UpgradIng Iron and reMovIng phoSphorUS of hIgh phoSphorUS oolItIc Iron ore By SegregatIon roaStIng wIth calcIUM chlorIde and calcIUM hypochlorIte

The iron-bearing ore, existing in the form of oolite, was mainly composed of hematite, limonite, daphnite, and collophane. The harmful element phosphorus content was 1.56%, belonging to high phosphorus ooliticiron ore in western Hubei. In this study, segregation roasting and low intensity magnetic separation techniques were applied for upgrading iron and removing phosphorus. The ores, the chlorinating agent, and the reducing agent were mixed into the roasting furnace for segregation roasting. After being transferred from the weak magnetic minerals to the strong ones, the iron was recovered by low intensity magnetic separation. During segregation roasting, new ore phases, metallic iron (Fe), a small amount of ferroferric oxide (Fe 3 O 4 ), and ferrous oxide (FeO) could be observed. The results showed that the iron concentrate with the Fe content of 90.3%, the phosphorus content of 0.15%, and the iron recovery of 92.9% were obtained under the segregation roasting temperature of 1273 K, and the roasting time of 90 min, CaCl 2 (calcium chloride) 20%, Ca (ClO) 2 (calcium hypochlorite) 3%, the dosage of coke 20%, and low intensity magnetic separation field intensity 0.12 T.


Introduction
The available reserves of high phosphorus oolitic iron ore [1] located in western Hubei are above 2 billion tons. The ore is considered as a chemical sedimentary iron ore, mainly composed of oolite and detritus. Specifically, oolite consists of oolitecore (quartz or iron material), collophane girdle, hematite, and limonite girdles. Detritus is composed of finegrained hematite and quartz feldspar. Generally, the iron concentrate consists of iron-rich oolite and some amount of hematite, with an iron grade of up to 55%. The phosphorus in the ore is mainly in the form of collophane, where the grain sizes of collophane girdle and monomer quartz are relatively small. The sizes of hematite and limonite in chilimonite are small and difficult to separate. Meanwhile, the low grade of aggregate iron makes it difficult for upgrading iron and removing phosphorus [2].
Conventionally, the direct utilization of oolitic iron ore without any pre-treatment [3][4][5] is not suitable, since its iron minerals, e.g., Fe 2 O 3 , have weak magnetic characteristic which cannot distinguish them from other gangue minerals. Meanwhile, due to its low Fe content, poor liberation, and complicated composition, satisfactory processing method with acceptable cost that can effectively utilize the ore is still lacking. However, it is feasible and sometimes necessary to modify the properties before direct beneficiation using various roasting methods [6][7][8][9], e.g., microwave roasting [10], salt roasting [11,12], sulfur-fixed roasting [13] and sintering roasting modelling [14], and sulphating roasting [15][16][17]. Segregation roasting, a technology that applies to the theory of chlorination metallurgy, shows excellent performance in extracting valuable compositions and removing harmful ones from complex refractory iron ore. The methods applied to processing high phosphorus oolitic iron ore are mainly magnetic roasting [18], direct reduction [19], deep reduction, and suspension roasting, along with others [20,21].
However, emphasize by segregation roasting on high phosphorus oolitic iron ore was rarely investigated. Hereby in this study, calcium chloride as a chlorinating reagent, calcium hypochlorite as an

J. Min. Metall. Sect. B-Metall. 55 (3) B (2019) 305 -314
auxiliary, and coke as a reducing agent were applied to treat the sedimentary high phosphorus oolitic iron ore in western Hubei by segregation roasting. This work was aimed at gaining a better understanding of the detailed characteristics of the iron phase change and microstructures under various conditions, e.g., segregation roasting temperature and coke dosage. The final chemical composition of iron concentrate with magnetic separation was also examined. The present study aims at providing a basis on upgrading iron and removing phosphorus for the commercial utilization of such ooliticiron ores.

