This study evaluated the efficiency of cerium reduction by grinding with microwave irradiation in mechanochemical processing. Grinding experiments with microwave irradiation were conducted using an agitating mixer. Since the structure of the ground samples was amorphous and the cerium concentration was much lower than those of other elements, the valence change and structural change of cerium after grinding with microwave irradiation were investigated using X-ray absorption fine structure (XAFS) analysis in the cerium K-edge. The X-ray absorption near-edge structure (XANES) analysis revealed that a portion of tetravalent cerium was reduced to trivalent cerium by grinding with microwave irradiation. In addition, it was confirmed by extended X-ray absorption fine structure (EXAFS) analysis that oxygen vacancies were produced as a result of the cerium reduction reaction. To evaluate the efficiency of cerium reduction efficiency, the percentage reduction by grinding with microwave irradiation was compared to that by planetary ball milling and microwave irradiation. As a result, it was revealed that the efficiency of cerium reduction via grinding with microwave irradiation was higher than that via microwave irradiation and the same as that via planetary ball milling. Moreover, a larger amount of tetravalent cerium could be reduced to trivalent cerium by grinding with microwave irradiation than when using planetary ball milling and microwave irradiation.

In this study, we investigated the localized and concomitant precipitation of calcium (Ca)/magnesium (Mg)-bearing species and iron oxides/oxyhydroxides, and their depression characteristics to the pyrite floatability in flotation process at pH 9 and pH 10.5 with prolonged pre-oxidation. Contrary to the depression characteristics at pH 9, the incipient (within aeration times of 30 min) depression of pyrite floatability in Ca/Mg-bearing solutions was more obvious at pH 10.5, while the subsequent decline was only slightly when the pre-oxidation time was expanded to 120 min and 360 min. The competitive adsorption among Ca/Mg-bearing species and potassium amyl xanthate (PAX, C6H11OS2K, collector) at specific sites onto the pyrite surface was demonstrated by the regularly decreased zeta potential of the pyrite surface pretreated in Ca/Mg-bearing solutions. Further scanning electron microscopy-energy dispersive spectrometry demonstrated the concomitant secondary Ca/Mg/Fe-bearing precipitates on the pyrite surface. X-Ray photoelectron spectroscopy suggested strong reprecipitation of iron oxides/oxyhydroxides on the pyrite surface via acid–base complexation among Ca/Mg hydroxy species and iron hydroxy species. Incipient occupation efficiency of specific reaction sites by Ca/Mg-bearing species, which were mainly controlled by the metastable distribution of Ca/Mg hydroxy species and their electrostatic affinity with pyrite surface, was the crucial factor that influenced the competitive adsorption of xanthate and pyrite floatability. More obvious incipient depression at pH 10.5 rather than at pH 9 contributed to more effective Ca/Mg-bearing species and their higher affinity to pyrite surface at pH 10.5. The localized and concomitant precipitation of secondary Ca/Mg/Fe-bearing species leads to a slightly increased hydrophilic coverage upon the pyrite surface, thus a slowly decreased pyrite floatability with increasing pre-oxidation time.

Manganese (Mn) is a major element in various aqueous and soil environments that is sometimes highly concentrated in mine water and other mineral processing wastewater. In this study, we investigated Mn removal from alkaline mine water (pH > 9) with an Mn-coated silica sand packed into a pilot-scale column reactor and examined the specific reaction mechanism using X-ray absorption near-edge structure (XANES) analysis and geochemical kinetic modeling. The kinetic effect of dissolved Mn(II) removal by birnessite (δ-Mn(IV)O2) at pH 6 and 8 was evaluated at different Mn(II)/Mn(IV) molar ratios of 0.1–10. Our results confirmed the positive effect of the presence of δ-MnO2 on the short-term removal (60 min) of dissolved Mn. XANES analysis results revealed that δ-MnO2 was more abundant than Mn(III)OOH in the reactor, which may have accumulated during a long-term reaction (4 months) after the reactor was turned on. A gradual decrease in dissolved Mn(II) concentration with depth was observed in the reactor, and comparison with the kinetic modeling result confirmed that δ-MnO2 interaction was the dominant Mn removal mechanism. Our results show that δ-MnO2 contents could play a significant role in controlling Mn removability from mine water in the reactor.

The wettability and floatability of oxidized chalcocite, bornite, and chalcopyrite with the conditions of sodium hydrosulfide (NaHS) dosages and pHs were studied by contact angle measurements, and single and mixture mineral flotation tests. To evaluate the results of the flotation, the flotation kinetic model for copper sulfide minerals treated by NaHS was derived. In this study, we focused on the activation and depression by NaHS, a well-known activator and depressant of copper minerals. The flotation results showed that there can be a threshold NaHS dosage to activate the mineral surfaces, as evidenced by the depression of the minerals and reduction of recoveries at higher dosages of NaHS. Chalcocite recoveries increased with an increase of NaHS dosage. Bornite recoveries tended to be depressed with a smaller amount of NaHS as pH increased. The recoveries of chalcopyrite increased as pH increased at an optimum NaHS dosage. Moreover, the flotation kinetic model that includes the surface properties and the reaction rate constant between the copper sulfide minerals and NaHS was derived. The trends of the flotation rate constants and mass fractions with NaHS dosages and pHs could quantitatively well-explain the flotation results.

The volume of spent photovoltaic (PV) panels is expected to grow exponentially in future decades. Substantial material resources such as silver (Ag), copper (Cu), aluminum (Al), silicon (Si), and glass can potentially be recovered from silicon-based PV panels. In this paper, we targeted the recovery of Cu and Ag from a cell sheet separated to a glass panel from a spent PV panel. The technical feasibility of a novel electrical dismantling method was experimentally studied. This method employed a pulsed power technology that releases high energy in a short time. It allowed a selective separation of the Cu/Ag wires from the sheet once per discharge in water. The experimental results indicated that 95.6% of the total Cu and 17.2% of the total Ag in the sample were successfully separated from the cell sheet using a 3.5-kJ capacitor bank circuit. Moreover, 3.66% of the total Si in the sample was contaminated by the separated Cu/Ag particles from the cell sheet, mainly by shockwaves generated by plasma expansion, and some of them formed a compound with Cu and Ag by eutectic melting, resulting in low liberation. At the lower energy of 3.5 kJ, eutectic melting of Cu and Ag with Si was more suppressed than 4.6 kJ, and 94.3% of Cu and 77.5% of Ag in the separated particles were liberated, which would be acceptable for further wet gravity and/or shape separation of Cu and Ag.

