Article pubs.acs.org/EF
Tribological Approach to Investigate the Interface Properties in Metallurgical Coke Richard Roest,*,† Hannah Lomas,† Kim Hockings,‡ and Merrick R. Mahoney† †
Centre for Ironmaking Materials Research, School of Engineering, The Newcastle Institute for Energy and Resources, University of Newcastle, University Drive, Callaghan, New South Wales 2308, Australia ‡ BHP Billiton, 480 Queen Street, Brisbane, Queensland 4000, Australia S Supporting Information *
ABSTRACT: Metallurgical coke is a brittle composite material comprising predominantly carbon derived from both reactive and inertinite coal macerals. During cokemaking, these macerals form the porous coke matrix. The interfaces between the distinct maceral derived components, known as textures in the final coke structure, are well-known to be a major source of weakness in some metallurgical cokes. In this paper, we present a novel approach which applies techniques used in tribology to investigate the strength of the interfaces as well as the wear properties of the different textural constituents in metallurgical coke, at room temperature. The wear mechanisms which occur in coke were investigated using three pilot oven cokes of different single coal origin. A range of high resolution analytical techniques, including coke petrography, optical microscopy, stereo microscopy, and scanning electron microscopy, were used to determine the wear modes. The technique has also proven to be an effective method to determine the relative strength of inerts and RMDC as well as the strength of the interface between these two textures, based on their susceptibility to different wear mechanisms. Further work is focused toward being able to predict the wear properties and relative strength of the inerts in a particular coke from their properties in the original coal(s).
1. INTRODUCTION Metallurgical coke is a complex heterogeneous material comprising both reactive maceral derived components (RMDC) and inertinite maceral derived components (IMDC) which originate from the parent coal(s). Previous research studies have consistently demonstrated that the grain boundaries (i.e., the boundaries between the RMDC and IMDC) are a common source of stress concentration (i.e., microstructural weakness) in metallurgical coke.1−5 Weak RMDC−IMDC interactions are due in part to insufficient dilatation during coking.6 The difference in the extent of contraction of IMDC and RMDC during coke making may also be a contributing factor.7 These stress enhancing interactions impact coke strength and breakage behavior; for example, they promote intergranular cracking, i.e., cracking at the boundary of IMDC, which reduces the final interfacial contact with the RMDC.8,9 Previous coke research conducted by Barriocanal and co-workers defined the quality of the interfaces present in the coke microstructure,10 but not the features which caused the reduction in interface quality. However, identification of these features is essential in order to link the quality of these interfaces to the properties of the original coals or coking conditions. We have recently developed an approach which uses tribology to better understand the nature and strength of the interfaces between the different solid phase components in coke. Tribology studies the wear properties of materials, and is a proven approach for investigating the grain boundary properties in other material systems, including metals and ceramics.11 In the tribology work presented here, rotational ball-on-disc tests were performed on polished blocks of cokes produced in © 2017 American Chemical Society
pilot scale ovens from single coals. In these experiments, the coke blocks were rotated at a constant speed and a ruby ball indenter was rubbed across the polished surface of the coke block under a fixed load. The wear path generated in the sample was then analyzed to identify the nature and degree of the damage to the coke surface. As for our previously reported fractographic analysis of coke to determine the mechanisms of coke breakage,8,9,12−14 the analysis was carried out on three levelsmacro (visual), micro (stereo and optical microscopy), and submicron (scanning electron microscopy (SEM)) scales. In addition, the images recorded were compared to petrographic images taken of the coke blocks prior to tribology testing, to link the wear properties of the coke to its distinct textural components and the interfaces between them. The wear which occurs in rotational tribology tests on coke is due to the progressive loss of surface material at the points at which the two surfaces (polished coke block and indenter) come into contact as they rub against each other. The main wear mechanisms which are relevant to analysis of coke cold strength are adhesive (transfer of material from the coke surface to the indenter), abrasive (“scratching” of the softer coke surface by the indenter), and delamination (subsurface cracks parallel to the surface) wear.15
2. MATERIALS AND METHODS 2.1. Metallurgical Coal Selection. The metallurgical coals used to form the cokes analyzed in this paper are summarized in Table 1. Received: November 3, 2016 Revised: January 8, 2017 Published: January 10, 2017 1422
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Energy & Fuels These coals were selected to cover a range of rank, petrographic composition and coking behavior.
