In the 1960s the "mini-mill" began to revolutionize the steel industry. The mini-mill is based an the dual concepts of recycling abundant, inexpensive steel scrap through a small, low capital steel mill consisting primarily of an electric arc furnace and a continuous caster. The process grew from obscurity until today nearly half of all steel in the United States is produced in EAFs. In the 1960s and 70s EAFs produced long products and growth was primarily at the expense of the open hearth. But with the advent of thin slab casting its growth has continued into the flat roll market at the expense of the BOF. Today there are approximately 100 mini-mills in the U.S. capable of producing 50 million or more tons per year.

New technology has vastly increased EAF productivity. Originally production ranged from 10-30 tons/hour but today there are numerous furnaces producing in excess of 100 tons per hour. The "mini-mill" has grown from a plant producing 250,000 tons per year to plants producing in excess of 2 million tons per year. Once relegated to producing inexpensive concrete reinforcing bar, today mini-mills can produce over 80% of all steel products.

Although EAF productivity has significantly increased, steelmakers must still optimize the EAF with the finishing operations so their production rates and sequencing are the same.

Fig. 1: EAF Evolution

EAF energy consumption is generally reported in kWh per liquid ton. The electrical energy is only about 65% of the total energy input. The other 35% comes from chemical energy generated by the exothermic oxidation of carbon and from and by oxy-fuel or natural gas burners. Schematically, the energy balance for an EAF is shown in Fig. 2. The tapped steel and slag require a specific amount of energy (approximately 70% of the input), regardless of heat time; heat losses to waste gas, cooling water, and radiation, which are all directly related to heat time and directly account for the remaining 30%.

Fig. 2: EAF Energy Input/Output [1]

Figure 1 shows some of the new technologies that have propelled this growth and indicates the reduction in tap-to-tap times, electrical energy and electrode consumption.

Consequently, there has been a relentless drive to shear minutes from the process by maximizing the rate of energy input when the Power is an and to minimize the Power-off time. As a result, the terms Power Utilization and Time Utilization have been coined. The former is the average power Input/maximum power input when power is on. The latter is the percent of tap-to-tap time when the Power is on. All EAF developments are directed at maximizing the product of Power Utilization and Time Utilization.

Raw materials and operating practices affect EAF efficiency and yield. The traditional EAF charge has been 100% cold scrap. Even in 1995, less than 1 million out of the 40 million tons of metallics charged to domestic EAFs was direct reduced iron (DRI), hot briquette iron (HBI), and iron carbide.

The iron unit situation is critical for several reasons:

• The product mix served by EAFs is moving more towards value-added steels, which are specified with low metallic residuals and low nitrogen levels (automotive flat rolled, cold heading-rolled and wire).
• The availability of scrap needed to meet these requirements is limited to prompt scrap, which is decreasing as more and more near-net-shape metalworking operations appear.
• Yield and energy consumption are both strongly dependent an the quality and physical characteristics of the from units available.

The following topics are of central importance to raw material issues: supply of manufactured iron unit (which includes DRI, HBI, carbide, and pig iron), upgrading of purchased scrap, and the physical nature of purchased scrap. These topics are discussed in more detail below. Process yield is not discussed because process yield has been determined to be a function of charged metallics quality, rather than of the process itself, and intrinsic iron losses are not likely to be further reduced.

1.1 Trends and Drivers

Manufactured iron unit supply is a major concern. As the new greenfield mills increase their output, demand for prime scrap will begin to outstrip supply. Pig fron is an excellent charge material for an EAF because of its high density, low melting point, carbon contribution ability, and low metallic residuals. However, the availability is low as integrated producers consume nearly all they produce.

Upgrading of purchased scrap has been another way to increase raw material quality by controlling residuals, including S, P, Sn, and Cu. Several chemical approaches to remove the copper that is physically associated with junked cars (shredded scrap, #2 bundles) have been developed. However, physical separation (shaking, magnets) seems preferable to any of the chemical methods, all of which either create environmental problems (coping with HZS, chlorine) or require auxiliary operations (molten aluminum bath). Phosphorus in DRI is tied up as an oxide and thus enters the slag, probably remaining there since normal distribution ratios for phosphorus in melting are well below equilibrium ( i.e., phosphorus in pig from will enter the steel, not the Blag, and must be oxidized out like phosphorus from scrap).

Sizing of scrap is important to maximizing bucket density and minimizing energy losses. Proper scrap sizing limits the number of required recharges, thereby saving energy lost during roof swings, and minimizes refractory damage due to impact of heavy pieces at Charge and flare from uneven charges.

