Development of equipment and technology for pelletizing iron ore charge in production of pellets

Introduction

The process of pelletizing an iron ore charge in the production of pellets represents the initial phase of agglomerating raw materials derived from iron ore. This process facilitates the formation of a wet bulk mass, its primary structuring, and subsequent hardening [1; 2]. The primary objective of pelletization is to create round-shaped pellets possessing maximum possible strength, allowing them to withstand transportation and endure thermal operations without softening. The wet charge forming process in pellet production initiates with nucleation and culminates in the final pelletizing of nuclei. In traditional pellet production technology, impacting the nucleation process using existing technical means without involving auxiliary physical fields proves challenging [3]. However, a recent proposal aims to enhance the functionality of the pelletizing section by employing thermal power spraying of the wet charge onto the pelletizer skull. This method endows it with additional shape-generating and structure-forming functionalities during pellet production [4; 5]. Known as the technology of induced nucleation by spraying and final pelletizing (NSF) of nuclei, it significantly transforms the processes of nucleation and pelletization of the iron ore charge. This technology offers a diverse array of tools to influence pellet structural properties and the parameters of pelletizer production [4 – 6]. According to this technology, the initial stage of raw pellets production involves the formation of a dense sprayed layer (SL) of the charge using an air-charge jet (ACJ) within the idle zone of the rotating disc pelletizer. To create an embryonic mass, the SL within the same pelletizer zone is mechanically divided into solid nuclei, having shapes akin to spherocubes or spheroparallelepipeds. During the subsequent forming stage within the pelletizer’s working zone containing lumpy materials, the corners and faces of these nuclei are crumpled to form a round shape. Simultaneously, the nuclei undergo final pelletizing while mixed with moistened charge in a rolling mode, thereby forming the pellet shell [6 – 8]. The central part of two-layer pellets exhibits lower moisture content and is characterized by higher porosity, featuring an increased proportion of open pores. The low moisture content of such raw pellets, coupled with this porosity pattern, mitigates crack formation and significantly reduces the temperature of shock fracture during drying [4; 5]. Consequently, the likelihood of a decrease in strength of annealed pellets during subsequent metallurgical treatment diminishes. Even after undergoing intense firing, the structure of pellets maintains an augmented number of permeable pores open to reducing gases [5]. This specific pellet structure minimizes diffusion limitations during subsequent reduction processes and enhances the reactivity of agglomerated raw materials. Similar structural properties of pellets can also be achieved by utilizing pore-forming biomass [7; 8] or complementary technologies [9 – 11]. Figs. 1 and 2 depict the schematic representation of NSF technology and the macrostructure of the materials involved in wet charge forming. Implementing the production scheme can be readily accomplished within existing sites possessing available manufacturing areas and technical capabilities. The NSF technology has laid the groundwork for various technical solutions [4; 5] that facilitate the control of nucleation processes, pellet formation, and their physical properties. By analyzing the operational processes of nuclei and pellets, one can formulate the general principles governing nucleation and structurization of the lumpy mass within this technology.

The primary aim of this paper is to analyze the technical solutions directed towards equipment and technology development for pelletizing iron ore charge during pellet production based on induced nucleation.

Materials and methods

Fig. 3 display technical diagrams of devices used in pellet production, showcasing different methods of forming the dense SL by spraying the wet charge onto the skull. Meanwhile, Fig. 4 present schematic representations of the diverse methods employed to divide the SL into nuclei within these devices. It’s important to note that the author has secured patents from the Russian Federation for the depicted technical schemes (Figs. 3, 4). For the implementation of the pellet production technical schemes, the laboratory semi-industrial disc pelletizer served as the foundational apparatus (diameter 0.62 m, disc inclination angle γ = 45°, number of revolutions n = 12 rpm). The pelletizer is equipped with a jet unit (JU) (diameter d_JU = 0.02 m, charge flow rate G_sh = 0.03 – 0.04 kg/s, pressure P_a = 0.2 MPa, compressed air flow rate V_a = 0.6 m³/min) and variously designed devices for dividing the SL into nuclei. To implement the multi-jet technical schemes initially, three JUs, each with a diameter of 0.02 m, were utilized, all operating under identical initial conditions and maintaining the same charge flow rate. The wet charge employed in this process consisted of iron-ore concentrate sourced from the Tei deposit (d_h = 0.068 mm) and 1 % bentonite. A 5-kg charge was sprayed over a period of 60 s onto the 30 mm thick charge skull (CS) (ρ_CS = 2230 kg/m³, W_CS = 8.14 %) present in the idle zone of the disc, positioned at Θ_L = 25 (Θ_L = L/d_JU – dimensionless distance, L = 0.5 m). For the final pelletizing of nuclei and the formation of standard pellets, an additional 5 kg of wet charge were introduced into the pelletizer’s working zone. Average strength and moisture content measurements were conducted for all formed materials. The dimensions of the SL were measured (diameter d_SL, m and SL height on its axis h_t, m). Subsequently, the embryonic mass and the array of pellets were sieved. The sampling methodology is detailed in [4; 5].

