|Oxidizing roasting of PbS with following reduction of PbO obtaining crude lead ("bullion") followed by refining (purification) of it is the main way of lead production. The roasting is processed in sinter pallet continuous-type machines. The main roasting reaction is:|
2PbS + 3O2 = 2PbO + 2SO2.
Some amount of lead sulphate PbSO4 is transferred into silicate PbSiO3 by adding some quartz sand into the furnace-charge. At the same time sulphides of other metal impurities (copper, zinc, and iron) are also oxidized. As a result of roasting agglomerate, a porous sintered mass, consisting mostly of PbO, CuO, ZnO, Fe2O3, is obtained instead of powder mixture. The pieces of agglomerate are mixed with coke and limestone, after which a water-jacket furnace is loaded by this mixture which is blown through by compressed air delivered from the bottom piping ("tuyere"). Lead is reduced from PbO by coke and carbon monoxide at relatively low temperatures up to 500°C. Copper and iron impurities are removed by liquation. Tin, arsenic, antimony are eliminated by air blast through the molten metal. Silver and gold are extracted by adding zinc which forms silver-gold-zinc crust consisting of metals with less weight than lead and melting temperature at 600-700°C. Surplus of zinc is removed from molten lead by air, water vapour or chorine blasted through. Bismuth is removed by adding calcium or magnesium which form high-melting compounds Ca3Bi2 and Mg3Bi2. Lead obtained by this method has 99.8-99.9% purity. Further purification is performed by electrolysis after which 99.99% purification may be reached.
The Metallurgy of Lead
|Owing to its easy reducibility lead was extracted from its ores in very early times. The Romans manufactured lead in Britain, and the remains of rough furnaces exist in Derbyshire and elsewhere, in which the ore was reduced by means of charcoal. Until the middle of the eighteenth century a kind of blast-furnace was in use in England, but about this time reverberatory furnaces, previously employed in Flintshire, were introduced into this country. |
Three distinct processes are now employed in the metallurgy of lead
- The air reduction process.
- The carbon reduction process.
- The precipitation process.
The Air Reduction Process is applicable to ores consisting chiefly of galena, and free from other metallic sulphides and silica. The term " air reduction " seems anomalous. It applies to the preferential oxidation of sulphur, whereby from galena, metallic lead and sulphur dioxide result thus
PbS + O2 = Pb + SO2.
This reaction is not, however, directly realised; but the following reactions of oxidation first take place
2PbS + 3O2 = 2PbO + 2SO2
PbS + 2O2 = PbSO4;
and these are followed by the interaction of unchanged lead sulphide with oxide and sulphate thus
PbS + 2PbO = 3Pb + SO2
PbS + PbSO4 ⇔ 2Pb + 2SO2.
The latter reaction, however, is liable to be reversed by the action of excess of sulphur dioxide on the reduced lead.
Four conditions of equilibrium, indeed, are possible, and are represented by the equations
PbS + PbSO4 ⇔ 2Pb + 2SO2
PbS + 2PbO ⇔ 3Pb + SO2
PbS + 3PbSO4 ⇔ 4PbO + 4SO2
Pb + PbSO4 ⇔ 2PbO + SO2.
According to Reinders, in the interaction of lead sulphide with lead sulphate, and the formation of metallic lead and sulphur dioxide, the five following univalent systems, each consisting of three solid phases, successively occur, and mark the gradual eliiiiination of sulphur
- PbS - PbSO4 - PbO.PbSO4
- Pb - PbS - PbO.PbSO4
- Pb - PbO.PbSO4 - 2PbO.PbSO4
- Pb - 2PbO.PbSO4 - 3PbO.PbSO4
- Pb - 3PbO.PbSO4 – PbO.
From the vapour pressure curve of the first system the thermal value of the reaction
PbS + PbSO4 = 2Pb + 2SO2
is calculated to be - 99,543 calories; and from the heats of formation of the compounds concerned to be -92,470 calories at 20° C.
