Tribological Testing of Metallurgical Coke - ACS Publications

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Tribological Testing of Metallurgical Coke: Coefficient of Friction and Relation to Coal Properties Hannah Lomas, Richard Roest, Tizshauna Thorley, Adam Wells, Hui Wu, Zhengyi Jiang, Richard Sakurovs, Sharna Wotherspoon, Richard A. Pearson, and Merrick R. Mahoney Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01339 • Publication Date (Web): 30 Oct 2018 Downloaded from http://pubs.acs.org on October 31, 2018

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Tribological Testing of Metallurgical Coke: Coefficient of Friction and Relation to Coal Properties Hannah Lomas,*,† Richard Roest,† Tizshauna Thorley,† Adam Wells,† Hui Wu,‡ Zhengyi Jiang,‡ Richard Sakurovs,║ Sharna Wotherspoon,† Richard A. Pearson,╫ 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, NSW 2308, Australia ‡School

of Mechanical, Materials, Mechatronic and Biomedical Engineering, University of

Wollongong, Northfields Avenue, Wollongong, NSW 2522, Australia ║CSIRO

╫

Energy, 11 Julius Avenue, North Ryde, NSW 2113, Australia

Pearson Coal Petrography, 1-740 Discovery Street, Victoria, BC, Canada

ABSTRACT

ACS Paragon Plus Environment

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Tribological testing and analysis of metallurgical-grade cokes were conducted to elucidate the nature of the surface of each coke and the influence of the surface on coke abrasion resistance. The coefficient of friction (COF) was contrasted between cokes of different coal origins. The results indicate that the COF increases with (i) parent coal vitrinite content and (ii) decreasing rank of the parent coal. The amount of ultrafine (i.e. < 10 µm at the longest dimension) material produced by coke during continuous rotational tribological testing increased as the rank of the parent coal increased. Above a parent coal mean maximum vitrinite reflectance of 1.37 %, the COF began to decrease slightly beyond the first 40 to 60 seconds of testing, which can be attributed to the anisotropic, graphitic ultrafines acting as a surface modifier or lubricant. This has implications for the abrasion resistance of coke under blast furnace conditions, in which graphitic ultrafines acting a lubricant would likely reduce the degradation rate of the coke. For the pilot oven coke from the blend examined, the COF at all stages of the experiment was approximately linear with blend composition. The exploratory study described in this paper indicates that the wear characteristics of inertinite maceral derived constituents (IMDC) and reactive maceral derived constituents (RMDC) are different and that the wear behaviour of the RMDC is rank dependent, whilst that of the IMDC is less sensitive to the rank of the initial coal.

1.

INTRODUCTION

Metallurgical coke is a macro-porous carbonaceous material that is required to be strong and resistant to chemical attack for its use in a blast furnace. The material is heterogeneous and contains different forms of carbon that are easily distinguishable under the optical microscope, known as microtextures. These microtextures can be conveniently grouped


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into two components: the reactive maceral derived components (RMDC), which are the solidified product of organic material that fuses and acts as a binder during coke formation, and the inertinite maceral derived components (IMDC), which are derived from the nonfusible and semi-fusible coal macerals that appear largely structurally unaltered by the coke making process. Coke is used as a permeable support to maintain the process of iron production in the blast furnace. To act as a strong support, it thus requires a high physical abrasion resistance. Coke mechanical strength tests are generally used to measure the resistance of a coke to size degradation and abrasion, but these tests are largely empirical in nature.1 Consequently, there are times where these tests fail to accurately predict the strength and behaviour of a coke, in particular the behaviour of coke in the blast furnace. Thus there is a need for further research into an assessment of coke quality that can be applied to industry. The mechanisms by which coke degrades include both volume breakage and surface breakage.2 The former is often due to the propagation of existing fissures3 or flaws in the coke, whilst surface breakage is typically the result of abrasion to the coke, for example when coke lumps move against each other in the blast furnace. Research into the factors that influence the abrasion resistance of a coke of specified parent coal properties has been limited. The abrasion resistance of a coke is typically measured by industry using one of a number of tumble drum indices.2 However, these indices are an indirect measure of coke’s abrasive strength,2 and the majority of these indices, such as the Micum M10 index, are not a measure of “pure” abrasive strength, but are also influenced by the coke’s volume breakage strength on impact.4 Further, effective models to predict abrasive strength indices, such as ASTM hardness and the Irsid I10 index, from a variety of input variables including


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initial coal properties and coking parameters, have to date remained elusive.5 This suggests that there are factors controlling coke abrasive strength which are yet to be identified or are not available in the databases of coal properties and coking parameters typically used by industry to predict coke strength.5 Further, tumble drum tests to measure coke abrasion resistance do not pinpoint the reasons for the coke degradation, i.e. the mechanism(s) of degradation and the specific microtextural or microstructural feature in the coke responsible for the degradation. For example, the interfaces between the RMDC and the IMDC are often a source of weakness in the coke at which degradation can occur,6-8 which may be due to, for example, insufficient dilatation during coking.9 It is therefore essential to better understand the mechanisms by which coke abrasion occurs, the microstructural features or microtextural attributes in the coke that influence the coke abrasion, and the link to parent coal properties that influence coke abrasion. This will facilitate the design of coal blends to form a coke with stronger interface properties and/or higher abrasion resistance under the conditions to which the coke is subjected in the blast furnace. Recently, a tribological approach has been developed to elucidate the nature and strength of the microtextural components in coke and the interfaces between them.10 Our tribological testing approach uses a typical ball-on-disc test setup, in which a stationary ball indenter is under a controlled load in contact with a rotating polished block of coke. It is necessary to test polished flat (< 3 µm) samples of coke, as horizontally flat surfaces are important for repeatability of the tribological testing. The resulting wear track in the coke surface is then analysed to determine the degree and nature of the damage to the surface. The wear that occurs in rotational tribological tests is due to the progressive loss of surface


