Rotary Hearth Furnace Process Research Articles

Process and mechanism of preparing metallized blast furnace burden from metallurgical dust and sludge

Metallurgical dust and sludge are solid waste resources with recycling value. In recent years, rotary hearth furnace has become the most important means to treat metallurgical dust and sludge because of its wide range of raw materials and strong treatment capacity. In this study blast furnace ash and converter sludge were selected as the research objects, and high-quality metallized pellets were prepared based on the rotary hearth furnace process. The strength changed of pellets, the reduction process of iron oxides and the removal process of zinc during the roasting of pellets in rotary hearth furnace were studied. To explore the reasonable roasting condition for preparing metallized pellets in rotary hearth furnace. The optimum roasting temperature of the pellets was 1250℃ and the roasting time was 25 min. The compressive strength, metallization rate and dezincification rate of metallized pellets reached 1361N, 97.44% and 95.67%, respectively. The efficient resource utilization of various metallurgical dust and sludge is realized.

Volatilization behavior of lead, zinc and sulfur from flotation products of low-grade Pb-Zn oxide ore by carbothermic reduction

Existing methods are difficult to directly treat the tailings of Lanping Pb-Zn deposit because of low Pb + Zn content. According to the characteristics of high CaO content (11%) in the flotation products of Lanping low-grade Pb-Zn oxide ore, a carbon thermal reduction method based on rotary hearth furnace (RHF) process was developed. When reaction temperature was above 1000 °C, CaO could realize the transformation of metal sulfides to oxides. Increasing reducing atmosphere was favor of the efficient volatilization of Zn and Pb and the trapping of S. The volatiles in reducing agents could promote the volatilization of zinc. The reduction effect of anthracite and thermal coal was better than that of charcoal. The volatilization rates of Zn and Pb exceeded 98% and 96%, respectively, under the conditions of 1250 °C, 30 min, 20% thermal coal addition and air atmosphere. High quality ZnO dust with a ZnO purity of 83% was obtained.

Enhanced reduction of multiple layers carbon containing pellets in rotary hearth furnace

To enhance the reduction of multiple layers carbon containing pellets in rotary hearth furnace (RHF) process, a three-layer bed pellets were investigated using an electric furnace by simulating the RHF process. It was found that the metallization rate of the pellets on the upper layer was greatly higher than that on the middle and lower layers due to the low reduction efficiency and heat transfer efficiency. Various types and amount of reducing agent in carbon containing pellets showed different metallization rate and shrinkage rate, which affected the reduction efficiency and heat transfer efficiency of pellets. By optimization the type and amount of reducing agent, the metallization rates of reduced pellets for each layer of the bed all achieved more than 85% at 1300 °C and holding 20min, the metallization rates of reduced pellets for each layer of the bed reached over 90% at 1300 °C and holding 25 min.

Optimized rotary hearth furnace utilization with blast furnace and electric arc furnace: Techno-economics, CO2 reduction

Two strategies to use a rotary hearth furnace (RHF) are discussed from the perspectives of techno-economics and CO2 reduction. One is to use the RHF to produce low-reduced iron (LRI), which is then used in a blast furnace (BF); the other is to use the RHF to produce direct reduced iron (DRI) and use it in an electric arc furnace (EAF). Using a phenomenological model of the furnaces, we compared the mass and energy balances of the two strategies and estimated the unit production cost of the products. The main result is that integration of the RHF with the BF (193 US$ t−1 manufacturing cost; 0.75 tCO2-eq t−1 carbon emission) was more beneficial than with the EAF (656 US$ t−1; 2.62 tCO2-eq t−1). The key difference was the productivity because the two utilization routes require different operating times to reach a specified metallization degree. To optimize the operation of the RHF process, dual objectives of minimizing cost and minimizing CO2 emission were simulated. The Pareto front showed that the emission was reduced by 30% by increasing the cost by 35%.

