MASTER’S THESIS
2005:029 CIV
JOUNI YLIPEKKALA
Quality Management of Chromium Containing Steel Slags from
Melt Phase to Cooling
MASTER OF SCIENCE PROGRAMME Luleå University of Technology
Department of Chemical Engineering and Geosciences Division of Process Metallurgy
2005:029 CIV • ISSN: 1402 - 1617 • ISRN: LTU - EX - - 05/29 - - SE
Foreword
This master’s thesis concludes my studies for a master’s of science in Process metallurgy and mineral technology at Luleå University of technology. This thesis has been carried out at MiMer in co-operation with Outokumpu Stainless Oy, Tornio Finland. The investigation was made during the period of January to August 2004 at Outokumpu Stainless Oy in Tornio and some of the analyses (XRD) were made at Luleå University of technology.
I will thank my supervisor Dr. Margareta Lidström-Larsson at MiMeR/ Division of process metallurgy at university of technology in Luleå and my instructor M.Sc. Juha Roininen at Outokumpu Stainless Oy. Special thanks go to personell at Tornio Research Centre and steel melting shop who have helped me with my work, with analyses and practical tests. I would also like to thank M.Sc. Fredrik Engström at MiMeR for all his help with the analyses I have made in Luleå.
I will also thank my family, Marja-Riitta and Anna, at home in Haparanda who have support me during my studies.
Haparanda December 2004
Jouni Ylipekkala
Abstract
Outokumpu Stainless Oy Tornio works produces in close future around 1.7 million tonnes steel slabs annually. As all in the metallurgical processes the production of stainless steel producing by-products as slags. The different kinds of processes produce about 300 000 tonnes slags annually, almost the whole amount of this has been deposited after the metal separation up to recent days. The project to utilize the steel slag started during 2001 with the aim to increase the amount of recycled by-products and to develop new slag-products.
By-products are produced from the different process part as Electric Arc Furnaces (EAF1, EAF2), Argon Oxygen Decarburization converters (AOD1, AOD2), ladle stations and Chromium Converter (CRC). The chemical composition of slags from a process has a slight variation, but between the different processes the variations in
analyses are greater. The chemical composition of the slags is dependent to the characters of the slag, as volume stability by basicity of the material. The aim of this work was to investigate the quality control of chromium containing steel slags during cooling into solid phase. The product should be an aggregate with sufficient hardness and low leaching behaviors.
The investigation started when the slag was poured into a slag pot at melting shop, any addition of stabilizers or other chemical components has not been done. The aim was to investigate the dependence of different cooling methods to spinel forming. The physical properties of the product of the tests were tested by Nordic ball mill test ( prEN 1097-9).
The chemical analyses were: total analyses (XRF), and leaching test (Shaking test prEN 12457-3). A powder X-ray diffraction meter analyses (XRD) was used to estimate the mineralogy of the samples. The solid aggregate samples were studied with Scanning Electron Microscope (SEM). Simulations with respective chemical analysis of the slags were done by the data program FactSage. The slags, included in this work, were mainly EAF2 and CRC slags because of less variation in chemical analysis of these slags. These slags contain chromium, a minor part of chromium is not bound in spinels, which is a reason for leaching. During the first tests, which were carried out by water-cooling, no significant decreasing of leaching of chromium was observed. When pouring out the slag as a thin bed the leaching of chromium was lower compared to the massive bed of the same slag part. Lower leaching of chromium from the massive material from the thin bed can be caused by the smaller active surface of material. Reference test with semi-
quenched slag were made and they showed low leaching values compared with the slags from normally pouring practices. Semi-quenched slag is porous and cannot be analyzed in SEM and was hence milled as powder and analyzed in XRD. The mineralogy of semi- quenched slag is varying in some degree from the mineralogy of normally poured slag.
The semi-quenched slag, pumice, is partly amorphous which can be the reason for lower leaching values of chromium. Granulation tests have been made earlier with all of the three types of slag, but all of these showed higher leaching values than those of pumice.
During granulation material may come in contact with air and become oxidized and chromium oxides leach more than other compounds of chromium.
The lowest leaching values of slag can be attained by semi-quenching of material but the product is not an aggregate and cannot be used in all of the civil engineering applications.
Aggregates can be produced by cooling on the slabs, but the slag bed should be
maximum 10 cm thick, so that the gases can flow out before solidification of material.
Sammanfattning
Outokumpu Stainless Oy Torneå verket kommer under de närmste åren att producera ca 1,7 miljoner ton rostfria stålämnen årligen. Liksom i alla metallurgiska processer innebär tillverkning av rostfritt stål även produktion av biprodukter, slagg. De olika
delprocesserna, inom ståltillverkningen, producerar ca 300 000 ton slagg som deponerats som blandslagg efter metallseparering. För att minska mängden deponerad slagg startades under 2001 projektet för produktifiering av stålslagger. Målet var att öka mängden
återanvänd stålslagg genom att utveckla nya slaggprodukter.
Biprodukter framställs i delprocesser som ljusbågsugn (LB1, LB2), AOD-konverter (AOD1 och AOD2), kromkonverter (CRK) och från skänkstationer. Variationen i den kemiska sammansättningen inom respektive delprocess är liten, men skiljer sig mellan de olika processerna. Den kemiska sammansättningen påverkar biprodukternas
volymstabilitet, halten lakbara ämnen mm. Målet med detta arbete är att finna metoder att kontrollera kvalitén på kromhaltiga stålslagger med hjälp av olika avsvalningsmetoder.
Den eftersökta produkten skall vara ett tillräckligt stabilt aggregat med låga lakningsvärden på krom och andra tungmetaller.
Undersökningen började då slaggen hade tippats i en slaggryta vid stålverket, inga kemiska tillsatser i den smälta slaggen användes. Avsvalningshastighetens påverkan på spinellbildning undersöktes. Erhållen produkt testades fysikaliskt med nordiska
kulkvarnstestet (prEN 1097-9). Kemiska analyser som gjordes var: totalanalys (XRF) och lakningtest ( SKAKTEST prEN 12457-3). Mineralogin hos pulverformiga biprodukter undersöktes med röntgen diffraktometer (XRD) och de aggregatformiga proven med svepelektron mikroskop (SEM). Med datorprogram FactSage gjordes några simuleringar med respektive sammansätting av olika testslagger. De undersökta slaggerna kom i huvudsak från LB2 och CRK, eftersom processtabiliteten är bra och fluktuationen i de kemiska analyserna är liten. Dessa slagger innehåller krom och en del av kromet finns inte i spinellstrukturer vilket innebär att kromet delvis är i lättlakad form. Under de första testerna, vilka utfördes med vattenkylning av slaggbädd kunde inte någon signifikant minskning i lakningen observeras. När slaggen tippades till en tunn bädd var material tätt och hårt och lakningen mindre än från en tjockare bädd av samma slaggparti. Den lägre lakningen kan vara resultatet av den mindre specifika ytan hos det täta materialet. Vid referenstester med snabbkylning med vatten (semi-quenched), som ger en produkt liknande pimpsten sk. lättstensmaterial, erhålles låga lakningsvärden. Denna produkt undersöktes med XRD eftersom den inte går att undersöka med SEM pga. materialets porositet. Mineralogin hos lättsten varierar något mot den på marken normalt tippad slaggen. Lättstenmaterial är partiellt ett amorft material. Granulationstester för slagger har gjorts tidigare, men resultaten av dessa visar på högre lakningsvärden av krom jämfört med semi-quenched material. Orsaken till detta kan vara oxidering under själva granulationen då de kylda slaggdropparna flyger genom luften till en vattendamm.
