Pulse iron ore sintering—A novel approach to reduce sinter fines

Abstract

Iron ore sintering is carried out under negative downdraft suction in a packed bed system. Normal operation results in non-uniform temperature distribution in sintered bed resulting in the generation of sinter return fines (−5 mm) which reduces the process yield. In the present work, a novel approach of pulsating suction through the generation of palpitate downdraft suction flow and its effect on sinter bed and synthesized sinter properties is explored. Pilot pot sinter experiments involving varied pulsation magnitudes through variation in valve closure (30%, 60%, 100%) for a specified time are performed. An increment in the pulsation magnitude results in a higher pressure drop (50–200 mm) occurring due to alteration in local flow dynamics. The pulsation mechanism of the downdraft suction leads to the broadening of the flame and increased sinter bed temperature for a prolonged duration. At 100% value closure better sinter properties are realized due to improved heat transfer and gas–solid reaction kinetics. Enhancement in the sinter properties like tumbler index from 69.42% to 75.82%, reduction degradation index from 24% to 19%, reducibility index from 74.25% to 76.22% is attained. In addition, overall sinter return fines are reduced from 22.3% to 22.12%. Mineralogical investigation of the pulse sintered product reveals that the increased formation of silico-ferrite of calcium and aluminum (SFCA and SFCA-I) improved the sinter properties.

Keywords

Iron ore sintering pulsed suction flame front sinter quality

Introduction

Iron ore sintering is a critical process in ironmaking, where fine raw materials are transformed into a cohesive, porous mass suitable for use in blast furnaces. The process involves a mixture of primary raw materials, such as iron ore and limestone, and recycled return fines-undersized sinter particles smaller than 5 mm. As India solidifies its position as the world's second-largest producer of crude steel, optimizing the sintering process has become vital for improving productivity, resource efficiency, and reducing environmental impacts.¹

Sinter return fines (SRFs), accounting for 20–30% of the sinter product, significantly impact process efficiency by reducing yield, throughput, and energy consumption.^2–5 Their generation is closely tied to the mechanical strength of the sinter, particularly in the upper layers of the sinter bed, where uneven heat distribution weakens the structure.^6–11 As the flame front moves downward, the top layer experiences thermal shock and a sharp temperature drop, making it more fragile and prone to fines generation. Additionally, the higher oxygen partial pressure in the top layer accelerates coke combustion, converting wustite to hematite and creating a porous, fragile composition that further contributes to sinter fines. Studies have identified key factors influencing sinter strength, such as heat transfer, mineral composition, porosity, and pore size distribution. Garbers-Craig et al.⁸ observed that sinter strength increases from the upper to lower layers of the sinter bed due to improved thermal conditions. Oyama et al.¹² identified pore size distribution as a critical factor, accounting for up to 60% of the overall impact, alongside mineral phases like calcium ferrite and amorphous silicate. Sato et al.¹³ emphasized the role of porosity, mineral composition, and melting ratio in determining strength. Higuchi and Heerema¹⁴ demonstrated the significance of pore distribution, particularly the matrix length between adjacent pores. Kasai et al.¹⁵ linked shatter strength to the fluidity of liquid phases, noting that chemical composition, large pores, and the loss on ignition (LOI) of core particles are also crucial. Additionally, Loo and Wong¹⁶ highlighted that variable such as silica, alumina, magnesia, basicity, and peak sintering temperature influence bonding phases and pore structures through the reshaping and coalescence of melts and bubbles.

All the parameters influencing sinter strength ultimately converge on a few critical aspects of sintering operations, including sintering rate (suction, burn-through length, flame front speed),¹⁷ green mix packing density,¹⁷ basicity,^18,19 ignition conditions,^20,21 and sintering temperature.^16,22 Enhancing reaction kinetics and optimizing heat and mass transfer mechanisms are crucial for controlling these parameters and reducing return fines. Essential gas-solid reactions, such as coke combustion, carbonate dissociation, and iron ore reduction, are highly dependent on efficient heat and mass transfer within the densely packed sinter bed. Enhancements in these areas accelerate reaction rates, ensure complete reactions, and facilitate the formation of robust mineral phases, effectively minimizing the generation of return fines.

To address these challenges, this study investigates pulse sintering, an innovative technique introducing flow or pressure pulsations during the sintering process. Pressure pulsations offer significant potential to enhance heat and mass transfer rates, leading to improved process efficiency. By influencing gas-solid reactions, they alter the concentration gradient near solid surfaces, which enhances reaction kinetics. Periodic pressure variations have been shown to improve intra-particle transport and increase convective effects, leading to faster reaction rates.²³

Applications in related fields, such as limestone calcination and packed bed heat transfer, have demonstrated the benefits of pulsating pressure in improving thermal conductivity, heat transfer coefficients, and reaction efficiency.^24,25 When applied to the sintering process, these pulsations create localized flows around particles, inducing periodic forced convection. This results in higher heat transfer coefficients compared to steady-state conditions, leading to stronger and more consistent sintered material while minimizing fines generation.

