Mechanisms in oxidation and sintering of magnetite iron ore green pellets

Thermal volume changes and oxidation mechanisms in magnetite iron ore green pellets balled with 0.5% bentonite binder, as a function of raw material fineness and pellet porosity, are shown. When a pellet starts to oxidize, a shell of hematite is formed around the pellet while the core still is magnetite. Dilatation curves were measured under non-oxidizing and oxidizing atmospheres to separately describe thermal volume changes in these two phases. Dilatation measurements showed contraction during oxidation between 330 and 900 °C by 0.5%. The extent of contraction was not influenced by the raw material fineness or the original porosity in pellets. Sintering started earlier in the magnetite phase (950 °C) compared to the hematite phase (1100 °C). The sintering rate increased with increasing fineness in the magnetite concentrate. A finer grind in the raw material would, therefore, promote the formation of duplex structures with a more heavily sintered core pulling away from the less sintered outer shell.At constant porosity in green pellets, the oxidation time became longer as the magnetite concentrate became finer, because of the enhanced sintering. In practical balling, however, the increase in fineness would necessitate the use of more water in balling, which results in an increase in green pellet porosity. These two opposite effects levelled out and the oxidation time became constant when green pellets were balled at constant plasticity. Combining the results from the oxidation and dilatation studies revealed new information on the rate limiting factors in oxidation of iron ore pellets. At 1100 °C, the diffusion rate of oxygen was limited by sintering in the magnetite core, taking place before oxidation rather than by the diffusion rate of oxygen through the oxidized hematite shell, as has been claimed in earlier literature. The oxidation rate was at maximum at around 1100 °C. At 1200 °C, the rate of oxidation substantially decreased because both the hematite shell and the magnetite core show heavy sintering at this temperature.Dilatometer measurements showed large thermal volume changes in the presence of olivine, at temperatures above 1200 °C. This is explained by the dissociation of hematite back to magnetite. Dissociation leads to an increase in the volume of the oxidized shell, while sintering of the magnetite core is further enhanced by the olivine additive.     

Metal-Ceramic Interactions:IV, Absolute Measurement of Metal-Ceramic Interfacial Energy and the Interfacial Adsorption of Silicon from Iron-Silicon Alloys

The surface and interfacial energies in the system Ni-A1₂O₃ md Ni-ZrO₂ were determined at elevated temperatures by means of equilibrium interfacial angle measurements. Coincidental measurements of surface tension and contact angle for iron and iron-silicon alloys in Al₂O₃ give values of interfacial energy. Application of Gibbs' isotherm indicates that interfacial adsorption of silicon takes place in this system.     

The role of normal and activated bentonite on the pelletization of barite iron ore concentrate and the quality of pellets

The essential parameters affecting the pelletization process of high barite iron ore concentrate were studied using the Egyptian normal and activated bentonite as binder materials. The metallurgical properties of green, dried and fired pellets were studied using chemical and X-ray analyses. The average strength of fired pellets 1.5% normal bentonite and fired 1300 °C for 25 min exceeded 200 kg/pellet. Using activated bentonite produced a lower kg/pellet value. Meanwhile, the productivity of green pellets decreased when the last binder was used.     

腾冲高泥氧化锌矿选矿实验研究

针对腾冲某氧化锌矿嵌布粒度细、含泥量高的技术难题进行了选矿实验研究. 结果表明,该矿中锌品位为6.65%、铁品位为12.31%,锌氧化率达92.83%,锌主要以异极矿形式存在,铁主要以磁铁矿形式存在. 经多个实验流程对比分析,采用预先洗矿(洗去小于37 mm粒级的颗粒)、沉砂磨矿后先磁选后浮选的流程,确定最佳磨矿细度为小于75 mm的颗粒达91.74%,弱磁选电流为4 A,粗选药剂用量为碳酸钠2 kg/t、六偏磷酸钠2 kg/t、硫化钠13 kg/t、KPR 2 kg/t. 采用闭路选矿流程,得到锌品位为27.89%、回收率75.26%的氧化锌精矿和铁品位为61.25%、回收率51.65%的铁精矿. 通过Zeta电位分析捕收剂KPR与异极矿的作用机理.     

