J. Cent. South Univ. Technol. (2011) 18: 83-88

DOI: 10.1007/s11771-011-0663-0

Thermodynamics of chromite ore oxidative roasting process

QI Tian-gui(齐天贵), LIU Nan(刘楠), LI Xiao-bin(李小斌),

PENG Zhi-hong(彭志宏), LIU Gui-hua(刘桂华), ZHOU Qiu-sheng(周秋生)

School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2011

Abstract: To explicate the thermodynamics of the chromite ore lime-free roasting process, the thermodynamics of reactions involved in this process was calculated and the phrases of sinter with different roasting times were studied. The thermodynamics calculation shows that all the standard Gibbs free energy changes of the reactions to form Na₂CrO₄, Na₂O·Fe₂O₃, Na₂O·Al₂O₃ and Na₂O·SiO₂ via chromite ore and Na₂CO₃ are negative, and the standard Gibbs free energy changes of the reactions between MgO, Fe₂O₃ and SiO₂ released from chromite spinel to form MgO·Fe₂O₃ and MgO·SiO₂ are also negative at the oxidative roasting temperatures (1 173-   1 473 K). The phrase analysis of the sinter in lime-free roasting process shows that Na₂O·Fe₂O₃, Na₂O·Al₂O₃ and Na₂O·SiO₂ can be formed in the first 20 min, but they decrease in contents and finally disappear with the increase of roasting time. The final phase compositions of the sinter are Na₂CrO₄, MgO·Fe₂O₃, MgO·SiO₂ and MgO. The results indicate that Na₂CrO₄ can be formed easily via the reaction of Na₂CO₃ with chromite ore. Na₂O·Fe₂O₃, Na₂O·Al₂O₃ and Na₂O·SiO₂ can be formed as intermediate compounds in the roasting process and they can further react with chromite ore to form Na₂CrO₄. MgO released from chromite ore may react with iron oxides and silicon oxide to form stable compounds of MgO·Fe₂O₃ and MgO·SiO₂, respectively.

Key words: chromite ore; oxidative roasting; thermodynamics; sodium chromate

1 Introduction

Soda-ash roasting of the chromite mineral is almost the unique method used worldwide for the production of water soluble sodium chromate [1]. The soda-ash roasting process is divided into lime-based roasting process and lime-free roasting process according to the composition of filler materials. In the roasting process, chromite ore and soda ash are mixed and subsequently roasted at 1 173-1 473 K in oxidative atmosphere to oxidize insoluble trivalent chromium spinels to water-soluble hexavalent chromate [2-3]. In traditional lime-based roasting process for manufacturing chromate, a mass of calcium-contained materials are used as fillers and large quantities of chromite ore processing residues (COPR) containing a lot of highly toxic, carcinogenic and mutagenic hexavalent chromium are produced, causing serious environmental pollution [4-5]. The alternative lime-free roasting process can overcome the disadvantages of low resource efficiency and the pollution of toxic residues in lime-based roasting process because there are no calcium-containing materials in this process, which makes the lime-free process become the most popular chromate production method [1, 6].

The chromite ore oxidative reactions in lime-free roasting process are complicated gas-liquid-solid heterogeneous reaction processes. When the roasting temperature is as high as 1 373 K, the impurities of silicon, ferrate, aluminum and magnesium present in chromite ore can also react with Na₂CO₃ as well as chromium. These subsidiary reactions affect the chromium oxidation seriously and make the roasting process more complex, so it is very important to reveal the rule of reactions in lime-free roasting process. In the past, the lime-free roasting process have been studied, the preliminary thermodynamics of the reactions of chromite ore with Na₂CO₃ has been investigated, and the molten salt phrases during the roasting process have been estimated [2, 6-7]. However, the main thrusts of previous research are to optimize the process parameters for improving kiln performance and to detail the kinetics, the reaction thermodynamics and the mechanism of the reactions between Na₂CO₃ and chromite ore. In this work, the thermodynamics of the reactions involved in lime-free roasting process is concentrated to make the reaction mechanism more clear.

2 Assumptions and method

It is assumed that the whole roasting process and all reactions are carried out at a constant temperature and

under the atmospheric pressure, and all compounds and substances in chromite ore are pure spinels. So, the standard Gibbs free energy change of all reactions can be calculated according to the classical thermodynamics theory related in Ref.[8]. Most of the thermodynamic data required for the calculation are drawn from Refs.[9-11]. For some complicated compounds, the thermodynamic data are calculated and estimated by the simulation models proposed in Refs.[12-13].

