Smältkoncept/New melting processes EAF

Retention, recovery and recycling of valuable metals

Project leader: Seshadri Seetharaman, Kungliga Tekniska Högskolan (KTH), MSE

Introduction

Steel is a general term covering a wide variety of iron-based alloys with a number of alloying elements such as V, Ti, Cr, Mn, Mo and Ni apart from C. Steel is produced either from the iron ore by the blast furnace-converter route, or by the secondary route from scrap-based metallurgy through Electric Arc Furnace (EAF) technology.
Apart from the main steel product, the steel industry produces slag, dust and off-gases as by products. As a result of this, the processing of slag, dust and off-gases, and effective utilization of the same, are of equal great importance. Due to increasing energy demand and environmental restrictions, the steel industry has, however, felt the need to focus upon the removal of sulphur and other impurities from the fuel gases. Recovery and reutilization of the valuables in the slag, as well as the dust, have not progressed to the same extent. In recent years, the processing of dusts has, however, been considered by several industries. Recovery of Zn from EAF dusts have received considerable attention, as well as the recovery and containment of trace amounts of Hg and to some extent even Cd.
Metallurgical slags are generally alkaline earth silicates containing several valuable alloying elements. The occurrence of these elements is of course dependent on the steel grade manufactured, as well as the process conditions. Due to environmental regulations, recycling of slag for road construction or as filling materials is not permitted in Sweden. In other word, for recycling to be an option all hazardous elements must be removed from the slag. This does of course add to the economic burden of the producer resulting in that the slag often is dumped.

Goals

General
The objective of the present proposal is the retention of the metallic valuables in the steel cycle and optimum utilization of the same. Minimization of the loss of valuable elements to the dump through the slag phase, and recovery of these elements from the slag rejects is therefore an important goal of this project. The development of a new generation of technologies leading to optimal utilization of metal values in steel scrap will be initiated.

Industrial
To find process routes to keep valuable alloys in the steel and recycle rest elements remaining in the slag.

Environmental
To decrease the consumption of chrome and molybdenum by 160 tons/year and 60 tons/year respectively. (Estimated effect +10 years from 2004, for Swedish steel- and scrap industry).

Work plan
The following strategy will be adopted, see Figure 1.

Retaining metal values in steel during processing
As steelmaking involves alternative reduction and oxidation processes it is extremely important to have efficient control of the oxygen partial pressures in the system. The use of buffered systems to generate and maintain the desired oxygen partial pressures in the reactors, as well as how to monitor and steer these buffers will be investigated. The use of CO₂, wherever there is a need to generate a lower oxygen partial pressure, will be considered and investigated. How to control and manipulate the slag chemistry will also be investigated by targeting curtain specific steel/slag reactions.

Recovery of impurities from slags and scrap
Steel scrap contains, apart from the iron value, several important alloy elements. When the unsorted scrap is fed into the EAF, it is likely that these valuables (due to their extreme reactivity) are lost in the slag phase. A certain elements distribution between the metal phase and the slag phase is in most cases extremely sensitive to different process parameters. For example, dephosphorization of stainless steel would require oxidizing conditions to remove P as PO₄^3-. On the other hand, if the oxygen potential exceeds critical limits, Cr could follow into the slag phase leading to high losses of Cr. This dilemma of dephosphorization of high alloyed steels will be investigated.

Suitable process routes towards recovery and reutilization of metal valuables will be developed. One of the most important steps in this regard is to identify a more effective way of sorting out the various types of scrap. The effect of surface contaminants on scrap is another issue that will be taken into consideration and investigation.

Recirculation/utilization of the slag in the process chain will be investigated from a broad metallurgical view point. In other words, utilization of slags from non-ferrous industries in steelmaking will be considered.

The proposed research can be summarised as follows:
Development of new process concepts, with thermodynamic, mass transfer and kinetic considerations, to selectively withhold the valuables and remove the undesirable impurities. The strategy adopted will differ significantly from the present day practice in the sense that the selective retention will be based on fine tuning of process variables.

