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Article

Study on Oxidation-Roasting Performance and Consolidation Mechanism of Phosphate Ore Pellets

by
Yulong Cen
,
Feng Zhang
*,
Xianghong Jiang
,
Zhuowei Lei
and
Zichun Chen
College of Materials and Metallurgy, Guizhou University, Guiyang 550025, China
*
Author to whom correspondence should be addressed.
Minerals 2026, 16(5), 433; https://doi.org/10.3390/min16050433
Submission received: 20 March 2026 / Revised: 18 April 2026 / Accepted: 20 April 2026 / Published: 22 April 2026
(This article belongs to the Section Mineral Processing and Extractive Metallurgy)
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Abstract

Pelletizing is an effective way of converting abundant phosphate ore fines into usable feedstocks for yellow-phosphorus production. In this work, the oxidation-roasting behavior of siliceous–calcareous phosphate ore pellets and siliceous phosphate ore pellets was evaluated in a laboratory tube furnace. The consolidation mechanisms were revealed using optical microscopy, X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy. The results indicate that siliceous phosphate ore pellets exhibit superior oxidation-roasting performance relative to siliceous–calcareous phosphate ore pellets. After roasting, oxidized siliceous–calcareous phosphate ore pellets show a loose and porous framework with large pores, thin walls, and occasional surface cracking. The consolidation of siliceous–calcareous phosphate ore pellets is mainly governed by the recrystallization bonding of silicon–magnesium-bearing fluorapatite. In contrast, oxidized siliceous phosphate ore pellets display a denser microstructure and stronger intergranular bonding. The dominant bonding forms are the recrystallization bonding of silicon-bearing fluorapatite and solid-state bonding between silicon-bearing fluorapatite particles and quartz particles. Furthermore, carbonate gangue minerals are detrimental to strength development because CO2 release during roasting promotes the development of interconnected porosity and defects, thereby reducing the compressive strength of oxidized phosphate ore pellets.

1. Introduction

Yellow phosphorus is an indispensable basic raw material in the contemporary phosphorus chemical industry [1]. It is widely used in many fields, such as agriculture, metal smelting, the fine-chemical industry, food additive production and the development of new-energy materials [2,3,4,5]. It plays an important role in the development of national economic security and strategic emerging industries [6,7,8]. At present, the main method used for the industrial production of yellow phosphorus is the electric furnace process [9,10]. This process is mature and highly adaptable, but it imposes strict requirements on the particle size, compressive strength, permeability, and thermal stability of the raw materials loaded into the furnace [11]. Usually, the electric-furnace burden should have a suitable lump size, sufficient compressive strength, good permeability, and adequate thermal stability so as to maintain stable furnace operation during charging and subsequent high-temperature smelting [12,13]. Among these properties, compressive strength is particularly important because insufficient compressive strength may lead to pellet breakage and the generation of fines during handling and charging, which can further deteriorate burden permeability and affect furnace stability. In addition, compressive strength is closely related to the internal structure of pellets, especially their porosity and pore connectivity, because these factors affect the continuity of the load-bearing skeleton and the degree of interparticle bonding.
However, the processes of mining, crushing and beneficiation are inevitably accompanied by the production of a large amount of phosphate ore fines with a particle size less than 5 mm, accounting for 20% to 50% of the total amount of raw ore [14,15]. It is estimated that the output of phosphate ore fines in China exceeds 20 million tons every year [16,17]. For a long time, phosphate ore fines could only be stockpiled or landfilled, leading to resource wastage and an increased environmental burden [18,19]. With the gradual depletion of high-grade lumpy phosphate resources, the supply of suitable phosphate feed for yellow-phosphorus production is becoming increasingly constrained. Efficient utilization of the large quantities of phosphate ore fines generated during mining and beneficiation has therefore become a key issue for the sustainable development of the phosphorus chemical industry [20]. However, it is difficult for these phosphate ore fines to meet the requirements of electric-furnace smelting regarding the permeability and structural strength of the burden due to their small particles, strong fluidity and irregular reaction surfaces. To meet the requirements of the electric-furnace smelting process, phosphate ore fines need to be agglomerated [21].
Sintering, briquetting and pelletizing are the three main methods for agglomerating phosphate ore fines. The sintering method is characterized by high energy consumption and significant emissions of flue gas and fluorine-bearing dust. Moreover, the size and pore structure of the sinter are not sufficiently uniform, which may lead to fluctuations in burden permeability in the furnace [22]. The briquetting method features a simple flowsheet and high shaping efficiency, but cold-pressed briquettes often exhibit inhomogeneous internal density and are prone to cracking and disintegration at high temperatures, making it difficult to maintain a stable burden column over long campaigns [23,24]. In comparison, the pelletizing method can produce fired pellets with a narrow size distribution, adjustable porosity, and high compressive strength and is therefore regarded as one of the most promising agglomeration routes for high-value utilization of fine phosphate ores [25,26].
In recent years, research has been carried out on the pelletizing of phosphate fines. The existing studies have mainly examined the effects of grinding fineness, moisture, pellet size, binders, and thermal treatment conditions on pellet quality and strength [26,27] and shown that pellet consolidation is closely related to phase transformation, sintering behavior, and pore-structure evolution [25,28,29]. Generally speaking, the balling performance of siliceous phosphate ore fines is superior to that of siliceous–calcareous phosphate ore fines and phosphate concentrates [30]. The balling performance and roasting properties of phosphate ore fines can be improved through strengthening measures such as pre-grinding, optimizing the ore blending scheme, and adding binders [30,31,32]. However, systematic comparisons of the oxidation-roasting behavior of different phosphate ore fines are still limited, and the mechanisms of phase evolution, pore-structure development, and strength formation during oxidation roasting remain insufficiently understood, especially for siliceous–calcareous phosphate ore fines. Therefore, it is necessary to clarify how ore type influences roasting behavior, phase evolution, microstructure development, and strength formation during oxidation roasting.
In this work, two representative phosphate ore fines were systematically investigated. Their oxidation-roasting performance was evaluated, and the consolidation mechanisms of the corresponding oxidized phosphate ore pellets were elucidated through phase and microstructural characterization. The unique aspect of this study lies in its direct comparison of two representative phosphate ore fines to clarify the effect of ore type on oxidation-roasting performance and pellet consolidation, thereby advancing the scientific understanding of the oxidation-roasting behavior of phosphate ore pellets.