Materials and methods

Raw materials

The formulation of the sinter mixture entails the utilization of specific raw materials, including iron ore fines sourced from the Joda region of Odisha, India, as well as limestone, pyroxenite, dolomite, burnt lime, SRFs, and coke breeze. A comprehensive chemical analysis of the fluxes and fuel utilized in the sinter test is presented in Table 1. For a detailed examination of the particle size distribution of the iron-bearing raw materials, fluxes, and fuels, readers are referred to the author's prior research work.²⁶

Table 1.

Chemical analysis of raw material used in sinter making (wt. %).

Material	T. Fe	CaO	SiO₂	MgO	Al₂O₃	FeO	Loss on ignition (LOI)
JODA iron ore	62.93	–	0.08	0.01	2.58	2.77	3.36
Pyroxenite	5.26	68.90	39.43	50.12	1.25	0.65	1.57
Lime fines	0.17	45.25	2.12	0.56	1.31	0.56	27.54
Limestone	0.00	36.25	5.62	7.49	2.12	–	48.25
Dolomite	0.83	–	2.14	18.62	4.00	0.67	43.65
Coke breeze	–		12.5	0.24	5.2	–	79.22

Methods

In this study, four sets of sinter pot tests were conducted to replicate both the conventional sintering method and the innovative approach introduced in this research depicted in Figure 1. The impact of the proposed pulse sintering method, involving the generation of controlled pulses in the suction flow during sinter making, with adjustments made to both decrease and increase the suction flow by specific magnitudes, was systematically assessed in terms of its influence on sinter process parameters and physical properties. The test was broadly divided in two to categorize without pulsation and with pulsation. Base case test was in the without pulsation category while experiments 2,3 and 4 come under with pulsation category.

The iron ore used in the preparation of the sinter base mix initially underwent screening, with the −10 mm fraction retained. These materials were meticulously combined in the desired proportions to achieve a CaO/SiO₂ basicity ratio of approximately 2.6. Moisture, approximately 6–7%, was introduced during the blending process. The resulting green mix exhibited an average particle size of 2–3 mm.

Pot sinter test

Four sets of pot grate sintering tests were conducted. The sinter was produced to match the target chemistry, with a CaO/SiO₂ basicity ratio of 2.3 and MgO content at 1.9%. The coke rate was held constant at 6.5% across all experimental sets. In terms of the bulk density of the green sinter bed within the 600 mm pot, it was maintained within the range of 1.8 to 2 g/cc, whereas in traditional sintering, this range typically falls between 2.2 and 2.4 g/cc. Measuring the total enclosed area above 1100 °C quantifies heat input during melt formation. When sintering a bed with low bulk density, the thickness of this region within the flame front is less sensitive to flame front speed. However, when sintering denser ores, this region is initially broad at lower flame front speeds but decreases in size as the flame front speed increases. To mitigate these challenges, low bulk density ores are employed.²⁷

A total of 100 kg of sinter raw mix was blended in a mixer drum, with moisture added to transform the fines into micro balls characterized by a mean particle size of 2.5 mm. The resulting green sinter mix was subsequently loaded into the pot sinter as shown in the schematic Figure 2. Throughout the trial sets, the initial suction rate and ignition flame temperature, essential components of the sintering process, remained consistent at 1200 mm of water column and 1100 °C, respectively.

Pot sinter test without pulsation effect

Laboratory sintering programs typically focus on assessing variations in comparison to a base case, as indicated in previous studies.²⁸ To evaluate the impact of pulsed air suction through the sintering bed, initial testing of the base blend is conducted under simulated plant conditions. This involves replicating parameters such as bed height and uniform suction across the bed to establish the foundational dataset.

Pot sinter test with pulsation effect

Apart from the base case test, three distinct experiments were conducted, each involving the manipulation of the valve or damper of a digitally operated PULSE generator to generate pulses. These experiments were executed as shown in Table 2.

Table 2.

Details of pot sinter experiments.

Experiment ids	Experiment conditions
Experiment 1	Base case-standard sintering was carried out with normal downdraft suction at 1200 mm water column.
Experiment 2	The damper valve of the Pulse generator closed to 30% for 20 s and then reopened to 100% for 60 s.
Experiment 3	The damper valve of the pulse generator closed to 60% for 20 s, followed by reopening it to 100% for 60 s.
Experiment 4	The damper valve closed to 100% (resulting in zero suction) and subsequently reopening it to 100% normal suction

The system comprises a damper valve that can be fully or partially opened according to the specific pulse requirements for the sintering process. When the damper in the pulse generator is fully open at 100%, it signifies a constant air suction without the presence of pulses. To generate pulses, the valve is periodically closed to different extents, such as 30%, 60%, and 100%, in intervals of 1–2 min. After a duration of 20–30 s, the damper is then fully reopened to the 100% position. This controlled valve manipulation creates the desired pulsation effect in the system are shown in Figure 3.

The fourth thermocouple plays an essential role in evaluating off-gas temperature, specifically measuring the burn-through temperature (BTP), which serves as an indicator of process completion. Once the BTP is reached, the pulse generator is turned off. At this point, the damper is fully opened, returning suction to its initial state at the beginning of the sintering process.

