Sintering Of Agglomerates Research Articles

Phase evolution of stainless-steel pickling sludge and blast-furnace gravity dust during high-temperature process

Stainless-steel pickling sludge (SSPS) and blast-furnace gravity dust (BFGD) are solid wastes and hazardous materials produced during iron- and steel-making processes and are important secondary metallurgical resources. The comprehensive use of these two materials shows important environmental significance and economic value. Herein, the elemental content, phase composition, and microstructure of SSPS and BFGD samples were then analyzed. Then, a high-temperature process for SSPS, BFGD, and their mixtures (mass ratio of 1:1) in air and N2 was analyzed by thermogravimetry and differential scanning calorimetry, and the high-temperature calcination products were identified and analyzed using X-ray diffraction. In air, the mixture of SSPS and BFGD can generate SFCA after roasting at 1200 °C. In N2, the carbon in the BFGD reduced the CaSO4 in SSPS to CaS. Based on the high-temperature phase-transformation behavior of the SSPS and BFGD mixture under aerobic and anaerobic conditions, a novel process flow of mixture pelletizing‒sintering agglomeration–blast furnace smelting was proposed for the synergistic treatment of SSPS and BFGD. This process is expected to provide innovation for stainless steel production enterprises to realize the closed circuit recycling of solid waste without leaving the factory.

Modernization of the agglomeration factory of PJSC “KAMET-STEEL”: organizational, technological, environmental and technical and economic aspects

A short-term industrial project of modernization of the agglomeration factory of PJSC “KAMET-STEEL” (Ukraine, Kamianske) is proposed. The implementation of the project is carried out in three directions — improvement of agglomerate sintering technology, overhaul of agglomeration machine No. 7 and improvement of the gas purification process of process gases and aspiration of the main dust removal units. The implementation of the project involves the following stages: engineering works; supply of equipment and production of technological metal structures; construction and assembly works and putting the modernized agglomeration machine into operation. For the capital repair of agglomeration machine No. 7, it is proposed to carry out: repair of the longitudinal and transverse seals of the agglomeration machine; repair of the gas collector, vacuum chambers, dust bags, compensators in the sintering zone, vacuum chambers in the cooling zone; restoration of thermal insulation on the gas collector, vacuum chambers, dust bags. For the purification of process gases at the sintering plant of PJSC “KAMET-STEEL” it is proposed to: replace the exhauster; installation of a new gas treatment; improvement of aspiration systems; dismantling of quenching and return cooling drums. An opportunity is shown by improving the process of gas purification of process gases and aspiration of the main dust removal units, to achieve a dust content of no more than 30 mg/m3, a CO content — of up to 6500 mg/m3, an SOx content — of no more than 500 mg/m3, and to reduce the overall dustiness at the sintering plant by 20 %. The technical and economic substantiation of the feasibility of modernization of the agglomeration factory of PJSC “KAMET-STEEL” is given. It has been established that the introduction of agglomerate sintering technology due to the use of energy-saving measures will lead to a 1.08 % decrease in the cost of 1 ton of agglomerate, which in turn will affect the decrease in the cost of 1 ton of cast iron.

Heterogeneities and defects in powder compacts and sintered alumina bodies visualized by using the synchrotron X-ray CT

The uniaxial die compaction and sintering is an important industrial production method for metal and ceramic components. While submicron- and nano-sized powders are finding increasing use for making dense materials with finest grain sizes, the uniformity of microstructure, and then, the reliability of the products depend on the heterogeneity in powder packing structure. Here we applied the synchrotron X-ray multiscale-CT to characterize the complex domain structures, i.e., agglomerates, in powder compacts, and revealed how heterogeneous distribution of fine residual pores is formed by the differential sintering of agglomerates by using a submicron alumina powder as a model. A crack-like defect was identified as the largest defect. This defect was formed along the boundary between a plate-shaped agglomerate and its surroundings. A technique was also proposed to visualize the heterogeneous distribution of residual pores by using SEM. These characterization methods will open up new possibilities for the optimization of ceramic processing.

