Speakers Focus on Improving Melt Efficiency, Process Control.
"This conference is dedicated to the memory of John M Svoboda. A long-time member of AFS, Dr. Svoboda helped coordinate much of this program. His service to our industry will long be remembered and his contributions will be missed."
With these words, Ezra Kotzin, AFS, greeted the more than 60 attendees, including speakers and exhibitors, who assembled for the first annual John M. Svoboda AFS International Conference on Steel Melting & Refining. Held June 13-15 in Rosemont, Illinois, the conference reflected Svoboda's lifetime involvement with steel and dedication to improving the metal's properties.
The 21 technical presentations, some which credited Svoboda as a co-author, included insight into improving the physical chemistry of melting steel, metal quality, economics and productivity, energy and the environment, and emerging technologies in steel melting and refining. This article presents information from three practical talks from the conference.
"Oxygen (O) is not good or bad-it's just necessary for refining and controlling steel properties," said Lawrence Heaslip, Advent Process Engineering, in his discussion of O control in steel melting, which included information provided by Svoboda. "Although we look at slag as nasty stuff, it is absolutely essential to making steel. We make more steel by controlling the metal's environment, and the only thing that really differentiates 'slag' from what we call 'refractories' is purity."
Steel foundries with electric arc furnaces (EAFs) have adopted melting practices that utilize large amounts of O injection in the furnace, and these practices offer greater opportunities for increased melting productivity and cleaner steel production, according to Heaslip. Acid refractories are not as stable as basic refractories, and acid slags that are produced in such furnaces do not offer the same opportunities for producing clean steel.
Basic melting allows higher power densities in the furnace and the opportunity for higher levels of O usage in the furnace with the following benefits:
* faster melting;
* reduced electrical power, electrode and refractory consumption;
* lower levels of dissolved hydrogen and nitrogen in the steel;
* better slag removal from the furnace, with the related benefits of improved alloy recoveries and enhanced steel cleanliness.
In order to see the benefits of O injection, according to Heaslip, foundrymen must meet carbon (C) requirements, achieve a vigorous C boil and foam the slag. C is required (in addition to O) for heat generation during melting and superheating to tap temperature and to achieve produce CO for slag foaming and the resulting clean steel.
First, to meet the C requirement, Heaslip said that sufficient C must be added to the charge to achieve a C level in the bath at melt down of 35-40 points above the aim level for tapping. The amount of C that is sufficient is determined by the type of C that is used and the amount of C that is burnt during melt down, he said. O lancing during melting will increase the amount of C required and reduce the melting time.
The choice of material used for charge C depends on C chemistry (fixed C, ash, moisture and sulfur contents), sizing and geographic location of the plant. Coal can fluctuate widely in chemistry, whereas metallurgical coke produced from coke ovens is consistent and high in fixed C. However, metallurgical coke is quenched with water as it is pushed from the ovens, so it can have a high moisture content, Therefore, depending on particle size, moisture should be reduced to less than 3-4%, he said, adding that finer coke, which contains particles less than 0.125 in., will almost certainly be dried because the fines will hold high levels of water and can freeze in the winter, and even coarser coke of up to 0.188 in. will have moisture contents from 8-12%.
Next, to achieve a vigorous C boil, Heaslip said that the rate of O injection should be matched to the desired superheating time. This process makes carbon monoxide (CO) gas in the melt, helping to rinse the steel, homogenize temperature and chemistry of the bath, and foam the slag. The heat of combustion of CO to carbon dioxide (CO) is three times greater than the heat of combustion of C to CO, so this represents a large potential energy source for the EAF. If the CO is burned above the melt, it is possible to recover the heat into the slag and metal, thus reducing the heat load that the off-gas system must handle.
Most of the CO is generated near the O lance or within the slag and metal, therefore, if a post-combustion O lance is located near the existing O lance, most of the CO can be oxidized before it leaves the furnace, according to Heaslip. This post-combustion lance is best located above the existing O lance and low in the furnace to allow for more residence time for the CO to oxidize into [CO.sub.2] and transfer the energy into the bath and not the furnace roof or ducts.
The C boil, which removes C and helps rinse out dissolved gases, can foam the slag so that it can be flushed from the furnace as tap temperature is approached. In some cases, to increase the heat recovery to the bath and shorten the superheating time, post-combustion can be optimized by O injection into the foamy slag. Also, for even higher energy input and increased foam depth, the post-combustion O injection rate into the slag can be increased and another lance can be used for either straight C injection or C plus lime injection.