Pig Iron Production (Post Blast Furnace Era)

It has become an environmental drive world-wide to reduce the production of Carbon Dioxide (CO2) in the steelmaking process. Alternative reductants other than coal-based materials has been found to be a true replacement with resultant major reduction in CO2, by using “cleaner” reductants like natural gas. Coke consumption in the Blast Furnace requires coke to produce pig iron for which alternative reductant routes are now sought. It has been proven that natural gas is a suitable reductant in the production of Direct Reduced Iron (DRI) in a shaft furnace or a Rotary Hearth Furnace (RHF) process. This DRI can then be used as raw material for pig-iron production in a closed Submerged-Arc Furnace (SAF), (being a circular AC Furnace, DC Furnace, or a large 6-in-line Rectangular Furnace), where a reduced amount of reductant is required for final reduction and introducing additional carbon required for the subsequent steelmaking processes. The SAF also acts as a slag modifier which adds value to this traditional waste product. Instead of producing aggregate, high quality granulated slag is produced which can be used as a clinker replacement material in the classical cement production. This contributes to the reduction of CO2 gasses by reducing traditional cement fabrication. Fossil-based reductant are still required through this process route, but the sum equates to a reduction of fossil-based reductants. The use of renewable energy as a source of electricity further adds to the reduction of emissions adding even more value to the whole production chain. A natural progression from this process step would be the introduction of environmentally friendly Hydrogen (H2) in the future.

Corrosion behavior of Al2O3 substrate by SiO2–MgO–FeO–CaO–Al2O3 slag

Alumina–based refractory is widely used as lining material of rotary hearth furnace (RHF) in the direct reduction process. However, corrosion of refractory by the burden is a crucial problem which affects the RHF production. In this study, a sessile drop technique was used to investigate the wetting and corrosion behavior of Al2O3 substrate against SiO2–MgO–FeO–CaO–Al2O3 slag, whose composition was determined according to the slag obtained from smelting nickel laterite in the RHF process. The experimental results showed that the slags with basicity ((MgO+CaO)/SiO2) of 0.43 and 0.63 had a low apparent contact angle of approximately 25° and 30°, respectively. It indicated the slag samples could wet well on the Al2O3 substrate. The spreading of slag on the substrate was mainly ascribed to the decrease of surface tension at the initial stage. As the wetting process proceeded, Al2O3 dissolved into the slag forming an interfacial layer which gradually covered the substrate surface under the droplet and inhibited further dissolution of Al2O3. Good wettability and the dissolution of Al2O3 are two main factors that caused the corrosion of substrate. Separating the burden from the Al2O3 substrate was restrained after cooling, which would possibly bring severe sticking problems between the burden and alumina refractories.

Study on the formation of direct reduced iron by using biomass as reductants of carbon containing pellets in RHF process

To investigate the possibility of using biomass reductants instead of fossil fuels in the formation of direct reduced iron (DRI) in rotary hearth furnace (RHF), three kinds of cheap biomass such as bamboo char, straw fiber and charcoal were selected as reductants to compare with anthracite and graphite. The effect of reductants, temperature, dwelling time, C/O ratio and particle sizes on the shrinkage, strength and metallization of the biomass/ore pellets was investigated and discussed. The optimized reduction conditions were obtained as 1200 °C, 20min, C/O = 0.7 and the particle size range of <0.15 mm, 1–2 mm and 0.15–0.3 mm for straw fiber, bamboo char and charcoal respectively. The results showed that straw fiber was considered as an optimal reductant in the formation of direct reduced iron instead of fossil fuels. By the techniques of scanning electron microscope (SEM), thermogravimetric analysis (TGA), and X-ray diffraction (XRD), the mechanism of biomass as reductants was revealed. Volatile matter was confirmed to play an important role in the formation of DRI when biomass used as reductants.