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material at the points at which the two surfaces (the polished coke block and the indenter) come into contact as they rub against each other. The setup for tribological testing of coke samples is shown in Figure 1a, and Figure 1b displays a schematic diagram of a rotational tribological test, in which Fz is the loading force and Fx the frictional force, which acts in the opposite direction to that in which the sample rotates. The tribological wear and fracture mechanisms are linked to the different carbon forms in a coke. There are three main wear mechanisms11 relevant to the analysis of coke cold strength that have been observed during tribological testing of coke10: adhesive wear is the transfer of material from the coke surface to the indenter; delamination wear refers to the subsurface cracks parallel to the surface; and abrasive wear a “scratching” and microcracking of the softer coke surface by the indenter. Abrasive wear becomes increasingly dominant as the tribological test progresses, as the ejected coke particles are transferred to the indenter and track surface and act as abrasion particles, causing further damage to the coke bulk surface. The relationship between each of these surface breakage mechanisms is clarified in Figure 2. The figure shows that both adhesive and delamination wear can increase coke abrasive wear. Like volume breakage, delamination is often the result of the growth of existing flaws in the microstructure; however, in tribological testing, the surface breakage effects of delamination can be significant due to the compressive force of the ball indenter on the surface as the sample is rotated. One of the key measurements that can be obtained from tribological testing is the coefficient of friction (COF). The frictional force between two opposing surfaces, i.e. the


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polished coke block and the indenter, shows the resistance of these two surfaces to relative motion, and indicates the susceptibility of the coke to tribological wear. The COF is calculated by dividing the frictional force by the normal (loading) force. A COF value of zero indicates a frictionless surface. The higher the COF, the greater the efficiency in transferring mechanical energy to the coke that can weaken or break its surface structure. In other words, at a constant loading force, a higher COF indicates that the frictional force is higher: in a tribological test, this higher frictional force is typically linked to increased damage to the surface structure of the material being tested. Rather than being a material property, the COF values reflect the properties of the tribological system, i.e. both the test material and the indenter. Thus its primary use is as a comparative tool. The COF value also depends on several additional factors, including the degree of lubrication,12,13 and surface roughness.11 Throughout this study, the tribological testing was thus consistently conducted in the absence of added lubricant and using horizontally flat, polished samples. Our previous study demonstrated that tribological testing can be used as an effective tool for measuring the microtextural interface characteristics and wear behaviour of metallurgical cokes from single coals of different properties.10 This was the first example of applying tribological testing to coke to better understand the strength of its microtextural interfaces. However, our earlier work found no significant differences between the COF values of the three cokes tested. This was due to the limit of the load cell capacity for the instrumentation available at the time. In this current paper, we apply a higher loading force to the coke samples during tribological testing to allow differences between cokes to be observed. These include differences in the measured COF value over time and differences


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in the severity of the damage to the coke surface in the tribological tests (assessed statistically using the Kruskal-Wallis test – data not shown). Moreover, in this present paper, we link the COF results and types of damage observed in rotational tribological tests to key parent coal properties. We assess the impact of (1) coal blending, (2) coal rank, and (3) coal petrographic composition, on (i) the COF over time under continuous rotational tribological testing of the coke samples, and (ii) the degree and nature of the damage to the surface of the coke samples during tribological testing. 2. MATERIALS AND METHODS 2.1. Coal and Coke Selection Five pilot-scale oven cokes and three laboratory-scale cokes were examined in this study. Four of the pilot oven cokes were formed from single coals and the other was formed from a 50:50 binary blend of two of the single coals. These cokes were produced in two separate pilot coke ovens, the conditions for which can be found in reference.14 The main difference between the two pilot-scale ovens was their size. Cokes LRHV, LRHV 50:50 HRLV and HRLV were produced in a 7 kg coke oven, whereas MRHV and MRLV were produced in a 400 kg coke oven. The laboratory-scale oven cokes were produced in a custom-built soleheated oven15-19 from blends of maceral concentrates, produced via the density-based separation of two coals (see Section 2.2). Table 1 lists the key parent coal properties (i.e. the properties of the coals from which the cokes were made) for the pilot oven cokes produced from them, and coke strength indices, where measured. Parent coal properties for the laboratory-scale oven cokes are listed in Table 2. N.B. (1) Validation of laboratoryscale coke, produced in UoN’s sole-heated oven, through comparison with its pilot oven


