*

The Ropp straight-line furnace has proved one of the most successful of all mechanical furnaces on leady material. Some of the details of the furnace will be understood from reference to Figs. 49 and 50. The rabble blades are supported on carriages which, instead of running on a track level with the hearth, and only separated from it by means of a sheet-iron curtain (as in the Brown-Allen O'Hara furnace), run in a vault beneath the arch, connection being made by vertical arms which pass through a continuous cast-iron slot in the centre of the hearth. These arms are attached to a wire rope instead of to a chain, and while both rope and carriages are much better protected from the heat of the hearth than in the O'Hara furnace, they are more than half their time outside the hearth altogether, so that they get thoroughly cooled down at each revolution, which takes three and a half minutes. The ropes pass round two large sheaves at the furnace ends, and the track for the stirrer carriages bends round outside the furnace, as shown in the figures, so that the carriages run continuously with the rope without any jerking or irregularity of motion. The length of hearth in the ordinary or small-sized furnace is 105 feet by 11 feet wide in the clear, with four stirrer carriages and usually three fireplaces, arranged as shown in the figure. The ore is fed in from hoppers at one end of the furnace by means of Challenge automatic feeders, and discharged into roasted ore hoppers at the other end. There is no complication of superposed hearths, and the stability of the principal arch is in no way interfered with by openings; the furnace, therefore, suffers less from racking and wear and tear than most others, while it is easily accessible for repairs. The usual consumption of coal is 4 short tons per twenty-four hours, and 4 H.P. is required for moving the rabbles. Each furnace contains 66,000 lbs. of ironwork (costing £1150 f.o.b. San Francisco), 54,000 red bricks, and 23,000 firebricks. With red brick at 37s. 6d. per 1000, and firebrick at £5s 5s., the total cost of one furnace ready for work should not exceed £1800.

Some data of the performance of the furnace at the Selby Smelting Works (Cal.), compared with the work done by the Pearce Turret furnace--both furnaces running side by side at Selby on the same ore-are given in Table XIV.

On these figures there would seem to be some saving in cost in favour of the "Straight-line," but evidently its principal advantages are simpler construction and better cooling of the rabble blades (which means less cost for repairs) and the smaller amount of flue dust produced. It is no doubt the use of a blast for cooling the rabbles which increases the amount of flue dust produced by the Turret furnace. As regards economy of fuel, neither furnace seems to reach a high standard, and no doubt any of the more complicated furnaces with superposed hearths * E. and M. J., July 12, 1895; also Private communications.

TABLE XIV.*-COMPARISON BETWEEN PEARCE TURRET AND ROPP STRAIGHT-LINE FURNACE.

would give better results in this respect if they could always be depended upon to handle leady ores without clotting.

Some much larger furnaces of the Ropp type (150 feet by 14 feet) have been erected lately at the Port Pirie Works of the Broken Hill Proprietary Company for roasting rich galena concentrates from the dressing works at Broken Hill. Considerable difficulty is reported to have been experienced in roasting material so rich in lead owing to the clotting and incipient sintering which takes place, but detailed results are not yet obtainable.

At the Hanauer Works (Salt Lake City) a large Ropp furnace 180 by 16 feet is used for roasting all kinds of leady ore mixtures and is very much liked. The average charge contains 25 to 30 per cent. S, which is roasted down to 5 per cent. at the average rate of 35 tons per day, with a consumption of 5 tons (or 14·3 per cent.) of best nut coal. The amount of repairs is very slight, chiefly consisting in renewing the rabbles on the firebox side about once in six months, those on the opposite side lasting a year or more. To obviate trouble from sintering it is found advisable to use rabbles on the firebox side an inch shorter than those on the opposite side.

Mechanical Reverberatories with Multiple Hearths.The best-known furnaces of this class are the Spence† and

* Private communication from A. V. I. Ropp, Nov., 1896.
tr. Peters, op. cit., 1895, p. 220.

Keller furnaces. Both have superposed hearths (four in the Spence, five in the Keller) traversed by rakes having a reciprocating motion; but in the latter the mechanism and stirrer carriages are altogether outside the furnace, only the rabble-arm itself entering the hearth by means of a slotted wall. These furnaces are doing splendid work at Butte on pyritic copper ores and concentrates, but have not, so far as the author is aware, come into use for leady ores, trials at the Germania Works having proved unsatisfactory.

The Wethey furnace † may be regarded as a four-hearth furnace on the O'Hara principle. It consists of four superposed hearths, each 50 feet long by 10 feet wide, heated by a firebox at the side, and the ore is worked forward from the top hearth to the bottom by the usual stirrers of rabble blades fixed to pipes resting on carriages, which run on tracks fixed outside the furnace. The peculiarity which distinguishes this from the Keller furnace is that the rabbles, instead of having a reciprocating motion and returning upon their tracks, turn completely over and return upon the hearth immediately below, so completing the circuit and carrying the ore on each hearth continuously in one direction. The furnace is therefore virtually a four-hearth O'Hara.

A furnace of this type at the Butte Reduction Works, working on zincy and coppery pyritic ores, treats 30 to 35 tons per twenty-four hours, bringing down the percentage of sulphur from 40 to 6 or 8 per cent.; it consumes only 2 cwts. of slack coal to the ton, or 10 per cent. The cost for fuel and labour is only 1s. 6d. per ton, and the repairs in two years have been nominal in amount.

Of course, the results on lead ores would not be nearly so good as the above, yet it should be possible to work them; in fact, the author is informed that a furnace of this type is actually at work in an English works roasting galena concentrates from Broken Hill, but no details as to performance are available.

