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It might be supposed that thin iron sheets would rust very quickly, but this is not found to be the case where the gases are properly cooled, and where they pass through a long length of flue before reaching that part where the plates are placed, so that all the moisture is condensed out of them before they come in contact with the iron.

An installation of Freudenberg plates put in at the Muldner Hütte (Freiberg) in 1890 is described by Bauer as a series of twenty-one parallel flues, each 33 feet long, forming part of the condensing system of the blast furnace and refining departments. The total surface of these flues is 1520 square metres; they are constructed of iron sheets fixed about 20 cm. apart. The sheets above the springing of the arch are fixtures, those in the central portion of the flue being suspended like those at Ems. The result of the insertion of the sheets was, that with an increase in ore smelted of 15 per cent., the flue-dust collected in the whole system showed an increase of 45 per cent. ; the increase in that portion of the main condensing chamber containing the flues was no less than 82 per cent., while nearer the stack the quantity condensed was actually less than before. Unfortunately, there is no excess of chimney draught as at Ems; in fact, a Guibal fan has always been required to assist the natural draught, and it was found that an increase of 40 per cent. in the work of the fan was required to overcome the increased resistance caused by the plates. The Guibal fan used is 20 feet diameter and 3 feet 6 inches wide; it takes about 7 H.P. to drive it at the rate of 65 to 85 revolutions per minute, which

Support

I beam

Suspended wires

Wire netting

Fig. 107.-Rösing Wires.

gives a pressure equal to 34 mm. (13 inches) of water, about 20 mm. of which corresponds to the resistance introduced by the plates. The cost of 20 mm. pressure is about £250 per annum, which, together with the interest and depreciation on the plant, must be set against the saving effected.

4. Rösing Wires.-Instead of sheets of iron, wires may be used as first tried by Rösing. The wires used are of No. 10 gauge, and three-fourths the full height of the flue; they hang in parallel rows from a wire netting of 1 inch mesh, which is rivetted to I beams running across *Jahrbuch für das Berg- und Hütten-wesen im K. Sachsen, 1894, p. 39. + Saeger, B- u. H. Zeitung, 1894, p. 299.

the top of the flue as shown in Fig. 106. At the Tarnowitz. Works an installation on this system in 1888 reduced the total amount of solid matter escaping during the year from 54 to 40 tons per furnace, without perceptibly influencing the draught, the extra material collected being very rich in lead.

5. Use of Electricity.*-Since the discoveries of Lodge and Clark in 1884, it has been known in a general way that discharges of static electricity would rapidly cause all dust to settle in a closed chamber, and patents were taken out by the firm of Walker, Parker & Co. in 1885 for the purpose of utilising these discoveries practically. No success has hitherto attended experiments in this direction, because electrification of the dust particles can only take place in stagnant air; as soon as there is anything like a current of gases the discharge seems to be dissipated.

6. Filtering Methods. So far as the author is aware we owe the first suggestion of filtering methods to Percy,† but it was not until recently that Bartlett experimented on the subject with satisfactory results. The Lewis and Bartlett bag process has been already described in Chapter v. As applied to blast furnace fumes the appliances employed have taken a somewhat different form, as, for example, at the Globe Works (Denver). The flue dust recovered at these works includes the whole of the lead fumes from the blast furnaces and roasters, and even that from the lead wells and slag taps, which is drawn off through an overhead main with funnel-shaped ventilators. It is ignited on the floor of the bag house, giving a porous grey cinder containing as oxides and sulphates Pb 75 per cent., Zn 3 per cent., Fe 0.5 per cent., As 1.3 per cent., and 4 ozs. silver per ton, which forms part of the ordinary smelting charge.