3 Thermodynamic calculation and discussion

3.1 Thermal decomposition of chromite ore

Natural chromite ore with spinel structure, has a formula of (Fe²⁺,Mg)(Cr,Al,Fe³⁺)₂O₄, which can be regard as a solid solution of FeO·Cr₂O₃, MgO·Cr₂O₃, FeO·Al₂O₃ and MgO·Al₂O₃. Meanwhile, some impurities such as 3MgO·2SiO₂·2H₂O, SiO₂ and Fe₂O₃ are packed in the ore [14]. Generally, the chromite oxidative roasting process is operated at a high temperature, and some thermal decomposition reactions would be carried out under this condition. To determine all the original reactants in roasting process, it is necessary to probe the thermal decomposition reactions of chromite ore. The main decomposition reactions in this process can be described as reactions (1)-(4):

FeO·Cr₂O₃=FeO+Cr₂O₃                       (1)

MgO·Cr₂O₃=MgO+Cr₂O₃                      (2)

FeO·Al₂O₃=FeO+Al₂O₃                       (3)

MgO·Al₂O₃=MgO+Al₂O₃                      (4)

FeO·Cr₂O₃+1/4O₂=1/2Fe₂O₃+Cr₂O₃             (5)

FeO·Al₂O₃+1/4O₂=1/2Fe₂O₃+Al₂O₃              (6)

The relationship between standard Gibbs free energy change of reactions (1)-(6) and temperature are plotted in Fig.1. Fig.1 shows that the standard Gibbs free energy changes of decomposition reactions of spinel compounds of FeO·Cr₂O₃, MgO·Cr₂O₃, FeO·Al₂O₃ and MgO·Al₂O₃ in neutral or reductive atmosphere are all positive. It is evident that chromite ore is hard to decompose without oxygen. Actually, the chromite roasting process is carried out in oxidative atmosphere. Under oxidative roasting condition, decomposition of chromite can be described as reactions (5) and (6). Fig.1 shows that standard Gibbs free energy change of reaction (5) is negative while that of reaction (6) is positive. This suggests that FeO·Cr₂O₃ may be decomposed to Fe₂O₃ and Cr₂O₃ in oxidative roasting process, but FeO·Al₂O₃ cannot be decomposed under oxidative, neutral and reductive roasting atmosphere.

Fig.1 Relationships between standard Gibbs free energy change of reactions (1)-(6) and temperature

3.2 Thermodynamics of chromium oxidation

The main target of chromite ore oxidative roasting process is to make the insoluble chromium transform to water-soluble sodium chromate. The possible oxidation reactions of chromite spinel in roasting process are listed as reactions (7)-(12). The relationships between standard Gibbs free energy changes of these reactions and temperature are plotted in Fig.2.

Na₂CO₃+1/2FeO·Cr₂O₃+7/8O₂=1/4Fe₂O₃+Na₂CrO₄+CO₂                   (7)

Na₂CO₃+1/2MgO·Cr₂O₃+3/4O₂=1/2MgO+Na₂CrO₄+CO₂                    (8)

Na₂CO₃+1/2Cr₂O₃+3/4O₂=Na₂CrO₄+CO₂         (9)

Na₂CO₃+FeO·Cr₂O₃=FeO+Na₂O·Cr₂O₃+CO₂     (10)

Na₂CO₃+MgO·Cr₂O₃=MgO+Na₂O·Cr₂O₃+CO₂   (11)

Na₂CO₃+Na₂O·Cr₂O₃+3/2O₂=2Na₂CrO₄+CO₂     (12)

Fig.2 Relationships between standard Gibbs free energy change of reactions (7)-(12) and temperature

Fig.2 shows that all the standard Gibbs free energy changes of reactions (7)-(9) are negative at normal roasting temperatures (1 173-1 473 K), which indicates that chromite spinel and chrominum oxide can be decomposed and oxidized to form Na₂CrO₄ under oxidizing condition with sodium carbonate. As we all know, the more negative the Gibbs free energy change is, the higher the reaction tendency will be. With this thermodynamic criterion, the thermal oxidative reaction tendency sequence of chromite spinel is FeO·Cr₂O₃> Cr₂O₃>MgO·Cr₂O₃. The standard Gibbs free energy changes of reactions (10) and (11) shown in Fig.2 are reduced rapidly with the temperature rising. All the values are negative at the roasting temperature, which proves that the chromite spinel may react with Na₂CO₃ to form Na₂O·Cr₂O₃. The standard Gibbs free energy change of reaction (12) shown in Fig.2 suggests that Na₂O·Cr₂O₃ can be oxidized easily when oxygen and sodium carbonate are sufficient.

3.3 Thermodynamics of sodium carbonate

Sodium carbonate is the most important reactant in the roasting process and its reaction rules will affect the roasting process directly. Under the conventional lime-free roasting conditions, sodium carbonate not only reacts with chromite spinel but also reacts with the impurities present in chromite ore. Among them, the reactions of sodium carbonate with the impurities of FeO·Al₂O₃, MgO·Al₂O₃, Fe₂O₃ and SiO₂ to form Na₂O·Al₂O₃, Na₂O·Fe₂O₃ and Na₂O·SiO₂ have received extensive attention. The subsidiary reactions between Na₂CO₃ and impurities are listed as reactions (13)-(16), and the relationships between changes of standard Gibbs energy and temperature for those reactions are plotted in Fig.3.