<p>Since the enactment of the "Feed-in Tariff" scheme in 2012, the solar power generation capacity in Japan has been steadily growing. Therefore, in the near future, the demand for the mass processing of spent photovoltaic (PV) panels is expected to increase. Secondary batteries, especially lithium-ion batteries (LiBs), have become important products for vehicles and mobile devices. The production of LiBs is also expected to significantly increase in the near future. In this study, we address the design of recycling systems for such emerging technologies. From life cycle perspectives, the requirements for the assessment of these technology systems are carefully defined through a bibliometric analysis of technology assessments, critical reviews of current research and developments in the recycling of PV panels and LiBs, and analysis of the intensities of life cycle impacts (such as greenhouse gas emissions and resource use). The necessities for life cycle assessments, material flow analyses, and other assessment methods are clarified, along with the conditions to be examined using these assessment methods.</p>

Mesoporous zerovalent iron-magnetite nanocomposites (ZVI-MNCs) were developed to circumvent the limitations of magnetite, such as its susceptibility to phase transition in air-water interfaces. High-resolution transmission electron microscopy images revealed the presence of Fe0 and Fe3O4 in the as-prepared adsorbent. High-resolution X-ray photoelectron spectroscopy (HR-XPS) Fe 2p deconvoluted spectra showed that electron transfer between Fe0 and Fe3O4 controlled the magnetite transformation. The isotherm equilibrium data for As(III) and As(V) are described by the Sips model, which suggests single- and multilayer formation onto a heterogeneous surface with different binding sites, whereas adsorption is controlled by a pseudo-second-order kinetic model, which indicates chemisorption. The maximum sorption capacities (qm) for As(III) and As(V) are 632.6 and 1000 μmol g-1, respectively, which are larger than the qm of similar adsorbents. The greater qm for As(V) is attributed to a higher multilayer formation and a stronger bonding force compared with As(III). The arsenic uptake capacity showed that the as-prepared adsorbent was effective over a wide pH range, and an optimal uptake capacity was recorded between pH 5.0 and 9.0 for As(III) and 3.0 and 7.0 for As(V). The adsorbent exhibited a remarkable regeneration performance for As(III) and As(V) uptake. Several microscopic analytical tools, including Fourier transform infrared spectroscopy, HR-XPS, and X-ray absorption near-edge structure together with zeta potential, confirmed that the binding mode of As(III) and As(V) on ZVI-MNCs was predominantly inner-sphere coordination. Partial redox transformation occurred for As(III) and As(V) on nearly 10 nm of the adsorbent, which indicates that a surface redox mechanism contributed partially to arsenic uptake on the near surface of the ZVI-MNCs. Extended X-ray absorption fine structure spectral analysis proposed that a corner-sharing monodentate mononuclear (1V) complex occurred for As(III) with a small portion of a corner-sharing bidentate binuclear (2C) complex, whereas As(V) formed a corner-sharing bidentate binuclear (2C) complex with octahedral Fe bonding.

We elucidate the removal mechanism of Cd by birnessite (delta-MnO2) in adsorption and coprecipitation processes in the context of acid mine drainage (AMD) treatments. The removal mechanism was studied through batch removal experiments at different initial Cd/Mn molar ratios (0-2) by zeta potential measurements, X-ray diffraction (XRD), small-angle X-ray scattering (SAXS), and X-ray absorption fine structure (XAFS) analysis. The sorption isotherm and zeta potential measurements suggest that surface complex formation is the dominant mechanism, and that surface precipitation and/or intercalation also occur in the coprecipitation process when the initial Cd/Mn molar ratio is high (1-2). Increasing the initial Cd/Mn molar ratio to above 0.5 decreased the particle size of delta-MnO2 and shifted its (001) XRD peak to lower angles, suggesting that the delta-MnO2 interlayer ((001) and (002) planes) was expanded and the growth of delta-MnO2 crystals was inhibited in the coprecipitation process. The results of XAFS analysis revealed the production of Mn(III) precipitates and surface complex formation with Cd at high Cd/Mn molar ratio condition ( < 1). No significant changes in the crystalline structures of delta-MnO2 over the entire range of initial Cd/Mn molar ratios were observed in the adsorption process, confirming that Cd could be adsorbed by triple-corner-sharing at neutral pH.

The development of efficient and inexpensive boron removal techniques is required to treat wastewater from many industrial processes. We investigated the removal mechanism of boron from wastewater by calcinated ettringite, a concrete sludge, and compared the advantage of this method to coprecipitation with ettringite. The amount of boron remaining in solution decreased slightly from an initial concentration of 2.3 mmol/dm(3) to 1.3-1.4 mmol/dm(3) after reaction for 60 min with ettringites calcinated at 55 and 65 degrees C. The boron removal efficiency increased greatly, with only 0.30 mmol/dm(3) remaining in solution after treatment for 60 min with samples calcinated at 75 degrees C, but samples calcinated at 95 and 175 degrees C removed slightly less boron than those calcinated at 75 degrees C. This enhancement of boron removal performance is due to the amorphization of the ettringite structure by calcination. However, excess heating inhibits ion exchange between the sulfate ions in the ettringite and the borate ions in the solution. The coprecipitation method also achieved efficient boron removal at high Al/B molar ratio (0.44 mmol/dm(3) boron remaining after 60 min). However, uptake by a different amorphous precipitant became the dominant removal mechanism. Large amounts of sludge are generated by this method. Adsorption using calcinated ettringite is therefore the optimal method for the efficient and inexpensive removal of boron from wastewater.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. There are about 100 sites of acid mine drainage (AMD) from abandoned/closed mines in Japan. For their sustainable treatment, future prediction of AMD quantity is crucial. In this study, AMD quantity was predicted for two closed mines in Japan based on a series tank model in three stages. The tank model parameters were determined from the relationship between the observed AMD quantity and the inflow of rainfall and snowmelt by using the Kalman filter and particle swarm optimization methods. The Automated Meteorological Data Acquisition System (AMeDAS) data of rainfall were corrected for elevation and by the statistical daily fluctuation model. The snowmelt was estimated from the AMeDAS data of rainfall, temperature, and sunshine duration by using mass and heat balance of snow. Fitting with one year of daily data was sufficient to obtain the AMD quantity model. Future AMD quantity was predicted by the constructed model using the forecast data of rainfall and temperature proposed by the Max Planck Institute–Earth System Model (MPI–ESM), based on the Intergovernmental Panel on Climate Change (IPCC) representative concentration pathway (RCP) 2.6 and RCP8.5 scenarios. The results showed that global warming causes an increase in the quantity and fluctuation of AMD, especially for large reservoirs and residence time of AMD. There is a concern that for mines with large AMD quantities, AMD treatment will be unstable due to future global warming.

© 2020 Elsevier Ltd. High contaminated levels of fluoride (F) in groundwater and drinking water are of significant concern for human health in many countries. This study examined adsorption and coprecipitation for effective F- removal from high F-containing water using inexpensive magnesium hydroxide, Mg(OH)2. Fluoride solutions were reacted with Mg2+-or Mg(OH)2-containing solution at different F/Mg molar ratios of 0.05-2, and the F- removal performance and mechanisms were investigated in both processes. As a result, the residual amount of F- in the solution decreased with an increase in Mg2+ concentrations for both processes, and the sorption isotherms followed the Langmuir type. Mg(OH)2 was precipitated only after F- removal, which suggests that adsorption to Mg(OH)2 was the main removal mechanism in both processes. However, the saturated adsorption capacity in the coprecipitation process was two times higher than that in the adsorption process. The chemical equilibrium calculation results implied that the surface complexation of MgF° and MgOH2F° was the dominant mechanism in the adsorption process whereas multiple complexations of Mg-MgF3° and Mg-MgF4- occurred in the coprecipitation process. This complex formation improves F- removal; hence, coprecipitation with Mg(OH)2 exhibited better potential as an efficient process for the treatment of industrial wastewater that contains F-.