Mechanism and Influencing Factors of Iron Nuggets Forming in Rotary Hearth Furnace Process at Lower Temperature

In order to improve the efficiency of slag and iron separation, a new idea of "the separation of slag (solid state) and iron (molten state) in rotary hearth furnace process at lower temperature" is put forward. In this paper, the forming process of iron nuggets has been investigated. Based on those results, the forming mechanisms and influencing factors of iron nugget at low temperature are discussed experimentally using an electric resistance furnace simulating a rotary hearth furnace process. Results show that the reduction of iron ore, carburization of reduced iron, and the composition and quantity of slag are very important for producing iron nuggets at lower temperature. Reduction reaction of carbon-containing pellets is mainly at 1273 K and 1473 K (1000 A degrees C and 1200 A degrees C). When the temperature is above 1473 K (1200 A degrees C), the metallization rate of carbon-containing pellets exceeds 93 pct, and the reduction reaction is substantially complete. Direct carburization is the main method for carburization of reduced iron. This reaction occurs above 1273 K (1000 A degrees C), with carburization degree increasing greatly at 1473 K and 1573 K (1200 A degrees C and 1300 A degrees C) after particular holding times. Besides, to achieve the "slag (solid state) and iron (molten state) separation," the melting point of the slag phase should be increased. Slag (solid state) and iron (molten state) separation can be achieved below 1573 K (1300 A degrees C), and when the holding time is 20 minutes, C/O is 0.7, basicity is less than 0.5 and a Na2CO3 level of 3 pct, the recovery rate of iron can reach 90 pct, with a proportion of iron nuggets more than 3.15 mm of nearly 90 pct. This study can provide theoretical and technical basis for iron nugget production.

Biomass reducing agent utilisation in rotary hearth furnace process for DRI production

This study aims to provide a theoretical and technical basis for utilisation of biomass (bamboo char, charcoal and straw fibre) in the rotary hearth furnace process to produce direct reduced iron. Each biomass material has strengths and weaknesses. The bamboo char pellets possess high fixed carbon content, but the lower strength and the expansion rate of pellets make it difficult to reach production requirements. Although the charcoal based pellets are similar to coal based pellets, their strength is the lowest of these reducing agents. The straw fibre based pellets possess high strength and large volume shrinkage, but the low fixed carbon content results in low productivity. Using biomass in a combination of these alternatives can enable coal to be replaced and produce pellets with a higher metallization than with coal.

Exploration of Straw Fiber as Reducing Agent Utilization in Rotary Hearth Furnace Process for Direct Reduced Iron Production

Straw fiber is a waste obtained by jet milling after extracting valuable substances form corn straw through steam explosion-washing-mechanical carding process. At the same time, it is also a clean and renewable reducing agent. In this study, physical and chemical characteristics of straw fiber were investigated firstly. The usage of straw fiber in rotary hearth furnace process for direct reduced iron production was then discussed. The results show that on one hand, fixed carbon content in straw fiber is low, which is unfavorable for the productivity of direct reduced iron; on the other hand, ash and sulfur content is also low, which improves the quality of direct reduced iron. Besides, the fibrous structure is conducive to improving the pelletizing performance of carbon-containing pellets, thus reducing an additional amount of binder. Compared with common reducing agents, straw fiber provides better reduction effect, and straw fiber carbon-containing pellets possess higher compressive strength and larger volumetric shrinkage, which is beneficial to increase the thickness of the whole material layer in rotary hearth furnace. In addition, these carbon-containing pellets also have advantages of lowering C/O ratio and shortening reduction time. The appropriate C/O and reduction time are 0.8 and 15 min, respectively, and the particle size of straw fiber below 0.15 mm has better reduction effect. This study aims to provide theoretical and technical basis for the utilization of straw fiber in rotary hearth furnace process.

Comparison of electric arc furnace dust treatment technologies using exergy efficiency

Abstract Multiple methods exist to treat electric arc furnace dust (EAFD), aiming at high metal recovery and low landfilling needs. However, a technology that is energy efficient and recovers both Zn and Fe has only recently been developed to a commercial scale with the Rotary Hearth Furnace. Various other technologies have been proposed as alternatives to the historically preferred Waelz Kiln process. This gate-to-gate study presents an objective method to compare the overall thermodynamic performance and efficiency of treatment technologies through the use of an exergy analysis incorporating the dominant factors without the need of examining every process detail. In addition, the influence of the EAFD zinc content on the exergy efficiency of both the Waelz Kiln and Rotary Hearth Furnace process was studied. The efficiencies of these high temperature metal recovery processes were also compared to the efficiency of the newly proposed In-Process Separation technology.