These reactions are carried out in a reverberatory furnace (Fig.). The charge of ore, consisting of from 12 to 21 cwt., is introduced through the hopper on to the hearth of the furnace, which is hollowed so as to allow the molten lead to collect above the tapping hole, and flow through it at the proper time into an iron pot. There are several doors surrounding the hearth which allow the temperature to be regulated and the mass to be rabbled at intervals.
|Flintshire reverberatory furnace for lead smelting. |
A. Hopper. B. Hollowed bed of furnace. C. Fireplace. D. Flue.
In the first part of the operation the ore is heated below a full red heat, and not allowed to clot; and thus absorption of oxygen takes place with the formation of oxide and sulphate according to the first two reactions. When this oxidation is at an end the doors of the furnace are closed, and the temperature is raised to a full red heat, and thus maintained for about half an hour. The mass now becomes plastic, reduction to metallic lead takes place, and the molten metal collects in the well of the furnace. In order, however, to separate the metal from unreduced ore, the furnace is allowed to cool somewhat, and lime is added and mixed with the charge by means of a rake. This makes a stiff slag, from which the molten metal more easily separates, and also serves to combine with any silica present, and liberate lead oxide with which the silica may have been combined. Lastly, the temperature is again raised, and more lime is added, so as further to stiffen the slag; then the molten metal is run off from the taphole, and the solid slag, technically known as grey slag, raked out from the furnace. Some slag, however, still remains with the metal, and this is eliminated by stirring coal slack into the mass by means of a paddle. The combustion of the coal melts and liberates the metal confined in the slag, and the latter is then skimmed off from the surface of the metal.
The slag may contain as much as 40 per cent, of lead, which is recovered by treatment in a special blast-furnace called a slag-hearth, or in the blast-furnace employed in method (ii). Ores smelted by the air reduction process in Flintshire contain from 75 to 80 per cent, of lead, 90 per cent, of which is obtained directly from the reverberatory furnace, and the remainder by the subsequent treatment of the slag.
The Carbon Reduction Process Ores in which lead exists as oxide or carbonate can readily be reduced by carbon, after the manner in which iron ores are reduced in the blast-furnace. Moreover, lead sulphide, occurring as galena, may be converted into oxide by efficient roasting, and this oxide may subsequently be reduced in the same way. Besides this, the method of carbon reduction in a blast-furnace is applicable to ores containing silica, iron and copper pyrites, and other minerals. Consequently, the carbon reduction process is now perhaps the most important part of the metallurgy of lead.
The blast-furnace (Fig.) in which lead ores are smelted is rectangular, and about 3 feet 6 inches wide, and 12 feet high above the tuyeres. The lower part or bosh of the furnace is made of cast-iron or steel, and surrounded by a water-jacket through which the tuyeres pass; the rest of the furnace is lined with fire-brick.
|Blast-furnace for lead smelting. |
A. Charging floor; B. Charging pipe; C. Blast-main; D. Tuyeres, or blast-pipes; E. Water-jacket surrounding hearth of furnace; F. Slag lip; G. Tapholes.
The preliminary process of roasting is carried out in a reverberatory furnace, or in a vessel shaped like a Bessemer converter, through which air is blown. When the lead sulphide has been converted into oxide and sulphate, the roasted ore is heated to incipient fusion which causes combination between lead oxide and silica. If copper is present in the ore, the sulphur is not all oxidised by roasting, but some is left to combine with the copper and form a matte. The roasted ore is then smelted in the blast-furnace with coke and a flux consisting essentially of iron itself or compounds of iron. The iron may be in the form of ore, which may or may not have been originally associated with the lead, or in the form of basic iron silicates derived from the refining or puddling of iron, or from copper smelting. In the smelting process some of the lead oxide is directly reduced by the carbon of the fuel, but for the most part the iron is first reduced, and this in turn reduces the lead. Moreover, if lead sulphide is present metallic iron will react with it, forming iron sulphide and metallic lead; the iron will also decompose lead silicate, liberating the metal. And since material poor in lead can be smelted in this furnace, the grey slags formed by the smelting of galena in the reverberatory furnace may be reduced here.
