The present study concerns itself on the adverse effects of alkaline elements like sodium and potassium on blast furnace cokes. To achieve a deeper insight on the effects of alkaline elements on coke reactivity and strength, industrial coke samples impregnated with different alkaline species in various amounts have been tested under standard conditions to find out their Coke Reactivity Index (CRI) and Coke Strength after Reaction (CSR) values. Scanning electron microscopy, petrographic and Raman Spectrometric investigations demonstrate the change of structural properties. The mechanism of catalysis has been postulated

A. Bhattacharyya

C. Thaler

G. Rantitsch

H. Stocker

J. Schenk

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Effect of alkaline elements on the reactivity, strength and structural properties of blast furnace cokes

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503METALURGIJA 54 (2015) 3, 503-506A. BHATTACHARYYA, J. SCHENK, G. RANTITSCH, C. THALER, H. STOCKEREFFECT OF ALKALINE ELEMENTS ON THE REACTIVITY, STRENGTH AND STRUCTURAL PROPERTIES OF BLAST FURNACE COKESReceived – Prispjelo: 2014-01-29Accepted – Prihvaćeno: 2014-12-20Preliminary Note – Prethodno priopćenjeISSN 0543-5846METABK 54(3) 503-506 (2015)UDC – UDK  669.1:662.7:546.32/.33=111A. Bhattacaharyya, J. Schenk, G. Rantitsch, Montanuniversität Leo-ben, Austria, C. Thaler, voestalpine Stahl GmbH, Austria, H. Stocker, voestalpine Stahl Donawitz GmbH, AustriaThe present study concerns itself on the adverse effects of alkaline elements like sodium and potassium on blast furnace cokes. To achieve a deeper insight on the effects of alkaline elements on coke reactivity and strength, indus-trial coke samples impregnated with different alkaline species in various amounts have been tested under standard conditions to find out their Coke Reactivity Index (CRI) and Coke Strength after Reaction (CSR) values. Scanning electron microscopy, petrographic and Raman Spectrometric investigations demonstrate the change of structural properties. The mechanism of catalysis has been postulated.Key Words: blast furnace cokes, alkali elements, boudouard reaction, CRI and CSR, coke characterizationINTRODUCTIONThe properties of different raw materials have great impact on the efficiency and productivity of blast fur-nace ironmaking process. The operational requirements of raw materials are significantly deteriorated in pres-ence of very small amount of alkalis. Alkaline elements like sodium (Na) and potassium (K) have been marked as detrimental for furnace operations because they cata-lyze the Boudouard Reaction [1, 2] and reduce the me-chanical strength of coke in the lower zone of the fur-nace [3], which in turn lead to furnace irregularities re-sulting in lower productivity and economic loss. The major sources of alkalis are coke ash and gangue mate-rials present in the ores, which makes it impossible to eliminate them from the actual process. Different stud-ies have shown the catalytic effect of various amounts of potassium on Boudouard Reaction under laboratory conditions [1, 2, 4].The aim of this study is to evaluate the effect of both sodium and potassium on coke reactivity and strength under standard conditions and further investigation of the structural change of coke samples by different meth-ods of characterization.METHODOLOGYArtificial Alkali ImpregnationThree different industrial coke samples (C1, C2 and C3) were used for the purpose of experiments. The proximate analysis of the coke samples are shown in Table 1.Table 1 Proximate analysis / mass %C1 C2 C3Ash 10,70 10,10 9,35Volatile Matter 0,19 0,37 0,65Moisture 3,4 3,15 2,04Fixed Carbon 85,71 86,38 87,96The coke samples were artificially impregnated by alkalis by leaching in different alkaline solutions (NaOH or KOH) containing the respective alkaline species.The coke pieces submerged in the solution were then placed in a vacuum chamber and deaerated until bubbling stopped. This method ensures the entry of the alkaline solution in all possible open and connected pores and removal of entrapped air. Then the excess so-lution was removed and the pieces were dried in a hot air oven at 125 oC for 10 hours for complete removal of water. Figure 1 shows schematically the process of arti-ficial alkali impregnation.An attempt was made to maintain the amount of added alkali close to industrial values. However, exact control of alkaline content in the samples is not possible because of different surface areas due to variable nature of porosities in