MATERIALS TRANSACTIONS
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ONLINE ISSN: 1347-5320
PRINT ISSN: 1345-9678

MATERIALS TRANSACTIONS Vol. 55 (2014), No. 6

  • Thermoelectric Properties of a T-Shaped Quantum Dot with Relatively Strong On-Site Coulomb Interaction

    pp. 861-865

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    DOI:10.2320/matertrans.M2013460

    The thermoelectric properties of a correlated T-shaped quantum dot system symmetrically coupled to two metallic leads are studied. This system is described by a generalized Anderson model and an approximation of correlation dynamics is employed to derive the corresponding Green’s functions. The properties are investigated by calculating the electrical conductance G, thermal conductance κ, thermopower S and figure of merit ZT for different gate voltages and temperatures. At last but not least, our numerical calculation results show that the thermoelectric properties of the T-shaped quantum dot with relatively strong on-site Coulomb interaction are mainly influenced by the bipolar effect, which is consistent with some theoretical and experimental results.
  • Structure of an Al–Cu–Co Decagonal Quasicrystal Studied by Cs-Corrected STEM

    pp. 866-870

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    DOI:10.2320/matertrans.M2014008

    The structure of an Al–Cu–Co decagonal quasicrystal (DQC) with two quasiperiodic planes in an annealed Al64Cu22Co14 alloy has been studied by spherical aberration (Cs)-corrected scanning transmission electron microscopy (STEM) with high-angle annular detector dark-field (HAADF) and annular bright-field (ABF) techniques. The observed HAADF-STEM images taken with the incident beam parallel to the periodic axis clearly represent individual transition-metal (TM) Cu/Co atoms and mixed sites (MSs) of Al and TM atoms as separated bright dots, and consequently arrangements of TM atoms and MSs on the two quasiperiodic planes can be directly determined. The TM atoms on the two quasiperiodic planes are arranged in pentagonal tiling with an edge-length of 0.76 nm, and also the TM atoms and MSs are located at vertices of Penrose tiling with an edge-length of 0.25 nm, and so they are arranged with a bond orientational order (BOO). Pentagonal frames with definite directions in the pentagonal tiling of TM atoms are also arranged in τ2-inflated pentagonal tiling with an edge-length of 2 nm. Arrangements of TM atoms derived from the observed HAADF-STEM image are placed in ideal pentagonal Penrose tiling with an edge-length of 2 nm, which is generated by the projection of five-dimensional (5D) hyper-cubic lattice, and are projected on occupation domains (ODs) in the perpendicular space. The arrangement of Al atoms and TM atoms and MSs in a well-symmetric region is interpreted from the observed HAADF- and ABF-STEM images.
  • Structure of the Passive Film formed on Fe–Mn–Si–Cr–Ni Shape Memory Alloy after Wet and Dry Corrosion Test

    pp. 871-876

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    DOI:10.2320/matertrans.M2014029

    The corrosion resistance of an Fe–Mn–Si–Cr–Ni shape memory (16 Mn) alloy was estimated by the wet and dry corrosion tests, and the structure of the passive film formed on the alloy was examined by primary EELS (Electron Energy Loss Spectroscopy) using TEM (Transmission Electron Microscopy) analysis. The electrochemical behavior of 16 Mn alloy was investigated by EIS (Electrochemical Impedance Spectroscopy). 16 Mn alloy showed much higher corrosion resistance than the carbon steel (SM) in wet and dry corrosion tests. In EIS measurement following corrosion test, 16 Mn alloy had much larger value of corrosion resistance (Z1mHz) by forming the passive film as compared to SM. In AES (Auger Electron Spectroscopy) and XPS (X-ray Photoelectron Spectroscopy) analysis, the passive film of 16 Mn alloy was shown to contain Fe, Mn, Cr, Si and Ni. From TEM-EELS, the passive film was found to consist of 2 layers of a Cr–Si-rich in inner and an Mn-rich outer layer of Fe oxide. It was found that 16 Mn alloy could maintain the passive film which composed of effective elements in the wet and dry environment.
  • Effect of High-Pressure Torsion Process on Precipitation Behavior of α Phase in β-Type Ti–15Mo Alloy

    pp. 877-884

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    DOI:10.2320/matertrans.M2013469

