Binding mechanisms in wet iron ore green pellets with a bentonite binder

Fundamental research during the past decade has been focussed on understanding the role of viscous forces on agglomerate deformability and strength. Much of this work has been done on glass spheres using Newtonian liquids as a binder. In this work, we show the variations in plasticity and strength of magnetite iron ore green pellets with varying liquid saturations and binder dosages (viscosities). For this purpose, a new measuring instrument was built to analyze the green pellet wet compression strength, plastic deformation and breakage pattern.Industrial iron ore green pellets are over-saturated and a supporting “network” of viscous liquid is formed on the green pellet surface. At least half, probably more, of the total binding force appeared to be due to the cohesive force of the network. The other half (or less) of the total compression strength can be explained by the capillary force. Due to irregularities on green pellet surfaces, both fully developed concave pore openings and saturated areas are expected to be found at the same time.Wet green pellets started showing plastic behaviour as they became over-saturated. In over-saturated green pellets, an explosive increase in plasticity with increasing moisture content was seen, due to the contemporary increase in porosity. Plasticity is an important green pellet property in balling and should gain the status of a standard method in green pellet characterization. It is suggested that the control strategy for the balling circuits be based on plastic deformation and compression strength of green pellets instead of the rather inaccurate drop number. The results also point out the importance of knowing whether the balling process should be controlled by adjusting the moisture content (plasticity) or by adjusting the bentonite dosage (viscosity). These two operations are not interchangeable—even if they would compensate in growth rate, the green pellet properties would differ.A new green pellet growth mechanism is suggested, based on the measured over-saturation. Firstly, green pellet plasticity needs to exceed a minimum level to enable growth. This limiting plasticity defines the material-specific moisture content needed in balling. Secondly, it is suggested that the growth rate be controlled by the viscosity of the superficial water layer rather than by the mobility of the pore water.     

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.     

Effect of iron ore properties on its balling behaviour

The pelletization behavior of an iron ore as well as the properties of the pellets produced are affected by the various properties of the ore. The main effective ore properties are: its specific surface area, porosity, mineralogical composition, and particle size distribution of the fine material to be pelletized. The effects of these properties are reflected in the amount of moisture needed for proper pelletization, and the sensitivity of balling kinetics to changes in moisture content and particle size distribution of the feed. The material properties also affect the quality of the product pellets both wet and dry. The properties of the feed material that improve the quality of wet pellets may not improve the quality of dry pellets.     

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.     

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.     

Pelletization kinetics of an earthy iron ore and the physical properties of the pellets produced

The iron ore sample used in this investigation was brought from the El-Gedida iron ore deposit, Baharia Oasis, Egypt. This ore is porous, earthy, hard, and has a relatively high specific surface area.The batch balling kinetics of this ore show that the ball growth rate increased by increasing the moisture content. The water content required for pelletizing this ore ranged between 16 and 19% of the dry weight of the charge. As expected, increase in bentonite content retarded the ball growth. Finer feed produced by more dry grinding increased the ball growth rate.The average drop number of pellets was improved by increasing the moisture content to a certain limit, after which the quality of the pellets decreased. The drop number also went up as the amount of bentonite was increased. Increasing the degree of fineness of ore improved the level of drop number.The crushing strength of dried pellets improved with increasing water content to a certain limit, then the trend reversed. Bentonite addition slightly improved the crushing strength of pellets. Increasing the degree of fineness of the ore decreased the crushing strength of dry pellets.The bulk density of pellets increased with higher moisture content to a certain limit and then the trend reversed. Small amounts of bentonite addition decreased the bulk density, but when bentonite exceeded 0.5% the bulk density slightly increased with increasing amount of bentonite. Denser pellets were produced when finer feed was used.     

Dry Strength of Pelletized Spheres

Three limestone powders, differing only in specific surface area, were agglomerated in a balling drum to form pellets of various sizes. The pellets were tested for dry compressive strength at different rates of deformation. A limiting elastic energy criterion for failure was used to derive the relation (analogous to the classical Hertz relation for strength of an elastic isotropic sphere) between the pellet size, the total deformation, and the load at failure. The surface area of the powder, the porosity of the pellet, and its compressive strength are analytically related; the results are in good agreement with the experimental data.     

Quantitative image analysis of bubble cavities in iron ore green pellets

Scanning electron microscopy and image analysis was used for quantitative analysis of bubble cavities in iron ore green pellets. Two types of pellets prepared with and without addition of flotation reagent prior to balling were studied. The bubble cavity porosity amounted to 2.8% in the pellets prepared without addition of flotation reagent prior to balling. When flotation reagent was added prior to balling, the bubble cavity porosity increased by a factor of 2.4 and the median bubble diameter was decreased slightly. It was also shown that mercury intrusion porosimetry is not suitable for determination of the distribution of bubble cavities. Finally, our data suggested that the difference in total porosity determined by mercury intrusion porosimetry and pycnometry between the two types of pellets was due to the bubble cavities.     

