LIST OF PUBLICATION

A. Research Publication

1. Panda C R, Mishra K K, Nayak B D, Rao D S & Nayak B B Chromium release

behavior of Ferrochrome Slag Int. J. Environmental Technology and Management,

Vol. 15, Nos. 3/4/5/6, 2012, Inderscience Publication, UK.

2. Panda C R, Mishra K K, Nayak, Panda K C B D, & Nayak B B. ‘Environmental

and Technical Assessment of Ferrochrome Slag as Concrete Aggregate Material,

Construction& Building Material volume 49(2013)Elsevier Publication.

B. Conference/Seminar Presentation/publication

1. Panda C R et al Chromium Leaching behavior of Ferro-chrome slag, International

Seminar on Mineral Processing Technology, MPT-09, Oct 2009, IMMT,

Bhubaneswar.

2. Panda C R et al Environmental compatibility of ferrochrome slag as reusable

material, International Brainstorming conference on Waste to Energy, Mumbai

August, 2012

3. Panda C R et al Environmental Assessment of Ferrochrome Slag As Reusable

Material, Symposium on Sustainable Infrastructure Development, IIT,

Bhubaneswar, Feb 2013

4. Panda C R et al Utilization of ferrochrome slag as construction material, National

conference on green building design, Institution of engineers, Bhubaneswar, Aug

2013

Environmental and technical assessment of ferrochrome slag as concreteaggregate material

C.R. Panda a,⇑, K.K. Mishra b, K.C. Panda a, B.D. Nayak c, B.B. Nayak c

a Department of Civil Engineering, ITER, Siksha ’O’ Anusandhan University, Bhubaneswar 751030, Indiab Radhakrishna Institute of Technology and Engineering, Jatni, Bhubaneswar 752050, Indiac Institute of Minerals and Materials Technology, CSIR, Bhubaneswar 751013, India

h i g h l i g h t s

� Chromium release from the ferrochrome slag limits its utilization/disposal.� The slag material has all the desirable properties for its use as concrete aggregate.� Successful immobilization of chromium occurs in cement-concrete matrix.� Concrete with slag as aggregate satisfies the desired technical properties.� Concrete with slag is environmentally compatible with low chromium leaching.

a r t i c l e i n f o

Article history:Received 6 April 2013Received in revised form 5 August 2013Accepted 7 August 2013

Keywords:AggregateChromiumConcreteFerrochrome slagLeachingImmobilization

a b s t r a c t

Ferrochrome slag is a major solid waste generated from submerged electric arc furnaces during manufac-turing of ferrochrome alloy. The waste slag has excellent mechanical and engineering properties for uti-lization as concrete aggregate material. But it contains about 6–12% of residual chromium which has thepotentiality of releasing hazardous chromium compounds to the environment. This research work carriedout the mineralogical and chemical characterization study to find out the major chemical elements andmineral phases in the solid slag matrix. Physico-mechanical experimental studies indicated the suitabil-ity of the slag as aggregate material in concrete work. The concrete product with ferrochrome slag ascoarse and fine aggregate showed excellent results with respect to compressive strength and were foundto be suitable for general purpose concrete work. The standard leaching experimental results showed thatthe leachable chromium remains well immobilized in the cement and concrete matrix with very low tonon-detectable level of chromium leaching. The results indicated the technical acceptability and the envi-ronmental compatibility of the slag as concrete aggregate material.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

High Carbon Ferrochrome (HCFeCr) is the most common alloy-ing material for the production of different grades of stainless steel.Chromium and iron in chromite ore can form a continuous series ofsolid solution under certain conditions of heat treatment contain-ing 45–80% of chromium. It is manufactured through direct smelt-ing in Submerged Arc Furnace (SAF) at a temperature above1500 �C. The furnace has the suitable system for tapping heaviermetal and lighter slag and their handling. For the production ofeach Metric Ton (MT) of ferrochrome about 2.5–2.6 MT of chromiteore, 0.5–0.6 MT of coke and 0.3 MT of fluxing agents are required.

There is a generation of 1–1.2 MT of solid waste slag for each MTof ferrochrome product. The waste slag material can be made avail-able in different sizes under different cooling conditions and aftermaterial recovery. It contains about 6–12% deleterious substanceslike chromium as chromium oxide and has the potentiality ofreleasing hazardous chromium compounds to the environmentrestricting its use and disposal. Chromium is one of the most com-mon toxic heavy metal found in the environment. It exists in thecommon oxidation states of hexavalent chromium Cr(VI) and triva-lent chromium Cr(III). While chromium as Cr(III) is less mobile andless harmful, Cr(VI) is highly leachable and extremely toxic underall environmental conditions. As per Occupational Safety HealthAdministration (OSHA) [1], the major health effects associatedwith exposure to Cr(VI) include lung cancer, nasal septum ulcer-ations and perforations, skin ulcerations, and allergic and irritantcontact dermatitis etc. Toxicity of chromium ranges from pulmon-ary to dermatological problems. As per US EPA [2], it is a suspected

0950-0618/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.conbuildmat.2013.08.002

⇑ Corresponding author. Tel.: +91 9438148473.E-mail addresses: chiittaranjan.panda435@gmail.com (C.R. Panda), kkmishra06

@yahoo.co.in (K.K. Mishra), kishoriit@gmail.com (K.C. Panda), bdnayak@immt.res.in(B.D. Nayak), bbnayak@immt.res.in (B.B. Nayak).

Construction and Building Materials 49 (2013) 262–271

Contents lists available at ScienceDirect

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carcinogen. That is why there exists stringent Indian dischargestandard such as 2.0 mg/l for total chromium (total Cr) and0.1 mg/l for Cr (VI) [3].

In ferrochrome manufacturing process, the slag is a reactivemedium, where most of the reduction reaction takes place. Ferro-chrome (FeCr) slag is found to consist of mainly silica, alumina andmagnesia with significant amounts of chromium and iron oxides inthe form of Partially Altered Chromite (PAC) and entrained ferro-chrome alloy [4]. At smelting condition, chromium is reported toexist as divalent CrO above 1600 �C. It is converted into metallicand trivalent chromium on cooling under ambient condition. Theprecipitated Cr2O3 is found to be in the needle shape and themetallic chromium inclusions are normally associated with theseprecipitates. The Cr2O3 and metal phases are normally concomitantand are dispersed in silicate phase [5]. Typically the granulated slagis reported to have three different phases, namely the amorphousglass phases including that of magnesium and calcium silicates, zo-nal (Fe, Mg, Al, Cr) oxide spinel phase and entrained metallic ferro-chrome alloy [6]. A Scanning Electron Microscopy (SEM) studyshowed the slag having a partly crystalline and hypidiomorphicspinel (Mg, Fe) (Fe, Al, Cr)2O4 crystals totally enclosed in a con-densed and homogenous glass matrix [7]. An Electron Probe MicroAnalysis (EPMA) study result indicated that chromium spinelphase is found to exist in three modes as remnants of PAC havingirregular space and lying within the framework of original particleas rims of chromite particles in an earlier stage of alteration or asseparate euhedral crystals that recrystallized out of slag [8]. Asper Kimbrough, Cr(III) stability occurs over a wide Eh and pH rangeunder both reducing/oxidizing and acidic/alkaline conditions.Cr(VI) stability occurs in a much narrower zone like oxidizingand alkaline conditions. The hexavalent state is stable in an oxidiz-ing alkaline environment, whereas the trivalent state is stable in areducing acidic environment [9]. It is reported that, all hexavalentchromium salts except calcium, barium and lead chromate arefound to be soluble in water under all pH conditions. Chromiumbeing an amphoteric element is found to be insoluble as Cr(III) inthe intermediate pH range showing minimum value in the pHrange of 5.5–8.5. Above pH 5, chromium leaching is dominatedby hexavalent chromium and in acid medium, that is below pH5, total chromium in leachate is increased considerably becauseof dissolution of immobile Cr(III) from the slag. Cr(III) leachabilityis mostly controlled by the formation of insoluble oxides andhydroxide compounds and by the surface adsorption mechanisms.The mobility of chromium is determined by the competitionamong the mechanisms like dissolution/precipitation, redox trans-formation and adsorption/desorption [10,12,13]. Kilau reportedthe formation of picrochromite (MgO�Cr2O3) with the availabilitylarge amount of MgO preventing the release of chromium even un-der mild acid conditions [11]. Standard leaching tests indicatedonly very low leaching of chromium from ferrochrome slag be-cause of its inherent chromium fixing ability in to spinel phaseand of the structural encapsulation of the dispersed crystals insidean impermeable and chemically stable glass phase [7]. There is re-mote possibility of conversion of Cr(III) to Cr(VI), because of highlyreducing conditions existing in the furnace, and the high redox po-tential of Cr(VI)/Cr(III) couple. The oxidation of Cr(III) to Cr(VI) bythe naturally available oxidants like dissolved oxygen and MnO2

under field condition, is reported to be too slow to cause any signif-icant leaching of chromium [14,15].