The Brown-De-Camp fume collector is on the same principle, and with it, as with all other filtering processes, it is necessary to thoroughly cool the gases down to 150° F. or less before their arrival at the condensing chamber; and advisable to settle out as much as possible of the ore dust and coarser clots of fume in ordinary flues and chambers. At the Omaha Works (Neb.), where the process has been in use since 1894, this is done by passing the gases from ten large blast furnaces, each 42 inches by 120 inches at the tuyeres, through 800 feet of brick flues. Instead of woollen bags a heavy open woven canvas or duck is employed, arranged in four high peaked ceilings, each 29 feet high by 99 feet long, and any one of which can be cut out of the series. A framework of I beams and pipes supports a series of ceilings in each compartment, the total effective filtering area of which is 40,000 square feet, and the capacity of each compart*Hutchings, B. u. H. Zeitung, 1885, p. 253; also Iles, S. of M. Q., vol. xvii., pp. 109, et seq. Eng. and M. J., Oct. 19, 1895.

+ Metallurgy of Lead, p. 449.

ment below the ceilings is 60,000 cubic feet. It is found that, to ensure complete efficiency, discharging the gases quite colourless and without visible vapour, about 16,000 square feet are required for each furnace, the whole installation being thus sufficient for ten furnaces. The fan is a 12 feet Murphy ventilator, which, at 200 revolutions, takes the fumes from ten furnaces, and consumes, roughly, about 7 H.P. for each furnace running. Suspended between the converging walls of the peaked ceilings are oscillating shafts carrying long beaters, which can be put in motion by means of levers from the outside of the building as often as the pressure gauge in each compartment shows that the meshes of the fabric are becoming clogged, causing the beaters to strike the canvas and shake off the dust, which falls to the floor of the building.

When the dust has accumulated to a sufficient extent it can be burnt on the floor, a separate set of flues and a small chimney being provided to carry off the products of combustion. It is usually, however, wheeled to a separate closed chamber for ignition, which converts it into a grey or pinkish friable coke, suitable either for the blast furnaces or for working up in the refinery together with skimmings and drosses to a hard lead.

The actual saving when running all the gases from seven furnaces through two compartments of the above condenser averages about 100 tons of metallic lead per month, carrying about 10 ozs. silver per ton, which is 6 per cent. of all the lead charged into the furnaces; this yields a gross income of about £1200 per month, or very much more than the total expense for maintenance, depreciation, and interest.

Wet Condensation.*-Many attempts have been made to employ water for condensing lead fumes, but the great difficulty is the practical impossibility of getting contact between the water and the minute particles of fume, which, instead of being attracted, are repelled by a moist surface as first noticed by Percy. Three main varieties of wet process may be described:

1. Passing through Water. This is the most expensive, owing to the powerful fans required to force the fumes through the head of water. Simple bubbling through water under a diaphragm or through perforated pipes was soon found to be almost useless, but the method of the Sheffield Smelting Co. is very effective. The fumes are distributed in a closed tank by means of a network of perforated pipes 7 inches below the surface, and above the pipes are fixed several diaphragms of copper wire, gauge about 16-mesh, the action of which is to break up the bubbles and so increase the contacts of dust-laden gases with the water. The gases are forced along by means of Roots' blowers, and it is stated that the cost of the power required, v. Percy, Metallurgy of Lead, pp. 441-449. + Metallurgy of Lead, p. 450.

*

though considerable, is small compared with the value of the fume recovered.

2. Blowing in Steam. This is cheap enough, but all the processes which depend upon the use of steam have proved utter failures. A large part of the steam escapes condensation, and rather hinders than assists the segregation of the dust particles, while such portion as is condensed forms a very acid mud which rapidly attacks the materials of the flues and chambers.

3. Passing through a Water Spray. This process, though costly in plant, has given good results at several works. At the St. Louis Smelting and Refining Works the plant consisted of a chamber built of tiles, in sections containing shelves composed of drain tiles supported by planks borne on old iron rails. The smoke passed first up through one such tower and then down through another, being sprayed all the time through fine rose nozzles at the top of each tower. It was then driven through a long wooden tank containing water, and provided with a number of hanging curtains which forced the gases to come in contact with the surface of the water while they were again sprayed from above. The fans were two Sturtevants, 7 feet 6 inches diameter, and the fume was perfectly condensed, but whether the process ever paid or not is a moot point.

