A R C H I V E S O F M E T A L L U R G Y A N D M A T E R I A L S

Volume 55 2010 Issue 4

DOI: 10.2478/v10172-010-0012-6

L. D. TENG∗ S. SEETHARAMAN∗, M. NZOTTA∗∗ P.D. DONG∗ H.L. GE∗ L.J. WANG∗ H.J. WANG∗ A. CHYCHKO∗

RETENTION, RECOVERY AND RECYCLING OF METAL VALUES FROM HIGH ALLOYED STEEL SLAGS

RETENCJA, ODZYSK I RECYCLING METALU Z ŻUŻLI WYSOKOSTOPOWYCH STALI

The work was carried out in four parallel directions. The thermodynamic activities of oxides of Cr in steel slags weredetermined by slag-gas equilibration technique. The ratio of Cr2+/Cr3+ in CaO-MgO(-FeO)-AlO3-SiO2-CrOxsystem slags wasmeasured by X-ray absorption near edge spectra (XANES). High-temperature mass spectrometry method was also used toobtain the distribution of chromium oxides. A mathematical correlation was established for estimating the ratio of Cr2+/Cr3+

as a function of temperature, partial pressure of oxygen and slag basicity. Laboratory investigations of the decarburizationof high alloy steels under controlled oxygen potentials have been carried out to retain Cr in the steel phase. A mathematicalmodel has been developed for the decarburization process with controlled oxygen partial pressure. Experimental and theoreticalinvestigations have been carried out in optimizing the Mo-additions to steel in the EAF practice in Uddeholm Tooling AB.Substantial saving of Mo as well as less emissions of Mo-bearing dust are indicated in the study. A salt extraction process wasdeveloped to extract the metal values from steel slags. Successful extractions, followed by electrolysis indicate that this couldbe a viable route towards recovery of metals from metallurgical slags.

Keywords: retention, recovery, recycling, slag, chromium steel, decarburization

Praca była realizowana w czterech równoległych kierunkach. Termodynamiczne aktywności tlenków Cr w żużlu zostałyokreślone w stanie równowagi żużel-gaz. Stosunek Cr2+/Cr3+ w żużlach CaO-MgO-(-FeO)-AlO3-SiO2-CrOx mierzono metodąabsorpcji promieniowania rentgenowskiego w pobliżu krawędzi widm (XANES). W celu uzyskania rozdziału tlenków chromuwykorzystano wysokotemperaturową spektrometrię masową. Korelację matematyczną ustalono do oceny stosunku Cr2+/Cr3+,jako funkcji temperatury, ciśnienia parcjalnego tlenu i zasadowości żużla. Badania laboratoryjne odwęglania stali wysokosto-powych, kontrolowanego przez potencjał tlenowy przeprowadzono, aby pozostawić Cr w stali. Model matematyczny zostałopracowany dla procesu odwęglania z kontrolą parcjalnego ciśnienia tlenu. Eksperymentalne i teoretyczne badania zostałyprzeprowadzone przez optymalizację dodatku Mo do stali w procesie EAF w Uddeholm Tooling AB. Znaczne oszczędnościMo, a także mniejsza emisja pyłu zawierającego Mo są wskazane w badaniu. Proces ekstrakcji soli został stworzony, abywyodrębnić metal z żużli stalowniczych. Udane ekstrakcje, a następnie elektrolizy, wskazują, że mogłaby to być opłacalnadroga do odzysku metali z żużli metalurgicznych.