Use of novel polyetheralkanolamine comb polymers as pigment dispersants for aqueous coating systems

The adsorption of a series of polyetheralkanolamine comb polymers characterized by a different length of the hydrophilic tail has been investigated at the carbon black/water interface by measuring adsorption isotherm, contact angle, wetting rate, zeta potential, and particle size distribution. Zeta potential measurements and adsorption layer thickness results suggest that polyetheralkanolamines with high ethylene oxide (EO) content provide only steric stabilization and they adsorb at the interface with the ethylene oxide chains in a coil conformation. The thickness of the adsorbed layer increases with increasing EO units; however, the surface tension and interfacial tension decrease with increasing EO content. Adsorption isotherms show that most of the added polyetheralkanolamine adsorbs onto the carbon black and only a small amount stays in the water phase. When treated with a polyetheralkanolamine, carbon black dispersions show uniform (unimodal) and narrow particle size distribution with very small median sizes of about 0.10 μm. The pigment concentrates containing the polyetheralkanolamine show excellent color compatibility in various decorative commercial white paints containing a wide range of resins and exhibit low viscosity with nearly Newtonian flow behavior. Presented at the 2006 FutureCoat! Conference, sponsored by the Federation of Societies for Coatings Technology, in New Orleans, LA, on November 1–3, 2006.     

固体颗粒对油水界面性质及乳状液稳定性的影响

采用界面张力仪、表面粘弹性仪和Zeta电位仪,研究了固体颗粒对胜利原油油水界面性质及乳状液稳定性的影响。结果表明,固体颗粒的存在使得油水界面张力及界面剪切粘度增加,O/W型乳状液的稳定性增加,而且随固体颗粒浓度增加,乳状液稳定性增强;在0.21~500μm范围内,固体颗粒粒径减小,其与原油形成的O/W乳状液稳定性增强,乳状液内部油珠表面Zeta电位负值增加。     

The Phase Transformation Mechanism of Bentonite-Stabilized and Cetyltrimethylammonium Bromide-Stabilized Emulsions and Application in Reversible Emulsification Oil-Based Drilling Fluids

We obtained a reversible emulsion system induced by bentonite solid particles and surfactant Cetyltrimethylammonium bromide (CTAB, whose critical micelle concentration (CMC) value is 9.21 × 10⁻⁴ mol L⁻¹). In this study, the zeta potential and contact angle were used to characterize surface wettability of bentonite solid particles in the process of phase transmission behavior. The adsorption amount was calculated at different CTAB concentrations, and then the adsorption isotherm of surfactant CTAB at bentonite solid particles was also studied to confirm the adsorption behavior and adsorption layer structure. The electrical conductivity and microscopic analysis were employed to characterize the phase inversion behavior of emulsion. The results show that the wettability of bentonite particles can be reversed by changing the CTAB concentration, and then the two phase transition behavior of bentonite emulsion can be induced. Additionally, the surfactant CTAB can be used in oil-based drilling fluid systems for the inversion of the emulsion type. The research on the performance of the reversible emulsion oil-based drilling fluid system shows that it has a good thermal stability and a small amount of fluid loss, and successfully reduces the damage of the traditional oil-based drilling fluids on oil well completion.     

Effect of bentonite addition on fabrication of reticulated porous SiC ceramics for liquid metal infiltration

SiC with different particles and a clay mineral bentonite (montmorillonite) were mixed in water to prepare ceramic slurry. The slurry was then infiltrated high porous polyurethane sponge. Excess slurry was squeezed out to adjust ceramic rate in the infiltrated body. The pore walls were coated with ceramic mix after the infiltrated body was dried. The polyurethane containing SiC particles and bentonite was fired in a box furnace to burn out the polyurethane from the body at 500 °C for 30 min. The remaining porous ceramic bodies were sintered at elevated temperatures to give strength. SiC particles with bentonite surface coating took polyurethane pore forms after firing the sponge. Bentonite was both used as binder for ceramic slurry at room temperatures and the sintering additives at elevated temperatures. Therefore, increasing bentonite addition gives higher strength to the resulting ceramic performs.     

碱化膨润土-海藻酸钙固定化颗粒作为微生物载体对微污染废水的处理性能

近年来,微污染景观水体表现出氨氮、化学需氧量(COD)超标的水质恶化趋势。使用吸附材料是解决该现状的有效途径之一,但是这类材料往往不具备原位长期稳定的处理性能。针对这一问题,本工作选用生物相容性良好的海藻酸钙作为固定剂,制备得到碱化膨润土-海藻酸钙固定化颗粒,探究固定化颗粒作为微生物载体对模拟微污染水体中NH₃-N和COD的去除效果。通过溶胶凝胶法采用海藻酸钙固定不同浓度NaOH溶液碱化的膨润土合成系列碱化膨润土-海藻酸钙固定化颗粒。通过扫描电子显微镜(SEM)、傅里叶红外光谱(FTIR)、Zeta测试、BET比表面积测试对制备得到的系列碱化膨润土及固定化颗粒进行表征,并分析固定化颗粒作为微生物载体对微污染废水的处理性能。研究结果表明,0.5 mol/L NaOH溶液处理的膨润土具有最低的Zeta电位(-27.4 m V),其表面电负性增强,且表面片层状结构增加。制备的0.5B-CA固定化颗粒比表面积最大,为12.1m²/g,比膨润土原土合成的固定化颗粒(0B-CA)比表面积增加了28%,有利于微生物附着生存。0.5B-CA固定化颗粒作为微生...     

