The smelting of lead is one of the important metallurgical industries of the United States, and we are able to produce considerably more lead than any other single country. Workable deposits of lead ores are found throughout many western States, as well as in Wisconsin and in Missouri.
But, although the lead-smelting industry itself is of very considerable magnitude, its importance is still further enhanced because lead is used extensively as a collector for precious metals. By this, we mean that many gold and silver ores are used as fluxes in lead smelting, or even may be put thus into the lead smelting charge for the sole purpose of recovering their gold and silver contents along with the lead.
It is common to work up secondary precious metals by smelting industrial residues with leady materials at industrial centers, like Chicago and New York. Some western plants likewise are virtually gold and silver smelters. A large part of the tonnage smelting may have its main value in the precious metals, and barely enough lead is put in the charge to carry out the function properly in a lead blast furnace; that is, enough bullion to keep the crucible in good running order.
It has been described how the precious metals accompany copper throughout the winning of that metal, and in the same way the precious metals go through all the processes in the recovery of metallic lead and finally are separated in the pure state. Thus, in studying the metallurgy of copper and lead, we cover a large section relating to the winning of gold and silver.
The lead minerals occur as follows:
The universal lead mineral is galena, PbS, which occurs as the main one of all our deposits.
The carbonate, PbCO3, is a surface mineral commonly found as an oxidized ore over deeper deposits of original sulphides.
The sulphate, PbS04, is another oxidized mineral and may accompany lead ores in general to a slight extent.
The silicate, Pb2SiO4, possibly is a more important lead mineral and sometimes constitutes a rather important ingredient in the ores of certain mines.
The oxide, PbO, mixed with other metallic oxides may be found in some surface deposits.
All of the surface oxidized ores are reduced very easily in the simplest way and are used up only too quickly whenever the mines are vigorously exploited. Digging deeper into the ground, the miner most always begins to produce a larger and larger per cent of his material as sulphides which possibly are concentrated from a rather lean original ore. Thus the lead smelter is confronted in general with higher sulphur ores and finer materials. LEAD ORE REDUCTION Reverberatories. Considerable pioneer work has been done both in this country and in Europe to develop lead smelting in reverberatory furnaces, but the process is now about extinct; If it is attempted to smelt galena in a reverberatory, there first must be a roasting or oxidizing interval before the main reduction can be accomplished. The process, therefore, is out of line with reverberatory development so successfully evolved in the one-reaction processes with other metals.
Fig. 40. Men Working at Lead Ore Hearth.
The ore hearth is an ancient appliance somewhat modified by later improvements. It consists of a basin to hold the melted lead and to support the charge to be smelted.
Sides restrain the fire while air is blown in from the back through holes which are placed above the level of the lead but under the surface of the charge.
Fig. 40 shows this accurately. A kettle for the smelted lead, a car for the residue, and hoods and a pipe to lead away the smoke about complete the equipment. In the picture the men are shown in front of the fire spreading out the charge and stirring it over and over, as must be done unceasingly.
The ore hearth has the advantage of requiring no mechanical accessories besides the blower; it is started and stopped at a moment's notice and has an output depending only on the supply of ore and labor.
The great disadvantages of hearth smelting are that only a portion of the lead is recovered directly, while the remainder partly divides between the gray slag, the flue dust, and the lead fume which is produced in excessive quantities. Working up these latter materials requires blast-furnace smelting which thus has to constitute a part of the plant after all.
The ore hearth may be considered as a furnace in which considerable lead is recovered and the remainder of the charge is left in a roasted condition ready for normal blast furnace smelting; In this sense hearth smelting is treatment preparatory to blast furnace smelting.
Roasting was accomplished for many years in long hearth reverberatories by hand stirring of the charge (see Fig. 11). During the 90's a new sort of roasting was introduced from abroad where it had arisen, which consisted in blowing air through an ignited charge to both roast and sinter at the same time; the process was developed by Huntington and Heberlein and was called pot roasting for short. Within the last dozen years an improvement of this method rapidly has replaced [all the older ones in many plants.
