THE USE OF GASEOUS OXYGEN IN METALLURGICAL PROCESSES

The importance of using gaseous oxygen in place of air or metallic oxides in the oxidation reactions common to metallurgical processes has long been known. Many of the common metallurgical operations including those in the blast furnace, the Bessemer converter, and the open-hearth furnace were analysed, and it was decided that if the cheap gaseous oxygen is obtainable, it will be very useful for metallurgical processes.

There are two main types of using oxygen in metallurgical processes: (1) the use of very large amounts of low cost, low-purity oxygen (in the order of 100 to 3,000 tons of oxygen per day), and (2) the use of small amounts of high-purity oxygen (in the order of 10 to 1,000 tons per month). In the first case the oxygen is used to speed up the reactions by increasing the temperature of the flame heating the bath. In the second case it is used as an oxidizer to combine with the various chemical elements present in the molten metal.

Steel making in the open-hearth furnace is a very slow process, much slower than the Bessemer one. In the normal scrap-plus-hot-metal process, much time is required after hot metal has been added, to melt the scrap which is on the hearth, before the slag is formed. As the carbon content is lowered and the melting point is increased, it becomes very difficult to heat the bath to the required temperature.

Oxygen is applied to the hot metal to produce high temperature iron of a constant and low silicon content. Molten blast furnace iron, for use as hot metal in open-hearth steelmaking, should be low in silicon to reduce the amount of slag, and should be as hot as possible to reduce the energy requirements of the refining process. But it is known that in blast furnace practice very hot iron is usually high in silicon content and low-silicon iron means generally high-sulphur iron. And so low-silicon, low-sulphur, high-temperature hot metal may be produced by treating it with oxygen before pouring the metal into the open-hearth furnace.

When oxygen is blown into molten pig iron, the well known Bessemer reactions take place under conditions that are easy to control. Silicon begins to oxidize first, followed by manganese. When these reactions are about half finished, carbon begins to oxidize to form carbon oxide which passes as a gas through the slag. In these reactions much heat is produced and the temperature of molten metal is greatly increased.

In working the open-hearth heat, carbon and manganese are reduced by the combined action of the oxidizing gas atmosphere and by iron ore additions to the oxidizing slag. Thus, the mass of cold ore must be heated to the slag temperature and in addition to this the reaction itself take away much heat, thus lowering the temperature of the bath. If oxygen is used for reducing the carbon content it is possible to prevent this effect of decarburization and even to increase the temperature of the bath because during the reaction 2C + O2 = 2CO much heat is formed.

As an example we can take a typical 140-ton heat, melting at 1 per cent carbon, when 4,000 pounds of ore were added, after that the carbon content was decreasing at the rate of 0.30 per cent per hour. Oxygen was added 60 minutes later, when the bath had reached 0.70 per cent carbon, through a 1-inch steel pipe at the rate of 650 cubic feet per minute, and the rate of carbon content decrease immediately increased to 1.20 per cent per hour. Additional ore was added at 0.50 per cent carbon and after that the carbon content began decreasing at the rate of 2.40 per cent per hour. This combined ore-oxygen reaction gives many advantages over any other reaction of reducing carbon content.

As the carbon content of a heat becomes lower, it is more and more difficult to heat the bath because of the lowered bath activity and the higher melting point of the metal. This difficulty is present almost always in making low-carbon steels, where the melting point is very high. Heats in the 0.02 to 0.05 per cent carbon range usually require 1 to 5 hours longer finishing time than medium carbon steels. Often, when the bath temperature is not increased rapidly enough to compensate for the increased melting point, bath activity is reduced to the point where additional carbon must be added in the form of cold pig iron or hot metal.

The use of oxygen as a means of decarburizing low carbon heats gives so many advantages over ore practice, that it has become a standard operation. It uses small amounts of high-purity oxygen. Other applications of oxygen to the finishing period include its use with such materials as ferrosilicon for rapidly increasing the bath temperature. Open-hearth furnaces, having worked a long time, rapidly decrease their heating ability because of bad work of the regenerative system. Much time is therefore required in order to bring the bath to the required tapping temperature. Small additions of ferrosilicon combined with the controlled oxygen blow can be used to increase the bath temperature rapidly without changing the carbon content.

Date: 2016-01-14; view: 575