Table 1. Properties of Coals Used to Make Cokes in This Study (Maceral Analysis on a Mineral Matter Free Basis)
coal
vitrinite content (vol %)
liptinite content (vol %)
organic inertinite content (vol %)
volatile matter (%, dry basis)
ash content (%, dry basis)
mean max vitrinite reflectance (%)
A B C
77 55 68
2.8 0.0 0.0
20 45 32
32 19 18
7.6 8.6 10.3
0.95 1.39 1.55
2.2. Coke Preparation. Cokes were prepared in two separate pilot coke ovens under the same conditions (see Table 2).16
Table 2. Typical Coking Conditions for the Three Compartment Charge for Formation of Cokes Coal grind Charge moisture Oven bulk density Coking time Wall temperature Final coke temperature at oven center
Figure 1. Rotational ball-on-disc experiment using a CETR tribometer.
∼85% passing 3.35 mm 4−5% ∼830 kg m−3 (db) ∼20 h Initially 770 °C then ramped to ∼1050 °C ∼1030 to 1060 °C
coke; it was required that there was just sufficient wear to be able to differentiate between the interface properties of distinct inerts within each of the cokes studied. The frictional coefficient (COF) was determined for each experiment. All tests were performed at room temperature and in the absence of added lubricant. Following the tribology experiments, coke samples were analyzed using high resolution optical microscopy, stereo microscopy, and scanning electron microscopy. 2.6. High Resolution Optical Microscopy. High resolution optical micrographs were recorded using a Zeiss Axio Imager.Z1m microscope equipped with both a high and medium resolution camera used to capture images using AxioVision software. 840 individual images were stitched together to capture a high resolution image (150 pixels per inch) of the entire sample using the AxioVision Mosaic software. 2.7. Stereo Microscopy. Stereo microscopy was conducted using a Zeiss Stemi 2000-C microscope for micro analysis of fracture faces. Two adjustable fiber optic lights were used to illuminate the samples and a color video camera was used to capture images using AxioVision software. 2.8. Scanning Electron Microscopy (SEM). Samples were carbon coated and then mounted onto aluminum stubs using conductive silver dag. Samples were stored in a desiccator until analysis. SEM analysis was conducted at the Electron Microscope and X-ray Unit located at the University of Newcastle. Images were recorded using a Zeiss Sigma VP Field Emission SEM equipped with a Bruker light element SSD EDS detector, using an accelerating voltage of 1.2−5 kV for secondary electron (SE) images, and 12−15 kV to record images using the backscatter electron (BSE) detector. The SE images only are shown in this paper.