钙基膨润土提纯制备球团黏结剂试验

以朝阳钙基膨润土为原料,采用擦洗-分散-分选的流程对其进行提纯,并对提纯膨润土进行挤压钠化改型试验.试验结果表明,当选用朝阳钠化提纯膨润土做球团黏结剂,粘结剂添加量为1.1％时,生球的落下强度为4.0次,爆裂温度为525℃,干球抗压强度为306 N/个,可以满足冶金球团性能需要.采用钠化提纯膨润土制备球团黏结剂可以有效降低球团中黏结剂的添加量.     

Pelletization of magnetite ore with colemanite added organic binders

A new generation binder consisting of an organic binder and a borate salt was tested as an alternative to bentonite in magnetite ore pelletization. Carboxyl methyl cellulose (CMC), Ciba DPEP06-0007 and corn starch, and calcined colemanite were used as organic binders and the borate salt, respectively. They were added to the pellet feed separately and in different combinations at several addition levels. It was found that the use of organic binders is sufficient in terms of wet pellet quality; however, they fail to render the required compressive strength to pre-heated and fired pellets. Therefore, organic binders and calcined colemanite were used together so that wet pellets, pre-heated and fired pellets would be of the required quality. The results showed that the use of an organic binder together with calcined colemanite indeed yielded pellets with equal or better wet and indurated pellet qualities compared to the pellets produced with bentonite binder alone.     

Predictive models and operation guidance system for iron ore pellet induration in traveling grate–rotary kiln process

Thermal state of iron ore pellets in industrial traveling grate–rotary kiln process cannot be revealed straightforward, which is unfavorable for field operations. In this study, coupled predictive models of pellet thermal state within traveling grate and rotary kiln were established. Based on the calculated temperature profiles, predictive model of pellet compression strength was also established to assist in process optimization. All the models proposed were validated by the industrial data collected from a domestic plant, and the results show that grate model possesses a high accuracy, kiln model is considered to be accurate to within 10–15% of actual values, and strength model can identify the variation of pellet strength caused by the thermal changes. The proposed models were embodied into an operation guidance system developed for a large-scale pelletizing plant, and the system running results illustrate that the predictive models and expertise rules established can optimize the process very well.     

Modeling and simulation of iron ore pellet drying and induration process with accurate bed void fraction calculation

As key parameters of modeling iron ore pellet drying and induration process, the properties of a packing bed are calculated by our proposed method. Derived from realistic spheres-dropping phenomena, the dropping and rolling rules make the moving sphere finally reach its minimum gravitational potential energy. Furthermore, the plastic deformation of green pellets and mechanic vibration are also considered in the model. The model is based on the laws of mass and heat transfer, physical process, and chemical reaction. The partial differential equations of the process kinetic model involve derivatives in time and bed depth. Comparing the results with measured data proves that the model can be used for optimizing the iron ore pellet induration process and as an effective tool for other, similar thermodynamic process simulations.     

焦炭粉冷固成型用粘结剂——改性水玻璃的研制

探讨了聚丙烯酰胺 ,葡萄糖 ,高岭土 ,Al Cl3,腐植酸钠 ,水溶性苯酚 -甲醛树脂和自制的水溶性树脂 A(简称为树脂 A)等对水玻璃改性的影响。结果表明 ,树脂 A使焦炭粉冷固成型的球团的生球和干球强度明显提高 ,并确定了其较佳配方 :水与水玻璃的质量比为 4∶ 6,其总用量为 16% (占干焦炭粉的质量分数 ) ;树脂 A用量为0 .2 4% (占干焦炭粉的质量分数 )。在此较佳条件下 ,所制球团的生球和干球强度 ,与用未改性水玻璃 (其中水与水玻璃的质量比为 3∶ 7) ,其总用量为 16% (占干焦炭粉的质量分数 ) )所制球团的生球和干球强度 ,几乎相等 ,但前者球团的耐水性以及耐热性有明显提高。     

Wet granule breakage in a breakage only high-hear mixer: Effect of formulation properties on breakage behaviour

Wet granule breakage can occur in the granulation process, particularly in granulators with high agitation forces, such as high-shear mixers. In this paper, the granule breakage is studied in a breakage only high-shear mixer. Granule pellets made from different formulations with precisely controlled porosity and binder saturation were placed in a high-shear mixer in which the bulk medium is a non-granulating cohesive sand mixture. After subjecting the pellets to different mixing time in the granulator, the numbers of whole pellets without breakage are counted and taken as a measure of granule breakage. The experimental results showed that binder saturation, binder viscosity and surface tension as well as the primary powder size have significant influence on granule breakage behaviour. It is postulated that granule breakage is closely related to the granule yield strength, which can be calculated from a simple equation which includes both the capillary and viscous force of the liquid bridges in the granule. The Stokes deformation number calculated from the impact velocity and the granule dynamic strength gives a good prediction of whether the granule of certain formulation will break or not. The model is completely based on the physical properties of the formulations such as binder viscosity, surface tension, binder saturation, granule porosity and particle size as well as particle shape.     