    Precipitation behavior of α phase in Ti–15Mo alloy during isothermal aging after straining by high-pressure torsion (HPT) was investigated and compared with that without HPT process. Upon isothermal aging, ultrafine equiaxed α phase particles and acicular α phase plates were obtained in HPT-processed samples, while only the coarse acicular α phase plates were obtained in the samples without HPT process. The precipitation of equiaxed α phase occurred faster than the precipitation of acicular α phase in HPT-processed samples. It seemed that a high density of dislocations resulted in highly strained area and enhanced atomic diffusivity, promoting the formation of equiaxed α phase. The HPT-processed samples exhibited higher hardness and ultimate tensile strength but lower ductility than those of the specimens without HPT process.
  • Effect of Two-Step Aging on Cluster Formation in Al–Mg–Si Alloys

    pp. 885-891

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    DOI:10.2320/matertrans.M2013388

    The change in the state of a Mg–Si cluster with pre-aging at 363 K, followed by aging at 303 or 323 K, was studied by means of a tensile test, three-dimensional atom probe (3DAP), and differential scanning calorimetry (DSC) measurements. Mg–Si clusters formed during isothermal aging (one-step aging) at 363 K after solution heat treatment were different from the ones formed at 303 and 323 K. Furthermore, during aging at 303 and 323 K following pre-aging at 363 K (two-step aging), the clusters that were originally formed at 363 K (high-temperature clusters) grew in size and a new type of clusters (low-temperature cluster) were newly formed at 303 and 323 K. The increase in yield stress with aging time at 303 and 323 K was greater with the pre-aging at 363 K than without the pre-aging. The greater increase in the yield strength was attributed to the growth of the high-temperature clusters formed in the pre-aging and the nucleation and growth of the low-temperature clusters, both of which proceeded during the aging at 303 or 323 K.
  • Preparation and Hydrolysis of Aluminum Based Composites for Hydrogen Production in Pure Water

    pp. 892-898

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    DOI:10.2320/matertrans.M2013425

    A series of Al based composites were prepared using mechanical milling method in this paper. Effects of additives including CaO, NaCl salt and low melting point metals (Ga, In, and Sn) on the hydrolysis activity of Al based composites were evaluated in pure water. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques were used for the microstructure analysis of as-prepared samples and their hydrolysis products. The results showed that the addition of CaO, NaCl salt and low melting point metals can effectively improve the hydrolysis properties of Al based composites. Especially, Al alloys-CaO–NaCl composites exhibited a higher hydrogen yields than Al–CaO and Al–CaO–NaCl materials. The SEM images displayed that NaCl salt particles were homogeneously distributed on the surface of Al based composites and inserted into Al matrix, which may damage the surface oxide layer of Al. Furthermore, the size of NaCl salt particles was much smaller in Al alloys-CaO–NaCl composites than that in Al–CaO–NaCl composites. The XRD patterns identified that the hydrolysis products were mainly composed of AlO(OH) and Al(OH)3. The microstructure-related hydrolysis reaction mechanism of Al based composites was proposed finally.
  • High Temperature Tensile Properties and Its Mechanism in Low-Carbon Nb-Bearing Steels

    pp. 899-906

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    DOI:10.2320/matertrans.M2013393