For the technical schemes implementing of various methods of SL formation (Fig. 3), the spraying coefficient C_SL, %, was calculated as follows:

\[{C_{{\rm{SL}}}} = \frac{{{M_{\rm{sh}}} - {M_{\rm{m}}}}}{{{M_{\rm{sh}}}}},\]

where М_sh is the mass of the deposited charge, kg; М_м is the mass of the charge remaining after SL formation, kg.

For the technical schemes implementing various methods of SL division into nuclei (Fig. 4), the fractional composition of the embryonic mass and the nucleation coefficient C_nucl, %, were determined using the following formula:

\[{K_{{\rm{nucl}}}} = \frac{{{M_{{\rm{nucl}}}}}}{{{M_{\rm{sh}}}}} = \frac{{{M_{\rm{sh}}} - {M_{\rm{m}}} - {M_{\rm{n}}}}}{{{M_{\rm{sh}}}}},\]

where М_nucl is the mass of nuclei ranging from 2 to 10 mm in size, kg; М_n is the mass of nuclei fines smaller than 2 mm in size, kg.

Multiple experiments were conducted for each technical scheme, enabling the acquisition of average values for the parameters being analyzed. In order to provide a comprehensive assessment of the NSF technology’s efficiency and the properties of the SL, nuclei, and pellets, certain experimental outcomes were presented in dimensionless form, as discussed in [4; 5]. The relative strength of the molded materials, denoted as Θ_S, was determined using the equation

\[{\Theta _{S} = \frac{{{\Pi _{{\rm{av}}}}}}{{{\Pi _{{\rm{p}}}}}},\]

where S_av is the average strength of samples from SL, nuclei and pellets, kPa; S_p is the average strength of pellets, 12 – 16 mm in size, kPa, and S_p = 280 kPa.

The relative mass of molded materials (SL, nuclei, and pellets) Θ_M was calculated using the formula

\[{\Theta _M} = \frac{{{M_{\rm{m}}}}}{{{M_{\rm{t}}}}},\]

where М_m is the average mass of molded materials (SL, nuclei, and pellets), kg; М_t is the total mass of the charge used in pellet production, inclusive of the charge fed for the final pelletizing of nuclei in the pelletizer’s working zone, kg, М_t = 10 kg.

The relative moisture content of the molded materials Θ_W was evaluated using the equation

\[{\Theta _W} = \frac{{{W_{{\rm{av}}}}}}{{{W_{\rm{sh}}}}},\]

where W_ср is the average moisture content of the samples from SL, nuclei, and pellets, %; W_sh is the moisture content of the deposited charge, %, set at W_sh = 8.2 %.

The relative duration of the processes involving charge loading, depositing, SL division, and final pelletizing of nuclei, represented as Θ_τ, was calculated using the equation

\[{\Theta _\tau } = \frac{{{\tau _i}}}{{{\tau _{{\rm{p}}}}}},\]

where τ_i – is the duration encompassing charge loading, depositing, SL division, and final pelletizing of nuclei, s. The time periods for depositing, SL division, and final pelletizing of nuclei were to 15, 5 and 300 s, respectively. Meanwhile τ_p represent the total duration of pelletization, set at τ_p = 380 s.

To calculate the parameters of the embryonic mass (fractional composition, C_SL, Θ_М, Θ_S, Θ_W), formed in various ways (Fig. 3), the pelletizer was operated without dividers throughout the entire spraying period. In order to conduct a comparative analysis of the performance of these diverse technical schemes, we utilized the fundamental scheme (Fig. 3, a). This scheme relied on a single JU operation, spraying the wet charge onto an unprepared CS. It is noteworthy that most industrial depositing technologies entail numerous performance requirements concerning spraying devices and techniques, as indicated in [12 – 15].

Для расчета параметров зародышевой массы (фракционный состав, C_SL, Θ_М, Θ_S, Θ_W) acquired via various methods of SL division into nuclei (Fig. 4), the SL divider was utilized on the pelletizer subsequent to SL formation. For a comparative assessment of these technical schemes, we utilized the fundamental scheme (Figs. 1 and 3, a), which was additionally equipped with a divider featuring longitudinal (lamellar knives) and transverse (rotating drum with edges) dividers coated with bakelite varnish to prevent charge adhesion (Fig. 4, a).