Slag foaming can be improved by operating the furnace at flat bath conditions, Heaslip said. In smaller furnaces, O can be injected with a consumable lance through the slag door. Injecting O into theslag with a second lance in the same proximity as the first one can maintain foamy slag depth and achieve the best post combustion characteristics.
Simultaneous O and C injection into the slag can ensure foam high enough to cover the arc and allow for maximum electrical input during refining, he said, adding that, at this time, the furnace should be tilted back slightly to control slag depth and allow the furnace to be slagged off. As the final C level is approached, up to 70% of the slag can be removed. This reduces slag quantity which can be tapped into the ladle, stabilizing and improving alloy recovery, deoxidation and steel cleanliness.
In small furnaces, the evacuation system's off-gases and heat load must be considered. Modifications may be required if high O usage is planned, Heaslip said.
In a presentation co-authored by Eugene Pretorius and Robert Carlisle, Helmut Oltmann, Baker Refractories, covered slag fundamentals and their application to steel melting and refining. Slag, an ionic solution of molten metal oxides, sulphides and fluorides, is an integral part of metal refining, Oltmann said. In their pure state, the common oxides that make up slag have higher melting points than temperatures achieved in steelmaking (iron oxide and Ca[F.sub.2] being the exceptions). However, the different oxides have a fluxing effect on each other, lowering the melting point of the solution. Therefore, slags melt over a temperature range.
The components of a basic slag can be classified as refractory oxides (CaO and MgO) and fluxing oxides ([SiO.sub.2],[Al.sub.2][O.sub.3], [CaF.sub.2] and FeO). The fluxing oxides are not as strong as the refractory oxides and usually are more effective in combination.
Refractory linings used in steelmaking are made of high liquidus temperature oxides or mixtures that can dissolve in the liquid slag. One of the principles of slag engineering' is to manipulate the slag composition with additions to the point where the slag is saturated with the major refractory constituent to protect the refractories from chemical wear (Fig. 1). A slag with optimum foaming properties is achieved with a balance of both liquid and solid slag, which increases the viscosity to stabilize the foam. This can best be done using a mass balance approach.
Slag foaming characteristics improve with decreasing surface tension and increasing viscosity, but the presence of suspended second phase particles in the slag has a much greater impact on foaming, Oltmann said. Optimum slags are not completely liquid but saturated with respect to CaO ([Ca.sub.2][SiO.sub.4]) and/or MgO (magnesia-wustite solid solution). "These second phase particles serve as gas nucleation sites, leading to a high amount of favorable small gas bubbles in the foam."
As the effective viscosity increases, the residence time of gas bubbles in the slag is prolonged, extending the stability and life of the foam, he said. However, there is a maximum amount of second phase particles that is beneficial for foam stability, and once this point is achieved, the slag becomes too crusty (oversaturated), and the foaming index decreases.
Since most electric furnace slag-line refractories are basic, dual saturization of CaO and MgO (or at least MgO minimizes chemical attack of the refractories and promotes good foaming. The second essential factor for foaming is gas generation (bubbles) and sustaining these bubbles in the slag. Controlling the FeO content of the slag is key to predictable foaming behavior, he said. Since FeO is the major fluxing component in slag, it strongly influences effective viscosity. If the injected O is balanced with sufficient C injection, the FeO content will remain constant. However, if fluidity increases due to increased temperature or FeO content, a decrease in the foaming height could result. In this case, the effectiveness of the FeO reduction by C is diminished because the retention time of the CO in the slag is decreased.
Isothermal saturation diagrams (ISDs) map the phase relations at certain temperature and basicity as a function of the MgO content and the major flux FeO. In most conventional EAFs, basicity of the slag remains fairly constant throughout the heat and is determined by the silicon (Si) and aluminum (Al) levels in the scrap and the amount of refractory oxides added with the scrap.
The ISD for a basicity of 1.5 at 29 12F (1600C) is shown in Fig. 2. Slags (X, Y and Z) that fall in an all-liquid area will have poor foaming properties due to low viscosity and the lack of second phase particles. The position of the liquidus curves shows the maximum amount of FeO that can be tolerated before the slag will be fully liquid.
In the ISDs, the all-liquid area shrinks as the basicity of slag increases. The levels of FeO required to reach the liquidus boundary are higher with increasing basicity. The MgO solubility decreases as the basicity of the slag increases.
If the initial MgO level is too low, the window of effective foaming is small, Oltmann said. However, if the MgO content is too high, the slag could be too stiff to achieve optimum foaming. Slag can be engineered to foam at an FeO range close to the final FeO content by adjusting the basicity. For a high-C heat (low FeO content), the basicity should be lower and MgO higher so the optimal foaming is achieved at low FeO content. For a low-C heat, the basicity should be higher and MgO lower so that the high FeO content does not render the slag too fluid for foaming early in the heat.
