Application of Modified Humic Acid (MHA) Binder in the Pelletizing Of Fluxed Hematite Concentrate

Modified humic acid (MHA) binder consists of high molecular weight organic molecules and inorganic part. It is extracted from lignite coal with sodium hydroxide and used in pelletization of iron ore concentrates. Our previous study shows that MHA binder is also a potential binder instead of bentonite for pelletizing of fluxed hematite. For evaluating the use of MHA binder in pelletization of fluxed hematite, pelletizing tests were conducted to optimize parameters, such as dose, firing temperature and time. The results show that the qualities of green/dried balls with 0.6 wt% MHA binder are equal to or even better than that of 0.66 wt% bentonite pellets, and that both are much higher than the minimum requirements of the pellets’ strengths. The compression strength of fired pellets also suggested that MHA binder is promising to completely replace bentonite in pelletizing of fluxed hematite concentrate. However, the abrasion rate of the fired pellets with MHA binder is slightly higher than that of bentonite pellets.     

Humic Substance-based Binder In Iron Ore Pelletization: A Review

Due to the reducing ability and bonding effect, a humic substance-based binder has been added into iron ore pellets, both as a reductant and a binder. However, humic substance-based binders were not commonly used in pelletization since some pelletizing results indicated they were not as good asbentonite or other binders. Thus, there were few detailed investigations using humic substance-based binders in pelletization before the 1980s. Funa, which is a type of humic substance-based binder extracted from lignite, was successfully invented and applied in cold-bonded pellets in China. Since the organic fraction in the humic substance-based binder is burnt away during heating, leaving no contaminant in pellets and improving the iron content of the pellets, humic substance-based binders were also gradually utilized in oxidized pellets. On the basis of Funa binder research, the extracting procedure of a humic substance-based binder was modified, and a composite binder named Modified Humic Acid (MHA)was prepared for oxidized pellets, especially for making Vanadium-Titanium (V-Ti) magnetite pellets, and achieved qualified V-Ti magnetite oxidized pellets in industrial testing. The behaviors of humic substance-based binders in wet balls, dry balls and fired balls were well investigated. Moreover, MHA binder was gradually tested in a lab for pelletization of several other types of iron ore concentrates, such as magnetite from different districts, specularite and fluxed hematite, and good quality pellets were obtained. A review of the development of a humic substance-based binder and its characteristics, preparing method, and behaviors in pelletizing were considered in this paper.     

Hard Alloy WC – 24% Ni Obtained in the Solid Phase from Ultrafine WC, NiO, and C Powders. Part 1. Density and Structure of Specimens

We have studied the density and structure of specimens of the alloy WC – 24 mass% Ni, obtained by combining into one step the processes of synthesis of the metallic phase and compaction of the ultrafine mixture of WC – Ni powders by high-energy pressing and sintering. We have established that reduction of nickel monoxide by carbon occurs at temperatures of 650-750°C and does not affect the shrinkage process which in the case of sintering begins only at a temperature of 1050°C. High-energy pressing of briquettes sintered at the indicated temperature reduces their porosity from 30-25% down to 8-4%. Specimens of porosity <1% can be obtained by pressing at 1150°C or 1050°C in the case of triple pressing. Raising the temperature at which the briquettes are heated is accompanied by enlargement of the pores together with a decrease in the total porosity, but at temperatures of 1300°C (sintering) and 1250°C (pressing), the pore dimensions are sharply reduced. The high density of the specimens pressed at low temperature does not provide low electrical resistance, which suggests the presence of weakly connected boundaries. When the specimens are sintered and pressed in the solid phase, we observe the growth of tungsten carbide particles. It is most rapid at 1150-1250°C, while at 1050°C the particle growth process slows down. Reduction of the metal oxide when the powders are heated promotes formation of structure in the higher temperature range.     

Replacement of bentonite in hematite ore pelletisation using a combination of sodium lignosulphonate and copper smelting slag