Mechanistic study of adsorption of cetyltrimethyl ammonium bromide on high purity iron using contact angle, polarization resistance and XPS

The mechanism of inhibition of carbon dioxide (CO₂) corrosion of high purity iron by cetyl trimethyl ammonium bromide (CTAB) was investigated by contact angle, polarization resistance and X-ray photoelectron spectroscopy (XPS). The wetting properties, corrosion inhibitor performance and the adsorption of the inhibitor on a high purity iron surface were investigated. Test conditions during corrosion testing were 25 °C, 3 wt% NaCl brine, initial pH 3.9 and 1 bar CO₂ partial pressure. The samples were precorroded for 5 h before inhibitor was added.The inhibitor performance data showed that addition of CTAB significantly reduced the corrosion rate of high purity iron. The decreased corrosion rate seems to be caused by a blocking effect, where the CTAB forms a surface structure on the iron surface. Chemisorption of the CTAB molecules, which could lead to gradually increasing inhibitor efficiency, might be the reason for the slowly decreasing corrosion rate. The XPS data confirmed a gradual accumulation of inhibitor on the surface but did not show any distinct change between different mechanisms at different exposure times. The contact angle measurements showed that the iron was water wet, with a contact angle higher than 160°, at all CTAB concentrations.     

Use of NMR imaging to measure interfacial properties of asphalts

An NMR imaging method that allows for direct calculation of interfacial surface tensions of asphalts is described. The method is based upon acquiring NMR images of water drops on the surface of asphalt as a function of time. By expressing the contact angle for the water drop either in terms of Young's equation, or the liquid lens equation for the initial placement of the water drop on the asphalt surface, and for a later time when the water drop has penetrated below the asphalt surface, two equations that incorporate the asphalt-air surface and asphalt-water interfacial tensions are obtained which can be solved analytically. The NMR imaging method was used to determine the surface tensions of the eight Strategic Highway Research Program (SHRP) core asphalts at 25 °C. Asphalt-water interfacial tension values ranged from 25 to 40 dynes/cm, and asphalt-air surface tension values ranged from about 38-50 dynes/cm. These results are in general agreement with other asphalt surface tension measurements that have been made using the du Noüy ring tensiometer, or the Wilhelmy Plate method. Spreading coefficients and capillary numbers derived from the surface tension calculations are also reported.     

Sorptive and activity behavior of iron minerals for hydrocarbon synthesis

The sorptive behavior of hydrogen and of carbon monoxide and of their mixtures on a low grade iron ore, magnetite, hematite and a commercial synthetic NH₃—iron catalyst was studied in an attempt to describe their catalytic properties for Fischer-Tropsch hydrocarbon synthesis.In comparison to hydrogen and carbon monoxide adsorbed individually, the adsorption of both the gases is enhanced invariably when adsorbed simultaneously from the mixture. Sorption of hydrogen is enhanced by the presorption of carbon monoxide on the catalyst surface while a small amount of presorbed hydrogen hardly enhances the carbon monoxide adsorption.Initial rates of carbon monoxide hydrogenation, which are most representative of clean surfaces, measured over the temperature range 250–360°C, are in the order hematite > magnetite > synthetic-NH₃-iron > iron ore. From the initial rates, the selectivity for methane formation at 250°C for all the four catalysts is about 20 percent.     

Influence of oxide dopants on the wetting of doped graphite by cryolite/alumina melts

In the aluminium smelting industry, the wetting of the electrolyte on the carbon anode is an important property associated with the onset of the anode effect. The effect of dopants on the wettability of the anode was investigated in this study. The carbon material selected was graphite. The composition of the cryolite/alumina melts varied between a very low alumina content and 6 wt.% alumina. The sessile drop approach was adopted to measure the contact angle between the melt and the graphite at 1030 °C. The influence of oxide dopants, chromium III oxide and alumina, in the graphite on the wettability was studied. The wettability on a pure graphite surface depends to a small extent on the liquid surface tension but mostly on the liquid–solid interfacial tension that varies with the concentration of alumina in the liquid. The wettability on an oxide doped graphite surface depends on the dissolution of the oxide in the melt that changes the liquid–solid interfacial tension. The alumina dissolution has a double effect on the liquid–solid interfacial tension: the chemical reaction as well as the change in the oxy-anions concentration at the interface decrease the interfacial tension.     