Results and discussion

Fig. 5 illustrates the typical changes in parameters Θ_М, Θ_S, Θ_W during the process of charge formation, which includes charge deposition, SL formation, and its subsequent division into nuclei, followed by pelletization. These trends were observed in the basic spraying scheme (Figs. 3, a and 4, a). The experimental results are comprehensively presented in Tables 1 and 2. The dependencies obtained from these experiments, as depicted in Fig. 5, allow for the generalization of research outcomes and the identification of potential constraints within the NSF technology. Across all technical schemes detailed in Tables 1 and 2, a consistent observation is the rapid formation of the wet charge, showcasing an average nuclei mass growth rate exceeding 3.0 g/s. This rapid growth is accompanied by an increase in strength (Θ_S > 0.39) and relatively minor mass loss of the formed materials during the stages of charge deposition (up to Θ_M = 0.44) and SL division (up to Θ_M = 0.29) into nuclei (Fig. 5). It has been noted that enhancing the strength of the SL during charge deposition (Θ_S > 0.8 – 0.9) [4; 5] brings it closer to that of the whole pellet. However, this particular regime results in decreased porosity and a reduction in the proportion of open pores in the embryonic part of the pellet, which contradicts the fundamental structurization principles of the NSF technology. The slight increase in nuclei strength during SL division can be attributed to the mechanical compaction of the wet mass facilitated by dividers or supplementary devices. During the final pelletizing phase of the nuclei, both the mass and strength of the pellets increase as the shell forms in the rolling mode. The dehydration process of the SL is closely associated with barodiffusive moisture transfer facilitated by the ACJ, into which the SL and embryonic mass come into contact during the spraying process. The final pelletizing phase of nuclei is accompanied by the growth of parameters Θ_M, Θ_S, Θ_W, includes several stages and extends over a longer duration (300 s), significantly slowing down both the growth rate of mass (to 0.11 g/s) and strength [4; 5]. During this phase, the moisture content of pellets formed in the final pelletizing stage increases due to excessive moistening of the pellet shell. In the majority of technical schemes employing the NSF technology for final pellet production, the focus is on forming the maximum number of pellets sized between 12 to 16 mm (Θ_M > 0.7). These pellets exhibit an average mass growth rate (more than 0.3 g/s), attain the required strength (Θ_S = 1.0), and demonstrate reduced moisture content in their structure (Θ_W < 1). In the central embryonic part of the pellets, this decrease is even greater (Θ_W < 0.95).

An analysis of the efficiency of the reviewed technical schemes would be prudent. The scheme depicted in Fig. 3, b, deviates from the basic one by intensively moistening the deposition zone and the SL surface. This alteration allows for an increase in C_SL to 0.95 and augmentation of the SL’s geometrical dimensions and parameters Θ_M, Θ_S, Θ_W (Table 1). However, the excessive moistening of the SL to W_SL = 0.99W_sh and, consequently, the nuclei and resultant pellets, prolongs the subsequent drying duration of pellets during the heat treatment stage. Similarly, the scheme illustrated in Fig. 3, c, results in similar SL characteristics by premoistening the charge before spraying, differing from the basic solution. The scheme presented in Fig. 3, d, enhances the transverse dimensions and maintains a consistent SL thickness by overlapping boundary zones (δ = 1.0 – 0.8, where δ denotes the dimensionless radius of the SL). These SL zones exhibit high porosity and low strength. A similar effect on the spraying process is detailed in [16 – 20], associating control of the dispersed phase flow rate in the JU with variations in the rotation speed of the sprayed base. The scheme displayed in Fig. 3, e, is notably more intricate and diverges from the basic scheme by employing successive charge spraying using three JUs, thereby expanding the longitudinal deposition area. This extended exposure of the ACJ to the SL for approximately three times longer increases the SL strength by around 10 – 15 %. It intensifies moisture removal from the SL, maintaining indicator levels Θ_M, Θ_W closely resembling those of the basic scheme. The scheme shown in Fig. 3, f, focuses on improving the uniformity of SL thickness by utilizing a single JU operating alongside auxiliary fan-supplied air along the JU axis. This modification results in larger geometric dimensions of the SL due to the expansion angle of the jet increasing to 30°. However, a drawback of this scheme is the reduced strength properties of the SL due to decreased ACJ pressure. The scheme illustrated in Fig. 3, g, involves deflecting mechanical devices positioned in the path of the ACJ. This scheme has analogous disadvantages and facilitates the acquisition of SL with characteristics akin to the previously described scheme. The scheme presented in Fig. 3, h, deviates from the basic one by enabling the addition of auxiliary materials (strengthening, binding, hard-to-pelletize, and structure-forming additives) to the air-charge jet (ACJ). By introducing a relatively small amount (up to 1 – 2 %) of additives, such as an aqueous solution of liquid glass, into the deposited charge, the SL strength can be augmented by 10 – 15 % [4; 5]. The scheme shown in Fig. 3, i, incorporates additional measures beyond ACJ pressure. It employs extra strengthening and profiling (height aligning) loads formed by rotating drums mounted on the SL surface before its division. This approach enhances the SL strength and uniformity of its geometric dimensions. However, it entails increased technical complexity and is characterized by enhanced charge buildup on the metal drums.