Kromoxider är mer lättlakade än andra kromföreningar.
Lägsta lakningsvärden på krom hos produkter fås med semi-quenching, men då är materialet inte aggregatformigt utan liknar granulerad slagg och pimpsten. Semi-
quenched slagg har goda isolerings- och dräneringsegenskaper. Aggregatformigt material med lägsta möjliga lakning av krom erhålls med kylning från en riktning, t.ex. med hjälp av slabsbotten och relativt tunn bädd av tippad material, så att gaserna hinner strömma ut utan att göra materialet poröst.
Yhteenveto
Outokumpu Stainless Oy Tornion tehtaitten vuosittainen tuotanto nousee lähivuosina 1,7 miljoonaan tonniin teräsaihioita. Ruostumattoman teräkseen valmistusprosessiin kuten muihin metallurgisiin prosesseihin kuuluu vääjäämättä myös sivutuotteiden kuten kuonan valmistaminen. Eri osaprosesseista muodostuu vuosittain sivutuotteita noin 300 000 tonnia, josta suurin osa on loppusijoitettu sekakuonana metallinerotusprosessin jälkeen vielä näihin päiviin asti. Terässulattokuonien tuotteistamisprojekti aloitettiin vuonna 2001 tavoitteena lisätä hyötykäytettävän sivutuotemäärän osuutta ja kehittää uusia tuotteita.
Sivutuotteita muodostuu osaprosesseista kuten, valokaariuuneista (VKU1, VKU2), AOD- konverttereista (AOD1, AOD2), senkka-asemilta sekä kromikonvertterilta (CRK). Kuonien kemiallisen koostumuksen vaihtelu kussakin prosessissa on pientä, mutta eri prosessien välillä kemiallinen koostumus on erilainen. Kemiallinen koostumus vaikuttaa lähinnä emäksisyyden osalta sivutuotteen koossa pysymiseen tai mahdolliseen pulverisoitumiseen.
Tämän työn tarkoituksena on kromipitoisten teräskuonien laadun hallitseminen eri
jäähdytysmenetelmiä käyttäen. Tavoiteltu tuote tulisi olla aggregaatti joka olisi riittävän luja sekä kemiallisilta ominaisuuksiltaan sellainen että kromin ja muiden raskasmetallien
liukoisuus olisi alhainen.
Tutkittavana materiaalina oli sula kuona joka oli kaadettu prosessista kuonapataan, mitään lisäaineita ei käytetty sulassa. Tavoitteena oli tutkia kuonassa olevan kromin sitoutumista spinelleihin eri jäähdytysmenetelmillä. Saatua sivutuotetta tutkittiin myös fysikaalisesti, pohjoismaisen kuulamyllytestin (prEN 1097-9) avulla. Kemialliset analyysit olivat : kokonaisanalyysi (XRF) ja liukoisuustesti (ravistelutesti prEN 12457-3). Sivutuotteiden mineralogiaa jauhetuista näytteistä selvitettiin röntgendiffraktiometrin (XRD) ja
kappalemuotoisesta näytteestä pyyhkäisyelektronimikroskoopin (SEM) avulla. Lisäksi suoritettiin simulointeja FactSage ohjelmalla eri yhdisteiden aktiivisuuden tutkimiseksi.
Tutkitut sivutuotteet tulivat pääosin VKU2 ja CRK:lta joiden prosessitasapaino on hyvä eikä kemiallisen analyysin vaihtelu ole merkittävää. Kyseiset tuotteet sisältävät kromia ja osa siitä ei ole spinelleissä joten kromin liukenemista tapahtuu. Käytettäessä vesijäähdytystä kuonapatjan yläpinnan jäähdyttämiseksi ei merkittävää liukoisuuden pienenemistä havaittu . Kaadettaessa sula kuona aihion päälle ohueksi kerrokseksi tuli patjasta tiivis ja liukoisuus oli vähäisempää verrattuna samaan erään joka oli kaadettu paksummaksi patjaksi. Liukoisuuden pienenemiseen vaikuttanee eniten aktiivisen ominaispinta-alan pieneneminen tiiviissä
materiaalissa. Vertailukokeita tehtäessä pikajäähdyttämällä kuonaa ns. kevytkiveksi saatiin alhaisia liukoisuusarvoja kromin osalta. Kevytkiviaineksen mineralogiaa tutkittiin XRD:lllä koska sitä ei huokoisuuden vuoksi voi tutkia SEM:llä. Kevytkiviaines, joka on osittain amorfista, sisältää joitakin eri mineraaleja normaaliin maassa jäähdytettyihin kuoniin verrattuna. Kuonatyypeille on myös tehty granulointitestejä, jossa sula kuona jäähtyy nopeasti sulasta kiinteään muotoon. Granuloidun kuonan liukoisuusarvot ovat korkeammat kuin kevytkiven, joten on syytä epäillä hapettumisen tapahtuvan granuloinnin yhteydessä minkä vuoksi kromiyhdisteet ovat suurimmalta osalta oksideja jotka liukenevat.
Matalimmat liukoisuusarvot saadaan vesi-pikajäähdytyksen avulla mutta tuote ei ole silloin aggregaattimuotoista, vaan muistuttaa granuloitua kuonaa tai hohkakiveä. Haluttaessa aggregaattia, jossa kromin liukoisuus olisi vähäistä, olisi kuona kaadettava jäähdyttävän metallialustan päälle suhteellisen ohueksi kerrokseksi jotta jäähtyminen tapahtuisi yhdestä suunnasta ja kaasut ehtisivät virrata ylös materiaalista ennen yläpinnan muuttumista kiinteään muotoon.
