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Testwork Enhances Graphite Electrode Performance16.10.2019 | 15:25 Uhr | GlobeNewswire
TORONTO, Oct. 16, 2019 - Hexagon Resources Ltd. (ASX:HXG) (“Hexagon” or the “Company”) has completed key testwork directed at enhancing graphite electrodes used in electric-arc furnaces (“EAF”) worldwide. Graphite electrodes are an essential part of the EAF steel production process and comprise a significant portion of cost. The market for these graphite electrodes is growing and any technology advances that will extend their service life and lower consumption rates have significant market appeal. Hexagon is pleased to report the results of its successful preliminary downstream technical development work on the addition of its treated natural graphite as an additive to synthetic graphite electrodes utilized in the high-growth EAF market, to manufacture steel. Testing of flake graphite from its McIntosh Project treated with a proprietary ingredient and branded as “Performance+”, has demonstrated a positive and direct correlation between the addition of Performance+ and increased electrical conductivity and durability in synthetic graphite electrodes. EAF steel producers are the dominant consumers of graphite electrodes, accounting for 90% of all production (GrafTech International Ltd., 2019). With continued strong demand, prices are 135% higher than they were in Q1 2017 (Roskill, 2019). Graphite electrodes are consumed every 8 to 10 hours in EAF steel production and are therefore an essential input, the purchase of which alone accounts for 3 to 5% of steel manufacturing costs (GrafTech International Ltd., 2019). Testing indicated that Hexagon was able to successfully extend the service life of graphite electrodes by reducing electrode lateral consumption/erosion (oxidative degradation). This was achieved by pre-treating its purified graphite with the Company’s proprietary coating, then subsequently mixing the performance-enhancement additive with synthetic graphite to manufacture graphite electrodes. Extending Electrode Service Life and Lower Energy Consumption. With Hexagon’s specialized natural-graphite performance additive for EAF graphite electrodes, the Company was able to demonstrate reduced energy consumption whilst minimising electrode consumption in normal EAF operations. Graphite electrodes have high thermal shock resistance (structural integrity) and are used to conduct electricity, while maintaining the ultra-high temperatures (thermal conductivity) of molten steel during EAF steelmaking. Hexagon’s technical development work demonstrated a consistent increase in electrical conductivity and lower coefficient of thermal expansion, thereby maximising electrical efficiency and reducing energy consumption. Hexagon’s Managing Director, Mike Rosenstreich commented, “the results for Performance+ highlight the potential for reduced downtime and lower power consumption leading to reduced costs and smaller carbon footprints in the smelting industry. These are essential challenges facing the steel industry and leading to a major transitioning to EAF furnaces in China for example. This strongly endorses our strategy of seeking high-value, deep-market opportunities for our natural graphite as set out in our recent downstream scoping study. Indeed, we are focused on executing that strategy utilising key elements of the scoping study and the vital technical know-how gained by studying the natural graphite flake from the McIntosh Project, doped with a specific ingredient, which together, offer cost efficiencies in steelmaking.” KEY POINTS
Figure 1: https://www.globenewswire.com/NewsRoom/AttachmentNg/9f3a0de1-a458-475b-8ca8-58d1c92827ae ______________________ 1. COMMENTARY Electric-arc furnaces (“EAFs”) are used to manufacture steel and are regarded as the most efficient and environmentally sustainable production technology currently available. Due to unique physical properties, graphite electrodes are a critical, non-substitutable industrial consumable in EAF-based steel production. With an average selling price of approximately US$10,000 per tonne, worldwide graphite electrode production capacity was approximately 800,000 tonnes in 2018, forecast to reach 850,000 tonnes in 2019 (GrafTech International Ltd., 2019). Roskill (2019) reports that electrode production consumed approximately 750,000 tonnes of synthetic graphite in 2018, consistent with Graftech’s production estimates and underpinning the deep nature of this market opportunity Made from a petroleum coke precursor, synthetic graphite is engineered to exacting specifications with high purity and predictable electrical, thermal and mechanical properties, but is less conductive and significantly more expensive than natural graphite. Unlike synthetic graphite, natural flake graphite cannot be sintered (meaning, formed into blocks) and can therefore only be utilized as an additive for electrode applications. However, given the significant size and strong, enduring demand profile of the graphite electrode industry and in keeping with Hexagon’s stated focus on producing downstream highly specialized industrial and energy graphite products, the Company sought to develop a natural-graphite additive to enhance the electrical performance of the synthetic graphite electrodes. In addition to increasing electrical performance, Hexagon sought to extend the service life of graphite electrodes by partially inhibiting electrode decomposition through building an oxidation-resistant layer. Oxidation is the primary limitation to the operational life of graphite electrodes. 