Mechanical Roasting in Silver-Lead Smelting

Mechanical Roasting in Silver-Lead Smelting

What Colonel Dwight says regarding the treatment of oxidized ores holds true also of the silver-lead smelting operations in Utah. The ore sought for was such as would, with appropriate fluxes, yield slag, and “lead bullion,” as the lead reduced from the ore and carrying its gold and silver, was called. No matte, and probably little or no speiss, was yielded, and hence there was nothing to roast, either of ore or matte. But, by 1880, when I first took hold in smelting, there was quite a production of matte at the works of the Germania Smelting and Refining Co., Utah, because, by that time, ores containing some pyrite and galena were being smelted. This was due, as at the Grant Works, to the more attractive price paid for the treatment of such ores. The matte, containing about 5 per cent, copper, was accordingly heap-roasted and returned to the blast furnace for the recovery of its contained metals. By this time there had been built a long-bedded, hand-rabbled reverberatory roaster with a hearth of about 40 by 10, ft. in size, and in this matte and ore, crushed at the sampling mill, was treated. When, in 1892, I took control of this works, called, now the Germania Lead Works, three other roasters had been added, but the practice of heap-roasting continued. This had the merit of needing no equipment, and of doing good work, because the pieces that were imperfectly roasted went back upon the next roast heap. At this time, when some suitable lump ore was obtainable, I roasted in heaps, in a satisfactory way. By 1892, the Bruckner cylinder roaster had been installed, which, compared with the reverberatory roaster, reduced labor. As such a cylinder revolves, a talus of ore is formed within it and at the same time fresh surfaces are exposed to the air, but the trouble was that the air did not reach to the bottom of the cylinder. This we corrected by placing a steam jet at the fire-box end, to project the air downward upon this stagnant ore, thus increasing the speed and completeness of roasting without at the same time increasing dust formation. The firing, if skilfully done, gave a somewhat sintered product, but with certain ores a slight access of heat over the normal would cause accretions to form within the cylinder near the fire end. These could be cut off by running back the fire box and barring off the crusts.


It was in 1895 that the Bruckner cylinders at the Arkansas Valley plant, Leadville, were giving unsatisfactory service and I was called in to investigate. The steam jet was put in use as at the Germania Lead Works, but we found that, due to the altitude (10,500 ft.), combustion was feebler than at sea level or even at 4000 to 5000 ft. Later it appears, as Colonel Dwight states, that R. D. Rhodes and Klots sought to increase the air supply by admission through perforated tiles, but the trouble was due to the air itself.


I have mentioned the formation of speiss resulting from the smelting of arsenical ores, and sometimes forming bottoms 6 in. deep in the slag pot beneath the matte, and separating from it as a distinct layer. This was crushed and some fed to each Bruckner roaster charge, thereby eliminating the arsenic. At that time, also, I found beneath the ore-yard floor many thin speiss bottoms, that were buried there years before, since they were then practically a waste product. These were similarly treated to recover their gold and silver content of about $4 per ton.

Colonel Dwight makes allusion, to the Williams roaster at the Arkansas Valley smelter. This consisted of a grate sustaining the charge; blast air came up through the grate and hot charge, so that this was a kind of blast-roasting on the same principle as the B. & H. pots. I am reminded of my observations on the roasting operations at the zinc plant at Canyon City, Colo. The ore containing both zinc and lead got a preliminary roast in a blowing-up furnace, the blast rising through the ore bead as in the Williams roaster. The aim, however, was to roast in such a way as to volatilize all the zinc and lead possible. The product was then sent to the blast furnace and with fluxes was smelted to yield slag and matte, the latter containing the copper of the ore. The furnace column was but 18 in. deep, and again the endeavor was to volatilize as much lead and zinc as possible. The fumes, both from the blowing-up furnace and the blast furnace, were drawn away by a suction fan to be sent to a bag-house, one of the early installations of that apparatus. The fumes there collected were purified of carbon etc., and, were sold as a pigment.

It was in the 90’s, however, that the mechanical roasters began to come in. These were of both the straight and the circular hearth variety, generally of a single deck, the rabble being pulled through by endless – chain. At the same time, also, the multiple-hearth furnace with its central circular shaft was increasingly brought forward, and, as we later realized, was supplanting the other mechanical types. It was recognized that continuous exposure of the rabbles to the heat gradually destroyed them, and accordingly it was sought to keep these only part of the time in the furnace.