Throughout the firing stage of sintering, the timing for closing and reopening the damper remains constant. This uniform procedure ensures a consistent flame front speed during the entire sintering process. The deliberate closure of the damper for 20–30 s, followed by its full reopening to the standard setting, allows for sufficient retention time, enabling the flame and heat front to traverse the entire cross-section of the sinter bed across all three layers. This controlled manipulation of the damper contributes to process optimization and uniform heat distribution.

Stabilization of the fired sinter was achieved through four drops from a height of 2 m. After removing the −5 mm fines fraction, the remaining sinter within the size range of −40 to +10 mm underwent screening for tumbler and abrasion testing. The analysis covered reduction degradation index (RDI), reducibility index (RI), tumbler index (TI), Abrasion Index, sintering time, SRF Generation, bed shrinkage, and microstructural phase fractions. Concurrently, real-time monitoring of the upper sinter bed layer's top temperature provided insights into thermal dynamics.

Result and discussion

Effect of pressure pulsation on sintering process

This project suggests that introducing flow/pressure pulsations in the bed can enhance heat and mass transfer rates. Building on prior research, Hamer and Cormack²⁹ explored periodic external pressure to boost reaction rates. Sohn and Chaubal found that oscillating pressure in gas-solid systems, especially under pore diffusion, increased bulk flows, enhancing overall reaction rates.²³ Ballal showed cyclic pressure variations could enhance mass transfer.³⁰ Maithy²⁴ and Bedarkar²⁵ demonstrated increased rates and efficiency through pressure pulsation in limestone calcination and thermal conductivity, respectively.

Pressure pulsation in the sintering process has two significant effects:

It induces a local flow around each particle, leading to forced convection effects.

The induced local flow is periodic, and it's well-established that heat transfer coefficients in transient flows are consistently higher than in steady-state flows.

For a given amplitude of pulsation, higher frequencies result in greater flow velocity and acceleration. This leads to increased heat transfer rates, allowing for improved combustion propagation and reduced sintering time, thereby enhancing sintering efficiency. However, at very high frequencies, increased viscous and inertial effects can dampen the flow. Consequently, for a given sintering bed, there should be an optimal combination of frequency and amplitude of pulsation to achieve the desired effects. The selection of this optimum valve is determined by considering various factors, including the time-temperature characteristics, the amount of return fines generated from each layer, and other quality indices of the sinter.

Time–temperature distribution across sinter bed in constant suction sintering process

Figure 4(a) displays the temperature distribution measured by three thermocouples, while the fourth thermocouple records the off-gas temperature. The arrangement of thermocouples at different heights within the sinter bed is depicted in Figure 2. The graph indicates that bed temperature 1 represents the temperature of the bed at a depth of approximately 200 mm. The temperature profiles recorded by each thermocouple exhibit a consistent pattern. Initially, there is a rise to a stable value of 60–70 °C, indicating the beginning of water evaporation. Subsequently, the bed undergoes preheating by hot gases. The ignition of coke combustion occurs as the temperature reaches its ignition point. This was followed by a rapid increase in local temperature, signifying the completion of the combustion process. Finally, the bed temperature declines due to convective cooling induced by fresh air.³¹ The temperature of thermocouple 1 increases from room temperature to 1185 °C in 10 min, then drops to around 200–150 °C within a 5–5.5-min interval. Thermocouple 2, located below thermocouple 1, marked as bed temp 2, reaches a temperature close to 1250 °C in 14 min, dropping to 200–150 °C after 16–18 min. The thermocouple at the middle layer of the sinter bed experiences the longest sintering time, with temperatures rising to 1340–1360 °C. The total estimated sintering time is 26 min.

Time–temperature distribution across sinter bed in pulsed suction Process

Figure 4(b) depicts the temperature distribution during the PULSE sinter process with the valve closed to 30% of the total opening of the PULSE generator. The valve closing time is maximally 20 s. Bed temperature 1 represents the temperature of the bed at a depth of approximately 200 mm. The temperature of thermocouple 1 increases from room temperature to 1230–1240 °C within 18 min and drops to around 200–150 °C over a 6–7-min interval. Thermocouple 2, located below thermocouple 1 (bed temp 2), reaches a temperature close to 1260 °C in 14 min, dropping to 200–150 °C after 18–19 min. The thermocouple at the middle layer of the sinter bed experiences the longest sintering time, with temperatures rising to 1340–1360 °C. The total estimated sintering time is 24 min.

Figure 4(c) and 4(d) reveal the temperature distribution with different damper closure settings during the PULSE sinter process. In Figure 4(c), the valve is closed to 60% for 20 s, while Figure 4(d) shows the temperature distribution when the valve is fully closed to 100% for 20 s.

