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Developments In EAF And IF Stirring - A Literature Review
This paper discusses the process of gas stirring and the various means and methods to carry out stirring in Electric arc and Induction furnaces. It also briefly talks about the history and evolution of stirring in the furnace along with some secondary stirring in the ladle. The different types of stirring and the inherent advantages and limitations of each method are also presented. In addition, different stirring processes are compared for efficiency and cleanliness of steel. Some modeling data to prove the conclusions are offered also.
Electric Arc Furnace (EAF)
Anelectric arc furnace(EAF) is a furnace that heats charged material by means of an electric arc. The electric arc furnace operates as a batch melting process producing batches of molten steel known "heats". The electric arc furnace operating cycle is called the tap-to-tap cycle and is made up of the following operations:
- Furnace charging
- Furnace turn-around
Modern operations aim for a tap-to-tap time of less than 60 minutes. Some twin shell furnace operations are achieving tap-to-tap times of 35 to 40 minutes.
Induction Furnace (IF)
Aninduction furnaceis an electrical furnace in which the heat is applied by induction heating of a conductive medium (usually a metal) in a crucible placed in a water-cooled alternating current solenoid coil. The advantage of the induction furnace is a clean, energy-efficient and well-controllable melting process compared to most other means of metal melting. Most modern foundries use this type of furnace and now also more iron foundries are replacing cupolas with induction furnaces to melt cast iron, as the former emit lots of dust and other pollutants.
The one major drawback to induction furnace usage in a foundry is the lack of refining capacity; charge materials must be clean of oxidation products and of a known composition, and some alloying elements may be lost due to oxidation (and must be re-added to the melt).
Stirring And History
In conventional AC furnaces, there is little natural convection within the molten bath. As a result high temperature (Temperature gradients have been reported in the range of 40-70°C) and concentration gradients tend to exist within the bath. These contribute to increased energy consumption, reduced reaction rates and over or under reaction of some portions of the bath. Bath stirring helps reduce the effect of these gradients.
The concept of bath stirring is not new and in the beginning, electromagnetic coils were reported to have been used from as early as in 1933. Most EAF stirring operations today use inert gas as the stirring medium where the gas is introduced through the bottom of the furnace by means of tuyeres or porous plugs.
The number of elements used can vary greatly based on the design and gas flow rate but is usually in the range of one to six. Usually three plugs are used in a conventional AC furnace with a plug located midway between each of the Fe electrodes. Typically, the elements used to introduce the gas into the bath can be tuyeres, porous plugs or combinations of porous plugs with permeable refractory rammed into place over top of the plug. Noncontact elements have the advantage that they can be operated at higher turndown ratios without the fear of plugging. The size of the furnace and its design will also create different demands for the number of stirring elements to be used.
Argon and Nitrogen gases are mainly used but some trials have reported using natural gas and carbon dioxide also. There are many advantages for modern stirring practices, ranging from increase in yield, reduction in tap-to-tap time to energy savings and reduced electrode consumption
Yield increases of 0.5 to 1%, savings of about 5 minutes of about 10 to 20 kWh/ton
The various types of stirring are:
1. Inert Gas injection
a. Bottom stirring
b. Top stirring using Lance
c. Combination stirring
2. Electromagnetic stirring