Magnetization roasting experiment of a certain oolitic high phosphorus hematite and limonite in a rotary kiln

The steel industry is one of the pillar industries of the national economy, especially in China’s high-speed development of the national economy. There are two ways to solve the shortage of iron ore raw materials and fill the supply and demand gap. One is to search for and develop new iron ore raw material bases; The second is to continue to utilize foreign iron ore resources. In China’s iron ore resources, iron ore with characteristics such as easy selection, low impurity content, high iron content, and simple beneficiation process is gradually facing depletion; On the contrary, the refractory iron ore resources with high impurity content (mainly P and S), low iron content, and fine embedded particle size have not been reasonably developed and utilized.
At present, due to the complex beneficiation process, the oolitic high phosphorus hematite and limonite in difficult to select iron ores have low iron grade and high phosphorus content in the obtained iron concentrate products. There is still no reasonable beneficiation process to utilize this valuable iron ore resource. Therefore, developing a reasonable new beneficiation process to treat oolitic high phosphorus hematite and limonite has significant practical significance.
1、 Sample properties
This semi industrial test sample comes from a certain region in Sichuan, embedded with fine-grained high phosphorus oolitic hematite and limonite, which are in block shape and have high hardness. The maximum particle size of the raw ore is less than 50mm, accounting for about 20% of the total sample. Some are less than 25mm, accounting for about 35% of the total sample. The rest are all below m15mm. From the naked eye, there are many gangue minerals (quartz, calcite, etc.) in the raw ore, which are densely distributed and have obvious oolitic morphology. The iron grade of the raw ore is 39.38%, and the phosphorus content is 0.763%. The main iron mineral components of the ore are hematite and limonite, followed by magnetite, silicate iron ore, siderite, pyrite, etc; The main gangue minerals of the ore are quartz, calcite, diopside, pyroxene, chlorite, aragonite, garnet, etc. To meet the requirements of industrial experiments, the samples were prepared to be less than -10mm for spectral analysis, multi-element analysis, iron phase analysis, and screening tests. The test results are shown in Tables 1 to 4 in sequence.
From the spectral analysis results, multi-element analysis results, and iron phase analysis results in Tables 1 to 3, it can be seen that the main recovered element in the sample is iron, while other valuable elements such as copper, zinc, lead, molybdenum, nickel, cobalt, titanium, gold, and silver have low content and no comprehensive recovery value; The content of harmful elements such as sulfur and arsenic does not exceed the standard, but phosphorus severely exceeds the standard by 0.763%. The optional iron in the sample is hematite, limonite, siderite, and magnetic iron, accounting for 84.99% of the original ore. Therefore, the main purpose of this ore is to achieve qualified iron concentrate by increasing iron and reducing phosphorus.
From Table 4, it can be seen that the distribution of iron does not change significantly with particle size, while the distribution of phosphorus also changes relatively little with decreasing particle size.
2、 Main equipment and phosphorus reducing agents for testing
The main equipment for the test is φ 800mm × 9000mm rotary kiln, spiral conveyor feeder, jaw crusher, roller crusher, vibrating screen, Raymond mill, fine coal feeder, spiral classifier, hydrocyclone, 2 sets of 900mm × 1800mm ball mill, cylindrical magnetic separator (B=0.30T), permanent magnet cylindrical magnetic separator (B=0.15T), water quenched spiral continuous conveyor (self-developed) and auxiliary equipment.
This experiment used a rotary kiln magnetization roasting method. Through mineralogical research on the raw ore, it was found that phosphorus in the sample exists in the form of collophane, which is characterized by a relatively fine particle size and coexists with iron ore in the form of lattice substitution. At the same time, laser cutting 25cm 30cm thick iron plate is embedded in the ore in the form of eel, and the particle size is also relatively fine. This determines that conventional magnetization roasting is difficult to achieve the ideal effect of increasing iron and reducing phosphorus. Therefore, a self-developed composite roasting phosphorus reducing agent (code name LCP) is used for phosphorus reduction.
This agent belongs to the category of salt inorganic compounds, with characteristics such as low melting point, affinity for phosphorus minerals, and low interference. The main mechanism is to use the ore to rapidly react with phosphorus minerals in iron ore at a roasting temperature of 900-1100 ℃ to generate a new mineral, achieving effective transformation of phosphorus minerals and ultimately achieving effective separation from iron minerals.