Table of Contents
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1. INTRODUCTION ... 7
1.1. Introduction... 7
1.2. Aim of this work ... 7
1.3. Limitations ... 8
2. STEEL MAKING AND BY-PRODUCTS FROM STEEL PLANTS ... 9
2.1. Stainless steel production at the Tornio works ... 9
2.2. By-products from steel making... 10
3 THEORY ... 12
3.1. General slag properties ... 12
3.1.1. Slag ... 12
3.1.2. Thermodynamics of the stainless steel slags ... 12
3.1.3. Activity of chromium compounds in slags ... 14
3.1.4. Chromium in steel slags... 15
3.1.5. Density of slag ... 15
3.1.6. Viscosity ... 15
3.1.7. Basicity ... 15
3.1.8. Solubility of water... 16
3.1.9. Spinels... 16
4. LITERATURE STUDY... 18
4.1. Investigations of slag and their properties in Europe... 18
4.1.1. Reduction of EAF slag by carbon blowing or remelting in carbon crucible. ... 19
4.1.2. Modification of the basicity of the liquid slag at KTN... 20
4.1.3.Treatment of solid slags ... 20
4.2. Utilization of steel slags... 21
4.2.1. Utilization ... 21
4.2.2. Production and utilization of slag in Europe... 22
4.2.3. The Weathering process... 24
4.3. Analyses of slags and test methods for technical properties... 25
4.3.1. Test methods, technical properties... 25
4.3.2. Test methods, environmental properties ... 26
4.3.3. Technical properties of slags compared with natural materials... 27
4.4. Environmental considerations... 29
4.4.1. Finnish guidance for the use of secondary products in earth and road construction... 29
4.5. Mineralogical investigations of steel slags ... 30
4.5.1. Investigations at Acerinox and KTN ... 30
4.5.2. Mineralogical investigation in Tornio ... 31
5. METHODS and MATERIALS... 32
5.1. Test materials... 32
5.2. Physical properties ... 32
5.2.1. Nordic ball mill test prEN 1097-9 ... 32
5.2.2. Crushing properties... 35
5.3. Chemical and mineralogical properties ... 35
5.3.1. XRF... 35
5.3.2. The standard shaking-test prEN 124 57-3 ... 35
5.3.3. XRD ... 35
5.3.4. SEM analyses... 36
Table of Contents
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5.3.5. Simulation with FactSage ... 36
6. EXPERIMENTAL and VISUAL OBSERVATIONS from the tests ... 38
6.1. Test 0 and 1, FeCr slag by normal pouring... 39
6.2. Test 2, FeCr slag with water-cooling... 40
6.3. Test 3, with water- and metal-cooling ... 40
6.4. Test 4, EAF slag cooled in the slag pot ... 41
6.5. Test 5, EAF slag cooled with scrap ... 41
6.6. Test 6, EAF slag cooled with scrap ... 41
6.7. Test 7, CRC slag cooled with scrap ... 42
6.8. Test 8, CRC slag cooled with the scrap ... 42
6.9. Test 9, AOD slag poured on the slabs... 43
6.10. Test 10, EAF slag poured on the slabs... 44
6.11. Test 11, EAF slag poured on the slabs... 44
6.12. Test 12, CRC slag poured on the slabs ... 45
6.13. Test 13, CRC slag poured on the ground... 45
6.14. Test 14, semi-quenched slags ... 45
7. RESULTS ... 46
7.1. Earlier tests and investigations... 47
7.2. Nordic ball mill test prEN 1097-9 ... 48
7.3. Chemical composition from XRF analyses ... 50
7.4. Shaking test... 51
7.5. SEM analysis ... 55
7.5.1. Tests with EAF slag... 56
7.5.2. Test 7 with CRC slag, scrap-cooled... 60
7.5.3. Test 10 with EAF slag poured out on the slabs as a thin layer. ... 61
7.6. XRD analysis ... 64
7.7. Simulation with FactSage ... 67
8. DISCUSSION... 69
9. CONCLUSIONS... 75
REFERENCES ... 76
APPENDIX... 79
Introduction
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7 1. INTRODUCTION
1.1. Introduction
Steel slags have been utilized around the world for a long period of time. The economical loss in form of metals in the slag is a marked disadvantage with depositing these slags.
A high chromium recovery from the slag is necessary for overall process economy.
High leaching of chromium from the slag materials is not environmental friendly and should be stopped in the process by treatments. Spinels in the slag are binding the chromium hard and hence the leaching of chromium is low or negligible. The control of leaching behaviours in slags has become more essential due to the stringent requirements for several aggregates for road construction.
1.2. Aim of this work
The aim of this thesis is to study chemical and physical properties of chromium
containing steel slags. These properties can be made to vary by different cooling rate and chemical accessory substances, stabilizers. The goal was to establish a cooling method that produce slag materials with low leaching behaviours and sufficient hardness to be used as a rock material in road constructions and civil engineering applications. A proper cooling method can be able to relive the utilization of slag materials. Steel slags are utilized in road constructions, since its physical properties are quite similar to natural stone materials. Mineralogical and chemical properties of every test samples were studied to examine the variations in compositions of slags. Differences in element concentration can affect leaching behavior. In this study the effect of different cooling methods as to leaching of chromium was investigated. The leaching behaviors of heavy metals e.g. chromium and molybdenum are often limiting factors for the use of slag products, as construction materials. The results from other investigations of slag utilization and leaching behaviours were compared with results of this work. The different methods of handling and characterizing slags in several countries were also investigated. The cooling methods tested, were, cooling by water-spray, by scrap in a slag pot and by pouring into a basin with the foundation of slabs. The dependence of thickness of the slag bed was investigated for stone material class and leaching behaviours.
Introduction
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8 1.3. Limitations
EAF- and AOD slags from line 1 were not included in this investigation because of the larger fluctuations in the chemicals analysis of these slags compared with slags from line 2. Ladle slags, which mainly have the similar analysis as AOD slag, has not been
investigated. At moment the praxis in Tornio is to add sodium tetraborax into liquid AOD2 slag, which results into a stable stone material with low leaching behaviors.
Stabilized AOD slag is included in this investigation as a reference material, with low chromium content and for some Nordic ball mill tests. Addition of chemical compounds into the slag pot, for example adding spinel formers into CRC or EAF slag, at the melting shop has not occurred. The investigation started when the slag pot was coming out of the melting shop and was continued during the whole cooling period.
Steel making and by-products from steel plants
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9
2. STEEL MAKING AND BY-PRODUCTS FROM STEEL PLANTS 2.1. Stainless steel production at the Tornio works
In the close future the production capacity of the steel plant at the Tornio works is 1.7 million tonnes cast slabs annually. The steel melting shop consist of two lines, the charge weight of line No 1 is 95 tonnes and of line No 2 150 tonnes. The specialty of line 1 is a chromium converter for treating liquid Ferrochrome from the Ferrochrome melting shop (Outokumpu Chrome Oy). The stainless steel production is mostly scrap based and begins at the raw material yard. The raw materials are melted in an electric arc furnace (EAF). After melting, the liquid steel is charged in an Argon Oxygen Decarburisation- converter (AOD) to reach the final quality of the steel (Figure 1). The AOD No 1. is charged with liquid ferrochromium from the chromium converter(CRC) and molten steel from the Electric Arc Furnace No 1. During the AOD process first oxygen and the mixture of oxygen and an inert gas, as argon or nitrogen, are injected in the melt. Carbon content in the melt is reduced to specified limits and the desulphurisation process is reducing the sulphur content. Between the AOD batch process and continuous casting is a ladle station one for each line for adjusting the steel quality to final limits. Special alloy elements may also be added at this stage and the melt is homogenized by argon injection.
The required temperature for the continuous casting machine is adjusted at the ladle stations.