2. ‘PERFORMANCE+’ — DEVELOPMENT OBJECTIVES High-quality graphite electrodes have low electrical resistivity and strong durability. Hexagon believed it could improve both these performance characteristics by hybridising a natural-graphite additive in synthetic graphite electrodes. Purified natural flake graphite exhibits a much higher crystalline structure than synthetic and is therefore more electrically and thermally conductive. The fewer the elemental impurities in the graphite, the better its electrical conductivity. With this understanding, Hexagon sought to achieve maximum electrical conductivity utilising ultra-high-purity natural flake graphite, as opposed to non-purified materials. Graphite electrode consumption during smelting is a significant component of the cost of steel production in EAF operations. Increased electrical conductivity holds the potential to reduce consumption losses in electrodes by utilising higher voltages and lower currents (long-arc operation). To further enhance electrode service, Hexagon developed a low-consumption-rate antioxidant pre-treatment coating in order to reduce electrode consumption losses due to lateral oxidation. Oxidative degradation involves the disintegration of macromolecules by the action of oxygen on the graphite substrate of the electrode. Oxidation causes graphite to be consumed or burned off during use in the smelting process. When heated in air at elevated temperatures (e.g. when graphite electrodes are dipped into an EAF’s molten metal) graphite burns (or gets oxidized), leading to the formation of volatile carbon dioxide. This parasitic loss of carbon as volatile gases is the primary limitation to the operational life of graphite electrodes. By lowering the linear coefficient of thermal expansion (“CTE”), Hexagon’s antioxidant pre-treatment coating of the natural flake, improves electrode structural integrity by increasing density (reducing porosity). A low CTE minimizes electrode consumption by maximizing efficient use of electricity in the EAF while maintaining its structural integrity. Figure 2: https://www.globenewswire.com/NewsRoom/AttachmentNg/d71ac590-51f9-4ceb-b1c3-b92bb4691b49 3. BACKGROUND — INDUSTRIAL APPLICATION To provide context to the significance of the results described above and detailed further below it is useful to provide a brief overview of the industrial electrode manufacturing process and EAF utilisation in steel making — whilst acknowledging other EAF furnace applications may also be relevant. Manufacturing of graphite electrodes is a highly technical industrial process with very demanding technical specifications that require compliance to ensure efficient EAF operations. In size, the electrodes maybe up to ~81 cm (32 inches) in diameter, more than ~3.4 m (11 feet) in length and can weigh more than 2.6 tonnes. The manufacturing timeline is between 3 to 6 months. Operationally, electrode consumption varies between 2 to 3 kg per tonne of steel and 8 to 10 hours of production. 3.1 Graphite Electrode Manufacturing The graphite electrode manufacturing process includes the following main processes set out below with reference to the testwork samples:
3.2 EAF Steel Industry EAF steelmaking grew at an annual pace of approximately 14% in 2017, compared with 4% for steelmaking overall. As a result of the increasing global availability of steel scrap and the more resilient, high variable cost and environmentally friendly EAF model. Electric vehicle (“EV”) battery demand for petroleum needle coke has constrained supply and pushed market prices higher. Roskill reports (2018) that EAF steel production accounts for approximately 27% of global production, but only 7% of Chinese steel production. There is a positive growth outlook particularly in China where government initiatives are forcing a transition to EAF smelting from the historical dominance of basic oxygen furnace (“BOF”) steel producers. These initiatives are the result of efforts to eliminate excess steelmaking production capacity and to improve environmental conditions. The EAF method produces approximately 25% of the carbon dioxide (or CO2) emissions of a BOF facility and does not require the smelting of virgin iron ore or the burning of coal. Additionally, as a result of significantly increased steel production in China since 2000, the supply of Chinese scrap is expected to increase substantially, which may result in lower scrap prices and provide the Chinese steel manufacturing industry with local scrap feedstock that was not historically available. Hexagon believes these trends will allow EAF steel producers to increase their market share and grow at a faster rate than BOF steel producers, resulting in increasing demand for graphite electrodes, which in turn, will create a potentially significant demand and commercial adoption for EAF electrode enhancement additives that reduce costs and increase performance in EAF electrodes. 4. TEST RESULTS Performance+ testing in graphite electrodes indicated a direct relationship between the amount of natural graphite added to the electrode matrix and increased performance of several electrode properties, including (refer Tables 1-3):
