Recent Advances in Sintering Granulation Technology for Efficient Utilization of Refractory Ores in China: Addressing the Depletion of High-Quality Iron Ore

Abstract

With the increasing depletion of high-quality iron ore resources, the efficient utilization of refractory ores has become a critical challenge in the iron and steel industry. This review systematically examines recent advances in sintering granulation technologies aimed at enhancing the processing of such ores. The study focuses on two main categories: conventional intensification methods, including moisture optimization, binder application, and mixer improvements, and novel sintering processes such as press-briquetting, pelletized sintering, split-stream granulation, composite agglomeration (CAP), and pre-granulation sintering. Key findings indicate that while conventional techniques can partially improve granulation uniformity and sintering bed permeability, they remain inadequate for handling high proportions of ultrafine ores or secondary iron-bearing materials. In contrast, innovative processes like CAP and pre-granulation sintering demonstrate superior adaptability and efficiency by integrating pelletizing and sintering mechanisms, enabling the treatment of complex ores and industrial residues. The adoption of advanced intensification-granulation sintering technologies offers a viable pathway toward sustainable ironmaking, providing the steel industry with essential tools to mitigate raw material constraints, lower production costs, and support green transformation. Future efforts should prioritize intelligent control, process integration, and the scaling of emerging techniques to maximize their industrial potential.

1. Introduction

Steel, as an indispensable fundamental material in modern industrial systems, is pivotal to sectors as diverse as national economic construction, national defense, transportation, mechanical manufacturing, and infrastructure development. Globally, steel production processes can be primarily categorized into three types (as shown in Figure 1): the blast furnace–basic oxygen furnace (BF-BOF) route using iron ore as the main raw material, the scrap–electric arc furnace (scrap-EAF) route utilizing recycled waste steel, and the direct reduced iron–electric arc furnace (DRI-EAF) route employing direct reduced iron [1,2,3,4]. Among these, the BF-BOF route maintains its dominant position in global steel production due to its mature technological system, large-scale production capacity, and stable product quality [5,6]. This predominance is particularly pronounced in emerging industrialized countries like China.

As the world’s largest steel producer and consumer, China’s steel industry exhibits unique characteristics. Over 90% of China’s crude steel is produced via the BF-BOF route, significantly higher than the global average of approximately 70% [7]. This industrial structure characteristic closely aligns with China’s current economic development model driven by large-scale infrastructure construction [8]. As shown in Figure 2, China’s crude steel production has demonstrated an overall upward trend in recent years. Notably, in 2017, China’s output exceeded half of global production for the first time, reaching a historical peak of 1.065 billion tons in 2020, accounting for as high as 56.65% of the world’s total production. This production structure creates enormous demand for iron ore. China has maintained its position as the world’s top iron ore consumer for consecutive years, accounting for over 60% of global consumption [9,10]. However, China’s iron ore supply faces significant challenges: import dependence has consistently exceeded 80% over the long term, with imports escalating dramatically from 70 million tons in 2000 to 1.107 billion tons in 2022, a compound annual growth rate of 12.4%. The severe fluctuations in international iron ore price indices frequently place domestic steel enterprises in a passive position regarding production cost control [11,12,13,14]. While domestically produced iron ore can partially mitigate import reliance, inherent limitations in resource endowment pose substantial challenges, as Chinese iron ores are refractory ores, typically characterized by a relatively low grade (averaging around 30%), fine disseminated grain size, and complex mineral composition [15,16,17]. After beneficiation processing, the resulting iron concentrate powder typically contains over 90% particles passing 200 mesh. This ultrafine nature severely challenges the sintering mixture granulation process, which adversely affects permeability, reduces sintering speed, and lowers product yield [18,19,20].

The sintering process serves as a critical nexus between mineral beneficiation and blast furnace ironmaking [21]. Its foundational step, granulation, involves the adhesion of finer particles onto coarser nuclei through capillary forces and liquid bridges, ultimately forming a uniform and stable quasi-particle system [22,23,24]. The granulation performance directly determines the particle size distribution, mechanical strength of quasi-particles, and consequently the permeability of the sintering bed—a decisive factor for sinter productivity and quality [25]. However, significant variations in chemical composition, particle size distribution, and hydrophilicity among iron-bearing materials from different sources pose tremendous challenges to controlling granulation effectiveness [26]. Thus, it is of strategic importance to strengthen the granulation process, as it enables the increased utilization of low-cost yet difficult-to-granulate iron concentrates while ensuring stable operation and meeting target quality indicators [21,27,28,29,30].

While numerous studies have investigated individual aspects of granulation improvement, the field lacks a systematic framework that compares and evaluates both conventional intensification technologies and novel process innovations, hindering optimal technology selection. This review aims to bridge this gap by providing a thorough analysis of recent advances in iron ore sintering granulation technology. It systematically summarizes research progress across two main domains: conventional granulation-process intensification methods and novel intensified granulation–sintering processes. By establishing a theoretical foundation for selecting optimal industrial processes, this work seeks to bolster steelmakers’ resilience against the challenges posed by increasingly inferior iron ore resources. Furthermore, this analysis offers actionable insights to improve operational flexibility in raw material procurement and sinter formulation practices, ultimately contributing to the development of more sustainable and cost-effective ironmaking processes. The comparative framework presented herein not only addresses contemporary technological challenges but also provides strategic guidance for future research and industrial applications.

2. Conventional Granulation-Process Intensification Technologies

Flow diagrams of conventional sintering production and equipment are shown in Figure 3 and Figure 4. In the sintering process, raw materials are first proportioned and then subjected to a mixing stage with dual objectives: homogenizing all components and introducing moisture to granulate the mixture [25]. This process aims to produce a sinter feed with a suitable particle size distribution and optimal permeability, goals typically accomplished through a two-stage mixing system [28].

In the two-stage mixing system, materials are processed sequentially in two separate units. The first mixer primarily moistens and homogenizes the mixture to ensure uniformity, whereas the second focuses on granulation while further enhancing homogeneity [31]. The most common equipment for both stages is the drum mixer. This equipment operates via gear ring transmission and is installed at a slight incline (typically 1–3°) to ensure directional material flow. Water is sprayed into the drum from the discharge end via spray nozzles, and the mixed material is discharged through an outlet chute. Intensifying granulation during mixing promotes the adhesion of fine particles onto coarser nuclei, forming granules of controlled size, which significantly improves the permeability of the sinter bed and enhances sintering productivity. Beyond mixing and granulation, this process also facilitates the continued hydration of quicklime (CaO), releasing heat that preheats the mixture [32].