To establish an effective recycling process for waste appliances, the process of recovering printed circuit boards (PCBs) containing valuable elements in comminution was investigated and evaluated. The present study performed comminution tests using three different types of waste appliances: smartphones, microwave ovens and electrical rice cookers. Comminution tests showed that a drum-type agitation mill operated at a mid-range rotation speed could achieve a relatively high recovery ratio of PCBs and inhibit excessive breakage of PCBs. Following these experiments, simulations using the discrete element method with a particle-based rigid-body model were conducted to evaluate the comminution performance of the drum-type agitation mill. Experimental and simulation results confirm that the processes of detachment of PCBs from waste appliances and subsequent breakage can be expressed by kinetic equations related to collision energy. It is concluded from these results that the kinetic equations obtained in experiments and simulations can be used to evaluate the recovery process of PCBs from waste appliances.

In this work we investigated the influence of planetary ball milling and vertical stirred ball milling on the leaching of a copper ore containing copper sulfate and covellite. We used a mixed experimental-simulation approach to correlate the kinetic parameters of leaching to the collision energy during grinding. The effect of milling was studied by scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS). Results showed that both high-intensity grinding techniques resulted into a dramatic decrease of particle size. Furthermore, under specific grinding conditions, the planetary ball milling determined also the partial amorphization of covellite. The collision energies corresponding to specific grinding conditions in terms of rotational speed and number of grinding media were assessed by DEM simulations and were related to the specific surface area after grinding. The specific surface area of grinded samples was found to be directly proportional to the collision energy in grinding. The leaching of the ore occurred through three subsequent steps: (i) dissolution of copper sulfate, (ii) dissolution of amorphousized covellite and (iii) dissolution of residual crystalline covellite. The results of kinetic fitting highlighted an increase of the rate constants for the leaching of amorphous and crystalline covellite by intensifying the milling conditions. By correlating the collision energies from DEM simulation with the leaching rate constants, we confirmed that the rate constants for the leaching of covellite increased due to an occurred mechanochemical reaction. The mechanochemical reaction that determined the partial amorphization of covellite occurred above 0.25 J/s · g. On the other hand, the rate constants for the leaching of the residual crystalline covellite constantly and progressively increased with the collision energy, thus highlighting an improvement of leaching due to an increase of surface area.

Recently, the discrete element method (DEM) has been widely applied to investigate the influence of operating and design parameters on grinding performances. However, while most studies investigated the effects of such parameters on the fineness of milling products, the relationship between them and the size dispersion of milling products has not been elucidated yet. In this study, we investigated the influence that the direction of the agitator shaft has on grinding performance in a media stirring mill. First, we proved by milling experiments that the media stirring mill with the horizontal direction of the agitator shaft can provide better grinding performances. Then, we further elucidated this experimental evidence by applying DEM simulations to a media stirring milling process in a vertical and a horizontal stirred mill. According to the simulations, in the vertical shaft configuration, the motion of the grinding media in the lower section through the vertical direction was inhibited by a too low velocity. On the other hand, the grinding media in the horizontal stirred mill moved more uniformly but with a lower collision energy. Furthermore, the grinding media in the low sections actively mixed with the grinding media in the upper sections, thereby resulting in a more uniform energy transfer and in a better grinding process. Accordingly, this study demonstrated that not only the collision energy but also the uniformity of the movement of the medium particles should be evaluated in order to investigate the grinding performance in a media stirred mill by DEM simulation.

We investigated the recovery of calcium fluoride (CaF2) from highly concentrated hexafluorosilicic acid wastewater by neutralization- purification. Neutralization was achieved by dosing calcium hydroxide, whereas purification was carried out by alkaline leaching with sodium hydroxide. X-ray diffraction, X-ray absorption fine structure, mineral liberation analyzer, Fourier transform infrared spectroscopy and Inductively-coupled plasma atomic emission spectroscopy were used to quantify the neutralization and leaching performance and to elucidate their mechanisms. The precipitation behavior was strongly dependent on the calcium (Ca)/silicon (Si) molar ratio. For a Ca/Si ratio of 1.12, approximately 25% of the total fluorine was precipitated selectively as CaF2. By increasing the Ca/Si ratio to 3.91, the recovery yield increased to 100% because of the precipitation of CaSiF6 and the hydrolytic decomposition of hexafluorosilicate ion (SiF6 2-) to CaF2. The hydrolytic decomposition of SiF6 2- resulted in the precipitation of silicon dioxide on the surface of the previously formed CaF2. Alkaline leaching by sodium hydroxide at 70°C resulted in an efficient removal of the silicon phase from the neutralized sludge with the formation of a CaF2 product with a grade above 90%. Leaching parameters, such as the kinetic constant and the activation energy, were determined by assuming first-order kinetics. The residual silicon phase in the final product could not be dissolved because of the formation of non-leachable SiO3 2-.

Using N,N,N',N'-tetra-2-ethylhexyl-thiodiglycolamide (TEHTDGA) in n-dodecane as the extractant, we compared the percentages of Pd(II) extracted from HCl and HBr solutions, and analyzed the structures of the Pd(II)-extractant complexes. For comparison, similar experiments were performed with di-n-hexyl sulfide (DHS), a well-known sulfide-type extractant. TEHTDGA extracted Pd(II) from both HCl and HBr solutions much faster than DHS. The Pd(II)/(TEHTDGA or DHS) stoichiometry in the organic phase was 1:2. For TEHTDGA, the extractability of Pd(II) from HBr solution was inferior to that from HCl solution, whereas the opposite was true for DHS. However, FT-IR spectroscopy and EXAFS measurements indicated that the inner-sphere structure of Pd(II) in the TEHTDGA complex was almost the same as that in the DHS system: in both cases, two of the halide ions in the tetrachloro- or tetrabromopalladate(II) ion were replaced by the sulfur atoms of two extractant molecules.

In situ remediation of a bauxite residue by integrating a storage area with CSIRO's soda recovery process was evaluated. The process consisted of the following steps: bauxite residue is neutralized with sulfuric acid; the slurry is separated into a neutralized residue and a leachate. The, residue is sent to a storage area -and washed with rainfall. The leachate and drainage/runoff from the storage area are collected and purified; sodium sulfate in the solution is then split into sodium hydroxide and sulfuric acid using bipolar-membrane electrodialysis. Finally, the sodium hydroxide is used in a Bayer refinery and the sulfuric acid is used for neutralization of bauxite residue. This study investigated the optimum conditions and remediation of the bauxite residue. pH rebounding was observed after sulfuric acid neutralization of bauxite residue. Considering this rebound, the most suitable pH was set at 4.5. Washing remediated the residue to accomplish the goals for bauxite residue rehabilitation. Furthermore, sodium hydroxide and sulfuric acid were recovered by bipolar -membrane electrodialysis. Cost -benefit analysis for an assumed one million tons refinery showed 10% return on investment. Evaluation of the advantages of neutralized residue and technical improvements are expected to make this process feasible for remediation of bauxite residue. (C) 2016 Elsevier B.V. All rights reserved.