2.3. Preparation of Polished Coke Blocks. Coke samples were prepared from 39-mm-diameter-cored cylinders, which were cut to a maximum height of 10 mm using a diamond saw. The coke discs were then embedded in circular polyester resin mounts and polished to a 3 μm finish using an automatic polisher. 2.4. Coke Petrography. Coke petrographic analysis was carried out at Pearson Coal Petrography Inc. (Victoria, BC, Canada) using a previously established method.17 Imaging was performed using a Zeiss reflected light microscope equipped with a rotating polarizer in the incident light path. The mean maximum reflectance (Romax) of each pixel was combined with the bireflectance to produce a false colored masked interpretation of each pixel in the image using algorithms developed by Pearson Coal Petrography.17 The individual images (recorded at a resolution of 1 μm per pixel) were stitched together to form a mask mosaic image, which represents a classification of each pixel in the image into one of several carbon forms. Regions colored in (i) pink depict uniaxial negative carbon (i.e., carbon forms with low bireflectance and high Romax); (ii) violet show areas of isotropic carbon including unfused inertinite and carbon from very low rank vitrinite; and (iii) dark blue, green, and red correspond to fused vitrinite with low, medium, and high bireflectance, respectively. 2.5. Tribology Testing of Coke Samples. Rotational ball-on-disc experiments were conducted using a CETR tribometer, equipped with a load cell (DFH-5, load range 0.5−50 N) and a rotational stage (model S21M0) (Figure 1). The polished coke blocks were petrographically analyzed prior to tribological experiments. Samples were manually ground flat on the unpolished side to within 100 μm prior to experimentation. Holes were then drilled into the samples to fit the rotational stage of the tribometer. A ruby ball (4 mm diameter) was used as the indenter. Trial experiments determined the optimum experimental parameters, which were kept constant between experiments. The coke samples tested were placed on a disc rotating at 7.5 rpm, with the ruby ball in contact with and moving across the sample under a constant 30 N load.11 The experimental duration was 10 min, which was sufficient to generate a clear wear track visible by eye in the samples tested. The optimum experimental duration depended on the applied load and the revolution rate, and was determined by the extent of wear to the
3. RESULTS AND DISCUSSION 3.1. Frictional Coefficient Values. The frictional force between two opposing surfaces, i.e., the resistance of the two surfaces to relative motion, indicates the susceptibility of a test material to tribological wear. The frictional coefficient (COF) is a value which links the frictional force between two opposing surfaces to the “normal reaction” between opposing surfaces. The COF values obtained from the frictional force measured during tribology experiments thus give an indication of the wear properties of the coke,11 and were calculated for each of the pilot oven coke blocks studied. Rather than being a material property, the COF values are a reflection of the properties of the tribological system, i.e., both the test material and the indenter. The COF value also depends on several additional 1423
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Energy & Fuels factors, including the degree of lubrication, surface roughness, and surface chemistry. Table 3 shows the average COF values recorded over 8 s (i.e., 1 complete revolution), 5 and 10 min (i.e., the total Table 3. COF Values Measured for Each of the Pilot Oven Coke Samples as a Function of Experimental Duration, and I20 and I10 Tumble Drum Indices Measured for the Cokes coke sample
COF 8s
COF 5 min
COF 10 min
A1 A2 B2
0.144 0.143 0.141
0.167 0.163 0.162
0.180 0.169 0.177
C1 C2
0.158 0.152
0.193 0.184
0.206 0.202
I20 index
I10 index
77.2 77.2 Not measured 80.4 80.4
19.1 19.1 Not measured 18.1 18.1
experimental duration) time periods. The COF values rose with increasing experimental duration due to (1) the rise in friction with surface roughness and (2) the greater extent of erosion due to particle generation (through removal from the microstructure) and the surface asperities created through wear. However, no signif icant differences were observed on comparison of the COF values of the different cokes. The COF values were also compared to the IRSID tumble drum indices measured for cokes A and C; however, there was no clear correlation between the I10 abrasion index and its COF (see Table 3). Higher loads and testing a larger sample number for each coke may allow distinct differences between cokes to be observed, and will be investigated in further testing. In this paper, we have therefore focused on image analysis and the information the images provide for characterizing the coke textural interfaces. We present our approach and qualitative findings on pilot oven cokes from single coals. 3.2. Types of Wear Observed in Metallurgical Coke. The wear which occurs during rotational tribology tests on metallurgical coke is due to the progressive loss of surface material at the points at which the two surfaces (polished coke block and indenter) come into contact as they rub against each other. Abrasion, adhesion, and delamination were observed in the pilot oven cokes studied. Examples of these wear modes are shown below, using images acquired using different analytical techniques to illustrate. A stereo micrograph of a section of a coke B sample is shown in Figure 2, and illustrates that tribology can be used as an effective tool to distinguish between inerts of varying hardness. The inert hardness will in part determine the mechanism and energy with which a crack will propagate when it reaches that inert. In the case of the labeled inert, thinning of the wear path was observed in regions where the ball had passed over the more dense or hard areas of the inert. If the inert is hard or has weak grain boundary properties, i.e., a weak interaction with the RMDC, then the crack will propagate intergranularly, i.e., via the boundary of the inert. Conversely, if the inert is soft or shows a strong interaction with the RMDC then the crack will be more likely to propagate transgranularly, i.e., through the inert. In the case of a strong interaction with the RMDC, the inert may act as a crack terminator, or decelerate the growth of the crack, thereby strengthening the coke. As discussed in our previous papers on fractographic analysis of coke, cracks will follow the path of least resistance,8,9 and the mechanism by and
Figure 2. Stereo micrograph of a section of a coke B sample, showing the wear track generated during the tribology test.
