ISIJ International, Vol. 52 (2012), No. 10, pp. 1757–1763 Beneficiation of High Phosphorus Limonite Ore by Sodium- carbonate-added Carbothermic Reduction Shaojun BAI,1) Shuming WEN,1,2)* Dianwen LIU,1) Wenbin ZHANG1) and Qinbo CAO1) 1) Faculty of Land Resources Engineering, Kunming University of Science and Technology, Kunming, Yunnan, 650093 PR China. E-mail: [email protected], [email protected], [email protected], [email protected] 2) Engineering Research Center and Ministry of Education for Efficient Utilization of Mineral Resources in Western of China, Kunming 650093, PR China. (Received on March 1, 2012; accepted on June 6, 2012) The characteristics of Huimin high phosphorus limonite ore and the beneficiation of this iron ore by sodium-carbonate-added carbothermic reduction, ultrafine grinding and magnetic separation were investi- gated. Iron particle size in reduced ore without Na2CO3 additive is tiny and the fayalite is abundant. It is indicated that the formation of fayalite is the main hindrance to accelerate the reduction of limonite. With a mass ratio of Na2O3 to ore of 10% additive, the reduction of limonite can be reinforced. The reinforcing affect may be caused by the increase of the reducing reaction activity of FeO and the acceleration of the carbon gasification reaction rate. Fluorapatite were not reduced in the low temperature reduction process and entered to gangue phases, after ultrafine grinding-magnetic separation process, a qualified iron con- centrate with 76.47% Fe, a recovery of 73.20% is obtained with simultaneous decrease in the phosphorus content down to 0.25%. KEY WORDS: iron ore; beneficiation; high phosphorus limonite ore; sodium carbonate; carbothermic reduc- tion; metallic iron; ultrafine grinding. tions on the magnetic separation index, including reduction 1. Introduction temperature, reduction duration, C/Ore mass ratio and With the continuous and rapid development of iron and Na2CO3 dosage were studied. It is therefore believed steel industry in China, China has become one of the world’s emphasis is paid to the effect of Na2CO3 on the reduction of largest importers of iron ore and is facing the risks of raw limonite and the proposed in this paper will be of benefit to material shortage for the iron and steel industry.1) To elimi- the exploitation of similar high phosphorus limonite ore. nate such risks, China has to focus on the utilization of its domestic iron ore resources though most of them are either 2. Experimental refractory or of low grade.2–4) A large deposit of about 20 billion ton of high phosphorus iron ore is in the Huimin 2.1. Materials region of Yunnan Province,5) and the limonite ore with an The limonite ores with high content of phosphorus used average of 0.80% phosphorus and 34% iron contributes to in the present work were collected from Huimin iron ore about 20% of the Huimin iron reverse. The lack of under- deposits, in Yunnan Province of China. Table 1 shows the standing on the existence of phosphorus and iron oxides chemical composition of the high phosphorus limonite ore. coupled with the erratic distribution within some of the ore The pulverized coal used as the reductant with a particle size bodies in the Huimin iron ore has limited the development <1 mm was supplied by Kunming Iron And Steel Ltd, of appropriate beneficiation techniques. As a result, the Yunnan, China. The industrial and fusibility analyses results cheap and abundant high phosphorus limonite ore reserves are listed in Table 2. The reactivity of pulverized coal result in the Huimin iron have remained largely unexploited. is listed in Table 3. The solid Na2CO3 used as the alkaline Therefore, the development of an economically viable tech- additive in the experiments was of reagent grade (Tianjin nology for this cheap and abundant high phosphorus iron Chemical Plant). ore as a resource in iron making is of profound importance. The main objective of this research is to investigate the 2.2. Experimental Procedure carbothermic reduction behavior of high phosphorus limo- 2.2.1. Carbothermic Reduction Process nite ore with sodium carbonate. The phase transformation of Carbothermic reduction experiments were performed in a phosphorus compounds and the effects of reduction condi- muffle furnace (SRJX-8-13, Troody Shanghai Sales & Service Centre) with an average heating speed at 10°C/min. 50.0 g * Corresponding author: E-mail: [email protected] raw ore with a particle size <0.5 mm was mixed with pul- DOI: http://dx.doi.org/10.2355/isijinternational.52.1757 verized coal (or sodium carbonate) for each and every 1757 © 2012 ISIJ ISIJ International, Vol. 52 (2012), No. 10 Table 1. Chemical composition of the high phosphorus