3、 Semi industrial experimental research
After preliminary small-scale experimental research and expanded experimental research, it was found that the suitable process for this ore is magnetization roasting, one or two stages of grinding, and one or two magnetic separation processes. Through the magnetization roasting process, a self-developed LCP combination phosphorus reducing agent was added, resulting in a beneficiation index of 65% iron grade, phosphorus content ≤ 0.30%, and iron recovery rate ≥ 75%. Therefore, the process flow of magnetization roasting, grinding, and magnetic separation is adopted for the rotary kiln (small 800mm) × 9000mm) semi industrial experimental research, and adjust process parameters based on the problems and test results encountered during the semi industrial experimental process to seek the optimal process parameters to obtain ideal iron concentrate product indicators
（1） Baking condition test
Roasting is one of the key factors in the entire process, and the roasting conditions include roasting temperature, roasting time (the time difference between the material entering the rotary kiln and discharging), coke dosage, phosphorus reducing agent (LCP) dosage, coke particle size, and pellet diameter. The roasting temperature is reflected by temperature sensors (A, B, C, D, E) installed on the rotary kiln. The high-temperature zone is A-B, with a length of 2m, the roasting reaction zone is B-C, with a length of 4m, and the drying zone is C-E, with a length of 3m. The roasting time is controlled by adjusting the rotational speed of the rotary kiln. Different rotational speeds of the rotary kiln are achieved by adjusting the frequency f of the frequency converter. The relationship between different frequencies of the frequency converter and the roasting time is shown in Table 5.
1. Baking temperature test
The roasting temperature is controlled by the temperature sensor of the rotary kiln. Under the conditions of rotary kiln frequency converter f=30Hz (roasting time of 60min), LCP dosage of 10%, coke dosage of 8%, coke particle size of -1mm, pellet diameter of -20+5mm, weak magnetic separation magnetic induction intensity B1=0.30 T, B2=0.12 T, primary grinding fineness of -0.100 mm accounting for 95%, and secondary grinding fineness of -0.045 mm accounting for 80% or more, the roasting temperature test is carried out. The experimental process is shown in Figure 1,
Fe grade; ▲ – Fe recovery rate; ◆ – P grade（ × 10-2); ● – P recovery rate
From Figure 2, it can be seen that the temperature ranges from 900 ℃ to 1000 ℃. As the roasting temperature increases, the iron grade gradually increases, and the iron recovery rate also shows an increasing trend; When the temperature rises to 1050 ℃, the iron grade decreases and the iron recovery rate also decreases to a certain extent. The phosphorus content in iron concentrate shows a trend of first decreasing and then increasing with the increase of roasting temperature. Taking into account the roasting temperature of 1000 ℃, a beneficiation index with an iron grade of 65.74%, phosphorus content of 0.236%, and iron recovery rate of 78.11% can be obtained.
2. Baking time test
Through the roasting temperature test, it was found that a roasting temperature of 1000 ℃ is more suitable. Therefore, the roasting time test was conducted under the conditions of controlling the rotary kiln temperature to 1000 ℃, using 10% LCP, 8% coke, particle size -1mm, pellet diameter -20+5mm, weak magnetic separation magnetic induction intensity B1=0.30T, B2=0.12T, primary grinding fineness -0.100mm accounting for 95%, and secondary grinding fineness -0.045mm accounting for over 80%. The experimental process flow is shown in Figure 1. The test results are shown in Figure 3.
Fe grade; ▲ – Fe recovery rate; ◆ – P grade（ × 10-2); ● – P recovery rate
From Figure 3, it can be seen that as the roasting time increases, the iron grade gradually decreases, and the iron recovery rate also shows a decreasing trend. During the entire change process, when f=40Hz, an extreme point appears, corresponding to a roasting time of 45 minutes (Table 5); As time increases, the phosphorus grade increases, and as time decreases, the phosphorus grade also increases, showing a trend of low to high between the two ends. Choosing a roasting time of 45 minutes can yield a beneficiation index with an iron grade of 66.01%, phosphorus content of 0.225%, and iron recovery rate of 79.09%.