2. Materials and Methods

2.1. Raw Materials

Raw materials used in this investigation included two types of phosphate ore fines supplied by a yellow-phosphorus production enterprise in Sichuan Province, China. The two ore fines are hereafter referred to as phosphate ore A and phosphate ore B. The chemical compositions of the phosphate ore fines were determined by X-ray fluorescence (XRF) ARL PERFORM’X X-ray fluorescence spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). The XRF data were processed using OXSAS software with the UniQuant analysis package (v2.2.1.2511, Thermo Fisher Scientific, Waltham, MA, USA) with the X_UQ method under a vacuum X-ray path, and the results were calculated as oxides and are listed in Table 1. As shown in Table 1, phosphate ore A is characterized by a relatively high CaO/P2O5 ratio of 1.59 and a low SiO2 content of 12.04 wt.%, whereas phosphate ore B exhibits a lower CaO/P2O5 ratio of 1.37 and a much higher SiO2 content of 39.43 wt.%. Accordingly, phosphate ore A is classified as a siliceous–calcareous phosphate ore, while phosphate ore B is classified as a siliceous phosphate ore [33,34,35,36,37,38].
The particle size distribution and specific surface area (SSA) values of the two ores are summarized in Table 2. Phosphate ore A shows an SSA of 1568 cm2/g, which is slightly higher than that of phosphate ore B (1194 cm2/g). Both ores exhibit relatively coarse size distributions, and the fractions smaller than 0.075 mm amount to 63.43% (A) and 66.95% (B), respectively. The specific surface area was measured using the Blaine method in accordance with GB/T 8074-2008 [39], and the particle size distribution was determined via sieving. Each reported value represents the average of three parallel measurements.
The X-ray diffraction (XRD) patterns of the two phosphate ore fines are shown in Figure 1. As shown, phosphate ore A is mainly composed of fluorapatite [Ca5(PO4)3F], quartz (SiO2), dolomite [CaMg(CO3)2] and a minor amount of calcite (CaCO3). In contrast, phosphate ore B mainly consists of fluorapatite and quartz and contains a certain amount of dolomite, while no obvious diffraction peak of calcite was detected [6,26]. Phase identification was conducted using HighScore Plus software (Version 3.0e, PANalytical B.V., Almelo, The Netherlands) based on the COD database.
To further clarify the differences in mineral composition between the two phosphate ore fines, a semi-quantitative analysis based on the XRD patterns was performed, and the results are listed in Table 3. For phosphate ore A, dolomite is the predominant phase, accounting for approximately 47.6%, followed by fluorapatite and quartz at approximately 39.1% and 13.1%, respectively, while calcite accounts for only 0.2%. In phosphate ore B, the contents of fluorapatite and quartz are comparable, at approximately 42.1% and 40.2%, respectively, whereas the dolomite content is approximately 17.7%, and no obvious calcite content was detected. These results indicate that phosphate ore A exhibits more-pronounced carbonate mineral characteristics, whereas phosphate ore B is predominantly composed of fluorapatite and quartz and contains a relatively lower quantity of carbonate minerals. This observation is consistent with the chemical composition data presented in Table 1, namely, the higher CaO/P2O5 ratio and lower SiO2 content of phosphate ore A, as well as the higher SiO2 content and lower CaO/P2O5 ratio of phosphate ore B.

2.2. Experimental Methods

2.2.1. Green-Pellet Preparation

Green pellets were produced from the phosphate ore fines using a disc pelletizer (diameter: 1.0 m; rim depth: 0.22 m) operated at 30 r/min with an inclination angle of 45°. Green pellet types A and B were produced separately from phosphate ore A and phosphate ore B, respectively. The moisture of green pellets was kept at 9.0 wt.% (type A green pellets) and 10.0 wt.% (type B green pellets), respectively. The pelletizing time was fixed at 12 min for all trials. After pelletizing, the pellets were sieved, and the 10–15 mm fraction was collected. The selected pellets were then dried in an oven at 105 °C for 4 h (until a constant mass was achieved) and used as feedstock for subsequent preheating and oxidation-roasting tests.