rate at which a particular crack will grow depends on the total energy available as well as the precise nature of the coke microstructure. An example of adhesive and delamination wear in a coke C sample is shown in Figure 3. The combined wear resulted in
Figure 3. SEM micrograph showing an example of adhesive and delamination wear in a coke C sample.
removal of an inert from the sample surface during tribology testing, evidence for which was the existence of a cavity (∼50 μm) in the wear track. The adhesive wear is a consequence of weak grain boundary strength, while the delamination wear gives a further indication of the relatively weak RMDC−IMDC interaction strength as well as the difference in hardness between these two textures. The smooth finish to the left-hand side of the cavity indicates that interaction between the RMDC and the inert was virtually nonexistent in this region. The righthand side of the cavity has a much rougher finish, implying that the inert had been “torn” from the coke microstructure with a significant load required to break the bonds between the RMDC and IMDC in this region. Better understanding of the nature and strength of the interactions will allow for a more developed understanding of the coke microstructure and strength under varying loads. Moreover, prevention or reduction of this type of wear through stronger RMDC− IMDC bonding would help to increase the resistance of coke to 1424
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Energy & Fuels abrasive wear. This is important since the generation of fine material severely impedes the flow of molten iron and gases in the blast furnace and thus the efficiency of its operation. Another section of a coke B sample is shown in the stereo micrograph in Figure 4a, in which a cavity (∼0.5 mm2) was
(i.e., the degree of interfacial contact between the inert and the surrounding RMDC), the strength of inert interactions with the RMDC, and is also correlated with reduced coke compressive strength.9 Through further research it will be investigated whether reducing the agglomeration of pores at the interface of inerts strengthens the coke by helping to prevent adhesive wear of these inerts. Also of note is the propagation of a crack (>5 mm) from this cavity during tribology testing. This crack propagated via the linkage of adjacent pores in the RMDC and also grew through an inert (see Figure 4a). A high magnification SEM micrograph of this cavity is shown in Figure 4c. In addition, a high resolution stitched optical image of the wear track in the entire coke B sample is shown in Supporting Information Figure S1. 3.3. Further SEM Analysis of the Wear Mechanisms. The difference in the wear properties of the different coke textural constituents is highlighted by Figure 5. The labeled
Figure 5. SEM micrograph of a coke C sample showing the different wear properties of RMDC and IMDC. Inset: Hackle lines visible on the labeled inert.
inert has not been worn compared to the RMDC in the field of view, verified by its smooth finish compared to the rough surface appearance of the RMDC. This inert also shows a weak interaction with the RMDC, since it is only partially bound. Of additional note is the presence of hackle lines on this inert, revealed in the inset recorded at higher magnification. As discussed in our previous publications on fractographic analysis of coke,8,12 these are generally only visible on homogeneous areas, and can indicate the direction of fracture propagation and the magnitude of the associated stresses. However, at present, there is no evidence that the markings seen in Figure 5 were generated during the tribology test. It is highly probable that they were the result of previous crushing during coke making. A high magnification view of the floor of a cavity in the wear path of a coke B sample is shown in Figure 6. The smooth appearance of this cavity floor with evidence of fracture faces suggests that a thin surface layer of an inert at which large pores were agglomerated at its interface was removed from the structure due to predominantly adhesive wear during the tribology test, leaving behind the cavity. The topography of the cavity floor shows that pores were agglomerated within a thin plate-like structure in this region, as has also been seen in our fractographic analysis of certain cokes.12,13 Smooth areas were seen in the RMDC comprising the cavity floor, with evidence of flow lines which highlight the dynamic nature of the plastic stage of coke making. Fracture faces (labeled, and shown at higher magnification in Supporting Information Figure S2)
Figure 4. (a) Stereo micrograph of a section of a coke B sample, showing a cavity in the wear track. (b) (different scale) Corresponding petrographic mask mosaic colored image recorded before the tribology test. (c) SEM micrograph of the cavity labeled in (a).