The physical preparation of scrap is important for efficient preheating and fast melting. Furnace designs are not being tailored to optimize scrap handling, so a physically homogeneous Charge is desirable. To this extent, manufactured iron units are ideal. Iron carbide can and possibly must be injected, and rates of several hundred pounds per minute have been achieved. Although overall quantities may be limited as injection cannot be used throughout the entire process, alternate iron forms that can be used in the conventional scrap buckets will enable regular use in current shops.

1.2 New and Emerging Technologies

The Iron Dynamics plant, in production early in 2001, is based an RHF and SAF technology and is the newest entry into the North American manufactured iron arena. It produces liquid pig from for consumption in an EAF. Other RHF/SAF processes and advanced iron smelting processes are being considered by both EAF and integrated steelmakers to provide needed virgin iron units for their operations. Continued development and installation of these processes will help relieve pressure an future prime scrap supplies and lessen domestic dependence an imported cold pig Iron.

For chemical upgrading, a pilot plant for removing zinc from prime flat-rolled automotive sheet is in operation in Chicago. The process in use is based an a caustic solution of zinc coating followed by electrolytic recovery to remove the zinc. The economics of this operation seem favorable at this point, but are still questionable as long as zinc prices hold around $.50 / pound.

An economic cryogenic technology to break up scrap has not yet been developed. The use of hollow electrodes to feed Small-sized iron units has been explored in ferro-alloy operations, but electrode current-carrying capacity is then reduced.

Productivity is a function of the net rate of energy input. Efforts are ongoing to maximize the energy delivery rate and its effective use to achieve reduced heat times. Electrical energy is dominant an the input side and often cheaper than chemical energy when consumables are considered. Conservation of energy by minimizing heat time is critical because of the large heat loss per minute during the EAF process and significantly increased heat loss during the final stages of heat.

The limitations of the conventional EAF have been identified and are forcing the builders of "greenfield" furnaces and those operating less efficient EAFs to consider new and advanced designs. The new generation of EAFs covers a multitude of configurations. Another key issue in EAF efficiency is the ability to pace and balance EAF production with the other parts of the steelmaking process.

The following topics related to energy are discussed: chemical/electrical energy input ratios, AC/DC power, and energy load.

2.1 Trends and Drivers

There is increased emphasis an chemical energy input, which is generally concurrent with electrical energy input and thus supplements it to reduce heat time. The post combustion of CO and HZ gases leaving the furnace is an important issue. Ideally, the gases should be burned in the furnace with the resulting heat load applied to the Blag/metal system. The oxidation of CO while the scrap is still solid and relatively cold provides a better opportunity to capture the heat. The level of CO and HZ leaving the furnace through the off-gas duct provides some measure of the effectiveness of post combustion and are key process parameters for an online energy balance model for furnace practice optimization.

The introduction of direct current (DC) furnaces and the projected savings in electrode consumption has reinvigorated the EAF industry. Current carrying capacity depends an electrode diameter, and as furnaces have increased in size (greater than 150 tons), diameter has become a potential limiting factor. Large electrodes (greater than 32 inches) sell for a premium which may offset the electrode savings. One solution is dual- er multiple-electrode DC furnaces. At the same time, AC power circuits have been improved to compete with the electrical efficiency of DC power supplies.

Energy load can be reduced by reducing tap temperatures. A 100° F reduction in tap temperature is worth 10 kWh/ton. At the front end of the process, preheating of iron units (including scrap, alloys, and fluxes) can reduce energy requirements. This approach has been known for years, but both the economics and logistics defeated the simple approach (for example, heating the scrap in the bucket).

2.2 Technological Challenges

Bottom problems with DC furnaces are no longer a major deterrent to installing such furnaces, but bottom configurations could restrict bottom injection technology.

Capture of heat from the walte gas has been fraught with environmental and other problems, including difficult logistics, damaged buckets, and limited benefits. The typical savings is 25 kWh/ton, representing a poor return an investment.

2.3 New and Emerging Technologies

New and emerging technologies are mainly in the area of reducing energy load. One way to reduce load is to design the furnace to capture the waste gas heat in a scrap-filled shaft atop the roof. However, the hot walte gas is supplemented by natural gas burners in the shaft when using this method. The setup must be extremely hot to conserve that much electrical energy and therefore must be oxidized since the shaft atmosphere is oxidizing.

A second answer is to build twin furnace shells and pass hot walte gas to one vessel while a heat is being melted in the other. Burners can also be used. Additionally, continuous scrap feeding systems that eliminate top charging and its energy losses have been developed. All these processes also may reduce the EAF dust load an the baghouse by "filtering" some of the dust. This dust is returned to the furnace, and no data exist to show the net effect of dust reduction per ton for the problem of ZnO build-up.

[1] Steel Industry Technology Roadmap. AISI, 2000