    The aim of this study is to clarify the high-temperature strengthening mechanism of Nb-bearing ultra-low carbon steel, which is well-known as superior steel for high-temperature applications. Observations by Three-dimensional Atom Probe (3DAP) suggested that the Nb atoms are either distributed in a solid solution within the grain or segregated at the grain boundary after hot-rolling. The strength at 600°C increases significantly upon addition of Nb, and the corresponding dominant strengthening mechanism is considered to consist of the following: the resistance for the dislocation gliding motion due to solute Nb, the retardation of the dislocation climbing-up motion due to solute Nb and Nb–C dipoles, and the resistance of the dislocation motion caused by the Nb–C(N) clusters formed when the materials are heated up to 600°C within 10 s and then held for 600 s. Further, compared with Nb-free steel or 0.1% Nb-bearing steel, 0.3% Nb-bearing steel has considerably reduced ductility at 600°C. This is attributed to the retardation of recovery due to the Nb addition. TEM observations imply that the dynamic recovery takes place easily during the tensile deformation at 600°C in Nb-free steel or 0.1% Nb-bearing steel, whereas the tensile stress increases significantly because of the work hardening presumably caused by the retardation of the restoration process by further addition of Nb. Hence, a rupture followed by necking is thought to occur easily. Moreover, there is a possibility that the segregated Nb at the ferrite grain boundary might affect the dislocation behavior resulting in an increase in the steel strength at a high temperature and a retardation of the recovery process. This possibility will be investigated in a future work.
  • Atomic Group Rotation Mechanism for {10\bar{1}2} Twinning of HCP Crystal Materials

    pp. 907-910

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    DOI:10.2320/matertrans.M2013472

    In this paper the atomic motion law of the {10\bar{1}2} twinning in hexagonal close-packed (HCP) crystal materials was studied through the atomic group rotation (AGR) model. The research results show that the AGR mechanism has universal property to all the HCP crystal materials. Though the structure of the atomic group changes with the axial ratio, the atomic motion of {10\bar{1}2} twinning in all the HCP crystal materials can be ascribed to the rotational motion of the atomic groups. The relation between the rotational angle, the relative displacement magnitude and the axial ratio was obtained. With the growth of axial ratio, the rotational angle increases, but the relative displacement magnitude decreases.
  • Characteristics of Austenite Grain Refinement by High-Speed Large-Reduction Forging —Production Technology for Fine Grained Steel by Large Deformation Forging III—

    pp. 911-916

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    DOI:10.2320/matertrans.P-M2014820

    To obtain fine and uniform microstructures, appropriate forging conditions were examined using high-speed large-reduction forging. Swing-type forging equipment was employed in hot-forging experiments using low-carbon steel (0.14%C–0.64%Mn). Also, a microstructure analytical system was developed by combining thermomechanical FE analysis with microstructural analysis to discuss the metallurgical phenomena occurring during forging. It was observed that coarse austenite grains can be refined to 20–30 µm uniformly using the proposed forging technology in the case of 59% reduction in thickness at 1273 K. In this case, dynamic recrystallization is a dominant factor for obtaining fine and uniform austenite grains. In the case of 43% reduction in thickness, the recrystallization is inhomogeneous, resulting in an inhomogeneous microstructure. Thus, about 60% reduction in thickness is necessary to obtain fine and uniform microstructures using the proposed technology when applied to the roughing of hot strip mills.
  • Defects Modification of TiB2–TiC Composite Phase Coating Resistance Spot Welding Electrode via Friction Stir Processing

    pp. 917-920

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    DOI:10.2320/matertrans.M2013363

    In this work, surface modification was carried out using friction stir processing (FSP) technique to investigate on the electro-spark deposited (ESD) TiB2–TiC coated electrode caps for resistance spot welding. The morphology, microstructure, phase composition and resulting mechanical properties of the electrode were examined and characterized. The results showed that treatment with FSP significantly reduced the number of cracks formed on TiB2–TiC coating and enhanced the interfacial binding force between the TiB2–TiC coating and the substrate. The micro-hardness and bonding strength of the TiB2–TiC coating were also improved and the heat affected zone (HAZ) of the substrate disappeared due to the influence of FSP. The improvement mechanism are attributed to a decrease in the number of coating cracks, improved delamination, and refined substrate grains size near the interface.
  • Microstructure of Cr–V–Mo Steel Processed by Recrystallization and Partial Melting and Its Effect on Mechanical Properties

    pp. 921-929

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    DOI:10.2320/matertrans.M2013398