1. Introduction

With the increasing constraints on the steel indus-try with respect to the energy and environmental opti-mization, eco-steel production has received high priorityin Sweden, as exemplified by the national effort sup-ported by Swedish Environmental Research Foundation(MISTRA) and the Swedish Steel Producers Association(Jernkontoret). New strategies for retaining the dissolvedelements in the steel bath within specified intervals, es-pecially in the case of high alloy steel production, hasbeen one of the targets in this great effort. It is realizedthat drastic changes in steel processes towards environ-

mental improvements could not be easily accommodat-ed. Modifications of existing processes and design ofnew process steps can only be enabled using processfundamentals which would lead to a win-win situationdue to environmental as well as economic advantages. Itis common knowledge that alloying elements like Cr andV form stable oxides and recovery of these metals in theelement form is energy-intensive. A simple mass-balanceconsideration would reveal that

1) It would be useful if these elements can be retained inthe steel, minimizing the loss of the same to the slagor dust, referred to in this project as “Retention”,

∗ DIVISION OF MATERIALS PROCESS SCIENCE, ROYAL INSTITUTE OF TECHNOLOGY, SE-100 44 STOCKHOLM, SWEDEN.∗∗ PROCESS METALLURGY, RESEARCH AND DEVELOPMENT, UDDEHOLM TOOLING AB, SE-683 85 HAGFORS, SWEDEN.

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2) Recovery of the elements from the slag and dustphases in a suitable form needs to be enabled so thatthese valuable metals can be reused in the steelmak-ing cycle, referred to in this project as Recovery”and

3) The recovered metals as well as the rest products(slag and the dust) could be recycled, referred to inthis project as “Recycling”.

With the industrial expansion of progressing coun-tries like China and India and consequent increase in theprices of the alloying elements, there is a strong econom-ic motivation for the recovery of metal values from slagsand dust. For example, in the production of high alloysteel of 1 ton at Uddeholm Tooling AB, Hagfors, Sweden[1], the amount of Mo per ton of steel is about 13 kg. Theslag contains 0.4 kg Mo and the dust, about 1 kg. Apartfrom the fact that there is a strong economic motivation,about SEK 500 for Mo per ton steel produced which islost in the slag and the dust, this amount of Mo ends upin the environment. The situation is similar even in thecase of Cr, Nb and V. Design of suitable process stepstowards retention, recovery and recycling of these metalvalues is the primary motivation of the project.

2. Experimental methods

The strategy adopted in the present project has beentailored primarily to the EAF practice of Uddeholm

Tooling AB, Sweden[2]. The present project approachconsists of four essential components, viz.

2.1. Thermodynamic studies of Cr-bearing slags

The objective of present investigation is to measurethermodynamic activity of chromium, or vanadium ox-ide in CaO-SiO2-MgO-Al2O3-CrOX(VOx) melt by usingthe gas equilibrium technique. The PO2was controlled bythe mixture of CO-CO2-Ar. The present division has thenecessary instrumentation as well as experience in suchmeasurements [3]. The principle of gas-slag equilibra-tion method is to equilibrate the slag sample kept in a Ptcrucible with the above-mentioned gas mixture, so thatthe oxygen partial pressure was predetermined. From aknowledge of the thermodynamic activity of Cr in Cr-Ptalloy and the oxygen partial pressure in the gas stream,it is possible to calculate the thermodynamic activity ofchromium oxide in the slag. A similar procedure is ap-plicable for vanadium. The oxygen pressures used were10−8,10−9,10−10 bar and experiments were carried out at1823,1873 and 1923K. The gas-cleaning train used andthe experimental apparatus are presented in Figure 1 and2 respectively. By preliminary experiments, it was foundthat the equilibrium between the slag and gas phasescould be attained after 14 h; consequently, all the equi-libration experiments were carried out at a safe limit of20 h. The slags were then quenched in argon atmosphereand analyzed.