Zelic in his experimental study found that FeCr slag satisfied thecriteria as aggregate for concretes and the concrete prepared fromthe slag showed better compressive strength results and was foundsuitable in pavement and general purpose concrete work. The slagwas found to be particularly suitable for high concrete brands(M-50 and higher) where the natural aggregates could not providethe desired results [16]. The mechanical and the physical

properties of the FeCr slag satisfied the requirement of the aggre-gate for granular layers of flexible pavement [17]. Lind justifiedthe use of FeCr slag with minimum adverse environmental impactin road construction [18]. FeCr slag is found to be suitable as con-struction material due to its excellent technical and material prop-erties but its use has been limited by the environmental concern ofrelease of chromium from slag. Blended with fly ash there is a de-crease in chromium leaching from the slag matrix [19]. Korkut et al[20] reported the usage of FeCr slag in epoxy resin up to 50%improving the nuclear radiation shielding performance of theblended epoxy resin.

Immobilization of chromium in cement-concrete matrix de-pends upon its oxidation state. It has been extensively reportedin literature that Cr(III) is effectively immobilized in all types of ce-ment-concrete matrix reducing chromium leachability to mini-mum, well below the US EPA TCLP limit and Indian dischargestandards [21–25]. Cr(III) is immobilized in C3S in OPC matrixdue to formation of Cr(OH)3. Chromium as Cr3+ is found to besubstituted by Al3+, Si4+and Ca2+ in Calcium Aluminate Hydrate(CAH) phase and in Calcium Silicate Hydrate (CSH) phase. The nec-essary containment frame work is provided by the stable CSH net-work preventing chromium leaching from slag [26,27]. But it wasreported by Glasser that Cr(OH)3 is not formed in the hardenedproduct. Solubility in pore water is found to be much less thanthe solubility of Cr(OH)3 at the same pH. The cement hydrationproducts having octahedrally coordinated Al3+, is readily replacedby Cr3+. Chromium like aluminium is amphoteric, both remaininsoluble in the cement environment. As the solubility of alumin-ium in the normal cement of pH range of 11–13 is controlled bythe solubility of calcium aluminate hydrate and not by Al(OH)3,so also the solubility is limited by the solubility of calcium chro-mium aluminate hydrate and not by Cr(OH)3. Therefore it can beconcluded that chromium is immobilized and stabilized in the ce-ment and concrete matrix and thus reducing its leachability[23,28–31]. On the other hand Cr(VI) is not effectively immobilizedin cement-concrete matrix with Ordinary Portland Cement (OPC),and is found to be soluble in entire pH range [21,23,32,33]. Cr(VI)substitution in ettringite is not expected in OPC matrix with itsvery formation is inhibited by the presence of chromium. It is re-ported that the slag type cement with Granulated Blast FurnaceSlag (GBFS) is found to effectively immobilize Cr(VI) because ofthe low redox potential and reductive capacity of GBFS with sul-phide content [23,24,34,35].

The objective of this research work is to evaluate the technicalperformance of concrete material with the use of the ferrochromeslag as aggregate material and is to assess the environmentalcompatibility of the waste slag with chromium immobilization incement concrete matrix and its release behaviour from theconcrete monolith having slag as aggregate material.

2. Materials and methods

The ferrochrome slag samples were collected from the operating plants of fer-rochrome industry with submerged arc electric furnaces, situated in Kalinganagarindustrial complex, Odisha, India. Air cooled slag after size reduction and alloyrecovery in jigging process is available in the size range of 10–20 mm which hasbeen used as coarse aggregate in this study. Alternatively when the molten slagis subjected to high pressure water jet, the slag is available as granulated havingthe suitable grain size which has been used as fine aggregate in this study. The pic-tures of collected slag samples are shown in Figs. 1 and 2.

2.1. Aggregate properties specification

All the aggregate properties required for the concrete work were determined asper the procedures outlined in Indian Standard IS 383-1997 [36]. The results arepresented in Tables 1 and 2. FeCr slag has the desirable physical and mechanicalproperties to be used as coarse aggregate in concrete work as per IS 383-1997.Granulated FeCr slag has the desirable properties with size gradation correspondingto zone 1 to be used as fine aggregate in concrete as per IS 383-1997.

C.R. Panda et al. / Construction and Building Materials 49 (2013) 262–271 263

2.2. Specification of the cement used

Three different types of cements like Ordinary Portland Cement (OPC) andblended cements like Portland Slag Cement (PSC) and Portland Pozollana Cement(PPC) were used in the experimental works to evaluate the performance of concretematerials made with waste slag. Specification of the different cements used isshown in Table 3.

2.3. Chemical characterization studies

Chemical characterization studies were undertaken for the slag samples in or-der to find the major element concentration including that of chromium in the solidwaste slag matrix. The quantitative analysis of major elements for the samples wascarried out by X-Ray Fluorescence spectroscopy (XRF). For this purpose, pressedpowdered pellets were exposed to a Phillips PW-1400 X-Ray spectrometer with

scandium tube. The results are shown in Table 4. The results indicated the presenceof major chemical components like alumina, silica and magnesia including appre-ciable amount of chromium in the slag samples. The residual amount of chromiumin the slag is responsible for the leaching of chromium from the slag solid matrix.

2.4. Mineralogical characterization studies

The mineralogical characterization by X-Ray Diffraction (XRD) study of thegranulated slag was carried out by using a Cu target of X-Ray Diffractometer, makePANalytical model- X’pert PRO. Mineral phases were identified by comparing the d-spacings with those given in ASTM data cards and the result is shown in Fig. 3.

The mineralogical characterization by XRD indicated the presence of dominantmineral phases like Partially Altered Chromite (PAC), Spinel (MgO�Al2O3), Enstatite(MgFeSiO3), and Fayalite (Fe2SiO4). Spinel as the major mineral phase is found to oc-cur as euhedral grains. Significant amounts of chromium as metallic (Cr, Fe)7C3

phase is found to be present in the form of globular, elliptical and dendritic grains.Fig. 1. Ferrochrome slag as coarse aggregate.

Fig. 2. Ferrochrome slag as fine aggregate.

Table 1Properties of ferrochrome slag and natural stone as coarse aggregate.

Properties FeCr slag Natural stone IS specification

Specific gravity 2.84 2.72Abrasion resistance (%) 19.2 27.1 <50%Crushing strength (%) 25.1 31.9 <45%Impact strength (%) 23.2 26.5 <45%Size mm 8 to 20 6 to 20Water absorption (%) 0.42 0.32

Table 2Properties of ferrochrome slag and natural sand as fine aggregate.

Properties FeCr slag Natural sand

Specific gravity 2.72 2.67Fineness modulus 4.80 4.00Grading Zone I Zone I

Table 3Specification of the cement used.

Cement properties OPC PSC with 55–58%blast furnace slag

PPC with 28–30% fly ash

IS specification IS 269-2000 [37]

IS 455-2000 [38] IS1489 (partI)-2000 [39]

Initial setting time(min)

30 30 30

Final setting time(min)

600 600 600

Specific gravity 3.15 3.10 3.10Insoluble residue (%) 2.0 2.5 2.5MgO (%) 6 6 8SO3 (%) 2.75 2.75 3.0Sulphide sulphur (%) – – 1.5Loss of ignition (%) 5 5 4Specific surface

(cm2/g)2250 3000 2250

Compressive strength(28 days) (MPa)

43 31 33

Table 4Major chemical constituents from XRF analysis.

