The late Captain W. R. Jones, of the Edgar Thompson Works, solved the difficulty by the invention of the "mixer," a huge steel vessel lined with firebrick, mounted on trunnions and poured by tipping back and forth with a hydraulic cylinder. The first of these had a capacity of 150 or 200 tons of iron, but later ones have capacities up 600 and 800 tons.

All the iron from all the furnaces is poured into this mixer and that for all the converters is drawn from it. This provides a "cushion" to absorb the fluctuations in the quality of the metal and secures gradual variations of the metal delivered to the converter, which enables the converting mill to adapt itself to these changes without difficulty, while of course the mere fact of mixing the output of three or four furnaces necessarily reduces the average variation at least to one-third or one-fourth of that for any individual furnace, and generally much more. Thus the mixer exercises two good influences. It greatly reduces the absolute size of the variations, and it makes them gradual instead of sudden.

This improvement permitted the use of direct metal at the steel works, and practically all modern plants of combined blast-furnaces and steel works operate on this system. Where steel is made by the basic open-hearth process as well as by the Bessemer, a separate mixer is used for this iron, and the same advantages are secured in that practice.

Hot Metal Ladles

The use of "hot metal," as it is universally called, for the steel works involved the necessity of having ladle cars to transport it from the blast furnace to the mixer and thence to steel works. These started out quite small, ten or fifteen tons capacity, but have gradually grown with the increased use of hot metal until 45, 50 and 60-ton ladles are now considered standard.

Fig. 217. Pollock hot-metal car.

A ladle capable of carrying this amount of molten iron, some times for distances of several miles, with entire safety from breakage of the apparatus or liability to accidental tipping, and yet capable of being easily poured where desired, is an apparatus of no mean proportions.

Such a ladle built by the William B. Pollock Company is shown by Fig. 217. It will be noticed that this ladle is provided with five trunnions, two pairs below and a single one above on each side. It is also provided with a spout on each side and with a grip for a hook at the bottom immediately under the spout to provide a convenient hold for the tilting mechanism. The single trunnions above are used in carrying the ladle on cranes; the two lower sets are used for pouring from the frame of the ladle car itself. The ladle revolves during the early part of the tilting movement around the forward one of the bottom pair of trunnions, but if it continued to revolve about this until empty its spout would be very low, necessitating a heavy drop for the metal poured at first in order to make room for the spout in its low position. The purpose of the upper pair of trunnions, with the corresponding jaws on the ladle truck shown beneath them, is to prevent this low drop of the ladle spout, and it is obvious that when these upper trunnions reach a seat in the corresponding jaws on the truck the ladle revolves around them, the lower pair leaving their seats. This obviously throws the spout up much higher at the conclusion of the pouring operation. By this means the drop from the spout of the ladle is reduced several feet over what it formerly was, which is an advantage wherever it is used, but particularly in connection with casting machines to be described presently. The receiving end of these with the old style ladle had to be placed below the track, but with the style here shown, on account of the high position of the spout at the end of the pour, these machines can be raised above the track with the result of making them much more accessible and less expensive to install.

Fig. 218. Treadwell hot-metal car.

A hot metal ladle built by the Treadwell Engineering Company is shown by Fig. 218.