Trans. Nonferrous Met. Soc. China 25(2015) 3133-3138

Thermodynamic of selective reduction of laterite ore by reducing gases

Song CHEN, Shu-qiang GUO, Lan JIANG, Yu-ling XU, Wei-zhong DING

Materials Science and Engineering, Shanghai University, Shanghai 200072, China

Received 15 October 2014; accepted 25 March 2015

Abstract: As the sulfide ore deposits become less economical and environmental viable as a source of nickel, increasing attention is being paid to the laterite ores. But in the pyrometallurgical process of laterite, more efforts should be paid to control the reduction of iron oxide in order to get high nickel-content nickeliferous product. For these reasons, equilibrium condition of iron oxide when laterite ore was selectively reduced by CO₂/CO, H₂O/H₂ and CO₂/H₂ was studied from the perspective of iron activity with an assumption that the activities of FeO and Fe₃O₄ equal 1 in this work, and it well accounts for the inescapability of Fe metallization. Activity coefficient of iron in Ni-Fe binary solid alloy was calculated by Miedema model based on the known thermodynamics datum filed. According to Raoult’s law, the relationship among the Fe/Ni ratio, reduction temperature and reduction gas composition was calculated when laterite ore was selectively reduced by the three different reduction systems. The calculation result was discussed and also compared with the experimental result. The trend of metal iron content in the reduction product of laterite ore varying with temperature and gas composition was well predicted by the calculation result.

Key words: selective reduction; laterite ore; activity coefficient; Miedema model

1 Introduction

Nickel, one of the most important strategic metals, is widely applied to stainless steel, electroplating, catalyst and petrochemical industry [1]. In recent years, more and more attention has been paid to the research about exploitation and utilization of nickel laterite ore rather than nickel sulfide ore, due to the factors such as resources, environment, and the cost aspects [2]. By regulating the reduction condition like reduction temperature and oxygen potential in the system, most of the nickel oxides in the ore are reduced to metallic state, while the reduction of iron oxides is minimized. Selective reduction of laterite ore, as explained before, plays an important role in the treatment process of laterite ore [3-6].

Reduction temperature and gas composition are two main factors which significantly affect the efficiency of the selective reduction of laterite ore [7]. And the metal Fe in the reduction production of laterite ore tends to combine with metal Ni, which is the reduction production of NiO in laterite ore, to be Fe-Ni alloy [8]. Thus, the varying activity of Fe will greatly affect the stability of iron oxide. In OLLI et al’s study [9], the predominance programs for iron and its oxides in the presence of nickel when laterite ore was reduced by CO₂/CO and H₂O/H₂ mixed gas showed that the reduction of iron oxide could be affected by the variation of the activity of Fe. However, it cannot be used directly to predict the composition of nickeliferous produced from laterite due to the lack of the activity coefficient of Fe in Fe-Ni alloy. Based on their work, equilibrium conditions of iron oxide when laterite ore was selectively reduced by CO₂/CO, H₂O/H₂ and CO₂/H₂ was studied from the perspective of iron activity with an assumption that the activities of FeO and Fe₃O₄ equal 1 in this work. Activity coefficient of Fe in Fe-Ni alloy was calculated by Miedema model [10-12], which can well calculate the enthalpies of the formation of binary solid alloy based on the known thermodynamic data. Then, the relationship between the iron content in Fe-Ni alloy produced by the reduction of laterite Fe and reduction temperature as well as gas composition can be worked out by Raoult’s law, when laterite ore was reduced by CO₂/CO, H₂O/H₂, CO₂/H₂ mixed gas respectively. The calculation result was verified by reduction experimental result of Indonesia laterite ore with CO₂/H₂ mixed gas. A good interpretation of the inescapability of Fe metallization in the selective reduction process of laterite ore can be obtained in this work.

2 Analysis process

2.1 Calculation process of relationship of gas composition, temperature and activity of Fe in Fe-Ni alloy

Reactions as follows would take place when laterite ore was reduced by H₂O/H₂:

NiO(s)+H₂(g)=Ni(s)+H₂O(g),

=-13460-25.60T                        (1)

3Fe₂O₃(s)+H₂(g)=2Fe₃O₄(s)+H₂O(g),

=-41425-111.42T                       (2)

Fe₃O₄(s)+H₂(g)=3FeO(s)+H₂O(g),

=66105-69.16T                         (3)

FeO(s)+H₂(g)=Fe(s)+H₂O(g),

=17580-11.60T                          (4)

1/4Fe₃O₄(s)+H₂(g)=3/4Fe(s)+H₂O(g),

=29700-25.94T                          (5)

The reduction of iron complies with stepwise reaction mechanism. If the temperature is below 570 °C，reactions were carried out by two steps:  Fe₂O₃→Fe₃O₄→Fe; otherwise, reactions were carried out by three steps: Fe₂O₃→Fe₃O₄→FeO→Fe.