2.1. Optimization and Modification of Mixer Systems

The granulation efficiency in sintering is significantly influenced by key parameters of the drum mixer, including drum dimensions, filling ratio, and internal structure, which are typically determined through a combination of industrial trials and laboratory experiments. To enhance granulation performance based on the characteristics of sintering raw materials, several adjustable factors can be optimized: lining plate (angle, spacing, height, and positioning), drum inclination angle, filling ratio and rotational speed [33,34]. Among these, the filling ratio, lining plate (angle/height), drum radius, and rotational speed exhibit optimal operating ranges for maximizing granulation quality. Empirical studies suggest that these parameters must be carefully balanced to ensure efficient particle aggregation and improved sinter bed permeability [25]. The optimal filling ratio is generally no more than 15%, the drum inclination angle is usually set between 2.5° and 4°, and the total granulation time is approximately 5 min. However, these values can vary significantly when processing different ore types. While such modifications can enhance operational efficiency and intensification, frequent equipment adjustments entail significant time and retrofitting costs. Moreover, conventional drum mixing exhibits inherent limitations in processing fine-grained, hydrophobic iron ores that are inherently difficult to granulate.

With advancements in intensive mixing technology, high-speed intensive mixers have been introduced as alternatives to traditional equipment. In practical production, intensive mixers typically replace the first drum mixer to achieve superior homogenization [35]. Nippon Steel [36] pioneered the industrial application of intensive mixing technology, demonstrating a >10% increase in sintering speed compared to conventional granulation–sintering processes. Studies confirm that intensive mixers significantly improve granulation of hydrophobic fine iron ore powders, enhancing sinter-bed permeability, sinter-machine productivity, and sinter strength [35,37,38]. However, intensive mixers present challenges, including complex mechanical structures, severe material-induced wear, higher failure rates, and substantial initial investment and maintenance costs. These factors must be weighed against performance gains when considering process upgrades.

2.2. Prolongation of Granulation Duration

Extending the granulation time serves as a direct and effective intensification method by providing finer particles more opportunities to adhere to nuclei, thereby enhancing the average size and strength of quasi-particles [39]. Studies confirm that appropriately prolonging the second-stage mixing time can increase quasi-particle size by 5%–10%, significantly improve bed permeability and sintering speed, and ultimately enhance sinter strength and yield [25,40]. However, this approach introduces drawbacks such as higher energy consumption, lower production efficiency, particle abrasion, and uneven water evaporation. Therefore, practical application necessitates a careful trade-off between these benefits and operational costs.

Compared to merely extending granulation time, increasing the number of granulation stages constitutes a more systematic intensification strategy. Multi-stage (e.g., three-stage) processes enable the gradual refinement of granulation and homogenization of raw materials by applying distinct parameters, such as water addition method, rotation speed, and filling rate, at each stage. This approach not only enhances particle adhesion efficiency but also fosters a more stable granule structure [41]. For instance, an industrial trial demonstrated that adopting a three-stage process yielded a 12% increase in quasi-particle average size, a 15% improvement in bed permeability, an 8% higher sintering machine utilization coefficient, and a 5 kg/t reduction in solid fuel consumption.

Building upon multi-stage granulation, several advanced granulation methods have been derived, such as coating granulation and self-densification high-melting-point liquid-phase sintering [42]. The coating granulation process involves creating quasi-particles with a “core–shell” structure for difficult-to-granulate ores (e.g., limonite, goethite). The shell, usually composed of flux or fuel, effectively suppresses excessive melting and assimilation of the core particles, thereby enhancing the stability of the sintering process and the quality of the finished sinter [43]. For instance, addressing the issue of high crystalline water content in limonite, which tends to generate voids during sintering, researchers have used a combined coating of coke breeze and flux, significantly improving sintering performance, reducing the number of voids, and increasing the strength and reducibility of the sinter [44,45,46]. The self-densification high-melting-point liquid-phase sintering method, on the other hand, focuses on optimizing the melting behavior of particles through multi-stage granulation, promoting the formation of high-melting-point liquid phases, thereby resulting in a denser mineralogical structure during sintering [47]. This method is particularly suitable for processing ores with high Al₂O₃ content, as it effectively inhibits the premature formation of low-temperature liquid phases, improves bed permeability, and enhances the metallurgical properties of the sinter [48,49].

Despite its significant advantages in enhancing granulation and sintering performance, multi-stage granulation introduces considerable engineering challenges. These include substantially higher equipment investment and operational costs, as well as increased space requirements, due to the need for multiple mixers alongside their associated conveying and control systems. Furthermore, process control complexity is significantly heightened; parameters such as moisture, rotational speed, and filling rate across different stages require precise coordination to prevent a decline in effectiveness. The process can also exacerbate issues like material sticking and particle breakage when processing highly viscous or extreme-sized feedstocks. Moreover, the applicability of multi-stage granulation is constrained by raw material characteristics. For hydrophobic ores with smooth surfaces, such as specularite or magnetite, simply increasing the number of granulation stages proves ineffective. Maximum benefits are only realized when this approach is combined with binder optimization or pretreatment processes (e.g., pre-wetting, pre-briquetting). Consequently, the selection of granulation stages and process parameters must be flexibly adapted based on the physicochemical properties of the raw materials.

In summary, extending granulation time and increasing the number of granulation stages can, to a certain extent, intensify the granulation process and improve the quality of quasi-particles and sintering indicators. In particular, the combination of multi-stage granulation with coating technology and self-densification processes provides new technological pathways for handling difficult-to-granulate iron ores. However, challenges such as high cost, complex control, and limited applicability remain. Future efforts should focus on further cross-process collaborative optimization research, integrating intelligent control, material modification, and other means to advance multi-stage granulation technology towards greater efficiency, economy, and sustainability.

2.3. Application of Novel Binders and Additives

Binders play a crucial role in the iron ore sintering granulation process. Their primary functions include enhancing inter-particle adhesion, improving the hydrophilicity of the mixture, increasing the strength and stability of quasi-particles, and ultimately optimizing the permeability of the sinter bed and the quality of the sinter product [50,51,52,53]. Against the backdrop of increasingly inferior iron ore resources and a growing proportion of fine and hard-to-process ores, binder selection and optimization have emerged as vital research directions for process intensification. Moreover, binder application critically influences not only granulation efficiency but also sintering energy consumption, emissions, and the metallurgical properties of the sinter.

2.3.1. Application and Mechanism of Inorganic Binders

Quicklime (CaO), the most classical and widely used inorganic binder since the 1940s, functions through hydration upon contact with water to form calcium hydroxide (Ca(OH)₂). This hydrate exhibits high moisture capacity, strong viscosity, and good dispersibility, effectively promoting the adhesion of fine particles to nuclei and enhancing quasi-particle formation and strength [53,54]. Furthermore, during sintering, quicklime acts as a flux, participating in liquid-phase formation, adjusting basicity, and promoting the development of beneficial minerals like calcium ferrite, thereby significantly improving the sinter’s strength and metallurgical properties [55,56,57].