2.2.2. Experimental Procedure: Oxidation Roasting of Pellets

The oxidation-roasting tests of the dried pellets were conducted in a two-zone tube furnace (GSL–1400X–II, tube outer diameter: 60 mm, total length: 1200 mm) (Hefei Kejing Materials Technology Co., Ltd., Hefei, China). Although this equipment is a vacuum tube furnace by model designation, no vacuum was applied in this work, and all roasting experiments were carried out in naturally circulating air. The furnace consisted of a low-temperature zone and a high-temperature zone, corresponding to the preheating and roasting stages, respectively. To simulate the continuous movement of pellets through the preheating and roasting steps of the grate–kiln pelletization process, a two-stage oxidation-roasting procedure including preheating and roasting was designed. For each test, eight dried pellets were placed in a corundum porcelain boat. During the experiment, the boat was introduced from one end of the tube furnace and moved at a constant translational speed of 5 cm·min−1, sequentially passing through the preheating zone and roasting zone under the preset conditions, and it was then withdrawn from the opposite end at the same speed. The temperature distribution and corresponding residence times for the preheating and roasting stages are shown in Figure 2. The residence time in each stage was determined according to the axial temperature distribution in the furnace shown in Figure 2 and the boat’s translational speed. As shown in Figure 2, the pellets were first preheated at 850–1100 °C for 5–13 min and then roasted at 1140–1290 °C for 6–14 min. After roasting, the pellets were cooled to room temperature in air. Finally, six roasted pellets were randomly selected from each batch, and their compressive strengths were determined according to ISO 4700 [40]. The reported value was the average of the six pellets.
In the oxidation-roasting tests, preheating temperature, preheating time, roasting temperature, and roasting time were the independent variables of the process, and compressive strength was the primary dependent variable.

2.2.3. Microstructure Observations

The fired pellets obtained under the optimal oxidation-roasting conditions were embedded in epoxy resin and then polished on the central cross-sections. The microstructure and mineralogy of the fired pellets were examined using a Leica DMLP (Leica Microsystems GmbH, Wetzlar, Germany) optical microscope and a scanning electron microscope (SEM, TESCAN MIRA LMS, TESCAN, Brno, Czech Republic) capable of performing energy-dispersive X-ray spectroscopy. SEM-EDS was used to analyze the elemental distribution and the chemical composition of the constituent phases. The phase compositions of the samples were determined via X-ray diffraction (XRD, Rigaku Ultima IV, Rigaku Corporation, Akishima-shi, Tokyo, Japan; continuous scan; Cu Kα radiation; 40 kV/40 mA; divergence slit 1/2°; 2θ range of 10–80°; scan rate of 2°/min). The XRD results were used to identify the major and associated phases in the samples and to provide a basis for discussing phase transformation and slag phase formation during pellet roasting. Phase identification was performed using HighScore Plus software (Version 3.0e, PANalytical B.V., Almelo, The Netherlands) based on the COD database.

3. Results

3.1. Oxidation-Roasting Performance of Phosphate Ore Pellets

Figure 3 summarizes the effects of preheating and roasting parameters on the compressive strength of phosphate ore pellets. As shown in Figure 3a,b, the type B phosphate ore pellets exhibited better preheating performance than the type A phosphate ore pellets. For the type B phosphate ore pellets, increasing the preheating temperature and extending the preheating time markedly improved their compressive strength [26]. The optimal preheating condition was 1000 °C for 7 min, under which the compressive strength of the preheated type B pellets reached 544 N/pellet. This value is close to the targeted level (500 N/pellet) adopted in this study for handling and transfer after preheating [41]. In contrast, preheating temperature and time had a relatively minor effect on the preheated type A pellets. The compressive strength of the preheated type A pellets remained below 500 N/pellet even when the preheating temperature was increased and the preheating time was extended, indicating insufficient strength development at the preheating stage.
As shown in Figure 3c,d, the compressive strength of the fired type A and B pellets increases with both roasting temperature and roasting time. For the type B pellets, the compressive strength increases from 1513 N/pellet to 3483 N/pellet as the roasting temperature rises from 1140 °C to 1290 °C (Figure 3c). Notably, even at 1140 °C, the type B pellets already exceed the target strength level of 1000 N/pellet used in this study for engineering comparison. This value is used as an internal criterion to facilitate process screening and comparison rather than as a mandatory industrial specification. By contrast, the type A phosphate ore pellets exhibit poorer roasting performance. To meet the targeted strength level, roasting at ≥1230 °C is required.
Overall, the results shown in Figure 3 indicate that the type B phosphate ore pellets (siliceous phosphate ore) exhibit superior preheating and roasting performance relative to the type A phosphate ore pellets (siliceous–calcareous phosphate ore), demonstrating that ore type strongly affects oxidation-roasting behavior and consolidation. The following section therefore focuses on phase evolution and microstructural development to elucidate the controlling consolidation mechanisms for the two pellet systems.