Hexavalent chromium (Cr(VI)) attracted researchers' interest for its toxicity, natural availability and removal difficulty. Nevertheless, its sorption mechanism is not clearly understood yet. In this work, we elucidated the sorption mechanism of the co-precipitation of chromates with ferrihydrite through quantitative analysis. The influence of Cr/Fe molar ratio on sorption was investigated by zeta potential measurements, X-ray diffraction (XRD) and X-ray adsorption fine-structure analysis (XAFS). Coprecipitation at pH 5 showed almost twice the sorption density of adsorption at pH 5. In co-precipitation, a shift of the XRD peak due to inner-sphere sorption of chromate was observed at Cr/Fe molar ratio 0.5. For adsorption, the same peak shift was confirmed at Cr/Fe molar ratio of 1. Zeta potential at pH 5 suggested that the sorption mechanism changed at Cr/Fe molar ratio 0.25 for coprecipitation and at Cr/Fe molar ratio of 1 for adsorption. Fitting of Cr and Fe K-edge extended X-ray adsorption fine-structure suggested that ferrihydrite immobilized Cr(VI) via outer sphere surface complexation for lower Cr/Fe ratios and via inner-sphere surface complexation for higher molar ratios. At higher molar ratios, bidentate binuclear Cr Fe bonds were well established, thus resulting in the expansion of the ferrihydrite structure. (C) 2017 Elsevier B.V. All rights reserved.

The separation properties of Pt(IV) over Fe(III) in HCl solutions using N-2-ethylhexyl-bis(N-di-2-ethylhexyl-ethylamide)amine (EHBAA) were investigated and then compared with those using the conventional extractant tri-n-octylamine (TOA). Also, the structural analyses of Pt(IV) in both of the aqueous (HCl solution) and organic (EHBAA in n-dodecane-2-ethylhexanol solution) phases were performed with EXAFS spectroscopy. The extractability of Pt(IV) was much higher with EHBAA than with TOA in the studied HCI concentration range (0.2-8 M HCl); additionally, EHBAA selectively extracted Pt(IV) over Fe(III) under the condition of [EHBAA] &lt;= 0.1 M and [HCI] &lt;= 1 M. The Pt(IV) loading capacity of 0.1 M EHBAA was about 9.2 (about 0.05 M). Most of the Pt(IV) extracted with 0.1 M EHBAA from 1 M HCI was stripped with 0.1 M NaOH; the co-extracted Fe(III) was selectively scrubbed with distilled water. The structural studies indicated that the Pt(IV) extracted with EHBAA from 1 M HCl formed an ion-pair complex, [PtCl6].(EHBAA.H)(2). (C) 2017 Elsevier B.V. All rights reserved.

Bangladesh has emerged as a leading ship breaking nation. We conducted a material flow analysis of steel in Bangladesh with an emphasis on the ship breaking industry (SBI). The total aggregate domestic steel consumption in fiscal year (FY) 2010 was 2,930,000 tonnes (t) in Bangladesh; SBI met approximately 51% of the demand for raw materials and 37% of the demand for finished steel products. Rolling industries output in FY2010 was 1,451,000 t; 23% of the input for this production was from ship breaking sources. Dismantled ships also generate high-quality reusable steel scraps. SBI was found to be the sole source of scraps for small rerolling industries in Bangladesh, and their output in 2008 more than doubled as compared to 2005. Larger rolling industries fulfilled their input needs for steel scraps by using both SBI and imported materials. We found a sharp increase in input imports during the global ship breaking recession in 2003-2007 and when Bangladesh's SBI faced a temporary ban in 2010. Induction furnaces in Bangladesh in FY2010 produced a total of 787,000 t of billets; more than 40% was from ship-sourced scraps. In 2008, the country's steel consumption was 3,220,000 t, that is, 22 kilograms per person, and the intensity of steel use was 40 grams per U.S. dollar, which was much higher than that of other developing countries with a similar per capita gross domestic product (GDP). The country exhibited a high level of steel consumption relative to its GDP, which is indicative of the contribution of SBI.

The handling of wet particles in chemical engineering is often difficult because cohesive forces acting on wet particles cause particulate aggregation and the adherence of devices, unlike what occurs under a dry condition. The liquid-bridge force is introduced to the discrete element method in the investigation of the behavior of wet particles. However, existing numerical modeling has problems from the viewpoint of the effect of cohesion in contact states. Specifically, the cohesive force is treated as a constant value for contact states. This means the effect of the cohesive force is not only dependent on the spring constant in the discrete element simulation but is also frequently overestimated. To solve this problem, the present study developed a numerical contact model considering quantitatively the effect of the cohesive force in contact states and validated the model for a pan pelletizer. The behaviors and cascading angles of wet particles in simulation and experiments were in good agreement and the validity of the contact model was thus demonstrated. The present numerical contact model is thus a promising model for the numerical simulation of wet particles. (C) 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

For the establishment of appropriate recycling process, the delamination of electric components from printed circuit boards (PCBs), which are major components of c-waste, is expected, because some of useful critical metals are concentrated in specific components. However, the knowledge about the mechanism of the delamination process in a drum typed agitation mill is still limited and uncertain. To better comprehend the mechanism, in this study, the particle-based simulation and comminution tests using handmade PCBs were conducted. The behavior of PCBs and the air flow in the mill having flexible chains was simulated by the discrete element method (DEM) coupled with computational fluid dynamics (CFD). To model the shape of PCBs, the particle based rigid body model was introduced into the DEM. Since this model could not directly simulate the breakage phenomena, collision energy was calculated to qualitatively evaluate the comminution performance. In addition, the simulation results were compared with the experimental comminution tests, This study demonstrated the behavior of PCBs and the air flow in the mill, and the effect of the air to comminution process in the mill. It was also shown that the collision energy related to parts collision was better correlated with the experimental results and this correlation can be assumed using first order rate equation, which suggested part detachment was mainly brought by direct collision to parts in the mill.

Die filling is an important aspect of powder molding in chemical engineering. The discrete element method (DEM) has been applied to simulations of die filling systems in the literature. In these simulations, the die has been modeled by simple shapes such as cylinders and cuboids. However, industries require modeling of complex die shapes in the computations. In addition, the existing DEM is problematic from the viewpoint of industrial applications, since complexly shaped dies might not be modeled by existing technologies. To solve this problem, the signed distance function (SDF) model is applied to the DEM simulation (DEM/SDF) and the DEM/SDF approach is validated for arbitrarily shaped dies. Focusing on macroscopic powder flow, simulation results are compared with experimental results, and good agreement is confirmed for the spatial distribution of velocity, the projection areas of the shoe, and the final mass of filling particles. Therefore, the adequacy of the DEM/SDF model is newly demonstrated in the die filling system; i.e., the DEM/SDF method is shown to be an effective method for the numerical simulation of particle flow into arbitrarily shaped dies. (C) 2015 Elsevier Ltd. All rights reserved.

The sorption mechanism of dilute Zn [initial Zn(II) concentration up to 40 mg dm(-3)] on aluminum hydroxide was investigated. Adsorption and coprecipitation at pH 7 were compared. The adsorption process gave a Langmuir-type isotherm and the zeta potential of Zn(II)-adsorbed aluminum hydroxide decreased linearly with increasing sorption density of Zn on aluminum hydroxide. The adsorption mechanism is therefore mainly surface complexation. In contrast, in the coprecipitation process, a Brunauer-Emmett-Teller like isotherm was obtained; the slope of the zeta potential versus Zn(II) sorption density decreased when the initial Zn/Al molar ratio was greater than 0.5. The X-ray diffraction pattern of Zn(II)-coprecipitated aluminum hydroxide changed from that of poorly crystalline gibbsite to a Zn-Al layered double-hydroxide (LDH) when the initial Zn/Al molar ratio was greater than 0.5, showing that surface complexation was the main sorption mechanism, but surface precipitation of Zn-Al LDH was also involved when the initial Zn/Al molar ratio in the coprecipitation process was greater than 0.5. A quantitative diffuse-layer model was constructed. The aluminum hydroxide exchange capacity was set at 0.61 mol mol-Al-1, based on the experimentally determined surface area, 340 m(2) g(-1). Surface complexation coefficients for H+, OH-, and Zn(II) adsorption on aluminum hydroxide were determined by fitting to the experimental adsorption results. The obtained parameters were in excellent agreement with those previously reported for a database of gibbsite adsorption equilibrium constants. The pH edge for Zn(II) removal by aluminum hydroxide was successfully reproduced by the constructed model. (C) 2015 Elsevier B.V. All rights reserved.