Major chemicalconstituent

Water cooled granulated FeCrslag (%)

Air cooled FeCrslag (%)

Cr2O3 10.37 8.32Al2O3 19.57 22.84SiO2 27.33 28.87MgO 32.28 30.32CaO 2.49 2.96Fe2O3 4.12 2.85

Fig. 3. X-Ray Diffraction (XRD) study of the slag sample.

264 C.R. Panda et al. / Construction and Building Materials 49 (2013) 262–271

Besides these crystalline phases, the slag contains a non-crystalline glassy phasewhich forms the matrix. Mineralogy of slag is complex due to presence of chro-mium in a range of oxidation states particularly in +2, +3 and +6 state. The majorityamount of chromium remains as immobile Cr(III) in magnesiochromite spinelphase, which is responsible for low level chromium leaching from the slag.

2.5. Mixing, casting, curing and testing of concrete specimens

Concrete cubes of size 150 � 150 � 150 mm were cast with FeCr slag and nat-ural stone with different proportions as coarse aggregate with a mix design ratioof 1:1.68:3.17 at water to cement ratio of 0.5 as per IS 10262-1999 [40] using dif-ferent types of cements like OPC, PSC and PPC. Similarly concrete cubes were castwith FeCr slag and natural sand with different proportions as fine aggregate witha similar mix design at water cement ratio of 0.5 as per IS 10262 using differenttypes of cements like OPC, PSC and PPC. Concrete specimens with different percent-age of ferrochrome slag and natural stone as coarse aggregate with 0%, 20%, 40%,60%, 80% and 100% of slag substitution and natural sand as fine aggregate were pre-pared with different types of cements like OPC, PPC and PSC. The specimen samplesare accordingly coded with 1st two letters representing the type of cement, FS, CAand FA symbolising for ferrochrome slag, coarse aggregate and fine aggregaterespectively. Similarly Concrete specimens with different percentage of ferro-chrome slag and natural sand as fine aggregate with 0%, 20%, 40%, 60%, 80% and100% of slag substitution and natural stone as coarse aggregate were prepared withdifferent types of cements. The specimen codes are shown in Table 5.

Fresh concrete tests were carried out to evaluate the workability of the concreteproducts. Then the Unconfined Compressive Strengths (UCS) of concrete cube sam-ples were measured after 7 days and 28 days of curing.

2.6. Leaching characterization of slag samples for environmental assessment

Chromium has been identified as the major deleterious element present in theferrochrome slag which is likely to cause environmental pollution problem in usageand disposal scenario. Even though the slag samples contain appreciable amount ofresidual chromium, almost all chromium remains highly immobilized in spinelphase allowing negligible chromium release from the slag samples. In addition tophysico-chemical and mineralogical characterization, adequate leaching study isrequired to establish the environmental compatibility of the slag for utilization asconcrete aggregate material. Chromium leaching and extraction under different testprotocols were carried out. A three tier leaching test frame work formulated by Kos-san et al. and extensively deliberated upon by van der Sloot and others [41–46] wasadopted for environmental performance and impact evaluation study. As per theframe work, an availability screening test was carried out to estimate the maximumpotential for chromium release under anticipated adverse environmental condi-tions as per the protocol NEN-7341, 1994 [47] and AV002.1 [41]. Availability testswere carried out at pH 4 and 7 as per NEN 7341 and with 50 M EDTA solutions asper AV002.1 at L/S ratio of 50. Equilibrium characterization test was carried out forgranulated and crushed slag samples as per protocol SR002.1 [41] to assess the re-lease of chromium as function of pH. Mass transfer based compliance tests in theform of tank leach tests for monolithic concrete samples as per protocol NEN7375, 2004 [48] was conducted to assess time dependent release from the concretemonolithic samples made with FeCr slag as aggregate material. Cr(VI) concentrationwas measured in leachate samples by UV/Visible spectrophotometer Elico modelSL-164 and total chromium content was measured by Atomic Absorption Spectro-photometer Simadzu model AA-6300 as per the procedure outlined AmericanPublic Health Association (APHA) manual [49].

2.7. Toxicity characteristic leaching procedure

Regulatory test in the form of Toxicity Characteristic Leaching Procedure (TCLP)tests as per USEPA 1311 [50] method were carried out for the determination ofhexavalent and total chromium in the slag samples. 100 g of granulated andcrushed slag samples with size <9.5 mm were subjected to batch leaching withextraction liquid with an L/S ratio of 20 depending on different pH conditions. Con-tact time with agitation was maintained at 18 h. Extraction liquid of pH 4.93 pre-pared with 5.7 ml of glacial acetic acid and 64.3 ml of 1 M NaOH in 1 l distilledwater was used for this case. The leachate samples were analyzed for pH, Cr(VI)and total Cr concentration.

2.8. Compliance test for monolithic concrete specimen

Compliance test protocol as outlined in the method NEN 7375 was adopted formonolithic material like concrete blocks and compacted granular material. Thisprotocol consists of tank leaching of continuously water saturated monolithic mate-rial with periodic renewal of the leaching solution. The vessel and sample dimen-sions were so chosen to keep the sample fully immersed in the leaching solution.Sample sizes of 5 cm cubes with surface areas of 150 cm2 and volume of 125 cm3

were contacted with Deionised (DI) water at its own pH 6.68 and at pH 2.88 usingliquid to volume ratio of 5 ml for every cm3 of concrete volume. Leaching solutionwas exchanged with fresh leachant in 0.25, 1, 2.25, 4, 9, 16, 36 and 64 days in 8

extractions. The leachate samples were filtered with 0.45 lm filter papers. Twodays short tank leaching tests as suggested by van der Sloot [44,46] and as outlinedin the method NEN 7375 were carried out to compare the results with that of reg-ulatory discharge standards. The leachate samples were analyzed for pH, Cr(VI) andtotal Cr concentration.

3. Results and discussion

The experimental results with respect to fresh and hardenedConcrete tests are analyzed to evaluate the suitability of Ferro-chrome Slag as coarse as well as fine aggregate in concrete work.Different leaching study results are discussed to assess the envi-ronmental compatibility of the waste slag as concrete aggregatematerial.

3.1. Fresh concrete test

Fresh concrete test results indicate that the substitution of fer-rochrome slag as coarse aggregate and fine aggregate more or lessproduces similar results as that with natural aggregate. The slumpvalues are found to be within the range of 48–60 mm and compac-tion factors are found to be in between 0.88 and 0.92 indicatingmedium workability for the concrete products.

3.2. Compressive strength of concrete cubes

The compressive strength results after 7 days and 28 days ofcuring and corresponding weights of different samples having fer-rochrome slag as coarse and fine aggregate blended with naturalstone and sand with different types of cements are presented in Ta-ble 5. The UCS values were compared with different substitutionsof slag content as coarse aggregate with the three different typesof cements under similar mix design formulations. The resultsare graphically shown in Fig. 4. The results indicate the progressiveincrease in UCS with the increase in slag percentage. At 100% sub-stitution of slag, the value of UCS is found to be highest and is sig-nificantly higher than the target strength. These results indicatethe suitability of the FeCr slag as coarse aggregate for higherstrength concrete design and in many other construction activitiesin agreement with the studies by several researchers [16–19]. Butthe weight of the cube increases marginally with the increase inslag content. This better performance is attributed due to favour-able mechanical and engineering properties of the slag.

The 7 days and 28 days of UCS results are compared with differ-ent substitutions of slag content as fine aggregate with the threedifferent types of cements under similar mix design formulations.

Fig. 4. FeCr slag as coarse aggregate vs. compressive strength.

C.R. Panda et al. / Construction and Building Materials 49 (2013) 262–271 265

The results are shown graphically in Fig. 5. The results indicate thatthere is no significant change in UCS and unit weight with the in-crease in slag percentage. At 100% substitution of slag, there is nosignificant change in UCS and in unit weight as compared withcubes with natural sand as fine aggregate. Thus granulated FeCrslag can be used as fine aggregate in place of sand in general pur-pose concrete work.