limonite ore. Composition TFe P S SiO2 Al2O3 CaO MgO Na2O Content (mass%) 34.47 0.85 0.26 29.93 5.68 1.84 1.98 0.15 Table 2. Industrial and ash fusibility analyses of pulverized coal. Industrial analysis/mass% Ash fusibility/°C S/mass% Mad Ad Vd FCd DT ST HT MT 2.45 9.45 6.45 82.56 1 250 1 325 1 344 1 346 0.45 Mad: Moisture (dry basis), Ad: Ash (dry basis), Vd: Volatile Matter (dry basis), FCd: Fixed Carbon (dry basis). DT: Distortion Temperature, ST: Softening Temperature, HT: Hemispherical Temperature, MT: Melting Temperature Table 3. Reactivity of pulverized coal. Fig. 1. XRD pattern of raw limonite ore. Temperature/°C 1 000 1 050 1 100 1 150 1 200 α/% 13.46 22.68 38.65 68.95 88.65 powder XRD, as shown in Fig. 1. The gangue mineral is The reactivity of pulverized coal gives the degree of chemical reaction ability between the pulverized coal and carbon dioxide at certain tempera- mainly quartz and the fluorapatite constitutes the phospho- tures, where α is the degree-of-reduction for carbon dioxide. rus compounds. Chemical analysis of raw ore shows that the iron grade of run-of-mine is 34.47%, the average content of experiment. The mixture was placed in the graphite cruci- phosphorus is 0.85% and the content of silicon dioxide goes ble of 600 mm in diameter and 800 mm in length. The pro- up to 29.93%. As we all known, phosphorus is one of the cess temperature ranged from 800 to 1 100°C, the reduction most detrimental impurities in steels and it reduces the low- time ranged from 60 to 180 min, the C/Ore mass ratio temperature toughness of iron and steel products,6) while sil- ranged from 5% to 25%; and the mass ratio of Na2O3 to ore icon dioxide is apt to react with ferrous oxide and metallic ranged from 5% to 20%. Upon the completion of experi- iron, resulting in a decrease of metal ratio. Thus, phospho- ments, the samples were taken out from the furnace and rus-removal and reinforcing the reduction of iron oxide are cooled down to ambient temperature with N2 protective very important for the beneficiation of this iron ore with atmosphere (1 L/min). high content of phosphorus. Optical microscopic examina- tion and Energy Dispersive X-ray Spectroscopy (EDS) 2.2.2. Grinding and Magnetic Separation Process analysis were employed to determine the occurrence and A wet conical ball mill (ϕ240×90 mm) was used for the distribution of iron and phosphorus minerals. The results are reduced ores grinding process, and the grinding fineness of shown in Fig. 2. reduced ores were controlled to about 90% below 74 μm by The visual inspection of image indicates that dark grey water-sieving. Magnetic separation process was conducted particles limonite cements and grey particles limonite clas- in magnetic separator (ϕ50 mm) with a magnetic field inten- tics are clump together and they are the principal phase, sity of 238 kA/m. At the end of the experiment, the concen- while the dark particles quartz are the second phase and trate was filtered and TFe and P-content of concentrate were presents itself as either liberated particles or attached to the analyzed. The iron grade of magnetic concentrate, P-content edges of the dark grey particles (Fig. 2(a)). Simultaneously, in the iron concentrate and iron recovery were considered as the results of X-ray energy spectrum analysis show that the the main basis for evaluation of test results. dark grey particles and grey particles are composed of limo- nite minerals. The grey particles are limonite clastics and 2.3. Analysis and Characterization contain a small amount of Al, Si (Figs. 2(a), 2(c)), while the The reduced ores were divided into two portions. One dark grey particles are limonite cements and contain a small portion was crushed and milled to below 75 μm for chemi- amount of Al, Si, P (Figs. 2(a), 2(b)). The dark particles are cal and X-ray diffraction (XRD) analyses, and the other por- quartz of 10–30 μm in size with high content of quartz, and tion of reduced samples fracture surfaces was used for there is almost no other mineral (Figs. 2(a), 2(d)). microtopographic analysis. The chemical analyses were Due to its low grade and its complex mineral dissemina- conducted by the Kunming Metallurgy Research Institute. tion particle sizes and its high content of phosphorus, it is The XRD patterns of the raw material and the reduced sam- very difficult to beneficiate this iron ore by traditional dress- ples were acquired with a Rigaku diffractmeter using Cu Ka ing, such as magnetic separation, gravity concentration, or radiation, and the scanning angle was varied from 10 to flotation. Therefore, a new process using a carbothermic 90°(2θ) at a speed of 1.2° min–1. The microstructure of the reduction was studied.