Condensing Vapours

The fumes produced in the various metallurgical operations with lead contain in suspension extremely minute particles rich in lead, sometimes in such proportion as to amount to 7 Per cent. of the total product of the works. Much of the solid material of the fume given off from the smelting - furnaces and slag - hearth collects in the flues of the works, and being very rich in lead, is from time to time taken out and smelted, but separately from the ores, since it contains metallic impurities such as zinc, arsenic, and antimony, which render the lead produced from it hard and unfit to mix with lead produced in the earlier processes.

French and Wilson, of the Sheffield Smelting Co.'s works, have studied the physical nature of lead - fume as it proceeds from the blast furnace used at these works. It consists of polished spheroidal particles, very uniform in size for the specimens taken at the same time, although those taken at different times frequently differed greatly. The smallest were about 1/20000 in. in diameter. After passing along a flue for about 60 ft., these particles begin to aggregate into flakes, which become more numerous and larger as the distance from the furnace increases. Lead - fume appears to have no definite composition, as the proportion of its constituents varies in every specimen. French gives the following results of analysis of 2 specimens of fume proceeding from blast furnaces: -

Lead - fame may also contain a little gold, and as much even as 1/2 to 1 per cent. of silver. When fluorine is present in the ores, or when fluor - spar is used as a flax, silicon fluoride will be present as a gaseous element in the fames, but it will be decomposed by any watery vapour present, as it is in the flues of artificial manure - works.

Following is the composition of lead - fume collected in the flues of the Bagillt Lead Works, where reverberatory furnaces are used: -

The lead exists in the fume almost entirely as sulphate, so that the problem involved in its treatment for the recovery of the lead it contains is simply the reduction of that salt. Where the ore - hearth is in use, the material is merely wetted to make it agglutinate, and is then smelted much in the same way as the original ore. Where the reverberatory furnace is in use, it is smelted in this furnace. At Wass's Alderwasley Works, where poor ores and slags and various refuse matters are smelted down in a slag - hearth, more with the object of making fume (which is carefully collected) than to obtain lead directly, the smelting of the lead - fume collected and subsequently stored as a pasty mass, is conducted as follows: - It is charged on to the reverberatory furnace in the usual way, and heated or calcined for about an hour with occasional rabbling, until the whole has become melted and run down into the well. This part of the operation lasts about an hour. Fine slack is then added, and the whole is pushed back upon the floor of the furnace, and the heat is urged, when the lead separates and runs down into the well. The slag which forms is raked out by the doors opposite the tap - hole, and is added to the matters which are treated in the slag - hearth.

The operation lasts 3 1/2 hours and upwards. Care must be taken that solid fume does not pass off with the sulphurous gas into the air.

The essential element of condensation or collection of the fume is a very long flue, in the construction of which it is well to make provision for retardation of the current of gases through it in their way to the chimney. Such provision may be made by giving a large capacity to the flue, by giving it an angular or zig-zag course, by interpolating large chambers which are best placed at the far end of the flue, or zigzags in its course, by introducing baulks or mid - feathers here and there, against which the current may impinge, or by hanging within the flue such things as iron hooping, bushes, old nets, etc, upon which the solid matters may be deposited. These various contrivances are adopted at different works. French says that invariably the fume is most abundant where the gases have suffered the greatest friction and fall in temperature. That this fact causes the fume to settle is also proved by the increased escape into the air for some time after the flues have been swept out. This has been proved by assays of the smoke, and is apparent at the top of the chimney to the eye. At many works some kind of arrangement for washing or damping the fume is adopted in addition. As to the value of this, however, opinions among lead - smelters differ very remarkably.

Some of the largest smelters are satisfied that as good results are obtainable by dry flues as by the use of water in addition, A. O. Walker (who merely uses some steam produced by the evaporation of water in his flue) says that the damp acid gases have injured his brick chimney - shaft, through which obviously some percolation has taken place. French goes further, and says that the use of steam in the flues is positively injurious to the deposition. He has carefully examined flues in order to determine this point, and has found less deposit in the parts where the steam has condensed into water than in the drier parts. The following particulars of the methods of condensation adopted at some works where that subject is most carefully attended to may be usefully given. The figures are to be regarded mostly as approximate: -

1. Smelt by reverberatory furnace. Aggregate length of flues (much of them underground), 2 miles; for the most part 7 ft. high by 6 ft. wide, but in some parts they are 10 or 12 ft. high. Early in their course the fume is made to pass through a number of shallow flues, at the bottom of which a layer of water is kept constantly supplied; the heated air converts some of this water into steam, which is carried forward with the fume. The greater part of the flue is disposed in the form of a helix, from the first part of which a branch or "relief-flue" passes away to a zig-zag of 20 rows or bendings, each of which being 14 yd. long (the length of the zig-zag), gives a total of 280 yd. of this arrangement. Old iron hooping is festooned upon iron rods within the flues of the zig-zag. The zig-zag terminates near the last coil of the helix. From the helix a straight flue conducts the fume up the side of the hill to the summit, where is a brick chimney - shaft 267 ft. high.