the coke samples. Hence, the following values are defined as a range of added alkali content (Table 2).Table 2 Alkali content rangesNomenclature Artifi cially added Na or KNo added alkali 0 %Medium alkali content 0,6 – 1 %High alkali content 1,5 – 2 %504A. BHATTACHARYYA et al.: EFFECT OF ALKALINE ELEMENTS ON THE REACTIVITY, STRENGTH AND STRUCTURAL... METALURGIJA 54 (2015) 3, 503-506For CRI and CSR tests, the vertical retort furnace aggregate of the Chair of Ferrous Metallurgy, Montanu-niversität Leoben was used. The instrument has been constructed by Siemens VAI Metals Technologies GmbH, as well as the automation system. CSR tests have been done by a twin tumbler system (Model TB 5000 of R.B. Automazione, Genoa, Italy).  RESULTS AND DISCUSSIONS The CRI and CSR values of all three samples (with-out added alkali) agree with normally required values for industrial blast furnace operations. However, with little added alkali, enormous increase in CRI and de-crease in CSR has been observed in all cases. The effect of sodium on lowering the CSR is observed to be stronger than that of potassium. The results are depicted in four separate bar diagrams in Figure 2.Organic PetrologyThe microstructures (investigated for C1 and C3) show a large variety (Figure 3). During the experiments, vitrinite fuses to diverse mesophase structures. The presence of mesophase particles in untreated samples demonstrate that this transformation has already started CRI and CSR TestsCoke Reactivity Index (CRI) and Coke Strength af-ter Reaction (CSR) are internationally accepted testing standards for blast furnace cokes, described under ISO 18894 (2006). All the coke samples (with and without added alkali) are tested under standard conditions. For CRI, a test portion of the dried coke sample (200 ± 3 g) having a size range from 19,0 mm to 22,4 mm is heated in a reaction vessel to 1100 °C in a nitrogen atmosphere. For the test, the atmosphere is changed to carbon dioxide for exactly 2 hours. During this treatment, the carbon part of the coke sample is gasified by forming carbon monoxide. This is an endothermic reaction and known as Boudouard Reaction.C (solid) + CO2 (gas) = 2CO (gas) with ΔHorxn == +172,67 kJ/mol. [5]After the test, the reaction vessel is cooled down to about 50 °C in a nitrogen atmosphere. The comparison of the sample weight before and after the reaction deter-mines the CRI. The reacted mass is tumbled in a spe-cially designed tumbler for 30 minutes at 20 r.p.m. After tumbling, the percentage of the mass having a particle size greater than 10 mm to the reacted mass is defined as CSR. More details about the test could be found from the corresponding standard. [6]Figure 1 Steps of alkali impregnation (schematic)Figure 2 CRI and CSR Values505A. BHATTACHARYYA et al.: EFFECT OF ALKALINE ELEMENTS ON THE REACTIVITY, STRENGTH AND STRUCTURAL...METALURGIJA 54 (2015) 3, 503-506during coking. During the experiment, coke porosity in-creases by the disintegration of the coke matrix. Simul-taneously the microstructural composition of the carbo-naceous matter changes independently from the parent samples. Homogeneous particles evolve to granular and lenticular particles.Raman SpectrometryAs shown by Raman Spectroscopy [7], there is a major change of the bulk microstructure of the alkali treated samples during CRI test, whereas the untreated sample shows only minor changes. This is accompanied by the evolution of the coke porosity. The data of this study suggests that the microstructural evolution of the coke components (macerals) remains unaffected by al-kali treatment. The transformation of these components is more probably temperature controlled.Scanning Electron MicroscopyThe scanning electron microscopy (SEM) of repre-sentative untreated and treated samples was performed using Quanta 200 Mk2 (Manufacturer – FEI Company) and an attached Energy Dispersive X-Ray spectroscopy (EDX) device (INCA Energy 200, Oxford Instruments). SEM/EDX analyses of various alkali treated samples (after reaction) show fused aggregates (complex ox-ides) of alumina and silica containing Na and K, fre-quently scattered over the coke matrix. This type of structure is not observed in reacted samples without ad-ditionally added alkali. These aggregates have been generated by local fusion of the ash (which consists of basically alumina and silica) caused by alkaline oxides (by lowering the melting point). The phases (NaAlSiO4 and KAlSiO4) have been confirmed by X-Ray Diffrac-tometry (XRD) of reacted alkali treated