As weight percent addition of natural graphite to synthetic increased, the density of electrodes increased reaching an impressive 1.62 g/cm3 at 2.5 wt.% flake addition to the electrode mix. Further, testing consistently outperformed the all-synthetic control in density and conductivity. The following section discusses three key testwork parameters; Bulk Density, True Density and Electrical Conductivity. 4.1 Bulk Density The best result was a 4.5% improvement from 1.55 g/cm3 to 1.62 g/cm3 for the 2.5% Performance+ addition. Density can be both an indicator and result of the particle size, strength and porosity inherent in a specific graphite material, because the larger the particle size and more openings filled with air, the lower the density. The density of graphite can be adjusted by the raw materials, formulation and manufacturing processes used to create the specific material grade during initial production. The finished graphite material's density may also be increased though the use of additives and impregnations that will fill in the open porosity of the base graphite material. Porosity is an undesirable phenomenon in electrodes as porosity leads to reduced electrode density, and typically, lower mechanical strength and electrical conductivity. When graphite flakes have been compressed under a high pressure, each of them will come closer and, consequently, the density of bulk graphite becomes higher. Higher density of bulk graphite leads to higher electrical conductivity because there is greater connectivity for the electrons to move across the graphite particle. In contrast, a lower density of the bulk graphite indicates a high level of voids which strongly reduces electron mobility, thus, resulting in a lower electrical conductivity of the bulk graphite. Bulk density is typically how this value is reported on most graphite material specification sheets. Table 1: Bulk Density Determinations.
4.2 True Density Data True density is determined by taking the mass of a particle and dividing by its volume, excluding open and closed pores. A constant value for a matter, true density is the density of the near net shape. True density in this case is a measure of how graphitic a material is. The best result was a 12% increase from 1.95 g/cm3 in the control sample to 2.18 g/cm3 with a 5% Performance+ addition. The high true densities suggest that the electrodes are made of highly graphitized material that should be as conductive as possible to furnish the best level of current carrying capability. By testing true density, the graphitization degree variable was effectively eliminated. The highest density recorded was the 10% addition of Performance+, although 5% is ideal when the mechanical strengths component is added to the overall equation. Table 2: True Density Determinations
4.3 Electrical Resistivity Data Electrical resistivity (also referred to as resistivity, specific electrical resistance, or impedance), is an intrinsic property that quantifies how strongly a given material resists the flow of electric current. A low resistivity indicates a material that readily allows the flow of electric current. Electrical conductivity or specific conductance is the reciprocal of electrical resistivity and measures a material's ability to conduct an electric current. The best result was a 25% enhanced conductivity with resistivity (the inverse of conductivity) declining from 11.88 μΩ·m in the control sample to 9.01 μΩ·m in the electrode, with a 5% addition of Performance+. Electrical conductivity of bulk graphite is regarded as a function of its volume density and temperature. In general, increasing the compression pressure mechanically reduces the gaps between carbon particles, directly enhancing the electrical contact. The electrical conductivity of graphite depends on the separation distance between each particle and the average particle size. The higher the concentration of Hexagon’s natural flake graphite in the composition of extruded shapes, the lower the resistivity. This demonstrates that Hexagon’s Performance+ additive has a positive effect on the conductivity enhancement phenomenon in graphite electrodes. At 5% and higher, test series became better than the synthetic control with greater percent addition of flake additive to the electrode formulation. Refined flake reached the resistivity level of the 3PI control formulation at 5 wt. % addition of flake to synthetic. Table 3: Resistivity Determinations
5. TEST METHODS 5.1 Electrode Manufacturing Graphite electrodes were produced by first mixing petroleum tar pitch suspended in a compatible solvent system, varying amounts of synthetic graphite, Hexagon unpurified or thermally purified graphite, and the doping additive to form a thick paste. The resulting paste was passed through NAmLab’s proprietary extruder mixer to form elongated rods of uniform diameter as shown in Figure 3. These rods were calcined (i.e. baked) at 900° C in an inert nitrogen atmosphere for 15 hours. Following calcination, the graphite rods were soaked in a solution of tar pitch for 1 hour to allow the pitch to fill any voids, allowed to air dry and then calcined again to convert the tar pitch into pitch coke. Depending on the specific samples, the soaking and calcination steps were repeated one to two more times (i.e. 2PI or 3PI) to ensure that all gaps within the rods are filled with pitch coke binder. Heat treatment makes electrodes harder, but after 2nd