Compressive Strength and Metallurgical Properties of Pellets with Added Oolitic Hematite

Abstract

The influence of oolitic hematite on the compressive strength and metallurgical properties of oxidized pellets was examined. The experimental results indicate that when the proportion of oolitic hematite does not exceed 8%, the compressive strength of the pellets can reach over 2500 N when roasted at 1250 °C for 15 min. When the proportion is increased to 10%, the compressive strength remains above 2500 N after roasting at 1250 °C for 20 min. As the proportion of oolitic hematite increases, the reduction expansion rate of the pellets decreases; however, the reducibility also diminishes, and the softening and dripping performance deteriorates. Take into account the characteristics of the comprehensive burden structure in blast furnace operations, the proportion of oolitic hematite in pellet production can be increased to 10.0%.

1. Introduction

The rapid expansion of China’s iron and steel industry has resulted in a significant rise in iron ore demand, leading to a substantial increase in iron ore imports. In recent years, imported iron ore has constituted over 80% of China’s total iron ore consumption. To safeguard the security of iron ore supply within our country, it is imperative to enhance the exploration, extraction, and utilization of domestic resources. The reserves of ootic iron ore in our country are estimated to be between 3 and 4 billion tons. Therefore, the effective utilization of this resource holds significant importance for alleviating the supply constraints of iron ore in our country. The iron grade of oolitic hematite in western Hubei is relatively high, averaging 42.59%. However, the concentrations of harmful elements such as phosphorus, aluminum, and silicon are also elevated. Moreover, the ore exhibits a disseminated and concentric ring structure, wherein various minerals are interlocked [1,2], as illustrated in Figure 1. The distribution of phosphate-containing minerals is highly uneven, with extremely fine dispersion. These minerals coexist with other mineral phases and are irregularly distributed along the edges of hematite particles. They frequently occur within the entire oolitic structure in the form of acicular, prismatic, and aggregate crystals, making it challenging to achieve effective monomer liberation [3,4]. To date, this resource has not been extensively developed and remains classified as a “dormant” mineral resource. In recent years, scholars both domestically and internationally have conducted extensive research on oolitic hematite. Based on current findings, the research objectives can primarily be categorized into two areas: iron extraction and phosphorus reduction. However, these studies exhibit certain limitations, including high costs, lengthy timelines, and significant resource consumption, which hinder its large-scale industrial application.

In this study, an experimental study on pellets produced through the direct addition of oolitic hematite raw ore was conducted to determine the optimal addition amount and evaluate its application feasibility. This research provides both theoretical and practical foundations for the development and utilization of oolitic hematite, which holds significant practical importance given the current situation of increasingly scarce high-quality iron ore resources and rising prices.

2. Experimental Materials and Methods

2.1. Experimental Materials and Chemical Composition

The iron ore utilized in the pelletizing experiment comprised oolitic hematite sourced from Enshi Prefecture in western Hubei Province and iron concentrate, with Indonesian bentonite serving as the binder. The chemical composition of the raw materials is detailed in

Table 1. Chemical composition of raw materials utilized in the experiments/%.

Raw Materials	TFe	FeO	SiO2	CaO	MgO	Al2O3	P
Iron concentrate	65.58	23.40	2.49	0.015	0.038	0.07	0.03
Oolitic hematite	55.29	0.89	18.17	1.18	0.29	5.98	0.56
Bentonite	2.76	-	57.38	3.01	1.98	14.23	-

. As illustrated in Table 1, the gangue content of oolitic hematite is significantly elevated, with the SiO₂ content reaching 18.17% and phosphorus content reaching 0.56%.

2.2. Experimental Methods

In this study, the mass% of oolitic hematite was incrementally increased to 0%, 6%, 8%, 10%, and 12%, respectively. Concurrently, the mass% of bentonite was maintained at a constant level of 1.0%. The proportion of raw materials for pelletizing is detailed in

Table 2. Proportion of raw materials for pelletizing.

Scheme	Oolitic Hematite/%	Iron Concentrate/%	Bentonite/%
1	0	99.0	1.0
2	6.0	93.0	1.0
3	8.0	91.0	1.0
4	10.0	89.0	1.0
5	12.0	87.0	1.0

.

Green pellets were prepared in a disk pelletizer of 1.0 m in diameter, with a rotational speed of 20 r/min and a 45° horizontal incline. The green pellets were obtained at sizes of 8 mm to 16 mm in diameter by screening, and the moisture of green pellets was about 8–9%. The green pellets were dried in an oven set at 105 °C for 3 h and subsequently roasted in a muffle furnace. Once the furnace temperature reached 900 °C, the dried green pellets were introduced into the furnace, with air being supplied at a flow rate of 1.2 L/min. After 40 min, the furnace temperature was increased to 1250 °C. Pellets were roasted at 1250 °C for 10, 15, and 20 min, respectively, followed by cooling to ambient temperature within the furnace.