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analogue, will be addressed in a subsequent research paper.19 (2) Sample nomenclatures are shown in Tables 1, 2 and 3. 2.2. Coal Washing Coals MRMV and HRMV were washed using the following bench-scale procedure to generate maceral concentrates. Water and calcium chloride dihydrate (CaCl2.2H2O) were mixed in 32 L containers to specific gravities (s.g.) of 1.26 g/cm3 and 1.34 g/cm3, verified using a hydrometer. A mesh-lined colander with an aperture of 0.318 mm was placed into the 1.26 s.g. bath. A portion of coal was put in the colander and agitated to ensure wetting and separation. The surface was repeatedly skimmed onto a tray, until all floating particles were removed; this was labelled the low density fraction. This process was then repeated in the 1.34 s.g. bath, with the removed particles labelled the medium density fraction. Remaining particles in the bath were considered the sinks fraction. The bath densities were monitored and the process repeated until all the coal had been washed. It should be noted that due to the mesh size of the colander, coal particles under 0.318 mm in size were not collected and discarded. Each fraction underwent a rinsing process over 15 minutes, split between being rinsed under a tap and sitting in a bucket of fresh water. Rinsed coal was drained for 5 minutes, with a sample of water being analysed by a pool strip to test the water hardness (calcium ion content), with a tap water control strip used for comparison. Each fraction was spread over a tray and placed in a fume cabinet. When dry, each fraction was weighed, bagged and tagged before being stored in a freezer until required. 2.3. Coal Sampling


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For blending of the maceral concentrates and petrographic, ash chemistry and proximate analysis, each coal fraction was sampled using manual increment division in accordance with Australian Standard AS4264.1-2009. For blending, the three fraction components required in each blend were weighed out and set aside, then thoroughly mixed together and stored in labelled zip lock bags in the freezer until required. For petrographic, ash chemistry and proximate analysis (data for the latter two analyses not shown) each of the washed coal fractions and the parent coal HRMV were sampled. (The equivalent data for the MRMV head coal was made available by the coal supplier). The 20-25 g samples were sieved, with oversized particles transferred to the mortar and pestle for manual grinding. The screening and grinding were alternated until the entire sample had passed the mesh, which had an aperture size of 1 mm. 2.4. Coal Petrographic Analysis Petrographic analysis of the HRMV head coal and the inertinite-rich and vitrinite-rich fractions for both coals MRMV and HRMV following the density separation washing was carried out by the Australian Laboratory Services (ALS), Queensland, Australia. Sample preparation and measurement was carried out at ALS in accordance with Australian Standards AS2856.1, AS2856.2 and AS2856.3. For each of the low density (LD) and medium density (MD) fractions the vitrinite content was markedly higher than in the sinks fraction. Conversely the sinks fraction showed an increase in the inertinite and mineral content. Both the petrographic results for the coals before washing and fractions after washing are shown in Table 2.


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2.5. Coal Blending Program The data from the petrographic analysis and an Excel solver routine were used to calculate the mass required of each fraction component for each individual blend. The blends comprised an inertinite-rich fraction from one coal and a mix of both of the vitriniterich fractions from a second coal. Each blend targeted a vitrinite content of 60 % of the overall mass, which resulted in the rank of the coal blends varying from 1.22 (for coke MRMVv:HRMVi) to 1.44 (for coke HRMVv:HRMVi). The coke code names and the constituent coal fractions used in the coal blends are shown in Table 3. 2.6. Preparation of Small-Scale Cokes using a Sole-Heated Oven Each coal charge had a final mass of approximately 340 g, which included a moisture content of 5 %. This resulted in a dry bulk density of approximately 830 kg/m3, similar to that used by MacPhee et al.20 The charge was compacted until a bed height of 50 mm was obtained. The formation of the bench-scale cokes using a sole-heated oven followed by annealing at 1000°C in a muffle oven is detailed elsewhere,17,18 and will also be described in our subsequent publication.19 2.7. Preparation of Samples for Tribological Testing An illustration of the following sample preparation procedure is presented in the Supporting Information Figure S1. Three lumps of each coke were mounted in an epoxy resin mixture comprising hardener and a red pigment (to facilitate optical imaging) in plastic containers. The set samples were then cut into 10-20 mm wide samples using a LECO drop down saw. Slices with maximum coke coverage were cored using a bench drill


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with a 40 mm diamond coring bit attached. The cores were ground back to fit inside 40 mm diameter plastic containers and remounted in resin. These remounted samples were then smeared with resin to ensure the resin filled all capillaries and pores. Using a PICO 155 Precision Saw the samples were cut to a 10 mm thickness whilst placed in a 40 mm holder to ensure the upper and lower surfaces were completely parallel, as this is crucial for tribological testing. A Struers TegraSystem Autopolisher was used to polish the coke samples to a < 3 μm finish. 2.8. Rotational Tribological Testing Rotational ball-on-disk tribological experiments were conducted using an Rtec tribometer equipped with loading sensors (Fx: FXH-1kN-ARM-238 and Fz: FZHA-1kN-237) and a rotational stage. Holes were drilled into the samples to fit the rotational stage of the tribometer. A photograph of the rotational tribological test setup is shown in Figure 1a. A ruby ball (4 mm diameter) was used as the indenter due to its high hardness. Each coke sample was attached to the rotational stage using a custom-made screw. During testing, the stage was rotating at 13.65 rpm for the tests at a 14 mm radius from the centre, and at 11.94 rpm at a 16 mm radius from the centre (these values were chosen to provide a linear speed of 20 mm/s). The ruby ball was in contact with and moving tangentially across the sample under a constant 80 N load. A diagram of the rotational tribology test is shown in Figure 1b. The duration of each test was 5 minutes. These conditions were kept constant between tests, and were selected on the basis of (1) earlier uniaxial compressive strength tests conducted on the same/similar cokes,21 and (2) the generation of a clear wear track in the samples examined via tribological testing. For the latter point, it was important that distinct