    Recrystallization and partial melting (RAP) experiments were conducted on Cr–V–Mo steel using a multistage hot compression test machine. The dendritic microstructure of the cast material was refined owing to the recrystallization and austenization that occurred during RAP. The refinement of the microstructure is affected by the preparation of the initial cast billet. A higher cooling rate during casting results in a finer RAP-processed globular microstructure. Owing to the core segregation during casting and the phase segregation during RAP, alloying elements are distributed inhomogeneously in RAP-processed specimens. Optimal post heat treatments were designed to increase the homogeneity of the microstructure of RAP-processed specimens. Quenching from 1323 K followed by tempering at 833 K improve the consistency of the Vickers hardness, toughness, and high-temperature wear resistance of RAP-processed specimens.
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  • Macro-Segregation and Microstructural Characteristics in Rheo-Diecasting of a High Strength Al–4.8 mass%Si–0.7 mass%Mg Alloy

    pp. 930-936

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    DOI:10.2320/matertrans.M2014041

    Rheo-diecasting with electro-magnetic stirring has been carried out to investigate macro-segregation and microstructural characteristics of a high strength Al–Si–Mg alloy under various process conditions. The amount of initial heterogeneous nucleation and the degree of temperature uniformity in the slurry are dependent on the superheat of the melt at pouring into the slurry making vessel. The formation of macro-segregation in rheo-diecasting was closely related to the microstructural characteristics of semi-solid slurries. Slurries which have fine and globular α-Al particles prevent the formation of macro-segregation throughout the rheo-diecast specimens. However, coarse and non-globular α-Al particles result in the formation of macro-segregation, leading to a non-uniform hardness distribution, dependent on the thickness of the product. Various casting process parameters were examined to prevent the formation of macro-segregation, and the optimal ranges of the process conditions for rheo-diecasting were determined as follows: the injection velocity of 0.3–1.0 m·s−1 and the mold temperature above 150°C.
  • Influences of pH Value and Deposition Time on HA/TiO2 Coatings Deposited by Electrochemical Method

    pp. 937-941

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    DOI:10.2320/matertrans.M2013457

    With titanium alloy after micro-arc oxidation as substrate, HA/TiO2 coatings were prepared by electrochemical method. The coatings were characterized by means of SEM and XRD. Variation of pH value of electrode/electrolyte was measured by means of pH microprobe during treatment, and the influences of pH value and deposition time on HA/coatings by this method were investigated. The results show that micro-arc oxidation treatment are helpful to obtain a good adhesion between HA and substrate, and HA crystals are well-distributed on the TiO2 film surface. The increasing of the pH value of treatment solutions results in higher crystallinity of HA crystals. During the process of deposition, pH value on the surface of the electrode increases at first and then decreases with deposition time, but pH value of electrolyte decreases with the deposition time. pH value has obvious influence on the morphology of HA crystals, and the size of HA crystal increases with the deposition time within certain limits. However, the size of HA crystal is abnormally large if the deposition time is too long.
  • Measurement of Seebeck Coefficient and Conductive Behaviors of Bi2Te3−xSex (x = 0.15–0.6) Thermoelectric Semiconductors without Harmful Dopants

    pp. 942-946

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    DOI:10.2320/matertrans.MB201301

    A system for measuring Seebeck coefficient was constructed and applied to Bi2Te3−xSex (x = 0.15–0.6) samples without harmful dopants, prepared by mechanical alloying (MA) followed by hot pressing (HP). The constructed thermal contact method system, using single and multiple ΔT values, gave Seebeck coefficients of a standard reference material (SRM 3451) at room temperature confirmable as −230 ± 4 and −232 ± 1 µV/K, respectively. X-ray diffraction patterns and differential scanning calorimetry curves showed that the MA–HP-sintered samples of Bi2Te3−xSex were single-phase Bi2(Te,Se)3-related materials. All the Bi2Te3−xSex samples were n-type semiconductors. The maximum power factor was 1.4 × 10−3 W m−1 K−2 for Bi2Te2.8Se0.2 sintered at 623 K. These results indicated that doping with harmful materials of Bi2Te3−xSex compounds prepared by the MA–HP process is not necessary for carrier control.
  • Carbon-Dispersed WC–FeAl Hard Material Fabricated by Mechanical Milling and Subsequent Pulsed Current Sintering

    pp. 947-951

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    DOI:10.2320/matertrans.M2014081