Fig. 1. Schematic graph of gas cleaning system0 – flowmeter; 1 – silica gel; 2 – magnesium percholorate; 3 – ascarite; 4 – copper turnings; 5 – magnesium chips

Fig. 2. Schematic arrangement of furnace

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2.2. Retention of Cr in steel melt using oxygenpartial pressure control

In the case of the decarburization of alloy steelsespecially stainless steels the oxidation of carbon is car-ried out by blowing O2Ar mixture (AOD process) intothe steel melt While at the initial stages carbon is prefer-entially oxidized at later stages as the oxygen potential ofthe bath increases elements with strong affinity to oxy-gen for example Cr will be oxidized and lost to the slagphase. In order to minimize this loss, it was proposed inthe present work to use O2CO2(-Ar) gas mixtures withhigh oxygen contents at the beginning of blowing andhigh CO2contents at the finishing stages

Thermodynamic calculations were carried out to ex-amine the possibilities of the effect of decarburizationusing CO2

Experiment series 1: In this series a steel melt con-taining pre-determined amounts of Cr and C was decar-burized with oxygen and CO2blowing respectively

Experiment series 2: In this series steel meltswere exposed to gas mixtures with different oxygenpartial pressures; viz. pure O2 75%volO225%volCO2,50%volO250%volCO2 25%volO275%volCO2and pureCO2 The samples are withdrawn at different time in-tervals and the C as well as Cr contents are analyzedThe amounts of Cr and C in the melt as well as thetemperature are varied The aim of these experiments isto determine the mass transfer effects of reducing theoxygen partial pressure during decarburization

Model development: If the oxygen partial pressureis to be continuously decreased during the decarburiza-tion process it is essential that a mathematical modelthat could predict the decarburization and the Crloss byoxidation is developed

2.3. Minimization of metal losses in dusts byprocess modification

In the addition of alloying elements losses are en-countered especially in slag and dust Minimization ofthe loss of these metals from dust is very important fromenvironmental as well as economic view points as dustprocessing requires additional process steps Once againthe EAF practice in Uddeholm Tooling AB had beentaken up for investigation

An analysis of the dust samples at Uddeholm Tool-ing AB, EAF practice reveals that the total amount ofMo lost amounts to 72.64 kg/heat 60.94ton/year, 32 mil-lon EUR/year. Similar is the situation with respect toCr slag losses Total amount of Cr lost is 27.9 kg/heat,14054 ton/year, .2 millon EUR/year. This part of thepresent work was focused on Mo losses in the dust. Planttrials were conducted and samples of steel slag and dust

were collected systematically at the various stages ofthe process during regular campaigns over nearly twoweeks The measurements to increase the Mo yield wereconducted and promising results have been obtained

2.4. Extraction of metal values by using a saltextraction process

Despite the precautions taken in process optimiza-tion in view of the thermodynamic and kinetic con-straints valuable metals are always lost in the slag.Chromium in stainless steel making slag can be usually24% according to information from Outokumpu Avesta[5] and could go up to as high as 8%. The presence ofsuch metals apart from the economic losses, poses a se-rious challenge to the recycling of the slags. The presentgroup has been devoted to developing new process con-cept in this respect by extracting the metal values by amolten salt phase

For the extraction of valuable metals in steelmakingslags a salt extraction process has been designed by thepresent group In this process the slag is treated with asalt melt of suitable composition and fluxes are added inorder to extract the metals into this melt The salt phasecan be electrolyzed or if necessary leached to extract thevaluable metals in aqueous phase. The results have beenencouraging and a patent application has been approvedThe process parameters are currently being examined foroptimization

3. Results and discussion

The results obtained so far are summarized in thefollowing sections

3.1. Thermodynamic studies of Cr bearing slags

As part of the present project, thermodynamic ac-tivity measurements of the low Cr-containing slags bygasslag equilibration method were carried out

3.1.1. Activity of CrO in Cr-slags

2 different slag compositions with varying amountsof chromium oxide have been investigated in the presentwork The work at this stage was restricted to low Cr-contents and the slag consisted of CaO MgO Al2O3 andSiO2 apart from CrOx The Ptcrucibles were analyzedafter the experiment and the Cr-content of the alloy inequilibrium with the slag was determined The amountof chromium oxide retained by the slag was found to bestrongly dependent on the slag chemistry, above all, thebasicity. The thermodynamic activity of chromium oxide