Deliverables for 2004-2008
Fundamental studies will be carried out to determine the thermodynamic and kinetic limitations of the systems in question. Practices will be recommended based on the results achieved as well as the practical limitations of the process.

Results of the investigations slag-steel interactions (including thermodynamic and kinetic aspects).
Process design towards the recovery of valuables from metallurgical slags, reutilization of slags as well as other valuables in the steel cycle.
Process design towards optimized scrap utilization in the steel processes.
Total optimization of the steel processes with respect slags, scrap and dust.

Deliverables for 2004-2005

R & D activities

Status

Literature survey

The strategy of the project has been proposed according to literature survey.

Development of module structures, from both thermodynamic and kinetic perspectives, for iron and steelmaking.

1) Thermodynamic calculation of slag-steel system: Activity of metallic phase was calculated using Thermo-Steel. Comparative studies were made with Thermo-Calc. The results were in agreement.

2) The critical conditions for retention of Mo and Cr have been obtained. Slag-metal reactions under non-equilibrium conditions are being examined in the case of other heavy metals like Mo, Nb etc.

3) Thermodynamic analysis of oxygen impact on AOD process and Cr (or Mo) loss has been carried out. Preliminary kinetic analysis of Cr or Mo oxidation in slags was carried out. The rate controlling steps as reported in literature were taken into account. Computations indicate the impact of mass transfer on the process. The effect of controlling the oxygen partial pressures by means of using CO₂ were estimated theoretically. The results indicate that the dilution of Ar-O₂ mixture with CO₂ could lead to substantial gains with respect to process control and environmental issues. The calculations show that the proposed new technique in AOD process is theoretically applicable.

4) New process for extraction of Mo and Cr by melt salt from EAF slag was designed. Experimental work started from Oct. 2005.

5) The strategy for the retention of Mo and Cr in EAF furnace has been proposed.

Deliverables for 2006

R & D activities

Status

1. Retention of Mo and Cr in EAF 1.1 Optimization of charging state of raw materials for Mo and Cr. Effect of Mo oxide state (MoO₃, MoO₂ or Fe₂MoO₄, ...) on the evaporation of Mo in EAF.
1.2 The optimization of the slag composition.
1.3 The EAF process development( charging schedule of Mo and Cr; slag foaming practice)

2. Recovery of Mo and Cr from slag by SET process
2.1 Lab-scale experimental parameter determination: a) Extraction temperature; b) composition of the salt; c) the control of the oxygen partial pressure. This work is expected to be finished in Match of 2006.
2.2 Large lab-scale experimental studies of electroextraction of Mo and Cr by SET process.

3. The CO₂ decarburisation in lab scale
Modelling studies of ladle refining process by blowing CO₂.
4.Thermodynamic measurements
4.1 Thermodynamic activity measurements of MoO₃, Cr₂O₃by gas equilibrium method.
4.2 Thermodynamic activity measurements of Mo in the alloys by galvanic cell method.

1. Retention of Mo and Cr in EAF 1.1 Industrial research on the ‘Where does the Mo lose?' at UDDEHOLM TOOLING AB has been conducted during this year.
1.2 Slag and dust samples from Uddeholm AB have been continuously collected and analyzed. The results show that the Mo content is very low in the EAF slag phase but very high in the EAF dust (-4.83wt%). It can be concluded that two measurements should be considered for the Mo: one is how to decrease the evaporation of Mo and the other is how to recover the Mo from dust.
1.3 Process analysis has been carried out. The loss of Cr and Mo in the process has been calculated and the results have been reported to the industry and Mistra committee.