Ni/CeO2 prepared by improved polyol method for DRM with highly dispersed Ni

AbstractDry reforming of methane (DRM) of Ni‐based catalysts is a hot method to solve greenhouse gas problems. The improved polyol method could overcome the disadvantage of Ni agglomerate sintering by the impregnation method. In this work, Ni/CeO2 catalysts were prepared by an improved polyol method with different CeO2 support and applied for methane dry reforming, which showed excellent catalytic activity and stability. The X‐ray power diffraction (XRD), transmission electron microscopy (TEM), X‐ray photoelectron spectra (XPS), H2 temperature‐programmed reduction (H2‐TPR) were used to study the composition of Ni species, the Ni‐CeO2 interaction, and Ni dispersion, while textural properties, oxygen vacancy, and the behavior of CO2 adsorption and activity were research by N2 adsorption and desorption, Raman and CO2‐TPD. There in, Ni/CeO2‐R showed excellent catalytic activity (CH4 and CO2 conversion: 61.8% and 96.5%; H2/CO radio: 0.93) and remained stable after the 50‐hr stability test. Meanwhile, the formation and graphitization of deposited carbon were observed and studied by XRD, Raman, TG/DTA, and TEM, and the cause of the deactivation of the catalyst was analyzed. This work studied and demonstrated the advantages of the improved polyol method and the differences between different CeO2 supports and provided a new idea for the construction of the catalyst for methane dry reforming. © 2021 Society of Chemical Industry and John Wiley & Sons, Ltd.

A Simple Model for Gas-Phase Synthesis of Nickel Nanoparticles

Gas-phase synthesis of nickel (Ni) nanoparticles by thermal decomposition of nickel tetracarbonyl, Ni(CO)4, is simulated accounting for nucleation, surface growth, coagulation, and sintering. By detailed analysis of phenomenological expressions for sintering, it is shown that surface diffusion (SD) is the dominant sintering mechanism for Ni nanoparticles at low temperatures (T < 700 K) and early stages of sintering, but grain boundary diffusion (GBD) dominates as sintering progresses and at higher temperatures. This is consistent with molecular dynamics simulations of noble metal nanoparticle sintering. Using the average of the above SD and GBD characteristic sintering times for Ni as well as a monodisperse population balance model (MPBM) that accounts for agglomerate morphology, polydispersity, and evolving structure with scaling laws from mesoscale simulations, Ni agglomerate sintering is benchmarked with measurements of agglomerate mobility and primary particle diameters. The MPBM predictions are in good agreement with the measured concentration and sizes of Ni nanoparticles by thermal decomposition of Ni(CO)4 in a hot wall flow reactor.

Effect of heat treatment on structure of nickel-based catalysts

Nickel-based catalyst is the general-purpose catalyst with a wide application and good performance. Widely used in fine chemicals, petrochemicals and other industries, it has received continuous attention from researchers. Taking silica gel (SiO2) and HZSM-5 molecular sieves as carriers, and Ni(NO3)2 · 6H2O and Cu(NO3)2 · 3H2O as precursors, in the work we prepared a series of NiO/SiO2, NiO/HZSM-5 and NiCu/HZSM-5 catalysts by impregnation method. Focused on the effects of heat treatment (baking) temperature and time on the structure of the catalysts, the structure of the catalysts was characterized by X-ray diffraction (XRD) spectroscopy. The results showed that in terms of carriers, HZSM-5 had stronger interaction with active metal particles than that SiO2 presented. It could better prevent the sintering agglomeration of catalytic particles during heat treatment, which is beneficial to preparing catalytic particles with smaller particle size. SiO2 exhibited better thermal stability for higher heat treatment temperatures and longer heat treatment time. At the same time, the heat treatment at the high temperature for a long time was liable to cause sintering growth of the active metal particles of the catalysts. The selection of suitable heat treatment process conditions was critical to obtaining the highly active nickel-based catalysts. The work briefly discussed the effect of heat treatment temperature and time on the structure of the catalysts, expecting to provide the useful reference for the preparation technology of supported nickel-based catalysts.