PI and especially 3rd PI they also gain strength. After completion of the final calcination step, the dry weight of the un-graphitized electrodes was measured prior to being graphitized at 2,800° C. Once retrieved from the furnace, the mass of the electrodes was measured to assess the electrode weight loss during the graphitization process. The resistivity of the resulting graphitized electrodes was tested in accordance with ASTM C611, which required the machining of electrodes using a lathe and precision cutting tools, to a length diameter ratio of between 6:1 and 4:1. The true density of 23 electrodes was measured using the Quantachrome Helium Multipycnometer. Since this test required powdered material, testing the true density of the entire electrode was not possible. Instead, the leftover shavings produced during machining of the uneven electrode ends were set aside and crushed; the resulting powder was then used for true density testing. True density is a measure of how graphitic a material is; the theoretical true density of pure crystalline graphite is 2.254 g/cm3, while a material with a true density of 1.9 g/cm3 is synthetic and moderately graphitized. It has been argued that the theoretical true density of graphite cannot be measured via helium pycnometry due to the particles’ porosity but crushing the electrodes and pressing the powder allowed for a more accurate true density reading. Figure 3: https://www.globenewswire.com/NewsRoom/AttachmentNg/55572589-6a0a-4eb5-9b84-e10baf2c98fd 5.2 Bulk Density Determinations The bulk density of electrodes has been determined by dividing the weight of the electrode after graphitization by its volume. Utilising NAmLab’s in-house extruder, the measured values were close to the bulk densities of industrially made electrodes, although some density values were slightly lower. The lowest bulk density recorded in the study was 1.31 g/cm3 with the highest value at 1.63 g/cm3(refer to relevant results in Table 1). For reference, industrially made electrodes range in bulk density from 1.58 to 1.65 g/cm3. All electrodes produced in this study had a fixed diameter of 2.54 cm and varying length, which is indicated by the varying weight data in Table 1. Electrodes after a single pitch impregnation (denoted as 1PI) and a single calcination, lost up to 18-23% of mass upon graphitization. Those electrodes had the lowest density in a given test series, clearly revealing significant amounts of open porosity and are not reported further herein, as they are not relevant to the industry sector. The electrodes denoted as 2PI had the lowest mass reduction of volatile matter during graphitization (typically, 0.5 to 5 wt.%). Their resultant bulk density values where notably higher, possibly as a consequence of effective filling of pores in the extruded shapes by pitch. Importantly, as mass additions of Performance+ natural flake graphite additive to the synthetic graphite increased, the bulk density of graphitized electrodes increased. 5.3 True Density Determinations As part of this study. NAmLab sought to determine the density (specific gravity) of graphite materials using an analytical method of gas (helium) expansion pycnometry. This is a widely recognized technique for precision determination of volume of crystalline matter — such as graphite electrodes. Helium pycnometer operates on a principle of gas displacement and the volume-pressure relationship (Boyle's Law). Helium pycnometry is expected to deliver the value of 2.266 g/cm3 at 293K for a 100%-pure monocrystalline graphite. A Quantachrome Instruments’ He/N2 gas Multipycnometer was used in this study. The samples comprised powders made from the crushed machine shavings of the 23 electrodes tested. A minimum of two true density measurements were taken for each electrode as a minimum, and the densities were averaged to determine the final true density values as presented in Table 3. 5.4 Electrical Resistivity The resistivity of graphitized electrodes has been assessed in accordance with ASTM method C 611-98, entitled: “Electrical Resistivity of Manufactured Carbon and Graphite Articles at Room Temperature”. According to the aforementioned test method, a low electric current is run through the graphite electrode to prevent sample heating while the voltage across a specified length of the surface of the graphite electrode is measured to enable calculation of the resistivity. To account for anisotropy in the cylindrical graphite electrode, this measurement is repeated following sequential 90-degree rotations of the electrode and/or by testing of a machined shape in the form of a bar. If testing a cylinder, these four measurements are then repeated using a reversed current flow and switched voltmeter electrode configuration to account for any material memory effects and voltage measurement biases. The result is 16 separate resistivity measurements that, when averaged, provide a representative measure of the graphite electrode’s resistivity. A 4-point resistivity test was conducted on the electrodes and the results presented in Table 3. Exploration Results and Mineral Resource Estimates Metallurgical Test Work Outcomes About Hexagon Resources Limited Learn more at www.hexagonresources.com Forward-Looking Statements Contact North American Media and Investor Relations Contact: Dieser Artikel stammt von Rohstoff-Welt.de
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