Bentonite is the most common binder used in iron ore pelletisation owing to its good bonding properties in green and dry pellets at both ambient and elevated temperatures. However, due to its high alumina and silica content, it increases the slag volume and energy consumption in downstream processes. Organic binders may be used to replace bentonite; however, they fail to provide strength at a high temperature (700–900°C) due to poor thermal stability during pellet induration. In the present study, an organic binder Na lignosulphonate (NLS) has been used along with copper smelting slag (Cu-SS). FeO in Cu-SS provides diffusion bonding at high temperature and maintains the strength of pellets even after evaporation/burning of NLS. It also enhances recrystallisation bonding at relatively lower temperature to provide good strength. The study has been carried out with hematite ore and varying amounts of NLS and Cu-SS. Copper smelting slag (1.0%) addition with 0.5%NLS has been found to be optimum to provide very good green properties and ～300?kg/pellet cold crushing strength (CCS) at 1250°C induration temperature. However, hematite pellets of similar basicity with 0.5% bentonite requires higher induration temperature (1300°C) to achieve a similar CCS. The developed pellet also shows better reducibility (80%), similar reduction degradation index (18%) and swelling index (10%) to the usual bentonite pellet. Thus, the induration temperature of hematite pellet has been lowered by 50°C using a combination of NLS and Cu-SS eliminating bentonite completely, which can provide a considerable energy and cost saving.     

The Influence of Induction Sintering on Microstructure and Deformation Behavior of Ti-5Al-5Mo-5V-3Cr Alloy

The influence of the induction sintering process at different temperatures on the behavior of the powder metallurgy Ti-5Al-5Mo-5V-3Cr alloy was investigated. Material for the research was produced by elemental powder blending, followed by the uniaxial cold compacting process. Powder compacts were induction heated and sintered within the temperature range of 1000 °C to 1300 °C. The influences of process parameters on the material behavior during sintering and its properties were studied. The microstructure examination was performed with particular attention to the pore size and distribution as well as the homogenization of the microstructure. The sintering temperature of 1200 °C proved to be critical for the dissolution of most alloying powder particles. Hot compression tests were performed to determine the formability of the obtained material. Significant differences in flow stress behavior between samples sintered at temperatures below and above 1200 °C were observed. The mechanical properties of the material before and after deformation were compared. The evolution of the microstructure of sintered Ti-5Al-5Mo-5V-3Cr alloy after hot deformation was analyzed with an emphasis on its influence on the material properties. Based on the conducted research, it was found that the adequate homogenization of the chemical composition and microstructure was achieved at the temperature of 1250 °C, and a further increase did not reflect in a significant improvement.

     

Mill scale as a potential additive to improve the quality of hematite ore pellet

Hematite pellet is required to be indurated at very high temperature to achieve its good strength as there is no exothermic heat of oxidation unlike magnetite. As mill scale contains mainly FeO and Fe₃O₄, any minor amount of its addition in pellet can provide in situ heat and enhance diffusion bonding and sintering. In this study, the mill scale generated in steel plant is added as magnetite input in hematite pellet both in acidic and in basic condition. It has been found that in fluxed pellet, mill scale can improve the properties of pellet. In acidic pellet, the induration temperature has been reduced to a great extent (1250–1275°C) and all properties have been found to be improved due to the addition of 15% mill scale. Mill scale shows enough potential to eliminate the flux addition in producing blast furnace quality pellet from hematite ore. Thus, the flux free acidic pellet has been developed even at very low temperature (1275°C) of induration.     

Phase transformations during the oxidation of calcium-containing titanium-vanadium slags and their influence on the formation of calcium vanadates

The processes of oxidation of titanium-vanadium slags with the participation of CaO are studied in the temperature range 800–1250°C, and the effect of these processes on the formation of calcium vanadates, which are easily dissolved in weakly acid media, is analyzed. The temperature ranges of the processes related to the oxidation of vanadium-containing phases and the release of vanadium oxides are found. It is shown that anosovite oxides in the temperature range 800–950°C, spinelides oxidize in the range 900–1050°C, and vanadium from the slag phase of calcium aluminotitanate oxidizes at 1050–1250°C. Vanadium-containing perovskite is the most stable phase: it undergoes insignificant structural changes upon roasting. The degree of vanadium extraction from slags reaches 88–92.5% under favorable conditions. The effect of the CaO and FeO contents in a slag on the oxidation of vanadium-containing phases is investigated. A decrease in the FeO content in a slag to 8% or below is shown to negatively affect the oxidation of vanadium in slag phases. At 8.3 or 5.0% FeO in a slag, the maximum degree of vanadium extraction from the slag is 81 or 59%, respectively.     