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was calculated utilizing the thermodynamic informationfrom the PtCr system reported by Pretorius and Muan[4]It is well known that Cr exists as Cr2+ and Cr3+ in slagsIn the present work the amount of chromium oxide isvery low Further, the slag is in equilibrium with PtCralloy at low oxygen partial pressures in the gas phase.Hence, all Cr in the slag was assumed to be in thedivalent state. This is supported by the measurementsof Forssbacka and Holappa [5] as well as the experi-ments conducted in the present laboratory by indepen-dent technique. The relationship between the chromiumoxide content for the slag compositions investigated inthe present work and the thermodynamic activity of CrOis presented in Figure 3.

Fig. 3. Thermodynamic activity of CrO as a function of content ofCrOxin the slag

As can be seen the activity of CrO shows a positivedeviation from Raoults law in agreement with results re-ported earlier. The values at slightly higher Cr contentscould be affected by slight increase in the Cr3+ contentsof the slags

From the present results an expression for theamount of Cr in the slag as a function of the oxygenpartial pressure and basicity was worked out and wasgiven below:

log(%Cr)slag = 4.488 + 16907.82T + 2.555 × log[Cr]alloy+

+1.589 × log(PO2) − 8.483 × log B(1)

Where ‘B’ is the basicity defined as

B =wt%CaO + wt%MgOwt%SiO2 + wt%Al2O3

(2)

This expression can be valuable in assessing theamount of Cr lost in the slag under given basicity and

oxygen partial pressure if the activity of Cr in steel isknown

3.1.2. Cr valence states

The distributions of CrO and CrO1.5 in theCaOSiO2-CrOx and CaOMgO(FeO) Al2O3SiO2CrOxslag systems were measured in the present study bygasslag equilibrium method and by hightemperatureKnudsen cellmass spectrometry respectively The resultsindicate that ratio XCrO/XCrO1.5 increases with the in-creasing temperature, decreasing oxygen partial pres-sure and slag basicity. Combining the results obtainedin present study and selected literature values a math-ematical expression of XCrO/XCrO1.5 ratio described asfunction of temperature, oxygen partial pressure and slagbasicity was built up and is presented in equation (2).

log(

XCrO

XCrO1.5

)= −11534

T−0.25 · log(pO2)−0.203 · log(B)+5.74

(3)where T is temperature in K, PO2is oxygen partialpressure in Pa and B refers to basicty defined as(%CaO+%MgO)/(%Al2O3+%SiO2) The correlation iscompared with the experimental results from the presentas well as earlier works in Figure 4 It is seen that thecorrelation is satisfactory

Fig. 4. Comparisons between experimental and estimatedXCrO/XCrO1.5

Equation (2) can be utilized in estimating thechromium oxidation level in slags in industrial practice.With a knowledge of basicity oxygen partial pressureprevailing as well as temperature of the process the ex-tent of Cr(III) in the slag can be estimated This infor-mation is considered to be useful in process control

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3.2. Retention of Cr in steel melt using oxygenpartial pressure control

Retention of reactive metals like Cr requires low-er oxygen partial pressures, higher temperatures and /orhigher thermodynamic activities of the oxide in the slagDuring decarburization, the competition between carbonand the reactive metals is an important factor in deter-mining the loss of these metals to the slag

Preliminary laboratory experiments at 500 g scaleusing an induction furnace were carried out with Fe-Cr melts. The gases, O2or CO2 were blown through thesample A schematic diagram of the experimental setupis presented in Figure 5 Samples were taken periodicallyin order to analyze the C and Cr contents

Fig. 5. A schematic diagram of the induction furnace experiments

Fig. 6. Effect of gas composition on the Cr loss of high carbon alloy

Experiments were carried out to explore the effectof composition of the injected gas on oxidation of theFeCr-C melts having high (approximately 3 mass%) aswell as low (0.25-0.3 mass%) carbon contents with the

chromium range of 118155 mass% Figure 6 shows thevariation of Cr content with time for gas injection atdifferent gas flow rates and injected gas compositionsat high carbon level Although initial chromium contentwas different it is still evident that the rate of Crlossdecreases with increasing CO2 content in the injectedgas Figure 7 shows the change in carbon content withinjection time at gas flow rate at 136 Nml/min for meltshaving high carbon contents It can be seen that carboncontent in the melt decreases At this gas flow rate, thedecarburization is much faster when blowing with pureCO2.