2. Recovery of Mo and Cr from slag by SET process
The main work was focus on the Molten salt extraction (MSE) technique experiments during this year. An international patent will be applied to protect this new technique.
2.1 Specific fluxes for leaching the metal oxides (Cr₂O₃) from the slag phase have been found by thermodynamic calculation and experimental test.
2.2 Electrolysis set-up was designed and successfully applied for recovering of Cr from chromium oxide, slag and chromites.
2.3 Good results have been obtained for the extraction of chromium from slag and chromites.
2.4 The compositions of electro-deposited products have been analyzed by SEM and ED methods.
2.5 Suitable process parameters, such as current density, electrode material, and cell will continue to be studied.

3. The CO₂ decarburisation in lab scale
Experimental set-up for the CO2 decarburisation process has been designed. The modelling studies of AOD process by simulating the CO2 blowing will be conducted in the end of this year.
4. Thermodynamic measurements
Thermodynamic activity measurements of Cr in the Fe-Cr-C-N alloy have been conducted by galvanic cell method. A manuscript has been submitted to Metal. Mater. Tran. B for publication.The activity measurements of Cr2O3 and V2O3 in the EAF slag have been started.

Deliverables for 2007

R & D activities	Status
Thermodynamics measurements 1.1 Activity measurements of Cr2O3, MoO3, and V2O5 in the stainless steel slag systems 1.2 Optimization of the slag database involving Cr2O3, MoO3, V2O3. 1.3 The adjustment of EAF slag chemistry to retention of Cr, Mo in the EAF real process based on the thermodynamic modelling and experimental results. 1.4 Thermodynamic activity of Cr in the Fe-Cr-N system	Activity measurements in slags containing Cr and V have been carried out. A report is getting finalised and will be submitted by the end of October 2007. The work is currently under progress. Modelling work has been initiated. It is expected to be ready by the end of the year. Laboratory experiments will be completed by the end of October 2007. The plant trials are expected to be started after this. Completed and the paper has been sent for publication.
2. Implication of MSE process 2.1 Experimental study of recovery of Mo, Cr from the dust in lab scale. 2.2 Experimental study of recovery of valuable metal from mill scale. 2.3 Large scale experiments on the MSE process. 2.4 Industrial trials of salt process.	Experiments with the materials from Uddeholm Tooling are proceeding. Preliminary results are expected by the end of this year. Mill scale experiments are yet to be started due to lack of personnel. Large scale experiments are planned. Design and economic aspects are currently looked into. Trials with Uddeholm slags have been started.
3. Modelling study of AOD decarburisation by blowing CO₂3.1 The process time needed if using CO₂ to replace some of the O₂. 3.2 The temperature change if using CO₂ to replace some of the O₂. 3.3 Test of CO₂ decarburisation in lab scale.	A model for the Uddeholm EAF practice instead of AOD in accordance with the wishes of the committee has been started. The first version of the model should be ready by the beginning of 2008. An existing model with Uddeholm Technology is being tried just now. A lab. Scale experiment is being carried out in collaboration with KIMAB Large scale experiments are planned for the experiments in Hagfors.

Deliverables for 2008

R & D activities	Status
1. Model for EAF process for Uddeholm Tooling with oxygen partial pressure change as the important variable together with accompanying temperature changes and follow the chromium oxide content in the slag. 2. Optimized conditions for salt extraction process. 3. Mo retainment in EAF under optimized conditions. Process improvement to be implemented in Uddeholm Tooling.
Final report of the project