Magnetite powder and kaolinite derived from waste iron ore tailings for environmental applications

The present study demonstrated a feasible approach of utilizing waste iron ore tailings in synthesis of value added products for environmental applications. The ferric iron recovered from waste iron ore tailings via acid leaching followed by precipitation as ferric (hydr)oxide was used as feed stock to synthesis magnetite powder by its reductive-roasting with charcoal. Physicochemical characterizations revealed the formation of sufficiently pure magnetite powder with ferric (hydr)oxide to charcoal ratio 1:2 and calcined at 1200 °C for 4 h. The magnetic powder as such or coated with polyacrylic acid, was found efficient for removal of a model dye, methylene blue from its solution within 2 min. The acid insoluble residue, generated after iron recovery from iron ore tailings, was characterized as kaolinite. The residue with reasonably good surface area (119 m² g^− 1) was also found effective for adsorptive removal of dyes including methylene blue and can find large scale applications in wastewater treatment. Moreover the present study provided further inputs for possible alternative uses of waste iron ore tailings to partly reduce its associated disposal problems.     

Reaction of smectites with iron in aerobic conditions at 75 °C

The stability of six dioctahedral smectites with different crystal chemistry was studied in the presence of iron to simulate the possible reactions of clay minerals in contact with Fe canister in a nuclear waste repository. The batch experiments were performed at 75 °C for 35 days in air. The reaction products were examined by XRD, QXRD, FTIR, BWA analyses and UV–Vis spectroscopy. The reaction of bentonites with metal iron led to the consumption of iron and the formation of magnetite and 7-Å phyllosilicate. The original smectites were partially transformed from Al-rich to Fe-bearing. Appearance of the dioctahedral–trioctahedral domains in the octahedral sheets of smectites resulted in the partial destabilization and/or partial dissolution of the smectite structure indicated by the increase of the layer charge, splitting of the smectite particles and partial disruption of the smectite layers. Benefits of NIR spectroscopy were found in identification of FeMgOH and FeFeOH groups, which were not recognized in the MIR region of reacted iron–bentonite mixtures due to overlapping with other absorption bands. The comparison of the present study with the paper of Osacký et al. (2010) showed that smectites reacted differently with iron in air and nitrogen atmosphere. In the aerobic conditions more iron was consumed and less amount of magnetite was formed. The distinct experimental conditions (aerobic versus anaerobic) had the greatest effect on the layer charge and thickness of the smectite particles. Less pronounced changes of the layer charge and thickness of the smectite particles were observed for the iron–bentonite mixtures reacted in the aerobic conditions. It indicated that smectite destabilization was inhibited in the aerobic conditions.     

HPAM对孤岛原油及沥青质油水界面特性影响的研究

用界面张力仪、表面粘弹性仪和Zeta电位仪测定了孤岛原油及沥青质模型油的界面特性,研究了部分水解聚丙烯酰胺(HPAM)浓度对这些界面特性的影响。结果表明,加入HPAM会使孤岛原油模型油及沥青质模型油与模拟水间的界面张力上升,但随着HPAM浓度增加,界面张力基本保持不变。当水溶液中HPAM的质量浓度大于5.0mg/L时,孤岛原油模型油及沥青质模型油与模拟水的界面剪切粘度随HPAM浓度增大明显增加,随着体系老化时间从1h增加到12h,其界面剪切粘度也有所增加。另外,HPAM还使得原油及沥青质模型油与模拟水体系所形成的O/W型乳状液的油珠表面Zeta电位变大。     

Industrial fluidised bed direct reduction kinetics of hematite ore fines in H2 rich gases at elevated pressure

Reduction kinetics of iron ore fines were investigated under fluidised bed conditions with hydrogen-rich gas mixtures in the temperature range from 400 to 700 °C at a pressure of 10 bar. Kinetic parameters were determined for the reactions hematite to magnetite, magnetite to wuestite and wuestite to iron from experimental data. To be able to investigate the influence of both temperature and reducing gas composition on the reaction rate, a laboratory scale pressurised fluidised bed reactor was used which is able to apply process conditions close to those in industrial plants. Appropriate to heterogeneous reactions, a rate expression considering the specific surface area was derived. The time dependent function for the transient evolution of specific surface area was generated from measured mercury porosimetry data. Kinetic parameters such as frequency factor, energy of activation and reaction orders were determined for the consecutive reactions from hematite to magnetite, magnetite to wuestite and wuestite to iron by numerical integration and parameter estimation from measured rate data. A comparison to the evaluation of kinetic parameters without considering specific surface area confirm the results of this work.     