Oxidation of iron ore at moderate and high temperatures

The oxidation of magnetite and titanomagnetites in iron-ore sinter at moderate (400–1000°C) and high (1000–1350°C) temperatures is subjected to physicochemical analysis. The oxidation kinetics is studied on briquets of Olkhovsk magnetite concentrate and Kachkanar titanomagnetite concentrate, as well as samples of unfluxed Kachkanar pellets and pellets fluxed to a basicity of 1.3. At moderate temperatures, the limiting stage in oxidation is the diffusion of the reagent to sections of the surface smaller than the total spherical surface. At high temperatures, in both isothermal and nonisothermal conditions, the limiting stage in oxidation is the diffusion of oxygen in pellet pores. From the kinetic equations for isothermal oxidation, the apparent activation energy with the specified degree of conversion is calculated; its variation is associated with change in the type of reagent diffusion through the layer of reduction products. The apparent diffusion coefficients of oxygen in Kachkanar pellets are determined at 500–1000°C. A method has been developed for determining the degree of pellet oxidation as a function of the time and the temperature in nonisothermal conditions. This method may be used to calculate the oxidation of the pellets in roasting on conveyer machines. The results may be used to determine the degree of oxidation in the roasted pellet bed and to optimize the heat-treatment parameters in roasting systems.     

Phase Composition of Oxide Scales during Reheating in Hot Rolling of Low Carbon Steel

Low carbon steel was oxidized over the temperature range 1000‐1250°C in O₂‐CO₂‐H₂O‐N₂, O₂‐H₂O‐N₂, and O₂‐CO₂‐N₂ gas mixtures. Oxidation times were 12‐120 min. and the scales were 50‐2000 μm thick. The variations of these parameters were chosen to elucidate the phase composition of oxide scales under conditions similar to those of reheating furnaces in hot strip mills, using either thin slab casting or conventional casting and rolling technology. Two types of scales have been observed which are influenced by the furnace atmosphere, oxidation time, and temperature. The first type is a crystalline scale with an irregular outer surface, composed mostly of wustite (FeO), and a negligible amount of magnetite (Fe₃O₄). The second type is the classical three‐layer scale, composed of wustite (FeO), magnetite (Fe₃O₄), and hematite (Fe₂O₃). In general, the experiments showed that an increase in oxidation time decreased the percentage of wustite while the percentages of magnetite and hematite increased. A rise in oxygen concentration in the gas mixture increased the percentages of magnetite and hematite, confirming earlier experimental findings. In water vapour‐free atmospheres O₂‐CO₂‐N₂, the oxide scales had a low percentage of wustite, and high percentage of magnetite and hematite. Carbon dioxide showed a small influence at 1100°C, and a negligible one at 1250°C.     

Pellets Prepared with Mechanically Activated Iron Ore

This work analyses pellets prepared with iron ore that has been mechanically activated by high energy ball milling. Pellet feed iron ore was submitted to high‐energy ball milling for 60 minutes, and the resulting material was analysed through measurements of particle size and specific surface area, as well as X‐ray diffraction. Pellets were prepared from this material. The pellets were heated at temperatures ranging from 1000 to 1250°C in a muffle furnace, and submitted to the maximum temperature during 10‐12 minutes. The samples were then tested regarding crushing strength, densification and porosity, and were examined in a scanning electronic microscope. The results were compared to those obtained with similar samples made from non‐milled pellet feed. It has been shown that through high‐energy ball milling of iron ore it is possible to achieve pellets presenting high densification and compressive strength at firing temperatures lower than the usual ones.     

Sintering characteristics and kinetics of acidic haematite ore pellets with and without mill scale addition

Haematite ore pellets require very high induration temperature (>1573?K) while, magnetite ore pellets require much lower temperature due to the oxidation of magnetite during induration. Mixing of some magnetite in haematite ore can improve the sintering property of pellets during induration. Mill scale is a waste material of steel plant which contains mainly FeO and Fe₃O₄. It can also be blended in haematite ore pellet mix which can enhance diffusion bonding and recrystallisation bonding and facilitate sintering at the lower temperature like magnetite ore. The extent of improvement in sintering property, sintering mechanism and its kinetics in the presence of mill scale is very imperative to study. In current study, the sintering characteristics of acidic iron ore pellet with 15% mill scale and without mill scale has been studied separately through microstructure observation, apparent porosity measurement and volume change. The volume changes due to heating at varying temperature and time has been measured by mercury displacement method and the data has been exploited for sintering kinetics study, wherein, extent of sintering α has a power relation with time. Several kinetics parameters such as time exponent (n), rate constant (k) and activation energies have been estimated for above two pellets and compared. While acidic pellet without mill scale requires 385?k?cal?mol^?1, acidic pellet with 15% mill scale requires only 310?k?cal?mol^?1 activation energy.     