3.2. Consolidation Mechanism of Oxidized Phosphate Ore Pellets

3.2.1. Macroscopic Appearance and Microstructure of Oxidized Phosphate Ore Pellets

To facilitate a direct comparison of microstructure and mineralogy between the two ore types under identical roasting conditions, pellets roasted at 1230 °C for 10 min were selected for OM/SEM-EDS/XRD characterization. This condition was used solely as a common reference for mechanism comparison. In contrast, the process-screening results in Figure 3 define the optimal roasting windows for each pellet type based on strength development.
Figure 4 presents the compressive strength and macroscopic appearance of fired phosphate ore pellets. As shown in Figure 4, the compressive strength of both pellet types increases with roasting temperature, and this is accompanied by a gradual change in pellet color. Notably, the most pronounced changes in both color and compressive strength occur in the range of 1200–1230 °C. When the roasting temperature is raised from 1200 °C to 1230 °C, the compressive strength increases sharply. Meanwhile, the surface color of the type A and B pellets shifts rapidly from light gray and reddish-brown to yellowish-brown and dark gray, respectively. These concurrent macroscopic changes suggest that intensive phase evolution and bonding development take place near 1230 °C, leading to enhanced interparticle consolidation and a marked increase in pellet strength. In addition, visible surface cracks can be observed on the fired type A pellets. Such cracking is expected to reduce local integrity and may partially account for the comparatively lower strength of the type A pellets under identical roasting conditions.
The microstructures of the oxidized type A and B phosphate ore pellets are shown in Figure 5. As illustrated in Figure 5a,b, the type A phosphate ore pellets contain numerous interconnected pores (highlighted in yellow in Figure 5a) and only narrow necks between adjacent mineral particles (the red-circled area in Figure 5b). Overall, the type A phosphate ore pellets exhibit a highly porous framework with thin pore walls, which indicates limited neck growth and weak interparticle bonding, consistent with their relatively low compressive strength. By contrast, the type B phosphate ore pellets show a more compact microstructure with fewer and smaller pores than the type A phosphate ore pellets, as shown in Figure 5c. The necks between adjacent mineral particles are noticeably thicker and wider (as shown in the red-circled area in Figure 5d), suggesting more developed solid-state bonding and improved load transfer across particle contact. These features imply that the type B pellets achieved a denser microstructure and stronger intergranular bonding, which is consistent with their higher compressive strength relative to the type A pellets.
To clarify the origin of the abundant internal porosity observed in the oxidized type A pellets, the two raw phosphate ore fines were embedded in resin and prepared as polished sections for SEM-EDS analysis. The results are shown in Figure 6. As shown in Figure 6a, the type A phosphate ore fines contain substantial amounts of fluorapatite and dolomite, and these two phases show pronounced intergrowth and mutual interlocking. By contrast, Figure 6b indicates that the type B ore fines are dominated by quartz and fluorapatite; fluorapatite in the type B ore fines mainly occurs as discrete particles with only limited intergrowth with quartz.
The distinct mineral assemblages provide a plausible explanation for the microstructural differences after oxidation roasting. Dolomite in the type A ore fines is expected to decompose upon heating, releasing CO2, which can generate pores at phase boundaries and within the fluorapatite-rich framework. As a result, the oxidized type A pellets tend to develop a loose and porous microstructure with large pores and thin walls, consistent with the SEM observations in Figure 5. Moreover, CO2 release and migration may contribute to local stress development during heating and cooling, which could promote surface cracking in the type A pellets. In contrast, the type B ore fines contain little carbonate; therefore, gas generation from carbonate decomposition is limited, favoring the formation of a denser microstructure in the oxidized type B pellets.