Compressive strength testing of pellets was conducted in accordance with the GB/T 14201-2018 standard [5]. The reduction expansion rate of the pellets was evaluated in accordance with the GB/T 13240-2018 standard [6]. The reducibility was evaluated in accordance with the GB/T 13241-2017 standard [7]. In accordance with the GB/T 34211-2017 standard [8], the softening and dripping performance of pellets during high-temperature reduction under load was conducted. The roasted pellets were analysed with an X-ray diffractometer (XRD, X’Pert Pro, Bruker, Ettlingen, Germany).

3. Results and Discussion

3.1. Compressive Strength Results

The influence of oolitic hematite with different mass fractions and different roasting times on the compressive strength of pellets is shown in Figure 2. As can be seen from Figure 2, when the pellets are roasted at 1250 °C for 10 min, their compressive strength is relatively low, not exceeding 2500 N, which fails to meet the requirements of large blast furnaces. When the proportion of oolitic hematite does not exceed 8%, the compressive strength of the pellets exceeds 2500 N after being roasted at 1250 °C for 15 min. Additionally, the compressive strength exhibits minimal increase with extended roasting time. When the proportion of oolitic hematite increases to 10%, the pellets must be roasted at 1250 °C for 20 min for their compressive strength to exceed 2500 N. However, when the mass fraction of oolitic hematite increases to 12% (Scheme 5), the compressive strength of the pellets remains below 2500 N, even after being roasted at 1250 °C for 20 min.

3.2. Reduction Expansion Rate Test Results

In the metallurgical performance test of the pellets, the samples used were Scheme 1, Scheme 2, and Scheme 3 pellets roasted at 1250 °C for 15 min, while Scheme 4 was pellets roasted at 1250 °C for 20 min. Due to the low compressive strength of the Scheme 5 pellets, the relevant metallurgical properties were not tested.

The effect of oolitic hematite with different mass fractions on the expansion rate of pellets is shown in Figure 3.

As illustrated in Figure 3, the reduction expansion rate of the pellets decreases as the proportion of oolitic hematite increases. Specifically, when no oolitic hematite is added, the expansion rate for NO. 1 is 13.73%. Conversely, with the addition of 10% oolitic hematite, the expansion rate for NO. 4 drops to 9.51%.

3.3. Reducibility Test Results

The influence of oolitic hematite with different mass fractions on the reducibility of the pellets is shown in Figure 4.

It can be seen from Figure 4 that the reducibility of the pellets decreases with the increasing proportion of oolitic hematite. Specifically, when no oolitic hematite is added, the reduction degree of the pellets (NO. 1) is 80.65%. However, with the addition of 10% oolitic hematite, the reducibility of the pellets (NO. 4) drops to 76.18%.

3.4. Softening and Dripping Performance Test Results

In the test, the softening onset temperature is defined as T₁₀, which corresponds to the temperature at which the volumetric shrinkage rate reaches 10%. The softening completion temperature is defined as T₄₀, representing the temperature where the volumetric shrinkage rate attains 40%. The softening interval is ΔT, and ΔT = T₄₀ − T₁₀. The test results of softening and dripping performance are shown in

Table 3. Test results of softening and dripping performance.

Pellet Type	T10/°C	T40/°C	ΔT/°C	Dripping Temperature/°C
NO. 1	1079	1195	116	1232
NO. 2	1087	1210	123	1295
NO. 3	1104	1287	183	1338
NO. 4	1120	1362	242	1405

.

As shown in Table 3, with the increasing proportion of oolitic hematite, the softening interval of the pellets broadens and the dripping temperature rises. For instance, when the proportion of oolitic hematite increases to 10%, the softening interval extends to 242 °C, and the dripping temperature reaches 1405 °C. This adversely affects the permeability of the upper section of the blast furnace.

3.5. Results Analysis

As illustrated in Figure 2, the compressive strength of the pellets is influenced not only by the quantity of oolitic hematite added but also by the duration of roasting. When the pellets are roasted at 1250 °C for 10 min, the compressive strength is less than 2500 N, which cannot meet the requirements of large blast furnaces. On the other hand, when no oolitic hematite was added, the compressive strength of the pellets reached 2730 N after roasting at 1250 °C for 15 min. With the increase in oolitic hematite, the compressive strength of pellet decreases. Under the same roasting time, when the proportion of oolitic hematite is 10%, the compressive strength of pellet drops to 2481 N, which cannot meet the requirements of large blast furnaces. If the roasting time is extended to 20 min, the compressive strength increases to 2588 N, meeting the requirements of large blast furnaces. When the proportion of oolitic hematite increases to 12%, even after roasting at 1250 °C for 20 min, the compressive strength of the pellets is still less than 2500 N.