    A new metal mold material for pulsed current sintering (PCS) methods at high temperatures with higher electrical resistivity than cemented carbide molds, was fabricated using mechanical milling and PCS. The constituent (100 − x)(WC–10 or 30FeAl) + xC (x = 0–8; mass%) hard material with dispersed spherical carbon particles were synthesized via mechanical milling of the source powders and PCS. The three-point flexural strength and electrical resistivity of the hard material decreased and increased with an increase in the carbon content, respectively. The 98(WC–10FeAl) + 2C alloy showed a flexural strength of 1 GPa and resistivity of 0.59 × 10−4 Ω cm. The material did not exhibit plastic deformation at 1073 K at 100 MPa under compression. The metal mold made from the 97(WC–10FeAl) + 3C was used for PCS at 1073 K at 200 MPa, suggesting that the carbon-dispersed WC–FeAl hard material was capable of being employed as a new mold material for the PCS method.
  • Effect of Orifice Introduction on Floating Characteristics of Cuboid Particles Simulating Tantalum Capacitors in Pneumatic Separation Column

    pp. 952-957

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    DOI:10.2320/matertrans.M-M2014818

    To recycle important rare metals (such as tantalum) from the printed circuit boards of waste electronic equipment, devices must first be delaminated from the boards. The devices are then separated into individual device types. For the practical use of the separation process, the authors previously developed a double-tube pneumatic separator. One of the features of this equipment is the introduction of orifices in a pneumatic separation column; an air–solid multiphase flow simulation was conducted to clarify its effect. The effects of the orifices on the floating characteristics and floating rates of cuboid particles and spherical particles with the same solid volume and mass as the cuboid particles were investigated for cases in which the number of orifices and the volumetric airflow rate in the separation column were varied. The results showed that the floating characteristics of cuboid particles were much different from those of spherical particles. It was estimated that the volumetric airflow rate could be decreased by 14% with the cuboid particles and the same particle recovery could still be achieved. When the volumetric airflow rate was identical, the treatment throughput for the cuboid particles was expected to increase by 20–30%. These results were caused by the difference in the cross-sectional areas of the particles, which indicates that taking the particle shape into account is extremely important for the numerical simulation of pneumatic separation. To thoroughly recover the cuboid particles, it is ideal to use a volumetric airflow rate greater than 0.129 m3·s−1 or less than 0.0615 m3·s−1, regardless of the orifice introduction. The number of orifices had no effect on either the separation efficiency or floating rate in the range of 0.118–0.129 m3·s−1. On the other hand, it was shown that the floating rate clearly changed depending on whether the orifices were introduced or not for the 0.103 m3·s−1 case.
  • Crystallization Behaviors of Copper Smelter Slag Studied Using Time-Temperature-Transformation Diagram

    pp. 958-963

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    DOI:10.2320/matertrans.M-M2014819

    In recent years, while Cu production has increased, ore quality has degraded. Consequently, copper smelting industries generate large amounts of byproducts and wastes, including slag. However, the use of these byproducts and wastes involve high costs and most of the wastes are discarded in landfills after processing. In some cases, these byproducts and wastes contain valuable components, which may be profitable to recover. In other cases, toxic or hazardous chemicals are required in the treatment of these wastes to prevent their release. Therefore, the processing of byproducts is a significant activity, which determines the profitability of copper production facilities.
    Magnetic separation of precipitated magnetite (Fe3O4) crystals in the copper slag is one of the most effective methods to recover iron resources. It is preferable to convert molten fayalite slag to magnetite during the cooling of the slag with oxidation.
    With a time-temperature-transformation (TTT) diagram, the microstructure of the slag obtained after heat treatment could be estimated through a designed cooling path according to the purpose for which the slag is to be used. Slag recycling, mainly the recovery of the precipitated magnetite crystals, will be enhanced by controlling the slag cooling conditions.
    In the present study, using an infrared furnace, the crystallization behavior of copper smelter slag with regard to obtaining a TTT diagram was assessed by X-ray diffraction (using an internal standard), scanning electron microscopy and energy dispersive spectroscopy. Moreover, the distribution behaviors of the heavy metals such as Cu, Zn, As, and Cr were also studied.
  • The Effect of Pre-Strain on the Resistance to Hydrogen Embrittlement in 316L Austenitic Stainless Steel

    pp. 964-970

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    DOI:10.2320/matertrans.M2014036

