Drying, Roasting, and Calcining of Minerals Edited by: Thomas P. Battle, Jerome P. Downey, Lawrence D. May, Boyd Davis, Neale R. Neelameggham, Sergio Sanchez-Segado, and P. Chris Pistorius TMS (The Minerals, Metals & Materials Society), 2015
EFFECT OF ADDITIVES ON PHASE TRANSFORMATION OF NICKEL LATERITE ORE DURING LOW-TEMPERATURE REDUCTION ROASTING PROCESS USING CARBON MONOXIDE Shiwei Zhou 1, Bo Li *11, Yonggang Wei 1, Hua Wang 1, Chengyan Wang 2, Baozhong Ma 2 1 State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China 2 Beijing General Research Institute of Mining and Metallurgy, Beijing 100160, China Keywords: Nickel laterite ore; Additives; Phase transformation; Gas reductant
Abstract The effects of additives with Na2S, Na2SO4 and CaSO4 on phase transformation of nickel laterite ore in low-temperature reduction roasting process using carbon monoxide were investigated. The results showed that diffraction peaks of forsterite increased with increasing reduction temperature for 30min with the addition of 10wt% Na2SO4 and 50vol% CO, and diffraction peaks of forsterite increased with increasing the content of Na2SO4 at 900°C for 30min. Comparing the above three additives, phase transformation of nickel laterite ore were strongly influenced with the addition of 10 wt% Na2SO4 and 10 wt% Na2S. Calcium sulphide from the thermal decomposition of CaSO4 was carried out in the reaction product. SEM-EDS analysis result deduced that increasing the dosage of Na2SO4 from 10wt% to 15wt% could promote the polymerization of ferronickel and improve the grade of nickel in ferronickel. Introduction In recent years, there has been an increased focus on the utilisation of low-grade nickel laterite ore, along with a growing demand for stainless steel and a declining supply of sulphide ores [1]. Among the world’s nickel resources, nickel laterite ore comprises 73% and will be the dominant source of nickel in the future. With the continuous depletion of high-grade nickel ores such as millerite and niccolite, nickel laterite ores have become the major source for the production of nickel metal. However, currently, only 42% of the world’s production of nickel comes from nickel laterite ore because the concentration of nickel in the ore is low [2]. Therefore, in the long term, it will be necessary to optimise valuable metals extraction from nickel laterite ore. Corresponding author: Bo Li; Tel:+86-15987127468; E-mail:
[email protected]
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Several investigations of the additives effects and phase transformation on nickel laterite ore have been performed. For example, Jie Luet al. studied the effect of sodium sulphate on the hydrogen reduction process of nickel laterite ore [3]. The presence of sulfur has been found to markedly improve the Ni and Co recoveries from laterite ores by Valix, M. et al. (2002) [4]. Man Jiang et al completed a simulation study on mechanism of sodium sulfate in promoting selective reduction of nickel laterite ore during reduction roasting process [5]. The mineralogical composition and phase transformation of Greek nickeliferous laterites was investigated by Emmanuel N. Zevgoliset al (2010) during preheating and reduction with carbon monoxide [6]. Yongfeng Chang et al investigated the phase transformation in reduction process with microwave heating by XRD and the reduction degree of iron by chemical method [7]. In this study, the author conducted carbon monoxide reduction of low-grade nickel laterite ore with the addition of Na2S, Na2SO4 and CaSO4 to determine the effects of temperature and additives content on phase transformation. The phase transformation accompanying reduction and the catalytic mechanism of additives were analysed using instrumental analysis techniques such as XRD, SEM–EDS and thermodynamic calculations. Materials and methods Raw material The chemical composition of low-grade nickel laterite ore was analysed by chemical analysis, as shown in Table I. Table I. Chemical analysis of nickel laterite ore (mass fraction, %). TFe
Ni
Co
Al2O3
MgO
CaO
SiO2
Cr2O3
9.12
1.09
0.023
2.47
29.08
0.03
36.48
0.34
The additives of sodium sulphide (Na2S), sodium sulphate (Na2SO4) and calcium sulphate (CaSO4) used in this study were chemical grade. Reduction was achieved by mixing reducing gases, which included CO and CO2. Experimental methods Nickel laterite ores were ground to 95wt% passing 100 mesh-sizes using a laboratory scale ball mill, and then mixed with different additives, respectively. The sample was then granulated by tablet machine according to the experimental requirements. The experiments were carried out in a vertical tube furnace. The reducing gas, which total rate of 50 ml/min (CO: 25 ml/min, CO2: 25ml/min), were metered into the tube while it was heated to the required temperature. After 30 minutes of reduction at temperature, the reducing gas metered to the reactor was replaced with nitrogen.