Gibbs free energies of Fe₂O₃, Fe₃O₄, FeO, NiO and H₂O as a function of temperature are shown in Fig. 1. In the reduction system, the difficulty of Eq. (2), Eq. (1), Eq. (3), Eq. (4), and Eq. (5) increases successively. Therefore, virtually, selective reduction of the laterite ore is to inhibit the occurrences of Eq. (3), Eq. (4) and Eq. (5).

Fig. 1 Gibbs free energy of Fe₂O₃, Fe₃O₄, FeO, NiO and H₂O as function of temperature

For reactions from Eq. (2) to Eq. (5), the equilibrium constant expression can be stated as follows with an assumption that the activities of FeO, Fe₃O₄ and Fe₂O₃ equal 1.

                          (6)

                           (7)

                   (8)

           (9)

For Eq. (8) and Eq. (9), the activity of Fe depends on the corresponding equilibrium constant and the p_H2O/p_H2 ratio. According to Eq. (10), the relations between the partial pressure of hydrogen, the reduction temperature and the activity of Fe can be worked out based on the value of  at different temperatures.

                             (10)

There is something different in the analysis process when laterite ore was reduced by CO₂/H₂ mixed gas compared with H₂O/H₂ mixed gas and CO₂/CO mixed gas.

Reaction as Eq. (11) may take place when CO₂/H₂ mixed gas is used to reduce the laterite ore.

CO₂+H₂=CO+H₂O, =30459-28.14T                 (11)

The mole fraction of H₂ in initial mixed gas was supposed to be  Thus, the mole fraction of CO₂ in initial mixed gas was  When the reactions were balanced, conversion amount of H₂ was supposed to be x. Then equilibrium constants can be stated as follows.

        (12)

So,

  (13)

       (14)

At the same time,

                                (15)

where x(H₂O)^e and x(H₂)^e are the mole fractions of H₂O and H₂ when Eq. (11) is balanced, respectively. And they can be calculated by the corresponding equilibrium constant based on the known corresponding equilibrium constant value of at different temperatures. The relations between  reduction temperature and activity of Fe can be worked out by Eq. (14) and Eq. (15).

Actually, when laterite ore was reduced by CO₂/H₂ mixed gas, the reduction potential of this system was equivalent to the equilibrium component (including CO₂, CO, H₂O, H₂) as the reaction of CO₂ with H₂ is balanced at a certain temperature. That is to say, the calculation method also applies to CO₂/H₂/CO/CO₂ mixed gas.

2.2 Calculation process of activity coefficient of iron in Ni-Fe binary solid alloy

Due to its convenience and the high consistence of calculation result, Miedema model was widely used for thermodynamic calculation of binary alloy. Based on the known parameter of Miedema model, activity coefficient of iron in Ni-Fe binary solid alloy was calculated in this work.

The calculation expression of heat of formation of A-B binary alloy by Miedema model is [10-12]

   (16)

    (17)

where the x_A and x_B are the molar fractions of A and B; V_Aand V_B are the molar volumes of A and B; n_WSA and n_WSB are the electron densities of A and B in electrons per (0.529 )³; Δf(f_A-f_B) is the electro-negativity difference of A and B; p, q, r, μ, α are empirical constants.

For Ni-Fe binary solid alloy, p/q=9.4, α=1, r/p=1, p=10.6. And the relations among the partial molar excess free energy  the excess entropy  the enthalpy change (ΔH_AB) and the activity coefficient (γ) can be described by equations as follows [13]:

                                (18)

                      (19)

       (20)

where T_m(A) and T_m(B) are melting points in Kelvin of pure elements A and B, respectively. From Eq. (16) to Eq. (20), activity coefficient of iron in Ni-Fe binary solid alloy can be calculated by expression as follows [14]:

                  (21)

The relevant parameters are shown in Table 1.

Table 1 Relevant physical parameters used in calculation process of activity coefficient of Ni-Fe binary solid alloy [15]

Activity coefficient of iron in Ni-Fe binary solid alloy at different temperatures and different contents of metal Fe can be worked out by combining Eq. (16), Eq. (17), and Eq. (21) together. By setting pure substance as standard state, and combining the activity coefficient expression of iron and the relationship among reduction temperature, reducing gas partial pressure and activity of iron together, relations among reduction temperature, reducing gas partial pressure and metal iron content in reduction product of laterite ore can be worked out.

