Novel Process of Ferronickel Nugget Production from Nickel Laterite by Semi-molten State Reduction

Abstract

Rotary kiln-electric furnace (RKEF) process is the main technology to deal with nickel laterite for the ferronickel alloy production in the world. However, this process needs huge amount of electric power due to the large ratio of slag to metal. Therefore, a novel process was proposed to directly produce ferronickel alloy nugget at a related low temperature from nickel laterite by the semi-molten reduction in the reactor like the rotary hearth furnace (RHF). The effects of temperature and basicity of the slag on the separation between the slag and metal were investigated, the results revealed that it is reasonable to achieve the ferronickel alloy nugget directly at 1400°C when the quaternary basicity ((m_CaO+m_MgO)/(m_SiO2+m_Al2O3)) was fixed at 0.60. Bad wettability of the refractory by the slag is good for the discharge of product from RHF and avoiding the corrosion of the slag. The high-temp wettability of refractory materials by the molten slag was also carried out with the sessile drop method, the results shows that the wettability order of the refractory by the slag from good to bad is Al₂O₃, MgO and graphite. It seems that the graphite is the suitable refractor material for bottom of RHF. However, the anti-oxidation of the graphite in the charging and discharging area is another potential problem which needs further study.

1. Introduction

As a commercially valuable metal, the nickel is largely used in production of stainless steel or high temperature alloys. In the last decade, the rapid increase of demand for stainless steel has led to a significant rise in ferronickel production. And with the continuous depletion of sulfide ores, much more attention has been drawn to nickel laterite processing.¹⁾

In 2011, China imported approximate 50 million tone (green weight) nickel laterite ores and produced 12.59 million tones stainless steel, accounted for 39.2 percent of the total stainless steel production of the world. The rotary kiln-electric furnace (RKEF)^2,3) process is chosen by most Chinese ferronickel works due to its good adaptability for various nickel laterite. However, the high amount of water and gangue in laterite always need huge transport cost and energy consumption especially for the lower grade laterite ores.^4,5) In addition, more and more investments from Chinese enterprises have been attracted to the Southeast Asia to exploit the laterite mine, where is normally lack of electricity and have high price of the electricity. Hence, developing a novel process with low energy consumption becomes a hot topic in the production of ferronickel with laterite.

A few researchers^6,7,8) tried to get higher grade ferronickel concentrate firstly by magnetic separation after pre-reduction in rotary kiln. Nevertheless, to avoid forming a ring against the wall of kiln, it becomes too difficult to enrich and recovery more nickel coincidently, cause the aggregation and separation of metal cannot accomplish effectively without high enough temperature. In addition, the success of “Oheyama Process”⁹⁾ also indicates that only with the semi-fused state at around 1400°C can directly produce granular ferronickel by rotary kiln. However, this process is mostly suitable for the laterite ores with >2 mass% of Ni-grade, and the average particle size of the metal granular is only about ϕ1 mm.^10,11)

The rotary hearth furnace (RHF) has been tried to produce iron nugget directly from iron ore with coal which is regarded as the new iron-making technology (ITMK3).¹²⁾ Therefore, a similar process was proposed to prepare ferronickel alloy directly with the rotary hearth furnace in this study.

2. Experimental

The chemical composition of the nickel laterite and coal were shown in Tables 1 and 2, it was assumed that all the nickel and iron were existed as NiO and Fe₂O₃ individually, and the gaseous product of the reduction reaction was CO only. Thermodynamically, the nickel has higher reducibility than iron, which has been confirmed by the previous studies,¹¹⁾ so the partial reduction is suggested to upgrade the nickel content in the alloy. The possible reactions occurred are shown in Eqs. (1), (2), (3), (4). The partial reduction means that Eq. (4) was incompletely. In this study, the carbon dosage (C/O mole ratio) was kept at 0.67, which can reduce 100 mass% Ni and 60 mass% Fe, according to the thermodynamic calculation with FactSage.   

$$\text{NiO}+\text{C}=\text{Ni}+\text{CO}$$(1)

  

$$3{\text{Fe}}_2{\text{O}}_3+\text{C}=2{\text{Fe}}_3{\text{O}}_4+\text{CO}$$(2)

  

$${\text{Fe}}_3{\text{O}}_4+\text{C}=3\text{FeO}+\text{CO}$$(3)

  

$$\text{FeO}+\text{C}=\text{Fe}+\text{CO}$$(4)

Table 1. Chemical composition of nickel laterite (mass%).

Ni	TFe	FeO	SiO2	MgO	CaO	Al2O3	Cr2O3	P	S	Water
1.81	17.87	0.44	34.97	13.50	1.54	4.75	0.51	0.005	0.064	17.35

Water=free water+crystal water+hydroxy group

Table 2. Chemical composition of coal (mass%).

