The hollow axis of the agitator is connected with a pipe, by which carbonic acid gas is supplied to it under pressure sufficient to cause the gas to pass through the contents of the vat. This gas escapes at the lower ends of the pipes g, and ascends through the liquid in which the hydrate and acetate of lead are held suspended. A free flow of this gas should be maintained, and some excess allowed to pass away by a pipe on the cover into the ammonia catch box r. The pipe dips down into the acetic acid which this box contains, and any ammonia passing off with the carbonic acid gas is caught by the acid, and forms acetate of ammonia. When the hydrate and acetate of lead are completely converted into basic carbonate (and this the workman will know by its changed appearance), the motion of the agitator is caused to cease, and the white lead is allowed to settle.
There is a gauge glass on the side of the vat a, and in this glass the changes of appearance can be recognised by which the workman regulates the process.
When the white lead is deposited, the liquor is drawn off into the receiver i by opening the cock h. The plug p is then displaced, and the white lead is allowed to descend into the washing cistern m. Below the cock h is another cock or cocks, in order that any further quantity of liquor may be drawn off into the cistern i if required. The taper plug p is then raised, by being pushed up from underneath the vat a, through pipe l.
The white lead requires to be well washed with clean cold water.
When a fresh charge of monoxide of lead has been placed in the vat a, the liquor is pumped up on to it from the receiver i, and then a further quantity of solution is drawn from the cistern b, and also from the ammonia catch box r, until the proper quantity has been supplied, which is determined by the gauge.
Quite recently, in conjunction with Mr. Watson Smith, Mr. MacIvor has introduced further improvements in the production of “white lead” or basic carbonate of lead, by the treatment of oxide of lead (litharge or “massicot”) with a heated solution of acetate of ammonium, in a closed vessel (called the digestor), with agitation and under pressure, so that the oxide of lead is transformed with exceeding rapidity into basic acetate of lead, chiefly consisting of the tribasic acetate, ammonia being set free; and the subsequent treatment of the basic acetate thus obtained, after filtering or otherwise removing insoluble impurities, and after cooling with carbonic acid gas in a separate vessel (called the carbonator).
The acetate of ammonium is by preference first charged into the digestor, and then the oxide of lead (litharge or massicot) in fine powder, and in the equivalent proportions calculated to be at least somewhat slightly in excess of the quantity necessary to form with the acetic acid of the acetate of ammonium the tribasic acetate of lead Pb(C2H3O2)2 + 2 PbO or Pb3O2 (C2H3O2)2, is added (this being in allowance for the certain amount of insoluble and unconvertible matters in the litharge), the acetate of ammonium liquor of a strength preferably not below 5 per cent., though it may be stronger, being first, before the addition of the oxide of lead, set in vigorous agitation and circulation by the pump, and heat having been applied by means of a steam heater as shown in the drawing and as will be explained later on.
The digestor is closed, and the temperature rises. As the temperature rises, and approaches 212° F., and as the pressure, due more especially to the tension of vapour of ammonia set free during the reaction with the litharge, increases, so is the rapidity of the conversion of the oxide into basic acetate increased, and more and more of this basic acetate becomes dissolved, whilst at or about 212° F. it is entirely in a state of solution. The degree of heat, or the prolongation of the heat, depend of course upon the state of dilution of the acetate of ammonium used. Something more specific will be said later on regarding this question of degree of heat.
The clear liquor, together with extraneous coloured particles, red lead, dirt and undissolved matters, is pumped out through a suitable filter to one of the carbonators, being carried en route through a cooling-pipe system, and being let cool further if necessary in the carbonator itself. After cooling thoroughly, the cold basic acetate, which may now have crystallised or separated out more or less, according to the strength of the solution, is treated with carbonic acid gas, whereby the basic acetate, in the presence of the ammonia set free, is converted into basic carbonate of lead of exceptionally white colour, and with high basicity and covering power, proper care being taken as specially indicated later on. There are several reasons why greater whiteness of the product is secured, the principal one being that filtration from solid and insoluble coloured impurities has taken place previous to the carbonating process.
The advantages of the employment of heat and pressure for the formation of the basic acetate of lead from oxide of lead, and the advantage of thus carrying out the conversion of oxide into basic acetate of lead separately from the conversion of that basic acetate of lead into basic carbonate or white lead, are—