Materials and reagents
The sedimentary high phosphorus oolitic iron ore in western Hubei was sampled and processed to the size of -1 mm. The hematite and limonite in the samples exist in the form of oolite, concentric and ellipsoidal structures, taking up 64.26% of total ores. The iron-bearing minerals in the oolite were mainly hematite, limonite, daphnite and collophane. Scanning Electron Microscopy (SEM) and Energy Dispersive Spectrometer (EDS) images of oolite are shown in Fig. 1. SEM and EDS images of collophane are shown in Fig. 2. The main chemical composition of high phosphorous oolitic iron ores is shown in Table 1, and the iron phase analysis results of sample  are shown in Table 2.
The segregation reagents are presented in Table 3. All reagents are of analytical grade. The fineness of coke was -0.5 mm. The quality analysis of coke is listed in Table 4.

Material characterization
SDTQ600 differential thermal analyzer (TG-DTA) was used to characterize the thermal decomposition behaviors of the ore.
In this experimental study, advanced equipment, including X-ray photoelectron spectroscopy (XPS)-EDS, SEM-EDS, and electron probe EDS analyzer made by EDAX Inc, USA, were used to analyze the mineral morphology and mineral compositions.

Segregation roasting and low intensity magnetic separation
The ores were crushed to a certain particle size, then the chlorinating agent and reducing agent were mixed and put into the roasting furnace. These mixtures were heated to a certain temperature under the neutral or weak reducing atmosphere, where the chlorinating agent was used to form volatile metal chlorides. The products after roasting were ground to  a suitable fineness, then separated to produce magnetic products and non-magnetic ones through a XCGS-13 magnetic tube under certain magnetic field intensity. Then, the two products were filtered, dried, weighed, sampled, and tested for further evaluation based on the iron grade, phosphorus content and iron recovery in magnetic products.

Thermal analysis
TG-DTA of high phosphorous oolitic iron ores were conducted from 35 to 900°C in air at a heating rate of 10°C/min, and the results are presented in Fig.  3. It can be seen that the total heat absorption was 313.8 J/g with the temperature increase from ambient temperature 35°C to 900.0°C. The absorption process was mainly divided into three stages, of which the second and the third stage are more pronounced.
In the first stage, the temperature increased from 169.5 to 307.3°C, and the endothermic peak was 265.9°C, the heat absorption occupies 6.6% of the total heat absorption. It could be observed that a small amount of heat absorption occurred at this stage, and the process was relatively mild, which might be due to the moisture evaporation in the ore. For the second stage from 307.3°C to 588.8°C, the endothermic peak was 469.7°C, and the heat absorption was about 60% of the total heat absorption.
In the third stage, the temperature increased from 588.8°C to 721.9°C. The endothermic peak at this phase was 687.0°C, which was one of the important endothermic peaks. It was speculated that the hematite and limonite were converted into hematite and magnetite by absorbing heat [22]. At the temperature above 721.9°C, the reaction almost stopped and the amount of heat absorbed by the minerals turned to be small.

Effect of segregation roasting temperature
At the experimental condition of segregation roasting time of 90 min, the 15% dosage of calcium chloride the 5% dosage of calcium hypochlorite, the J. Xiao    15% dosage of coke , the SEM analysis, and the iron phase analysis results of different segregation roasting temperatures are shown in Fig. 4 and Fig. 5, respectively. It was found from Fig. 4 and Fig. 5 that the segregation roasting temperature is one of the key factors that determine whether the chlorinating agent can be decomposed and whether the Fe can be chlorinated to form volatile metal chloride. With the increase of temperature, the content of magnetite decreases dramatically. The content of magnetite increases slightly after the temperature exceeds 1270 K. The change trend of metallic iron is negatively correlated with the change trend of magnetite. An increase in the segregation roasting temperature has a significant effect on the generation of Fe and magnetite within the segregation roasting process [23,24]. When the segregation roasting temperature exceeded 1273 K, the proportion of Fe has a small range of fluctuations [25]. At 1173 K, the SEM spectra showed a dispersed granular shape which became more concentrated when the temperature rose to 1223 K. At 1273 K, the dispersed granules disappeared and the maps showed large aggregates. Fig. 5 can clearly show that this is the time when the metal iron content is the largest, and the bulk morphology is mainly the embodiment of metal iron at this time. Meanwhile, the SEM images showed that the content of Fe was also in well degree at segregation roasting temperature of 1273 K. It can be concluded that the segregation roasting temperature of 1273 K can be a suitable condition, under which Fe 32.67% and magnetite 5.68% could be obtained.