After ladle treatments, the melts are transferred to the continuous casting units. The melt is solidified and torch cut into 800-1620 mm wide slabs. Most of the slabs are transferred hot from continuous casting units to the hot rolling mill. Prior to hot rolling, possible surface defects are removed by grinding. Two of the four grinding machines are able to grind hot slabs. The hot rolling mill is able to roll the slabs to a black hot band with a thickness of a few millimeters. The 14-metre long slab is first rolled to 22 mm thickness in the roughing mill. The finishing is made in a Steckel-type finishing mill with a
maximum rolling speed of 600 m per minute. Leader strips are welded to the beginning and the end of each coil, increasing the amount of usable products in subsequent process stages. The black hot strips are first softened or annealed in the annealing furnace. The cold rolling is carried out in reversible Senzimir mills. The final product, stainless steel, is cut in specific dimension in sheets or coils, [1].
Steel making and by-products from steel plants
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10
Fig. 1. Flow sheet of the melting shop line 1 and 2, Outokumpu Stainless Oy
2.2. By-products from steel making
Slags are produced from EAF´s , AOD´s and (Ferro)chromium converter( CRC) and even from ladle stations. Slag from AOD 2 is stabilized at the moment by the addition of sodium tetraborax, Na2B4O7, during the slag tipping into the slag pot at the melt shop, but the other slags are poured out without any chemical additions. There are different types of technologies to make products of these slags, semi-quenching by strong water-jet results in a porous, pumice like product, also called “kevytkivi” and normally poured slag, which has to be crushed to smaller aggregates to be used in road constructions.
Slags from ladle stations have no applications as products but ideas of their recycling in melt shop exist.
When the steel production increases to 1.7 million tons, the production of the by-product of different slag species will be about 366 000 tonnes. A project “ Utilizing slag from melting shop to products” started at 2001, the aim for this project is to find new slag products mainly for road construction. Until the year of 2003 all slags from the melting shop were processed in the Bergslagen process to separate the metals from crushed and milled mixed slags (AOD´s, CRC, EAF´s and ladle slags). The total amount of mixed slag was pumped in a waste pond as slurry. The slag is final deposited in the pond. In the future feasible products are for road construction, cement factory and for the manufacture of bricks and concrete blocks.
Steel making and by-products from steel plants
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11
Oxidation of chrome, iron and molybdenum is undesirable. Reducing alloys to decrease the oxidization of these metals can be done in EAF. In EAF coke is charged during melting to reduce the scorification of metals. Limestone is used as a slagbuilder in EAF 1 and burned lime in EAF 2, [1, 11].
Theory
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12 3 THEORY
3.1. General slag properties 3.1.1. Slag
Slags contain most of the non-metallic compounds of the input materials charged into the metallurgical process, or metals, which have been oxidized during the process. Parts of the metallurgical slag also contain metallic droplets, which still are in the slag because of the viscosity of the slag.
Two different theories exist about the composition of slag, one is the molecule theory and the second the ionic theory. In pursuance of the molecule theory the components do not appear as a pure material, but as chemical compounds. According to the ionic theory the slag components are positive or negative ions and could be divided into three blocks, alkaline, acid and amphoteric ions. Alkaline ionic compounds can deliver an oxygen-ion.
Acid ionic complex can consume an oxygen ion and amphoteric ions can behave as an acid in alkaline slags and as a base in acid slags. Steel slags have some alkaline compound such as CaO, FeO, MgO and MnO. The most common of the acids are SiO2
and P2O5. In steel slags the common amphoteric compounds are Al2O3 and TiO2. The molecule theory has a more classical chemistry approach of stoichiometri and described the build-up of the slag as a compound of different components, which are in equilibrium with each other. In pursuance of ionic theory the SiO4 tetrahedron is the main-component, with four oxygen atoms around each silica-atom. These tetrahedrons form a regular lattice arrangement, which are broken in melt phase. This phenomenon is called depolymerisation of the silica network. Molten silica network is dissymmetrical compared to crystalline silica. Addition of an oxide of a divalent metaloxide such as CaO results in a breakdown of the lattice of molten silica. A SiO4 tetrahedron is a strong entity, which cannot breakdown into atoms in the slag, [2, 3].
3.1.2. Thermodynamics of the stainless steel slags
Chromium has four electrons in orbital 3d and two electron in orbital 4s, thus exists in a variety of oxidation states. The behaviour of oxides in metallurgical processes is very complex due to the co-existence of multivalent chromium ions, the high melting points of slags containing chromium oxides, the characteristic of chromium oxides volatization and the sophisticated structures of chromite materials, [4]. Three different chromium ions exist in different environment; blue Cr2+, green Cr3+ and yellow to red Cr6+. Cr2O3 is moderately stable with respect to its constituent’s elements. Quantitive data on the thermodynamics of chromium oxide in silica melts are practically very few, and the phase equilibrium data in systems containing chromium are available mostly at high oxygen pressures under poorly defined reducing conditions. All of the slags contain Cr3+,
Theory
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13
but normally none of them contain Cr2+ or Cr6+ in significant amount, [6]. The
constitution of each slag should be ascertained from the total chromium content and the amount of the redox-species, Cr2+ or Cr6+. Cr2+ is a very powerful reductant, which means that divalent chromium in the slag will easily be oxidised when the slag is dissolved into a solution. This will cause certain difficulties or errors in the chemical analysis of chromium. When iron and chromium exist in the slag at the same time two redox- systems occur together. Therefore Cr2+ will be oxidised to Cr3+ and the
corresponding amount of Fe3+ is reduced to Fe2+, [6].
Iron-Chromium equilibrium reaction;
x(FeO) + Cr = xFe + CrOx ( 1.1)
Slags containing chromium oxide have been investigated at temperatures between 1400 and 1650ºC under different oxygen partial pressures, ranging from a very strong reducing atmosphere to ambient air. The results showed that the chromium exists in the silicate slags as divalent, trivalent, pentavalent or hexavalent, depending on temperature, slag composition and partial pressure of oxygen. The chromium phase relations in slags are very complex caused by the multivalences of chromium. It has been observed that the existing form of chromium in air was hexavalent if the system was CaO-CrOx. In lime rich systems chromium is oxidised to hexavalent in air. When the molar ratio CaO/Cr2O3
is higher than 3, the chromium oxidation state becomes higher in air between
temperatures of 800 and 1000ºC. Trivalent chromium was found to be incorporated into the lattice of CaO at low chromium content, but pentavalent chromium was favoured when chromium oxide concentration was higher. Earlier investigations have showed that the ratio of Cr2+/Cr3+ increases with increasing temperature and decreasing oxygen partial pressure, as well as decreasing the slag basicity from 1.5 to 1.3, [6].
The chromium oxidation state has been observed to be a function of FeO content of the slags and Si content in the metal. Activity of chromium oxides is dependent on the partial pessure of oxygen.
2CrO + ½ O2 = Cr2O3 when ∆G1700o oC =−38654 ±2500cal (1.2.)
and 2 ½2 4
CrO O
Cr pO 1.9 10
a a 2 3
⋅
⋅
= (1.3.)