Research confirms that the binding efficiency of quicklime is significantly influenced by its activity. High-activity quicklime hydrates rapidly, releasing substantial heat that raises mixture temperature, reduces water condensation, and thereby improves granulation efficiency. Compared to its low-activity counterpart, it can elevate the sintering mix temperature by 5–10 °C, increase sintering speed by ~5%, boost calcium ferrite content, and improve tumbler strength by 1–2 percentage points [58]. However, the use of quicklime has limitations: excessive addition can raise sinter basicity excessively, impairing blast furnace slag fluidity, while its hygroscopic nature leads to premature slaking during storage, worsening handling conditions and causing material loss.

Besides quicklime, other inorganic binders such as hydrated lime (Ca(OH)₂), cement, and bentonite are also used. Hydrated lime does not require a hydration process and is convenient to use, but its binding capacity is relatively weak; cement offers good early-stage strength but at a higher cost; while bentonite can improve inter-particle bonding, it introduces impurities such as SiO₂ and Al₂O₃, which may reduce the grade of the sinter [50,59,60,61,62,63]. Therefore, selection must be based on raw material characteristics, process requirements, and cost considerations.

2.3.2. Development and Application of Organic Binders

Organic binders have garnered significant attention in recent years due to their unique properties, which generally include higher binding efficiency and superior environmental performance compared to inorganic binders. Common types, such as starch, cellulose, polyacrylamide (PAM), and lignosulfonates, function by forming polymer films on particle surfaces, thereby enhancing inter-particle bonding and granulation effectiveness [64,65,66]. Their principal advantages include the absence of inorganic impurities, which maintains the sinter iron grade, and complete combustion without ash formation, preventing detrimental effects on sinter quality and the environment. This combustion characteristic, exemplified by starch-based binders, also aids in fixing fine particles within the sinter bed, reducing dust emissions and the formation of pollutants like NO_x and SO₂ [67,68]. Furthermore, organic binders exhibit better performance with certain hard-to-process ores, such as hydrophobic specularite with smooth surfaces.

Nevertheless, organic binders present notable drawbacks. Their high cost renders large-scale applications economically challenging. Furthermore, their inferior thermal stability risks premature decomposition at sintering temperatures, which can compromise process stability. Finally, their sensitivity to ambient conditions demands strict control over storage and usage. Thus, these binders are presently employed primarily in niche applications demanding high-quality sinter or where critical environmental constraints must be met.

2.3.3. Development and Advantages of Composite Binders

To overcome the limitations of single-type binders, composite binders have been developed by combining inorganic and organic components in specific proportions. These hybrid binders synergistically incorporate the advantages of both parent materials: they retain the thermal stability and cost-effectiveness of inorganic binders while leveraging the high binding efficiency and environmental benefits of their organic counterparts. Studies demonstrate that composite binders can significantly improve the granulation of refractory iron ores. For instance, a blend of quicklime with starch or PAM enhances the uniformity of the particle size distribution and the strength of quasi-particles, thereby reducing the escape of fine particles during sintering while simultaneously improving bed permeability. An industrial trial confirmed these benefits, where adopting a quicklime–starch composite binder improved sinter bed permeability by 10%, increased sintering speed, reduced solid fuel consumption by 3–5 kg/t, and enhanced both sinter tumbler strength and yield [69,70].

Another advantage of composite binders is their flexible designability. By adjusting the ratio of inorganic to organic components, they can be customized for different raw material characteristics and process requirements. For instance, for limonite with high water content, the proportion of organic components can be increased to enhance water retention and binding force; for extremely fine magnetite concentrate, the ratio of inorganic components can be optimized to promote bridging between particles.

However, the widespread adoption of composite binders faces several hurdles. The compounding process itself is relatively complex and demands stringent control over production parameters. Meanwhile, their cost remains substantially higher than that of conventional inorganic binders. Furthermore, their potential long-term effects on sinter mineralogy and metallurgical properties necessitate further comprehensive investigation.

2.3.4. Selection Strategy and Development Trends for Binders

The selection of binders requires comprehensive consideration of raw material properties, process conditions, cost factors, and environmental requirements [71,72]. For conventional iron ore fines, quicklime remains the most cost-effective choice; for hard-to-granulate ores or situations with strict environmental requirements, organic or composite binders may be used. Additionally, the method of binder addition deserves optimization, such as using solution spraying or pre-coating, to improve dispersibility and efficiency.

Future binder research will increasingly prioritize multifunctionality and environmental sustainability. A key direction involves developing novel, cost-effective composite binders derived from industrial by-products (e.g., steel slag, fly ash), which can lower costs while promoting resource recycling [67,72,73]. Concurrently, smart responsive binders (e.g., temperature- or pH-sensitive types) represent an emerging frontier; these advanced materials can perform stage-specific functions during sintering, further enhancing granulation efficiency and product quality. In summary, binder optimization is pivotal for advancing iron ore sintering granulation. Through scientifically guided selection and innovation, it not only increases the utilization of fine iron ores but also drives the greening and intellectualization of sintering technology, thereby providing crucial technical support for the sustainable development of the iron and steel industry.

2.4. Optimization of Granulation Moisture

Moisture plays a vital role in iron ore sintering granulation, serving as the foundation for inter-particle capillary forces and critically influencing the pelletization rate, quasi-particle size distribution, and ultimately, sinter bed permeability. By wetting particle surfaces, moisture enables fine particles to adhere to coarser nuclei via liquid surface tension, thereby facilitating the formation of mechanically robust quasi-particles [74]. However, deviation from the optimal moisture content is detrimental. Insufficient moisture weakens capillary forces, leading to fine and weak quasi-particles, while excess moisture causes overwetting, forming large agglomerates that degrade bed permeability, increase solid fuel consumption, and promote heterogeneous sintering. Consequently, precise moisture control is essential for achieving efficient granulation [72].

2.4.1. Prediction and Modeling of Optimal Moisture Content

Determining the optimal granulation moisture content for varying raw material blends remains a persistent challenge in sintering production. Traditional approaches, which rely heavily on operator experience and repetitive field trials, are not only inefficient but also prone to inconsistent water utilization and control instability. In response, developing predictive models based on material characteristics has emerged as a key research focus [75]. These models typically employ the physical and chemical properties of raw materials—such as water-holding capacity, specific surface area, porosity, chemical composition (e.g., Fe, SiO₂, Al₂O₃), and particle size distribution—as inputs. By leveraging techniques like regression analysis and machine learning, they establish quantitative relationships to predict the optimal moisture content [76,77,78].

Empirical studies have established significant positive correlations, for instance, between optimal moisture and the content of sub −0.2 mm fine particles, while linear models based on maximum water-holding capacity have also proven effective [79,80]. The field has since evolved with the introduction of data-driven algorithms like Support Vector Machines (SVMs), Random Forests, and Artificial Neural Networks (ANNs) [81]. These advanced techniques excel at capturing the nonlinear relationships between material properties and moisture requirements, significantly outperforming traditional regression in prediction accuracy. However, their primary limitation lies in a heavy dependence on high-quality data and limited adaptability to sharp fluctuations in raw material composition. Therefore, future efforts should focus on integrating real-time online composition detection with adaptive algorithms to develop dynamic, self-adjusting prediction systems.