Preparation of nickel porous materials by sintering nickel oxalate and sodium chloride after blending and reduction

In this paper, sodium chloride(NaCl) was used as the separator agent and the pore-forming agent. Nickel porous materials were prepared by sintering nickel oxalate(NiC2O4) and NaCl after blending and reduction. The pore micro morphology, porosity, pore size, pore size distribution and permeability coefficient of nickel porous materials at different sintering temperatures were studied. The results show that NaCl as a separator can effectively prevent numerous direct contacts between the initial nickel particles and then reduce the sintering agglomeration during the process of blending. Thus, uniformly mixed fibrous nickel powders and NaCl powders are prepared. Nickel porous materials prepared by sintering nickel oxalate and NaCl after blending and reduction have a pore structure composed of lapped pores between the fibrous nickel particles and the prefabricated pores formed by the pore-forming agent. By changing the sintering temperature, the porosity and pore structure characteristics of the materials can be well controlled, and the nickel porous materials with good pore properties are obtained. When the sintering temperature increases from 450 °C to 650 °C, the open porosity is 71.7 % ∼ 79.5 %, the average pore size is from 0. 519 μm ∼ 1.043 μm, and the permeability is from 0.076×10−12 m2 ∼ 0.19×10−12 m2. The pore size distribution shows three peaks at low sintering temperatures. The distribution gradually turns into a double peak and eventually tends to a single peak as the sintering temperature increases.

ОСОБЛИВОСТІ СПІКАННЯ АГЛОМЕРАТУ ПРИ ВИКОРИСТАННІ ШИХТИ З ПОПЕРЕДНЬО ПІДГОТОВЛЕНИМИ КОМПОЗИТАМИ

У статті досліджували різні способи спільного, а також роздільного грудкування компонентів шихти. Задачею є розробка, теоретичне та експериментальне обґрунтування способу грудкування агломераційної шихти, який дозволить формувати гранули заданого гранулометричного та мінералогічного складу, створивши умови для отримання в процесі спікання агломерату блочної структури високої міцності. Проведено дослідження ефективності роздільного грудкування шихти з використанням композитів заданого складу з метою формування міцного агломерату блочної структури заданого хімічного та гранулометричного складу.Запропоновано спосіб роздільного грудкування з використанням композиту з концентрату, залізної руди крупністю 0-3 мм, вапна та вапняку, основність якого складатиме 0,9-1,0 од. Технологія передбачає дозування, змішування та грудкування даного композиту, при цьому залишкова шихта, основністю 1,6-1,8 од., дозується та змішується паралельно. Після цього відбувається спільна грануляція композитів в барабані-грануляторі. Паливо, крупністю 0-7 мм подається наприкінці грануляції. Виробництво агломерату запропонованим способом дозволяє збільшити вихід придатного агломерату на 10,29 %, і фракції +5 мм на 11,5% після випробування на міцність. Ця технологія призвела до зменшення кількості фракції 0-1 мм та збільшення еквівалентного діаметра гранул; зменшення середньо-квадратичного відхилення та коефіцієнту варіації. Найкраща якість агломерату була забезпечена при використанні композиту з «концентрату – руди – вапна – вапняку», основність якого складала 0,9-1,0 одиниць. Запропонована технологія передбачає дозування, змішування та грудкування даного композиту. Залишкова шихта, основністю 1,6-1,8 од., дозується та змішується паралельно. Після цього відбувається спільна грануляція композитів в барабані-грануляторі. Паливо, крупністю 0-7 мм подається наприкінці грануляції.

