire-cloth screen is 7s. 6d.; and such a screen lasts about six months. Sixty- four plants, with about 600 screens supplied, are in operation at present, giving the highest satisfaction. Exhauster Systems for Dust Treatment. An example was given of an exhauster system for the treatment of dust and small coal. But in such methods there is always the trouble of pulling over grainy coal with the dust that is below “the possible enrichment dimension.” This coal consequently passes out with the dust, and the action of this system is not so definite nor so efficient as percussive screening. With this apparatus about 65 per cent, of the dust from 1 millimetre down- wards can be extracted, but in the percussive screens up to 98 per cent, has been extracted where the dust is very dry; from 89 per cent, at 4 per cent, moisture-factor to 98 per cent, at 1| to 2 per cent, moisture-factor. The fine coal is fed past the screen where it meets the incoming blast. The dust is drawn over according to blast and regularity of feed, and drops into a delivery screen, from which it passes out, to be mixed with small wash coal, or to the bunkers as desired. Lancashire Slack. In the north-western and Lancashire area in particular, it appears that local owners have to meet the serious commercial proposition of supplying unwashed stock varying from 1| in. downwards to compete with other sources of supply offering from f to 1$ inches of washed slack. The Lancashire coal is the better steam-raiser, but there is the everlasting trouble of getting rid of the 0 to $ inch small coal and dust. The competing slacks are invariably offered by companies with coking plants, and therein lies their power to offer a generally lower rate for larger average sizes. If the Lancashire coal is sold without its dust addition, there is no question of its market, and until coking (where possible) is adopted, or some of the processes suggested herein later, are adopted, the competi- tion in this area will become daily keener, while the ever-present small coal and dust question will be a steadily recurring nightmare of ever increasing dimensions, due to deeper workings and poorer coals. It is a matter of interest to note the manner in which the actual small coaldust is related to the ash-percentage of a fixed quantity of coal; and an analysis and segregation of the various elements of a Lancashire slack distinctly illustrates the manner in which the small dust-coal must bear on the buyer’s aspect of the question:— Table I.—Physical and Ash Properties of a Lancashire Slack. Size. Weight. Inches. Per cent. 1| to I .............. 14*43 f „ I .............. 16 50 | „ f .............. 14 43 1 „ i .............. 1134 | „ 0 .............. 43 30 As an alternative, the fine coal | to 0*inch was further classified into the sorts, £ in. to 1 mm., 35 per cent., and dust, 1 to 0 millimetre, 65 per cent. Table II. shows the result obtained after washing the individual sorts:— Table II—Results Obtained after Washing Various Sorts of Coal. Coal. Dirt. r~ \ No. Sizes. In. Weight. P. cent. Ash. P. cent. No. Weight. Ash. P. cent. P. cent. 1 . .. 1^ to £ ... 1175 . .. 4*52 ... ... 6 . .. 165 ... 7000 2 . •• 8 >, f ■ ... 13*61 . .. 3*45 ... ... 7 . .. 2*47 ... 60 00 3 . •• f » t ... 11*75 . .. 5'37 ... ... 8 . .. 186 ... 75*00 4 . .. t „ i ... 8*66 . .. 5*41 ... ... 9 . .. 1*86 ... 75*00 5 . .. | „*0 ... 23*92 . ..11-18 ... ...10 . .. 9-48 ... 68*00 < .. Sludge. — . ) ...11 . .. 12’17 ... 29 51 j 70*51 29*49 The separate fine coal | in. to 1 mm. and 1 to 0 millimetre yielded the following results :— No. Size. Weight. Ash. No. Weight. Ash. P.c. P.c. P.c. P.c. 12 ...liin. to 1 mm. ...26’29... 5*81 .... 14 ...618... 75 13 ...1mm. to0mm. ...10 82...3274 .... — ...Unwashed. By this test the great advantage in screening the dry dust below 1 mm. is confirmed, since the total final cost of f to 0 millimetre averaged an ash content of about 10 per cent. ; whilst the fine coal from f in. to 1 mm. only showed an ash content of about 5*7 per cent. The evident value of first separating the dust and small coal under 1 mm. from the f to 0 in. is evident, as the small dust in this case is particularly charged with ash. As washing would not enrich this coal, except at a large cost, its removal is certainly to be desired before it contaminates the washing water, if the slack is to be washed. Again, if the slack is to be sold without washing, and trouble is experienced in keeping the ash percentage within a fixed limit, then dry percussive screening is evidently the means to adopt for abstracting the most objectionable portion which contains the maximum ash. The question which now naturally arises is : What is to be done with this dust when once it is removed ? The answer to this is that the quality of the dust must first be determined. While in the sample just quoted, where the ash percentage was very high, namely, 32 74 in the dust from 1 to 0 millimetre, this may be accepted as a fairly bad case. In another sample the following analysis shows what possibilities occasionally lie in that section of the coal which is called “ dust.” Table III.—Distillation Sample No. 1. Per cent. Test on Lancashire Coal. Analysis. Per cent. Coke yield .... 7410 Moisture 060 Sulphur salts .., Volatile matters .... 1’30 ... 25*90 Ash Coke yield (Muck 16*80 Tar Benzol .... 3-20 .... 0’83 method) Coke yield (dis- tilling test) 74-80 74-10 Sample No. 2, dated June, 1912. Analysis. Per cent. Per cent. Coke yield ... 75*1 Moisture 070 Sulphur salts ... 1*20 Ash 16*20 Volatile matters Tar ... 24*90 ... 2 90 Coke yield Coke yield (dis- 7500 Benzol ... 