3.2.2. Mineralogy of Oxidized Phosphate Ore Pellets

Figure 7 presents the XRD patterns of the raw type A phosphate ore fines and the oxidized type A phosphate ore pellets. As shown in Figure 7, the major mineral phases in the raw type A phosphate ore fines include fluorapatite, dolomite, quartz, and a minor amount of calcite. After oxidation roasting, the diffraction peaks of dolomite and calcite disappear in the oxidized type A phosphate ore pellets, indicating the decomposition of carbonate phases with concomitant CO2 release. This gas evolution provides a reasonable explanation for the development of internal porosity and surface defects observed in the oxidized type A phosphate ore pellets, which is also consistent with the observations previously shown in Figure 5.
To further clarify the phase composition of the oxidized type A phosphate ore pellets, a semi-quantitative analysis based on the XRD patterns was performed, and the results are shown in Table 4. It can be seen that the oxidized type A phosphate ore pellets are still dominated by apatite-related phases, with fluorapatite-related phases accounting for approximately 93.7 wt.% and quartz, residual dolomite, and diopside accounting for approximately 1.8 wt.%, 3.2 wt.%, and 1.4 wt.%, respectively, while calcite was not detected. Compared with the raw type A phosphate ore fines, the number of carbonate phases fell markedly after oxidation roasting, indicating that dolomite had undergone substantial decomposition, whereas calcite essentially disappeared. Meanwhile, the formation of a small amount of diopside indicates that part of the CaO and MgO generated from carbonate decomposition reacted with SiO2 to form diopside (Ca(Mg,Al)(Si,Al)2O6) [42]. Because diopside is present only in a minor amount, it is unlikely to dominate the overall mechanical response of the oxidized type A phosphate ore pellets [36].
In addition, the main diffraction peaks of fluorapatite in the oxidized type A phosphate ore pellets become sharper and more intense, accompanied by a slight shift toward lower diffraction angles (Figure 7). Such a peak shift may indicate compositional modification of fluorapatite during high-temperature roasting. Considering the coexistence of SiO2 and MgO adjacent to fluorapatite grains, a solid-state reaction among fluorapatite, quartz, and MgO (derived from dolomite decomposition) can lead to the formation of silicon–magnesium-bearing fluorapatite, which can be expressed as Ca5−yMgy(PO4)3−x(SiO4)xF. The feasibility of substitution between PO43− and SiO44−, as well as the incorporation of Mg2+ into apatite-related structures, has been reported previously [33,43,44,45]. The SEM and EDS results shown in Figure 8 further support the presence of silicon–magnesium-bearing fluorapatite. In Figure 8, points 1, 2, and 4 all show the coexistence of Ca, P, Si, and Mg, indicating that these regions are not simply original fluorapatite but instead are more likely silicon–magnesium-bearing fluorapatite formed through high-temperature solid-state reactions; point 3 is mainly quartz.
It should be noted that silicon–magnesium-bearing fluorapatite was not quantified separately as an independent phase in the present semi-quantitative XRD analysis. This is mainly because its diffraction features overlap strongly with those of fluorapatite, making reliable distinction and separate quantification difficult under the current analytical conditions. Therefore, it was included in the fluorapatite-related phases in the semi-quantitative analysis. Overall, the oxidized type A phosphate ore pellets are mainly composed of fluorapatite-related phases, together with a small amount of residual quartz, residual dolomite, and newly formed diopside. The consolidation of this system is mainly associated with the recrystallization bonding of silicon–magnesium-bearing fluorapatite. However, the high porosity and loose structure shown in Figure 8a, together with the formation of numerous interconnected pores caused by carbonate decomposition, still limit further improvement in the compressive strength of the oxidized type A phosphate ore pellets.
Figure 9 shows the XRD patterns of the raw type B phosphate ore fines and oxidized type B phosphate ore pellets. As shown in Figure 9, the raw type B phosphate ore fines are primarily composed of fluorapatite and quartz, with only a minor presence of carbonate minerals. After oxidation roasting, the main reflections of fluorapatite in the oxidized type B phosphate ore pellets become sharper and more intense, and this is accompanied by a slight shift toward lower diffraction angles. This peak shift may indicate compositional modification of fluorapatite at elevated temperatures. Considering the intimate contact between quartz and fluorapatite, partial substitution of PO43− by SiO44− within the fluorapatite lattice is feasible, leading to the formation of silicon-bearing fluorapatite, which can be expressed as Ca5(PO4)3−x(SiO4)xF [43,46]. The SEM-EDS results (Figure 10) further support the presence of silicon-bearing fluorapatite in the oxidized type B phosphate ore pellets (points 1, 3, and 4 in Figure 10).
To further clarify the phase composition of the oxidized type B phosphate ore pellets, a semi-quantitative analysis based on the XRD patterns was performed, and the results are shown in Table 5.
It can be seen that, after oxidation roasting, the oxidized type B phosphate ore pellets are still mainly composed of fluorapatite-related phases and quartz. Fluorapatite-related phases account for approximately 62.2 wt.%, while quartz accounts for approximately 37.1 wt.%. Only a trace amount of residual dolomite (0.1 wt.%) was detected. Compared with raw type B phosphate ore fines, the number of carbonate phases fell markedly after oxidation roasting, indicating that carbonate minerals underwent substantial decomposition during roasting. These results suggest that the predominant mineral phases in the oxidized type B phosphate ore pellets are silicon-bearing fluorapatite and residual quartz. Combined with the XRD peak shift and SEM-EDS evidence, part of the fluorapatite phase can be reasonably regarded as silicon-bearing fluorapatite.
As shown in Figure 10a, the oxidized type B phosphate ore pellets exhibit relatively low porosity, characterized mainly by small pores and closely packed mineral particles.
This dense framework is consistent with the mineralogy of the raw type B phosphate ore fines, which contain only a minor amount of carbonate minerals; therefore, gas generation associated with carbonate decomposition during roasting is limited, reducing the likelihood of the formation of abundant large pores and surface cracking as observed for the oxidized type A phosphate ore pellets. Both quartz-rich regions (point 2 in Figure 10b) and fluorapatite-rich regions (point 1 in Figure 10a) were observed. Notably, at the contact interfaces between quartz and fluorapatite, Si incorporation into fluorapatite is evidenced by the formation of silicon-bearing fluorapatite (points 3 and 4 in Figure 10b), suggesting interfacial solid-state reactions during high-temperature roasting. Meanwhile, the dense fluorapatite-rich regions also indicate that recrystallization bonding of silicon-bearing fluorapatite occurs during oxidation roasting. Therefore, the dominant bonding form in the oxidized type B phosphate ore pellets can be understood to be the recrystallization bonding of silicon-bearing fluorapatite, supplemented by solid-state bonding between silicon-bearing fluorapatite particles and quartz particles.

