THE METHOD OF OPERATING THE MACHINERY JUST DESCRIBED.
During the treatment of cast iron in the Bessemer apparatus, the operator concen trates his attention upon the following points:—the character of the flames and sparks that flow from the mouth of the converter; the character of the smoke that rises from the top of the chimney, which, by reason of the abundant sparks generated in the operation, cannot be studied below; the noise of the cast iron in the converter; the height of the air-gauge, which indicates the pressure necessary to force the air from the tuyers; and, finally, upon the duration of each of the phenomena of the process, and the total duration of the blowing.
Let us watch, for an example, the converting of a charge of three tons of red-hema tite charcoal iron through the process of conversion into steel. The air is blown at a pressure of 20 pounds to the square inch.
The converter is first heated white-hot before the metal is poured in, by filling it with coke, and blowing into it gently with the blast-engine. The coke is poured out, and the vessel returned to its erect position; the metal from a reverberatory furnace is then run by a trough into the converter, and the blast turned on. The blowing is continued twenty-two minutes, until over-refined iron is produced. The vessel is then turned to a horizontal position, the blast stopped, and an addition of 7 per cent. of Franklinite or other spathose iron containing manganese, in a fused condition, is made : this is mixed thoroughly, and the mixture poured into the ladle above described, and thence into the moulds. In looking more closely into the process, we may divide the twenty-two minutes during which the blast is being driven into the fused metal into four periods.
First. Lasting seven to eight minutes, being the period of sparks, and until flames commence.
Second. Period of eight minutes, during which flames augment and the " dart-like" flame is formed.
Third. Period of two minutes, of detonations and eruptions.
Fourth. Period of four or five minutes, increase of amount and brilliancy of flame until the flame falls.
Upon the air reaching the fused metal, large numbers of sparks are projected through the neck of the vessel and into the chimney by the violent blast. These are combustible
particles, that burn brilliantly in the oxidizing currents. The chimney as yet produces no smoke. The noise formed in the converter is dry and crackling, and indicates that the metal raised by the blast falls back upon itself. The noise grows gradually into a regular and dull clapping. The iron, during this period, becomes more and more hot and liquid. The combustion, however, is accompanied by no visible flame.
During this period the combustion, which causes the metal to grow hotter and hotter, does not appear to produce any combustible gas. The current flowing from the con verter appears to be still oxidizing, since the sparks continue to burn in the midst of the current. If the iron is poured now, it has the appearance of pig partly refined and having lost but little in weight. These facts lead us to believe that a part of the oxygen passes through the iron uncombined, and that the combustion is caused by the oxidation of a body more oxidizable than iron, without attacking sensibly as yet the carbon or the iron. The sparks are, indeed, as characteristic of the combustion of silicum and metallic manganese, as of iron.
We may, then, conclude that the heat at the beginning of the operation is furnished by the oxidation of a body more combustible than iron; that the carbon, if it burns at all, produces carbonic acid gas; and that the temperature of the mass is not high enough to absorb all the oxygen.
The sparks diminish, and are masked and replaced by flame. At six or seven minutes a red, dull, short flame begins to show itself ; at eight minutes it is more dis ,tinct, and begins to be yellowish and bright. At first only a few inches long, it gradually grows to be three feet in length. About the middle of the period, a dart like flame shows itself at the throat of the converter, resembling the flame of a candle, and continues during the remainder of this and a part of the following period. The noise in the converter becomes drier and drier, and less perceptible, and is replaced by a sound like the formation of innumerable bubbles of gas. No smoke yet issues from the chimney. The back-pressure of the air diminishes.