3.3. Leaching study results

The results of total content, availability tests, characterizationleaching tests at different pH conditions and regulatory TCLP testresults are shown in Table 6. The results of characterization leach-ing tests at different pH conditions are plotted in Fig. 6. As the slagis derived from the highly reducing conditions in submerged arcfurnace, there is very remote possibility of formation of oxidizedproduct of chromium like Cr(VI). Hence chromium in slag is almostin Cr(III) oxidation state and very little in Cr(VI) state. Availabilityof Cr(VI), is only 20–29% of the total content. Actual leaching ofCr(VI) under anticipated pH conditions is about 35–38% ofavailability. Therefore release of Cr(VI) from solid waste slagmatrix is negligibly small and is not likely to cause significantchromium pollution problem. On the other hand, the slag containsconsiderable amount of total chromium in the form of Cr(III), butits availability is only 0.022–0.046% of the total content. The avail-

Fig. 5. FeCr slag as fine aggregate vs. compressive strength. Fig. 6. Chromium release as function of pH.

Table 5Compressive strength of concrete cubes with FeCr slag as aggregate.

Specimencode

Weight of the cube (kg) 7 days UCS(MPa)

28 days UCS(MPa)

Specimencode

Weight of the cube (kg) 7 days UCS(MPa)

28 days UCS(MPa)

OPFSCA0 8.50 23.6 30.7 OPFSFA0 8.50 23.8 30.5OPFSCA20 8.52 24.5 31.4 OPFSFA20 8.44 22.7 29.4OPFSCA40 8.55 25.4 32.2 OPFSFA40 8.25 22.8 29.6OPFSCA60 8.58 25.9 33.7 OPFSFA60 8.22 22.5 29.8OPFSCA80 8.62 28.1 35.6 OPFSFA80 7.95 23.0 30.1OPFSCA100 8.68 29.7 37.2 OPFSFA100 7.88 22.9 30.0PSFSCA0 8.51 20.2 30.2 PSFSFA0 8.51 20.2 30.2PSFSCA20 8.51 20.3 30.8 PSFSFA20 8.45 19.8 29.9PSFSCA40 8.52 21.0 32.3 PSFSFA40 8.38 19.8. 30.0PSFSCA60 8.54 22.3 33.6 PSFSFA60 8.32 19.7 29.8PSFSCA80 8.56 23.7 35.5 PSFSFA80 8.26 20.1 29.9PSFSCA100 8.62 24.3 36.8 PSFSFA100 8.22 19.9 29.8PPFSCA0 8.55 21.1 30.1 PPFSFA0 8.44 21.5 30.3PPFSCA20 8.55 21.8 30.8 PPFSFA20 8.35 21.4 30.2PPFSCA40 8.58 22.7 32.1 PPFSFA40 8.28 21.1 29.9PPFSCA60 8.62 23.9 33.2 PPFSFA60 8.22 21.2 30.0PPFSCA80 8.66 25.1 35.8 PPFSFA80 8.16 20.9 29.9PPFSCA100 8.72 25.6 37.1 PPFSFA100 8.12 20.8 30.0

Table 6Chromium leaching and extraction results under different test protocols.

Testprotocol

Leachant pH L/SRatio

Measurementunit

Cr (VI) TotalCr

TotalcontentanalysisWetchemicalanalysis

– – – mg/kg 6.5–12.8

28464–35477

Release asfunctionof pHSR002.1[40]

DI waterwithHNO3/KOH

5.0–9.0

10 mg/kg 0.64–0.92

2.5–15.8

AvailabilitywithEDTAAV002.1[40]

50 mMEDTA

7.5 100 mg/kg 1.88–2.58

27.8–31.4

Availabilityat pH 4and 7 asper NEN7341

DI waterwithHNO3/KOH

4and7

50 mg/kg 1.66–2.13

24.7–26.8

US EPATCLPmethod1311

CH3COOHand NaOH

4.93 20 mg/l 0.048–0.084

0.55–0.88

266 C.R. Panda et al. / Construction and Building Materials 49 (2013) 262–271

ability of chromium indicating the potential leaching values repre-sent only a miniscule fraction of the total. Whereas the actual con-tent in the anticipated pH conditions is considerably less and isabout 10–50% of availability. These results infer that even thoughthe slag samples contain considerable amount of residual chro-mium, the available and actual leachable fractions are negligiblysmall. From the review of literature, the present slag characteriza-tion study and leaching analysis results conclude that the chro-mium in slag remains in Cr(III) stable oxidation state and remainin highly immobilized spinel phase, thus preventing significant re-lease of chromium from the slag solid matrix.

3.4. Leaching test results of samples having different % of FeCr slag ascoarse aggregate

Two days Short Tank Leaching Tests were performed on Mono-lithic cube samples having different percentage of FeCr slag ascoarse aggregate with normal distilled water at its own pH 6.68and with TCLP extraction fluid of pH 2.88. These results show thehigher chromium concentrations in leachate as compared to aver-age 64 days NEN 7375 test protocol [44,46] and hence can repre-sent the maximum release condition. The results are comparedwith the availability data and TCLP data for regulatory complianceto USEPA and Indian discharge standards. The 64 days leachingresults in mg/m2 are compared with Dutch Building MaterialDecree (BMD), 1995 [51] for environmental compatibility of the

slag in utilization scenario. The results of two days short tankleaching Tests with respect to leachate pH, Cr(VI) and total Cr con-centrations are presented in Table 7 and are graphically shown inFigs. 7–10.

Table 7Short tank leaching test results of concrete specimens with FeCr slag as aggregate.

Specimen code Leachate pH Cr(VI) in lg/l Total Cr in lg/l Leachate pH Cr(VI) in lg/l Total Cr in lg/l

US-EPA TCLP standard – 5000 – 5000Indian discharge standard 100 2000 100 2000TCLP analysis for FeCr slag sample as coarse aggregate 32 124 48 307TCLP analysis for FeCr slag sample as fine aggregate 76 288 94 1238

Leachant pH 6.68 Leachant pH 2.88OPFSCA0 10.93 21 25 8.50 25 28OPFSCA20 10.95 21 24 8.44 27 29OPFSCA40 11.12 23 28 8.25 28 32OPFSCA60 11.05 24 27 8.22 30 34OPFSCA80 10.95 25 28 7.95 28 35OPFSCA100 10.96 23 32 7.88 29 35PSFSCA0 10.05 12 16 7.89 14 16PSFSCA20 10.04 15 18 7.90 15 18PSFSCA40 10.02 14 17 7.92 14 17PSFSCA60 10.05 15 18 7.94 16 18PSFSCA80 10.08 16 18 7.92 18 20PSFSCA100 10.06 16 18 7.91 17 20PPFSCA0 10.32 22 30 8.22 24 26PPFSCA20 10.31 20 23 8.35 24 25PPFSCA40 10.36 21 24 8.28 24 26PPFSCA60 10.35 22 25 8.22 25 28PPFSCA80 10.37 23 25 8.26 26 29OPFSFA0 10.98 30 32 8.52 35 38OPFSFA20 11.00 31 34 8.54 34 38OPFSFA40 11.08 32 34 8.55 36 39OPFSFA60 11.00 33 35 8.52 37 40OPFSFA80 10.98 32 35 8.55 38 42OPFSFA100 11.02 34 36 8.54 37 42PSFSFA0 10.12 15 18 7.68 16 18PSFSFA20 10.14 16 18 7.72 20 22PSFSFA40 10.16 17 19 7.75 19 21PSFSFA60 10.14 19 22 7.76 19 22PSFSFA80 10.12 18 21 7.74 20 23PSFSFA100 10.15 19 22 7.75 20 24PPFSFA0 10.45 28 31 7.98 32 35PPFSFA20 10.45 28 32 7.95 33 35PPFSFA40 10.48 30 33 7.97 34 36PPFSFA60 10.48 29 32 7.95 33 32PPFSFA80 10.46 30 33 7.98 34 37PPFSFA100 10.47 30 34 7.96 34 38

Fig. 7. FeCr slag as coarse aggregate vs. leachate Cr(VI) concentration at pH 6.68.

C.R. Panda et al. / Construction and Building Materials 49 (2013) 262–271 267

3.5. Leaching test results samples having different % of FeCr slag as fineaggregate

Short tank leaching tests were performed on concrete cubesamples having different percentage of sand and FeCr slag as fineaggregate with normal distilled water at its own pH 6.68 and withTCLP extraction liquid at pH 2.88. The results of leachate pH, Cr(VI)

and total Cr concentrations are presented in Table 7. The results arecompared with regulatory TCLP standard limits and permissibledischarge standards prescribed in India, for regulatory compliancerequirement. The results are graphically shown in Figs. 11–14.

