Lead poisoning and lead absorption

by Sir Thomas Morison Legge; Kenneth Weldon Goadby

Lead ores, drosses, etc., on arrival at the factory, are, after sampling, deposited in bins or heaps (often in the open air), and watered to prevent dust. All ores may, and refractory ores (containing over 4 per cent. silica) and dross must, be smelted in a blast furnace by aid of coke. The bulk of the charge in a blast furnace may consist of more or less complex ores of the precious metals, especially silver.

When galena is treated in a blast furnace, preliminary roasting is indispensable, and in many smelting works its treatment takes place in a reverberatory or open-hearth furnace, and not in a blast furnace.

The three principal methods applicable to extraction of lead from ores are—(1) The roast and reaction method; (2) the roast and reduction method; and (3) the precipitation process.

By the roast and reaction method a part of the galena is first converted into oxide and sulphate of lead with access of air. Subsequently, on shutting off the air-supply and increasing the temperature, a reaction takes place. The sulphur in the unchanged sulphide combines with the oxygen of the oxide and sulphate to form sulphur dioxide, which is carried away by the draught into the bricked flue, leaving metallic lead behind. The process is carried on in a reverberatory or open-hearth furnace.

In the roast and reduction method the first portion of the process is carried out in a reverberatory furnace, the galena being roasted pretty completely to lead oxide and sulphate, which are then—usually in a blast furnace—reduced to the metallic state with coke and other reducing agents, such as iron.

By the precipitation process galena was decomposed at a high temperature by means of metallic iron, forming a mixture of iron and lead sulphide. This method was only applicable to rich lead ores, and is now given up.

The three methods are hardly ever independent of one another, as the rich slag or residues, for instance, which are obtained by the first method are retreated by the second, and the second is, as has been stated, almost always combined with the first.

On tapping the blast or reverberatory furnace, the lead is drawn off into a lead well or sump, from which, when cool, it is ladled into moulds, while the slag is run into movable metal pots or along specially-prepared channels. The slag run off from the reverberatory furnace contains much lead locked up as silicate, which requires to be retreated, usually in the blast furnace. During the roasting process much raking of the material is necessary. The slag from the blast furnace should contain less than 1 per cent. of lead.

On the Continent and in America, the Huntingdon-Heberlein process has been extensively adopted, with lessened incidence of poisoning, the result of mechanical methods of working, obviating hand labour, and the low temperature (diminishing risk from lead fume) at which the roasting is carried on. In this process the crushed ore is desulphurized by first mixing with lime and heating in presence of air in a revolving furnace, provided with automatic rabble, at moderate temperature (about 700° C.). Subsequently the roasted material is conveyed from closed bins, into which it falls automatically, by dust-proof elevators to a converter, in which atmospheric air at slight pressure is forced through it. The agglomerated mass so formed, when tipped out of the converter (in doing which there is risk from dust), is well damped, broken by hand, and charged with coke in the usual way into the blast furnace.

In some lead-smelting works the material arrives on the premises in the form of ingots of base bullion—i.e., impure lead rich in silver—the product of previous smelting of the ore where it is mined in Australia or Spain. And one of the main objects of the blast-furnace smelting of galena in the factory is to produce a base bullion rich in precious metals. The lead so obtained requires further softening or refining to get rid of copper, antimony, arsenic, and tin. This is effected in a reverberatory furnace, first at a low temperature to allow of formation of furnace dross, which is removed through the working doors, and secondly with increase of heat and access of air to oxidize, in the order named, the tin, arsenic, and antimony. Finally the lead is tapped into kettles or pots. If free from silver, such lead, when poured into moulds, is ready for the market; but if rich in silver, it is treated for the recovery of that metal either by (a) Pattinson’s process, depending on the higher temperature of crystallization of lead than of an alloy of lead and silver, which enables a separation of one from the other to be made by a process of ladling the crystalline from the liquid portion; or, much more commonly, by (b) Parkes’s process, depending on the formation, on addition of zinc to a pot of molten lead, of crusts consisting of an alloy of silver, lead, and zinc. The crusts obtained in the latter process, after cooling, are broken up, placed in a crucible, and the zinc driven off at a temperature of 1,000° C. in a dezincing Faber du Faur retort. The rich bullion, retained either in the last kettle by the Pattinson process, or remaining in the crucible after dezincing, next undergoes cupellation—i.e., exposure to a blast of air in a furnace. The lead is oxidized into litharge, which drops into a receptacle below the furnace, leaving the silver behind. In all lead-smelting works the draught from the furnace carries much dust of ore and fuel, and fume, consisting of sulphide, sulphate, and oxides of lead, into the flues. The dust is easily collected in dust chambers, but the fume requires ducts of great length—sometimes a mile or more—in which to deposit.