This process was introduced by white lead corroders some thirty years ago, probably much earlier, but if so it was kept very quiet, as it did not become generally known to the trade until twenty-five years since. Even then the process was by no means new, as it had been carried on for years, but on a very small scale, and for a different purpose. When it was adopted on a large scale it was to save time and expense of drying the pulp lead after washing, and taking it all in all, aside from economical reasons, there is another phase to the question, which may be stated in short as follows: - When the hydrated carbonate of lead is mixed with linseed oil by agitation, thus separating the water from the pulp and uniting lead and oil without more heat than is generated by the friction incidental to the agitation, the resulting product will if all the water is thoroughly expelled, be free from any tendency to become stringy or gum up, as it is liable to do when run through overheated stone mills. The process resolves itself into something rather simple. It requires a building with several strong floors to conduct the process in, as the only economical way is to do it by gravity. The pulp washing and storage tanks must be on the top floor, whence the well washed pulp containing at least 33 per cent. of pigment is run into another large tank of wood that contains a stirring device, also of wood and steam coils of leaden pipes for the purpose of bringing the pulp to a temperature of as much as 120 degrees F., because at this temperature the agitation and separation of water is effected much more rapidly. On the floor below there is a weighing box on a stationary scale, which has near its top an overflow pipe and on its bottorn at one end several discharge outlets with valves that are directly over the agitators. These, which may be eight or ten feet deep and three feet or more in diameter, are made of heavy timber having heavy shafts running in a step in bottom of tank, while very strong mixer arms with heavy blades are fastened to the end of shaft in bottom and about three feet from the bottom there is a heavy cross arm that holds the shaft in place, and at the same time contains blades that extend toward the bottom of the tank. On this cross bar rests a brass sleeve that reaches to the top of the tank, but is loose and can be started by means of a clutch on the shaft by dropping a lever. This brass sleeve has a number of brass blades that are so arranged as to give the oil a downward course while the agitation is going on. The lead in oil, after water has been drawn, is discharged from the agitators by means of gates and passed into other mixers below, when all the water possible is drawn off and passed over stone rollers on the lowest floor, which squeeze out all the remaining water if proper care is exercised. So far the apparatus, which, of course, varies in construction according to the facilities of the buildings in which it is located. Now as to the manipulation itself. The well-washed pulp is run or pumped into the stirring tank and there allowed to settle to some extent, so that the excess of water may be run or siphoned off. This done, the mechanical stirrer is placed in action and the steam turned on for warming up the pulp lead. When sufficiently heated enough of it is discharged into the weighing box below to fill it up to the overflow pipe referred to above, and the weight ascertained. In order to make this fully understood, we will say that the weighing box is of wood, very strongly constructed, resting on a framework that again rests on a large platform scale of say 5,000 pounds capacity, and the box is inclined toward the agitators, so that it may be flushed with clear water from a hose after being emptied of its charge of pulp lead. Assuming that the weighing box and frame work weighs 1,500 pounds, and that the box itself holds 1,000 pounds (120 gallons) of clear water when filled to the overflow, the dry weight of the lead contained in the pulp when the box is filled to the same point of overflow with pulp lead instead of water is figured on this equation:-Assuming that a batch of pulp lead in the box shows a gross weight of 3,500 pounds, we subtract the tare of frame work 1,500 pounds, leaving 2,000 pounds of pulp lead and water. From this we subtract 1,000 pounds for the capacity of the box when filled with water, leaving a balance of 1,000 pounds to be accounted for. Multiplying this by the figure 1.18, which has been ascertained to be the correct multiple, we have 1,180 pounds of dry lead in the 2,000 pounds of pulp. This having been ascertained, the pulp is run into one of the aforesaid agitators, and when the mixer is in motion a weighed portion of oil, amounting to not less than 9 per cent of the weight of the dry lead ascertained as above is run into the agitator in a thin stream.
The higher the temperature of the pulp lead in the agitator or separator, the quicker will be the union of lead and oil and the separation of the water, always providing that the oil is of the right character. At certain times the separation of the water is effected in as short a time as twenty minutes from the time of putting the mixer in motion, while sometimes it will require over an hour or more. The most rapid action takes place when well refined linseed oil is being used and the oil must be refined by the sulphuric acid process, well washed and clarified by settling before use. Raw linseed oil will not do the work no matter how well settled and clarified by age. It requires too much time to effect separation of the water, and the lead in oil so manipulated retains too great a percentage of the water. The process of separation, when properly attended to, is very interesting, because of the lightning-like rapidity of the clearing of the water and the uniting of the lead and oil. At first, as the stream of oil is run in the agitator on top of the pulp, it floats on top for a few minutes, then appears to form an emulsion with the water of the pulp, but all of a sudden the water clears, and on stopping the machine the lead in oil can be seen in the bottom of the mixer, so clear is the water, which is removed by means of a steam siphon and another small percentage of oil, say one-half of 1 per cent of the calculated weight of dry lead added, at times even more, as may be necessary. With this addition of oil the lead in bottom of mixer is agitated some more, and then enough drawn into the mixers above the roller mills to relieve the agitator sufficiently to admit of running in another batch of pulp. In the mixers above the roller mills the mixing obtains the finishing touch, because here any moisture that may still be present comes to the top and can be removed. This additional manipulation also imparts compactness or density to the product, which is always more loose than dry ground lead in oil. The further running of the pulp ground lead over stone rollers aids in still further imparting density as well as pressing out moisture should such be still present. In some instances the lead has been run over a double set of stone rollers, or six of them in all. Some one may ask wherein is the economy of this process with all the stages it has to go through, but any one that is or has been connected with the manufacture of white lead will readily see the benefit derived as against the process of drying the lead on steam jacketed copper pans or in drying rooms on wooden trays, the use of iron being out of the question because of discoloration from rust. Before the advent of the steel keg, wooden kegs and pails were glued on the inside to keep the oil from being absorbed by the staves, but most of the packages containing pulp ground lead in oil were not sized in order to permit the wood to absorb the moisture that had not been entirely expelled. As steel kegs and pails are now in general use, pulp ground lead must be manipulated much more carefully than it used to be years ago, as it is otherwise liable to discoloration.