The

At the Bimetallic Smelter (Leadville) a similar plant is at work, consisting of two circular towers, each 20 feet high by 16 feet diameter, through which the fumes from two large pyritic furnaces are forced by means of two Huyett & Smith fans, 10 feet diameter, running at 260 revolutions per minute. towers contain a great number of A-shaped tiles with serrated lower edges, supported in the form of shelves on old rails completely enclosed in sheet lead, the seams of which are "burnt" together, as are also those of the sheet-lead lining of the towers. Water is supplied from a shallow tank, forming the roof of the tower, through a network of 120 leaden pipes, and trickles down from shelf to shelf, falling from the serrated edges of one tile on to the ridges of those in the next row. The fumes enter the towers at a temperature of 350° to 400°, and leave at a temperature of 90°, being discharged through an octagonal stack 150 feet high built of planks heavily tarred. The condensed fume is washed out by the stream of water issuing from the bottom of the towers, settled in tanks, and sun-dried. It contains 25 to 40 per cent. Pb and about 25 ozs. Ag per ton.

Quantity of Flue-Dust Recoverable.—The quantity of fluedust produced is influenced by various factors, among which may be mentioned the amount of fine ore in the charge, degree of friability of the fuel, pressure of blast employed, amount of bosh in the furnace shaft, care in keeping the walls free from accretions and the top cool, and, especially, the height of the column of charge in the furnace.

With filtering methods, practically the whole of the lead volatilised can be collected as fume, but with all the other methods a variable amount always escapes, in many works little else but ore dust being collected.

At the Keld Head smelting works in 1857* the fume, collected by means of water-spray condensation from ores which yielded in the ore hearth 1374 tons of lead, gave 96 tons of fume lead, which is 7 per cent. of the ore yield, or about 5 per cent. of the total weight of ore charged, and over 6 per cent. of the weight of lead present.

Hofman + puts the average quantity of flue-dust formed in American works at 5 per cent. of the total weight of ore charged, which is probably an extreme figure for high modern furnaces; the quantity collected, however, averages barely half as much.

At Ems in ten months of 1881-82, the 14,605 tons of ore treated, containing 42.8 per cent. lead, yielded 931 tons of flue dust, averaging 56.9 per cent. of lead; the flue-dust actually collected, therefore, was 64 per cent. by weight of the ore treated, and contained 8.5 per cent. of the lead contained in it. At Freiberg in 1893 the weight of fume actually collected amounted to no less than 9.7 per cent. of the weight of ore smelted, and contained 1658 tons lead, 36,655 ozs. silver, and 917 tons of white arsenic, the total value being £13,636.

It has been already mentioned that at Omaha (with the new appliances) the fume collected, as an extra saving over and above the ordinary flue- and chamber-dust collected in 800 feet of flue, is equivalent to 6 per cent. of the lead charged into the furnace. It may, perhaps, be reckoned that, as a rule, in works smelting ores fairly rich in silver, from 2 to 3 per cent. of both lead and silver will be carried away mechanically as fine dust, which can all be settled out readily in suitable chambers; while from 6 to 10 per cent. of the lead, and to per cent. of the silver, will be volatilised, more or less of which is condensible according to the degree of perfection of the appliances employed. With ores poorer in silver, the quantity of that metal volatilised is much greater in proportion to the lead, and may reach, or even exceed, one per cent. of the total amount present.

TREATMENT OF FLUE-DUST.

The methods adopted for treating flue-dust have nothing to do with the mode of collection. The use of lead fume in its natural condition as paint is more than two centuries old, and at Linares and other places large quantities are sold for use as such, but the grey colour is frequently objectionable. The

Percy, op. cit., p. 456.

+ Op. cit., p. 282.

Egleston, Trans. A.I. M. E., vol. xi., p. 404.
Bauer, Jahrbuch f. d. B. u. H. W. im. K. Sachsen, 1894, p. 39.

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