Compared to the results observed with 30% damper closure and pulse generation, it's evident that in the 60% damper closure and 100% damper closure scenarios, the sinter bed retains temperatures above 1200 °C for a longer period. The sintering time is also shorter. An important observation from Figure 4(c) and 4(d) is that all three-bed thermocouples achieve a temperature of 1200 °C more quickly, retain at this temperature for more than 3–5 min, and then slowly cool down to 150–200 °C. The BTP is higher in the latter two cases, indicating improved sinter kinetic rates and reactions throughout the sinter bed.

Effect on sinter residence time and average area above 1200 °C

The influence of sintering suction on sinter residence time and the enclosed area above 1200 °C was step-by step examined. The quality of produced sinter is primarily governed by factors such as sintering time, the maximum temperature attained during sintering process, and the duration of residence at elevated temperatures.³¹ As the sintering process advances, the combustion of solid fuel and evaporation of water enhances porosity, promoting improved oxygen supply and bed permeability. Furthermore, prolonged preheating duration enhances combustion and heat utilization efficiencies. In conventional scenarios, uneven heat distribution leads to insufficient heating in the upper bed, which weakens sinter strength, while excessive heating in the lower bed causes unnecessary energy waste.³¹ The introduction of pulsed sintering ensures uniform heat distribution, contributing to the production of consistent sinter. The top layer of the sinter in the pulsed suction process reaches the desired 1200–1300 °C range and maintains this temperature for an extended period, even with a reduced sintering time of less than 23 min. Table 3. illustrates that an increase in pulsation magnitude extends the residence time above 1200 °C and result in slower cooling compared to conventional sintering.

Table 3.

Time intervals during which the thermocouples maintain temperatures exceeding 1200 °C and subsequently cool down to 150–200 °C at the conclusion of the sintering process.

	Time above 1200 °C, minutes
Process	Bed thermocouple1	Bed thermocouple 2	Bed thermocouple3	Total time
Normal sintering	Not reach the temp	2	7	9
Sintering with 30% valve closed pulse	2	4	4	10
Sintering with 60% valve closed pulse	3	5	5	13
Sintering with 100% valve closed pulse	4	5	5	14

The quantification of sinter strength often involves assessing the enclosed area above 1100 °C^27,32 a parameter linked to the melt quality index, as noted by Cheng et al.³¹ This metrics align with findings reported by Loo and Harvey. In this study, a high residence time at temperatures exceeding 1200 °C is believed to produce sinter with higher strength and yield.^9,33–37 In conventional sintering low temperature of the top portion indicates low agglomeration of granules hence lower yield.³¹ K. Hara et al. reported a lower yield (∼60%) in the top 10% part of the bed, this support Zhilong et al. report also.^31,37

Figure 5 provides a clear estimation of the positive impact of pulsation on the sintering process. In the last case, where the valve was closed and opened at 100% pulse, the maximum area coverage above 1200 °C at different heights of the sinter bed was achieved. This enhanced temperature distribution and increased coverage area at higher temperatures contributed to better microstructure and phase formation in the sinter. Additionally, the temperature gradient from the top to the bottom bed of the sinter was reduced. In comparison, the normal sintering process exhibited significantly lower area coverage at higher temperatures.

Pressure variation during sintering process

The negative effects of pressure drop during iron ore sintering process encompass various aspects that impact the overall efficiency and quality of the sinter produced. A significant pressure drop can lead to reduced airflow, hindering combustion and heat transfer within the sinter bed. This, in turn, may results in incomplete reactions, the formation of undesirable phases, uneven temperature distribution, increasing sintering time and decreasing productivity.

T. singh et al. emphasized that pressure drop in the iron ore sintering process is predominately influenced by the pre-melt reaction zone (PMRZ), with minimal contribution from fully sintered and un-sintered regions.³² The PMRZ involves crucial reactions such as dehumidification, dihydroxylation of goethite, flux calcination, combustion, and flux melting. The introduction of pulsed sintering, characterized by a 20-s closed valve followed by a 60-s opening valve, significantly reduces pressure variations. This is attributed to the increase in reaction kinetics and a decrease in resistance offered by the PMRZ. In contrast to conventional sintering, where negative pressure starts at 1200 WC and ends around 800–900 WC, pulsed sintering exhibit minimal variation, concluding with a drop of only 100–200 WC without extending the sintering time shown in Figure 6.

Mechanism of flame front travel during sintering process and its impact on key parameters

The distinctions between conventional and pulsed suction sintering, as discussed in the preceding sections, highlight several improvements brought about by the pulsed suction technique. From the time-temperature graph, it is evident that sintering time is shorter in pulsed sintering, yet the sinter bed remains above 1200 °C for a longer duration, with a higher BTP. Additionally, pulsed suction enables the top layer of the sinter bed to reach and maintain 1200 °C for an extended period without increasing the overall sintering duration. The enclosed area under the temperature curve above 1200 °C is also larger compared to conventional sintering. Furthermore, the final pressure drop during pulsed sintering is significantly reduced, ranging from 100 to 200 mmWC, as opposed to 400 to 500 mmWC in conventional sintering. These observations can be better understood by examining the mechanism of flame front travel during sintering.