Effects of the number of particles and coordination number on viscous-flow agglomerate sintering

The process of sintering of several particles in contact via a viscous flow mechanism was studied numerically using computational fluid dynamics. The volume of fluid technique within a finite volume method was used to simulate bridge formation between particles, as well as densification at different configurational states of the particles. The method was validated by comparing results for two-particle coalescence with the literature. The effect of the number of particles on agglomeration kinetics was studied by comparing bridge growth rate for systems having different numbers of particles in a chain. Although increasing the number of particles led to a decrease in the local bridge growth rate and to slower equilibration, there were no marked differences, when the overall volume of the system was considered. The effect of coordination number on the densification rate was directly studied by changing the number of particles in contact with a central particle. Increasing the coordination number caused the overall rate of densification to increase, but delayed equilibration, analogous to steric effects. These findings describe the configurational state of agglomerates, typical of mesoscale caking. In a multi-scale study, they can be used to characterize caking at a bulk scale to partly address the lack of experimental data in this field.

ON THE USE OF KOVDOR IRON ORE IN SINTERING PROCESS

Recently, the demand for iron ore concentrates of the Kovdor deposit has been maintained, despite the complexity of their use for the production of man-made raw materials. Magnetite of Kovdor ore has a heterogeneous structure and its iron does not participate in melt formation processes during sintering of agglomerates. Therefore, up to the basicity of 2.0, a silicate glass phase is a binder of ore grains of the finished product. With the increase in basicity above 2.0, the mineral composition and microstructure of the agglomerates change. Magnetite is oxidized to hematite, an alumino-siliciferite appears on the hematite contact with the high-calcium melt, the residual melt crystallizes to form a titanium-containing silicate.

Self-agglomeration mechanism of iron nanoparticles in a fluidized bed

Aided by self-agglomeration, a two-stage reduction process conducted at a higher temperature (600 °C) than the single-stage process resulted in an enhancement of the k constant to more than twice that of the single-stage process. A force balance model coupled with the reduction kinetics of Fe2O3 is first proposed to explain the self-agglomeration mechanism of iron nanoparticles (NPs) during the reduction. This force balance model successfully elucidates the reason for the prevention of defluidization via a two-stage fluidized bed method. At lower temperatures, there is a long stationary phase, which is of great importance in overcoming the sintering of agglomerates and in promoting the reduction reaction. At lower temperatures, the initial NPs first self-agglomerate into particles that are tens of microns in size; they then gradually agglomerate into larger particles (>100 μm) at higher temperatures. By contrast, the sudden growth of iron NP agglomerates causes sintering and defluidization in the single-stage fluidized bed method.

Behaviour of engineered nanoparticles in a lab-scale flame and combustion chamber

Nanostructured materials are widely used to improve the properties of consumer products such as tires, cosmetics, light weight equipment etc. Due to their complex composition these products are hardly recycled and thermal treatment is preferred. In this study we investigated the thermal stability and material balance of nanostructured metal oxides in flames, in a pilot scale combustion plant and an industrial hazardous waste incinerator. We studied the size distribution of nanostructured metal oxides (CeO2, TiO2, SiO2) in a flame reactor and in a heated reaction tube. In the premixed ethylene/air flame, nano-structured CeO2 partly evaporates forming a new particle mode. This is probably due to chemical reactions in the flame. In addition sintering of agglomerates takes place in the flame. In the electrically heated reaction tube however only sintering of the agglomerated nanomaterials is observed. Ceria has a low background in waste incinerators and is therefore a suitable tracer for investigating the fate of nanostructured materials. Low concentrations of Ceria were introduced by a two-phase nozzle into the post-combustion zone of a waste incinerator. By the incineration of coal dust in a burning chamber the Ceria nanoparticles are mainly found in the size range of the fly ash (1 – 10 µm) because of agglomeration. With gas as a fuel less agglomeration was observed and the Ceria nanoparticles were in the particle size range below 1 µm.