3. Coke dosage test
There are many types of reducing agents, such as brown coal, anthracite, bituminous coal, etc. These types of reducing agents generally contain high impurities (such as sulfur, phosphorus, arsenic, etc.) and are easily brought into the concentrate to affect product quality. Therefore, only coke is selected as the reducing agent for testing. Coke plays a dual role in providing a reducing atmosphere and reducing carrier throughout the entire roasting process, and the amount of coke directly affects the quality of the roasted product. Therefore, under the conditions of rotary kiln frequency converter f=40Hz (roasting time 45min), LCP dosage of 10%, coke particle size -1mm, pellet diameter -30+5mm, weak magnetic separation magnetic induction intensity B1=0.30T, B2=0.12T, primary grinding fineness -0.100mm accounting for 95%, and secondary grinding fineness -0.045mm accounting for more than 80%, the coke dosage test was conducted. The test process flow is shown in Figure 1, and the test results are shown in Figure 4.
Fe grade; ▲ – Fe recovery rate; ◆ – P grade（ × 10-2); ● – P recovery rate
From Figure 4, it can be seen that as the amount of coke increases, the iron grade increases, the phosphorus content decreases, and the iron recovery rate increases. However, when the amount increases to 8% and then continues to increase, the changes in iron grade, phosphorus grade, and iron recovery rate are relatively small. Therefore, choosing 8% of coke amount is more reasonable, and a beneficiation index with an iron grade of 65.98%, phosphorus content of 0.215%, and iron recovery rate of 78.89% can be obtained.
4. Coke particle size test
The particle size of coke is mainly reflected in its specific surface properties. The larger the particle size, the smaller the specific surface area; On the contrary, the larger the specific surface area. In addition, due to the need to pellet the sample, the larger the particle size, the less uniform the corresponding degree is; The finer the particle size, the larger the contact area with the sample. Under the conditions of roasting temperature 1000 ℃ (rotary kiln temperature sensor), rotary kiln frequency converter f=40Hz (roasting time 45 minutes), LCP dosage 10%, coke dosage 8%, pellet diameter -20+5mm, weak magnetic separation magnetic induction intensity B1=0.30T, B2=0.12T, primary grinding fineness -0.100mm accounting for 95%, secondary grinding fineness -0.045mm accounting for more than 80%, the coke dosage test is conducted. The test carbon fiber roller by precision cnc turning flow is shown in Figure 1, and the test results are shown in Figure 5.
Fe grade; ▲ – Fe recovery rate; ◆ – P grade（ × 10-2); ● – P recovery rate
From Figure 5, it can be seen that mineral processing indicators with iron grade greater than 65%, phosphorus content less than 0.3%, and iron recovery rate higher than 78% can be obtained for sizes below -1mm. When the coke size increases to+1mm, the phosphorus in the iron concentrate increases to 0.328%. Therefore, a coke particle size of -1mm is more reasonable.
5. Pellet diameter test
The size of the pellet diameter mainly affects the roasting time, and the larger the diameter, the longer the roasting time; On the contrary, the shorter the roasting time. In addition, the prolonged roasting time affects the unit processing capacity of the rotary kiln, which increases the beneficiation cost under the same conditions. Therefore, the diameter of the ball should not be too large or too small. Under the conditions of roasting temperature 1000 ℃, rotary kiln frequency converter f=40Hz (roasting time 45min), LCP dosage 10%, coke dosage 8%, coke particle size -1mm, weak magnetic separation magnetic field strength B1=0.30T, B2=0.12T, primary weak magnetic separation grinding fineness -0.100mm accounting for 95%, and secondary weak magnetic separation grinding fineness -0.045mm accounting for more than 80%, the pellet diameter test is conducted. The test process flow is shown in Figure 1, and the test results are shown in Figure 6.
Figure 6 Test results of pellet diameter size
Fe grade; ▲ – Fe recovery rate; ◆ – P grade（ × 10-2); ● – P recovery rate
From Figure 6, it can be seen that the diameter of the pellets is relatively suitable between -30+5mm. The iron grade in the obtained iron concentrate is greater than 65%, the phosphorus content is less than 0.3%, and the iron recovery rate is higher than 78%. However, during the roasting process, it was found that there was a phenomenon of “ring formation” at -10+5mm, so it is more reasonable to control the diameter of the pellets between -30+10mm. This can not only obtain better beneficiation indicators, but also reduce the degree of “ring formation” in the rotary kiln.