Leachate pH values by and large remain unaffected by the sub-stitution by the slag content. But the pH values are higher in case ofconcrete cubes made with OPC compared to other cubes made

Fig. 9. FeCr slag as coarse aggregate vs. leachate Cr(VI) concentration at pH 2.88.

Fig. 10. FeCr slag as coarse aggregate vs. leachate total Cr Concentration at pH 2.88.

Fig. 11. FeCr slag as fine aggregate vs. leachate Cr(VI) concentration at pH 6.68.

Fig. 12. FeCr slag as fine aggregate vs. leachate total Cr concentration at pH 6.68.

Fig. 8. FeCr slag as coarse aggregate vs. leachate total Cr Concentration at pH 6.68.

Fig. 13. FeCr slag as fine aggregate vs. leachate Cr(VI) concentration at pH 2.88.

268 C.R. Panda et al. / Construction and Building Materials 49 (2013) 262–271

with blended cements like PSC and PPC. This is because more CaOin OPC and the CaO content is substantially reduced in blended ce-ment because of substitution of CaO containing clinker by blastfurnace slag and fly ash.

From the experimental batch leaching studies of the slag sam-ples, it is found that there is significant leaching of Cr(VI) and totalchromium from the slag samples under regulatory TCLP test condi-tions. Even though leaching results are by and large low but often itexceeds the Indian regulatory discharge standard of 0.1 mg/l forCr(VI) and 2 mg/l for total chromium. The chromium leachingstudy results from concrete samples with ferrochrome slag ascoarse as well as fine aggregate showed very low level leachingwell within the US EPA and Indian regulatory discharge standardwith respect to both Cr(VI) and total chromium. While chromiumin the form of Cr(III) as mostly available in slag samples is wellimmobilized in all types of cement-concrete matrix, Cr(VI) is notsuitably immobilized in concrete matrix with ordinary Portland ce-ment and fly ash based portland pozollana cement. But slag basedPortland slag cement has been found to be most effective in immo-bilizing Cr(VI) as well as Cr(III). The reason attributed is that reduc-tion of Cr(VI) to Cr(III) by sulphide compounds and ferrous saltspresent in the blast furnace slag in slag cement followed by theimmobilization in the cement-concrete matrix. Thus the chromiumleaching from concrete specimens having FeCr slag as aggregatematerial with slag cement is quite low and is well below the USEPA and Indian regulatory discharge standard.

3.6. Ferrochrome slag as concrete aggregate material in the purview ofBuilding Material Decree (BMD) protocol

The environmental burdening of soil and surface water byleaching of substances is called immission. The Dutch BuildingMaterial Decree (BMD); 1995 protocol sets standards for inorganicsubstances in the building materials with regards to immission ofsuch substances in to soil and surface water. The 64 days tankleaching test results from monolithic concrete specimens having100% of substitution with FeCr slag as aggregate are comparedwith Dutch BMD 1995 protocol for category1 limit values and withSoil Quality Decree (SQD), 2007 [52] with respect to chromium forunrestricted use. The analysis of results is shown in Table 8 and isgraphically represented in Figs. 15 and 16. In comparison withDutch Building Material Decree and Soil Quality Decree (2007),the 64 days release of chromium from concrete samples, the ser-vice life of concrete is quite low and therefore the FeCr slag asbuilding material is not likely to create any significant chromiumpollution problem.

4. Conclusion

From the detailed review of the literature and the presentexperimental results of this research work, the following importantconclusions may be drawn.

� Ferrochrome slag, a major solid waste in ferrochromeindustries faces the disposal problem because of residualchromium content in it. Even though the slag has the

Table 8Comparison of 64 days Tank Leaching Test for Cr with BMD 1995 and SQD 2007.

Samplespecimencode

LeachantpH

Total chromium64 days release(mg/m2)

BMD 199564 days(mg/m2)

SQD 2007 in64 days (mg/m2)

OPFSCA100 6.68 8.4 140 120PSFSCA100 6.68 4.8 140 120PPFSCA100 6.68 7.2 140 120OPFSCA100 2.88 9.2 140 120PSFSCA100 2.88 5.0 140 120PPFSCA100 2.88 7.5 140 120OPFSFA100 6.68 9.6 140 120PSFSFA100 6.68 5.5 140 120PPFSFA100 6.68 8.8 140 120OPFSFA100 2.88 9.8 140 120PSFSFA100 2.88 5.6 140 120PPFSFA100 2.88 9.2 140 120

Fig. 15. Comparison of 64 day leaching at pH 6.68 with BMD and SQD protocol.

Fig. 16. Comparison of 64 day leaching at pH 2.88 with BMD and SQD protocol.

Fig. 14. FeCr slag as fine aggregate vs. leachate total Cr concentration at pH 2.88.

C.R. Panda et al. / Construction and Building Materials 49 (2013) 262–271 269

desired engineering and mechanical properties, its use hasbeen restricted because of leaching of chromium oftenexceeding the regulatory norms under adverse environ-mental conditions.

� From the experimental study in concrete work using slag asaggregate material, it is concluded that ferrochrome slagavailable as air cooled slag shows excellent results ascoarse aggregate in concrete work indicating its possibleutilization in high strength concrete work. From the exper-imental studies it is also concluded that ferrochrome slagavailable as water cooled granulated slag shows its suit-ability as fine aggregate in general purpose concrete work.

� From the detailed review and characterization studies inthe form of mineralogical and chemical characterizationstudies of the ferrochrome slag samples, it is found thatresidual chromium in the ferrochrome slag mostly remainsimmobilized as Cr(III) in highly stable spinel phases likeChromite or Magnesiochromite/Magnesium AluminiumChromite and thereby inhibiting chromium release fromslag matrix under ambient environmental conditions. Atthe same time the slag samples contain negligibly too smallamount of leachable Cr(VI) concentration to cause any sig-nificant pollution problem.

� From the elaborate leaching experimental study carried outwith in situ ferrochrome slag samples, it is found that thereis significant leaching of chromium from the slag samplesunder regulatory TCLP test conditions. With slag as aggre-gate in concrete product, the concrete provides suitablesolid matrix for immobilization of chromium. Whateverthe small amount of chromium leaching from the slag,mostly in the form of Cr(III), has been found to be success-fully immobilized in all types of cement and concretematrix resulting in very low level release from concretemonolith samples. The results from compliance monolithicleaching and regulatory leaching study indicate very lowlevel of chromium leaching under anticipated adverse envi-ronmental conditions and are well within the permissibleregulatory norm. Blast furnace slag based Portland slagcement has been found to be most effective in immobiliz-ing Cr(III) as well as Cr(VI).

� The results of 64 days release of chromium from standardtank leaching tests of concrete monolith samples show verylow values as compared with the Dutch Building MaterialDecree (1995) and Soil Quality Decree (2007). From theresults it is inferred that the service life of concrete is notlikely to create environmental pollution problem, indicat-ing the environmental compatibility of ferrochrome slagas concrete aggregate material.

Thus ferrochrome slag, a problematic solid waste having dis-posal problem can be suitably utilized as concrete aggregate mate-rial with chromium immobilization in cement concrete matrixwithout causing significant environmental pollution problem.

Acknowledgments

The authors gratefully acknowledge to Director IMMT for provid-ing the laboratory facilities for the characterization of the samples.The authors are thankful to the Dean, Institute of Technical Educa-tion and Research, Siksha O Anusandhan University, Bhubaneswar,India for laboratory and other facilities for this research work.

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Int. J. Environmental Technology and Management, Vol. 15, Nos. 3/4/5/6, 2012 261

Copyright © 2012 Inderscience Enterprises Ltd.

Release behaviour of chromium from ferrochrome slag

C.R. Panda* and K.K. Mishra Department of Civil engineering, Institute of Technical Education & Research, Siksha O Anusandhan University, Bhubaneswar 751030, Odisha, India Email: chittaranjan.panda435@gmail.com Email: kkmishra06@yahoo.co.in *Corresponding author

B.D. Nayak, D.S. Rao and B.B. Nayak Institute of Minerals and Materials Technology (IMMT), Council of Scientific & Industrial Research (CSIR), Bhubaneswar 751013, Odisha, India Email: bdnayak@immt.res.in Email: dsrao@immt.res.in Email: bbnayak@immt.res.in

Abstract: Huge amount of ferrochrome slag, a solid waste is being generated from submerged electric arc furnace during manufacturing of ferrochrome alloy. The waste slag contains about 6–12% chromium as chromium oxide and it has the potentiality of releasing hazardous chromium compounds. Although chromium exists in the slag as highly immobile Cr (III) state, there is possibility of chromium release under different adverse conditions inter alia its conversion to highly leachable Cr (VI) state. While Cr (III) is an essential trace element in glucose synthesis, the Cr (VI) compounds are very toxic and highly leachable. This leaching characteristic may classify the slag as hazardous waste and restricts its use and disposal. Release of chromium from granulated slag was evaluated under laboratory batch studies with varying different experimental conditions. The results were analysed to find the conditions for maximum release of total chromium from the slag.