identified in the wear path. Comparison with the petrographic mask mosaic image, which is a false colored reflectance map showing the degree of anisotropy of the different carbon domains in the coke, recorded prior to the tribology test (Figure 4b), reveals that the cavity is the result of adhesive and abrasive wear leading to the removal of an inert from the microstructure. Close examination of the petrographic inset shows micropores agglomerated at the interface of the inert indicating that it was an area of weakness in the coke microstructure. Its low mean maximum reflectance indicates the inert is isotropic and this implies that the inert did not soften and remained largely structurally unaltered during coke making. Our previous fractographic research (discussed further in section 3.4) has indicated that porosity at the RMDC−IMDC interface is one of the major factors in reducing inert wetting 1425
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the hard ruby ball scratching the softer coke surface, resulting in the generation of loose pieces of coke which caused further abrasive wear of the coke surface. In the majority of cases, the dominant wear mechanisms at the beginning of the test were adhesion and delamination, resulting in the production of particles which most likely became confined at the interface between the indenter and the surface, leading to so-called “three-body abrasive wear”.11 The damage to the coke microstructure is from the detached particles which then act as a contaminant at the interface between the interacting surfaces leading to increased gouging and surface damage of the microstructure due to the particle which is approximately the same hardness as the remaining microstructure. It should also be noted that no damage or wear to the ruby ball occurred following the 18 × 10 min tests, showing it is a suitable indenter for performing tribological analysis of metallurgical coke. In addition, the influence of the resin on the wear and fracture mechanisms was determined to be negligible, and as expected, test parameters, including load, speed, and experimental duration, were found to be critical to the extent and nature of the wear generated in the coke samples. 3.5. Linkage of Coke Tribological Wear Mechanisms to Other Coke Strength Measures. In the preliminary tribology tests presented in this paper, the combination of the difference in hardness between some inerts and the surrounding matrix as well as the adhesive and abrasive wear at the inert−RMDC interface led to fracturing in the coke microstructure and the removal of these inerts from the microstructure. In all three cokes studied, the displacement of inerts in this manner was observed, and was facilitated by the agglomeration of pores at the inert interface, evidence for which was given by the smooth surface of some of the cavity floors, and the pretribology petrographic analysis. This links well to our previously published fractographic analysis of these cokes,9 in which the agglomeration of pores at the inert interface was a dominant weakness in the microstructure of all three cokes. One of the differences observed in this tribology study was the depth and size of the cavities generated by inert displacement, with a number of cavities greater than 250 μm2 visible in the wear path of the coke B samples tested (example shown in Figure 4), while there were no cavities of comparable size present in the coke A and C samples tested. This again links well to our fractographic analysis of these cokes, in which the inert size range at the fracture faces was found to be higher in coke B than either coke A or coke C, as shown by the lack of overlap in the 95% confidence intervals for the mean for this parameter, assuming it is normally distributed.9 In future analysis, use of an optical profilometer will assist in quantifying the size and depth of the cavities generated by inert displacement. Another important step for this work will be to quantify the extent of each wear mechanism observed in each coke, in order to clarify and compare the wear modes to the breakage mechanisms identified in quantitative fractographic analysis of the same cokes.9 For example, the area of pore wall collapse and agglomerated voids observed in coke A fracture faces was found to be substantially higher than in coke C, which in turn was higher than in coke B. This implies that coke B has greater abrasion resistance than the other two cokes. Testing of a greater sample number is required to identify differences in the wear behavior between cokes, as well as quantify those
Figure 6. SEM micrograph of a coke B sample showing the floor of a cavity.
were also evident, suggesting that there were regions where the plate-like structure of the RMDC was bound to the IMDC surface layer, forming the cavity evident in the micrograph upon adhesive removal of the inert. The differing hardness of both the IMDC and the RMDC within an example coke A sample is highlighted in Figure 7.