Water based processing of iron powder utilising starch consolidation

Abstract

The effects of water based shaping, by means of starch consolidation (SC), of an iron powder system regarding oxygen/carbon content and sintering performance were evaluated. Specifically, the influence of the drying conditions and the use of two different thickeners, xanthan gum and cellulose ether, were studied. The results showed that cellulose ether gave lower sintered density than xanthan gum, mainly because of less favourable rheological impact and air/gas entrapment at mould filling and consolidation. Due to less oxidation at drying and less removal of carbon at sintering, freeze dried specimens sintered to a higher density than room temperature air dried ones. The degree of oxidation and removal of carbon also influenced the as sintered microstructure. Ferrite grains surrounded by iron phosphide were found in both air dried and freeze dried specimens. However, the higher carbon content in freeze dried specimens also resulted in a significant amount of iron carbide grains (inclusions), which can be a potential strength limiting factor.     

Use of Iron Ore Fines in Cold-Bonded Self-Reducing Composite Pellets

The feasibility of producing direct reduced iron from cold-bonded, self-reducing composite pellets, constituted from beneficiated iron ore slime, coke, and different binders (dextrin, bentonite, calcium lignosulfonate, and carboxymethyl-cellulose [CMC]) was studied. This was done using a design of experiments approach. It was found that as-received beneficiated iron ore slime is suitable as a raw material for the production of self-reducing composite pellets with carboxymethylcellulose as the most suitable binder. Dry strengths in excess of 300 N/pellet were attained by curing the pellets under ambient conditions. The composite pellets reduced within 20 min to degrees of metallization in excess of 90% at 1100°C, with decrepitation indices significantly below 5%. The degree of metallization of composite pellets increased with an increase in reduction temperature (from 1000 to 1100°C), reduction time (20 min. vs. 40 min), and coke quantity (15% vs. 20%). CMC was identified as the most economical and suitable binder for the Sishen concentrate.     

Formation and decomposition of C4F7 type calcium ferrites in superfluxed magnetite based pellets during oxidation and reduction

Abstract

In the current work the reactions of magnetite based pellets with large additions of calcite (3%CaO) during reduction have been investigated. This made it possible to use both X-ray diffraction (XRD) and scanning electron microscopy (SEM) to detect reaction phases that normally occur in very small amounts. The main binding phase in the pellets after oxidation was (CaO,MgO,FeO)₄(Fe₂O₃)₇, whereas the one commonly reported in the literature is (CaO)(Fe₂O₃)₂. During reduction at 500–700°C severe cracking occurred in these pellets, especially in the calcium ferrite phase. However, the decomposition of this phase began at 600°C, and therefore it is believed that the reason for the cracks is low strength of the phase itself, rather than weakness induced by reduction of the phase. Upon reduction of magnetite into wüstite at 800°C, the calcium began dissolving in the wüstite, and at 900°C porous calciowüstite had formed in the entire sample, except for some remaining magnetite left in the pellet cores.     

SINTERING KINETICS IN IRON-COPPER ALLOYS WITH AND WITHOUT A LIQUID PHASE

Abstract

Sintering of iron-copper alloys has been studied in the temperature range 950–1250°C. The factors involved include compacting pressure, sintering temperature, sintering time, and atmosphere. The results are interpreted as a decrease in pore volume due to the filling of voids between particles by a diffusion mechanism. An empirical equation of the Arrhenius type, based upon volume change as a function of sintering time, has been derived in order to evaluate the rate constant of the sintering process.

Volume diffusion is considered to be the primary mechanism of material transport in alloys containing 0·5–2·0% copper, when sintered in the range 950–1250°C, and in alloys containing 5·0–10·0% copper, when sintered in the range 950–1050°C. The activation energy derived for the sintering process is 53·4 kcal/mole. Surface diffusion appears to be the operative mechanism of material transport in alloys containing 5·0–10·0% copper, when sintered above the melting point of copper. The activation energy for this sintering process is 32·6 kcal/mole.     

Solid-phase prereduction of iron-vanadium concentrates and liquid-phase separation of the products of their reduction

The transformations that occur in ore grains during solid-phase carbon reduction of the metals from the iron-vanadium concentrates formed upon the beneficiation of the titanomagnetite ores from Southern Ural deposits are studied. Upon heating to 1000°C, the solid solution in titanomagnetite grains decomposes with the formation of magnetite and ilmenite; the reduction of iron begins in the temperature range 1080–1110°C, and the reduction of titanium begins at above 1215°C. The reaction mixture should be held at 1250°C for 45 min to ensure almost complete iron reduction and the minimum degree of titanium reduction. For rapid separation melting, this procedure results in vanadium-containing cast iron (0.43–0.5% V) with <0.15% Ti and a slag with 42–43% titanium oxides.     