The slag should consist of the silicates of iron, calcium, aluminium, and magnesium, and not contain more then 3 per cent, of lead; if, however, sulphur was still present in the ore, there will also be a matte or regulus of sulphides of lead, iron, and copper, together sometimes with silver and gold. The matte is resmelted to remove the lead and concentrate the copper. Lead obtained from matte, however, is very impure.
The Condensation of Lead Fume. - Owing to the volatility of lead some of it is carried away and oxidised in the gases passing from the different furnaces employed in the extraction and purification of the metal. Various compounds of lead with other products are deposited when these gases cool, and constitute " lead fume." This " fume " consists chiefly of lead oxide and sulphate, together with smaller quantities of lead sulphide, ferric oxide, alumina, zinc oxide, lime, and insoluble matter; and various arrangements have been adopted to collect this fume, and resmelt it. The method usually employed is to connect to the furnaces long flues which may be as much as 8 feet by 9 in cross-section, and from three to five miles long. Jets of water or steam may be made to enter the flues at intervals to assist precipitation of the fume; or a filtering arrangement of faggots, gauze, sawdust, or canvas bags, known as " bag houses," may be employed. An alternative method - that of Messrs. Wilson and French - is to pass the fume-laden gases, after cooling, through a condenser containing water in which the fume is deposited. The sludge is then removed to a settling tank, where the fume is separated from most of the water. It is subsequently dried and smelted in a blast-furnace.
The Precipitation Process This "process of affinity," which consists of the reaction
PbS + Fe = FeS + Pb,
plays some part in the smelting of lead in the blast-furnace already described; otherwise it is of relatively small importance. It has, however, been employed in France for the reduction of Spanish galena in a reverberatory furnace, but is said to be wasteful and expensive.
Purification of Lead
|Pig lead, obtained by any of the above processes, contains a number of impurities; these may be antimony, arsenic, copper, zinc, iron, tin, bismuth, silver, nickel, cobalt, and sulphur. The general effect of these impurities is to harden the lead; silver, moreover, may be recovered economically from the lead, much of this metal being now obtained from argentiferous galena. |
There are, therefore, two distinct branches of lead purification - softening and desilverisation.
Softening of Lead There are two processes of softening - liquation and oxidation. The process of liquation, which is employed especially to eliminate copper, consists in carefully heating the crude metal on the sloping bed of a reverberatory furnace. A purer lead melts and runs away, leaving behind an alloy which contains the copper together with nickel and cobalt, and sometimes arsenic and sulphur.
Other impurities are removed by oxidation. The metal is melted in a reverberatory furnace, and exposed at a red heat to the action of air. A scum forms on the surface, consisting of the more oxidisable impurities together with some oxide of lead; lime is sometimes added to stiffen the scum, which is removed from time to time; and the molten lead beneath it is tested at intervals, so that the process may be continued until oxidation of impurities is complete. The first oxidation products contain most of the tin, later ones the antimony. Bismuth is not removed in this way, since it is not more oxidisable than lead; but it is associated with the silver in the Pattinson process for desilverisation.
Desilverisation of Lead Originally the only process available for recovering silver from argentiferous lead was that of cupellation, by which all the lead was oxidised, and so needed again to be reduced to metal. Now three other processes are in use which are associated with the names of Pattinson, Rozan, and Parkes.
The Pattinson Process This process, sometimes spoken of as Pattinsonising or Pattinsonage, was invented by Hugh Lee Pattinson, of Newcastle-on-Tyne, who took out a patent, in October 1833, for " An improved method of separating silver from lead." The method depends upon the fact that when an alloy of lead with not more than 1.8 per cent, of silver is melted, and allowed to cool slowly, pure lead crystallises out, leaving a molten alloy richer in silver. The principle of the process is the same as that according to which pure ice separates when a dilute aqueous solution of a substance is sufficiently cooled. Moreover, pure lead is denser in the solid than in the liquid state, and therefore sinks to the bottom of the molten alloy.