3 Calculation result and discussion

The partial calculation results of iron activity coefficient by Miedema model are shown in Table 2. And they are consistent with the characteristics of Fe-Ni system, which can be considered to be the approximate ideal solution [16].

The relations among reduction temperature, initial reducing gas partial pressure and metal iron content in reduction product of laterite ore are shown in Figs. 2-4 when the initial mixed gas was CO₂/CO, H₂O/H₂, CO₂/H₂, respectively.

In the equilibrium diagram of iron oxide in reduction condition, the activity of Fe is 1, and the whole area is divided to Fe zone, Fe₃O₄ zone, FeO zone by curves ab, bc, and bd, which are the critical lines of transformation from Fe₃O₄ to Fe, Fe₃O₄ to FeO, and FeO to Fe, respectively. As the formation of Fe-Ni alloy when laterite is reduced by CO₂/CO, H₂O/H₂ and CO₂/H₂, the activity of Fe will vary with the alloy composition and temperature. At the same time, three zones become the stable zones of [Fe,Ni]_ss (zone A), Fe₃O₄+[Fe,Ni]_ss (zone B), and FeO+[Fe,Ni]_ss (zone C). As shown in Fig. 2 to Fig. 4, when the activity of Fe is reduced from 1 to 0.2, the positions of curves ab and bc will be down to be curve a_0.2b_0.2 and b_0.2c_0.2, and the area of zone A increases while the areas of zone B and zone C decrease accordingly. That also means the iron oxide will be unstable because of the reduction of Fe activity.

Table 2 Partial calculation results of iron activity coefficient

Fig. 2 Relations of reduction temperature, initial reducing gas partial pressure and metal iron content in product of laterite ore reduced by CO₂/CO mixed gas

Fig. 3 Relations of reduction temperature, initial reducing gas partial pressure and metal iron content in product of laterite ore reduced by H₂O/H₂ mixed gas

Fig. 4 Relations of reduction temperature, initial reducing gas partial pressure and metal iron content in product of laterite ore reduced by CO₂/H₂ mixed gas

When Figs. 2-4 are compared with each other, some visible differences could be found when laterite is selectively reduced by CO₂/CO, H₂O/H₂, CO₂/H₂. The area of zone A in Fig. 3 is smaller than the others, and the position of curve a_xb_x (0 < x <1) in Fig. 3 is the highest in the three figures at temperature below 810 °C. That means H₂O/H₂ mixed gas has the best selective ability to reduce the laterite ore at temperature below 810 °C, but the worst selective ability at temperature above 810 °C. Besides, in Fig. 2, the content of Fe in Fe-Ni alloy (x(Fe_[Ni,Fe]ss)) increases with the rising of the reduction temperature, especially in zone C. But in Fig. 3, the x(Fe_[Ni,Fe]ss) decreases obviously with the rising of the reduction temperature. For the case of that laterite is reduced by CO₂/H₂, the x(Fe_[Ni,Fe]ss) is not sharply affected by the reduction temperature, especially in zone C. A resemblance is that the x(Fe_[Ni,Fe]ss) increases obviously with the increase of the partial pressure of reduction gas.

In general, as the formation of Fe-Ni alloy, the activity of Fe varies with the x(Fe_[Ni,Fe]ss), and it is feasible in thermodynamic for iron oxide to be reduced to metal Fe, which will be combined with metal Ni to be Fe-Ni alloy. Thus, the activity of Fe increases, and the reduction of iron oxide becomes more difficult, while the activity of Ni decreases, and it is easier for the reduction of nickel oxide. Therefore, the reduction of iron oxide and nickel oxide is a mutually reinforcing process. In order to obtain a low-Fe Fe-Ni alloy product at low temperature, in contrast, H₂O/H₂ mixed gas would be the best reduction gas in this thermodynamic analysis.

Laterite ore from Indonesia was selectively reduced by CO₂/H₂ mixed gas with different CO₂/H₂ ratios at different temperatures for 1 h. The raw ore was broken, ground to 180 μm, thoroughly incorporated, and then calcined at 700 °C for 2 h before reduction. The reduction was carried out in a tube furnace shown in Fig. 5. About 1 g samples in a corundum crucible were roasted at 500, 600, 700, 800 and 900 °C respectively for 1 h with a fixed CO₂/H₂ ratio. Ni metallization ratios were detected by inductively coupled plasma atomic emission spectrometry after bromine-methanol extraction, and Fe metallization ratios were determined through titrating by potassium dichromate solution after ferric trichloride solution extraction. And the experimental result is shown in Fig. 6.