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differences in the width and depth of the tracks between cokes formed from coals with different properties were easily observed. The testing conditions differed from our previous publication10 as we were limited by the maximum loading capacity of the load cell available for those tests, therefore the conditions for the earlier tests were not optimal. All experiments were performed at room temperature and in the absence of added lubricant. Six tests per coke were conducted. 2.9. COF Analysis For each tribological test, the COF was calculated from the frictional force and loading force as the test progressed, accounting for the initial (static) frictional force when the indenter came into contact with the rotating sample. This initial frictional force varied depending on the angle at which the indenter came into contact with the sample surface, and thus affected the applied loading force. COF values were calculated for each rotational tribology test for 1 complete revolution (approximately 4.6 s, depending on the radius from the sample centre at which the test was conducted), 30 s, 60 s, 120 s, 180 s, 240 s, and 300 s (i.e. the total experimental duration for each sample). The mean COF values and standard deviation between tests were then calculated for each time interval for each coke from the six samples tested per coke, followed by subtraction of the COF contribution of the epoxy resin (see Figures 3, 4 and 5 and Table 4 for the results). The COF contribution of the resin was calculated as the mean COF of the resin multiplied by the mean porosity of the coke (expressed as a fraction), which was estimated via optical image analysis of the samples (see section 2.14). N.B. The samples comprising solely of epoxy resin were found to have a substantially


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lower COF than the cokes tested (see Supporting Information Table S1). This is due to a combination of the lubricity, hardness, and ductile nature of the resin. 2.10. Optical Microscopy High resolution optical micrographs were recorded both before and after tribological tests using a Zeiss Axio Imager.Z1m microscope equipped with an automatic stage and both high and medium resolution cameras were used to capture images using AxioVision software. Individual images (recorded at 5 x magnification) were stitched together to capture a high resolution image (150 dots per inch) of the entire sample using the AxioVision Mosaic software. The size of each stitched image was ~ 4070 * 4000 pixels. 2.11. Microtextural Analysis Coke microtextural analysis was carried out at Pearson Coal Petrography Inc. (Victoria, BC, Canada) using a previously established method.22 Imaging was performed using a Zeiss reflected light microscope equipped with a rotating polarizer in the incident light path. The maximum and minimum reflectance values for each pixel in the field of view were determined by rotating the polarizer on the microscope (18 times at 10° rotation per step) and recording the reflectance of each pixel at each step. The bireflectance values for each pixel were then obtained by subtracting the minimum from the maximum reflectance values. Quantitative identification of seven types of carbon was made from cross plots of coke bireflectance with coke maximum reflectance, using algorithms developed at Pearson Coal Petrography to distinguish between the different types. The results are shown in Table 5, and

gave an indication of the degree of anisotropy within the distinct forms of carbon in each coke.


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2.12. 3D Laser Scanning Microscopy A Keyence 3D laser scanning microscope (VK-X100 Series) with an automatic stage was used for 3D imaging (at 200 x magnification) of four points per wear track following tribological testing. Optical, laser and height profile images were recorded simultaneously. Analysis of the 3D images and profiles was carried out using VK Analyser software. 2.13. Scanning Electron Microscopy Samples were carbon coated using an SPI carbon coating unit and mounted onto aluminium stubs using a carbon tab. Conductive channels were added to the samples from their surface to the stub using carbon dag. Samples were then placed in an oven at 65°C for at least 30 minutes and stored in a desiccator until analysis. Scanning electron microscopy (SEM) analysis was conducted at the University of Newcastle Electron Microscope and X-ray Unit. 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 2-3 kV for secondary electron images. 2.14. Optical Image Analysis Optical image analysis was performed to measure the coke porosity using ImageJ software. The stitched optical image was converted to a monochrome image and thresholding was performed manually using the greyscale display. The overall porosity was estimated from the percentage area of the image that was above a set threshold on the greyscale. 3. RESULTS AND DISCUSSION


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3.1. Comparison between Cokes from Single Coals and a Binary Blend of those Coals The COF values during the first 30 seconds (s) of tribological testing were indistinguishable for cokes LRHV, HRLV and the coke from the LRHV 50:50 HRLV coal blend (see Figure 3), after accounting for the standard deviation between tests for each coke (see Table 4). This is highlighted by the similarity in the initial slope of the COF curves for all three cokes. With further rotation, the COF values for the three cokes started to differ: The COF for LRHV was higher than that for LRHV 50:50 HRLV, which in turn had a higher COF than HRLV. The difference in COF between these three cokes was accentuated with time as the tribological test progressed; however, beyond 200 s testing, the difference in COF between HRLV and LRHV 50:50 HRLV did not increase further. The actual COF over time for coke LRHV 50:50 HRLV was marginally lower than the COF obtained if the curves for LRHV and HRLV were simply averaged, i.e. if there was no interaction occurring between the components in the blend. From the data in Figure 3 and the values of the standard deviation in the COF measurements, we can conclude that the COF is approximately linear with blend composition in this case. Ascertaining whether the linearity is generally true would require the investigation of cokes from a greater number of blends and their constituent single coals. The increase in COF in the initial stages of the tribological test indicates increasing surface roughness caused by the formation of asperities, for example when large grains or particulates are displaced from the sample. These particulates will then break up into smaller pieces, and act as abrasives if they are harder than the rest of the coke (for example,


