Effects of SiO_(2) on the preparation and metallurgical properties of acid oxidized pellets

The effects of SiO_(2) content on the preparation process and metallurgical properties of acid oxidized pellets, including compressive strength, reduction, and softening–melting behaviors, were systematically investigated.Mineralogical structures, elemental distribution, and pore size distribution were varied to analyze the mechanism of the effects.The results show that with an increase in SiO_(2) content from 3.51 wt%to 7.18 wt%, compressive strength decreases from 3150 N/pellet to 2100 N/pellet and reducibility decreases from 76.5% to 71.4%.The microstructure showed that pellets with high SiO_(2) content contained more magnetite in the mineralogical structures.Additionally, some liquid phases appeared, which hindered the continuous crystallization of hematite.Also, the softening–melting properties of the pellets clearly deteriorated as the SiO_(2) content increased.With increasing SiO_(2) content, the temperature range of the softening–melting zone decreased, and the maximum differential pressure and the comprehensive permeability index increased significantly.When acid oxidized pellets are used as the raw materials for blast furnace smelting, it should be combined with high basicity sinters to improve the softening–melting behaviors of the comprehensive charge.

Mechanisms of Swelling of Iron Ore Oxidized Pellets in High Reduction Potential Atmosphere

The influences of the time, temperature and atmosphere on the reduction swelling of oxidized pellets were investigated by single factor experiments. The mechanisms of reduction swelling of oxidized pellets were analyzed and investigated by SEM （scanning electron microscopy） and XRD （X-ray diffractometer） analysis. The results show that the change rules of reduction swelling index of oxidized pellets in different reduction atmospheres are very similar. With the increase of reduction time, the reduction swelling index moves up firstly and then down. When the reduction temperature is above 900 ℃, α-quartz turns into α-tridymite, and the transition generates additional volume expansion effect. The reduction swelling index changes faster in H2 atmosphere than in CO atmosphere. Increasing Ha content in the reduction atmosphere is useful to decrease the reduction swelling index, but it is also easy to cause oxidized pellets cracking.

Reduction kinetics of iron oxide pellets with H_2 and CO mixtures

Reduction of hematite pellets using H2-CO mixtures with a wide range of H2/CO by molar （1：0, 3：1, 1：1, 1：3, and 0：1） at different reducing temperatures （1073, 1173, and 1273 K） was conducted in a program reducing furnace. Based on an unreacted core model, the effective diffusion coefficient and reaction rate constant in several cases were determined, and then the rate-control step and transition were analyzed. In the results, the effective diffusion coefficient and reaction rate constant increase with the rise in temperature or hydrogen content. Reduction of iron oxide pellets using an H2-CO mixture is a compound control system; the reaction rate is dominated by chemical reaction at the very beginning, competition during the reduction process subsequently, and internal gas diffusion at the end. At low hydrogen content, increasing temperature takes the transition point of the rate-control step to a high reduction degree, but at high hydrogen content, the effect of temperature on the transition point weakens.

Diffusion behavior and distribution regulation of MgO in MgO-bearing pellets

In this paper, the diffusion behavior between MgO and Fe2O3（the main iron oxide in pellets） is investigated using a diffusion couple method. In addition, the distribution regulation of MgO in MgO-bearing pellets is analyzed via pelletizing experiments. The results illustrate that MgO is prone to diffuse into Fe2O3 in the form of solid solution; the diffusion rate considered here is 13.64 μm·min^（-1）. Most MgO content distributes in the iron phase instead of the slag phase. The MF phase {（Mg（1-x）Fex）O·Fe2O3, x ≤ 1} is generated in the MgO-bearing pellets. However, the distribution of MgO in the radial direction of the pellets is inconsistent. The solid solution portion of MgO in the MF phase is larger in the outer layer of the pellets than in the inner layer. In this work, the approximate chemical composition of the MF phase in the outer layer of the pellets is {（Mg（0.35-0.77）·Fe（0.65-0.23）） O·Fe2O3} and in the inner layer is {（Mg（0.13-0.45）·Fe（0.87-0.55））O·Fe2O3}.

Effect of alumina occurrence form on metallurgical properties of hematite and magnetite pellets