Keywords: chromium (VI); chromium (III); ferrochrome slag; spinel; TCLP; redox chemistry.

Reference to this paper should be made as follows: Panda, C.R., Mishra, K.K., Nayak, B.D., Rao, D.S. and Nayak, B.B. (2012) ‘Release behaviour of chromium from ferrochrome slag’, Int. J. Environmental Technology and Management, Vol. 15, Nos. 3/4/5/6, pp.261–274.

Biographical notes: C.R. Panda presently is an Assistant Professor of Environmental Engineering in Institute Technical Education & Research, Bhubaneswar, India. He received his post graduate degree in Environmental engineering & management from IIT Kharagpur, India. He has more than 30 years of experiences in industry, consultancy and teaching. He has completed 28 EIA and EMP projects approved by Ministry of Environment & Forest, Government of India in core sectors like steel, cement, thermal power, mining and mineral beneficiation.

262 C.R. Panda et al.

K.K. Mishra presently is Director (Examination) in Institute Technical Education & Research, Bhubaneswar, India. He received his PhD degree in environmental engineering from Sambalpur University, India. He did his doctoral research work from IIT Kharagpur and Delft Holland. He has 40 years of teaching and research experience and was professor in the Department of Civil Engineering in National Institute of Technology, Rourkela, India. He has published more than 30 papers in national and international journals.

B.D. Nayak received his Post Graduate and PhD degree in Geo-sciences from Sambalpur University, India. After a brief stint in industry, he joined Institute of Minerals and Materials Technology (IMMT), CSIR, Bhubaneswar, India where he is continuing as Senior Scientist. He has completed a number of research and commercial projects in the field of solid waste utilisation particularly in the area of utilisation of industrial solid waste like fly ash, red mud, steel plant slag and tailing waste, etc. He has six patents granted to his credit.

D.S. Rao received his Post Graduate and PhD degree in Geo-sciences from Berhampur University, India. He joined National Metallurgical Laboratory a CSIR Laboratory in Chennai and later moved to Institute of Minerals and Materials Technology (IMMT), CSIR, Bhubaneswar, India, where he is continuing as Senior Scientist. His specialisation and research interest includes mineralogy and material characterisation study.

B.B. Nayak received his PhD degree in Chemistry from Utkal University, Bhubaneswar, India. He joined Institute of Minerals and Materials Technology (IMMT), CSIR, Bhubaneswar, India, where he is continuing as Senior Technical Officer. His specialisation and research interest includes chemical characterisation study and development of various analytical techniques.

1 Introduction

Chromium is one of the most common toxic heavy metal found in the environment. It exists in the common oxidation states as hexavalent Cr (VI) and trivalent Cr (III). Chromium as Cr (III) is highly immobilised in the solid waste slag matrix and hence does not leach out. Moreover trivalent chromium is far less toxic than its hexavalent counterpart. On the other hand Cr (VI) is highly mobile under all environmental conditions and it causes all the toxic effect. As per Occupational Safety Health Administration (OSHA), USA, the major health effects associated with exposure to Cr (VI) include lung cancer, nasal septum ulcerations and perforations, skin ulcerations and allergic and irritant contact dermatitis. Toxicity of chromium ranges from pulmonary to dermatological problems. As per EPA (2000), it is a suspected carcinogen. That is why there is stringent USEPA Toxicity Characterising Leaching Study (TCLP) standard such as 0.1mg/l for Cr (VI) and 5.0 mg/l for total chromium (USEPA, 1990).

Ferrochrome is the most common alloying material for the production of different grades of stainless steel. Chromium and iron in the chromite ore can form a continuous series of solid solution under certain condition of heat treatment containing 45–80% of chromium. Ferrochrome is manufactured through direct smelting in Sub-merged Arc Furnace (SAF) at a temperature above 1500C. The furnace has the suitable system for tapping heavier metal and lighter slag and their handling. For the production of each MT of ferrochrome about 2.5–2.6 MT of chromite ore, 0.5–0.6 MT of coke and 0.3 MT of

Release behaviour of chromium from ferrochrome slag 263

fluxing agent are required. There is generation of 1–1.2 MT of solid waste in the form of slag for every MT of ferrochrome product. The solid waste slag contains about 6–12% chromium as largely immobilised Cr (III) state. There is remote possibility of conversion of small amount of Cr (III) to higher oxidation state of Cr (VI) which is highly mobile and can leach into surface and ground water creating pollution problem. The purpose of this study is to evaluate the release behaviour of chromium from the granulated slag as available from the furnace.

2 Literature review on chromium leaching behaviour from solid waste slag

2.1 Generation and characterisation of ferrochrome slag

As per Coetzer et al. (1997) the following simple equation represents the production and different important phases in ferrochrome production in a SAF.

(Fe, Mg) (Cr, Al)2O4 + C (from coke) + fluxes = FeCr alloy + MgAl2O4 (Spinel) + Partially Altered Chromite (PAC) + Glass + Entrained metal

Slag in the ferrochrome manufacturing process is Al2O3-MgO-SiO2 based stable compound with significant amounts of Cr2O3. The slag is created in the process as a result of the reaction of gangue material in ore with flux burden. In ferrochrome process, the slag is a reactive medium where reduction takes place. It dissolves chromite pellets and lumpy ore until at the saturation point Cr2O3 is reduced by carbon to form metallic ferrochrome which coalesces into droplets (heavier phase) and is separated out of slag and settles to the bottom of furnace (Forsbacka and Holappa, 2004). The temperature of the slag in tapping is 1700°C and that of the ferrochrome 1600°C. The densities of metal and liquid slag are significantly different, so the slag is separated from the metal. The density of the ferrochrome slag is between 2.5–2.8 g/cm3 and that of ferrochrome 6.8 g/cm3. The slag is directly granulated during tapping where ferrochrome is tapped into ladles. The overflow from the ladles flows along the slag launder to the granulation pond, where high-pressure water breaks slag into small fractions and efficiently it cools down. Granulated slag is a very homogenous product of grain size is <6 mm. Typically the granulated slag includes three different phases, which are amorphous glass phase, crystalline Fe-Mg-Cr-Al-spinel and metal droplets (Niemela and Mauri, 2007). Chromium content in Fe-Cr slag has typically about 6–12% chromium as Cr2O3 of which significant part as metallic particles dispersed in the slag. At smelting condition chromium exists as divalent CrO above 1600°C. It converts into metallic and trivalent chromium on cooling under ambient condition (Holappa and Xiao, 2004).