2.4.2. Optimization Strategies for Moisture Addition Methods

In addition to controlling the total moisture content, the method of moisture addition also significantly affects granulation outcomes. Optimized water addition strategies aim to achieve uniform distribution of moisture in the mixture, avoiding localized overwetting or drying. Currently, the main optimization approaches include staged water addition, atomized water addition, and pre-wetting technology [72].

Staged water addition involves applying different proportions and timing of water during primary and secondary mixing. Studies have shown that focusing on mixing and adding 70%–80% of the total water during primary mixing, then supplementing the remaining moisture and enhancing granulation during secondary mixing, can significantly improve the particle size distribution and strength of quasi-particles [82,83]. Atomized water addition technology uses high-pressure nozzles to atomize water into micron-sized droplets, allowing for more uniform spraying into the material flow, effectively reducing the inhomogeneity of material moisture [84]. This is particularly suitable for fine-grained iron ores with poor hydrophilicity. Pre-wetting technology involves preliminary moisture adjustment of hard-to-wet materials (such as return fines and some coarse-grained ores) before they enter the mixer, shortening overall mixing time and improving granulation efficiency [77,85,86]. For example, pre-wetting return fines significantly reduces material sticking in the drum mixer and increases the pelletization rate.

Nevertheless, the implementation of these optimized strategies is not without challenges. For example, atomized water addition necessitates specialized nozzles and dedicated maintenance, increasing operational costs, while pre-wetting requires supplementary space and equipment. Consequently, the selection of an appropriate water addition method must be carefully evaluated against raw material properties, existing equipment capabilities, and overall economic viability.

2.4.3. Development of Intelligent Water Addition Control Systems

The ultimate goal of sintering moisture control is to achieve precise, stable, and adaptive automatic regulation. Early systems, primarily based on Proportional–Integral–Derivative (PID) control and offline moisture detection, were plagued by response lag and poor disturbance rejection. In contrast, modern intelligent water addition systems have become mainstream, integrating real-time data from online moisture detectors, flow sensors, and level gauges with advanced control algorithms to enable dynamic closed-loop moisture regulation [87,88].

Such systems typically employ multi-sensor information fusion technology to obtain key parameters in real time, including mixture moisture, material flow rate, and raw material composition. Decision-making is carried out using fuzzy control, model predictive control (MPC), or deep learning networks to implement feedforward–feedback composite control [89]. For example, researchers have proposed adaptive PID control systems based on neural networks, which can effectively overcome disturbances caused by fluctuations in raw material properties, significantly outperforming traditional methods in control precision [87]. Additionally, digital twin technology is being explored for full-process simulation and optimization of the water addition process, providing decision support for actual control by simulating different working conditions in a virtual space.

However, intelligent water addition systems also face challenges such as sensor reliability, system maintenance complexity, and high initial investment costs. Especially in the harsh environment of sintering sites, characterized by high temperatures and high dust, the long-term stability and measurement accuracy of sensors remain critical issues to be resolved. In the future, it will be necessary to further develop highly robust detection technologies and control architectures with stronger adaptive capabilities to promote the comprehensive intellectualization of moisture control in the sintering process.

Moisture regulation is a fundamental yet complex key aspect of sintering granulation. From the accurate prediction of optimal moisture to the optimization of addition strategies and the application of intelligent control systems, all demonstrate the importance of moisture management for improving sintering efficiency and quality. Future research should focus more on the integration of multiple technologies, such as combining real-time identification of raw material characteristics with adaptive modeling, or using machine vision to monitor pelletization status and provide feedback for water distribution control, thereby constructing more robust intelligent granulation moisture control systems. This will not only contribute to improving resource utilization and economic efficiency in sintering production but also provide important technical support for processing inferior iron ore resources.

3. Advances in Novel Intensification-Granulation Sintering Technologies

Conventional sintering processes face growing challenges from the declining quality of iron ore feeds and increasingly stringent environmental regulations. Their limitations, primarily inadequate granulation performance and poor sinter bed permeability, become critically apparent under specific conditions. This is particularly evident when processing refractory ores, such as fine-grained varieties or those containing high levels of crystal water. In response, several novel intensification-granulation sintering technologies have emerged. These innovative approaches aim to simultaneously enhance sintering efficiency and product quality by reengineering particle architecture, optimizing raw material allocation, and redesigning process flows.

This section introduces several representative intensified granulation sintering processes. These include pre-pressing sintering, pelletized sintering, split-stream granulation sintering, composite agglomeration, and pre-granulation sintering. Each method tackles the challenges of raw material adaptability and sintering quality control from a unique angle. Together, they contribute to higher utilization rates of fine iron ores and improved metallurgical performance of the sinter product. Moreover, these processes offer practical pathways to achieve efficient, low-consumption, and environmentally friendly sintering production. A clear grasp of their principles and characteristics is essential for promoting the advancement of sintering technology.

3.1. Press-Briquetting Sintering Process

The press-briquetting sintering process is an innovative technology designed to address the challenges of granulating fine-grained and difficult-to-process iron ores. Its core principle involves mechanically pre-briquetting the raw mix into briquettes or pellets with sufficient mechanical strength, which are then screened to produce granules with an optimal size distribution for sintering. By applying external pressure, this method enhances inter-particle contact, promotes van der Waals forces and mechanical interlocking, and significantly improves the granulation behavior of ultrafine materials that are poorly handled by conventional drum mixing [90].

The process flow diagram of press-briquetting sintering is shown in Figure 5. Key equipment includes high-pressure roller mills, double-roll briquetters, or mechanical presses, selected based on raw material characteristics and production scale. Critical process parameters such as roller pressure (3.5–5.5 kN/cm), gap width (20–50 mm), and roller speed (0.8–1.5 m/s) are tightly controlled to ensure that the bulk density of the material increases by 25%–40%, creating favorable conditions for subsequent granulation [91].

Research has demonstrated significant advantages, particularly in treating ores such as specularite and titanomagnetite. For example, one plant using this process with 45% specularite achieved a 12%–18% improvement in sinter bed permeability, a 15%–22% increase in vertical sintering speed, an 8%–15% rise in utilization coefficient, and a 4–6 kg/t reduction in solid fuel consumption. The process also reduces the proportion of ultrafine particles (<0.2 mm), thereby lowering dust emissions and providing environmental benefits [90,91,92].

However, widespread adoption of the pre-briquetting sintering process faces challenges, including high equipment investment and energy consumption, difficulties in maintaining iron-ore briquette quality consistency, and sensitivity to fluctuations in raw material moisture. Future development should focus on designing energy-efficient pressing equipment, establishing intelligent control models that link material properties to pressing parameters, integrating press-forming with other intensification technologies (e.g., binder optimization or pre-wetting), and optimizing crushing methods to improve the size distribution and surface characteristics of the processed material.