6. LCP phosphorus reducing agent dosage test
The LCP phosphorus reducing agent belongs to the category of composite agents. Based on the market price of its components, the comprehensive price is about 400 yuan/t. The amount used not only affects the phosphorus content in the iron concentrate, but also affects the beneficiation cost. Under the conditions of roasting temperature 1000 ℃, rotary kiln frequency converter f=40Hz (roasting time 45min), coke dosage 8%, coke particle size -1mm, pellet diameter -30+10mm, weak magnetic separation magnetic induction intensity B1=0.30T, B2=0.12T, primary grinding fineness -0.100mm accounting for 95%, and secondary grinding fineness -0.045mm accounting for more than 80%, the pellet diameter size test is conducted. The experimental process flow is shown in Figure 1, and the test results are shown in Figure 7.
Fe grade; ▲ – Fe recovery rate; ◆ – P grade（ × 10-2); ● – P recovery rate
From Figure 7, it can be seen that as the amount of LCP increases, the phosphorus content in the iron concentrate gradually decreases to 0.109%, but the iron grade and iron recovery rate show a trend of first increasing and then decreasing. When the LCP dosage is 15%, the iron grade is 63.65%, the phosphorus content is 0.109%, and the iron recovery rate is 71.68%. Therefore, considering factors such as iron concentrate grade, iron recovery rate, and phosphorus content, selecting a 10% LCP dosage can obtain beneficiation indicators such as iron grade of 65.71%, phosphorus content of 0.223%, and iron recovery rate of 78.91%.
（2） Continuous roasting full process test
The roasting conditions of the magnetization roasting weak magnetic separation (stage grinding stage separation) process were obtained through experiments on the main process parameters of rotary kiln roasting: roasting temperature 1000 ℃, f=40 Hz (roasting time 45 minutes), coke dosage 8%, coke particle size -1mm, pellet diameter -30+10mm, LCP dosage 10%, weak magnetic separation magnetic induction intensity Bl=0.30T, B2=0.12T, primary grinding fineness -0.100mm accounting for 95%, The second stage grinding fineness -0.045 mm accounts for over 80%. To investigate the reliability and stability of the obtained process parameters, a continuous 72 hour full process test was conducted under the obtained roasting conditions to test the process
It can be seen that a beneficiation index with a yield of 50.41%, an iron grade of 65.93%, phosphorus content of 0.225%, and an iron recovery rate of 78.91% can be obtained. Compared with the roasting condition test, this index has a small difference, so the obtained process parameters are more reliable, repeatable, and the product index is stable; In addition, there was no “ring formation” phenomenon during the continuous 72 hour rotary kiln roasting process, and the entire continuous process equipment operated normally.
4、 Conclusion
（1） By φ 800 mm × Industrial experimental research on magnetization roasting in a 9000mm rotary kiln has obtained beneficiation indicators with iron grade greater than 65%, phosphorus content less than 0.25%, and iron recovery rate higher than 78%.
（2） The use of self-developed LCP composite phosphorus reducing agent effectively reduced the phosphorus content in iron concentrate, resulting in high-quality iron concentrate products. LCP has the characteristics of low melting point, cheap price, convenient source, and low pollution. Adding a certain amount during the roasting process of high phosphorus iron ore can effectively reduce the phosphorus content in the iron concentrate. In addition, a large number of experimental studies have been conducted on other types of high phosphorus iron ore using LCP, and good phosphorus reduction effects have also been achieved.
（3） Magnetization roasting (added

（3） The successful process of magnetization roasting (adding LCP to reduce phosphorus) – weak magnetic separation (stage grinding stage separation) provides a new idea for the development and utilization of difficult to select high phosphorus iron ore.
（4） In the situation of scarce iron ore resources that are easy to select, high in iron content, low in impurities, and simple in process, the development and utilization of iron ore resources that are difficult to select and high in impurities is an inevitable trend. Therefore, developing new technologies and processes to treat this valuable iron ore resource will have significant practical significance.