So far as the preservation of the rabble arms was concerned, the trouble was overcome in the multiple-
hearth furnaces by water-cooling these arms, or by air-cooling them at the hearths where temperatures were lower. Wedge made his central shaft a hollow cylinder so large that a man might ascend in it in case of need. He covered it with tile as a protection against the heat. Of course, the rabble blades had to take the exposure and wear, but they could be promptly replaced without having to cool down the furnace.


When operating the Standard Works, Durango, Colo., in 1895, where copper smelting had been formerly performed, I made use of a Brown-O’Hara mechanical roaster on lead-bearing sulfides; but a good roast could be had only by long exposure to the heat, so that considerable ferric oxide was formed, anything but good for the lead blast furnace to follow. Also, the chains and rabbles gave down and caused expensive delays.

As to the flue dust of the multiple-hearth furnace, it does not seem to embarrass the designers of a recent one called the Queen roaster. This has large dropholes, and much of the roasting is done on the ore as it falls from hearth to hearth. One may say, however, that the flue dust then formed is quite recovered at a Cottrell treater. Due also to the increased area of the dropholes, the velocity of the upward-flowing gasses has been cut to one half, and so their carrying power to one-fourth, as compared with the older roasters. When Henry A. Vezin designed the Pierce turret roaster in the early 90’s, he devised admission of air from the rabble arms to the hearth. This has been well carried out on the Queen roaster, where the arms are air-cooled, and the air, thus heated, is admitted to the hearth by separate air arms.

In lead-smelting practice, where the ores carry lead; any excessive heat softens the ore, making it form accretions or crusts, as already mentioned in the Bruckner cylinder. Delays caused by the necessity of barring off resulted, as well as a serious cooling of the furnace (in roasting copper ores, this trouble can not occur) ; and so matters proceeded. Close roasting could not be well done in a mechanical furnace, and so the older long-bedded reverberatory roasting persisted, not to be abandoned until after 1910.

About this time sintering methods began to come forward, first of all with the use of the Huntington-Heberlein pots. In these receptacles, holding about 10 tons, blast roasting was done. The air under pressure, coming up through a perforated bottom, burned the sulfur in the already heated ore, and roasted upward through the mass. The product was dumped out and broken up for use. Looking back at it, one notes the crudeness of the operation, and yet it gave a sintered product well suited to the following smelting. About this time, also, Messrs. Dwight and Lloyd had developed a machine in which the imperfectly roasted ore from a mechanical furnace was sintered. In this machine we see a uniform layer of ore, set fire to at its upper surface, being roasted by a downward draft of air sucked through the filter bed of the ore itself, so that little flue dust followed the blast downward. The ore had been fed upon an endless-chain grate, the coarser particles of the feed falling upon the grate. Thus it resulted that the finest material, even flue dust, could be roasted with the production of little dust. Of course the charge had to be made up so that it could sinter, but the product could be put through the blast-furnace, giving but little flue dust.

Consider the program! Sulfide ore imperfectly roasted, to say 13 percent, sulfur, is made up into a sinter charge with ores lower in sulfur, even with oxidized fines, with crushed matte, and with flue dust, but in such proportions that it will sinter. This is fed to a machine that treats 150 tons per day, that gives a product that is easily reduced, that can be smelted rapidly, and that will give little flue dust. The roasting, however, must be so complete that the matte production of the blast furnace shall be kept at a minimum—say 10 per cent. Of course some matte must be yielded so as to insure clean slags; that has become the established practice, in later years.


It has been found that, with sinter, the blast furnace can be run at a speed that would have horrified the early metallurgist. It was a prime principle with him that time must be given for good reduction. The shape of the furnace was to conform to this, having a good bosh and an expanding shaft. Now, however, this idea has changed, the furnace is straighter, the smelting column low, and yet the slags keep clean.

Supplementing these operations, nothing must be lost. The flue dust from the blast furnace is caught in a baghouse, that from the roasters and sintering machine in a Cottrell electrostatic treater.