Xiao has studied the equilibrium of chromium containing slags in metallic chromium crucible at 1500, 1550 and 1600ºC temperatures [6]. The system CaO-SiO2-CrOx was analysed by wet chemical method. The effects of addition of MgO and Al2O3, slag basicity and temperature were investigated. The results showed that increasing
temperature increased the fraction of divalent chromium, but increasing of the basicity decreased the fraction of divalent chromium.
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Partially substituting CaO for MgO did not result in any significant change in the
oxidation state of chromium. Increasing of the Al2O3 content from 0 to 10 mol-% resulted in a lower divalent chromium fraction at the slag basicity of 1.0 mole ratio. A further increasing in the Al2O3 content didn’t cause any obvious change in the oxidation state of chromium, [5, 6].
Fig. 2. The phase-system CaO-Cr2O3-Al2O3 versus temperature(Ford and Rees. 1958), [6]
In Figure 2. the formation of the intermediate phase 10CaO* 8Al2O3* 2CrO3* Cr2O3 at the temperature about 1400ºC, can be seen. This indicates that the chromium exists with an oxidation state of Cr6+ while CaO * Al2O3 content is more than 15 wt.-%. The
intermediate phase exist with a solid solution (ss) CaO * Al2O3, when the CaO* Al2O3
content is higher than 75%.
3.1.3. Activity of chromium compounds in slags
Although the activity measurements in chromium containing slags have been an
interesting topic for over 20 years, the existing activity data in the literature are still very restricted especially for the slag system under stainless steel production circumstances.
Studies of solubility of MgCr2O4 in CaO-MgO-Al2O3-SiO2 slags at 1600ºC have shown that when %CaO/%SiO2 ratio is constant the chromium solubility decreases with increasing Al2O3 content in the melt. The activity of CrO decreases weakly with increased temperature, but increasing basicity is increasing the activity. The activity of chromium is strongly decreased by building spinels between Cr and Magnesia or Iron. [6, 10].
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15 3.1.4. Chromium in steel slags
Chromium exists universally in slags as Cr2O3 or bounded in mineral structure by
substituting several equivalent cations. Chromium exists in slags as tri- or divalent cation.
The hexavalent cation can be produced by oxidation with atmospheric oxygen and contact with free lime, [6, 12].
3.1.5. Density of slag
With increasing temperature the density of material is often linearly decreasing, because of the oscillating motion of atoms are longer and the distance between atoms are
increasing. The temperature has quite a small effect on the density. Adding some material, which changes the composition of slag, can vary the density. The density of EAF-slag is about 2.6 – 3.0 Mg/m3 at 1500ºC, [2].
3.1.6. Viscosity
The viscosity is describing the friction forces between particles in a liquid, which could stop the particles moving. Most of the steel slags are Newton’s liquids.
The viscosity is a function of temperature, chemical composition and pressure and is decreasing with increasing temperature. Increasing the concentration of SiO2 and Al2O3
increases the viscosity, because of more of the aluminate- silicate chains. Addition of metal oxides brakes down the silica network and thereby decreases the viscosity. Chrome oxides, occurring in slag, CrO and Cr2O3, have different functions. CrO decreases the viscosity and Cr2O3 increases. When viscosity is low metal droplets can easily fall down through the melt; at high viscosity more of the droplets stay in the slag and therefore cause higher metal losses, [2].
3.1.7. Basicity
The basicity of calcium aluminate slag can be calculated by several formulas but is often calculated by formula:
B4 =
) O
%Al (%SiO
CaO
%
3 2
2⋅ (3.1.)
The availability of chromium in slags is correlated to the basicity. The leachability decreases with a rise of basicity above 1. The lowest leaching values were obtained at
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basicity around 2 and leachability of chromium increases again at higher basicity. The formula (3.1.) was developed for systems where all of the components are totally dissoluted in the slag. In practice all steel slags contain CaO and MgO, not dissoluted in the molten slag. The content of chromium in the steel slags is strongly correlated with the bacisity of the slag, at higher basicity than 2.0 chromium mainly exist as CaCr2O4. In alkaline circumstances and with presence of atmospheric oxygen CaCr2O4 can easily be oxidized to CaCrO4, a compound that is easily leaching chromium and building
hexavalent chromium from divalent or trivalent chromium.
With basicity higher than 1.0 (B4) chromium content is low and increasing with low basicity. The desirable basicity of AOD slag in Tornio is about 2.2 and 1.5 for EAF slag, when basicity index calculated by formula (3.2.), [2, 12]:
) O Al (SiO
MgO) B (CaO
3 2 2 +
= + (3.2.)
3.1.8. Solubility of water
To be soluble in slag a gaseous element must be in ionic phase. This solubility reaction is strongly depending on the basicity of the system. Solubility of H2O is increasing strongly when the mole fraction of CaO/ SiO2 is higher than 1.0 and is approximately 500 ppm at mole fraction 1.3. At the mole fraction 1.0 the water solubility is lowest, about 375 ppm, [2].
3.1.9. Spinels
The spinel group comprises a large number of binary oxide minerals, such as spinel (MgAl2O4), chromite (FeCr2O4) and hercynite (FeAl2O4).
The spinel structure AB2O4 consist of a face centered cubic (fcc) array of oxide ions (O2-) in which the A cations occupy one-eighth of the tetrahedral holes and the B cations occupy the octahedral holes. Face centered cubic is also called cubic close packed.
Examples of compounds that have spinel structures include some of d-block oxides, such as Fe3O4 and Mn3O4, where A and B are the same element.
Lattice enthalpy calculations based on a simple ionic model indicate that for A2+ and B3+
the normal spinel structure is more stable than the invers spinel structure B[AB]O4. The occupation factor, λ, of a spinel describes the degree of occupation of B atoms in the tetrahedral sites. For a normal spinel is λ = 0 and is often depending on the temperature.
When B is Cr3+ the probable A2+ atoms, which correspond to a normal spinel are Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+ and Zn2+. Cr2O3 (Eskolaite) is known to exhibit
nonstoichiometric spinel modifications whereby the normally six-coordinate cations
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occupy both octahedral and tetrahedral sites. This structural change is decreasing the overall packing efficiency of the structure, [2, 3].
In a face centered cubic structure atoms are packed at maximal possible density, of 74%
packing density.
The activity of Cr2O3 in a spinel phase, FeO* Cr2O3 or MgO* Cr2O3, is reduced and formation Cr6+ is suppressed. When the analysis of a slag is known factor sp. can be calculated by formula (3.3.), [9, 20]:
Factor sp. = 0,2⋅MgO+1,0⋅Al2O3⋅(n⋅Fetot)−0,5⋅Cr2O3(in wt.-%) (3.3.) Factor sp, which is an empiric formula, shows the amount of possible spinelphases in a system based on the stoichiometri of the elements (MgO, Al2O3, Fen and Cr2O3) in slags.
The formula has been formulated by FehS in Germany, [9]. The coefficient for Fe has been changed between 1 and 4 and in the latest version of formula Fe has the coefficient n. When factor sp is higher than 5 leaching of chromium is low because of the spinel formations in the slag (Figure 3.).