3.2. Pelletized Sintering Process

3.2.1. Technical Principle, Characteristics and Innovations

The pelletized sintering process is an innovative agglomeration technology that integrates principles of both sintering and pelletizing. Its core concept involves forming green pellets with adequate mechanical strength from the raw mix, which are then consolidated through sintering. Unlike conventional sintering that relies primarily on melt-phase bonding, this method utilizes both solid-phase diffusion and liquid-phase formation to achieve inter-particle bonding, significantly improving the permeability and thermal efficiency of the sinter bed [93,94]. This process was originally developed by Nippon Kokan (NKK) in the 1980s to efficiently process fine iron concentrates [95].

Figure 6 shows a flow diagram of the pelletized sintering process. The typical process includes raw material pretreatment, mixing, pelletizing, distribution, ignition and sintering. Pelletizing is commonly performed using disc or drum pelletizers, where moisture content and pelletizing time are controlled to produce uniform green pellets with a size range of 5–12 mm [96]. Generally, it is necessary to add a certain amount of binder. The types of binder are divided into two categories: inorganic (such as bentonite) and organic (such as humic acid). The pellets are dried before being fed into the sintering machine, where they are sintered under downdraft conditions.

The pelletized sintering process offers multiple advantages over conventional sintering. The regular arrangement of spherical pellets greatly improves bed permeability, increasing sintering speed by 10%–15%. The uniformity of the green pellets ensures even thermal distribution, reducing the occurrence of over-melting and unsintered zones, thereby enhancing sinter quality consistency. Most importantly, this method can effectively process ultra-fine concentrates with more than 80% of particles below 0.045 mm, significantly expanding the range of utilizable resources [97,98].

In terms of technological innovation, the Hybrid Pelletized Sintering (HPS) process represents a major advancement [95]. HPS employs layered pelletizing to form a unique core–shell structure: coarse ore particles as the core, fine ore and flux as the intermediate layer, and fuel as the outer layer. The structure diagram of the green pellet is shown in Figure 7. This design enables staged combustion of fuel and targeted melting of flux, not only improving thermal efficiency but also promoting the formation of desirable mineral phases. Studies have shown that the HPS process can reduce solid fuel consumption by 5–8 kg/t while increasing sinter reducibility by 8%–12% [99].

3.2.2. Industrial Applications and Performance

Industrial practices worldwide have demonstrated the outstanding performance of pelletized sintering in processing special types of ores. Steel plants applied this process to treat a high proportion of limonite (up to 35%), achieving remarkable results: the utilization coefficient increased by 0.3–0.5 t/(m²·h), tumbler strength of the sinter improved by 3%–5%, and FeO content decreased by 1.5%–2.0%. More importantly, due to improved bed permeability, the sintering machine could operate with ultra-thick beds (reaching 900–1000 mm), further reducing solid fuel consumption [98,100,101,102,103].

Environmental benefits have also been observed [94,97,99]. The spherical pellets reduce dust carry-over in flue gas, lowering dust emissions by 30%–40%. Meanwhile, more complete combustion results in corresponding reductions in sulfur dioxide and nitrogen oxide emissions. Data from one manufacturer showed that adopting this process reduced total pollutant emissions from sintering by 25%, delivering significant environmental benefits.

3.2.3. Challenges and Future Directions

Despite its demonstrated benefits, the full-scale industrial implementation of pelletized sintering remains hampered by several critical challenges. The process complexity is significantly increased due to auxiliary steps such as the external coating of coke breeze, the laying of the side edge return-ore layer to prevent gas flow segregation, and the incorporation of a dedicated drying section to avoid green pellet bursting. For existing sintering plants, the structural modifications required to accommodate these additional units entail prohibitively high retrofitting costs. Furthermore, the grape-like clusters of pellets, which are bonded by point contacts, are structurally weak. They are prone to breakage during crushing and transportation, generating a significant number of individual pellets. The presence of these single pellets can fill the voids within the sinter bed, severely impairing its permeability and destabilizing the entire sintering process.

Future development efforts should prioritize several strategic directions: advancing energy-efficient pelletizing systems to reduce overall energy consumption and operational expenditures; investigating the correlation between green pellet strength and sintering parameters to establish intelligent control models for process optimization; and optimizing material distribution systems to minimize pellet degradation and enhance operational reliability. The process shows great potential in processing secondary iron-bearing resources such as dust and sludge, offering a new technical pathway for green development in the iron and steel industry [94,98].

3.3. Split-Stream Granulation Sintering Process

3.3.1. Technical Principle and Process Innovation

The split-stream granulation sintering process is an innovative technology that classifies and processes raw materials based on their particle size characteristics and granulation properties. This approach overcomes the key limitations of conventional uniform mixing and granulation, in which fine particles fail to adhere effectively while coarse particles demonstrate poor granulation efficiency, thus compromising overall sintering performance. The core concept of the split-stream granulation sintering process involves dividing iron ore raw materials into two or more parallel processing lines according to differences in size distribution and granulation behavior. By adopting a “classified processing, synergistic optimization” strategy, it significantly enhances overall granulation effectiveness and sintering efficiency [41,104,105].

A flow diagram of the split-stream granulation sintering process is shown in Figure 8. The typical process flow comprises three main stages: split-stream processing, mixing and granulation, and distribution and sintering. Specifically, the raw materials are separated into two main streams: difficult-to-granulate fine particles and easy-to-granulate coarse particles. The fine fraction is directed to a dedicated intensification system, through which pretreatment measures are employed to form well-structured quasi-particles, such as binder addition, extended granulation time, or increased mixing intensity. The coarse fraction undergoes conventional granulation processing. After separate granulation, the two streams are precisely proportioned through metering and conveying systems before being blended and fed to the sintering machine.

3.3.2. Application Performance and Advantages

Industrial practices have demonstrated significant advantages of split-stream granulation in processing complex raw material formulations. Steel plants applying this process to treat high-proportion fine magnetite (up to 30%) achieved remarkable results: sinter bed permeability improved by 15%–25%, vertical sintering speed increased by 20%–30%, utilization coefficient rose by 10%–18%, and solid fuel consumption decreased by 5–8 kg/t. Simultaneously, sinter quality showed notable improvement, with tumbler strength increasing by 3%–6% and the low-temperature reduction degradation index (RDI) decreasing by 4–8 percentage points [41,104,105,106,107,108]. Environmental benefits are equally impressive. Effective processing of the fine iron ore reduces dust emissions in flue gas by 30%–50%. Improved combustion conditions also lead to corresponding reductions in sulfur dioxide and nitrogen oxide emissions by 15%–25% [105,109].