Development Prospect of Rotary Hearth Furnace Process in China

Summarized the development situation of rotary hearth furnace (RHF) direct reduction technology, ore resource allocation situation and direct reduction iron demand. The survey results show that: China's iron ore resource allocation heavily rely on imported iron ore, gas-based direct reduction process (MIDREX, HYL-III, FINMET) is not likely to be the mainly direct reduced iron (DRI) process in China. However, non coking coal resources is very rich in China, research and development of coal-based direct reduction process (such as FASTMET and ITMK3 process) has important practical significance, it can strengthen the comprehensive utilization on low grade iron ore, associated mineral resources and iron &amp; steel plant dust and sludge. It has great significance to alleviate steel scrap demand and stable development of the iron and steel industry and protect the national economic security in China.

A Mathematical Model for Carbothermic Reduction of Dust-Carbon Composite Agglomerates

Recycling iron–bearing dust from steel mills has gained a considerable attention in the past two decades to recover the valuable metals in dust while improving the sustainability of steel production. As a method for extracting the metals from dust, the reduction of dust–carbon composite agglomerates using a rotary hearth furnace (RHF) has been practiced in the steel industry. The use of low–grade carbonaceous reductants, such as low–rank coal and waste plastic, is of steelmakers’ interest to further enhance the waste recycling using the RHF process. However, applying these materials as reductants has proven to be a challenging task since the impact of the released volatile gas from such reductants on reduction reactions is not predictable. In addition, the reduction kinetics of dust pellet in RHF is more complicated than the reaction behaviour of sintered ore in blast furnace due to the higher furnace temperature, faster reduction and rapid gas evolution inside the agglomerate. To predict the reaction behaviour of the dust–carbon composite in RHF, a mathematical model was developed. The model takes into consideration heat and mass transfer as well as the reduction reaction of iron and zinc oxides, gasification of carbon and release of volatiles. The simulated behaviour of a dust pellet by the proposed model provides beneficial information to promote recycling an expanded range of waste materials in the RHF process. The modelling approach and calculation results are discussed in the present paper.

Exploration of biomass char for CO2 reduction in RHF process for steel production

The application of biomass char as a substitute for coke in the rotary hearth furnace (RHF) process for producing direct-reduced iron (DRI) is developed. The proposed method reduces the amount of required coal, production cost, and carbon emissions. Experiment results show that coconut crust carbon and bamboo carbon have good metallurgical qualities, namely high fixed carbon content, high volatile matter content, high caloric value, low ash and sulfur content. Coconut crust, as a waste product in Taiwan, is an ideal raw material for biomass char. In the RHF process, the proper molar ratio of biomass char to iron ore is (C/O)mol=1. The crushing strength of DRI can be increased by adding 2.5% bentonite as binder. For a raw materials ratio of (C/O)mol=1, the optimal reaction time is 15min.

Effects of the Composition of Residual Materials from Steelwork on the Crushing Strength of DRI

The goal of this study is to investigate the effects of the composition of residual materials, such as the basicity and (C/Ored)mol, as well as the reduction temperature and time, on the crushing strength of direct reduced iron (DRI) under the carbothermic reduction through the rotary hearth furnace (RHF) process. Laboratory- scale experiments were executed to examine the DRI quality in the RHF operation. It is observed that when the composition of residual materials varies or the reduction temperature and time decrease, DRI crushing strength becomes lower than the 60 kg/piece requirement for blast furnace feed stock. Both basicity and (C/Ored)mol of the residual materials would affect the crushing strength of DRI. As B2 increase, the crushing strength of DRI also rises, and the maximum value is when the basicity is 1.4. The result shows that as the value of (C/Ored)mol rose above 1.20, the DRI strength declined.