Effect of coke dosage
SEM images of products using various coke dosages are shown in Fig. 6, and the iron phase analysis results are shown in Fig. 7. These results were obtained at conditions of the segregation roasting temperature of 1273 K, the roasting time of 90 min, the 15% dosage of calcium chloride, and the 5% dosage of calcium hypochlorite.  During the segregation roasting process (Fig. 6 and Fig. 7), the coke reacted with water vapor to form H 2 and CO, and volatile FeCl 3 was reduced to Fe particles by H 2 over the coke surface, as shown in Eq.
(1) and Eq. (2). (1) As the coke dosage increased, the Fe content in the segregation roasting ore gradually increased. Under the condition that the coke dosage reached 20% or above, the increasing of Fe stopped. For Fe 3 O 4 (Fig. 7, red line), when the coke dosage was below 20%, the smaller coke dosage could lead to the increased amount of magnetite in the roasting ore. Taking costs into account, the excessive or insufficient coke dosage was not beneficial, and the 20% coke dosage could be a choice.

Effect of calcium chloride
At the condition of the segregation roasting temperature of 1273 K, the roasting time of 90 min, 5% calcium hypochlorite and 20% coke dosage, the SEM images and the iron phase analyses of calcium chloride are shown in Fig. 8 and Fig. 9.   Firstly, Fe content increased significantly then decreased slightly (Fig. 8 and Fig. 9), with the increasing calcium chloride dosage in the segregation roasting process. Along with the gradually decreased generation amount of magnetite, which indicates that increasing the amount of calcium chloride in a certain range will promote the formation of iron, but after exceeding a certain limit, the amount of calcium chloride is increased, and the formation of iron is not changed much and tend to be stable. During the segregation roasting process, the calcium chloride reacted with the acidic oxides in the ore (e.g., quartz and aluminosilicate ) and H 2 O, forming hydrogen chloride, as shown in Eq. 3 and Eq.4 [26].
If calcium chloride reached an excessive content, the amount of hydrogen chloride generated in the system would increase. FeCl 3 could be generated by chlorination reaction between the Fe and hydrogen chloride. FeCl 3 was easily reduced by H 2 to metal particles and then adsorbed on the surface of the coke, due to its instability [27,28]. However, if calcium chloride is not sufficient, the amount of hydrogen chloride generated would be reduced, which is not conducive to the formation of Fe in the segregation roasting process. It showed that the dosage of calcium chloride 20% could be a good choice.

Effect of calcium hypochlorite
SEM images and iron phase analysis results after segregation roasting with various calcium hypochlorite dosages are shown in Fig. 10 and Fig.  11, at the conditions of the segregation roasting temperature of 1273 K, the roasting time of 90 min, calcium chloride 20%, and coke 20%.
With the increasing of calcium hypochlorite dosage, a gradual increase of Fe content and a reduction of magnetite in the segregation roasting process (Fig. 10, Fig. 11) can be found. A chemical reaction between calcium hypochlorite (an auxiliary) and hydrogen chloride in the system occurred, forming calcium chloride and chlorine, as shown in Eq. 5 [29].
With the increased dosage of calcium hypochlorite, the chlorine generated by the reaction accelerated the chlorination reaction of Fe to a certain extent, contributing to the formation of FeCl 2 . In addition, it can be found that excessive calcium hypochlorite might lead to excessive consumption of hydrogen chloride. On the other side, hydrogen chloride involved in the chlorination reaction during the segregation roasting process [30] is the most important chemical reaction.

Effect of segregation roasting time
At the condition of segregation roasting temperature of 1273 K, calcium chloride 20%, calcium hypochlorite 3%, and coke of 20%, the SEM analysis and the iron phase analysis results of different segregation roasting time are shown in Fig.  12 and Fig. 13, respectively.
The extending segregation roasting time was effective for the formation of Fe (Fig. 12, Fig. 13), but when the roasting time was increased to 120 min, the increasing of Fe stopped and decreased slightly. The segregation roasting process had multiple chemical reactions occurring over time, e.g., the decomposition, the chlorination, and the reduction reactions; it also involved a complex phase change. A shorter roasting time will result in an insufficient and non-effective chemical reaction progress [30].