Figure 7. SEM micrograph of a coke A sample showing the differing hardness of both the RMDC and IMDC.
This figure also exemplifies the varying interaction properties between these two distinct textural constituents. Inert 1 is well bound/fused to the RMDC and we can also surmise that it is relatively hard since it did not undergo abrasive wear or microcracking during tribology testing. Inert 2, in contrast, fractured transgranularly (examples labeled) during the tribology test, indicating that it was at least partially well bound to the RMDC since it was not removed from the microstructure; however, there was also evidence of intergranular crack growth (labeled) at its interface with the RMDC. Microcracking within the RMDC was observed as a result of its abrasive and delamination wear (example labeled). Of additional significance was the greater extent of wear to the RMDC at its interface with this inert, as would be expected based on their contrasting hardness. 3.4. Discussion of the Observed Coke Wear Mechanisms. In our tribology tests, abrasive wear was the result of 1426
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differences and compare them to traditional coke strength measures, for example, tumble drum indices. When combined with quantitative elemental mapping, there is potential for tribological analysis of coke to become a powerful tool in better understanding (i) the wear properties of a range of cokes of varying coal properties, (ii) how these properties relate to the physical and chemical properties of the specific coal macerals, and (iii) how to optimize blending of coals in order to remove or reduce the occurrence of specific wear mechanisms in the resulting coke. Parallel research in progress is studying the wear mechanisms of blast furnace coke collected from different locations of a tuyere-drilling, to investigate the effect of the passage of coke through the furnace on its wear characteristics.
We gratefully acknowledge BHP Billiton and the Australian Coal Association Research Program (ACARP − project C22036) for funding this research work. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to David Phelan at the Electron Microscope and X-ray Unit, University of Newcastle (UoN), for advice and training in SEM analysis, Prof. Liangchi Zhang, Department of Mechanical and Manufacturing Engineering, University of New South Wales (UNSW) for access to his tribology equipment, and the technical assistance of Sanjeewa Herath and administrative assistance of Evan Yang (UNSW) during tribology tests. We are also grateful to Pearson Coal Petrography for the analysis of coke microtexture, Robert Fetscher and Leanne Matthews (UoN) for assistance with sample preparation, and Gareth Penny (UoN) for assistance with optical microscopy.
4. CONCLUSIONS The key findings of this preliminary study are as follows: Tribology can be used as an effective tool to distinguish between inerts based on their differing hardness and/or the strength of their boundary interface with the RMDC. This demonstrates the potential to provide an input to improve predictions of coke strength through greater understanding of the properties of the parent coals, in particular, the chemical and structural composition of the different inerts, and how this influences the nature and strength of the inert interactions with the RMDC. In this study, the observed tribological wear and fracture mechanisms were linked to high resolution coke petrographic images, and thus the structural forms of carbon present within the different textural constituents. Through further understanding of the link between coke microtexture and both textural hardness and interface strength, and subsequent linking to the original coal properties, we anticipate being able to apply the findings of tribological testing to predict the inerts in the coke microstructure that will be susceptible to adhesive wear (through weak grain boundary properties) and/ or abrasive wear (through determination of inert hardness relative to the surrounding RMDC). Further development of this technique will require an effective quantification.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b02902. (1) High resolution optical image of a coke B sample showing the wear path through the entire sample; (2) fracture faces at the floor of a cavity generated by displacement of an inert (PDF)
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REFERENCES
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[email protected]. Tel.: +61 414260361. ORCID
Richard Roest: 0000-0002-7018-5472 Author Contributions
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