Dismantling (or "breaking") of ships invigorates global shipping by replacing older ships and recycling or reusing as much as 95 % of their materials. Recently, Bangladesh is dominating global ship breaking. Here, we aimed basically at expounding the inflows of the ship breaking industry (SBI) in Bangladesh from the perspectives of origin, types, and lifespan of dismantled ships in order to develop a better understanding of this industry. The inflows in SBI have been increasing and the tonnage dismantled rose to a record high of 2,308,525 LDT (light displacement tonnage) in 2009. Interestingly, ships dismantled in Bangladesh were relatively younger having the mean age of 27 years. Of the outflows from ships dismantled, reusable/recyclable steel comprised 85 % of the total weight of ships; the rest were machinery, hardware, fittings, and consumable items. The major hurdle we faced was the unwillingness of stakeholders in disclosing available but relevant information due to widespread suspicion. We suggest that keeping organized records and their disclosure is critical to developing a comprehensive understanding of this industry. We are hopeful that this research will pave the path for continued objective scientific investigations into the contributions, both positive and negative, as well as the evolution of SBI in Bangladesh.

The removal mechanisms of silicate using an aluminum hydroxide co-precipitation process was investigated and compared with an adsorption process, in order to establish an effective and validated method for silicate removal from wastewater. Adsorption isotherms, XRD and FT-IR analyses showed that silicate uptake occurred by adsorption to boehmite for initial Si/Al molar ratios smaller than two, but by precipitation of poorly crystalline kaolinite for the ratios larger than two, in both co-precipitation and adsorption processes. Silicate was removed by two steps: (i) an initial rapid uptake in a few seconds; and (ii) a slow uptake over several hours in both processes. The uptake rate in the first step was higher in the co-precipitation process than in adsorption process, presumably due to increased silicate adsorption to boehmite and rapid precipitation of kaolinite. These results suggest that silicate removal using aluminum salts could be effectively achieved if the pH adjustment and aluminum concentration are strictly controlled.

The effects of NO3- and SO42- ions on chloride ion removal from neutralized sludge were investigated to determine if removal of chloride ions enabled the use of sludge for cement production. An artificial sludge prepared from iron, calcium and chloride that primarily consisted of two-line ferrihydrite was prepared. Chloride ions in the sludge were easily washed out by distilled water, NO3- water or SO42- water to levels below those specified in the standard for Eco-cement production (1,000 mg/kg), but the washed sludges did not meet the standard for ordinary Portland cement (350 mg/kg). Conversely, artificial sludge prepared from aluminum, calcium and chloride mainly consisted of low crystalline boehmite with other minor components. Chloride ions in this sludge were only reduced to below those specified in the standard for the ordinary cement production if SO42 water was used to wash the sludge. The filtration rate during washing using SO42 water was faster than when distilled water or NO3- water was used because SO42 ions were adsorbed onto the sludge particles and the absolute value of the zeta potential of the sludge particles decreased. Overall, sludge washing using SO42-water was the best process for chloride ion removal and efficient filtration.

This study investigates the removal mechanism of arsenate, i.e., As(V), by coprecipitation with aluminum hydroxide in solution at pH 5. Both the filtrates and precipitates were analyzed. The removal mechanisms of As(V) were investigated by constructing sorption isotherms and by analyses of X-ray diffraction (XRD) patterns, zeta potentials, and Fourier-transform infrared (FT-IR) spectra. The sorption isotherms showed Brunauer-Emmett-Teller (BET) behavior. The sorption densities showed a rapid uniform increase when the initial As/Al solution molar ratio was more than 1.5. This suggests two sorption mechanisms of As(V), namely surface complexation and three-dimensional uptake, which were dependent on the initial As/Al molar ratio. It is important to note that the As(V) and Al(III) concentrations were unable to reach the thermodynamic solubility product of aluminum arsenate in the bulk solution. The three-dimensional uptake was therefore estimated to be not bulk precipitation but surface precipitation of aluminum arsenate. Surface precipitation, generation of a new surface phase, is known to be derived from surface complexes. XRD, zeta potential, and FT-IR analyses of the precipitates for each As/Al molar ratio confirmed that As(V) was predominantly removed by the adsorption of the As(V) oxyanions to the aluminum hydroxide surface when the initial As/Al molar ratio was less than 1.5. At an initial As/Al molar ratio greater than 1.5 at pH 5, As(V) was removed mostly by surface precipitation, evidently by poorly crystalline aluminum arsenate on the surface. This sorption as surface precipitation is a phenomenon specific to the coprecipitation process.

Conversion of carbon dioxide to useful chemicals or fuels has become an active area of research. However, most methods used in these studies need expensive or precious reagents, such as hydrogen or rare metals. The objective of this study is the reduction of carbon dioxide to organic compounds using a cyclic Fenton reaction that uses only inexpensive reagents. We achieved carbon dioxide reduction to methanol and ethanol using this reaction at ambient temperatures and pressures. The Fenton reaction was promoted by the repetitive addition of H2O2 and Fe(II) salts, resulting in the synthesis of methanol/ethanol and formic/acetic acids from carbon dioxide saturated water. In addition, it was found that H2O2 and Fe(II) in a molar ratio of 1 or 2 is optimal for the reaction as this increase the generation of OH. The catalytic effects of magnetite, pyrite, sodium hypophosphite and copper sulfate were also investigated in the reduction of carbon dioxide to acetic acid. All reagents acted as a catalyst, and the concentration of TOC and acetic acid was increased. The highest concentration of acetic acid was achieved with the addition of copper sulfate. This enhanced the generation of H or H− as Cu(II) forms more stable complexes with oxalic acid than Fe(II)/Fe(III), meaning more free Fe(II)/Fe(III) ions were present in the solution. © 2013, The Resources Processing Society of Japan. All rights reserved.

The reaction mechanism of H3BO3/B(OH)(4)(-) with MgO in aqueous phase was investigated using sorption isotherm, XRD, B-11-NMR and FTIR. Release of Mg2+ was observed soon after contact of MgO with H3BO3 and maximum released Mg2+ was proportional to the initial boron concentration, suggesting ligand-promoted dissolution of MgO by H3BO3. The molecular form of H3BO3 was more reactive with MgO in releasing Mg2+ ions than B(OH)(4)(-). B-11-NMR results indicated that trigonal B (B-[3]) was predominant over tetrahedral B (B-[4]) in solid residues after sorption of H3BO3. The molar ratio of B-[4]/B-[3] increased with H3BO3 sorption density. XRD patterns for the solid residues were assigned to Mg(OH)(2) and peaks broadened with increasing H3BO3 sorption density, except for (hk0) planes due to c-axis lattice strain induced by incorporation of H3BO3 between layers. These results indicated that H3BO3 interfered in the c-axis stacking of in Mg(OH)(2). Molecular H3BO3 acted as a trigger when reacting with the MgO surface, releasing Mg2+ to produce an unstable complex leading to the precipitation formation of Mg(OH)(2), which is a sink for the immobilization of H3BO3/B(OH)(4)(-).