The experimental result shows that the Fe content in Fe-Ni alloy increases with the rising of the partial pressure of H₂, and it is consistent with the calculation result described before. For low temperature such as 500-600 °C，the content of Fe in Fe-Ni alloy is not affected by the partial pressure of H₂, it is perhaps because 1 h is not enough for CO₂/H₂ mixed gas to bring out all its reduction potential at relative low temperature. For reduction temperature of 800 °C, the content of Fe in Fe-Ni alloy increases with the rising CO₂/H₂ ratio, the experimental result value of content of Fe in Fe-Ni alloy is higher with a CO₂/H₂ ratio no less than 1, but lower with a CO₂/H₂ ratio less than 1, when it is compared with the calculation result. Three reasons could be used to account for this phenomenon.

1) The metal products of this reduction reaction are not only Fe and Ni, but also Co. And metal Co is easy to combine with metal Fe and metal Ni to be alloy. But in this calculation work, the effect of Co was not taken into account.

2) The reactions of laterite’s selective reduction complies with the rule of gas-solid reaction, the diffusion of metallic element is a critical factor for the formation of the alloy, and it also has a great influence on the alloy composition.

3) The reduction time of 1 h may be not enough for the mixed gas to bring out all its reduction potential, thus lead to this deviation.

It is worth mentioning that at the beginning of the calculation, the activities of FeO, Fe₃O₄ and Fe₂O₃ were assumed to be 1. But in fact, the activities of FeO, Fe₃O₄ and Fe₂O₃ may change with the varying reduction potential. When laterite ore is reduced at temperature above 570 °C, the reduction of FeO, actually, is the reduction of wusitite (Fe_xO). Fe_xO could be considered as solid solution, which is the production of Fe₃O₄ dissolved into FeO. Thus, the activity of FeO would change with the varying of wusitite composition. When laterite ore is selectively reduced by reducing agent with a certain reduction potential, the smaller the FeO activity is, the more difficult the reduction of wusitite to Fe would be. This variation reflected in Fig. 2 to Fig. 4 is that the position of curve a_xb_xc_x (0 < x <1) would rise in some degrees.

Fig. 5 Schematic diagram of roasting apparatus

Fig. 6 Experimental result of laterite ore reduced by CO₂/H₂ mixed gas

4 Conclusions

1) The reduction of iron oxide and nickel oxide is mutually reinforcing process.

2) The trend of the content of Fe in Fe-Ni alloy varying with the partial pressure of H₂ in experiment is well predicted by the calculation result.

3) In order to better predict the relations of reduction temperature, initial reducing gas partial pressure and metal iron content in reduction product of laterite, more factors such as the variation of FeO activity, the effect of Co and the diffusion of metal element, should be taken into consideration.

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气体选择性还原红土镍矿的热力学

陈 松，郭曙强，姜 兰，徐玉棱，丁伟中

上海大学 材料科学与工程学院，上海 200072

摘  要：由于硫化镍矿生产镍铁在经济和环境上不断出现的问题，采用红土镍矿生产镍铁越来越受到重视。但是红土镍矿制备镍铁的火法工艺中，在提高铁镍产品中的镍含量方面的理论研究仍存在许多不足。出于这方面的考虑，假设Fe₂O₃、FeO和Fe₃O₄的活度为1，计算了CO₂/CO、H₂O/H₂和CO₂/H₂三种气氛下选择性还原红土镍矿时，不同铁活度下铁-铁氧化物的平衡条件。从已有的热力学数据出发，利用Miedema二元合金生成热模型，计算了Ni-Fe固态二元合金中铁的活度系数。并以活度系数为纽带，最终计算出这三种还原气氛下，镍铁合金产物中的铁含量与还原气体分压、还原温度的关系。并用CO₂/H₂还原红土镍矿，得到的实验数据与理论值进行了对比分析与讨论，热力学计算结果很好地解释了选择性还原红土镍矿时铁金属化无法避免的原因，并较好地预测了红土镍矿还原产物中铁含量随温度和气体组分的变化趋势。

关键词：选择性还原；红土镍矿；活度系数；Miedema模型

(Edited by Yun-bin HE)

Foundation item: Project (2012CB722805) supported by the National Basic Research Program of China

Corresponding author: Shu-qiang GUO; Tel: +86-13916144589; Email: sqguo@shu.edu.cn

DOI: 10.1016/S1003-6326(15)63943-7