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if the particulates comprise mineral matter), thereby further raising the measured COF. The variation in the curves beyond the first 30 s of testing indicates that coke LRHV has a greater surface roughness after testing than coke HRLV. After several revolutions, the COF started to briefly level off for coke HRLV, indicating an equilibrium surface roughness had been reached. However, there was then a tendency for the COF of coke HRLV to decrease before stabilising again. The observed decrease in the COF for coke HRLV prior to stabilising was likely due to this coke being made from a coal of higher rank (with a mean maximum vitrinite reflectance (MMR) of 1.39 %) than the coal or coal blend used to form cokes LRHV and LRHV 50:50 HRLV, respectively. Research led by Andriopoulos2,23 showed that the percentage of ultrafines (< 10 μm, assessed via a Rosin-Rammler cumulative size distribution of the fines, measured using a Malvern Mastersizer E2) generated from coarse mosaic (in other words, relatively high anisotropy quotient (AQ), assessed using a pointcounting technique24) RMDC was greater than that generated by fine mosaic (low AQ) RMDC. This finding corroborated Sakurovs’ earlier ACARP study25 in which the presence of high rank coal in a blend decreased the abrasion resistance of the resulting coke. It was hypothesised that the large anisotropic domains in the cokes made from the highest rank coal used in that study flaked readily during extended abrasion testing, generating fines. This hypothesis was explored further by testing additional cokes from coal blends of relatively high rank (see Section 3.2). Under continuous tribological testing, graphitic ultrafines would act as a surface modifier or lubricant, thereby lowering the coefficient of friction as they are softer than the bulk coke and resin surface. This is explained further in Section 3.2.


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3.2. Comparison between Cokes from Blends of Coal Macerals with different Rank and similar Maceral Composition Beyond the first 40 s of testing, a decrease in the COF over time, was observed for two of the three cokes from blends of coal maceral concentrates, HRMVv:HRMVi and HRMVv:MRMVi (see Figure 4), before a levelling off towards the end of the test was observed. This follows the behaviour observed for pilot oven coke HRLV, which was also formed from a coal of relatively high rank (see Section 3.1). We can thus conclude that this behaviour is likely to be a consequence of the cokes being generated from coal or coal blends with a mean maximum vitrinite reflectance (MMR) of 1.38 % or above: The highlyordered graphitic structure within the RMDC of these cokes generates abraded graphitic ultrafines that act as a lubricant or surface modifier since they adhere to the indenter. This effect was accentuated for cokes HRMVv:HRMVi and HRMVv:MRMVi compared to HRLV due to the substantially greater proportion of the high bireflectance (ribbon) carbon form in the former two cokes (see microtextural analysis results in Table 5). It was noted during testing that the coke samples from coals or coal blends of relatively high rank powdered readily compared to cokes from lower rank coals. This finding is in agreement with both Andriopoulos’2 and Sakurovs’25 earlier studies. The former study showed that RMDC with a relatively high degree of anisotropy were able to absorb more energy on sub-micron indentation.2 This was attributed to microcracking, which, despite increasing the fracture toughness of this microtextural constituent, increased the generation of graphitic ultrafines. As mentioned above, Sakurovs and co-workers’ previous study25 found that the presence of high rank coal in a blend decreased the abrasion resistance of the


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resulting coke when the overall MMR and fluidity were kept constant. On the other hand, variation in the amount of the high rank coal did not affect other coke cold strength indices. We hypothesise that higher AQ RMDC is preferentially abraded via breakage between graphitic planes. Examination of the fine and ultrafine materials produced during tribological testing using either Raman spectroscopy or X-ray diffraction (XRD) would allow this hypothesis to be examined. However, an insufficient quantity of fine and ultrafine materials was collected from each tribological test to allow either of these techniques to be used for analysis. Based on the findings of this present study and the previous study conducted by Andriopoulos,2,23 we predict that the amount of ultrafine material generated by a coke during tribological testing is approximately linear with the proportion of high rank coal in the blend, if coal maceral composition is kept a constant. 3.3. Comparison between Cokes from Single Coals of different Rank and similar Maceral Composition As mentioned above, the fracture toughness of the RMDC was found to decrease with decreasing anisotropy in sub-micron indentation tests conducted by Andriopoulos et al9 (although the literature suggests some variation in relationships depending on the loading method). This decrease in fracture toughness presumably results in the RMDC being more susceptible to damage from the rotational load under tribological testing. Figure 5 shows the COF curves over time for two cokes from coals of similar petrographic composition but different rank (see Table 1): LRHV and MRHV. The COF beyond the first 30 s of testing is higher for coke LRHV from relatively low rank coal (MMR 0.85 %) compared to coke MRHV from medium rank coal (MMR 1.24 %). The


