Lead poisoning and lead absorption

by Sir Thomas Morison Legge; Kenneth Weldon Goadby

Dangers and Prevention.

—In casting the strips, risk does not arise from lead fume, as the temperature at which this is effected (350° C.) is too low for appreciable fume to be given off. Danger here is from skimmings and deposit of them on the floor or into a receptacle unprovided with exhaust draught. Pots in which remelting of the uncorroded cores (returns) is done should be provided with hoods and exhaust, because of the dust given off in stirring and skimming and the spurting which occurs as they are thrown in wet. In making the blue beds, dust arises from particles of white lead adhering to the pots and in the tan bark. Pots, on removal from the white beds, should have all white lead inside them removed by washing in a tank. Screening of the bark should be dispensed with. Emptying the white beds accounts, perhaps, for the largest number of cases, owing to the impossibility, in the present state of knowledge, of dealing with the dust by means of exhaust ventilation, or quite adequately by watering or wearing of respirators. Watering by means of a hosepipe with rose attached is, however, the main safeguard. Substitution of the square cockney pots for the long castle pot is also of moment, as the flat plates of lead form a denser and more porcellanous corrosion than that of the grids in the castle pots. Moreover, in stripping the beds the flat corrosions can be lifted into trays without creating dust, whereas to dislodge the contents of the castle pots may require a sharp tap, and the unglazed portion of the interior surface of the latter retains some carbonate when moistened. Watering requires to be thorough and done with care, or else the softer material of the corrosion may be washed into the tan. No less important is it to water the layers of tan, and at a time while they are yet warm and slightly damp, otherwise the tan becomes so dry that the water runs through, and does not adequately prevent dust formation on its removal. A requirement of the special rules is that the trays for collecting the corrosions shall not stand directly upon the beds. When corrosions contain an undue proportion of lead acetate, they are termed technically “floury,” and much dust may arise from them on watering unless this be done with a very fine rose.

Dust at the rollers and wash becks is usually checked by preliminary immersion of the tray of corrosions in a trough of water, but the extra weight of the water causes this sometimes, in the absence of mechanical arrangements for immersion, to be done perfunctorily. Where there are rollers, the tray is inserted in a small opening above them, the contents saturated by a spray, and then tipped over. This method also may fail to control the dust, as, unless the men engaged in washing the corrosions in the wash beck keep the mass tipped in from piling up in a heap, the contents of the trays are discharged on to the heap, and not into water. In some factories exhaust ventilation at the rollers or wash becks has been necessary.

During subsequent wet processes of grinding danger is mainly from splashing. From the settling tanks the white lead is pumped into filter presses, and the resulting cake is dried. Here the considerable risk from splashing is again almost unavoidable. Concrete floors are necessary. Emptying the stoves involves much handling, with inevitable creation of dust, especially when the bowls are withdrawn from the racks. Risk has been greatly lessened by reducing the height of the shelves to 10 feet and prohibiting the piling of one bowl upon another. Mechanical drying stoves into which men need not enter either for filling or drawing are now commonly met with. Of these there are various types—(1) Horses similar to those common in laundries, which can be withdrawn on rails; (2) small chambers built up one upon another somewhat in the form of gas retorts in a gasworks, heated by steam jackets and coils, each chamber containing only two or three cakes of white lead pulp, the cakes themselves being removed by a mechanical process from the press into the drying chambers; (3) bogies carrying the white lead in bowls on racks made to pass through the tunnel-like stove; (4) drying machines—i.e., closed cylinders fitted with a series of platforms so arranged that they may be charged with white lead on one side, and so fixed as to be turned round by means of mechanical appliances. When dry, the material is discharged into a chute by a series of scrapers into a small enclosed compartment, holding the barrel to be filled. With the drying machine, however, there is considerable risk of dust leakage, especially when the doors are opened. In packing by hand, safety depends on efficient exhaust ventilation when the contents are tipped into the barrel, but a danger constantly present is that, to get through the work quickly, the bowls may be withdrawn from the influence of the exhaust before the last traces of dust have been removed from the bowl. Mechanical packing, by means of a large bladed screw forcing the white lead into the barrel, which as it becomes filled is lowered automatically, is everywhere desirable. An essential condition of this method is that a dust-proof collar should connect the automatic packer with the barrel. Some dust almost inevitably escapes, and a hood and exhaust should be provided, however perfectly the machine is said to act.

Much of the white lead is converted into paint on the premises, being ground in oil either in pug mills, Torrance mills, or under edge-runners. A negative pressure must be maintained inside the casing, which must enclose the stones. Here the conditions are precisely those described under the manufacture of paints and colours. In some white lead works conversion into paint is done without the dangerous process of stove drying, either by drying under a vacuum or by mixing the white lead directly with oil. In the process of grinding the oil incorporates itself with the white lead, and the water is forced out, running away in a clear stream.

Chamber Process.

—In this method, almost universally used in Germany, and adopted in at least one large white lead works in this country, a chamber arranged with numerous sets of parallel bars on which the thin strips of lead are set saddle-wise takes the place of the stack in the Dutch process. Carbonic acid gas and acetic acid vapour act on and corrode the strips. In a period of from eight to ten weeks the corrosions mostly fall to the ground. Such of the strips as do not fall have to be lifted off the bars, having been previously well saturated with water from a hosepipe, and are dropped on to the floor of the chamber. We are not satisfied that working in the dark, confined chamber by artificial light is less dangerous than working on the stacks. Chamber-made lead undergoes practically the same subsequent processes as have been described.

Precipitation Processes.

—These also dispense with stacks and the consequent risks attending work on the blue and white beds, but they substitute another—namely, use of oxide of lead (litharge or the suboxide) as the initial product to be carbonated, with the inevitable danger, in the absence of mechanical contrivances entirely closed in, of shovelling dusty material. In many of these methods, however, mechanical arrangements obviate hand labour or contact with dust in all but the first process.

The Brimsdown process[21], for instance, is automatic and free from dust, except in the initial stage of preparation of litharge in cupellation furnaces. The great risk from the disintegration of this material (see [p. 250]) by turning it out on the floors is obviated by allowing disintegration to take place in the pots (which must not, therefore, be completely filled), and tipping these when cooled directly into a breaker under powerful exhaust draught. From the bin into which it falls the material is conveyed by dust-proof elevators to (a) screens and packing arrangements when the object is flaked litharge, or (b), in the case of the bulk of the material for manufacture of white lead, by enclosed conveyors to reducers and mixing mills, where reduction and hydration take place. It is then charged automatically into weak solution of acetic acid, and by agitation with carbonic acid gas converted into basic carbonate of lead. From the carbonators pumps force it into filter presses, where the acetate is drained off and washed out by pure water. The cakes of white lead are fed into mixing machines and pugged with linseed-oil until the water has been entirely removed, and finally passed through the roller mills to be packed in casks.