3.2.3. Analysis of the Consolidation Mechanism of Oxidized Phosphate Ore Pellets

Based on the results from Section 3.2.1 and Section 3.2.2, the mineral reactions and structural evolution of phosphate ore pellets during oxidation roasting are schematically summarized in Figure 11. As shown in Figure 11, the raw type A phosphate ore fines are classified as siliceous–calcareous phosphate ore and mainly composed of fluorapatite, dolomite, quartz, and a minor amount of calcite. During oxidation roasting, the carbonate minerals (dolomite and calcite) in the raw type A phosphate ore fines decompose at elevated temperatures, releasing CO2 and producing reactive oxides (CaO and MgO). The release and outward migration of CO2 provide a reasonable explanation for the formation of internal pores and surface defects in the oxidized type A phosphate ore pellets, which limits densification and is consistent with their comparatively low compressive strength. Moreover, the decomposition of carbonate minerals leads to the formation of a large number of interconnected pores inside the pellets, which is unfavorable for pellet densification and consolidation. Meanwhile, the decomposition-derived oxides can participate in high-temperature solid-state reactions with adjacent silicate/phosphate phases. In particular, contact between MgO and quartz and fluorapatite can promote the recrystallization and compositional modification of fluorapatite, leading to the formation of silicon–magnesium-bearing fluorapatite [Ca5−yMgy(PO4)3−x(SiO4)xF]. In addition, a minor amount of diopside can form through reactions among CaO/MgO and SiO2 during roasting at 1230 °C. Consequently, the mineral assemblage of the oxidized type A phosphate ore pellets can be described as being predominantly composed of silicon–magnesium-bearing fluorapatite, together with residual quartz and a minor amount of diopside. Overall, the consolidation of the oxidized type A phosphate ore pellets is mainly governed by the recrystallization bonding of silicon–magnesium-bearing fluorapatite. However, the release of CO2 from carbonate decomposition leads to the formation of numerous interconnected pores, which is unfavorable for pellet densification and consolidation.
In contrast, the raw type B phosphate ore fines are classified as siliceous phosphate ore and mainly consist of fluorapatite and quartz with only minor quantities of carbonate minerals. During oxidation roasting, interfacial solid-state reactions between fluorapatite and quartz promote the incorporation of Si into fluorapatite, resulting in the formation of silicon-bearing fluorapatite [Ca5(PO4)3−x(SiO4)xF] and strengthening the bonding at points of fluorapatite–quartz contact. In parallel, the low carbonate content in the raw type B phosphate ore fines limits CO2 generation during roasting, thereby reducing gas-induced pore formation and favoring a denser microstructure in the oxidized type B phosphate ore pellets. The combination of strengthened interfacial bonding and reduced porosity improves load transfer across particle contact points, leading to a markedly higher compressive strength of the oxidized type B phosphate ore pellets compared with the oxidized type A phosphate ore pellets. Overall, the consolidation of the oxidized type B phosphate ore pellets is mainly governed by the recrystallization bonding of silicon-bearing fluorapatite and the solid-state bonding between silicon-bearing fluorapatite particles and quartz particles.

4. Conclusions

The oxidation-roasting behavior of two representative phosphate ore pellet types was systematically investigated, and the corresponding consolidation mechanisms were discussed based on strength measurements and phase/microstructural characterization. The main conclusions are as follows:
  • Compared with siliceous–calcareous phosphate ore pellets, siliceous phosphate ore pellets showed superior oxidation-roasting performance, as reflected by the higher compressive strengths of the preheated and oxidized pellets under lower roasting temperatures, together with a denser roasted structure. For siliceous phosphate ore pellets, the optimal parameters within the experimental range investigated in this study were preheating at 1000 °C for 7 min and roasting at 1140 °C for 10 min, yielding compressive strengths of 544 N/pellet (preheated pellets) and 1513 N/pellet (oxidized pellets). For siliceous–calcareous phosphate ore pellets, the optimal parameters within the experimental range investigated were preheating at 1000 °C for 7 min and roasting at 1230 °C for 10 min, resulting in compressive strengths of 264 N/pellet (preheated pellets) and 1017 N/pellet (oxidized pellets). Notably, the preheated strength of siliceous–calcareous pellets remained relatively low even under intensified preheating conditions.
  • Oxidized siliceous–calcareous phosphate ore pellets showed a porous microstructure with large pores, thin pore walls, and visible surface cracking. The predominant phases in these pellets were silicon–magnesium-bearing fluorapatite, together with minor amounts of quartz and diopside. In contrast, oxidized siliceous phosphate ore pellets exhibited lower porosity and a denser framework, and their main phases were silicon-bearing fluorapatite and quartz. These microstructural differences help explain why siliceous phosphate ore pellets are more favorable for subsequent high-temperature utilization, as they provide higher strength and a more stable pellet framework after oxidation roasting.
  • The consolidation of the siliceous–calcareous pellets was mainly characterized by the recrystallization bonding of silicon–magnesium-bearing fluorapatite, whereas siliceous pellets were mainly characterized by the recrystallization bonding of silicon-bearing fluorapatite and the solid-state bonding between silicon-bearing fluorapatite particles and quartz particles.
  • Carbonate decomposition had a detrimental impact on densification and strength development. CO2 release during oxidation roasting promotes pore formation and can introduce surface defects, which reduces effective particle–particle bonding and limits the compressive strength of the oxidized pellets.