If the scrap quality in terms of Si and Al content that contribute [SiO.sub.2] and [Al.sub.2][O.sub.3] to the slag is repeatable, then low-tap C requires less dolomite and more lime, while high C requires less lime and more dolomite to achieve the desired basicity.
In deoxidation and alloying practice, Al can reduce all oxides in the slag, leading to Si and manganese reversion and the corresponding loss of Al from the steel as it reacts with [SiO.sub.2] to become [Al.sub.2][O.sub.3], he said. If this is not floated out and absorbed by the slag during argon rinsing, it will remain in the steel, making it "dirty" or clogging the nozzle at the caster.
Fading can be minimized or eliminated by deoxidizing the slag with Al shot or FeSi fines and [CaC.sub.2] orSiC added to the slag, however, the C-containing materials will generate some [CO.sub.2] gas that can excessively foam the slag leading to C pickup, according to Oltmann.
While oxidizing conditions are beneficial to phosphorous removal in the slag, reducing conditions or a deoxidized system aids sulfur (S) removal. Some S is removed in the furnace, the advantage being the high slag-to-metal ratio, although the S distribution ratio is small. Scrap selection and desulfurization in the ladle are the best options for realizing low S content.
Efficient Induction Melting
"While arc furnaces remain an important tool for steel melting, new induction technologies, including high-power-density power supplies and furnaces and multiple-output power systems, are making steel melting systems more productive and more efficient," said Paul Cervellero, Inductotherm Corp., in his discussion of efficient steel melting with induction furnaces.
Today's power units are larger than 5000 kW and are able to apply 700-1000 kW/ton of metal in the furnace, Cervellero said.
Dual-output power units make it possible to simultaneously melt in one furnace and hold with power in a second pouring furnace, increasing production up to 40% at the same KVA compared to a single furnace/single power unit system and by up to 20% compared to a 'butterfly' batch melting system with two furnaces and a single power supply. According to Cervellero, a recent installation in Tennessee is using a 9000-kW triple-output unit to power three 10-metric-ton furnaces, allowing the foundry to melt in one furnace and hold at the desired temperature in the other furnaces.
Further, he said that a 35,500-kW triple-output batch melting system that has three 20-ton furnaces with a 36-min melt and 18-min pour time will produce 65 tons of steel/hr. "And the power utilization continues at 100%."
Other advantages to multiple-output induction power systems include:
* the ability to sinter or cold-start multiple furnaces at once or to sinter one furnace while melting in others;
* significant savings in installation and melting cost achieved from the unit's single set of power and water connectors;
* equipment utilization approaching 100% for maximum output per KVA;
* minimum investment per ton of metal poured.
There are nearly 500 dual- and multiple-output induction systems operating worldwide, Cervellero said.
He stressed that an induction system stirs the molten metal, producing a homogenous metal bath--an advantage to the system. However, a disadvantage (if the furnace is not properly designed) to this stirring is excessive wear to the refractory. He said that modern high-power-density melt quickly, producing less wear on the furnace as well as less gas picked up by the metal. Automated lining installation, lining pushout systems and a broad range of alumina-based lining materials reduce the impact of lining changes on steel production.
To illustrate the advantages of induction melting of steel, Cervellero detailed an installation at Metaltec Steel Abrasives Co., a Michigan-based producer of steel shot that melts using a dual 5000-kW induction system operating two 6-metric-ton coreless steel shell furnaces.
Charging and melting takes 35 min until the bath is brought to the pouring temperature of 3100F (1704C). Slagging and sampling take 10 min, while pouring takes 20 min. Because of the furnaces' minimum heel extended coil design, power can be maintained until 90% of the metal has been poured.
Metaltec's furnaces are lined with a fused alumina-base compound with 85% [Al.sub.2][O.sub.3] and 13% MgO enriched with magnesia to form a spinel bond. The normal lining campaign totals 135-140 heats, with about 840 metric tons of metal for each lining. A lining pushout system allows changeover in 24 hr.
In addition, Metaltec reports that rapid melting results in the recovery of virtually all of the 1.5% manganese content of its charge materials, reducing alloy costs.
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|Title Annotation:||furnace oxygen control, slag engineering, and improving steel casting production with induction melting methods|
|Comment:||Speakers Focus on Improving Melt Efficiency, Process Control.(furnace oxygen control, slag engineering, and improving steel casting production with induction melting methods)|
|Date:||Nov 1, 2000|
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