The reduction of iron oxides by volatiles in a rotary hearth furnace process: Part III. The simulation of volatile reduction in a multi-layer rotary hearth furnace process

For reduction of iron oxides by volatiles from coal, the major reductant was found to be H2, and it can affect the overall reduction of iron oxides. In this study, the reduction by actual volatiles of composite pellets at 1000 °C was studied. The volatile reduction of the hand-packed Fe2O3/coal composite pellet as it is devolatilizing out of the pellet was found to be negligible. However, the reduction of iron oxide pellets at the top layer by volatiles from the bottom layers of a three-layer pellet geometry was observed to be about 15 pct. From the morphological observations of partially reduced pellets and the computed rates of bulk mass transfer, volatile reduction appears to be controlled by a mixed-controlled mechanism of bulk gas mass transfer and the limited-mixed control reduction kinetics. Using the reduction rate obtained from the single pellet experiments with pure hydrogen and extrapolating this rate to an H2 partial pressure corresponding to the H2 from the volatiles, an empirical relationship was obtained to approximately predict the amount of volatile reduction up to 20 pct.

The reduction of iron oxides by volatiles in a rotary hearth furnace process: Part II. The reduction of iron oxide/carbon composites

The reduction of iron oxide/carbon composite pellets with hydrogen at 900 °C to 1000 °C was studied. Compared to hydrogen, the reduction by carbon was negligible at 900 °C and below. However, significant carbon oxidation of the iron oxide/graphite pellets by H2O generated from the reduction of Fe2O3 by H2 was observed. At higher temperatures, reduction by carbon complicates the overall reduction mechanism, with the iron oxide/graphite composite pellet found to be more reactive than the iron oxide/char composite pellet. From the scanning electron micrographs, partially reduced composite pellets showed a typical topochemical interface with an intermediate region between an oxygen-rich unreacted core and an iron-rich outer shell. To determine the possibility of reduction by volatiles, a layer of iron oxide powders was spread on top of a high volatile containing bituminous coal and heated inside a reactor using infra-red radiation. By separating the individual reactions involved for an iron oxide/coal mixture where a complex set of reactions occur simultaneously, it was possible to determine the sole effect of volatile reduction. It was found that the light reducing gases evolve initially and react with the iron oxide, with complex hydrocarbons evolving at the later stages. The volatiles caused about 20 to 50 pct reduction of the iron oxide.

The reduction of iron oxides by volatiles in a rotary hearth furnace process: Part I. The role and kinetics of volatile reduction

With iron ore reduction processes using coal-ore pellets or mixtures, it is possible that volatiles can contribute to reduction. By simulating the constituents of the individual reducing species in the volatiles, the rates for H2 and CO were investigated in the temperature and reduction range of interest; hydrogen is the major reductant and was studied in detail. The kinetics of the reduction by H2 has been found to be a complex mechanism with, initially, nucleation and growth controlling the rate. There is a catalytic effect by the existing iron nuclei, followed by a mixed control of chemical kinetics and pore diffusion. This results in a topochemical reduction of these iron oxide particles. Up to 1173 K, reduction by H2 is considerably faster than by carbon in the pellet/mixture or by CO. It was also found that H2S, which is involved with the volatiles, does not affect the rate at the reduction range of interest.

Rotary hearth furnace process for steel mill waste recycling and direct reduced iron making

For the promotion of zero emission society, the recycling of dust and sludge produced in the steel making process, leading to the recovery of valuable metals like Iron, Zinc, Nickel and Chromium, has highly attracted public attention. The Plant and Machinery Division of Nippon Steel Corporation has introduced the basic technologies of the rotary hearth furnace, DryIronTM Process, from Maumee Research & Engineering Inc. in the United States, and developed the process in order to recycle such dust and sludge effectively. In addition, the DryIronTM Process can also produce direct reduced iron from virgin iron ore and non-coking coal. In this paper, the features of the DryIronTM Process and the machine outline in Hikari Works of Nippon Steel Corporation are described.