During the sintering process, the flame front progresses as the thin layer of solids immediately ahead of it reaches the spontaneous coke combustion temperature, approximately 1100 °C.³⁸ Convection is the primary mode of heat transfer, carrying heat from the flame front to this critical layer. The time required for this layer to reach the combustion temperature is directly influenced by airflow rates, which, in turn, affect both the flame front speed and the overall sintering duration.

Key observations from the sinter bed

Figure 7(a) illustrates the movement of the flame front and heat front from the top to the bottom of the sinter bed, disrupting equilibrium as it progresses. The retention time of sinter in the top layer (300–400 mm of the total 600 mm height) is shorter compared to the middle and bottom layers. Temperature distribution data from embedded thermocouples reveal that the area under the temperature curve is smaller for the top layer, leading to weaker sinter compared to the other layers. Strong suction prevents the top bed from retaining temperatures of 1200–1250 °C. However, hot gases descending from the top to the middle and bottom layers provide sufficient heat to these lower layers, enhancing their sintering quality.

Figure 7(b) shows that in pulsed sintering, the flame front becomes broader compared to conventional sintering. This broadening increases the area under the temperature curve across the sinter bed, indicating prolonged exposure to higher temperatures. The top layer reaches higher temperatures, favorable for forming beneficial sinter phases and reducing thermal shock. The heated gases descending from the top layer also enhance the temperatures of the middle and bottom layers, achieving uniformity without exceeding 1290–1310 °C. This temperature range is ideal for the sustained formation of silico-ferrite of calcium and aluminum (SFCA-I) phases.

Impact of pulsed suction on flame front expansion

The pulsed sintering technique introduces minor disturbances in suction flow, synchronizing the flame front and heat front to create a broader flame front. This synchronization allows the sinter bed to reach higher temperatures at the flame front, increasing the area under the temperature curve and aligning the thermal conditions across adjacent layers. As a result, the sintering process becomes more efficient, yielding stronger and more uniform sinter with reduced thermal shock and fewer return fines. The benefits of this mechanism, as outlined above, are further validated through key quality indices such as improved sinter strength and reduced return fines. The following sections discuss these indices in detail, providing a comparative analysis of conventional and pulsed sintering techniques.

Sinter Microstructure

The iron ore sintering process is a combination of oxidation and reduction processes. In conventional sintering, the top layer experiences rapid oxidation due to the high oxidation potential at the top of the sinter bed. This leads to the formation of a porous and fragile layer, indicative of increased fines generation. Micrographs illustrating different mineralogical structures under conditions of base case, 30%, 60%, and 100% closed valve pulse operations are presented in Figure 8.

The sinter microstructure in base case Figure 8(a). shows more porous structure with SFCA phases both in columnar and dendritic form and increased content of magnetite phase in middle portion of the sinter Figure 8(aⁱ). During pulse sintering the micrographs got drastically transform into better morphology of SFCA and SFCA-1. Sinter having lesser interconnected porosity and SFCA as columnar and acicular form Figure 8(b) which is desirable microstructure for sinter low temperature properties. The hematite and magnetite grains were found to be sufficiently surrounded by SFCA Figure 8(bⁱ). With further increase in magnitude of pulsation from 30% to 100% closed conditions acicular ferrite is observed in larger proportion in sinter and a low amount of magnetite and columnar SFCA is visible Figure 8(d) and (dⁱ). It is evident that pulsation sintering helps to avoid the formation of Columnar/dendritic SFCA structure and reduce sinter interconnected porosity.

Micrographs of sinter with 30% and 60% closed valve pulsation exhibited crystalline and secondary hematite phases. In contrast, micrographs with 100% closed valve pulsation showed massive hematite. In normal sintering, the predominant phase was columnar SFCA, with a minor quantity of acicular SFCA. The longer residence time at higher temperatures during normal sintering caused the acicular SFCA phase to melt and re-precipitate as columnar SFCA during cooling. Micro-graphs with 60% closed valve suction showed a higher proportion of acicular SFCA phase, while those with 100% pulsed suction had a greater quantity of SFCA phase compared to the 60% case.

Sinter porosity is primarily influenced by the sinter bed temperature, with lower temperatures leading to higher porosity. As the airflow rate decreases, the sinter bed temperature increases, resulting in a higher slag phase. When the sinter remained at temperatures of 1340–1350 °C for over 2–4 min, more non-beneficial secondary hematite was generated, leading to the formation of additional glassy phases due to rapid cooling from high temperatures. As depicted from Figure 9 porosity and secondary hematite content are decreasing with increase in SFCA content as the magnitude of pulsation increasing.

Influence of pulsed suction on SRFs

In this study the 600 mm height bed was segmented in to three 200 mm sections, and the return fines in the top layer exhibited a substantial 6–8% reduction, as illustrated in Figure 10, signifying enhanced productivity.

The comparison of SRF data for the top and middle layers accounted for approximately 33.3% of the entire sinter produced. To understand the influence of pulsation across the entire bed, SRF calculations were extended to cover the entire cross-section of the sinter bed. These findings are presented in Figure 11.