Acid mine drainage (AMD) from about 80 abandoned or closed mines in Japan are actively treated by neutralization; the government spends billions of yen every year to protect the environment from these discharges, and treatment will have to continue for many years. However, it is difficult to predict the future AMD chemistry, neutralizer requirements, and volume of sludge to establish future costs, chemical inputs, and wastes. The water monitoring data or source rock samples for conventional predictive methods are not available. In this study, a predictive model for AMD composition and the neutralization process was constructed based on geochemical modeling, assuming first order kinetics for the dissolution of several minerals. The solution composition, neutralizer requirements, and sludge generation were estimated for case studies at two abandoned mines in Japan: a sulfur mine, and a copper and iron mine. The model effectively predicted AMD composition after termination of mining and was also useful for estimating future neutralizer requirements and sludge volumes. This research will increase understanding of the lifecycle environmental costs in the mining industry.

The effects of attrition on sandy and silty Pb-contaminated soil from shooting ranges were quantitatively investigated. The abrasion rate could be quantitatively evaluated using a population balance model from the size distribution of soil particles before and after attrition in an intensive mixer. The maximum abrasion rate was 0.047 mu m/s in the first stage of attrition (1 mm of grinding), and then declined between 2 and 4 min of grinding. The two samples had similar maximum abrasion rates, but the decline in the abrasion rate was larger in the silty soil, which would have a thinner soft layer on the surface of the soil particles, than the sandy soil. The incremental change in the mass fraction of Pb in the &lt;20 mu m particle size fraction increased proportionally with the abrasion thickness from the surface of the soil particles. Although the Japanese regulatory standard for the Pb concentration in soil (150 mg/kg) was not met for either sample, the Japanese regulatory standard for the concentration of leached Pb (0.01 mg/L) was met for silty soil after attrition. Sequential extraction using Tessier's procedure for the &gt;20 mu m size fraction before and after attrition showed the attrition was more effective for the oxides or organic-bound species and carbonate species of Pb chemical forms. [doi:10.2320/matertrans. M2012240]

Recycling of printed circuit boards (PCBs) is an important subject not only from the treatment of waste but also from the recovery of valuable materials including rare metals. We have tried a special and selective grinding such as parts detachment from the board using drum typed agitation mill to concentrate a part of “rare metals”. The objective of this study is revealing the mechanism of parts detachment process from PCBs. A simulant PCBs on which nine capacitors were solder-mounted were used for the communition test using the agitation mill. Effects of rotation speed and the number of boards for parts detachment or board breakage were investigated. Both of parts detachment rate and board breakage rate were increased as rotation speed increased. On the other hand, as the number of boards increased, parts detachment rate was increased while board breakage rate was slightly increased. DEM simulation with and without particle breakage model was conducted. DEM simulation with spherical particle model (without particle breakage model) showed that board breakage was strongly affected by collision of PCBs to the wall in the normal direction while parts detachment was affected by both of collision of PCBs to the wall in the tangential direction and interaction between PCBs. To investigate the mechanism of parts detachment process more directly, DEM simulation with particle breakage model was also conducted. Simulation results obtained from DEM with particle breakage model corresponded to these experimental trends successfully. © 2012, The Resources Processing Society of Japan. All rights reserved.

This paper describes the possibility of concentrating minor rare metals contained in cellular phones by applying a novel comminution and the combination of physical separation technologies, which were followed by metallurgical processes to extract them. We compared two kinds of comminution processes, conventional and novel, from the viewpoint of compositional separation and combined two kinds of physical separation categories, &amp;ldquo;Device Separation&amp;rdquo; and &amp;ldquo;Powder Separation&amp;rdquo; to achieve an effective concentration of rare metals. It was demonstrated that the novel comminution process could separate printed circuit board (PCB) from the body in the first stage and detach the installed electronic devices from the board in the second stage and that several kinds of rare metals could be concentrated by combining the novel comminution and &amp;ldquo;Device/Powder Separation&amp;rdquo;. The &amp;ldquo;Device&amp;rdquo; and &amp;ldquo;Powder&amp;rdquo; products concentrated are expected to be further treated by hydrometallurgical processes to extract rare metals and/or stocked as &amp;ldquo;Artificial Deposits&amp;rdquo; in accordance with the concept of &amp;ldquo;RtoS (Reserve to Stock)&amp;rdquo;.

Gallium metal under the freezing point was observed to maintain its liquid state by dispersing silica nanoparticles of a given particle size and concentration. Though the freezing point of pure gallium is 302.9 K, the liquid gallium maintained its supercooling state at 276-277 K for more than 400 days by dispersing 1.0 wt. % of silica nanoparticles (10 nm in size). Extended x-ray absorption fine structure analysis shows that the supercooled gallium liquid has a beta-Ga-like feature, and the nearest neighboring atom distance is 0.1 angstrom larger than that of pure liquid gallium. This method opens the way to use liquid gallium as a promising fluid carrier in energy conversion devices. (C) 2011 American Institute of Physics. [doi:10.1063/1.3645596]

The mechanism for removal of dilute As (V) by coprecipitation with aluminum hydroxide was investigated using sorption isotherms, X-ray diffraction (XRD) patterns, and zeta potential analysis. The results from the coprecipitation experiments were compared with those from simple adsorption experiments. &lt;BR&gt;During simple adsorption of As (V) on aluminum hydroxide, the sorption density obeyed a Langmuir-type isotherm. XRD patterns of the adsorbed As (V) were independent of the mass adsorbed. The relationship between zeta potential and sorption density was linear. These results suggest that As (V) was removed mainly by surface complexation.&lt;BR&gt;In comparison, during As (V) co-precipitation with aluminum hydroxide, the sorption density followed a BET-type isotherm at pH 5 and 7, and there was a steep increase in sorption density at a transition point. XRD patterns and zeta potential analysis also supported the existence of a transition point during As (V) co-precipitation. The transition was from surface complexation to precipitation, and occurred when the initial molar ratio was As/Al=1.5. These results suggest that during coprecipitation As (V) is removed by surface complexation at molar ratios of As/Al &amp;le; 1.5, and by the formation of amorphous aluminum arsenate at As/Al &amp;ge; 1.5.