3CrO → Cr + Cr2O3

Increasing the slag basicity up to 1.4–1.6 reduces chromium loss to slag. Addition of MgO enhances the chromium recovery from slag. Chromium in slag after recovery of metal remains mostly in Cr (III) state in the form of stable and immobile spinel structure and a very small amount remains in mobile state. Cr (III) stability occurs over a wide Eh and pH range under both reducing and oxidising and acid and alkali conditions. Cr (VI) stability occurs in a much narrower zone like oxidising and alkaline conditions. The hexavalent state is stable in an oxidising alkaline environment, whereas the trivalent

264 C.R. Panda et al.

state is stable in a reducing acidic environment (Kimbrough et al., 1999). Almost all hexavalent chromium salts except calcium, barium and lead chromate are soluble in water under ambient conditions. The Cr (VI) state as negatively charged chromate and dichromate ions are not controlled by sorption mechanism and are mobile under a wide Eh and pH range (Dermatas and Moon, 2006) The commonly available Cr (III) compounds like Cr (III) chloride, Cr (III) nitrate, potassium chromium (III) sulphate, etc are soluble in water (Motzer, 2005). pH is the single most important factor influencing the chromium release behaviour from the slag. Chromium being an amphoteric element, it is highly insoluble in the intermediate pH range and is soluble under very low and very high pH conditions. The solubility of chromium decreases as pH increases and shows minimum value in the pH range of 5.5–8.5 (Richard and Bourg, 1991). Several other factors affect the release of chromium species from slag under different environmental conditions. The physical factors are particle size, contact time, homogeneity and liquid to solid ratio (L/S ratio), porosity, partitioning, temperature, agitation/mixing and type of flow. Among the major chemical influencing factors are redox potential, complexation, precipitation and dissolution, organic carbon content, alkalinity and common ion effect, etc. Some researchers reported that microbial action also enhances the leaching of chromium from slag. Kilau and Shah (1983) found that presence of sufficient amount of MgO in the slag facilitates the formation of Picrochromite (MgO.Cr2O3) which prevents the dissolution of chromium under mild acid medium. Above pH 5, chromium leaching is dominated by hexavalent chromium and in acid medium that is below pH 5, total chromium content increases considerably because of dissolution of immobile Cr (III) from the slag (Wazne et al., 2005). The mobility of chromium is determined by the competition among the mechanisms like dissolution/precipitation, redox transformation and adsorption/desorption mechanisms. The Cr (III) leachability is mostly controlled by the formation of insoluble oxides and hydroxide compounds and the surface adsorption mechanisms (Richard and Bourg, 1991). Examination of the CaO-SiO2-Cr2O3 system delineated by Glasser and Osborn (1964) indicated the importance of waste slag chromium leachability. Basicity of the phase with respect to CaO/SiO2 plays an important role in the Cr6+ formation (Kilau and Shah, 1983). From these studies a generalised sequence of trend of most easily leachable to most stable phases of the ferrochrome slag is, silicates (magnesium, aluminium, calcium, silicate with chromium) oxides (chromite with or without iron content and spinel) (forsterite) Fe-Cr metal.

2.2 Redox chemistry of chromium

The redox potential of Cr (VI)/Cr (III) couple is so high, that there are few oxidants in the neutral environmental system capable of oxidising Cr (III) to toxic Cr (VI). Naturally pH can be reduced by CO2 absorption, acid rain, microbial metabolic action and anaerobic decomposition, facilitating the dissolution of Cr (III) into water environment. Molecular oxygen and manganese oxides are commonly and naturally occurring oxidising agents which can oxidise Cr (III) to Cr (VI). Molecular oxygen oxidises very slowly at pH > 9 (Rinehart et al., 1997; Deakin et al., 2001). Manganese oxides particularly MnO2 is the most important oxidising agent available in slag and other steel plant waste which may oxidise Cr (III) to toxic Cr (VI) at all pH conditions. The rate of oxidation tends to increase with solution alkalinity and rapid oxidation of Cr (III) to Cr (VI) occurs at pH > 9 (Kimbrough et al., 1999). Based on the information reported by several workers that oxidation of Cr (III) to potentially toxic Cr (VI) is possible in natural aerobic environment in presence of manganese dioxide particularly MnO2.Under

Release behaviour of chromium from ferrochrome slag 265

moderately oxidising to reducing conditions Cr (III) precipitates controls the solubility of chromium and Cr (III) is thermodynamically more stable throughout the redox potential and pH range (Richard and Bourg, 1991; Kimbrough et al., 1999).

Ageing of the slag sample has considerable bearing on the oxidation of Cr (III) to Cr (VI). The levels of Cr (VI) are found to increase with the ageing of the sample (Pillay et al., 2003). This aspect requires a long term leaching modelling study of the ferrochrome slag.

3 Materials and methods

The ferrochrome granulated slag sample was collected from the operating plants of ferrochrome industry with submerged arc electric arc furnaces, situated in Kalinganagar industrial complex, Odisha, in the eastern part of India.

3.1 Mineralogical characterisation

The mineralogical characterisation by X-ray Diffraction (XRD) study of the granulated slag was carried out by using a Cu target of X-ray diffractometer, make – Pan Analytical model – Expert Pro. Mineral phases were identified by comparing the d-spacing with those given in ASTM data cards and the result is shown in Figure 2.

3.2 Major chemical characterisation by X-ray fluorescence (XRF) analysis

The quantitative analysis of major and minor elements for some of the samples was carried out by X-ray Fluorescence spectroscopy (XRF). For this purpose, pressed powdered pellets were exposed to a Phillips PW-1400 X-ray spectrometer with scandium tube. The results are shown in Table 1.

3.3 Particle size distribution (PSD)

Particle Size Distribution (PSD) study was done with different sized standard sieves. The results are expressed in terms of mass retained and the cumulative mass retained and the results are shown in Table 2 and illustrated in Figure 3. The batch leaching study was carried out with the different size samples and the leaching results are shown in Table 4.

3.4 Experimental batch study with different L/S ratio

L/S ratio varies from 1:1 to 500:1. L/S values depend on different experimental condition (Deakin et al., 2001). The volume of leachate can also be calculated from simple empirical formula.

10 100L MC MD

L = Leachate volume in litre

MD = Dry mass in kg

MC = Moisture content ratio (%)

266 C.R. Panda et al.

50 gm of dry solid sample were taken in 1 litre bottle. Different L/S ratio of 5, 8, 10, 15, 20 were chosen for the study. The samples after seven days were taken for the analysis of hexavalent chromium and total chromium. Hexavalent chromium was analysed by colour development with diphenyl carbazide and absorbance study by spectrophotometer of Systronic-106 India. Total chromium analysis was done by Atomic Absorption Spectrophotometer (AAS) model (SIMADZU, AA-6300) of as per the procedure outlined in American Public Health Association (APHA, 2005) manual on water and wastewater analysis). The results are presented in Table 3 and are graphically illustrated in Figure 4.

3.5 Batch leaching study under different pH conditions

Batch leaching study under different pH conditions was conducted as per the procedure outlined by Kossan et al. (2002). 50 gm of sample were taken in 1 litre bottle with L/S ratio of 10. The samples were acidified with 2N Nitric acid for different low pH experimental conditions. Similarly some other samples were treated with 1 N KOH to maintain different high pH environment. Finally leachates were collected after 48 hours for the determination of hexavalent chromium by UV visible spectrophotometer and total chromium content by AAS. The results are tabulated in Table 5 and illustrated in Figure 5.

3.6 Batch leaching study under different contact time

50 gm of sample were taken in eight 1 litre bottles with L/S ratio of 10 at different contact time in 1, 2, 4, 8, 16, 24, 48, 120 and 168 hours. Each sample was analysed for hexavalent chromium and total chromium content. The results are shown in Table 6 and Figure 6.

Figure 1 Batch leaching experimental set-up (see online version for colours)

3.7 Batch leaching study under high temperature condition

Four 50 gm of samples were taken in 1 litre glass jar with L/S ratio of 10 and is kept in a muffle furnace at a temperature about 700–800C for about 4–5 hours with leachant make up from time to time and the leachate samples were analysed for hexavalent chromium and total chromium content.

Release behaviour of chromium from ferrochrome slag 267

3.8 Toxicity characterising leaching study (TCLP)

TCLP tests as per USEPA 1311 method were carried out for hexavalent and total chromium. 100 gm of granulated slag with size < 9.5 mm was subjected to batch leaching with extraction liquid with an L/S ratio of 20 depending on different pH conditions. Contact time with agitation was maintained at 18 hours. Extraction liquid of pH 4.93 prepared with 5.7 ml of glacial acetic acid and 64.3 ml of 1 M NaOH in 1 litre distilled water was used for this case.

4 Results and discussion

4.1 XRD results

The mineralogical characterisation by XRD indicated the dominant mineral phases that are Forsterite (Mg2SiO4), Magnesiochromite Spinel with Aluminium (Mg (Cr.Al)2O4) and Fayalite (Fe2SiO4). Spinel is the major mineral phase and occurs as euhedral grains. Besides the slag contains chromium metal as Chromoferide (Cr, Fe)7C3 in the form of globular, elliptical and dendritic grains. Besides these crystalline phases, the slag contains a non-crystalline glassy phase which forms the matrix. Mineralogy of slag is complex due to presence of chromium in a range of oxidation states particularly in +2, +3 and +6 state. The majority amount of chromium remains in immobile Cr (III) spinel structure, which is responsible for low-level chromium leaching.