Furthermore, the process demonstrates remarkable raw material flexibility. By adjusting split ratios and granulation parameters, it can handle various materials from conventional iron ores to special types (such as limonite and siderite), providing steelmakers with greater flexibility in optimizing raw material formulations. Particularly when processing secondary resources like iron-bearing dust and sludge at high proportions, the process enables separate granulation of these materials before blending, ensuring sinter quality while facilitating resource recycling.

3.3.3. Technical Challenges and Development Prospects

The widespread industrial adoption of split-stream granulation still faces several challenges. The additional classification step requires extra equipment investment and plant space, increasing initial capital costs. The complexity of the process demands higher operational expertise, necessitating comprehensive training programs. Moreover, the precision of material splitting and proportional mixing directly affects process performance, requiring advanced detection and control systems. Process stability can be compromised by fluctuations in raw material characteristics, and the separate treatment of material streams may lead to inconsistencies in final blend homogeneity if not properly controlled.

Future development should prioritize several interconnected strategic pathways, beginning with the design of efficient and cost-effective classification equipment to reduce both capital investment and operational expenditures. Subsequently, efforts should establish robust correlations between raw material properties and splitting parameters to facilitate intelligent decision-support systems. Furthermore, research must explore integrated applications with other advanced technologies—such as flue gas recycling and waste heat recovery—to develop synergistic and comprehensive process solutions. Finally, continuous optimization of control systems remains essential to achieve superior classification accuracy and mixing homogeneity.

In summary, as an innovative technology, split-stream granulation sintering offers significant advantages in enhancing process efficiency, improving sinter quality, and reducing environmental footprint. With continuous technological refinement and cost reduction, this process is poised to find wider application in the steel industry, particularly in processing complex ores and achieving green production objectives. Through sustained innovation and engineering optimization, split-stream granulation will provide robust technical support for the sustainable development of the iron and steel industry.

3.4. Composite Agglomeration Process (CAP)

3.4.1. Technical Principle and Process Philosophy

The Composite Agglomeration Process (CAP) is an innovative iron ore agglomeration technology that transcends the conventional boundaries between sintering and pelletizing. Its core concept lies in the separate treatment of different types of raw materials and their synergistic consolidation, enabling the highly efficient utilization of feedstocks with diverse characteristics. This process creatively addresses the contradiction between the poor sinterability of fine concentrates and the poor pelletizability of coarse ore powders by proposing a “raw material classification—separate treatment—combined roasting—composite mineralization” strategy [72,110].

The technological foundation of CAP capitalizes on the distinct advantages of different mineral components: fine concentrates, which are difficult to sinter, are pelletized into acidic pellets that gain strength through solid-phase consolidation, while more fusible coarse ore powders are granulated into high-basicity sinter feed, achieving bonding via melt-phase sintering [72]. The two types of agglomerates are mixed in a specific ratio and, during sintering, form a unique “matrix-embedded” structure. The high-basicity sinter feed acts as a matrix, providing a consolidation environment and mechanical support for the acidic pellets. Simultaneously, the acidic pellets effectively regulate the overall basicity, improving the metallurgical properties of the final product. This synergistic consolidation mechanism not only overcomes the inherent limitations of single-process approaches but also achieves a synergistic effect where the combined outcome is greater than the sum of its parts [110].

The flow diagram of CAP is shown in Figure 9. The process flow of the CAP comprises four critical stages: raw material classification, parallel pelletizing (for traditional iron concentrate, refractory complex concentrate and various fine-grained iron-bearing secondary resources) and granulation (for all coarse iron ore fines, solvents, fuels and return ores), blending and distribution, and synergistic sintering. Initially, raw materials are classified based on particle size characteristics: fine concentrates are routed to the pelletizing system, where they are formed into acidic pellets measuring 8–16 mm using disc pelletizers; coarse ore powders are directed to the granulation system, where they are processed into high-basicity sinter feed with a size range of 3–8 mm in drum mixers. After separate preparation, the two streams are uniformly blended in optimized proportions (typically 30%–50% pellets and 50%–70% sinter feed) using dedicated mixing equipment, followed by layered distribution and sintering.

Key equipment systems include parallel pelletizing and granulation devices, precision mixing systems, and specialized distribution installations. The pelletizing system employs large-diameter disc pelletizers (typically ≥6 m in diameter), featuring variable-frequency drives and automatic water addition systems. The granulation system utilizes intensified drum mixers with length-to-diameter ratios controlled between 3 and 4. Mixing is carried out in double-shaft paddle mixers, ensuring a blending homogeneity exceeding 95%. The distribution system employs multi-roll feeders coupled with deflector plates to achieve uniform burden distribution.

Process parameter control is critical. In the pelletizing system, moisture is maintained at 8%–10%, with the drop strength of green pellets required to be ≥3–4 times/pellet. The granulation system operates at a moisture content of 6%–8%, with quasi-particle average size controlled within 3–5 mm. The basicity of the mixed burden is regulated between 1.8 and 2.2 to ensure favorable sintering performance. During sintering, temperature is controlled at 1250–1350 °C with a holding time of 15–25 min, ensuring complete synergistic consolidation.

3.4.2. Technical Advantages and Application Performance

Scholars have employed the CAP to treat fine iron concentrates such as titanium-bearing magnetite and hematite, significantly increasing the proportion of fine iron materials in the sintering mixture. Studies have shown that in this process, the high-basicity matrix provides a favorable consolidation environment for the acidic pellets, promoting their solid-phase consolidation. As a result, under equivalent conditions, the pellet strength achieved through CAP is higher than that of conventional pellets [111,112]. Moreover, with the same proportion of refractory iron ores in the mixture, the sintering performance indicators of CAP are markedly superior to those of traditional sintering. Specific research data demonstrate that for titanium-bearing magnetite concentrates, CAP improves the yield by 11–12 percentage points, increases the sinter tumbler strength by 7–12 percentage points, raises the utilization coefficient by 0.2–0.5 t/(m²·h), and reduces solid fuel consumption by 3–9 kg/t compared to conventional sintering [113,114,115,116,117]. For other fine iron concentrates, the process also exhibits excellent performance: yield is enhanced by 9–11 percentage points, tumbler strength is improved by 9–11 percentage points, the utilization coefficient is elevated by 0.3–0.5 t/(m²·h), and fuel consumption is lowered by 5–12 kg_coke/t_product [118,119]. Furthermore, this process provides an effective solution for the recycling of detrimental secondary resources, including red mud and virulent chromium slag. It achieves the transformation of these deficient wastes into high-value iron-bearing raw materials, thereby establishing a technical basis for the efficient utilization of these strategic resources [120,121,122]. Therefore, regarding resource utilization, the process demonstrates outstanding adaptability. It can treat various refractory iron ores, including hematite, limonite, and siderite, and accommodates significant proportions (up to 20%–30%) of iron-bearing secondary materials such as mill scale, dust, and steelmaking sludge.