加入聚乙烯的含碳铁矿球团的直接还原

以包头地区褐铁矿和无烟煤为主要原料,加入聚乙烯颗粒制成含碳铁矿球团,直接还原制备珠铁. 考察了还原温度、还原时间、配碳量及聚乙烯加入量对含碳铁矿球团直接还原的影响. 结果表明,影响含碳铁矿球团还原率的因素为还原温度、还原时间、配碳量、聚乙烯加入量. 最佳还原条件为C/O摩尔比1.2,加入聚乙烯量4%(w), 1350℃下保温5 min. 该条件下产物还原率最高,达99.87%. 加入一定量聚乙烯可缩短球团还原时间、降低还原温度、提高还原效率. 添加2% CaF2不仅使渣铁分离效果明显,且分离的渣可自然粉化,有利于筛分得到高品质珠铁.     

磷矿-碳复合球团冷固结成型试验研究

为了促进磷矿-碳复合球团在黄磷生产中的应用,进行了磷矿粉-碳质还原剂冷压造球试验研究,考查了黏结剂种类及配比、原料粒度、配碳量等因素对生球性能的影响以及生球烘干过程的脱水行为和强度变化.试验结果表明:随着腐植酸钠配比从0.5％增至2.0％,落下强度和抗压强度均先升高后降低;随着膨润土配比从0.5％增至2.0％,抗压强度...     

高炉瓦斯灰含碳球团粘结剂研究

在掌握高炉瓦斯灰和粘结剂特性的基础上,通过测试生球、干球和焙烧后球团的抗压强度和落下强度,实验考察单一粘结剂和复合粘结剂对高炉瓦斯灰含碳球团强度的影响. 结果表明,加入淀粉类粘结剂能改善球团的低温强度,生球的抗压和落下强度分别达到72 N/个和5.9次/0.5 m;干球的抗压和落下强度分别达到58 N/个和4.3次/0.5 m;但焙烧后球团的抗压强度相对较低. 加入水玻璃含硅类粘结剂能改善其高温强度,焙烧后球团抗压强度最高达到1764 N/个,但生球和干球的强度较低,达不到生产要求. 加入玉米淀粉和水玻璃组成的复合粘结剂后球团强度的改善效果更明显,生球的抗压和落下强度最高达到60 N/个和5.5次/0.5 m;干球的抗压和落下强度达到55 N/个和3.4次/0.5 m;焙烧后球团的抗压强度最高达到1958 N/个.     

Effect of spherical-agglomerate strength on the distribution of force during uniaxial compression

We employ the carbon paper technique with the aim of investigating the effect of spherical-agglomerate (pellet) strength on force distributions, through confined compression of approximately 1 mm sized pellets formed from microcrystalline cellulose and polyethylene glycol. The carbon paper technique relies on the transference of imprints from compressed pellets onto white photo quality paper, which are digitised and processed via image processing software. The investigated pellets can both deform plastically and develop localised cracks in response to an applied stress, while remaining largely intact during confined compression. Our results indicate that such crack formation - henceforth referred to as fracture - has a decisive influence on force distributions. Previous work on non-fracturing systems has found that the distribution of normalized forces tends to narrow with increasing particle deformation. No narrowing is observed after the point of fracture in this study and the width of the distributions - as quantified by the standard deviation of non-normalized forces - is found to increase with the difference between non-normalized mean force and fracture force. Additional corroborative results show that spatial force-force correlations typically exhibit a marked change once the fracture force is exceeded.     

Controlled relative humidity storage for high toughness and strength of binderless green pellets

An equation was developed to predict fracture toughness of green powder compacts. The model combines crack tip toughness predicted by Kendall's model with crack tip shielding due to bridging of moisture meniscuses across the crack. The model predicts that crack tip shielding due to moisture should be dominant. Fracture tests on ceria green pellets verified that storing pellets at a high relative humidity (98% RH) for an extended period of time led to fracture strength more than double those stored at lower RH. However, at lower RH there is no significant increase in fracture strength with increased RH as predicted by the model. The lower strength at low RH is due to insufficient capillary and surface forces but may also be related to the lack of sufficient adsorbed moisture to form bridging meniscuses. The high green strengths achieved by storing pellets at a high RH suggest a method of strengthening green parts without adding binder.     

The measurement of porosity for an individual taconite pellet

The knowledge of the variability in the properties of individual taconite pellets as a function of processing conditions may lead to the improvement of the production of iron. An important property is porosity. Pellets with high porosity are desirable for the reduction to iron in the blast furnace. There is a published work describing the measurement of porosity on collections of pellets. Here I describe a method for the determination of the porosity of an individual pellet.Recent determinations of porosity have used the measurements of the skeletal volume and the envelope volume. Helium pycnometry is the method of choice for the measurement of the skeletal volume, whereas volumetric displacement of dry material is now the preferred method for the envelope volume, requiring collections of pellets. I have adapted a method of silhouettes, developed for single items of fruit, to measure the envelope volume of a single pellet. The porosities of six individual sintered pellets from a facility in North-Eastern Minnesota, USA, range from 33 to 38% with a relative uncertainty of about 1%. Certain pellets have significantly different porosities from each other. The magnitudes are comparable to published porosities on green pellets, ranging from 30 to 36%, and to fired pellets, ranging from 28-38%.