Fixed Carbon	Ash	Volatile	S
71.41	17.41	11.02	0.48

The experimental procedure was shown in Fig. 1, after drying and grinding, the raw ores and coal were briquetted by powder tablet press machine with the pressure of 15 MPa, and then the green pellets were dried and weighed. The reduction process was carried out in a resistance furnace with MoSi₂ as the heaters (Fig. 2) in the N₂ atmosphere. The temperature was adjusted by two thermocouples to ensure the temperature error was ≤1°C before charging green pellets. The mass of sample is 55.00 g ± 1.00 g. After reduction of 30 min, the corundum crucible with the pellets reduced would be quickly taken out and quenched with water to room temperature. Finally, all the products were collected and manually crushed for magnetic separation, and the magnetic intensity was detected by a telemeter (Bokles, BK-8390, range: 0–1.999 T). Furthermore, some experiments would be done twice for the reproduction; the mineralogical observation and the chemical composition analysis were carried out.

Fig. 1.

Experimental procedure.

Fig. 2.

Schematic diagram of experimental apparatus.

Herein, the basicity and recovery ratio of nickel/iron were defined as follows:   

$$Basicity=\frac{{\mathrm{m}}_{\mathrm{C}\mathrm{a}\mathrm{O}}+{\mathrm{m}}_{\mathrm{M}\mathrm{g}\mathrm{O}}}{{\mathrm{m}}_{\mathrm{S}\mathrm{i}{\mathrm{O}}_2}+{\mathrm{m}}_{\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3}}$$(5)

Where: m_CaO, m_MgO, m_SiO2, m_Al2O3 were the mass fraction of CaO, MgO, SiO₂ and Al₂O₃ in the samples for reduction;   

$$\text{Recovery ratio}=\frac{{\text{m}}_{\text{i}}}{{\text{m}}_0}\times 100\%$$(6)

Where: m_i—mass of metal in alloy; m₀—mass of metal in green pellets.

In order to investigate the influence of temperature and basicity, two groups of experiments were designed as Tables 3 and 4 shown, and the basicity was modified by adding analysis grade CaO from original 0.40 to 0.60 by every 0.05.

Table 3. Scheme of experiments on temperature.

Basicity	0.40	Time/min		30
Temperature/°C	1380	1400	1420	1440

Table 4. Scheme of experiments on basicity.

Temperature/°C	1400		Time/min	30
Basicity	0.40	0.45	0.50	0.55	0.60

The wettability of the refractory materials by the molten slag is an important issue for the industrial practice of the RHF process. The good wettability would cause the difficulty of discharging and serious corrosion. Therefore, the slag after separation would be prepared as 3×3×3 mm samples for the measurement of contact angle to define the high-temp wettability between refractory materials and molten slag with the sessile drop method. And the weight of sample is 0.1640 g ± 0.0015 g in the wettability experiment, and the wettability experiment was carried out in a high-vacuum at 5×10^–4 Pa.

3. Results and Discussion

3.1. Effect of Temperature and Basicity on the Separation

3.1.1. Mineralogical Observation

The experiments on temperature in Table 3 were carried out firstly, and the mineral phases of cross section view of reduction samples were shown in Fig. 3, where the white areas were metal, and the gray and black regions were slag and pores respectively. It was obvious that raising the temperature could enhance the aggregation of metal. When the temperature was 1380°C, some few of the fine metals were much less than 0.1 mm and still comparatively dispersive, however, after 1400°C there would be a notable growth via diffusion coalesces among the metals, and the size of metal could reach about 0.3 mm in Fig. 3(d).

Fig. 3.

Mineral phase of cross section view of reduction samples at 1380°C, 1400°C, 1420°C, 1440°C (M-metal, S-slag, P-pore).

According to the Fe–Ni–C phase diagram calculated by FactSage in Fig. 4, which indicates that the ferronickel alloy is not possible to be melted as liquid without sufficient carburization in previous work, since the carbon dosage is even inadequate for reduction; that is to say, herein, the growth of metal was largely dependent on the solid state diffusion in molten slag. Hence, it becomes indispensable to reduce the melting point of slag.

Fig. 4.

Phase diagram of Fe–Ni–C.

Additionally, in view of the temperature operability of rotary hearth furnace, it is also very important to add flux to enhance the metal-slag separation. Therefore, the experiments in Table 4 were done. Meanwhile, on the basis of the calculation of the basicity, the chemical composition of SiO₂, CaO, MgO and Al₂O₃ were shown in Fig. 5, It can be estimated that the melting point can be brought down to nearly 1350°C by increasing the basicity. The mineral phase or morphology of reduction samples were shown in Fig. 6, of which a significant growth of the metal could be observed with the basicity raise, and when the basicity was 0.60, centimeter-level metal shell was formed on the surface and the metal could be very easily separated from the slag. The possible reasons are as below: first, the slag can be formed at relative lower temperature, which means much easier for the metal aggregation in the same period; second, the fluidity of slag could be improved by CaO addition, which is favorable for the growth of metal.

Fig. 5.

Phase diagram of SiO₂–MgO–CaO–Al₂O₃.

Fig. 6.

Mineral phase or morphology of reduction samples with different amount of CaO at 1400°C (M-metal, S-slag, P-pore).