The effect of alumina occurrence form on the metallurgical properties of both hematite and magnetite pellets was investigated at the same Al_(2)O level of 2 wt.%,including reduction index(RI),low-temperature reduction disintegration index(RDI),reduction swelling index(RSI),and high-temperature softening-dripping performance.The mineralogy of fired pellets was also studied to reveal the influence of alumina occurrence form on the phase composition and microstructure.From the results,the alumina occurrence form presents tremendous impacts on the metallurgical perfor-mance of both magnetite and hematite pellets.Addition of all alumina occurrence forms contributes to inferior reducibility of pellets,especially in the case of gibbsite for magnetite pellets with a RI of 58.4%and kaolinite for hematite pellets with a RI of 56.8%.However,addition of all alumina occurrence forms improves the RDI of magnetite pellets,while there is no significant difference among various alumina occurrence forms.In contrast,alumina occurrence forms have little influence on the RDI of hematite pellets.The presence of free alumina,gibbsite,and kaolinite tends to improve the RSI of hematite and magnetite pellets,whereas hercynite gives the opposite trend with a RSI of 25.6%.For softening-dripping performance of magnetite pellets,all alumina occurrence forms contribute to narrower softening-melting interval.Meanwhile,alumina,gibbsite,and kaolinite give narrower softening-dripping interval,at 229,217,and 88℃,respectively,whereas addition of hercynite results in the largest melting range at 276℃ due to its high melting point.Regarding hematite pellets,free alumina,gibbsite,and hercynite tend to enlarge melting range,whereas kaolinite contributes to lower dripping temperature of 1148℃ and narrow softening-dripping interval of 88℃ due to the formation of a greater amount of slag phase at high temperatures.

Effects of Temperature and Atmosphere on Pellets Reduction Swelling Index

After taking into account the conditions of the domestic iron resources and the non-coking coal resources, the process of coal gasification-shaft furnace is an effective way to develop direct reduction iron in China. The following tasks are very critical to choose suitable process of shaft furnace and gasification, including the production of oxidized pellets with excellent comprehensive properties as well as the study of the reaction behavior and mechanism of swelling. The results showed that the oxidized pellets of using domestic magnetic iron concentrate as raw materials have favorable comprehensive properties, including higher mechanical strength both before and after reduction, faster reduction rate and lower reduction swelling index (RSI). All of these properties can meet the shaft furnace yielding requirement. When the temperature was below 1 223 K, the pellets′ RSI was lower than 20%. With increasing of the content of H2 in atmosphere, the pellets reaction rate accelerated, crushing strength enhanced and RSI decreased. The RSI dropped to 10.26% at 1 323 K in 100% H2 atmosphere, and it is up to 39.88% in 100% CO atmosphere. The iron grains mainly presented in platelike when pellets were reduced by H2, however, in CO atmosphere the iron grains were precipitated in flocculent. The whisker shape of iron grains and heating effects of reduction reaction are the major factors leading to the poor pellets strength and increase of RSI. Appropriately controlling the temperature and increasing the ratio of H2 to CO in atmosphere are good for dropping the RSI.

Non-Isothermal Kinetics of Reduction Reaction of Oxidized Pellet Under Microwave Irradiation

The microwave heating characteristics of the mixture with oxidized pellet and coal was studied, and the non-isothermal reduction dynamics is discussed. The results show that, the slow-heating stage of the temperature rising process can be segmented into two heating temperature curves approximately that have good linear relation- ship. They can be seen as temperature programming. In the first stage, between 827 and 1073 K, the reaction mechanism obeys diffusion controlled model. In the second stage, between 1 093 and 1 323 K, the reaction mecha- nism also obeys diffusion controlled model. The apparent activation energies are found to be 75.13 kJ/mol for the first stage and 53.17 kJ/mol for the second stage. That is lower than the apparent activation energy under conven- tional heating. The microstructure of the reduced pellets shows that microwave can improve the kinetics of the reduc- tion. Microwave has anxo-action to the reaction obviously.

Temperature Field Simulation Model for Rotary Kiln of Iron Ore Oxidized Pellet

Based on conduction,convection and radiation heat transfer among the flue gas,kiln wall,and the pellet bed material,and on the basis of the coal combustion and analysis of reaction heat of pellet induration in the rotary kiln,the temperature field model of rotary kiln was established.Using visual studio net,matlab and open source computer vision library as development tools,combining with the ActiveX data objects database technology,the temperature field simulation system for rotary kiln of iron ore oxidized pellet production was developed.Temperature distribution of pellet and flue gas in rotary kiln was dynamically displayed.

Analysis of Gas Thermodynamic Utilization and Reaction Kinetic Mechanism in Shaft Furnace

The technology of coal gasification in shaft furnace is an effective way to develop direct reduction iron in China. In order to clarify the process of the reduction of oxidized pellets in shaft furnace by carbon monoxide or hydrogen in two ways, i.e. thermodynamics and kinetics, the gas utilization and reaction mechanism were studied by theoretical computations and isothermal thermogravimetric experiment. The results showed that the gas utilization increased with the rise of temperature when xH2/xco≥1 and with the increase of xco/（xH2 ＋xco） when temperature is less than 1073 K. The water-gas shift reaction restrains efficient utilization of gas, particularly in high tem- perature and hydrogen-rich gas. The gas utilization dropped with increase of carburization quantity of direct reduction iron （DRI） and oxygen potential of atmosphere. With the increase of both temperature and content of H2 in inlet gas, the reaction rate increased. At 100% Hz atmosphere, the interfacial chemical reaction is the dominant reaction re- stricted step. For the H2-CO mixture atmosphere, the reduction process is controlled by both interfacial chemical reaction and internal diffusion