Fig. 7. Effect of gas composition on the decarburization rate of highC alloy

3.3. Minimization of metal losses in dust by processmodification

Plant trials were conducted in the EAF at UddeholmTooling AB, in collaboration with Dr Nzotta [1], in orderto investigate the Molosses in the slag and dust A massbalance for Mo worked out from the results is shown inFigure 8.

Fig. 8. Mo mass balance in EAF practice at UTAB

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The present investigation focuses on the evaporationof MoO3 from various stabilizing sources during contin-uous heating of the materials from room temperature upto 1300◦C by thermogravimetric analysis (TGA). Theresults from the nonisothermal TGA investigations ofthe mixtures of MgO CaO and CaCO3 with MoO3 arepresented in Fig. 9 It can be seen that in the case ofmixtures of MoO3 with CaO or MgO only a minor partof MoO3 (wt loss was about 1.8%) evaporated below1373 K.

Fig. 9. The mass changes for MgMoO4 and CaMoO4 precursors byTGA studies

The evaporation rate of pure MoO3was studied byTGA and compared with the evaporation rates from mix-tures of MoO3with CaO CaCO3 MgO MoO3can be sta-bilized by mixing with CaO MgO or CaCO3 and heat-ing the mixtures as the result of appropriate molybdatesformation In the case of CaMoO4 and MgMoO4 the de-composition rate is increased with increasing temper-ature and the evaporation rate of MoO3would be pro-portional to the equilibrium partial pressures of MoO3.Liquid steel treatments with different sources of Mo ina small induction furnace in argon atmosphere showedthe Mo yield in the case of MoO3+C was similar tothat of CaMoO4+C and also MoO3+CaO+C mixturesCaO+MoO3mixture can be used instead of CaMoO4asthe Mo yield is approximately the same. The lowest Moyield is found for MgO+MoO3mixture. Fast evaporationof MoO3from the mixture with MgO+C was found tobe the reason for very low yield for this mixture Fur-ther work towards stabilization of molybdenum oxide iscurrently in progress.

3.4. Extraction of metal values by using a saltextraction process

A salt extraction process was designed to extractthe metal values from slag rests Attention was mainlyfocused on Cr in the slag The flow sheet for the ex-

traction part of Cr from the slag is presented in Fig-ure 10. Experiments were conducted with Crslags fromUddeholm Tooling AB by extracting the Cr-values byNaClKCl melts at 1023 K at different time intervals Theinvestigations were carried out in order to optimize theprocess parameters for dissolution of slag/chromite in theeutectic salt mixtures of NaClKCl containing a patentedflux AlCl3. Based on the optimized conditions labscaleinvestigations of electrolysis were carried out applying acell voltage of 2.8V Ferrochrome was recovered as thecathode deposit.

Fig. 10. Schematic diagram of Salt Extraction Process

The theoretical decomposition voltage plotted inFigure 11 can give the deposition trend as follows:CrCl3 >FeCl2 >MnCl2 >AlCl3 >MgCl2 >NaCl>CaCl2 >KCl It shows that Fe and Cr will deposit first Takingaccount of the higher cell voltage requirement and thelower content of Mn2+ and Mg2+ in salt phase, under acell voltage of 28V ferrochromium alloy is likely to bethe main product It should be noted that the electrode-position of metals can be affected by a number of factorseven in pure molten salt systems as for example, overvoltage, current density current efficiency bath electricconductivity the nature and surface of the cathode ma-terial and distance between electrodes etc In the presentwork several electrolysis tests on EAF slag/chromite arebeing carried out with promising results.