Figure 5 shows the main phase of the roasted pellets. It can be observed from the figure that when the roasting time of the pellets reaches 15 min or longer, the phases are primarily composed of Fe₂O₃ and SiO₂, with no detectable unoxidized Fe₃O₄ phase. Conversely, for a shorter roasting time, incomplete oxidation of the pellets results in the presence of Fe₃O₄.

To further elucidate the reasons for the variations in the compressive strength of the roasted pellets, an analysis of the optical microstructure of different types of pellets was conducted, with the results presented in Figure 6.

Figure 6a shows pellets roasted for 10 min. Due to the short roasting time, residual raw ore structures are evident, with Fe₃O₄ not fully oxidized to Fe₂O₃. Consequently, the structure remains porous and loose, resulting in low compressive strength of the pellets. In contrast, pellets subjected to adequate roasting exhibit a phase structure primarily composed of hematite, gangue, and pores, which is characteristic of a typical oxidized pellet [9,10], as shown in Figure 6b–e. The majority of hematite grains exhibit interlinking characteristics with well-developed crystallization and relatively large grain sizes. During the roasting process of magnetite pellets, magnetite (Fe₃O₄) undergoes oxidation to form hematite (Fe₂O₃). This transformation results in a significant change in lattice structure, endowing the newly generated hematite with substantial migration capability. Under high-temperature conditions, solid-phase diffusion facilitates the formation of hematite crystal bridges between particles, thereby interconnecting them and imparting high strength to the pellets [11].

As shown in Figure 3 and Figure 4 and Table 3, the metallurgical performance tests of the pellets indicate that an increase in the proportion of oolitic hematite results in a decrease in the reduction expansion rate, which is beneficial for blast furnace operation. However, this increase also leads to a reduced reducibility, wider softening interval, and higher dripping temperature, all of which negatively impact blast furnace operation.

Table 4. Chemical composition of roasted pellets/%.

Pellet Type	SiO2	Al2O3	CaO	MgO	TFe	FeO
NO. 1	2.98	0.16	0.10	0.15	65.13	2.28
NO. 2	5.32	1.97	0.19	0.20	64.09	0.70
NO. 3	7.07	3.25	0.22	0.31	63.78	0.68
NO. 4	9.12	4.50	0.31	0.45	62.83	0.69

shows the chemical composition analysis results of the roasted pellets. It can be seen from Table 4 that with the increase in the proportion of oolitic hematite, the content of SiO₂ in the roasted pellets increases. The increase in SiO₂ content in the pellets hindered the growth of hematite grain. At the same time, when the content of SiO₂ is too high, the quartz and glass in the pellets increase, which reduces the compressive strength and leads to a reduced reducibility and wider softening interval [12,13]. Furthermore, SiO₂ reacts with FeO generated during the reduction process to form 2FeO·SiO₂; 2FeO·SiO₂ is more difficult to reduce than FeO [14], which adversely affects the softening and dripping performance of the pellets. In summary, it is evident that an increase in SiO₂ content is detrimental to the metallurgical properties of pellets.

In terms of the metallurgical properties of the pellets alone, the proportion of oolitic hematite ideally not exceed 8.0%. This is because a higher proportion, such as 10.0%, results in a softening interval exceeding 200 °C, along with an elevated dripping temperature. However, the charge structure in modern blast furnaces typically consists of high-basicity sinter combined with acidic pellets and lumps, which compensates for the drawbacks associated with pellets. Consequently, the proportion of oolitic hematite in pellet production can be increased to 10.0%. The focus of further investigation is to explore the optimal ratio between pellets containing oolitic hematite (for instance, 10%) and sinter, aiming to achieve superior metallurgical properties.

4. Conclusions

(1) Under identical temperature and roasting time conditions, the compressive strength of the pellets decreases as the proportion of oolitic hematite increases. Specifically, when the proportion of oolitic hematite does not exceed 8%, and the pellets are roasted at 1250 °C for 15 min, their compressive strength consistently exceeds 2500 N. However, when the proportion of oolitic hematite rises to 10%, the pellets require a roasting time extension to 20 min at 1250 °C to achieve a compressive strength greater than 2500 N. Nevertheless, as the mass fraction of oolitic hematite continues to increase, even after roasting at 1250 °C for 20 min, the compressive strength of the pellets remains below 2500 N.

(2) With the increasing proportion of oolitic hematite, the SiO₂ content in the roasted pellets also increases, potentially leading to effects on the metallurgical properties of the pellets. Specifically, the reduction expansion rate decreases, but reducibility diminishes, the softening interval widens, and the dripping temperature rises.

(3) From the perspective of the metallurgical performance of the pellets alone, the proportion of oolitic hematite should not exceed 8.0%. However, by leveraging the complementary nature of high-basicity sinter and acidic pellets within the comprehensive burden structure of the blast furnace, the proportion of oolitic hematite in pellet production can be increased to 10.0%.