The rapidly cooled slag has an amorphous structure, which has no sharply defined melting point, transition from solid to liquid state occurs gradually. One can say that an amorphous solid is a liquid with extremely high viscosity at room temperature. The slowly cooled slag is crystalline solid; atoms are homogenously distributed into the structure. The atoms mobility is limited to small vibrations about fixed equilibrium sites, [4, 20].
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18 4. LITERATURE STUDY
4.1. Investigations of slag and their properties in Europe
Steel is infinitely recyclable and also its by-products, as slag, should be recycled, not deposited to establish a good environmental awareness. In stainless steel production the raw material cost dominate the total production cost for the primary product. Since chrome is one of the major constituents of stainless steel, it represents a large portion of the raw material cost. The most significant disadvantage with classification the slag as waste and the deposition of slag is the enormous economical losses of metals and alloys in the slagheaps, which have been deposited. A high chromium recovery is essential for overall process economy. Investigations of properties and different behaviours of slag have been done in Europe especially research on decreasing of scorification of chrome.
These tests were done at Forschungsgemeinschaft Eisenhüttenschlacken e. V. (FEhS) in Germany, at Krupp Thyssen Nirosta (KTN) in Germany, at Centro Sviluppo Materiali S.p.a. (CSM) in Italy and at ACERINOX (ACN) in Spain. The results of these
investigations were accounted in a report EUR 19382. The final aim in EAF practice was to avoid and/or decrease the scorification of chromium with addition of mixtures of slag forming materials. In AOD process operationally tests have been made to optimise the process in order to blowing and Ar/O2-ratio. Additions of different slag forming materials to avoid the chrome scorification have been tested in both the EAF- and AOD process, with the aim of binding chromium into stable mineral phases in the slag.
One method has been the addition of carbon dioxide, resulting in a lower partial pressure of the oxygen and oxidation potential. The injection of reduction materials, as Al, and Si, into furnace has been investigated. The materials have been added in the liquid slag for the desirable result with stable minerals phases with low content of chromium in leachate.
In AOD practice at KTN the decreasing of chromiumoxides in the slag was made with the optimisation of the blowing by top-lance and Ar/O2- ratio. At FEhS liquid and solid slag has been treated with different methods:
- Reducing of EAF- and AOD –slag - Mixing of solid slag with FeSO4 .
- Addition of spinel forming compounds in liquid slag.
Addition of iron sulphate to solid slag prohibited the leaching of chromium reducing the chrome in leachate. The hexavalent chromium cannot be stable with divalent iron (formula 4.1.).
3Fe2+ + Cr6+ Æ 3Fe3+ + Cr3+ (4.1.)
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There are several treatments of liquid EAF-slag that can be used in remelted or primary molten slag:
- Remelting of high alloy steelmaking slag under different atmospheres and crucibles (carbon) and addition of coal to reduce the chrome in the slag.
- Modification of the liquid slag with sand or lime - Addition of pure oxides into the slag.
- Addition of bauxite or Al2O3- MgO and FeOn-containing residues.
The results of the operational tests with EAF-slags have shown that an increased content of MgO and Al2O3 can decrease leaching of chromium. Injection of reducing agents such as FeSi into the steel bath has shown to be successful. Laboratory tests at FehS
concerning the binding of chromium into stable mineral phases have been confirmed by operational tests at KTN’s EAF. The addition of bauxite has given good results when bauxite is added into transfer ladle when steel and slag are tapped into it at the same time.
Laboratory and operational tests have shown a correlation between MgO-, Al2O3- and FeOn content and leaching behaviours. Laboratory investigations have shown that the influence of these compounds to leaching of chromium is different. The Factor sp (Figure 3, formula 3.3) for EAF slag has been formulated; when the factor is lower than 5 the chromium leaching is high but with the value higher than 25 is the leaching very low.
(Below detection limit 0.01 mg/l)
The test at KTN had been planned with the knowledge of factor sp. In practice bauxite was added into the transfer-ladle slag. By SEM analyses they have demonstrated that the chrome is bound in spinel phases if the spinels are of type Me’O*Me2’’O3, where Me’ is Mg2+,Fe2+ and Me’’Fe3+, Al3+or Cr3+. Chromium (Cr3+) is placed on the lattice places formerly used by Fe3+ or Al3+. The investigations by FehS have showed that there is a strong relationship between the MgO-, Al2O3-, FeOn- and Cr2O3-content in the slag and the leaching behaviours of chromium, [9].
4.1.1. Reduction of EAF slag by carbon blowing or remelting in carbon crucible.
The reduction of slag with coal can be achieved by injection coal into the liquid slag with the chemical reaction:
2Cr2O3 + 3C Æ 4Cr + 3CO2 (4.2)
The remaining chrome in the slag was found in small metal droplets, whose number and size increased with the blowing time. Spherical pores in the reduced slag were observed after the solidification that indicated the not complete reaction and CO-gas formation.
CO-formation took place during the solidification and formed pores in the EAF-slag, [9].
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Fig. 3. Correlation between factor-sp and leaching of chromium [9]
Observe the coefficient for Fetotal, which is 4.0 in this formula but is in the latest investigations n.
Each point or square is a specific slag sample.
4.1.2. Modification of the basicity of the liquid slag at KTN
The tests were carried out by adding lime or sand at 1650ºC with 10 min holding time.
Modifying of the basicity of EAF-slag by adding lime or sand had no effects on the leaching of chromium of the slag. Lower basicity is of advantage when the desirable result is a foamy slag, but the basicity of the slag has no dependence on the leaching behaviours of chromium from slag. KTN had in tests been able to vary the cooling rate in the slag pit to determine its influence on the stability and leaching behaviour of the slag, [9].
4.1.3.Treatment of solid slags
Solid slag can be treated by mixing with FeSO4. It was possible to decrease the leaching of chromium with addition of iron(II)sulfate, as chromium (Cr6+) always reduces into Cr3+ in the presence of Fe2+. Leaching of chromium increases as soon as the iron (Fe2+) is oxidized into Fe3+. Addition of iron-sulfate results also in leaching of sulfate from the slag, which can be defined as an undesirable effect. The result of long-term investigations
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has shown that, when all of the Fe2+ have been oxidized to Fe3+ the Cr-leaching has increased to the same level as the untreated slag. The final result is that the iron sulphate addition will reduce the chromium leaching only for a period a time.
Another method of treatment with solid slag is the mixing of solid slag with cement or ground granulated blast furnace slag. The aim with this mix is to form a compound, which is like concrete with low active surface of chromium leaching. The leaching process includes both surface reaction and diffusion and will be started by surface reaction. A diffusion process follows and this process is very dependent of temperature.
The surface can be minimized by briquetting or by mixing e.g. cement or other hydraulically binding agents.
A good environmental compatibility over long-term conditions can be obtained only with the modification of liquid slag. Additions of bauxite to the liquid EAF-slag do not
influence their mechanical and technical properties. They are suitable for utilization in road constructions, [9].
4.2. Utilization of steel slags 4.2.1. Utilization
Slag is a multifunctional material, which is used across many fields of application.
Utilization of slag products helps to conserve finite, maiden field, natural resources and also minimizes the emissions to atmosphere. Slag produced by right methods can be considered as an environmentally friendly raw material, [13, 16].