0*72 tilling test) 7510 It must be borne in mind, however, that samples 1 and 2 had not been over percussion screens, but were, however, distinctly within that classification of dust which comes under the heading of “ dust,” and to which a cotton mill engineer or large coal consumer would take objection when present in too large quantities in his slack. The calorific value of such small coal in the past has been by no means related to the proportion of its value on a selling basis, and recent developments of special stokers and dust-fired boilers are slowly proving this assertion. In Mr. H. V. Hart-Davis’ recent paper describing the Bettington boiler, the capacity of dust for boiler firing is excellently demonstrated, and prospective developments of dust-fired systems promise to bring this matter to a yet more successful issue. Ordinary Boiler Fired with Dust Coal. In this case the forcing of the fires is fatal, and the only method of securing efficiency is to have ample fire and heat- ing surface for the duty desired, and to operate in duplicate, to allow coking of one fire, while the other is forced by breaking up. In simple language, it means that, as against standard slack conditions, the boiler capacity has practically to be duplicated. Where the dust will not coke and avail- able heat surface is limited, then stokers must be used. Excellent papers on dust-fired boilers have also been given by Mr. Pritchard, the chief engineer of the United Alkali Company, before the Liverpool Engineering Association, and by W. B. Harrison before the Leeds University Engineering Society. In both papers the advantages of dust firing is amply demonstrated, and each bore special reference to the Schwartskopff coaldust firing system. The great drawback to dust-spray-fired boilers and the commercial disadvantage under which they labour is the charge for pulverising the coal; but in the dust available in many mines, both from the roads, suction apparatus, screens, &c., if primarily passed over a percussive system of the type already mentioned, with a mesh of 0’5 mm., a large percentage could be deleted (which seriously militates against the efficient crushing of the larger particles), and is virtually ready at hand for such systems of firing (suggested combined furnaces gas or oil and dust). Dust coal of 1 mm. and under will find a fairly ready market in large foundries, either for sale as a medium to produce blacking for the face of moulds, or for reforming foundry sand; and various compounds for this purpose are but little removed from the original dust while their market value may reach as high as 10s. per cwt. Disintegrated Coal for Chemical Purposes. For many processes porous and “ sintering ” coal is desir- able, but the principal stated demand is a reasonable ash, so long as too much small dust is not included. By the screening method already advanced a small coal of t in. to £ mm. is obtained, which is within the permissible ash-limit, and should also meet the conditions as to size. In view of the enormous quantities of coal which is pulverised, and used in the chemical industry, the writer can hardly appre- ciate the attitude of the chemical industry to the small dust, and the ridiculous prices (in view of calorific and other powers in the same) that this same dust coal has previously been bought at, unless solely on the basis of the higher ash percentage ratio, which is always associated with the dust. Doubtless this attitude has helped to build the enormous profits that have been accumulated from this much-despised dust. Coal for Coke and By-Products. The question of coking is too well known to mining engineers for the writer to add much of import on technical matters ; but the fact that one coking company states that small dust cannot be used for coking, while another states that the largest coking plant in the world has decided that almost sand-like dust is the best material for coking, may come as a surprise to some, and cause an increased va|ue to be attached to the dust. The writer has already suggested that groups of collieries should be formed to pool their dust at a common centre for by-product work, power, or illuminant gas supply. Further, the introduction of central heating and hot water systems in private houses has greatly increased the demand for a moderate quality of coke. If collieries took up the utilisation of their own dust for the distillation of oils, &c., the modern market would probably provide a consumer for the coalite coke which would be produced. As an instance of the increased value obtainable from 1 ton of coal the members are asked to consider the samples already^cited and to take average figures. s. d. Per cent. Coke........ 75 at 12s. per ton.............. 9 0 Sulphate salts 125 at .£15 per ton.............. 3 3 vr .. nr (50 p. c. gas for sale ) 5,000 ) , o Volfc ........ 25 i 50 p.c. used in coking) at 3d.]1 8 Tar.......... 3 .£1 15s. per ton.............. 5 0 Benzol ...... — Is. per gallon................ 2 0 Approximate.............. 16 0 It is within the writer’s knowledge that coking plants have cleared from £2,000 to £3,000 per month on by-products alone, outside of coke sales, and this on a basis of 200 to 300 tons of coal carbonised per day. On an average the returns available by coking approximate for each ton of coal carbonised is as follows :— Coke, in hundredweights ........ 12 Tar, in pounds.................. 100 Ammonia salts, in pounds........ 30 Light oils, in gallons............ — Crude benzol, in gallons ........ 2*5 . Available gas, in cubic feet...... 5,000 The commercial value of these items must necessarily be fixed by coal qualities, locality and transport conditions. Power-gas Systems. For the purposes hereinbefore suggested, a fairly good quality and condition of coal is essential. But the residual dust, high ash fuels and shaly coal should be used if in any way possible. In the majority of cases it is possible to do so by the employment of the gas-producer on the system originated by the late Dr. Mond, which will convert these fuels, down to the last traces of carbon content, into a gas which is perhaps the most useful and tractable for power purposes, whether consumed under boilers or in gas engines. It is not suggested that the refuse, comprising simply dust and dirt, can be so used alone, but it may be judiciously mixed with a proportion of better fuel and consumed to great advantage. High ash content is not a great hindrance to this procedure, as latterly very successful types of producers with mechanical grates and ash - delivery