The data from the graph demonstrates a consistent reduction in SRF generation throughout the entire bed as pulsation intensity increases. This decrease is indicative of the favorable development of a robust bonding phase mineralogy. The prolonged exposure of the bed to temperature above 1200 °C due to pulsation contributes to increased formation of acicular SFCA without a concurrent rise in the FeO content.

Influence of pulsed suction on sintering time and bed shrinkage

Sintering time is notably influenced by suction pressure, as observed by Singh et al. and Cheng et al.³¹ with higher pressure promoting increased sintering speed and reduced time.³⁹ Optimal productivity, identified by T. Singh et al. is associated with lower air flow resistance at 14 kPa suction pressure, leading to decreased sintering time. Loo et al.²⁷ supported this, noting that higher suction values result in an elevated flame front.³⁹ Umadevi et al.⁴⁰ reported an increase in sinter strength with suction pressure up to 13 kPa, followed by a decline at 17 kPa. In contrast, Cheng et al.³¹ highlighted the drawbacks of increased sintering pressure, causing accelerated flame front descent but compromising cooling rates. Excessive pressure adversely affected sinter strength, while low sintering speed reduced overall productivity.

Pulsed suction eliminates the discrepancies associated with choosing high or low suction pressures in the sintering process. By introducing pulses during suction phase, the speed of the flame front is significantly enhanced, both horizontally and vertically. This results in a faster sintering process, reducing sintering time. While typical sintering takes around 26 min, pulse sintering reduces this to between 22 and 24 min, depending on the damper closure, as shown in Figure 12(a). This improvement boost productivity.

The study also finds that prolonged exposure to temperatures above 1200 °C in the top layer facilitates better assimilation, leading to greater bed shrinkage compared to conventional sintering. This method results in a bed shrinkage of 87 mm, compared to the 70 mm observed with conventional sintering, as shown in Figure 12(b). This increased shrinkage is linked to improved product yield and sinter strength, leading to a more durable final product.⁴⁰

Influence of pulsed suction on sinter strength and mean size

Pulse sintering elevates the sinter's TI by 5–6 points, particularly at higher pulse magnitudes. This enhancement is linked to the pulse's effectiveness in sustaining elevated flame front temperatures and speeds, covering the entire sinter bed in a shorter duration. The prolonged exposure of sinter to elevated temperatures for over 5–6 min promotes the formation of SFCA/SFCA-I and primary hematite phases. These factors collectively contribute to the heightened tumbler strength and improved abrasion index, as visually depicted in Figure 13(a) and (b).

The relationship between Pulsation effect and sinter mean size is depicted in the Figure 14. An increase in pulsation leads to a rise in sinter mean size, reaching its peak at the 60% closed valve condition, after which it declines. The mean size of the sinter is contingent on its strength, with lower quality sinter yielding smaller sizes. Both conventional sintering and the 30% closed valve condition result in lower quality sinter with an increased fines fraction, subsequently reducing the mean particle size.

Influence of pulsed suction on sinter RDI and RI

The RDI measures the potential disintegration of sinter in the upper part of the blast furnace after partial reduction.³⁹ As shown in Figure 15(a), pulsation significantly reduces magnetite content while increasing hematite content. The RDI improves (decreases) due to higher temperatures and sufficient retention time, which promote densification and melt formation. This is reflected in the increased presence of SFCA and glassy phases. The optimal RDI of 19% is achieved under the condition of 100% closed valve.

As depicted in Figure 15(b), the impact of pulsed suction on sinter reducibility is evident, with reducibility increasing alongside an increase in pulsation. Sinter reducibility is chiefly influenced by the microstructural phases within the sinter. An increase in pulsation leads to higher hematite content and lower magnetite content. Hematite reduction occurs at a faster rate than magnetite and wustite. Additionally, sinter reducibility rises with higher porosity. The reducibility of the sinter is also influenced by the morphology of SFCA. Acicular SFCA facilitates the uniform accommodation of a large number of pores. Therefore, increased hematite content and porosity, along with decreased magnetite content, contribute to enhanced sinter reducibility.

Implication of pulsed sintering at different sinter plant of TATA steel

This study aims to validate the practical implications of pulsed sintering by implementing these methods in various sinter plants (SPs) within the Tata Steel industry. Schematic representations Figures 16 and 17, were used to delineate the operational differences between conventional and pulsed sintering methods.

The experimental design for the pulsed suction test in the plant relies on two critical factors: the timing and magnitude of pressure fluctuations during pulsation. To determine the pulsation timing, calculations were based on the horizontal dimensions of wind boxes in our SP-1 of Tata Steel. With machine speeds ranging from 1.5 to 1.7 m per minute and a wind box dimension of 1.79 m, the pulsation times were determined accordingly (e.g., 1.19 min for a machine speed of 1.5 m/min and so on, as detailed in Table 4.).

Table 4.

Detailed design of experiment is given by considering varying sintering machine speed.