Thermal Stability and Material Balance of Nanomaterials in Waste Incineration

Nanostructured materials are widely used to improve the properties of consumer products such as tires, cosmetics, light weight equipment etc. Due to their complex composition these products are hardly recycled and thermal treatment is preferred. In this study we investigated the thermal stability and material balance of nanostructured metal oxides in flames and in an industrial waste incinerator. We studied the size distribution of nanostructured metal oxides (CeO2, TiO2, SiO2) in a flame reactor and in a heated reaction tube. In the premixed ethylene/air flame, nano-structured CeO2 partly evaporates forming a new particle mode. This is probably due to chemical reactions in the flame. In addition sintering of agglomerates takes place in the flame. In the electrically heated reaction tube however only sintering of the agglomerated nanomaterials is observed. Ceria has a low background in waste incinerators and is therefore a suitable tracer for investigating the fate of nanostructured materials. Low concentrations of Ceria were introduced by a two-phase nozzle into the post-combustion zone of a waste incinerator. By the incineration of coal dust in a burning chamber the Ceria nanoparticles are mainly found in the size range of the fly ash (1 – 10 µm) because of agglomeration. With gas as a fuel less agglomeration was observed and the Ceria nanoparticles were in the particle size range below 1 µm.

Incorporation of Zr into Calcium Oxide for CO2 Capture by a Simple and Facile Sol–Gel Method

A series of Zr-doped CaO-based sorbents were synthesized by a simple and facile sol–gel method, and sample of Ca/Zr (30:1) exhibited excellent CO2 capture capacity (0.64 g CO2/g sorbent) and favorable stability (up to 18 cycles). The performance of sorbent still remained stable when tested under a high decarbonation temperature of 900 °C. The structure–property relationship of the sorbents was explored by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), transmission electron microscope (TEM), and N2 physisorption measurement, respectively. The favorable CO2 capture characteristics of CaO-based sorbents were attributed to the CaZrO3 nanoparticles, which created an inert barrier preventing the sintering–agglomeration, the nanosize of the particles, optimized pore size distributions, and uniform distribution of Ca and Zr in the sorbent.

Morphological changes of nano-Al agglomerates during reaction and its effect on combustion

Agglomeration of aluminum nanoparticles (nano-Al) is inevitable due to the strong van der Waals forces. It is supposed to affect the combustion characteristics of nano-Al. However, the agglomeration effect is not fully understood yet due to the worse description of the morphological evolution of agglomerates during reaction. In this paper the morphological changes of agglomerated nano-Al, which mainly results from the oxide shell thickening when the particle temperature is below the melting point of alumina, are confirmed in the thermogravimetric analyzer (TGA) and Hencken experiments. Three factors (fA, fht, and fsp) are defined to describe the morphological changes quantitatively through proposed formulas. Thus the agglomerate could be equivalent to an effective spherical particle to help studying the effect of agglomeration on combustion. The results indicate two combustion modes for different agglomerates. The small Al agglomerate is hard to combust fully due to the strong heat transfer with ambience, corresponding to the high-temperature oxidation mode. However, at the same environment, as the agglomerate becomes larger, the maximum particle temperature (Tp,max) increases until that the agglomerate burns fully, corresponding to the fully-fledged combustion mode. At that mode, the burning particle temperature is mainly controlled by the viscous sintering of agglomerates.

EFFECT OF NATURAL ORE FORMATION ON MINERAL COMPOSITION AND COLD STRENGTH OF IRON ORE FLUXED AGGLOMERATES

The article displays the determining infl uence of natural iron ore mineralization on the sintering process and the strength of fl uxed sinter. Magnetite crystals with homogeneous structure actively move into the melt, forming the silicate binder ore grains in agglomerates of low basicity. With the growth of the basicity the change of silicate ligaments to ferrite ones can be determined by the amount of iron in the melt and high oxidation potential of the gas phase. The participation of magnetite of heterogeneous structure in processes of melt formation was limited. During the sintering of agglomerates the low iron content in the melt expanded the range of crystallization of silicate ligaments, shifting the start of the process of ferrite formation toward to the agglomerates of high basicity.