The process is carried out in a series of iron pans, each heated over a fire. The lead is melted in the first pan, skimmed, and then allowed to cool; the sprinkling of water on the surface helps the cooling. As solid lead separates it is pushed beneath the surface of the molten metal, so that it may redissolve if possible. After a time crystals of lead accumulate at the bottom of the pan and are then removed by a perforated ladle, allowed to drain from the liquid argentiferous lead on their surface, and then placed in the next pan, which has already been made hot enough to melt them. In this way two-thirds, or even as much as seven-eighths, of the metal is removed from the first pan, leaving behind a correspondingly enriched alloy. In practice alternate pans of different qualities are generally being crystallised simultaneously, and the rich remainder of one pan is mixed in the intervening pan with the purer lead from the other pan. By successive crystallisations and removals of lead from the various pans, the original alloy is gradually separated into purified lead at one end of the series of pans, and an enriched alloy at the other end. So from a lead containing only 10 ounces of silver per ton, a rich alloy containing from 600 to 700 ounces per ton is separated; this is then cupelled. The Pattinson process is generally employed for lead too poor in silver to be profitably dealt with by any other process.
The Rozan Process or Pattinsonising by Steam This process, introduced in the works of Luce and Rozan at Marseilles, has been described by Cookson. Two pots only are required, a melting pot and a crystallising pot. The molten alloy, after removal from the melting to the crystallising pot, is stirred up in the latter by high- pressure steam, while the surface of the metal is cooled by water. About two-thirds of the lead is allowed to crystallise, and then the still liquid alloy is drained off from the crystals through spouts protected by perforated plates, which prevent solid lead from leaving the pot. Another charge of lead, of silver content equal to that of the crystals remaining in the crystallising pot, is added from the melting pot, the whole remelted, and the operation repeated. It is claimed for this process that it obviates the need for previous softening of the lead, since the steam, either itself, or by the air it carries into the pot, causes the oxidation of impurities; also that its use effects a great saving in labour, fuel, and the amount of dross formed.
The Parkes Process - Desilverisation by Zinc Molten lead and zinc are partially miscible liquids, like water and ether. Equilibrium is established between the two liquids when the lead has dissolved 1.6 per cent, of zinc, and the zinc 1.2 per cent, of lead. It was observed by Karsten in 1842 that silver is more soluble in molten zinc than in molten lead, so that zinc when melted with argentiferous lead will remove and dissolve its silver; just as, for example, aniline is more soluble in ether than in water, and ether when shaken with water containing aniline will consequently dissolve out the aniline.
Practical use was made of these facts by Alexander Parkes, of Birmingham, who, in the years 1850 to 1852, took out patents for the desilverisation of lead by zinc.
The lead is melted in one of a series of pots, and heated to the melting-point of zinc. Slabs of zinc are then added, and the contents of the pot are stirred until the zinc is melted. The amount of zinc used varies between 1¼ and 2 per cent, of the lead, according to the quantity of silver present; and it is generally added in three successive quantities, the separated and solidified zinc, containing the silver, being removed to a smaller pot before more zinc is added.
The zinc remaining in the desilverised lead is removed by oxidation with air or steam, followed by skimming; or by the aid of an alloy of copper or copper-aluminium with lead, which takes up the zinc and forms a crust on the surface of the lead. The zinc is then recovered from the alloy by distillation.
From the zinc-silver alloy, some of the lead that it contains is removed by liquation. The alloy is then heated in a fire-clay retort, by which means the zinc is distilled and recovered; and the rich silver-lead alloy remaining is then cupelled.
If gold and copper are present in market lead they are also separated fiom it by the Parkes process.
Electrorefining of Lead Lead is refined by electrolysis, impure lead being made the anode, and a thin slab of pure lead the cathode, in a cell containing a suitable lead solution as the electrolyte. The most successful process is that of Betts, wherein the electrolyte consists of a solution of lead silicifluoride, formed by dissolving the carbonate in aqueous hydrofluosilicic acid, to which is added 1 part of gelatine in 5000 in order to secure a coherent deposit of lead. The electro- deposition of lead is also very satisfactorily carried out by the employment of a solution of the perchlorate.