The scheme illustrated in Fig. 4, b, diverges from the basic one due to the inclusion of composite drums. These composite drums account for the differences in the circumferential velocities of the SL circumferential velocities easing the division of larger diameter SL. Consequently, this modification leads to a 5 % increase in the strength of the nuclei and a reduction in fines content to 33.2 % (Table 2). The scheme displayed in Fig. 4, c, uses the pressure from the layer of materials circulating in the pelletizer’s working zone, which possesses substantial height and mass. This pressure aids in hardening the SL, facilitating its division upon exiting the lumpy materials layer [4; 5]. This approach results in a 5 – 10 % increase in nuclei strength while maintaining a relatively uniform fractional composition. The pelletizer divider shown in Fig. 4, d, employs a curved plow divider with a simplified structure. It enables the generation of nuclei in various sizes, encompassing a significant amount of fines (0 – 2 mm) at 43.4 %, along with larger pellets exceeding 10 mm. The scheme depicted in Fig. 4, e, involves a thin metal string stretched across a conical drum, serving as a divider for SL. This innovative design enables the simultaneous division of SL in both longitudinal and transverse directions, requiring significantly less force. It boasts features such as minimal mass buildup, low metal consumption, and high technological effectiveness [4; 5]. Similar devices are utilized in the ceramic industry for mass division before molding. In Fig. 4, f, the SL division scheme incorporates a cylindrical drum with wave-shaped ribs acting as a divider to form pelletized nuclei. To counter increased charge buildup, this scheme implements intensive moistening of the drum ribs before SL division, consequently elevating the moisture content of the nuclei. the SL and charge skull profiling schemes depicted in Fig. 4, g and h, respectively, ensure a constant thickness of the SL, thereby enhancing the uniformity of the fractional composition and resulting in larger average nuclei size Anucl of 6.22 mm. The SL division scheme in Fig. 4, i, employs a divider equipped with a system of rods to create a specialized nuclei structure. However, this configuration results in significantly lower strength values compared to those of the basic scheme.

Considering the characteristics of SL and embryonic mass (Θ_М, Θ_S, Θ_W, Table 1, 2) derived from the implemented technical schemes, along with an evaluation of the technological effectiveness of the devices (such as charge buildup, additional equipment material consumption, design complexity, reliability, and operational stability), a combined technological scheme for pellet production is recommended for practical implementation. This recommended scheme encompasses the induced nucleation NSF technology based on SL formation by a single ACJ (Fig. 3, a), enabling the utilization of strengthening additives in the deposited charge (Fig. 3, h). In this setup, the material is sprayed onto the pre-profiled charge skull (Fig. 4, g), and the SL is divided into nuclei using the conical drum equipped with a metal string (Fig. 4, e). Implementing the NSF technology based on these components facilitates the production of pellets characterized by reduced moisture content (Θ_W = 0.97), evenly distributed across the cross-section, achieving necessary and sufficient strength (Θ_S ≥ 1.0), favorable pore structure, and a maximum yield of pellets sized between 12 to 16 mm (Θ_M = 0.72). These parameters are much lower when conventional nucleation and pelletization technology is used (Θ_W = 1.1, Θ_M = 0.33) [4; 5]. Based on these findings, it is reasonable to anticipate higher pelletizer performance, along with lower energy consumption for thermal drying of pellets during subsequent heat treatment.

Conclusions

The results of studies investigating the performance of technical schemes based on NSF technology, which enables the control of the nucleation and pelletization processes, were analyzed. General principles governing the nucleation and structurization of lumpy mass within this technology framework have been formulated. We assessed the typical variations in parameters Θ_М, Θ_S, Θ_W during the charge formation and pelletization processes for both the basic scheme and several technical schemes under investigation. Through a comprehensive evaluation considering the indicators of NSF technology and technological effectiveness (e.g., charge buildup level, material consumption of additional equipment, design complexity, reliability, and operational stability), specific technical schemes were appraised, and the most efficient solutions were identified. The recommended combined scheme for pellet production utilizes NSF technology based on SL formation by a single air-charge jet (ACJ), permitting the incorporation of strengthening additives. In this approach, the material is sprayed onto a pre-profiled charge skull, and the SL is divided into nuclei using a conical drum equipped with a metal string. Implementing the NSF technology based on these elements allows for the production of pellets characterized by reduced moisture content (Θ_W = 0.97) evenly distributed across the cross-section, achieving necessary and sufficient strength (Θ_S ≥ 1.0) and yielding maximum pellets sized between 12 to 16 mm (Θ_M = 0.72). These findings provide grounds to anticipate enhanced pelletizer performance and reduced energy consumption during subsequent pellet heat treatment.