Identification of the compensation effect in the characteristic sintering time model for population balances

The sintering of agglomerates under high temperature determines the diameter and morphology of particles. Accurate sintering model is essential to the process simulation for the particle dynamics. A method of combining population balance modeling and inverse problem methodology was applied in sintering simulation process to investigate relationship between effective kinetic parameters in the characteristic sintering time model, i.e., two dynamic parameters (the pre-exponential factor A and the apparent activation energy E). The polydisperse primary particle (PP) model was introduced to consider the inhomogeneous structure in agglomerates. Two inverse problem methodologies, tabulation method and response surface method, were employed by fitting simulation results to experimental measurements. A contour map about the difference between simulation results and experimental measurements as a function of various parameter sets was obtained. Optimal values were obtained when the difference is small. A linear relationship between the two uncertain kinetic parameters was identified, which is similar to the kinetic compensation effect in the Arrhenius equation for reaction rate. The linear relationship holds true for the sintering of both TiO2 and SiO2 agglomerates at least, which are dominated by the surface diffusion mechanism and the viscous flow transport mechanism, individually.

Consolidation of Hierarchy-Structured Nanopowder Agglomerates and Its Application to Net-Shaping Nanopowder Materials

This paper provides an overview on our recent investigations on the consolidation of hierarchy-structured nanopowder agglomerates and related applications to net-shaping nanopowder materials. Understanding the nanopowder agglomerate sintering (NAS) process is essential to processing of net-shaped nanopowder materials and components with small and complex shape. The key concept of the NAS process is to enhance material transport through controlling the powder interface volume of nanopowder agglomerates. Based upon this concept, we have suggested a new idea of full density processing for fabricating micro-powder injection molded part using metal nanopowder agglomerates produced by hydrogen reduction of metal oxide powders. Studies on the full density sintering of die compacted- and powder injection molded iron base nano-agglomerate powders are introduced and discussed in terms of densification process and microstructure.

Consolidation of Iron Nanopowder by Nanopowder-Agglomerate Sintering at Elevated Temperature

【The key concept of nanopowder agglomerate sintering (NAS) is to enhance material transport by controlling the powder interface volume of nanopowder agglomerates. Using this concept, we developed a new approach to full density processing for the fabrication of pure iron nanomaterial using Fe nanopowder agglomerates from oxide powders. Full density processing of pure iron nanopowders was introduced in which the powder interface volume is manipulated in order to control the densification process and its corresponding microstructures. The full density sintering behavior of Fe nanopowders optimally size-controlled by wet-milling treatment was discussed in terms of densification process and microstructures.】

Aggregate morphology evolution by sintering: Number and diameter of primary particles

The structure of fractal-like agglomerates (physically bonded) and aggregates (chemically or sinter-bonded) is important in aerosol synthesis of nanoparticles, and in monitoring combustion emissions and atmospheric particles. It also influences particle mobility, scattering and eventually performance of nanocomposites, suspensions and devices made with such particles. Here, aggregate sintering by viscous flow of amorphous materials (silica, polymers) and grain boundary diffusion of crystalline ceramics (titania, alumina) or metals (Ni, Fe, Ag, etc.) is investigated. A scaling law is found between average aggregate projected area and equivalent number of constituent primary particles during sintering: from fractal-like agglomerates to aggregates and eventually compact particles (e.g. spheres). This is essentially a relation independent of time, material properties and sintering mechanisms. It is used to estimate the equivalent primary particle diameter and number in aggregates. The evolution of aggregate morphology or structure is quantified by the effective fractal dimension (Df) and mass–mobility exponent (Dfm) and the corresponding prefactors. The Dfm increases monotonically during sintering converging to 3 for a compact particle. Therefore Dfm and its prefactor could be used to gage the degree or extent of sintering of agglomerates made by a known collision mechanism. This analysis is exemplified by comparison to experiments of silver nanoparticle aggregates sintered at different temperatures in an electric tube furnace.