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XRD and SEM-EDS analysis The XRD experiments were performed on a Japan Science D/max-R diffractometer apparatus with Cu Kα radiation (k = 1.5406 Å), an operating voltage of 40 kV and a current of 40 mA. The diffraction angle (2θ) was scanned from 5° to 90°. The morphology and particle size were determined by Scanning Electron Microscopy (Philip XL30) equipped with an Energy Dispersive X-ray Spectroscopy. Results and discussion Effect of reduction temperature The laterite ores with the additive of 10 wt% Na2SO4 were reduced at temperatures from 600 to 1000°C. The XRD patterns of the roasted ore in different conditions are shown in Figure 1. The peaks intensity of forsterite increased with increased reduction temperature from 600 to 900°C, but it had no obvious increase when the reduction temperature was above 900°C. The theoretical decomposition temperature of lizardite was 585°C [3]. But, when the reduction temperature was 600°C the diffraction peaks of forsterite was so weak, it was deduced that the lizardite was not decomposed and the actual decomposition temperature was higher than theoretical decomposition temperature above 600°C. In addition, the quartz can be observed in XRD pattern under 800°C, and the reasons may be attributed to the presence of quartz in raw laterite ores and the thermal decomposition of lizardite [8]. However, it disappeared above 800°C. According to the thermodynamic calculation, quartz could react with Na2SO4 at 684°C in the CO atmosphere and the equation was an endothermic reaction (enthalpy ΔH > 0). Therefore, increasing the reduction temperature could accelerate the reaction and resulted in the amount of quartz decreased gradually. As revealed from the XRD, the peak intensity of quartz decreased gradually, and finally disappeared above 800°C. Thus, the reaction equation could be represented by [9] Na2SO4 + SiO2 + CO(g) - Na2SiO3 + SO2(g) + CO2(g)
GT =137582.64-143.63T
(1)
Effect of additive dosage The laterite ores with different dosage of Na2SO4 from 0 to 15wt% were reduced at the reduction temperature of 900°C. The XRD pattern of the roasted ore in different conditions is shown in Figure 2. With the increase of Na2SO4 dosages from 0 to 10wt%, the peak intensity of forsterite increased. However, when the dosage of Na2SO4 was increased to 15wt%, the diffraction peaks of forsterite had no obvious change.
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-Mg2SiO4
-{Fe,Ni}
1000℃
Counts (A.U.)
-SiO2
SiO2
15wt% 10wt%
Counts (A.U.)
900℃
Mg2SiO4
800℃
700℃
5wt%
2wt%
600℃ 10
20
30
40
50
60
70
80
0wt% 10
90
20
30
40
50
60
70
80
90
Diffraction Angle (2)
Diffraction Angle (2)
Figure 1. XRD patterns of roasted ore at Figure 2. XRD patterns of roasted ore varying temperatures and 10wt% Na2SO4. at 900°C and varying dosages of Na2SO4. Effect of different additives The laterite ores with different additives of 10wt% were reduced at the reduction temperature of 900°C and the XRD pattern of the roasted ore is shown in Figure 3. As revealed from the XRD, the peaks intensity of forsterite varies with the kinds of additive. The diffraction peaks of forsterite with additive of Na2S had an increase, in contrast to the XRD pattern of the roasted ore with the additives of Na2SO4 and CaSO4. The result illustrated that Na2S had more effect on decomposition of lizardite. Moreover, the XRD pattern of roasted ore with the additive of CaSO4 included calcium sulphide, and the reason may be attributed to the thermal decomposition of CaSO4. The reaction equation is shown below [9]:
GT = -10435.98+0.17T
1/4CaSO4 + CO(g) - 1/4CaS + CO2(g)
(2)
-Mg2SiO4 -SiO2
Counts (A.U.)
-CaS
10wt% Na2S
10wt% Na2SO4
10wt% CaSO4
0wt% 10
20
30
40
50
60
70
80
90
Diffraction Angle (2)
Figure 3. XRD patterns of roasted ore with different additions at 900°C.
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SEM-EDS analysis The microstructure of the roasted ores with different additives and dosages was analyzed by SEM and EDS, as shown in Figure 4. These figures from (a) to (d) were the microstructures of the roasted ore and other figures from (a’) to (d’) were the magnified view of the marked areas in the figures from (a) to (d), respectively.
Figure 4. SEM images of the roasted ores with different additives at 900°C: (a): 10wt% of Na2SO4; (b): 15wt% of Na2SO4; (c) and (d): 10wt% of Na2S. A-FeS, B-forterite, C-quartz, D-wustite. Figure 4 (a) and (b) showed the microstructure of the roasted ores with 10wt% and 15wt% Na2SO4, respectively. As revealed from the figures, forsterite and ferronickel
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polymer were observed. The compositions of ferronickel polymer were determined by EDS, and the contents of Ni were 1.58wt% and 6.28wt%, as shown in Figure 5 and 6 respectively. This result illustrated that increasing the dosage of Na2SO4 could promote the polymerization of metal nickel, and the reason was that the thermal decomposition of Na2SO4 generated sulfur, which could reduce the surface tension and make the metal particle size grow [10]. Figure 6 shows that ferronickel polymer interlinked with the forsterite, and the reason was the most of Ni and Fe existed in silicates in the form of isomorphism. Figure 4 (c) and (d) show the microstructure of the roasted ores with 10wt% Na2S. Wustite, quartz and forsterite were observed in these figures. Removal of crystal water for goethite formed ferric oxide during the drying and roasting process [11-14] and the ferric oxide was reduced by gaseous reductant (CO/CO2), and then formed ferroferric oxide. Nevertheless, further reduction of ferroferric oxide in the reducing atmosphere would form wustite. But it was hard to form metallic iron [8]. The aforementioned equations are as follow [8, 9]: 3Fe2O3 + CO(g) - 2Fe3O4 + CO2
GT =-52131-41.0T
(3)
Fe3O4 + CO(g) - 3FeO + CO2(g)
GT =35380-40.16T
(4)
FeO + CO(g) - Fe + CO2(g)
GT =-22800+24.26T
(5)
According to the SEM-EDS analysis of the roasted ore, it was noted that wustite and ferronickel polymer were detected by SEM-EDS, but were not identified by XRD analysis, as shown in figures. This was attributed to the fact that those phases were present in a low content.