    The effect of pre-strain before hydrogen charging on the resistance to hydrogen embrittlement (HE) in the 316L austenitic stainless steel was investigated through the slow strain rate tensile test (SSRT), transmission electron microscopy, and thermal desorption analysis (TDA). The pre-strain suppressed mechanical twinning during the SSRT, regardless of hydrogen charging. However, it accelerated the ε-martensitic transformation in hydrogen-charged specimens. The TDA revealed that whereas hydrogen atoms migrated from grain boundaries and dislocations mainly to the austenite (γ)/ε interfaces in pre-strained specimens during the SSRTs, they moved to the boundaries of fresh mechanical twins, which newly formed during the SSRTs, in the annealed specimen. The elongation loss by hydrogen charging became greater with increasing the pre-strain, indicating that pre-straining deteriorated the resistance to HE. This elongation loss by pre-strain resulted from both the increase in fraction of ε-martensite with pre-strain and the segregation of hydrogen atoms to the γ/ε interfaces.
  • Effect of Cu on the Precipitation of Deleterious Phases and the Mechanical Properties of 27Cr–7Ni Hyper Duplex Stainless Steels

    pp. 971-977

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    DOI:10.2320/matertrans.M2013471

    Cu addition to the base alloy reduces the total amount of deleterious phases such as chromium nitride and sigma and chi phases. In particular, Cu addition to the base alloy results in pronounced suppression of the amount of sigma phase whereas it slightly facilitates the precipitation of chromium nitride and chi phase along the phase boundaries and within ferrite grains. During the initial stage of aging, the preferential precipitation of chromium nitride and chi phase seems to be closely associated with the retardation of the precipitation of the sigma phase. The preferential precipitation of chromium nitride and the chi phase inhibits the nucleation and growth of the sigma phase by depleting the Cr adjacent to the chromium nitride particles and depleting the Mo and W adjacent to the chi phase. Thus, the addition of Cu to the base alloy reduces its embrittlement owing to the delayed precipitation of these deleterious phases.
  • Surface-Grinding Kinetics for the Concentration of PGMs from Spent Automobile Catalysts by Attritor Surface Grinding

    pp. 978-985

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    DOI:10.2320/matertrans.M2014082

    Spent automobile catalyst is an important secondary resource of PGMs (platinum group metals), and the recovery is fairly significant. Various recovery technologies have been reported and applied in practical production, including pyro- and hydrometallurgical processes. Because the grade of PGMs in the catalysts is quite low, approximately 0.5 mass%, present recycling technologies involve high energy consumption. Enriching PGMs with energy-saving physical separation methods as a pre-treatment is therefore of great significance.
    Automobile catalyst is usually composed of cordierite phase as a substrate and an alumina/ceria phase, with PGMs, which covers the cordierite phase. In this research, a combined flowsheet, jaw crusher, crushing rolls, and attritor, was used to perform selective grinding and concentrate the alumina/ceria phase into finer size ranges. High-grade alumina/ceria phase was recovered into a −0.3 mm size fraction with 57% recovery by using the jaw crusher and crushing rolls for pre-crushing. The coarser size fractions, in which the alumina/ceria phase was still present, was fed to the following surface-grinding step with an attritor to concentrate alumina/ceria phase further. By combining pre-crushing and surface grinding as described above, an alumina/ceria concentrate was produced with a grade of 66%, recovery of 76%, and separation efficiency of 52%.
    This research used the Ouchiyama model, a well-known surface-grinding model, to evaluate attritor surface grinding for various feed size fractions. Results of kinetic analyses and experiments demonstrated that surface grinding became predominant for larger feed size fractions under the condition of low ball-media content because of collisions between sample particles, but for smaller feed size fractions, surface grinding occurred mainly by collisions of sample particles with the grinding media because of the small size of the sample particles. Surface grinding of the cordierite phase was also observed, especially for coarser feed size particles for which the exposure ratio of the alumina coat layer was lower compared with finer feed particles.

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