Low intensity magnetic separation
At condition of the roasting temperature of 1273 K, the roasting time of 90 min, calcium chloride 20%, calcium hypochlorite 3%, coke 20%, 36.18% Fe, and 1.89% magnetite were in the roasted ore (the total was 38.07%), which accounts for 91.29% of all the iron. It is known that the factors affecting low intensity magnetic separation for the roasted ore are mainly grinding fineness and magnetic field intensity. Under the above conditions involved in the segregation roasting process, with the low intensity magnetic separation field intensity of 0.10T, the effect of grinding fineness of low intensity magnetic separation is presented in Table 5.
From the data in Table 5, with the increase of the grinding fineness, a regular increase of the iron grade and the reduction of the phosphorus content in the iron concentrate can be observed. Due to the increased grinding fineness, the difficulty in the liberation of mineral monomer increased. However, excessively higher grinding fineness may lead to the reduction of iron recovery in the iron concentrate, along with the significantly increased grinding costs. The effect of field intensity is shown in Fig.14, with the grinding fineness less than 0.019 mm accounting for 98%.
From Fig. 14, with the increased magnetic field intensity, the regular decrease of the iron grade and the increase of both the phosphorus content and the iron recovery can be observed. With the decreased magnetic field intensity, the materials will undergo a combined action by mechanical and magnetic forces during the separation process. Under the condition of the magnetic field intensity 0.12T, the iron grade of the iron concentrate decreased to 86.54%, the phosphorus content increased to 0.27%, while the iron recovery was 96.0%, which indicated that the increasing magnetic field intensity had a positive influence on the increase of overall iron recovery. But it was not beneficial to improve the iron grade of the concentrate.

Chemical composition of iron concentrate
The effects of CaCl 2 and Ca(ClO) 2 on upgrading iron and removing phosphorus of high phosphorus oolitic iron ore in western Hubei by segregation roasting were tested. The iron concentrate with Fe content of 90.3%, the phosphorus content of 0.15%, and the iron recovery of 93.0% were obtained under the segregation roasting temperature of 1273 K, the segregation roasting time of 90 min, the dosage of calcium chloride of 20%, the dosage of calcium hypochlorite of 3%, the dosage of coke of 20%, and the low intensity magnetic separation field intensity of 0.12 T. To further investigate the final quality of the iron concentrate, chemical composition analysis, phase analysis, SEM, and electronic probe composition analysis were performed. The results are shown in Table 6 to Table 8, and Fig. 15.
It could be seen ( Table 6, Table 7) that the new ore phases were dominated by Fe (86.91%), a small amount of FeO and Fe 3 O 4 . The iron phase changed compared with the iron phase of the crude ore, which changed from limonite (Fe 2 O 3 , Fe 2 O 3 ·nH 2 O) to a new iron phase dominated by metallic iron. Based on the SEM images and electronic probe composition analysis in Fig. 15 and Table 8, remarkable changes on the iron phases of high phosphorus oolitic iron ore after segregation roasting process can be observed.

conclusions
High phosphorus oolitic iron from western Hubei, China, was investigated using segregation roasting and low intensity magnetic separation method to upgrade iron and remove phosphorus. The ironbearing ores in in the oolite are mainly hematite, limonite, chlorite and collophanite. The detailed behaviors of the iron phase change and microstructures during the segregation roasting were examined at various conditions, including the effect of the roasting temperature, the effects of various roasting reagents, e.g., coke, calcium chloride, and calcium hypochlorite. The low intensity magnetic separation after the segregation roasting showed that the iron content of 90.3%, the phosphorus content of 0.15%, and the iron recovery of 93.0% were obtained for the iron concentrate under the segregation roasting temperature of 1273 K, the segregation roasting time of 90 min, the dosage of calcium chloride of 20%, the dosage of calcium hypochlorite of 3%, the dosage of coke of 20%, the low intensity magnetic separation field intensity of 0.12 T and 98% grinding fineness of less than 0.019 mm. The chemical composition analysis, the iron phase analysis, SEM, the electron probe analysis and EDS of the iron concentrate, showed that new ore phases were dominated by Fe