Going back to the matter of sinter roasting, Colonel Dwight has insufficiently emphasized the Dwight- Lloyd sintering machine, but has given us a good idea of its capacity and its efficiency. It is proper to add, however, that it has revolutionized the methods of silver-lead smelting, and has so added to capacity and assured continuity of operation that, were it not for this improvement, this industry would have been handicapped recently to the point of extinction, especially when we consider the growing expense of operation in the immediate past.

H. O. Hofman, in the early edition of his book on lead smelting, published in 1894, describes only the reverberatory roaster and the Bruckner cylinder. In the edition of 1906 we find, in addition, the Ropp straight line, the Pierce turret, the Brown horseshoe, the O’Hara, the Keller automatic and the Wethey described and discussed. The McDougall circular multiple-hearth type was not in use in lead smelting. The author stated that fusing the lead-bearing ore had been largely given up due to serious volatilization of the contained silver and that there was no satisfactory way of condensing the roaster fumes; that bricking the roasted ore was effective, but that a low roast was hard to get. In the 1918 edition we find that the Dwight and Lloyd roasting and sintering machine had taken first place, and that the multiple-hearth roasters were giving a preliminary roast.

With renewed activity in custom silver-lead smelting, one can only surmise the direction of improvement. Will ore, as received, be less crushed, less handled, less thrown about, and so saved rather than carried away by the wind? As a whole there should be less flue dust made, but all that is made will be caught. Will the ore be roasted in two stages on the one machine ? Will it be done, perhaps, at the top of the blast furnace, and there be down-drafted to meet the rising blast, all escaping by ports in the side walls? Will the reverberatory supercede the blast furnace, as in copper metallurgy? Will roasting and smelting be continuous in the same reverberatory? To what extent will selective precipitation be carried in the Cottrell treater? Will smelter fumes be rendered innoxious or at least invisible?

Steel Pipe Concrete Jacketed and Concrete Lined

The Catskill Aqueduct System is one of the greatest engineering feats in the world, and for magnitude and cost, for variety and complexity of engineering problems encountered, it stands without equal. It supplies New York City, including the Boroughs of Manhattan, Bronx, Queens and Richmond with about 650,000,000 gal. of water per day. This water, which is collected back of the Ashokan Dam, 92 miles north of New York City and on the opposite side of the Hudson River, is conducted through the Catskill Aqueduct, passing under mountains, valleys and streams until it reaches the west bank of the Hudson River near Storm King Mountain where it then drops down through a shaft cut in the solid rock for a distance of 1114 ft., crosses under the river and comes up on the opposite side in another shaft. From there it ultimately reaches New York through various reservoirs, including those of Kensico and Hill View. At the Kensico Reservoir it is sterilized by the introduction of chlorine gas in a liquid state. It then goes on to New York City through pipe lines, tunnels, etc., and is distributed through the various boroughs by 35 miles of tunnels, some of which are 700 ft. beneath the surface of Manhattan Island. pipe for catskill aqneduct system lead smelting

The pipe which constitutes the siphons of the two new lines that are being laid for the Catskill Aqueduct System is being built by the New York Engineering Co., of which A. C. Ludlum is president. They have devised an entirely new method of manufacturing the pipe and it would seem that this method will eventually supplant the one now in use by all others, inasmuch as it offers so many advantages such as lower costs and better workmanship. These lines extend from the Kensico Lake Dam in Westchester County down to Yonkers. The pipe is laid on concrete saddles and after all the joints are riveted together and caulked tight in the field, it is then filled with water and the pressure test applied. After all leaks are made tight and while the pressure is still in the pipe, in order to maintain it in its normal shape, a heavy concrete envelope is built around the outside of the pipe. After this sets permanently, the water is removed from the pipe and the interior is lined with two inches of concrete.

This pipe is 11 ft. 3 in. in diameter and is made from open-hearth tested steel plate ½ in. thick. The 20,000 ft. of pipe being built will require over 10,000 tons of plate. The sheets are 7 ft. 6 in. wide and 34 ft. long and are first planed on all edges to accurate dimensions.