samples. Figure 4 shows a microscopic image of reacted and potassium treated coke sample. Apart from mechanical weakening, alkali treated and reacted coke samples generate a lot of fines during tumbling. All of these phenomena can be explained as a result of local fusion caused by lowering of the melting temperature of the ash phase by alkaline elements which may generate internal stress in the structure and subse-quently make the structure weak.Catalysis of Boudouard Reaction: Hypothesis on the Theory of Ionic RadiiNo chemically stable (or metastable) intermediate catalytic phase was found by SEM/EDX and XRD anal-ysis. However, various literatures confirm XRD peak broadening, i.e. change of lattice parameters of organic crystals treated with alkali [3, 8]. On the other hand, the degradation of hematite pellets under alkaline influence has been explained in terms of the ionic radii of the con-cerned elements [9]. Table 3 demonstrates effective ionic radii of Na+, K+ and C4+.Table 3 Effective Ionic Radii / nm [10]Elements Na+ K+ C4+Ionic Radii 0,102 0,138 0,016The ionic radius of carbon is much smaller than that of sodium or potassium. At higher temperatures (1 100 oC and above), pronounced lattice disturbance is possibly caused by the rapid diffusion of Na or K in the graphite crystal system (which has much lower atomic Figure 3  Microstructure of the investigated coke samples (a= large fused (isotropic) particle in fine and lenticular anisotropic matrix, b= fused vitrinite with degassing pores, c= fused vitrinite with degassing pores, c= incipient anisotropy, d= decomposition of vitrinite into an anisotropic matrix at a pore contact, e= lenticular anisotropy developing out of a circular Anisotropy, f= graphitized lamella with undulose darkening)Figure 4  SEM image showing fused aggregate of KAlSiO4 on the dark coke matrix506A. BHATTACHARYYA et al.: EFFECT OF ALKALINE ELEMENTS ON THE REACTIVITY, STRENGTH AND STRUCTURAL... METALURGIJA 54 (2015) 3, 503-506radius) followed by consequent expansion and distor-tion of the unit cells. This process of distortion caused by rapid diffusion of alkali atoms is likely to cause the higher reactivity and weakening of the carbon structure.CONCLUSIONSBoth sodium and potassium, when present in very little amounts, bring a tremendous change to the CRI and CSR. On one hand, reactivity increases steeply and on the other hand, the coke becomes more and more weak. In actual blast furnace process, higher alkali in-put increases the coke consumption rate, causing eco-nomic loss and also lower the stability of the burden resulting in various operational irregularities. The effect of sodium on lowering the CSR is somewhat stronger than that of potassium. Cokes with higher porosities pick up more alkali when leached. Also in industrial process cokes with more porosity will pick up more al-kali from the system, hence making it weaker. Local fusion caused by lowering of the melting temperature of the ash phase by alkaline elements is likely to generate internal stress in the structure and subsequently make the structure instable.REFERENCES[1] T. Lindstad, M. Syvertsen, R. J. Ishak, H. B. Arntzen, P. O. Grontvedt, Proceedings: INFACON X, Cape Town, 2004, 261-271.[2] J. Kaczorowski, T. Lindstad, M. Syvertsen, ISIJ Internatio-nal, 47 (2007) 11, 1599-1604.[3] T. Hilding, S. Gupta, V. Sahajwalla, B. Björkman, J. Wik-ström, ISIJ International, 45 (2005) 7, 1041-1050.[4] J. Kaczorowski, T. Lindstad, Proceedings: INFACON XI, New Delhi, 2007, 584-593.[5] D.A. Bell, B.F. Towler, M. Fan, Coal Gasification and its Applications. Elsevier, 2011, pp. 399-400.[6] ISO 18894, 2006. Coke – Determination of Coke Reactivi-ty Index (CRI) and Coke Strength after Reaction (CSR). International Organization for Standardization.[7] G. Rantitsch, A. Bhattacharyya, J. Schenk, N. K. Lünsdorf, International Journal of Coal Geology, 130 (2014), 1-7.[8] S. Gupta, D. French, R. Sakurovs, M. Grigore, H. Sun, T. Cham, T. Hilding, M. Hallin, B. Lindblom, V. Sahajwalla, Progress in Energy and Combustion Science, 34 (2008) 2, 155-197.[9] A. K. Biswas, Principles of Blast Furnace Ironmaking, SBA Publications, Calcutta, 1984, pp. 233-235.[10] R. D. Shannon, Acta Crystallographica A32 (1976), 751-767.Note:  Dr. S. De, (University of Eastern Finland, Kuopio, Finla nd) is responsible for the English language content of this paper

Effect of alkaline elements on the reactivity, strength and structural properties of blast furnace cokes

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