Author Contributions

Conceptualization, F.Z.; methodology, Y.C. and F.Z.; software, Y.C.; validation, F.Z.; formal analysis, Y.C.; investigation, Y.C., X.J., Z.L. and Z.C.; data curation, Y.C., X.J., Z.L. and Z.C.; resources, F.Z.; visualization, F.Z.; writing—original draft preparation, Y.C.; writing—review and editing, F.Z.; supervision, F.Z.; project administration, F.Z.; funding acquisition, F.Z. All authors have read and agreed to the published version of the manuscript.

Funding

The research was financially supported by Zhongye Changtian International Engineering Co., Ltd. (No. 2023JCYJ04).

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

This study received funding/financial support from the company Zhongye Changtian International Engineering Co., Ltd. The funding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

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Minerals 16 00433 g001
Figure 1. XRD patterns of two phosphate ore fines.
Figure 1. XRD patterns of two phosphate ore fines.
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Figure 2. Heating profile of the oxidation-roasting process for green pellets.
Figure 2. Heating profile of the oxidation-roasting process for green pellets.
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Figure 3. Effect of preheating and roasting conditions on the compressive strength of type A and B phosphate ore pellets: (a) preheating for 7 min; (b) preheating at 1000 °C; (c) preheating at 1000 °C for 7 min, roasting for 10 min; (d) preheating at 1000 °C for 7 min, roasting at 1230 °C.
Figure 3. Effect of preheating and roasting conditions on the compressive strength of type A and B phosphate ore pellets: (a) preheating for 7 min; (b) preheating at 1000 °C; (c) preheating at 1000 °C for 7 min, roasting for 10 min; (d) preheating at 1000 °C for 7 min, roasting at 1230 °C.
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Figure 4. Compressive strength and morphological characteristics of fired phosphate ore pellets at different roasting temperatures (preheating at 1000 °C for 7 min, roasting for 10 min).
Figure 4. Compressive strength and morphological characteristics of fired phosphate ore pellets at different roasting temperatures (preheating at 1000 °C for 7 min, roasting for 10 min).
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Figure 5. Microstructures of oxidized phosphate ore pellets ((a) oxidized type A phosphate ore pellets; (b) enlarged view of the selected area in (a); (c) oxidized type B phosphate ore pellets; (d) enlarged view of the selected area in (c) (conditions: preheating at 1000 °C for 7 min, roasting at 1230 °C for 10 min)).
Figure 5. Microstructures of oxidized phosphate ore pellets ((a) oxidized type A phosphate ore pellets; (b) enlarged view of the selected area in (a); (c) oxidized type B phosphate ore pellets; (d) enlarged view of the selected area in (c) (conditions: preheating at 1000 °C for 7 min, roasting at 1230 °C for 10 min)).
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Figure 6. SEM images of the main mineral phases in raw phosphate ore fines and corresponding EDS analysis. (a) Type A phosphate ore fines; (b) type B phosphate ore fines. F: Fluorapatite; Q: quartz; D: dolomite. Points 1–4 indicate the EDS analysis positions, where 1 denotes dolomite; 2 and 3 denote fluorapatite; and 4 denotes quartz.
Figure 6. SEM images of the main mineral phases in raw phosphate ore fines and corresponding EDS analysis. (a) Type A phosphate ore fines; (b) type B phosphate ore fines. F: Fluorapatite; Q: quartz; D: dolomite. Points 1–4 indicate the EDS analysis positions, where 1 denotes dolomite; 2 and 3 denote fluorapatite; and 4 denotes quartz.
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Figure 7. XRD patterns of raw type A phosphate ore fines and oxidized type A phosphate ore pellets (conditions to which the oxidized pellets were exposed: preheating at 1000 °C for 7 min, roasting at 1230 °C for 10 min).
Figure 7. XRD patterns of raw type A phosphate ore fines and oxidized type A phosphate ore pellets (conditions to which the oxidized pellets were exposed: preheating at 1000 °C for 7 min, roasting at 1230 °C for 10 min).
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Figure 8. SEM images and EDS analysis of oxidized type A phosphate ore pellets. (a) Oxidized type A phosphate ore pellets. (b) Enlarged view of the selected area in (a). F: Silicon–magnesium-bearing fluorapatite; Q: quartz; P: pore. Points 1–4 indicate the EDS analysis positions, where 1, 2 and 4 denote silicon–magnesium-bearing fluorapatite and 3 denotes quartz. Conditions: preheating at 1000 °C for 7 min, roasting at 1230 °C for 10 min.
Figure 8. SEM images and EDS analysis of oxidized type A phosphate ore pellets. (a) Oxidized type A phosphate ore pellets. (b) Enlarged view of the selected area in (a). F: Silicon–magnesium-bearing fluorapatite; Q: quartz; P: pore. Points 1–4 indicate the EDS analysis positions, where 1, 2 and 4 denote silicon–magnesium-bearing fluorapatite and 3 denotes quartz. Conditions: preheating at 1000 °C for 7 min, roasting at 1230 °C for 10 min.