Experiment no.	Pulse timing with varying Sinter machine speed (minutes)	% Closing of orifice plate in suction system
Base case	Full suction 1200 mmWC	–
Expt −1	1.19 @ 1.5 m/min	30%
Expt −2	1.11 @ 1.6 m/min
Expt −3	1.05 @ 1.7 m/min
Expt −4	1.19 @ 1.5 m/min	60%
Expt −5	1.11@ 1.6 m/min
Expt −6	1.05 @ 1.7 m/min

During experiments, it was observed that with a 60% closed valve, the pressure drop across the sinter bed drops from the set pressure of 1200 mm water Columns (mmWC) to a range of 100–200 mmWC. When this pressure drop occurred, pulsation was automatically stopped to allow the pressure to normalize. Once the pressure returned to the desired level, the pulsation was reinitiated. The study encompassed varied closed valve conditions, primarily 30% and 60%, due to the impracticalities and challenges associated with the 100% closed valve conditions. The findings suggested that the 60% closed valve condition showed promising outcomes, leading to favorable results compared to other settings. Detailed observations and underlying mechanisms were documented for further comprehensive analysis.

Conclusions

In this novel PULSE sintering method for iron ore fines, improvements were made to the downdraft suction technique in sintering machines. Pulsations in the suction field were generated during the sintering process by alternately increasing and decreasing the downdraft suction rate.

Key findings include:

The optimal pulsation sequence involved 20 s of pulse generation and 1.5 min of normal suction, continuing for a total of 16 min. Pulses were generated by partially closing the damper/valve, typically to 60%.

Flame front speed increased to 23–25 mm/min, up from 23 mm/min in the normal sintering process, while maintaining peak temperature and the phase formation area above 1200 °C.

The top sinter bed retained temperatures above 1200 °C for 3 min, while the middle and bottom layers did so for 5 min. Approximately 30% of the area on the temperature-time distribution curve exceeded 1200 °C.

Other quality improvements include reduced return fine generation (6–8 points), increased TI (4–7 points), greater bed shrinkage (15–17 mm), and decreased RDI (4–5 points) and RI (2–3 points). These enhancements were achieved even with higher alumina and silica content and combined moisture, leading to improved quality and increased productivity.

The sinter mineralogy exhibited more primary hematite (32–48%), SFCA phase (28–42%), and uniform porosity (22–26%). Pulse sintering resulted in increased SFCA and SFCA-I structure with decreased porosity in sinter which was otherwise not obtained in the absence of pulsation.

Footnotes

Acknowledgments

The authors wish to thank the management of Tata Steel, especially Mr Vinay V Mahashabde (Chief R&D and Product Technology) and Mr Uttam Singh (Vice President—Operation TSM), for the permission to use the necessary data in this publication. Thanks, are also due to Mr Suraj Choudhary, Mr Danish Equebal, and Mr Shailendra Kumar Jha for experimental support.

The authors also want to thank Mr Subodh Pandey (Vice President—Technology, R&D, NMB and Graphene) for their support of this project.

Author contributions

Dhiraj M Kadhe: Conceptualization, methodology, formal analysis, investigation, writing-original draft preparation, writing-review and editing, visualization; Manoj K Sahoo: Methodology, formal analysis, investigation, writing-original draft preparation, writing—review and editing; Aditya Sarda: Resources, plant trial support; Prateek Jivrajka: Resources, plant trial support; Manoj K Patel: Resources, supervision, plant trail support; Jay P Behura: Resources, supervision, plant trail support; Srinivas Dwarapudi: Writing—review and editing, supervision; Siddhartha misra: Resources, writing—review and editing, supervision; Pradeep Choudhary: Plant trial supervision; Surajit Sinha: Plant trial supervision.

Declaration of conflicting interest

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Dhiraj M Kadhe

References

Crude steel production, https://worldsteel.org (July 2023).

Sanematsu

Toyozawa

. On the quantity of return fines in sinter mixture. Tetsu-to-Hagane 1960; 46: 243–250.

Ando

Sato

Kawabata

. Effect of adding crushed product of magnetite ore and return fines on sintering characteristics. Tetsu-to-Hagane 1964; 50: 1579–1581.

Zhu

Shi

, et al. Study on formation mechanism of return fines in sintering process. In: Technical contribution to the 6th International Congress on the Science and Technology of Ironmaking—ICSTI 2012, pp.437–449.

Bhagat

. Factors affecting return sinter fines regimed and strand productivity in iron ore sintering. ISIJ Int 1999; 39: 889–895.

Gao

Wang

Pan

, et al. A forecast model of the sinter tumble strength in iron ore fines sintering process. Powder Technol 2021; 390: 256–267.

Zhu

Fan

, et al. Sintering behavior of Return Fines and their effective utilization method. ISIJ Int 2013; 53: 1561–1570.

Garbers-Craig

Geldenhuis

JMA

Jordaan

, et al. The influence of increased air flow on the spatial variation of iron sinter quality. J South Afr Inst Min Metall 2003; 103: 645–650.

Loo

. A perspective of goethitic ore sintering fundamentals. ISIJ Int 2005; 45: 436–448.

10.

Zhou

Zhao

Loo

, et al. Numerical modeling of the iron ore sintering process. ISIJ Int 2012; 52: 1550–1558.

11.