Revolving Bed Furnaces with Fixed Ploughs. Examples of this type are the Brunton, which, though the earliest of all mechanical furnaces (patented 1828), is still in use for roasting arsenical ores, and the Bunker Hill § rotary pan furnace, a very poor device, combining the disadvantages of hand and mechanical furnaces. Neither of these, however, is used for roasting lead

ores.

The best furnace on this principle appears to be that of Blake,||

*Douglas, Journ. Soc. Arts, Aug. 16, 1895; and Peters, op. cit., pp. 214 to 218.

+ Private communication, A. B. Wethey, Aug., 1896.

Foster, Ore and Stone Mining, p. 614.

§ Rose, Metallurgy of Gold, p. 228. Trans. A.1.M.E., vol. xxi., p. 943.

which is on the principle of the Brunton, but instead of being supported on a central axis it runs on cast-iron balls in an annular groove 12 feet in diameter. It differs radically from the Brunton in two respects:-1. In having on its surfaces a series of concentric terraces, one above the other, so that each particle of ore pushed off one terrace by the fixed plough has actually to fall through the air on to the next terrace, instead of being merely pushed sideways still enveloped in sulphurous vapours. 2. In delivering upon the charge an independent stream of hot air unmixed with reducing gases. The heating of this air is effected by a pair of Siemens' chambers built in the usual way of brick chequer work, and heated, not by the waste gases, but by a separate fire, as the heat of the former was considered by the inventor to be insufficient. The air arrives upon the hearth in a nearly red hot condition, and thus very little fuel is required in the fireboxes. Its quantity being capable of accurate regulation the temperature of the hearth is perfectly under controlso much so that in treating a mixture of blende, galena, and pyrites the latter can be, if required, completely oxidised at a dull red heat without tarnishing the blende or fusing the galena.

The ease with which the temperature and rate of oxidation can be regulated with this furnace would seem to render success quite probable even on rich galena ores where only plain roasting without agglomeration is desired.

Revolving Cylinder Furnaces.-There are two main types of cylinder furnace, namely, the Brückner,* in which charging and discharging are intermittent, a single charge being put in and rotated until finished; and the Orland † and White cylinders, in which charging and discharging are automatic and continuous, the inclination of the axis and speed of rotation combining to deliver the roasted ore at a constant though adjustable rate. Both types are often referred to as American inventions, but they are both of English origin, the Brückner cylinder being nothing more or less than Mactear's modification of Elliott and Russell's revolving black ash barrel, while the White furnace is practically identical with the Oxland and Hocking calciner, patented and erected in England several years before the first American "invention" of a revolving furnace. Both the continuous and intermittent cylinders are described and illustrated in Part ii., Chap ix. of this work.

Although giving very good results with argentiferous copper ores and mattes the continuous cylinders have not been tried on leady material, owing to the temperature not being sufficiently under control. Their capacity, however, is great; thus at Briton Ferry (Cape Copper Company) Oxland calciners, 60 feet long and

Rose, Metallurgy of Gold, p. 229; also part ii., figs. 209 and 210. + Foster, op. cit., p. 615. Rose, op. cit., p. 235.

5 feet 6 inches inside diameter, roast daily 9 tons of white metal dead or 18 tons of coarse metal down to 10 per cent. sulphur.

Brückner Cylinders.-Many attempts have been made to apply these to the roasting of lead ores. At the Germania works (Utah) Brückner cylinders, 22 feet long by 8 feet in diameter, gear-driven, are used for roasting both ore and matte, and put through 8 tons each per twenty-four hours of pyritiferous ores containing 20 per cent. lead, roasting down to 2 or 3 per cent. sulphur, while ores with 30 per cent. lead roast down to 5 per cent. sulphur in the same time; the fuel consumption in each case being about 20 per cent. of coal by weight. Even ores containing as high as 35 per cent. lead can be successfully roasted by adopting special precautions to keep the temperature low, especially at the neck of the cylinder and at the beginning of the roast. The marked success of the Brückner cylinders at these works, compared with results obtained elsewhere, is due partly to the unusually large diameter of the cylinders themselves, partly to greater care in regulating the firing, and partly to the adoption of special devices for equalising the temperature inside the cylinder, which the author is not at liberty to publish. By means of these precautions and devices sintering, or "balling," of the charge is of exceedingly rare occurrence, and even when it does occur accidentally the form of the cylinder permits of ready access for " barring off," &c. The revolving cylinder, however, has the disadvantage of making a very large proportion of flue dust and of being rather heavy in repairs, while, unless in the hands of a skilful foreman, it is certain to give trouble by its irregular results.

Attempts have been made to combine revolving cylinder roasters with independent smelting hearths, on which the roasted charge could be sintered or fused, as suggested by Brückner years ago; but such attempts have all been complete failures. The fact that no systematic sintering of the charge is possible in any kind of mechanical furnace is no great drawback, for the plain roasted charge can be removed in an iron hopper truck and dropped, still red hot, into a separate smelting

hearth.

Shaft Furnaces.-The only furnace of this kind which has been successfully used on pyritous lead ores is the well-known Gerstenhofer, which at Freiberg puts through in twenty-four hours about 60 tons of a pyritous ore containing 18 per cent. galena, 60 per cent. pyrites, and 22 per cent. gangue, and roasts it down to about 7 per cent. sulphur. Ordinary lead ores, properly so called, could not, of course, be roasted in such a furnace at all.

Muffle Furnaces.-The same remark applies to the use of v. Schnabel, Handbuch der Metallhüttenkunde, vol. i., p. 55.

*