In terms of product quality, sinter produced via the CAP exhibits a distinctive and advantageous mineralogical structure. The pelletized component is predominantly composed of hematite and magnetite, while the matrix phase is characterized by an interwoven network of silico-ferrite of calcium and aluminum (SFCA), accompanied by minor amounts of hematite and magnetite. Notably, in the transitional region between the pelletized and matrix components, iron oxides are intricately interlocked with acicular SFCA phases, creating a robust interfacial bond that ensures strong integration between the two constituents. This unique microstructure facilitates the production of high-quality, medium-to-low basicity blast furnace burdens through CAP. Such structural characteristics are exceptionally well-suited to the intensified smelting demands of large-scale blast furnaces, promoting stable and efficient furnace operation [110,113,118,123].

3.4.3. Development and Future Directions

Future development of the CAP should prioritize its integration with clean energy sources and the advancement of equipment automation and intelligent process control. The synergy of CAP with hydrogen-fueled sintering presents a viable pathway to substantially reduce carbon emissions, while its coupling with carbon capture technologies could achieve near-zero emissions from the sintering process, positioning CAP as a cornerstone for the green transformation of the iron and steel industry. Moreover, from a decarbonization perspective, CAP reduces solid fuel consumption by 5–12 kg/t, cutting direct CO₂ emissions by approximately 10%–15% compared with conventional sintering. Its improved bed permeability and thermal uniformity also enable better combustion efficiency and a more concentrated CO₂ off-gas, which enhances compatibility with hydrogen-based sintering and post-combustion carbon capture (CCUS). Future work should quantify the CO₂ reduction potential under different hydrogen-injection scenarios and assess CAP integration with CCUS systems.

Concurrently, advancements in digital technologies offer significant opportunities for enhancing CAP. The implementation of machine vision enables real-time monitoring of green pellet quality and quasi-particle characteristics. When combined with digital twins for process simulation and AI algorithms for parameter optimization, these innovations facilitate intelligent and precise control of the entire agglomeration system. Such digital integration is anticipated to significantly improve the techno-economic performance and market competitiveness of CAP.

In summary, as a revolutionary agglomeration technology, CAP demonstrates comprehensive advantages in resource utilization, product quality, and energy conservation. As the technology matures and costs decrease through continuous innovation, it is poised to become a mainstream raw material processing technology within the steel industry. Its unique capability to efficiently treat low-grade ores and secondary resources will make it indispensable for the high-quality development and sustainable transformation of the sector.

3.4.4. Industrial Deployment Considerations of CAP

Although the Composite Agglomeration Process (CAP) exhibits outstanding technical advantages in treating refractory iron ores and secondary resources, its industrial deployment entails several practical challenges that must be carefully evaluated.

From an industrial deployment perspective, the Composite Agglomeration Process (CAP) entails several practical challenges. Spatially, CAP requires parallel pelletizing and granulation lines, precision mixing, and dedicated distribution systems, typically increasing plant footprint by 15%–25% compared with conventional sintering, which may necessitate substantial layout reconfiguration for space-limited plants. Capital intensity is also higher: additional disc pelletizers (≥6 m diameter), high-precision mixers, multi-roll feeders, and advanced control systems can raise initial equipment costs by approximately 20%–30%, and retrofitting often involves extended shutdowns. Control complexity is significantly increased because two parallel material streams must be precisely proportioned and blended, demanding real-time monitoring, online basicity adjustment, and advanced automation (e.g., AI-based optimization, digital twins), as well as higher operator training levels. Sensitivity to raw material fluctuations is another concern: while CAP tolerates moderate ore type variations, sudden large changes in ultrafine concentrate proportion or chemical composition (Al₂O₃, SiO₂, LOI) can upset the optimal pellet-to-granule ratio, leading to incomplete consolidation or over-melting; therefore, a consistent feed or online characterization tools are beneficial.

Despite these challenges, industrial practices (e.g., at Baotou Steel) have shown robust CAP operation with proper design and control, and future efforts on modularized units and intelligent control systems can further mitigate these deployment hurdles.

3.5. Pre-Granulation Sintering Process

The steadily increasing output of fine iron concentrates continues to exert pressure on pellet production systems, necessitating the processing of surplus fine concentrates via sintering methods. Against this background, the effective integration of pelletizing—an important and efficient agglomeration method—into sintering production to utilize fine iron concentrates has emerged as a highly promising research direction. Innovative processes such as the CAP have demonstrated significant potential in this field.

However, the CAP requires pellets with a size range of 8–16 mm. These larger pellets are prone to breakage in common sintering plant distribution systems, such as shuttle distributors, rotary table feeders, and reflector plates (or multi-roller distributors). Furthermore, as the CAP production flow lacks a dedicated drying stage, the rapid dehydration of green pellets within a short time frame easily leads to fragmentation, imposing stricter requirements on pellet quality. Additionally, during the application of CAP at Baotou Steel, segregation occurred during the distribution of pelletized material [124]. These issues collectively indicate that the challenge of green pellet breakage must be prioritized for resolution if the distribution system remains unmodified.

To overcome the aforementioned limitations, the pre-granulation sintering process introduces critical improvements based on the CAP: difficult-to-granulate raw materials are first formed into 3–8 mm pellets in a drum or disc pelletizer before being mixed with the matrix feed for further granulation [125]. This optimized pellet size allows the coarser, high-basicity matrix granules to provide effective support and buffering for the smaller pellets. The matrix, with its larger particle size, acts as a protective cushion, significantly reducing the breakage rate of green pellets during distribution [126]. This enhancement not only improves mechanical stability but also ensures compatibility with conventional sintering distribution systems, such as reflector plates or multi-roller feeders, thereby substantially increasing the process’s industrial applicability.

Research findings demonstrate that the pre-granulation sintering process effectively addresses the issue of deteriorated permeability in thicker beds, enabling an increase in bed height while maintaining sinter productivity and quality. Specific improvements include a 3.2% increase in yield, a 7 kg/t reduction in solid fuel consumption, and a 0.7 t/(m²·h) gain in the utilization coefficient [125]. Furthermore, a novel quality evaluation method for sintering pellets has been proposed, providing a representative and effective approach for characterizing pellet quality during the sintering process [127]. This methodology offers crucial guidance for quality control and presents a viable solution for treating fine, easily slurried iron concentrates.

Currently, research on this method has been confined to some specific types of magnetite concentrate. Future studies should expand the range of raw material sources to verify the adaptability of the pre-granulation sintering process to different ore types. This will further enhance the iron and steel industry’s utilization rate of fine, difficult-to-granulate iron concentrates and promote the broader industrial application of this technology.

3.6. Comparison of Novel Intensification-Granulation Sintering Technologies

The aforementioned novel intensification-granulation sintering technologies significantly enhance the granulation efficiency of sintering raw materials, improve sinter bed permeability, and consequently increase sinter yield and quality. Compared to conventional granulation-process intensification technologies, these novel processes offer diversified solutions tailored to specific bottlenecks.

