In the United States about 20,000,000 tons of steel are made annually by the basic open-hearth process while only 1,100,000 tons are produced by the acid open-hearth process.
The two processes are practically the same except that by the basic process the phosphorus as well as the silicon, manganese, and carbon are reduced or eliminated. In order to take out the phosphorus, additions of lime (i. e., calcium oxide or calcium carbonate) are made just as occurred with the basic Bessemer process.
Should we use lime in a furnace having an acid lining, much of the lime, which is a “base,” would react with the “acid” (silica) bricks of the lining, and, becoming neutralized, would not do its work. So, as in the basic Bessemer process, we here have to use either “basic” or “neutral” lining.
The material generally used is burnt magnesium carbonate which is known as “magnesite.” Dolomite, which is a combination of the carbonates of calcium and magnesium, is sometimes used. Chrome bricks, the usual neutral material, are rather too expensive for extensive use. The best magnesite comes from Austria and is usually not very cheap. As acid materials (those of silica or clay) are cheaper and mechanically stronger, a compromise is ofttimes effected by using basic materials for the furnace bottom and acid bricks for the sidewalls and roof. A few rows of chrome bricks may be put in to form a neutral dividing line just at and above the edge of the bath where the action of the slag is the most severe. It also serves to keep the basic and acid materials apart and from reacting with each other.
At the commencement of charging, limestone or sometimes burnt lime is shoveled in upon the bottom or “hearth” of the white-hot furnace.
When cold metal is charged, the pigs of iron are conveyed into the furnace by the melter and his helpers by means of long handled flat iron tools called “peels.” This is followed by charging some or all of the scrap or iron which is to be made a part of the charge.
Even in the smaller 15 or 25–ton furnaces hand charging takes a great deal of time, sometimes as much as six or eight hours, and the labor cost as well as the heat loss is therefore excessive. Modern machine charging which requires not more than an hour is therefore highly desirable.
Row of Open-Hearth Furnaces Showing Pit or Tapping Side
During the melting down of the pig iron with the scrap that has been charged, the air and flame burn out about half of the silicon and manganese of the metal. To remove the remainder of these and the carbon of the charge, additions are made from time to time of sufficient ore to keep the bath “boiling.” This phenomenon results from the giving off of carbon monoxide gas formed from the oxygen of the iron ore and the carbon of the metal, just as happened when the puddler in the manufacture of wrought iron used iron ore in his furnace. The covering of slag which forms and protects the bath from the flame undoubtedly transfers oxygen from the furnace gases to the bath and this helps to burn out the carbon.