Slags from different melting plants and processes have different chemical and physical properties and will influence on the utilization possibilities of slags. In production of stainless steel EAF- and converter slags are produced as co-products. One of the most important properties is the volume stability of slag used as aggregates in road
construction. Fine milled quenched slag powder can be utilised in cement industries.
Slags can be used in different fields of applications. The use of steel slags for road construction is well experienced and has a long tradition in Europe. Many roads, ways and airfields in the neighbourhood of steel plants have been built on slag aggregates. The Canadian company succeeded in recovering the remaining stainless steel from the slag, using the Swedish patented technology. The principle of this technology is mainly the wet grinding followed of magnetic separation of the metals. Steel slag has even been used as a fertilizer, as recycled materials in melt shop and as aggregates in road construction.
In Germany during the 1960´s the use of steel slag for the production of fertilizer
amounted to more than 3 million tonnes per year. Today only some blast furnace (BF)-, basic oxygen steelmaking (BOS)- and secondary steel making slags are used in the production of lime fertilizer.
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Table 1. Chemical composition of several stainless steel slag (CANADA)[14], in Italy, Germany and Spain [9] and Outokumpu Stainless Oy
Average chemical analyses
EAF CaO SiO2 Al2O3 MgO Cr2O3 Fe2O3 B4 Factor sp EAF´s in Canada 33.4 30.8 6.0 5.9 7.8 3.2 1.07 6.5 Outokumpu Stainless 43.3 26.9 7.0 5.6 2.2 0.9 1.44 7.6 Acerinox 42.8 31.3 7.8 10.2 3.4 0.5 1.36 8.6 CSM 46.0 33.0 4.0 3.1 7.0 1.5 1.33 2.6 KTM (1998) 39.3 28.5 6.5 11.2 3.0 0.6 1.44 7.8
An application for iron and steel slags is the use as a fertilizer. These fertilizers have shown good effects on the growing of plants. Test fields, which have been under investigations for nearly 100 years indicate no harmful effects on the soil and plants.
These types of utilization of slags make it valuable and sustainable products. Slags from different melting plants have varying contents of metals and calcium that is an important component for a fertilizer (Table 1), [14].
4.2.2. Production and utilization of slag in Europe
Fig. 4. Use of steel slag in Europe /Euroslag 2000
Use of Steel slag in Europe 2000 (Euroslag)
final deposid cement production road construction hydraulic engineering fertilizer
internal recycling interim storage
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In the year 2000 in Europe was around 25 millions tonnes blast furnace slag was produced, about 29% of this as a slow cooled crystalline, 69% as granulated and 2%
pelletised slag. In Germany it is expected that in 2007 nearly 98% of the blast furnace slag will be granulated due to environmental reasons, [22]. Figure 4 shows that the largest amount of steel slag was used in road construction followed by final deposit and internal utilisation in the steel plant. Granulation of blast furnace slag generates a slag with latent hydraulic properties. Approximately 56 % of produced blast furnace slag in Europe is used in cement production and 39.5 % in road construction. In Europe were 16.8 million tonnes steel slag produced in 2000 and 59 % of this amount was basic oxygen furnace (BOF) slag and 28 % EAF-slag. As a road construction material 39 % of 16.8 million tonnes was used and 24 % was finally deposed. Some countries have a utilization rate higher than 95% for the steel slags.
Air-cooled blast furnace and steel slag has a long and qualified application field in road constructions in Europe. Slags as a raw material are processed in plants by crushing and screening simultaneous with natural stone material. The speciality of steel slag is its high density, which can be a considerable factor for transportations and for the necessary amount of bitumen in bituminous bound layers. The high density can even open new possibilities for the use of slag, as high-density concrete for special applications. One potential object for this building-material could be the foundation of the wind power plants at sea. There are two current trials of four pre-cast dollos, large blocks used for sea defense essentially to replace natural aggregates used for armourstone. These blocks have been exposed in severe wetting and drying environments for about eighteen months. A visual monitoring has not showed any cracking or spalling of these blocks.
The first standard for blast furnace cement was established in 1917. In Germany more than 60% of the blast furnace slag was granulated in 1999. In 1936 a part of motor highway in Saxonia was built with a Portland-slag cement, it has constantly been used for 55 years, every winter salted and has endured about 6000 freeze-thaw cycles.
The capillary porosity of concrete with blast furnace cement is lower than that of concrete with Portland cement, chemical resistance is in general also higher. One of the most important properties for concrete is durability. Blast furnace cement improves its durability in a field of applications. Low energy, sustainable, materials incorporating blast furnace and basic oxygen steel slag for highway construction and maintenance are used in the UK. In Germany steel slag products utilization was 5.61 Mt by the latest statistics (2000). 12.6% of this amount has been recycled in metallurgical processes, 6 % have been used as fertilizer and 11.4% had to be deposited. BOS-slag is particularly well suited to the new generation of “quiet” asphalt thin surfacing because of its high abrasion resistance and aggregate shape that contributes to surface structure, a key requirement for high speed skidding resistance
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In Denmark EAF slag has been used as aggregate in high performance asphalt surface course. The surface of EAF slag has a structure, which is very resistant to stripping of binding agent by moisture and mechanical wear off the surface by traffic with studded tyres. This results in environmental benefits; the amount of deposited slag is lower and the asphalt pavements withstand for a longer time wearing. Slag aggregates have been exported from Denmark to a number of different countries. EAF slags are high quality building materials and cannot be classified as a waste material. The content of free lime and the basicity of the slag are two important properties for slag being used as a
pavement. The basicity of the slag is one of the most important chemical properties, when the basicity is more than 1.55 the amount of free lime causes a volume expansion and further decomposition of the slag. Slag can be deposited as a waste as in part of Russia, Ukraine and the USA. In Japan, Korea and many of the EU-countries is slag used as a component in cement production. Slag is even equal with cement in several countries as in China and India, countries with scarce capital and raw material. Some countries, with high population density and countries with high environmental awareness as Taiwan, Belgium and Holland, favour development and utilization of slag products, [16].
Several” environmentally friendly” materials have been reviewed; cold mix steel slag asphalt wearing course, slag bound materials as alternative sub-base, road base and base course layer and hot mix steel slag wearing courses for use on heavily trafficked roads.
Low energy slag bound materials (SBM). European standard for slag bound mixtures is EN 227 402.
The utilization and recycling of steel slag is not as well developed as for blast furnace slag. The recycling grade of steel slag varies from country to country mostly because of chemical composition and local environmental regulations. Problems with the use of steel slag aggregates concerns the large amount of CaO (free lime and even MgO), which especially EAF-slag is containing (Figure 5). Compositions of certain slags and natural stone minerals are rather equal, [16].