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higher COF for LRHV indicates that at constant loading conditions there is more efficient transfer of mechanical energy to this coke to break its surface structure, which may be attributed to its lower fracture toughness on the basis of the lower anisotropy of its RMDC, i.e. the higher proportion of the low bireflectance (circular) carbon form in coke LRHV (59.9 %) compared to coke MRHV (48.7%) (see Table 5). Moreover, the COF over time stabilises more rapidly for coke MRHV than LRHV: The COF stabilises within the first 90 s of testing for coke MRHV, whereas it continues to increase at the end of the 300 s testing period for coke LRHV. This may be due to the small proportion of anisotropic, graphitic ultrafines in the former coke (0.8 % high bireflectance (ribbon) form and 15.0 % medium bireflectance (lenticular) form in coke MRHV compared to 0.0 % ribbon form and 3.0 % lenticular form in coke LRHV (see Table 5)) acting as a surface modifier or lubricant, thereby retarding further damage and preventing the COF from increasing further. The different wear behaviours of the RMDC in cokes LRHV and MRHV are highlighted by the SEM images shown in Figure 6. At the surface of LRHV, the RMDC has broken up extensively into relatively large pieces of up to 100 µm at the longest dimension. Conversely, the RMDC of MRHV has undergone subsurface cracking, resulting in delamination. Again, we hypothesise that this is a consequence of its higher degree of anisotropy compared to the RMDC of LRHV.

3.4.

Comparison between Cokes from Single Coals of similar Rank and different Maceral Composition

Despite being formed from coals of similar rank, coke MRHV had a slightly higher COF than coke MRLV (see Figure 5), suggesting the former had a rougher surface following the


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five minute testing period. The optical micrographs shown in Figure 7 highlight that the rotational wear paths of MRHV after five minutes’ tribological testing were less consistent than those of MRLV. The greater surface roughness of MRHV after testing is likely due to the substantially higher vitrinite content of the coal used to form this coke (80 %) compared with coke MRLV (44 %), and thus the higher ratio of RMDC to IMDC in the former. IMDC are generally harder26 than medium AQ RMDC, and have also been shown to have higher abrasion resistance, due to greater cross-linking27 and thus fewer extended planes of weakness compared to the RMDC. It is thus plausible that there was less damage overall to coke MRLV than coke MRHV due to the greater inertinite content of the former coke. In the optical micrographs shown in Figure 7, there are numerous examples of thinning of the wear track at IMDC (examples labelled). The lower degree of wear to the IMDC compared to the RMDC is supported by concurrent findings that the severity of damage to IMDC is less than RMDC at comparable loading force,18 which will be the subject of a subsequent publication. Nevertheless, it should be noted that the larger variation in the width of the track for coke MRHV is not reflected in the standard deviation values. MRLV has a much larger standard deviation than that of MRHV at each of the time intervals for which it was calculated (see Table 4), as would be expected on the basis of it being a more heterogeneous coke than MRHV. The COF values for cokes MRLV and MRHV were compared to traditional abrasion indices determined via ASTM tumble drum tests. The ASTM hardness index and COF value showed opposing trends for these two cokes, with MRLV having a higher ASTM hardness index than MRHV (see Table 1). This further indicates that the abrasion resistance


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of MRLV is greater than MRHV. Comparison of the COF with the surface breakage rate constant,25 a test specifically designed as an abrasion measure,28 would be a further useful comparison. The higher abrasion resistance of MRLV compared to MRHV indicates that it is the IMDC that is resistant to wear rather than the RMDC (at comparable parent coal rank). We have qualitatively assessed the abrasion resistance of each of these microtextural components using a combination of optical image analysis (see Figure 7) and 3D laser scanning microscopy.29 Ideally, the COF of the IMDC and RMDC would be individually assessed quantitatively. Due to the difficulty in obtaining such a measurement for individual coke microtextures, we have instead assessed the severity of damage to both RMDC and IMDC as a function of parent coal properties, which will be detailed in a later publication. Figure 8 summarises the dominant factors contributing to the COF value during tribological testing of coke.

3.5.

Potential Advantages of a Tribological Approach to assess Coke Strength and Future Directions

Our tribological approach offers several potential advantages over current alternative techniques to assess coke strength. Unlike tumble drum tests, tribological tests can be used to distinguish between coke components in a test, and may be able to quantify the susceptibility of the different coke components to abrasion. One of the current issues facing the coal technical marketing industry is the accurate prediction of the abrasion resistance of cokes from coal blends. Tribological testing offers a promising approach to directly


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measure the influence of microtextural interfaces on abrasion resistance. Combined with coke microtextural analysis, tribological testing could also elucidate the abrasion behaviour at the interfaces between regions of different anisotropy within the RMDC itself, which are often a consequence of coal blending.

4. CONCLUSIONS AND DISCUSSION The coefficient of friction (COF) was contrasted between cokes of different coal origins using tribological testing and analysis. The amount of ultrafine material produced by the cokes during continuous rotational tribological testing increased as the rank of the parent coal or coal blend increased. The ultrafines appeared to act as a surface modifier or lubricant on continuous rotational tribological testing, which suggests that (i) the ultrafines are graphitic (i.e. anisotropic) in nature, and (ii) that the production of such ultrafines may modify the abrasion resistance and resistance to degradation of coke under blast furnace conditions. The results presented indicate that the COF increases with (i) parent coal vitrinite content and thus coke RMDC content, due to RMDC generally having lower hardness than IMDC, and (ii) decreasing rank of the parent coal, due to a combination of the reduction in RMDC fracture toughness with decreasing RMDC anisotropy and the hypothesised surface modifying / lubricating effect of the graphitic ultrafines produced by the RMDC of cokes formed from high rank coal(s).