Figure 2 XRD results of ferrochrome slag (see online version for colours)

4.2 XRF analysis results

XRF analysis result indicates the major chemical components in the slag sample. In addition to major elements like aluminium, silicon and magnesium, significant amounts of chromium present in the slag. This residual amount of chromium in the slag is responsible for the leaching of chromium.

268 C.R. Panda et al.

Table 1 XRF analysis results of ferrochrome slag

Analyte Concentration % Analyte Concentration %

Na2O 1.19 TiO2 0.21 MgO 33.23 Cr2O3 10.67 Al2O3 19.59 MnO 0.37 SiO2 26.27 Fe2O3 3.73 SO3 1.68 NiO 0.04 K2O 0.26 SrO 0.01 CaO 2.70 ZrO2 0.007

4.3 Particle size distribution analysis

The PSD measurement of in situ granulated slag indicates the majority of the particle remains in the average size range of 1180–2360 μm by different sieve sizes as per Indian Standard Code.

The finer particle size favours the release of chromium from the slag.

Table 2 Particle size distribution (PSD)

size (micron) mass (gm) cum mass (gm) mass % cum mass %

50 3 3 0.5 0.5 75 7 10 1.17 1.67

212 6 16 1 2.67

300 6 22 1 3.67

425 29 51 4.83 8.5 600 84 135 14 22.5

1180 273 408 45.5 68

2360 139 547 23.17 91.17

4750 53 600 8.83 100

Figure 3 Cumulative mass retained % vs. screen size (see online version for colours)

4.4 Leaching data at different L/S ratios

Optimum L/S ratio was found to be 10:1 at which maximum leaching of chromium occurs. At this ratio, the pH changes from 7.3 to 7.8 and conductivity change from 133 μS/cm to

Release behaviour of chromium from ferrochrome slag 269

370 μS/cm indicating that there is dissolution of some salts including particularly chloride and sulphates of magnesium and calcium and that of trace amounts of chromium salt. This optimum L/S ratio was chosen for the subsequent experimental works.

Table 3 Concentration under different L/S ratio

L/S ratio Cr(VI) mg/kg Total Cr mg/kg

5 0.330 1.095

8 0.728 1.126

10 0.884 2.880

15 0.630 1.840

20 0.785 1.760

Figure 4 Chromium concentration in mg/l at different L/S ratio (see online version for colours)

4.5 Leaching results at different particle sizes

The finer particle size favours the release of chromium from the slag.

Table 4 Chromium leaching with respect to different particle sizes

size (micron) Cr (VI) mg/kg Total Cr mg/kg

100 2.95 31.251

250 2.84 15.426

355 1.56 11.569

850 1.31 5.712

1000 0.99 2.88

4.6 Leaching results under different pH values

The batch leaching under different pH conditions showed the expected results. The results are in agreement with the results obtained by various researchers like Rai et al. (1989), Kimbrough et al. (1999), Weng et al. (2001) and Rinehart et al. (1997). At high

270 C.R. Panda et al.

pH > 6.3 up to 11.5 Cr (III) exists mostly as insoluble Cr(OH)3 and as slightly soluble Cr(OH)4

– above pH 11.5. The experimental results confirmed the increase chromium concentration under high pH conditions especially after pH 11. It is concluded that under ambient to high pH conditions Cr (III) forms highly insoluble precipitate as Cr(OH)3 in the pH range of 6–11. Amorphous Fe (III) and Cr (III) hydroxide are probably the main chromium solubility controlling phase in natural environment. Rai and Eary (1987) suggested that the mixed hydroxide is readily formed under ambient conditions and most of them remain as insoluble precipitate of (Fe1 – x.Crx)(OH)3 and Cr(OH)3 in near neutral to alkaline pH range of 6.5–9.5. They reported the formula for mixed hydroxide as (Fe0.75.Cr0.25)(OH)3 and this precipitate is even more insoluble than pure hydroxides. Thus aqueous solubility of Cr (III) is a function of degree of hydroxylation of chromium which in turn depends upon pH. Under neutral to basic condition Cr (III) shall precipitate out, while under acidic condition it will go in to solution. Cr (VI) in the form of chromates and dichromate are soluble in water at all pH except some of the chromates of calcium, barium, strontium, lead, etc.

Table 5 Leachate chromium concentrations at different pH values

pH Cr (VI) mg/kg Total Cr mg/kg

0.11 14.53 14,320

0.8 14.23 3220

1.03 14.12 1472

2.23 12.56 39

3.9 4.23 17.5

5.12 1.55 14.5

7.02 0.78 2.33

7.96 0.86 2.45

8.75 1.56 3.2

9.44 4.89 4.5

10.98 8.96 5.3

12.37 8.98 16.4

Figure 5 Leachate chromium concentration at different pH values (see online version for colours)

Release behaviour of chromium from ferrochrome slag 271

Figure 5 Leachate chromium concentration at different pH values (see online version for colours) (continued)

4.7 Leaching results at different contact time

The release of chromium from the slag was found to depend on contact time. There is a major initial percent of release in the first eight hours and the rate of decrease falls with the increase in time. It reaches almost constant value after seven days of period.

Table 6 Chromium concentration at different contact time

contact time (hour) Cr (VI) mg/kg Total Cr mg/kg

8 0.878 2.16

16 0.916 2.36

24 0.945 2.64

48 0.991 2.87

120 0.992 2.92

168 0.992 2.92

Figure 6 Chromium concentration at different contact times (see online version for colours)

272 C.R. Panda et al.

4.8 Regulatory TCLP test results

TCLP is used for characterising a waste as hazardous or non-hazardous. It provides a worst case scenario for leaching that of consistent acid attack on a material which has limited physical integrity and low buffering capacity that is relative inability to resist acidic and alkaline influences.

The regulatory TCLP study indicated the release of chromium values in the range of 0.076–0.158 mg/l for Cr (VI) and 0.58–1.12 mg/l for total Cr and largely remained within the statutory USEPA norms. The large increase in total chromium in the leachate is due to dissolution of Cr (III) under TCLP low pH conditions. But with the ageing and in the long run, the TCLP leachate may exceed the regulatory standard.

The experimental batch study under high temperature results indicated that the temperature has minimum influence on chromium release from slag. This result is anticipated as the slag is derived from very high temperature conditions of the SAF.

5 Conclusion

Residual chromium in the ferrochrome slag mostly remains entrapped in immobile spinel phase like Chromite or Magnesiochromite/Magnesium Aluminium Chromite structure and thereby inhibiting chromium release from slag matrix under ambient environmental conditions. The leaching of chromium may be due to presence of some mobile trivalent chromium and their subsequent natural oxidation to hexavalent chromium. But under ambient conditions the level of chromium leaching is quite low and within the permissible regulatory norm. Chromium release from ferrochrome slag is influenced by several important factors like pH, particle size reduction, L/S ratio, ageing, etc. Experimental investigation indicated leaching is minimum in the ambient pH range of 5.5–8.5. Drastic chromium leaching occurs at very low pH conditions because of dissolution of trivalent chromium. Some appreciable leaching occurs under high pH conditions, because of formation of leachable chromium salts. There is initial increase in chromium leaching and it becomes constant after almost seven days of contact time. Reduction in particle size enhances the chromium release. Therefore ageing induced particle size reduction is likely to cause long-term leaching. High temperature has no effect on leaching. Once in aqueous solution Cr (III) has the possibility of converting into highly leachable and toxic Cr (VI) under the natural ambient environment with the available oxidising agents. While under the natural conditions, it is recognised that commonly Fe (II) and organic matter influence the reduction kinetics of Cr (VI) to Cr (III), the presence of dissolved oxygen and manganese dioxide enhances the oxidation potential of Cr (III) to Cr (VI). The regulatory USEPA TCLP values by and large remained within the permissible limits. The mechanisms like redox chemistry, precipitation and dissolution reactions, sorption and desorption, complexation and diffusion, etc. control the leaching, transformation and distribution of chromium in the natural environment.

Release behaviour of chromium from ferrochrome slag 273

Acknowledgements

The authors gratefully acknowledge to the director IMMT, for providing laboratory facilities for the characterisation of the samples. They are thankful to the Dean, Institute of Technical Education & Research, Siksha O Anusandhan University, Bhubaneswar, India for permitting to publish this paper.

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