4.2.3. The Weathering process
Generally the slag aggregates must be aged before utilization in civil engineering applications. The weathering is one of the most common methods for ageing. The
traditional weathering occurs during storage outdoors for several months, which has often yielded fairly good results, example lower volume expansion. One condition for that is that the slag is not crushed to smaller size, causing new crush surfaces. New surface always leach more elements, weakly bound in the mineral structure, by the diffusion theory. A series of weathering trials have been carried out at several works locations. The size of the heaps has varied widely. Samples were then taken when the heaps were constructed and subsequently at time intervals of up to 2 years. The results have clearly
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demonstrated that hydration of free CaO occurred progressively within the weathering heaps. It has been shown that it would be preferable to weather slag in its final size, that should not exceed 2 cm. The undersized material needs to be screened out from the product. Crushing the product material after weathering make new surfaces that have higher leaching compared to aged surfaces. This particle size, 2 cm, is too small for using as a construction material in certain of the road layers.
One method developed in Germany is injection of sand and oxygen into the molten slag.
Free lime can be stabilized with injection of sand and oxygen: [20]
O2 + 2FeO Æ Fe2O3 + energy 2 CaOfree + SiO2 Æ 2CaO * SiO2
2 CaOfree + Fe2O3 Æ 2CaO * Fe2O3
4.3. Analyses of slags and test methods for technical properties
There are many test methods to investigate chemical and physical properties of slags and other constructions materials. In the following paragraphs these methods are described.
4.3.1. Test methods, technical properties
The most important properties for road construction and civil engineering materials are the resistance to weathering, the shape and the grain size distribution. Several other properties, which may be investigated are: Bulk density, shape, impact value crushing value, 10%-fines, polishing, water absorption resistance to freeze-thaw and binder adhesion, [20].
Several standardized methods for analysis of properties have been used:
Technical Terms of delivery for Armourstones by definition for special test for steel slag armourstones. At least 20 pieces of natural size are to be placed in water at the temperature of 25 degrees for 20 days. If less than 5% of the slag pieces have got cracks or are disintegrated the material is sufficient to be used as a road construction aggregate.
Water absorption test, prEN 1097-6, where the sample is saturated in water for 24 hours. After this time the weight is measured. The wet surface of the sample is dried with a swab and the sample mass will be weighed again. Then the sample will be place warm, until the mass of the sample is constant. The difference between the wet mass and the final mass gives the water absorption of the material.
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Heat conductive test (Sond method) and the freezing test by Hermansson are certain tests, which are used on road construction materials. Dynamic triaxis test has been used often with investigation of aggregates used in road construction, [8, 10, 18].
The volume stability test, used for example in the Netherlands, slag pieces are placed in boiling water for 8 hours and after that time a limited quantity of disintegration cracks and loss of mass is allowed.
One of the test methods for volume stability is the steam test. The volume stability is correlating with the MgO- and the free lime content in the slag. The steam test, that at the present time is a European aggregate standard test method for steel slags (prEN 1744- 1) has in Germany been used from 1980 as a test for quality control of slags. In the steam test are pieces of slag placed over a heating element and water, in steam flow. The grain size distribution of the sample is from 0 to 22mm. The steam test evaluates the volume increase of sample in percent.
The resistance against abrasion of steel slags and natural rock materials, as basalt and diabase, has been tested by test in a rotary drum, Nordic ball mill test, prEN 1097-9. The results have confirmed that steel slags have better resistance to abrasion than several natural rock materials.
4.3.2. Test methods, environmental properties
Environmental properties are content of harmful elements, their availability and leaching in naturally circumstances. The active surface area has a correlation to the leachability, therefore the measuring of pore volume and surface area are important.
Leaching behaviors are detected by two-step batch leaching test prEN 12457-3 by L/S 2 and 8 or column test NEN 7343 or NT ENVIR 002, [8, 17].
Dutch availability test (NEN 7341) is carried out in two steps at L/S 100. The amount of test material used is 8± 0.01g with d95 = 125µm. During the test is pH constant of 7.0 for 3 hours in the first step and at pH 4.0 for 4 hours in the second step. Between the two steps the liquid is filtered and the residue is added in the second step. The sample and leachant are well mixed during the whole test period, [8].
To simulate the natural behaviours a Lysimeter test has been developed. In the lysimeter test the leachant is continuously circulated in the material and the amount of test material is rather high, about 15 m3, which has been packed as a bed to decrease the flow rate of the leachant, [7, 8].
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Determination of the total grain surface area, make it possible to estimate the leaching behaviours. The determination can be carried out in a gas absorption apparatuses, in which the in sample absorpd gas volume can be measured and specific area can be calculated as m2/g of sample material, [12].
Pore volume can be measured with Quatachrome Autoscan, which forces mercury in the pores in the material and the mercury volume can be measured after the measuring.
The tank-leaching test seems to be more practically orientated than the shaking test, but the shaking tests are widely used with investigation of the leaching behavior of rock materials. In Netherlands the diffusions analysis prEN 7345 is used by investigation of bounded materials, which is even made by Finnish technical research center, [17].
The mineralogy of slags has been investigated by Scanning electron microscope SEM and by X-ray diffractmeter XRD. Total analyses of chemical components have been done by X-ray fluorescence (XRF)
Determination of the total amount of harmful metal is made by the prEN 13657 or prEN 13656 methods. A sample is taken from the fines from the crush, of grain size less than 4mm. Like all samples it’s divided into the right amount, 2kg. The metals must be defined before tests and this process occurs by standard prEN 12506 and prEN 13370.
4.3.3. Technical properties of slags compared with natural materials
Table 2. Technical properties of processed steel slag aggregates [25]
Characteristics Aggregates
Property Dimension BOF-slag EAF-slag Granite Flint gravel Bulk density (g/cm3) 3.3 3.5 2.5 2.6
Shape (%) < 10 < 10 < 10 < 10
Impact value (% by weight) 22 18 12 21 Crushing value (% by weight) 15 13 17 21
10%-fines (KN) 320 350 260 250
Polishing (PSV) 58 61 48 45
Water absorption (% by weight) 1.0 0.7 < 0.5 < 0.5 Resistance to freeze-thaw (% by weight) < 0.5 < 0.5 < 0.5 < 1 Binder adhesion (%) > 90 >90 > 90 > 85
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The mineralogy of slags and natural stones is quite similar, but there are differences in slag compositions between different steel plants and processes. The mineralogy of natural bedrock also differs in different regions on Earth. Some elements such as chromium exist essentially at a low amount but is in several regions a general component in bedrock. In the Table 2 several properties are compared between natural materials and slags. The density of slag is always higher than natural stones, but the water absorption is higher by slags because of the pores in the slags. One of the most important factors for leaching behavior is the porosity of material; material with high porosity has a large active surface for leaching.
Fällman has investigated the dependence of micro- and macropores in leaching of chromium from steels slags. Discrimination between solubility controlled leaching and diffusion controlled leaching release from micro as well macropores, [8]. The leaching estimations of heavy metals from a material should be measured and calculated as a degree of the total amount of respective element, not only as a objective leaching values.
For example, the degree in ppm estimates the leaching properties better than the objective leaching values in dimension (kg/l), [19].
Fig. 5. Comparison of the composition of iron and steelmaking slags with natural stones, shown in the system CaO-SiO2-MgO-Al2O3-Fe2O3-Cr2O3. [25]