They are then assembled in packages of four; a steel drilling template is placed on top of the package of plates and all rivet holes are drilled accordingly. This template is built up from 6 in. angle irons and is rigidly braced so there is no chance for any deviation from its original shape. Holes are drilled in the templet and these are lined with a hardened steel bushing through which the drills pass.

The package of plates and its templet are all bolted together and are put on a runway which contains many rollers and the package travels the length of the runway over these rollers, being moved by power. Two large radial drills are located on each side of the runway and these four drills all operate at the same time on the one package of plates, drilling all four plates at once. After the plates are drilled, instead of being rolled up into circular section, which is the usual practice, they are pressed up in a vertical pneumatically operated press of 400 tons capacity.

It would be a most difficult matter to handle these long sheets with the usual setting of horizontal rollers on account of the large overhang of the plate. While vertical rolls could be used, still it is necessary to press the first part of the plate which is not bent in passing through the rollers. The vertical press crimps or bends the edge of the plate to its true circle as well as the balance of the plate and it is only necessary to pass the plate through the press once. The face of the dies of the press are 16 in. wide and the plate is fed forward through the press at the rate of about 10 in. for each stroke of the press. The plate being handled on edge simplifies matters a great deal as small trucks are placed under the edge of the plate and the plate is fed through the press by a cable attached to the straight end of the plate, this cable in turn being automatically wound up on a drum operated by power.

The circular seams are double riveted and the horizontal seams are of the double butt strap triple riveted type; rivet holes are drilled; all of which is in accord with the best known boiler practice. The entire sheet is dipped in a bath of diluted sulfuric acid, in order to remove all scale, and is then riveted by the hydraulic riveting method.

Hoover and Wheeler at Chemical Exposition

The, 1922 National Exposition of Chemical Industries to be held during the week of Sept. 11-16 at the Grand Central Palace in New York will be a great scientific gathering. Herbert Hoover, Secretary of Commerce, frequently referred to as “the business man of the Cabinet ” will speak before the annual convention of the Salesmen’s Association of the Chemical Industry which will be held in conjunction with the Exposition. Mr. Hoover will talk on “Standardization, and What It Can Do for the American Chemical Industry.” Wayne B. Wheeler, general counsel for the Anti-Saloon League, considered one of the foremost authorities in the United States on alcohol and prohibition legislation, and active in the rigid enforcement of the Volstead Act, will address the assemblage at the Exposition on “ The Attitude of the Anti-Saloon League Toward Industrial Alcohol.”

Each afternoon there will be a program arranged by one of the scientific or business associations of the chemical industry, according to the plans which have been tentatively outlined. The standardization program with its many different phases will fill one afternoon session. The evening sessions will be given exclusively to motion pictures of which a number will be illustrated lectures on various developments in the chemical and allied industries. All meetings will be held in a special auditorium in the Grand Central Palace.

Every branch of the chemical and chemical equipment industries will be represented among the exhibitors, including those interested in metallurgical chemistry, as well as numerous allied houses. About 400 firms have contracted for space at the Exposition and basing predictions on present figures about 450 are looked for by the opening day. These figures compare with 83 exhibitors at the original exposition in 1915 and 427 in 1921.

Acetylene as a Precipitant for Cyanide Solutions

Cyanide manufactured from cyanimide contains a small amount of carbide, which upon dissolving produces acetylene, and this has led some metallurgists to believe that this acetylene might be capable of precipitating precious metals from the cyanide solutions during the ore treatment. The results obtained at The Rare and Precious Metals Station of the U. S. Bureau of Mines show that acetylene resulting from the presence of calcium carbide in the cyanide is inert as a precipitant and that any precipitation of silver is due to small unimportant quantities of hydrogen sulfide generated from sulfur compounds present in the carbide as impurities. Since there are standard methods of disposing of soluble sulfides in cyanide solutions, difficulty from this source need not be feared.

The acetylene used in the tests by the Bureau of Mines was made from a commercial calcium carbide. A number of experiments resulted in more or less complete precipitation of silver, but in no case was any gold precipitated. Silver acetylide is yellowish white, easily soluble in cyanide and highly explosive, but the precipitate obtained from the cyanide solutions was black, not soluble in cyanide and not explosive, and after appropriate tests was proved to be silver sulfide.—Serial No. 2346, Reports of Investigations, U. S. Bureau of Mines.