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Finally, the results of this study show that, for the pilot oven coke from a coal blend investigated, the COF of the coke over time was approximately linear with the parent coal blend composition; however, the analysis of additional cokes from coal blends is required to test the generality of this relationship.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. The contents include (1) procedure of sample preparation for tribological testing; (2) mean COF and standard deviation of epoxy resin during tribological testing.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Tel.: +61 458181880 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources We gratefully acknowledge ACARP (project C25043) for funding this research work.


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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We gratefully acknowledge ACARP (project C25043) for financial support and the four coal companies that supplied the pilot oven coke and coal samples. We thank Robert Fetscher and Leanne Matthews, University of Newcastle (UoN), for their valuable assistance, including with sample preparation, Pearson Coal Petrography for coke microtextural analysis, and Xing Xing, UNSW Sydney, for advice on image analysis and estimation of coke porosity. The Electron Microscopy and X-ray Unit, UoN, is thanked for SEM access, with a particular thank you to Yun Lin.

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Diez, M. A.; Alvarez, R.; Barriocanal, C. Coal for metallurgical coke production:

predictions of coke quality and future requirements for cokemaking. International Journal of Coal Geology 2002, 50, 389-412. (2)

Andriopoulos, N.; Dukino, R.; Sakurovs, R. The Strength Controlling Properties of

Coke and Their Relationship to Tumble Drum Indices and Coal Type: ACARP Project C9060 Final Report 2002. (3)

Jenkins, D. R.; Shaw, D. E.; Mahoney, M. R. Fissure formation in coke. 3: Coke

size distribution and statistical analysis. Fuel 2010, 89, 1675-1689. (4)

Gibson, J.: Dilatometry and the prediction of coke quality, 1972.

(5)

North, L.; Blackmore, K.; Nesbitt, K.; Mahoney, M. R. Models of coke quality

prediction and the relationships to input variables: A review. Fuel 2018, 219, 446-466. (6)

Hiraki, K.; Hayashizaki, H.; Ogata, T.; Yamazaki, Y.; Matsushita, Y.; Aoki, H.;

Miura, T.; Fukuda, K.; Matsudaira, K. Analysis of Fracture Behavior of Coke with Non-


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adhesion Region Boundaries by Rigid Bodies-Spring Model. Tetsu-to-Hagane 2010, 96, 313318. (7)

Kanai, T.; Yamazaki, Y.; Zhang, X.; Uchida, A.; Saito, Y.; Shoji, M.; Aoki, H.;

Nomura, S.; Kubota, Y.; Miyashita, H. H. S. Quantification of the Existence Ratio of NonAdhesion Grain Boundaries and Factors Governing the Strength of Coke Containing LowQuality Coal. J. Thermal Sci. Tech. 2012, 7, 351-363. (8)

Roest, R.; Lomas, H.; Hockings, K.; Mahoney, M. R. Fractographic Approach to

Metallurgical Coke Failure Analysis. Part 1: Cokes of Single Coal Origin. Fuel 2016, 180, 785-793. (9)

Nomura, S.; Arima, T.; Kato, K. Coal blending theory for dry coal charging

process. Fuel 2004, 83, 1771-1776. (10)

Roest, R.; Lomas, H.; Hockings, K.; Mahoney, M. R. Tribological Approach to

Investigate the Interface Properties in Metallurgical Coke. Energy and Fuels 2017, 31, 14221428. (11)

Bhushan, B.: Introduction to Tribology; 2nd ed.; Wiley: New York, 2013.

(12)

Cheng, X.; Jiang, Z.; Zhao, J.; Wei, D.; Hao, L.; Peng, J.; Luo, M.; Ma, L.; Luo, S.;

Jiang, L. Investigation of oxide scale on ferritic stainless steel B445J1M and its tribological effect in hot rolling. Wear 2015, 338-339, 178-188. (13)

Xia, W.; Zhao, J.; Wu, H.; Zhao, X.; Zhang, X.; Xu, J.; Hee, A. C.; Jiang, Z.

Effects of Nano-TiO2 Additive in Oil-in-Water Lubricant on Contact Angle and Antiscratch Behavior. Tribology Transactions 2017, 60, 362-372. (14)

Mahoney, M.; Andriopoulos, N.; Keating, J.; Loo, C. E.; McGuire, S. Pilot scale

simulation of cokemaking in integrated steelworks. Ironmaking and Steelmaking 2005, 32, 468-478. (15)

Mahoney, M.; Roest, R.; Lomas, H.; Fetscher, R.; Jenkins, D. R.; Pearce, R.; Mayo,

S.: Examination of coke formation through microstructure of sole-heated oven tests. In 9th International Metallurgical Technology Trade Fair with Congresses and 2nd European Steel Technology and Application Days (METEC-ESTAD), 15-19 June 2015: Dusseldorf, Germany. (16)

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Development Of Coke Microstructure by 3D Imaging of Quenched Semicoke-Plastic LayerCoal Beds. In ICCS&T/ACSE Conference 27 Sept - 01 Oct 2015: Melbourne, Australia. (17)

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Undergraduate Research Project Thesis. University of Newcastle, 2016.