A typical photograph of the cathode product afterelectrolysis of the slag is shown in Figure 12 (a). Thedeposit consists of Fe-Cr alloy with a low amount ofoxygen and salt mixtures with some undissolved slagAfter removing the adhered product and washing thecathode with ethanol a typical SEM image of the de-posited product could be obtained which is shown in

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Fig. 12 (b). A dendritelike structure of the crystal depositcould be observed in this figure. Further EDS analysisof the dendritic crystal shows that it consists of Fe andCr along with low content of oxygen (below 10%) and asmall amount of Ca, Na, K and Cl It should be noted thatthe oxygen content in this EDS analysis is not accurateThe present experiments show clearly that ferrochromecan be recovered from the slag by the combined extrac-tionelectrolysis process XRD results of the adhered sub-stances on the cathode confirm the formation of Fe-Cr,as shown in Fig. 12 (c). The cathode products consistmainly of FeCr, inclusions of NaCl and KCl from theelectrolyte apart minor levels of other impurities.

Fig. 11. The theoretical decomposition voltage of different metalchlorides

Fig. 12. Analysis of the cathode deposit after salt extraction and electrolysis (a) Cathode deposit product photo; (b) SEM analysis; (c) XRDanalysis of the cathode deposit with different flux/slag ratio

4. Conclusions

4.1 Thermodynamic activities of chromium oxidesin slags were measured by gasslag equilibration methodActivity of chromium oxide shows positive deviationfrom ideality it decreases with increasing temperatureand decreasing content of chromium oxide in slag andoxygen partial pressure. The oxidation states of CrOxcontaining slag have been studied.

A mathematical correlation was established for es-timating the ratio of Cr2+/Cr3+ as a function of temper-ature partial pressure of oxygen and slag basicity.

4.2 The second part viz decarburization under con-trolled oxygen partial pressure was dealt with both theo-retically and experimentally A theoretical generic modeldescribing the mass transfer phenomena between risinggas bubbles and a metal bath has been developed by thepresent authors to predict the composition change in themelt as a consequence of blowing different oxidant gasesThe experimental results also indicate that the Crlosses

can be significantly lowered by replacing the oxygen withCO2in the injected gas specifically for FeCrC melts withcarbon levels higher than about

.8 mass%.

4.3 In the third part of the project viz modificationof the Mo precursor additions in EAF, theoretical assess-ment of Mo additions and plant trials are being carriedout Synthesis of stable Mocompounds from lowgradeMosources were carried out Liquid steel treatments withdifferent sources of Mo in a small induction furnacein argon atmosphere showed the Mo yield could be in-creased by using proper Mo source.

4.4 In the fourth part viz salt extraction of valuablemetals from slags successful extraction of Cr and Mo us-ing suitable fluxes have been carried out The electrolysisof the extract to get Fe-Cr has also been demonstrated.

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REFERENCES

[1] M. N z o t t a, L. T e n g, S. S e e t h a r a m a n, Un-published work 2006.

[2] S. S e e t h a r a m a n, Project application Stal kretslop-pet, MISTRA 2004.

[3] P. F r e d r i k s s o n, S. S e e t h a r a m a n, Ther-modynamic Activities of “FeO” in some Ternary“FeO”-containing Slags Steel Research International 75,357-365 (2003, 2004).

[4] E. P r e t o r i u s, A. M u a n, Activity-Composition Re-lations of Chromium Oxide in Silica Melts at 150◦Cunder Strongly Reducing Conditions, JAm.CeramSocl75[6], 1364-77 (1992).

[5] L. F o r s s b a c k a, L. H o l a p p a, Viscosity of SiO2CaO-CrOx slags in contact with metallic chromium andapplication of the Iida model VII International Con-ference on Molten Slags Fluxes and Salts Cape townSAIMM, Johannesburg 129-136 (2004).

Received: 10 April 2010.