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Figure 9. XRD patterns of raw type B phosphate ore fines and oxidized type B phosphate ore pellets. Conditions for oxidized pellets: preheating at 1000 °C for 7 min, roasting at 1230 °C for 10 min.
Figure 9. XRD patterns of raw type B phosphate ore fines and oxidized type B phosphate ore pellets. Conditions for oxidized pellets: preheating at 1000 °C for 7 min, roasting at 1230 °C for 10 min.
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Figure 10. SEM images and EDS analysis of oxidized type B phosphate ore pellets. (a) Oxidized type B phosphate ore pellets; (b) enlarged view of the selected area in (a). F: Silicon-bearing fluorapatite; Q: quartz; P: pore. Points 1–4 indicate the EDS analysis positions, where 1, 3, and 4 denote silicon-bearing fluorapatite and 2 denotes quartz. Conditions: preheating at 1000 °C for 7 min, roasting at 1230 °C for 10 min).
Figure 10. SEM images and EDS analysis of oxidized type B phosphate ore pellets. (a) Oxidized type B phosphate ore pellets; (b) enlarged view of the selected area in (a). F: Silicon-bearing fluorapatite; Q: quartz; P: pore. Points 1–4 indicate the EDS analysis positions, where 1, 3, and 4 denote silicon-bearing fluorapatite and 2 denotes quartz. Conditions: preheating at 1000 °C for 7 min, roasting at 1230 °C for 10 min).
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Figure 11. Schematic illustration of phase evolution and consolidation mechanisms of phosphate ore pellets during oxidation roasting for different ore types.
Figure 11. Schematic illustration of phase evolution and consolidation mechanisms of phosphate ore pellets during oxidation roasting for different ore types.
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Table 1. Chemical compositions of phosphate ore fines (wt.%).
Table 1. Chemical compositions of phosphate ore fines (wt.%).
CompositionP2O5SiO2CaOMgOAl2O3Fe2O3FLOI
Phosphate ore A26.9812.0442.982.193.091.521.958.52
Phosphate ore B21.9339.4330.120.312.162.101.682.26
Table 2. Size distribution and specific surface area values of phosphate ore fines.
Table 2. Size distribution and specific surface area values of phosphate ore fines.
SampleSpecific Surface Area (cm2/g)Particle Size Distribution (%)
≥0.075 mm0.045–0.075 mm≤0.045 mm
Phosphate ore A156836.5715.3148.12
Phosphate ore B119433.0517.6549.30
Table 3. Major mineral phases and their relative contents in the two phosphate ore fines based on semi-quantitative XRD analysis.
Table 3. Major mineral phases and their relative contents in the two phosphate ore fines based on semi-quantitative XRD analysis.
SampleQuartz (SiO2)/%Fluorapatite [Ca5(PO4)3F]/%Dolomite [CaMg(CO3)2]/%Calcite (CaCO3)/%
Phosphate ore A13.139.147.60.2
Phosphate ore B40.242.117.7–
Table 4. Major mineral phases and their relative contents in raw type A phosphate ore fines and oxidized type A phosphate ore pellets based on semi-quantitative XRD analysis.
Table 4. Major mineral phases and their relative contents in raw type A phosphate ore fines and oxidized type A phosphate ore pellets based on semi-quantitative XRD analysis.
SampleQuartz/%Fluorapatite/%Dolomite/%Calcite/%Diopside/%
Phosphate ore A13.139.147.60.2–
Oxidized type A phosphate ore pellets1.893.73.2–1.4
Table 5. Major mineral phases and their relative contents in raw type B phosphate ore fines and oxidized type B phosphate ore pellets based on semi-quantitative XRD analysis.
Table 5. Major mineral phases and their relative contents in raw type B phosphate ore fines and oxidized type B phosphate ore pellets based on semi-quantitative XRD analysis.
SampleQuartz/%Fluorapatite/%Dolomite/%
Phosphate ore B40.242.117.7
Oxidized type B phosphate ore pellets37.162.20.1
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Cen, Y.; Zhang, F.; Jiang, X.; Lei, Z.; Chen, Z. Study on Oxidation-Roasting Performance and Consolidation Mechanism of Phosphate Ore Pellets. Minerals 2026, 16, 433. https://doi.org/10.3390/min16050433

AMA Style

Cen Y, Zhang F, Jiang X, Lei Z, Chen Z. Study on Oxidation-Roasting Performance and Consolidation Mechanism of Phosphate Ore Pellets. Minerals. 2026; 16(5):433. https://doi.org/10.3390/min16050433

Chicago/Turabian Style

Cen, Yulong, Feng Zhang, Xianghong Jiang, Zhuowei Lei, and Zichun Chen. 2026. "Study on Oxidation-Roasting Performance and Consolidation Mechanism of Phosphate Ore Pellets" Minerals 16, no. 5: 433. https://doi.org/10.3390/min16050433

APA Style

Cen, Y., Zhang, F., Jiang, X., Lei, Z., & Chen, Z. (2026). Study on Oxidation-Roasting Performance and Consolidation Mechanism of Phosphate Ore Pellets. Minerals, 16(5), 433. https://doi.org/10.3390/min16050433

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