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coke and their relationship to tumble drum indices and coal type. In ISStech 2003 conference proceedings, 2003; pp 13-23. (24)

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TABLES Table 1. Key coal properties and coke strength indices for the pilot oven cokes. N.B. Coke strength indices were not measured for LRHV, HRLV and LRHV 50:50 HRLV. Coal Property

LRHV

LRHV 50:50 HRLV

MRLV

MRHV

HRLV

Mean Maximum Reflectance (%)

0.85

1.12

1.23

1.24

1.39

Vitrinite (%)

78.5

65.8

44.3

80.4

53.0

Inertinite (%)

13.8

28.9

53.0

15.3

43.9

Mineral (%)

3.3

3.2

2.7

3.6

3.1

ASTM Stability

59.2

57.3

ASTM Hardness

66.0

60.6

Coke strength index


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Table 2. Coal composition pre and post density-based washing. N.B. LD = low density fraction; MD = medium density fraction. Starting compositions

MRMV

HRMV

MRMV

HRMV

LD

MD

Sinks

LD

MD

Sinks

1.17

1.50

1.17

1.16

1.15

1.43

1.44

1.45

66.0

69.4

66.7

69.2

29.7

81.8

80.3

40

Liptinite (%)

0.4

0.0

1.2

0.8

1.8

0.0

0.0

0.0

Inertinite (%)

30.4

27.2

28.6

27.7

63

16

16.5

55.2

Mineral (%)

3.1

3.4

3.5

2.3

5.5

2.2

3.2

4.8

Mass of sample (g)

3000

3500

857

567

641

320

1118

958

28.6

18.9

21.4

9.1

31.9

27.4

Mean Maximum Reflectance (%) Vitrinite (%)

Yield (%)


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Table 3. Coke code names and composition of the constituent coal blends, for the smallscale cokes produced in a sole-heated oven.

Coke code

Vitrinite-rich coal fraction

Inertinite-rich coal fraction

HRMVv:HRMVi

HRMV

HRMV

MRMVv:HRMVi

MRMV

HRMV

HRMVv:MRMVi

HRMV

MRMV


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Table 4. Standard deviation in the COF measurements between tribological tests for each coke. Coke

Standard deviation between tests for the COF as a function of time 1 revolution

30 s

60 s

120 s

180 s

240 s

300 s

LRHV

0.02

0.01

0.00

0.01

0.02

0.02

0.02

LRHV 50:50 HRLV

0.01

0.01

0.01

0.02

0.02

0.03

0.03

HRLV

0.06

0.06

0.05

0.05

0.04

0.04

0.04

MRMVv:HRMVi

0.00

0.01

0.01

0.01

0.01

0.01

0.01

HRMVv:MRMVi

0.02

0.02

0.02

0.02

0.02

0.02

0.02

HRMVv:HRMVi

0.01

0.00

0.00

0.00

0.00

0.00

0.00

MRHV

0.01

0.00

0.00

0.00

0.00

0.00

0.00

MRLV

0.06

0.05

0.04

0.04

0.05

0.05

0.04


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Table 5. Coke microtextural analysis, conducted by Pearson Coal Petrography. Coke Component Isotropic (includes both inertinite derived and low rank vitrinite derived material) Fused inertinite (uniaxial negative) Pyrolytic carbon (uniaxial positive) Spherulytic carbon High bireflectance (ribbon form) Medium bireflectance (lenticular form) Low bireflectance (circular form)

LRHV

LRHV 50:50 HRLV

HRLV

37.1

27.7

22.3

19.2

16.6

0.0

0.8

1.0

1.6

0.0

0.0

0.0

0.0

0.0

0.0

MRMVv:HRMVi HRMVv:MRMVi HRMVv:HRMVi

MRHV

MRLV

17.6

35.4

29.2

5.0

4.3

0,1

0.5

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

1.9

2.2

5.2

16.9

17.8

0.8

2.5

3.0

18.2

31.0

35.5

32.0

35.2

15.0

27.9

59.9

51.5

43.5

38.4

29.6

25.1

48.7

39.9


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FIGURES

Figure 1. (a) Rotational tribological test setup using a RTEC tribometer, with a coke sample post-testing visible; (b) diagram of a rotational tribological test, in which Fz is the loading force and Fx the frictional force.


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Figure 2. Relationships between mechanisms of coke degradation during tribological testing.


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Figure 3. Mean COF over time for cokes from single coals, and the binary blend of those single coals (both actual results and averaged results using the curves for the cokes from the constituent single coals).


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Figure 4. Mean COF over time for cokes from blends of coal macerals.


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Figure 5. Mean COF over time for cokes from different single coals.


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Figure 6. SEM images of the wear tracks in samples of (a) coke LRHV and (b) coke MRHV, highlighting the differences in the mechanisms of damage to the RMDC.


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Figure 7. 2D optical micrographs of (a) coke MRHV, showing some variation in the wear paths for the rotational tribological tests, and (b) coke MRLV, highlighting the relative consistency in the rotational wear paths for this coke. N.B. The cores visible are to attach


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the samples to the rotational drive of the tribometer, and the scratches were other tests conducted on the samples (reported elsewhere29).

Figure 8. Dominant factors on which the COF is dependent during tribological testing of coke.


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(This graphic is for the table of contents)


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Tribological Testing of Metallurgical Coke - ACS Publications

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