Nath

Mitra

. Optimisation of suction pressure for iron ore sintering by genetic algorithm. Ironmak Steelmak 2004; 31: 199–206.

12.

Oyama

Nushiro

Konishi

. Influence of matrix strength and pore structure on sinter cake strength. Tetsu-to-Hagane 1996; 82: 719–724.

13.

Sato

Kawaguchi

Ichidate

. Strength controlling factors and their model for iron ore sinter. Tetsu-to-Hagane 1987; 73: 964–971.

14.

Higuchi

Heerema

. Influence of artificially induced porosity on strength and reduction behavior of hematite compacts. ISIJ Int 2005; 45: 574–581.

15.

Kasai

Omori

. Factors governing the strength of agglomerated granules after sintering. ISIJ Int 1991; 31: 17–23.

16.

Loo

Wong

. Fundamental insights into the sintering behavior of goethitic ore blends. ISIJ Int 2005; 45: 459–468.

17.

Mohanan

Mohapatra

Arun Kumar

, et al. Prediction and optimization of internal return fines generation in iron ore sintering using machine learning. Adv Mater 2021; 10: 42–47. doi: 10.11648/j.am.20211003.12

18.

Feng

Zhang

. Influence of basicity on the low silicon sinter strength. Sinter Pellet 2004; 29: 9–11.

19.

Zhang

Feng

. Study on strengthening of low silica sinter and application in production. Iron Steel 2004; 39: 38–42.

20.

Kodama

. Investigation of the reduction of return fine sinter operation. Tetsu-to-Hagane 1984; 70: 27.

21.

. Research of igniting system in sintering machine at panzhihua steel. Iron Steel 2004; 39: 1–3.

22.

Nigo

Kimura

Ichimiya

. Improvement for sintering plant operation by means of A mathematical model. Tetsu-to-Hagane 1982; 68: 2182–2189.

23.

Sohn

Chaubal

. Rate enhancement of the gaseous reduction of iron oxide pellets by pressure cycling. ISIJ Int 1984; 24: 387–395.

24.

Maithy

. Effect of pressure pulsation on gas solid reaction in packed beds . M. Tech. Thesis, Department of Metallurgical Engineering and Material Sciences, Indian Institute of Technology, Bombay, 1997.

25.

Bedarkar

Viswanathan

Ballal

. Measurement of thermal conductivity along the radial direction in a vertical cylindrical packed bed. J Powder Technol 2015; 2015: 584538.

26.

Kadhe

Sekhar

Sahoo

. Development and application of novel fluxing compound for improving sintering performance of high alumina iron ore. Ironmak Steelmak 2021; 48: 1261–1276.

27.

Loo

Tame

Penny

. Effects of iron ores and sintering conditions on flame front properties. ISIJ Int 2012; 52: 967–976.

28.

Witchard

. Sintering of blends containing magnetite concentrate and hematite or/and goethite ores. ISIJ Int 1998; 38: 1069–1076.

29.

Hamer

Cormack

. Influence of oscillating external pressure on gas phase reactions in porous catalysts. Chem Eng Sci 1978; 33: 935–944.

30.

Ballal

. Mass transfer in porous solids under pulsating pressure conditions. ISIJ Int 1995; 35: 446–448.

31.

Cheng

Yang

Zhou

, et al. Sinter strength evaluation using process parameters under different conditions in iron ore sintering process. Appl Therm Eng 2016; 105: 894–904.

32.

Singh

Zhang

, et al. Iron ore sintering in Milli-pot: comparison to pilot scale and identification of Maximum resistance to air flow. ISIJ Int 2021; 61: 1469–1478.

33.

Harvey

Pownceby

Chen

, et al. Effect of temperature, time, and cooling rate on the mineralogy, morphology, and reducibility of iron ore sinter analogues. Journals of Metals 2021; 73: 345–355.

34.

Sasaki

Hida

. Consideration on the properties of sinter from the point of sintering reaction. Tetsu-to-Hagané 1982; 68: 563–571.

35.

Dawson

. Recent development in iron ore sintering. II. Research studies on sintering and sinter quality. Ironmaking and Steelmaking (UK) 1993; 20: 37–143.

36.

Oyama

Iwami

Yamamoto

, et al. Development of secondary-fuel injection technology for energy reduction in the iron ore sintering process. ISIJ Int 2011; 51: 913–921.

37.

Hara

Matsumura

Higuchi

. Shrinkage distribution of sintering bed evaluated by X-ray CT observation. ISIJ Int 2019; 59: 1966–1971.

38.

Loo

. Changes in heat transfer when sintering porous goethitic iron ores. Miner Process Extr Metall 2000: 109: 11–22.

39.

Singh

Mitra

O’dea

, et al. Quantification of resistance and pressure drop at high temperature for Various suction pressures during iron ore sintering. ISIJ Int 2022; 62: 1768–1776.

40.

Umadevi

Brahmacharyulu

Sah

, et al. Influence of sinter grate suction pressure (flame front speed) on microstructure, productivity and quality of iron ore sinter. Ironmak Steelmak 2014; 41: 410–417.