Element Wt% At%
FeK 64.85 51.96
SK 33.57 46.84
NiK 1.58 1.20
Figure 5. EDS result of roasted ore grain with 10wt% Na2SO4 at 900°C.
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Element FeK
SK
OK
NaK
MgK
AlK
SiK
NiK
Wt%
29.69
18.65
15.03
4.02
12.76
0.79
12.78
6.28
At%
15.90
17.40
28.09
5.22
15.69
0.88
13.61
3.20
Figure 6. EDS result of roasted ore grain with 15wt% Na2SO4 at 900°C.
Conclusions This article was focused on the phase transformation of nickel laterite ore, which added different additives, during low-temperature reduction roasting process with a gaseous reducing mixture (50vol.% CO and 50vol.% CO2) for 30min. The results of XRD analysis in this study demonstrate that use of selected additives promoted phase transformation in nickel laterite ore. Diffraction peaks of forsterite increased with increasing reduction temperature with the addition of 10wt% Na2SO4, and it was also increased with higher amounts of Na2SO4 at 900°C. Comparing the three additives (Na2SO4, Na2S and CaSO4), Na2S had more promoting effect on the phase transformation of nickel laterite ore. Meanwhile, the CaSO4 would decompose to form CaS in the reduction atmosphere. As it was verified by SEM-EDS, increasing the dosage of Na2SO4 could promote the polymerization of ferronickel and improve the grade of nickel in ferronickel.
Acknowledgments
Financial support for this study was supplied from the National Natural Science Foundation of China (Project Nos. U1302274 and 51304091), the Applied Basic Research Program of Yunnan Province (No. 2013FZ007 and 2013FD009), the Candidate Talents Training Fund of Yunnan Province (2012HB009) and the Analysis and Testing Foundation of Kunming University of Science and Technology.
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References 1. Y.C. Zhai et al., A green process for recovering nickel from nickeliferous laterite ores. Transactions of the Nonferrous Metals Society of China. 20(2010), 65–70. 2. K. Jungah et al., Calcination of low-grade laterite for concentration of Ni by magnetic separation. Minerals Engineering, 23(2010), 282–288. 3. J. Lu et al., The effect of sodium sulphate on the hydrogen reduction process of nickel laterite ore, Minerals Engineering. 49(2013), 154-164 4. M. Valix , W.H. Cheung. Effect of sulfur on the mineral phases of laterite ores at high temperature reduction. Minerals Engineering. 15(2002), 523-530 5. J. Man et al., Mechanism of sodium sulfate in promoting selective reduction of nickel laterite ore during reduction roasting process. International Journal of Mineral Processing. 123(2013), 32-38 6. Emmanuel N. Zevgolis et al., Phase transformations of nickeliferous laterites during preheating and reduction with carbon monoxide. J Therm Anal Calorim. 100(2010), 133-139 7. Y.F. Chang et al., Phase transformation in reductive roasting of laterite ore with microwave heating. Transactions of Nonferrous Metals Society of China. 18(2008), 969-973 8. Li Bo, Wang Hua, Wei Yonggang, The reduction of nickel from low-grade nickel laterite ore using a solid-state deoxidization method. Minerals Engineering. 24(2011), 1556–1562. 9. Shi Tangming, Research on the Solid·State Reduction Roasting of Nickeliferous Laterite Ore Intensified by Sulfur-bearing Additives. 10. Nagata, K., Bolsaitis, P., 1986. Selective removal of iron oxide from laterite by sulfurization and chlorination. International Journal of Mineral Processing. 19(1987), 157–172. 11. O’Connor F, Cheung WH, Valix M. Reduction roasting of limonite ores: effect of dehydroxylation. International Journal of Mineral Processing. 80(2006), 88–99. 12. Swamy YV, Kar BB, Mohanty JK. Physico-chemical characterization and sulphatization roasting of low-grade nickeliferous laterites. Hydrometallurgy. 69(2003), 89–98. 13. Mackenzie R. Differential thermal analysis, London: Academic Press; 1973. 14. Walter D, Buxbaum G, Laqua WJ. The mechanism of the thermal transformation from goethithe to hematite. J Therm Anal Calorim. 63(2001), 733–48.
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