The press-briquetting sintering process introduces a pre-treatment concept for raw materials. However, the resulting briquettes often suffer from compositional heterogeneity and significant size variation, limiting their suitability for large-scale industrial applications. The pelletized sintering process innovatively converts all raw materials into pellets for sintering. Nevertheless, its integration into conventional sintering necessitates additional steps such as external coke breeze coating, laying of the side edge return-ore layer, and a dedicated drying section, leading to increased process complexity.

The split-stream granulation sintering process further advances raw material pre-treatment. Departing from the conventional uniform mixing and granulation approach, it classifies and processes materials based on their characteristics, creating favorable conditions for subsequent composite agglomeration. The composite agglomeration process (CAP) effectively combines the advantages of split-stream granulation and pelletized sintering. It provides a robust solution for utilizing fine iron concentrates and various iron-bearing secondary resources, overcoming the limitations of single-process methods. The pre-granulation sintering process introduces key improvements based on CAP. By optimizing pellet size, it enhances compatibility with conventional sintering distribution systems and reduces green pellet breakage, offering greater adaptability to existing sintering plants and equipment.

Each technology exhibits unique advantages in addressing specific bottleneck challenges. The rational selection of an appropriate technology, based on raw material characteristics and production objectives, is essential for stabilizing sintering operations. In particular, the Composite Agglomeration Process (CAP) and the pre-granulation sintering process demonstrate exceptional capability in treating the majority of fine-grained, complex, and difficult-to-granulate iron ores, as well as various iron-bearing secondary resources. These processes establish a necessary technical foundation for iron and steel enterprises to enhance the utilization rate of cost-effective raw materials, reduce production costs, and achieve sustainable development. A detailed comparison of these novel technologies, covering aspects such as raw material types, pelletizing/granulation methods, fuel addition techniques, product morphology, and basicity, is provided in

Table 1. Comparison of different novel intensification-granulation sintering technologies.

Comparative Items	Conventional Sintering	Conventional Pelleting	Press-Briquetting Sintering	Pelletized Sintering	Split-Stream Granulation Sintering	Composite Agglomeration Process	Pre-Granulation Sintering
Raw material type	Powder ore, concentrate ore	Concentrate ore	Powder ore, concentrate ore	Concentrate ore, fine powder ore	Powder ore, concentrate ore	Powder ore, concentrate ore, iron-bearing dust, etc.	Powder ore, concentrate ore, iron-bearing dust, etc.
Raw material particle size range	≤10 mm	<0.045 accounted for 80%–90%	≤10 mm	0–5 mm	≤10 mm	Pelleting material: <0.075 accounted for 60%–90%; coarse material: ≤10 mm	Pelleting material: <0.075 accounted for 60%–90%; coarse material: ≤10 mm
Granulation/pelletizing material particle size	Granulation: 3–10 mm	Pelletizing: 12–16 mm	Briquetting: 2–5 cm; Granulation: 3–10 mm	Pelletizing: 5–10 mm	Granulation: 3–10 mm	Pelletizing: 8–16 mm; Granulation: 3–10 mm	Pelletizing: 3–8 mm; Granulation: 3–10 mm
Fuel addition manner	All directly into the mixture	External heating	All directly into the mixture	Some directly into the mixture and some adhered to the surface of the pellets	All directly into the mixture	All directly into the matrix mixture	All directly into the matrix mixture
Drying section	Without	With	Without	With	Without	Without	Without
Side edge return-ore layer	Without	Determined by the roasting equipment	Without	With	Without	Without	Without
Consolidation mechanism	Melt-phase bonding	Solid-phase consolidation	Melt-phase bonding	Solid-phase consolidation with sintering melt-phase bonding	Melt-phase bonding	Solid-phase consolidation with sintering melt-phase bonding	Solid-phase consolidation with sintering melt-phase bonding
Product shape	Irregular block	Spherical shape	Irregular block	grape-like pellet aggregates connected in dots	Irregular block	Irregular block with a “matrix-embedded” structure	Irregular block with a “matrix-embedded” structure
Product alkalinity	1.8–2.2	<0.2	1.7–2.2	<0.2 or >2.0	1.8–2.2	1.2–2.2	1.2–2.2

.

4. Conclusions

This paper systematically reviews the research progress in granulation intensification technologies for iron ore sintering, focusing on two main directions: conventional granulation-process intensification technologies and novel intensification-granulation sintering processes. Conclusions are as follows:

(1)

In response to the declining quality of iron ore and increasingly complex raw material blends, the optimization and innovation of granulation processes have become pivotal for enhancing sintering efficiency and final product quality. Conventional granulation-process intensification techniques such as moisture control, binder modification, and equipment optimization can, to a certain extent, improve bed permeability and granulation uniformity, particularly under relatively stable raw material conditions. Meanwhile, granulation-process intensification technologies are rapidly developing toward intelligence, precision, and high efficiency. Intelligent moisture control, functionalized binders, and high-intensity mixing equipment, as well as parameter optimization and model prediction for the granulation process, have significantly enhanced control accuracy and process stability. In particular, the application of intelligent water addition systems and composite binders and the promotion of high-intensity mixers provide technical support for the granulation process to cope with raw material fluctuations. However, these conventional granulation-process intensification methods remain insufficient to meet the demands of utilizing high proportions of fine iron concentrates and difficult-to-process secondary iron-bearing resources. Breakthroughs must rely on new processes and methods.

(2)

Novel intensification-granulation sintering processes significantly expand the applicability of sintering raw materials and improve granulation effectiveness and sintering indicators through structural design and process innovation. The press-briquetting sintering process enhances the nucleation performance of fine-grained raw materials via mechanical compression; the pelletized sintering process optimizes particle structure and combustion efficiency through full raw material pelletizing and separate fuel addition; the split-stream granulation process enhances granulation specificity through material classification; the composite agglomeration process (CAP) cleverly combines pelletizing solid-phase consolidation with sintering melt-phase bonding, enabling synergistic utilization of high- and low-basicity materials; the pre-granulation sintering process further optimizes pellet size and burden distribution adaptability based on the composite agglomeration process, improving industrial feasibility. These processes each have distinct features and can be flexibly selected according to raw material characteristics and production objectives, providing diverse solutions for the efficient utilization of inferior iron ores and iron-bearing dust and sludge.

(3)

Among the novel intensification-granulation sintering processes, the CAP and pre-granulation sintering process demonstrate remarkable capabilities in expanding raw material adaptability and enhancing process efficiency. These two processes exhibit outstanding performance in treating fine-grained, complex refractory iron ores and various iron-bearing secondary resources. Their implementation establishes a crucial technical foundation for iron and steel enterprises to improve utilization of cost-effective raw materials, reduce production costs, and achieve sustainable development objectives.