3.1.2. Metal Grade and Recovery Ratio of the Samples

After mineral observation, the experiments on basicity were repeated for chemical composition analysis, which was shown in Fig. 7, and then on the basis of formula (6), the recovery ratio of nickel and iron were also obtained.

Fig. 7.

Effect of basicity on metal grade and recovery ratio at 1400°C with corundom crucible.

It can be seen that the nickel grade was only 2.91% when the basicty was 0.40, even though the recovery ratio was 91.45%; while the grade and recovery ratio of iron were 28.21% and 91.34% separately. That is to say, most metal was still hidden in the slag and the separation was not sufficient. After that, both the nickel and iron would be upgraded with the raise of basicity, and the recovery ratio of nickel kept growing gently. When the basicity was 0.60, the nickel grade could reach 12.63% and the recovery ratio was 96.21%, meanwhile, the iron grade was 81.10% and its recovery ratio was 62.96%. Herein, it is worth to note that the recovery ratio of iron decreased approximately 30%, which is largely due to the incompletely reduction. Anyhow, it is no doubt that, although the metal may still keep solid state at 1400°C, it is workable to produce ferronickel directly by raising the basicity to enhance the separation of metal-slag.

3.2. Wettability of Refractory/Molten Slag

It is evident that the ferronickel alloy nuggets can be achieved directly with incompletely reduction and slag with higher basicity. However, the sticking between molten slag and refractory came to be the crucial problem. Therefore, the high temperature wettability between different refractory materials and slag were carried out by using sessile drop method from room temperature to 1450°C, the slag was from former experiments of which the slag basicity was 0.60. As shown in Figs. 8(a) and 8(b), obvious wetting would take place when Al₂O₃ and MgO were chosen as the substrate; while, the contact angle between the graphite substrate and molten slag in Fig. 8(c) was 140°, i.e., graphite was nearly nonwetting with the slag. Consequently, another five experiments were carried out again by using graphite crucible based on Table 4 so as to explore the problem of slag sticking. The Fig. 9 showed the similar tendency with Fig. 7, the nickel grade could reach 11.33% and the recovery ratio was 98.59% when the basicity was 0.60; whereas the iron grade was 85.06% and its recovery ratio was only 63.72%. Furthermore, contrast with corundum crucible, centimeter-level spherical metal would appear when the basicity was 0.50, and the sphere became bigger with the raise of basicity, which was possible due to the decline of melting points by carburization.

Fig. 8.

Wettability of refractory materials and molten slag (basicity=0.60, T=1450°C), (a) Al₂O₃, (b) MgO, (c) graphite.

Fig. 9.

Effect of basicity on metal grade and recovery ratio at 1400°C with graphite crucible.

3.3. C, S, P in the Metal

Apart from the grade of iron and nickel, the content of carbon, sulfur and phosphorous were also analyzed, and the results were showed as Figs. 10 and 11. Theoretically, increasing the basicity is favorable for the desulfurization and dephosphorization of the metal. However, herein, it was very interesting that the sulfur and phosphorus in the metal showed apparent raise, which were mostly contributed to the amount of slag in the metal. As for the carbon, the fluidity of slag was improved by raising the basicity and led to better carburization. Particularly, when the basicity was 0.50, the carbon in metal reached 3.27% which meant that the metal would certainly melt since its melting point can be less than 1300°C as Fig. 4 showed.

Fig. 10.

Content of C, S, P in metal with Al₂O₃ crucible.

Fig. 11.

Content of C, S, P in metal with graphite crucible.

Moreover, compared to the corundum crucible, it can be seen from Fig. 11 that there was about 0.02 percent descent of sulfur and a little ascent of phosphorous by using graphite crucible; to a large extent, this was caused by the excess reduction of wustite in slag and lowered the oxygen potential. Anyhow, it is still need further study on the element distribution between slag and metal.

4. Conclusions

A novel process of ferronickel nugget production with laterite at a lower temperature than RKEF process was investigated in the present study, and the conclusions can be summarized as follows:

(1) In order to produce ferronickel nugget from laterite, it is necessary to supply high enough temperature to ensure the aggregation of metal, which should not lower than 1400°C.

(2) The separation between metal and slag can be enhanced by increasing the quaternary basicity to 0.60. Meanwhile, the partical reduction of iron can successfully improve the grade of nickel in ferronickel alloy.

(3) The experiments of high temperature wettability of refractory/slag indicate that the wettability order of the refractory by the semi-molten slag from good to poor is Graphite, Al₂O₃, MgO and graphite; the carbonaceous material is likely to be the best refractory material.

(4) The experimental results shows that when the temperature is 1400°C and the basicity is 0.60, the nickel content could reach 11.53% in the alloy nugget with the nickel laterite of 1.81 mass% nickel and the recovery ratio of nickel was 98.59%, while the grade and recovery ratio of Fe was 84.16% and 68.72%, respectively.

Acknowledgement

The authors are especially grateful to Key Programme of National Natural Science Foundation of China (Grant No. 51234010).

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