Wetting and adsorption properties of cetyltrimethylammonium bromide and Triton X-100 mixture with short-chain alcohol in polymer–solution–air system

Measurements of the contact angle of the aqueous solutions of Triton X-100 (TX-100) and cetyltrimethylammonium bromide (CTAB) mixture with methanol or propanol on the polytetrafluoroethylene (PTFE) and nylon-6 surfaces were made. On the basis of the obtained results, the Gibbs surface excess concentration of alcohol and TX-100 + CTAB mixture at the polymer–solution and polymer–air interfaces was calculated and compared to that at the solution–air one. The standard Gibbs free energy of alcohol adsorption was determined by different methods. For TX-100 and CTAB mixture, this energy was calculated using the values of critical micelle concentration (CMC) of that mixture, the surface tension and contact angle of aqueous solution of alcohol as well as the surface tension and contact angle of the aqueous solution of TX-100 and CTAB mixture with alcohol at CMC. The polymer–solution interfacial tension, the adhesion tension, and the adhesion work of the studied solutions to the polymer surface were also determined. From the obtained data, it results that the studied solutions can wet completely only the nylon-6 surface and that below alcohol critical aggregation concentration the adsorption of surfactants and alcohols at the polymer–water and water–air interfaces is similar for PTFE and different for nylon-6.     

A study on plasticity and compression strength in wet iron ore green pellets related to real process variations in raw material fineness

The main binding force in wet iron ore green pellets has been found to be the cohesive force of the viscous binder. The wet compression strength (wet-CS) in green pellets is, however, also influenced by the green pellet plasticity. A certain degree of plasticity is needed to sustain the green pellet growth rate. Too much plasticity results in decreased bed permeability and production problems. As the plasticity increases, wet-CS decreases. The amount of moisture needed to create a given degree of plasticity depends on particle properties and on the particle size distribution. Therefore, it was of interest to study how wet-CS would be influenced by variations in raw material fineness, if the green pellet plasticity was kept constant, i.e. the green pellet properties would be compared under relevant industrial balling conditions. For this purpose, magnetite concentrates of different particle size distributions were balled in a laboratory drum and the moisture content for constant plasticity was determined for each of the materials.No difference in green pellet wet-CS as a function of the raw material fineness was found when the bentonite binder was used and the plasticity was adjusted to a constant level. Green pellets prepared of raw materials with narrow size distributions were just as strong as those with broader ones. This is because the main binding force is the cohesive force of the viscous binder. In green pellets balled without the bentonite binder, wet-CS increased with increasing specific surface area in the raw material, in a similar manner as has been shown in earlier agglomeration literature. In this case, the capillary forces prevail. Comparison of wet-CS at constant moisture, instead of constant plasticity, would lead to erroneous conclusions. Fineness, or rather the slope of the particle size distribution curve, had a major impact on the moisture content needed for constant plasticity. If the slope increases, more water is needed to keep the plasticity on a constant level. Implications of these results in control of industrial iron ore balling circuits are discussed.     

pH-mediated interfacial chemistry and particle interactions in aqueous chlorite dispersions

Interfacial chemistry and particle interactions of polydispersed chlorite clay mineral dispersions as a function of pH, solid content and ageing time have been investigated in the pH range 2–9 at 25 °C. Particle zeta potential, reflecting interfacial chemistry indicates a strong pH history and solid loading dependency. Zeta potential trends observed from high to low pH sweep show that an isoelectric points of pH ≈ 2.5 and 5, respectively for dilute (0.05 wt.%) and both 8 and 57 wt.% suspension. An electrokinetic potential bifurcation is observed upon reverse pH from low (e.g. 2) to high value (e.g. 9), indicative of pH-mediated interfacial chemistry modification. Particle interactions measured through dispersion shear yield stress show a similar pH-history dependency and compliance with DLVO theory. Supernatant analysis suggests that the suspension pH dependent behaviour may be attributed to the leach of Mg(II), Fe(II/III), Al(III) and Si(IV) ions from the chlorite particles at lower pH. Formation and specific adsorption of hydrolysed metal complexes onto particles which occurs at higher pH values manifest in the observed interfacial chemistry and particle interactions. The findings enable us to accurately rationalise the nature of the inter-particle forces underpinning particle network structure and strength as a function of dispersion conditions during aqueous processing of clay-based minerals.