Fragmentering
/Shredding
... är en teknik som används för sönderdelning av skrot innan det smälts om till nytt stål. Genom att utveckla tekniken att finfördela skrotet kan variationerna i dess samman-sättning minskas och dess densitet ökas. Detta innebär att stålindustrin vid omsmältning av skrotet kan reducera både sitt elbehov och sina utsläpp av koldioxid.
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Laseranalys
/Laser analysis
... är en teknik som nu utvecklas för snabb bestämning av skrotets metallinnehåll. Stålindustrin väntas här få tillgång till en teknologi som kan komma att reducera behovet av jungfruliga metaller vid användning av skrot för tillverkning av nytt stål.
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Skrotflödet i samhället
/Recycling of steel
... är trots den höga återvinningsgraden föga dokumenterat. Med bättre kunskaper skapas förutsättningar för ökad återvinning och bättre styrning av skrotsorter för optimerad skrot-användning. Stålindustrin får möjlighet att effektivisera sin energianvänd-ning, stora ekonomiska värden tas till vara och miljöbelastningen minskar.
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Ytrening av skrot
/Surface cleaning of scrap
... är en teknik som skall bredda användningen av färg- och metallbelagt skrot som råvara till ett ökat antal stålsorter. För stålindustrin innebär användning av ytrenat och förvärmt skrot att både energi-användning och koldioxidutsläpp kan reduceras. Dessutom kan den avdrivna zinkbeläggningen komma att återanvändas inom zinkindustrin.
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Vanadinutvinning
/Recovery of Vanadium
Det finns låga halter av vanadin i svensk järnmalm. Genom utveckling av metoder för utvinning av vanadinet får en redan exploaterad naturresurs ett ökat miljövärde.
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Ökat metallutbyte vid smältning av stål/New melting processes EAF
... är teoretiskt möjligt genom metallurgisk manipulering under smältförloppet. Målet är att skapa sådana förhållanden att metaller överförs till stålet i stället för till slaggen. Detta innebär minskat behov av jungfruliga metaller, samtidigt som slaggen kan ges nya användnings-områden.
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Återvinning av slagg
/New slag systems EAF
... är tekniskt möjligt genom utveckling av nya metoder för lakning av olika metaller ur slagger, t.ex vanadin, och behandling av slagger så de kan användas som t.ex isoleringsmaterial eller cementråvara. Vissa slagger kan också användas som ny slaggbildare i stålprocessen, vilket minskar behovet av ny kalk. Återvinning av slagg miinskar både behovet av deponering och förbrukningen av nya naturresurser vid ståltillverkningen.
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Bevarande av alla ämnen i skrotet
/Tolerance of recycled elements
... är en teoretisk studie som skall söka klarlägga hur dessa kan nyttjas för att ge den färdiga stålproduktens önskade egenskaper. Om sådan "självlegering" blir praktisk möjlig kan de metalliska ämnen som finns i skrotet, och som normalt avlägsnas i processen, bevaras i det nya stålet. Detta innebär att förlusten av metaller ur stålets kretslopp kan reduceras.
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Nya höghållfasta stålprodukter
/Optimisation of hot rolling
... är ett forskningsprojekt som skall identifiera de processparametrar som vid värmning och valsning har störst inverkan på materialegenskaperna. De nya rönen kan ge stålindustrin möjligheter att med ökad precision framställa nya avancerade stålsorter och samtidigt minska förbrukningen av fossila bränslen.
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Metoder för lättare stålkonstruktioner
/High strength steel structures
...ska för avancerade höghållfasta stål utmynna i att stålets egenskaper kopplas till produkternas användning och återvinning ur ett livscykelperspektiv. Genom denna metodutveckling kan stål-, bygg- och verkstadsindustrin få nya infallsvinklar på användandet av stål som både kan minska behovet av jungfruliga metaller och reducera emissionerna vid såväl tillverkning som användning av produkterna.
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Miljövärderingsanalyser
/Environmental evaluation

I. Methodology for evaluation of environmental impact
...ska påvisa Stålkretsloppets miljövärde med beaktande av ekologiska, tekniska och sociala faktorer. Miljövärderingar ger forskare, industri, användare och samhället i övrigt möjlighet att värdera såväl stålets nytta för miljön som dess belastning på miljön ur ett livscykelperspektiv. En metod som gör miljövärderingar tillgängliga för objektiva beslut.
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II. Conjoint analysis as a decision tool for evaluation of environmental performance of for instance experts and consumers preferences
... tar fram en metod för att mäta olika experters och andra intressenters attityder till och värdering av komplexa miljöfrågor.
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