The Reaction Behavior of Direct Reduced Iron (DRI) in Steelmaking Slags: Effect of DRI Carbon and Preheating Temperature

An experimental study was conducted to quantify the rate of direct reduced iron (DRI) decarburization in a steelmaking slag using the constant volume pressure increase technique. Experiments were conducted by dropping DRI pellets into molten slag at temperatures from 1773 K to 1873 K (1500 °C to 1600 °C). Subsequent experiments were carried out in which the DRI pellets were preheated while the slag temperature remained constant. The effect of the initial carbon content and the preheating temperature of the DRI on the reaction rate was investigated. The decarburization of DRI seems to comprise two stages, a reaction between the FeO and DRI followed by decarburization through the iron oxide of slag. Carbon has a significant effect on the kinetics of both stages, whereas the preheating temperature mainly influences the rate of decarburization between FeO and carbon inside the pellet.     

Can Modified Starch be Used As A Binder For Iron Ore Pellets?

Organic binders are often desired when making low-silica iron ore pellets. Corn and wheat are grown in large quantities near certain iron ore pelletizing facilities and their starches are easily modified to form cold-water-soluble powders that can be used as binders. We investigated how starch cold-water solubility, starch dose, starch hydration time, green ball moisture content, and firing temperature affected pellet quality. With a fluxed, hematite concentrate, the high-soluble starch led to good wet and dry balls, but weak and friable indurated pellets compared to the standard binder, bentonite clay. As expected, the low-soluble starch did not make as good green balls as the high-soluble starch. Thermogravimetric analyses of unfired pellets and their abrasion products showed that modified starch was inhomogeneously distributed in pellets, with a high concentration near the ball surfaces. The surface concentration increased, and the core concentration decreased, as pellets grew during the pelletizing process. This suggests that starch enrichment near surfaces is a consequence of the agglomeration–compaction process, and may occur with other pelletizing binders. Abrasion mass losses were 81% greater with modified starch than with bentonite at 1100°C, and 31% greater at 1250°C. However, starch contents near the surfaces did not qualitatively correlate to roughness, as the highest starch dose tested gave the smoothest and least dusty pellets.     

Rate of the reduction of the iron oxides in red mud by hydrogen and converted gas

The drying and gas reduction of the iron oxides in the red mud of bauxite processing are studied. It is shown that at most 25% of aluminum oxide are fixed by iron oxides in this red mud, and the other 75% are fixed by sodium aluminosilicates. A software package is developed to calculate the gas reduction of iron oxides, including those in mud. Small hematite samples fully transform into magnetite in hydrogen at a temperature below 300°C and a heating rate of 500 K/h, and complete reduction of magnetite to metallic iron takes place below 420°C. The densification of a thin red mud layer weakly affects the character and temperature range of magnetizing calcination, and the rate of reduction to iron decreases approximately twofold and reduction covers a high-temperature range (above 900°C). The substitution of a converted natural gas for hydrogen results in a certain delay in magnetite formation and an increase in the temperature of the end of reaction to 375°C. In the temperature range 450–550°C, the transformation of hematite into magnetite in red mud pellets 1 cm in diameter in a converted natural gas is 30–90 faster than the reduction of hematite to iron in hydrogen. The hematite-magnetite transformation rate in pellets is almost constant in the temperature range under study, and reduction occurs in a diffusion mode. At a temperature of ～500°C, the reaction layer thickness of pellets in a shaft process is calculated to be ～1 m at a converted-gas flow rate of 0.1 m³/(m² s) and ～2.5 m at a flow rate of 0.25 m³/(m² s). The specific capacity of 1 m² of the shaft cross section under these conditions is 240 and 600 t/day, respectively. The use of low-temperature gas reduction processes is promising for the development of an in situ optimum red mud utilization technology.     

Influence of Phase Transitions on the Macrokinetics of the Gaseous Reduction of Iron Oxide

The reduction of magnetite pellets is studied by thermogravimetry in a hydrogen flow upon linear heating. A multiple decrease in the reduction rate is observed at the degrees of metallization higher than 70% in a temperature range of 920–950°C. The observed effect of a decrease in the reduction rate by four to five times on heating is related to the α → γ phase transition of iron. Temperature cycling (heating–holding–heating–holding…) in the phase transition range shows that this effect is reversible; i.e., the reduction rate increases on cooling, unlike that on heating. The average temperature of the beginning of the effect in heating–cooling cycles (913°C) turns out to be close to the thermodynamic temperature (911°C) of the α → γ phase transition of iron. Isothermal studies of the reduction rate of pellets in a temperature range of 900–1000°C also confirm this phenomenon. The effect of a decrease in the reduction rate is assumed to appear due to a decrease in the effective coefficient of gas diffusion in γ-iron as compared to α-iron.