Down-draft roast-sintering is a truly remarkable process for desulphurizing and sintering lead ores preparatory to blast-furnace reduction. The charge is made up with the idea of having just enough metallic sulphides present to support a progressive combustion and to agglomerate the material fully. Fig. 13 is to be studied in detail again, while Fig. 41 is a side view of the same machine in action.
At the very left of the picture and at the top is seen the feed hopper which is fed with a conveyor with the well-mixed and moist charge. As the line of pallets, each carrying 3 grates, moves under this hopper it becomes burdened with a layer of charge and passes next under the small oil burner. From the burner the grates slowly move toward the right over the suction box, seen between the upper and lower lines of pallets, while the fire eats its way down through the cake and should be through by the time the material has passed over the box and is to be dumped off into the car which is in waiting.
Fig. 41. side View of Continuous Rout-Sintering Machine.
The grates pass around the sprocket wheel at the discharge end and return under the suction box to come up again for a fresh layer of charge in passing under the feed hopper. The roast-sintering process is striking in its simplicity and is highly satisfactory as to the results obtained.
Aside from the lead made in ore hearths all our lead is now won in blast furnaces. This makes blast-furnace smelting of lead ores an extremely important topic.
The lead blast-furnace plant has its departments for sampling the materials, for storage of everything, for sintering, for power, repairs, and means to settle flue dust and to cool and filter the gases. American plants usually dross the lead in 30- to 50-ton kettles, and then send it to the refinery.
Both round and rectangular furnaces are common, the latter being customary in large plants. The peculiar features developed in America have been the siphon tap for the lead, full water-jacketing of the fusion zone, patent tuyeres, and mechanical feeding. Fig. 42 is an excellent picture of a round furnace in an European plant as partly improved with siphon tap, and partly water cooling.
At the base of the furnace is the steel-bound crucible and the enlargement on the side which constitutes the lead well out of which the lead is pouring into the small kettle. The circular shape of the entire shaft may be seen plainly; the cast-iron posts support the bustle pipe and the steel shell of the top. This furnace appears to be water cooled only about the breast and the tuyeres. The slag tap is on the right-hand side of the furnace, while the lead which runs out the well is ladled into the molds seen in a row in the immediate foreground.
In the lead blast furnace the strength of reduction is not nearly so great as in the iron blast furnace; the temperature in the smelting zone is not nearly so high, and the fuel required on the charge is several times less. A calcium-iron silicate slag always is made; a little matte always is formed to settle out in the forehearth or the pot settler below the slag. This matte carries the copper of the charge, as well as lead and values, and so is separated carefully and worked over for its metals.
Flue dust is recovered in large brick or steel chambers and ducts, the latter made long enough so that the gases will be cool enough to enter safely the cotton or woolen bags at the bag house. All United States lead plants have bag houses.
Bag houses are wooden, iron, or brick structures surrounding enough long porous bags to filter the solid particles from whatever gas may have to be treated. The cellar compartments receive the fume-laden gases and the collected lead fume falls down into the same cellar whenever the bags above are shaken. The top of the cellar is the floor of the bag room; this floor is hardly more than a support for row upon row of iron thimbles about which the bottom ends of the bags may be tied. The main chamber of the bag house is hung thickly with bags 30 feet long and 18 inches in diameter, which bags may be of cotton or of woolen fabric. Such a bag room may contain from 100 to 1,000, or more, bags; large plants commonly put partitions through so that one section may be repaired or cleaned while the others carry the load.
Fig. 43 indicates crudely something of the nature of this accessory. More often the structures have stacks instead of the opening along the roof as indicated. Bags may be shaken by hand, by mechanical means, and by reversing the draft with an auxiliary fan. The frequency of shaking may be once a day or every few hours.
Fig. 43. Diagram of Bag House Courtesy of Amirian Intitute of Mining Engineers.
The bag house is firmly established as an integral part of all lead smelting plants, both small and large, and is justified fully from the financial point of view as well as by being necessary from the hygienic point of view.