Comparison of removal characteristics between coprecipitation and adsorption method of aluminum hydroxide for wastewater containing F (-I) was attempted by sorption isotherms, X-ray diffraction (XRD) patterns, and zeta potential analysis. &lt;BR&gt;When F (-I) was coprecipitated with aluminum hydroxide, the sorption density was higher at pH 7 than at pH 5. The sorption density at both pH values followed a BET-type isotherm. High sorption density was obtained when the initial F/Al molar ratio was high, and some of the aluminum hydroxide dissolved. The sorption density in adsorption treatment was slightly lower than that in coprecipitation treatment. These results suggest that the mechanism of F&lt;SUP&gt;&amp;mdash;&lt;/SUP&gt; coprecipitation was not simply surface complexation of F&lt;SUP&gt;&amp;mdash;&lt;/SUP&gt;.&lt;BR&gt;At pH 7, the XRD patterns for F (-I) coprecipitates and aluminum hydroxide were identical. The slope between zeta potential and sorption density changed when the initial molar ratio was F/Al=3 in both coprecipitation and adsorption experiments. This F/Al=3 transition point corresponded to the point where the sorption density increased abruptly in the sorption isotherm. These results reveal that F (-I) was primarily adsorbed as a surface complex to aluminum hydroxide when the initial molar ratio was F/Al&amp;lsaquo;3. In contrast, when the initial molar ratio was F/Al&amp;rsaquo;3, amorphous compounds of F&lt;SUP&gt;&amp;mdash;&lt;/SUP&gt; and Al (III) formed or F-Al complexes such as AlF&lt;SUP&gt;2-&lt;/SUP&gt;, AlF&lt;SUB&gt;2&lt;/SUB&gt;&lt;SUP&gt;+&lt;/SUP&gt;, AlF&lt;SUB&gt;3&lt;/SUB&gt;&lt;SUP&gt;0&lt;/SUP&gt; and AlF&lt;SUB&gt;4&lt;/SUB&gt;&lt;SUP&gt;-&lt;/SUP&gt; were adsorbed.

Dilute arsenate (As(V)) coprecipitation by ferrihydrite was investigated to determine if treatment of acid mine drainage containing dilute As(V) using coprecipitation is feasible. The sorption density obtained at pH 5 and 7 was nearly identical when As(V) was coprecipitated with ferrihydrite, while it was higher at pH 5 when As(V) was adsorbed on the ferrihydrite. The high sorption density of As(V) to ferrihydrite in coprecipitation with 1-h reaction time suggested that coprecipitation Occurs via both adsorption and precipitation. Furthermore, the relationship between residual As(V) and sorption density revealed a BET-type isotherm, with a transition point from a low residual As(V) concentration to a high residual As(V) concentration being observed for all initial As(V) concentrations between 0.15 and 0.44 mmol/dm(3) when the initial molar ratio was 0.56 at pH 5 and 7 X-ray diffraction and the xi potential revealed that the transition point from surface complexation to precipitation was obtained when the initial As/Fe ratio was 0.4 or 0.5. When dilute As(V) was coprecipitated with ferrihydrite at pH 5 and 7, it was primarily adsorbed as a surface complex when the initial molar ratio was As/Fe &lt; 0.4, while a ferric arsenate and surface complex was formed when this ratio was &gt;= 0.4.

In this work, the size classification of a mixture of submicron and micron size Ni particles dispersed in aqueous solution was investigated. The suspension of Ni particles was allowed to flow, under laminar condition, through a burette filled with an adhesion medium (i.e. substrate). The difference between zeta potentials of larger particles and adhesion medium created a base for the heterocoagulation. Thus, the larger particles move towards the medium assisted by the Saffman lift force and are attracted to its surface. The finer particles, on the other hand, flow off the burette and were collected as a separate fraction. This phenomenon was then explained by numerical simulating the particle's motion and showing that a stronger Saffman lift force will act in the direction of the substrate only for large particles, leaving the smaller ones unaffected as they sediment out. (c) 2006 Elsevier B.V All rights reserved.

The feasibility of making ferrite at ambient temperature from aqueous solutions of simulated acid mine drainage was investigated following our previous report. In this paper, qualitative and quantitative effects of coexisting ions such as Mn(II), Zn(II), Al(III) and Si(IV), on ferrite formation were investigated.&lt;BR&gt;As the first step, influences of several coexisting ions on the settling volume of the precipitates were investigated. In the case of Mn(II) or Zn(II) coexisting system, which are well known to have the capacity to substitute to the part of the lattice of ferrite, the settling volume increased a little, but in the case of Al(III) or Si(IV) coexisting system, which are known to impede ferrite formation, the settling volume considerably increased. However, the interference of Al(III) or Si(IV) to ferrite formation was weakened in the sludge recycling system.&lt;BR&gt;Secondly, their influences on the yield of magnetite or ferrite, which are calculated by the total oxygen consumption in the aerial oxidation reaction test, were investigated. The yield of magnetite decreases with an increase of Al(III) or Si(IV), but their interferences on ferrite formation were weakened in the presence of restored magnetite seed.&lt;BR&gt;Thirdly, their influences on the reaction rate in the aging process were also examined by solid-solid reaction test in which Fe(OH)&lt;SUB&gt;2&lt;/SUB&gt;, FeOOH and coexisting ions reacted directly in reduction atmosphere and at a fixed pH. The reaction coefficient considerably decreased not only by Al(III) or Si(IV), but also by Mn(II) or Zn(II), which are estimated not to impede ferrite formation.

The removal of iron and various heavy-metal ions in waste water into stable ferrite-type precipitates at ambient temperature is a promising alternative to clean up large volumes of polluted effluents. In this study, quantitative evaluation of ferrite formation method from aqueous solutions containing iron and sulfate at ambient temperature was considered, toward future application of this method to the treatment of acid mine drainage containing iron. In order to evaluate suitable conditions of Fe&lt;SUB&gt;3&lt;/SUB&gt;O&lt;SUB&gt;4&lt;/SUB&gt; formation quantitatively, these formation process was considered to be divided into two processes, i.e. oxidation process and aging process.&lt;BR&gt;The reaction rate in the oxidation process was examined by aerial oxidation reaction test. The results showed that the oxidation rate of ferrous iron was determined by the mass transfer of oxygen from atmosphere to solution, so the yield of Fe&lt;SUB&gt;3&lt;/SUB&gt;O&lt;SUB&gt;4&lt;/SUB&gt; in precipitation could be evaluated quantitatively by the total oxygen consumption which could be estimated from the oxygen transfer coefficient and the oxidation time.&lt;BR&gt;The reaction rate in the aging process was also examined by solid-solid reaction test in which Fe(OH)&lt;SUB&gt;2&lt;/SUB&gt; and FeOOH reacted directly in reduction atmosphere and at a fixed pH. At the initial stage of the reaction, the concentration change of FeOOH followed the first-order reaction concerning with FeOOH concentration. Surface complexation model which Fe(II) adsorbed to Fe(III) oxide could describe this phenomenon qualitatively.&lt;BR&gt;Relation between quantitative indexes which investigated in this study and several reaction conditions such as pH, Fe concentration and stirring intensity agrees with the results of Fe&lt;SUB&gt;3&lt;/SUB&gt;O&lt;SUB&gt;4&lt;/SUB&gt; forming reaction from aqueous solutions at 25 &amp;deg;c which have been investigated by us or other researchers. These quantitative indexes are useful to understand suitable conditions or mechanisms of ferrite formation.

酸性坑廃水の最適中和処理法の確立を目指し，旧幌別硫黄鉱山からの酸性坑廃水処理への水酸化カルシウム (Ca(OH)&lt;SUB&gt;2&lt;/SUB&gt;)，現状の2段中和 (CaCO&lt;SUB&gt;3&lt;/SUB&gt;およびCa(OH)&lt;SUB&gt;2&lt;/SUB&gt;)，および酸化マグネシウム (MgO) の利用を比較検討した。中和処理により，溶液中のヒ素はAs(III) あるいはAs(V) のどちらの形態で存在している場合でも，Fe(III) との共沈あるいは吸着によって，pH7で排水基準値以下に処理されることが確認された。Ca(OH)&lt;SUB&gt;2&lt;/SUB&gt;および2段中和を用いて殿物繰り返し法を行なった場合には，殿物の大半は水酸化第二鉄および石膏 (CaSO&lt;SUB&gt;4&lt;/SUB&gt;･2H&lt;SUB&gt;2&lt;/SUB&gt;O) であり，中和剤の未溶解成分はわずかであることがわかった。一方，MgOを用いて殿物繰り返しを行なった場合には，殿物は塩基性硫酸鉄とわずかなMgOの未溶解成分から成ることがわかった。殿物の沈降速度は，繰り返し回数が増加するにしたがってやや減少したが，MgOを用いた場合の殿物の沈降速度は，Ca(OH)&lt;SUB&gt;2&lt;/SUB&gt;や2段中和を用いた場合に比べて速くなる傾向が見られた。MgOを用いた場合には，他の中和剤を用いた場合に比べて高密度の殿物が得られ，繰り返し回数が増加するにしたがってその密度は更に増大し，最適な中和法であることが示唆された。

The diffuse layer model (DLM) of surface complexation modeling (SCM) was applied to discuss in the removal of dilute arsenic in wastewater. The equilibrium constants for complexation reactions of arsenic to ferric/aluminum hydroxide were based on several reference values and the total site capacity of ferric/aluminum hydroxide was determined by fitting to experimental results, since it is difficult to obtain surface area of amorphous hydroxide in wastewater system directly by experiment. The fitting results turns out that the total site capacity ratio becomes smaller as the concentration of ferric or aluminum hydroxide precipitates in wastewater increases. The experimental approximation formula between the total site capacity and the concentration of ferric or aluminum hydroxide precipitates was obtained. The model gave a quantitative correct description of experimental results for As(V) removal by ferric or aluminum hydroxide.&lt;BR&gt;The above-mentioned examination was also applied to As(V) removal in waste water with SO&lt;SUB&gt;4&lt;/SUB&gt;. In Fe(III) salts, the total site capacities in wastewater with SO&lt;SUB&gt;4&lt;/SUB&gt; became a little smaller than them in wastewater without SO&lt;SUB&gt;4&lt;/SUB&gt;, but in Al(III) salts, they are almost same value regardless of coexistence of SO&lt;SUB&gt;4&lt;/SUB&gt;. The removal of As(V) by ferric hydroxide at pH6 were predicted quantitatively by the model, also when SO&lt;SUB&gt;4&lt;/SUB&gt; is contained.

Dry gravity concentration method using a fluidized bed is being investigated. We employ polystyrene (PS) balls and glass beads (GB) as a fluidizing medium. Pieces of corrugated cardboard, paulownia wood, polypropylene (PP), polyvinyl chloride (PVC) and Al are employed as test materials. The separation efficiency is experimented and vertical position of separated materials in the fluidization bed is simulated, by changing the superficial velocity. It is found by experiment that high recovery (&gt;90%) of sink products and float products is achieved, even if the density difference between them is slight. These high recoveries are achievable when the superficial velocity is slightly higher than the minimum fluidization velocity.&lt;BR&gt;The recovery of float products is found to be sensitive to the superficial velocity: i.e., the recovery is low when the superficial velocity is either too high or too low. In order to examine the influence of the superficial velocity on the recovery of float products in detail, we employ two-dimensional simulation based on the discrete element method (DEM). The grater the superficial velocity is, the wider the vertical positioning of float materials becomes. It is found by DEM simulation that the recovery of float products tends to be low when the superficial velocity is too high, because the lowest vertical position during separation test is lower than that when the superficial velocity is optimal.&lt;BR&gt;It is found from these series of experiments and simulation in this study that high efficient separation by dry gravity concentration using a fluidized bed is possible.

The boundary conditions for an axial diffusion model were overviewed and then modified concerning the pulp layer boundaries in a flotation column. It has been confirmed through the transfer function representation that the four general boundary conditions for an axial diffusion model correspond to the particular cases of Wehner and Wilhelm's, which considers not only the reaction section itself but also those of its top and bottom sides.
Various mathematical models have been proposed for the pulp layer in a flotation column and the most popular one may be an axial diffusion with first order reaction model of one stage. They are, however, unable to estimate the behaviors of free particles which have not attached to bubbles at the pulp-froth interface.
In this study, therefore, the authors, represented the transportation of free particles in the pulp layer of a flotation column by the two-stage axial diffusion with first order reaction model. The Wehner and Wilhelm's, as well as Danckwerts' boundary conditions have been partly modified and then applied to the feed point, the pulp-froth interface and the tailing discharge point of the flotation column. The proposed models and their boundary conditions made it possible to estimate the behaviors of free particles at the pulp-froth interface. (C) 2000 Elsevier Science Ltd. All rights reserved.

The boundary conditions for an axial diffusion model are overviewed and modified concerning the pulp layer in a column flotation cell. It is confirmed by the transfer function representation that the four general boundary conditions for an axial diffusion model correspond to the particular cases of Wehner and Wilhelm&#039;s, which considers not only the reaction section itself but also its fore and after ones. &lt;br&gt; Various mathematical models for the pulp layer in a column flotation cell have been proposed and the most popular one of them is an axial diffusion with first order reaction model of one stage. However, they can&#039;t estimate the behaviors of non-adhesive particles at the pulp level. &lt;br&gt; In this paper, the authors, therefore, represented the transportation of non-adhesive particles in the pulp layer of a column flotation cell by the two-stage axial diffusion with first order reaction model. The Wehner and Wilhelm&#039;s as well as Danckwerts&#039; boundary conditions have been partly modified and then applied to the feed point, the pulp level and the tailing point of the column flotation. The proposed models with their appropriate boundary conditions make it possible to estimate the behaviors of non-adhesive particles at the pulp level.&lt;br&gt;

The authors proposed the mathematical model for the pulp layer in a flotation column by the two-stage axial diffusion with first order reaction model in the former paper. The &lt;i&gt;Wehner&lt;/i&gt; and &lt;i&gt;Wilhelm&lt;/i&gt;&#039;s as well as &lt;i&gt;Danckwerts&lt;/i&gt;&#039; boundary conditions have been partly modified and then applied to the feed point, the pulp level and the tailing point of the column. This model makes it possible to estimate the behaviors of particles that haven&#039;t yet attached (free particles) at the pulp level. &lt;br&gt; The proposed model has three unknown parameters, that is, the Peclet &amp;middot; Bodenstein number, the collection rate constant and the concentration of drainage particles from the froth layer. From the sensitivity analysis, it was found that the proposed model has little sensitivity to the Peclet &amp;middot; Bodenstein number, but some sensitivity to other parameters, especially to the concentration of drainage particles. In this study, the Peclet &amp;middot; Bodenstein number was assumed from the empirical expression derived by &lt;i&gt;Mankosa&lt;/i&gt; et al.(1992), and the collection rate constant and the concentration of drainage particles were estimated by a least squares regression fit to the data sampled from an industrial operation, concerning the superficial concentration of attached particles as well as free particles. It is estimated by investigation of those parameters that the cleaning effect prevails exclusively not in the pulp layer but in the froth layer.&lt;br&gt;