Na₂CO₃+FeO·Al₂O₃+1/4O₂=1/2Fe₂O₃+Na₂O·Al₂O₃+CO₂                (13)

Na₂CO₃+MgO·Al₂O₃=MgO+Na₂O·Al₂O₃+CO₂   (14)

Na₂CO₃+Fe₂O₃=Na₂O·Fe₂O₃+CO₂             (15)

Fig.3 Relationships between standard Gibbs free energy change of reactions (13)-(16) and temperature

Na₂CO₃+SiO₂=Na₂O·SiO₂+CO₂               (16)

Fig.3 shows that all the standard Gibbs free energy changes of reactions (13)-(16) become lower with the increase of temperature and all of them are negative above 1 200 K, which indicates that the formation reactions of Na₂O·Fe₂O₃, Na₂O·Al₂O₃ and Na₂O·SiO₂ via Na₂CO₃ and impurities are possible under the normal chromite roasting temperature.

By comparing the values of standard Gibbs free energy change of reactions (13)-(16) in Fig.3 and those of reactions (7)-(9) in Fig.2, it will be found that the standard Gibbs free energy changes of reactions (7)-(9) are more negative than those of reactions (13)-(16), which indicates that the reaction tendency for Na₂CO₃ to form Na₂CrO₄ is higher than the tendency to form Na₂O·Fe₂O₃, Na₂O·Al₂O₃ and Na₂O·SiO₂. This means that Na₂CrO₄ is more stable than Na₂O·Fe₂O₃, Na₂O·Al₂O₃ and Na₂O·SiO₂ in roasting process. But, some dynamics research shows that the formation reactions of Na₂O·Fe₂O₃, Na₂O·Al₂O₃ and Na₂O·SiO₂ have large reaction rates [7,15]. So, the thermodynamic and kinetic sequence may make some Na₂CO₃ form the intermediate compounds of sodium salts of ferrite, aluminate and silicate and then those intermediate compounds replacing part of Na₂CO₃ react with chromite to form Na₂CrO₄ in oxidative atmosphere. Those reactions can be described as reactions (17)-(22). The relationships between standard Gibbs energy change and temperature for those reactions are plotted in Fig.4.

Na₂O·Fe₂O₃+1/2FeO·Cr₂O₃+7/8O₂=5/4Fe₂O₃+Na₂CrO₄                    (17)

Na₂O·Al₂O₃+1/2FeO·Cr₂O₃+7/8O₂=1/4Fe₂O₃+Na₂CrO₄+Al₂O₃                 (18)

Na₂O·SiO₂+1/2FeO·Cr₂O₃+7/8O₂=1/4Fe₂O₃+Na₂CrO₄+SiO₂                  (19)

Na₂O·Fe₂O₃+1/2MgO·Cr₂O₃+3/4O₂= 1/2MgO+Na₂CrO₄+Fe₂O₃                (20)

Fig.4 Relationships between standard Gibbs free energy change of reactions (17)-(22) and temperature

Na₂O·Al₂O₃+1/2MgO·Cr₂O₃+3/4O₂= 1/2MgO+Na₂CrO₄+Al₂O₃                (21)

Na₂O·SiO₂+1/2MgO·Cr₂O₃+3/4O₂= 1/2MgO+Na₂CrO₄+SiO₂                 (22)

The standard Gibbs free energy changes of reactions (17)-(22) shown in Fig.4 are negative in an extensive range of temperature, suggesting that chromite spinel can seize the alkali in Na₂O·Fe₂O₃, Na₂O·Al₂O₃ and Na₂O·SiO₂ to form Na₂CrO₄ under oxidative atmosphere.

All the analyses above show that Na₂CrO₄ is more stable than Na₂O·Fe₂O₃, Na₂O·Al₂O₃ and Na₂O·SiO₂, while small amounts of Na₂O·Fe₂O₃, Na₂O·Al₂O₃ and Na₂O·SiO₂ can be formed as intermediate compounds in practical roasting process because of the kinetic factors. To explicate the rule of Na₂CrO₄ formation process, the chromium oxidation rate with different roasting temperatures and roasting times is investigated, then the oxidative reaction kinetic parameters of the reaction of chromite ore with Na₂CO₃ and Na₂O·Fe₂O₃ in oxidative atmosphere are calculated. The results show that the apparent reaction rate constant of Na₂CrO₄ formation reaction via Na₂CO₃ and chromite is about an order larger than that of the reaction via Na₂O·Fe₂O₃ and chromite (5×10^-2 min^-1 and 3.6×10^-3 min^-1 at roasting temperature of 1 323 K, respectively). This implies that the reaction rate of Na₂CrO₄ formation reaction through Na₂O·Fe₂O₃ is significantly lower than that of the reaction through Na₂CO₃. So, the formation of the intermediate compounds containing alkali (sodium ferrite, etc) may be one of the important causes of low chromium oxidation rate, especially in the later roasting process when the Na₂CO₃ is exhausted.

3.4 Thermodynamics of magnesium oxide

In the chromite ore oxidative roasting process, the chromite spinels are oxidized and the MgO and iron oxides are released. MgO is the unique alkali compound except Na₂CO₃ in lime-free roasting process and it can react with most acid compounds at high temperature. The previous study results show that all the FeO, Fe₃O₄ and Fe₂O₃ can react with MgO in oxidative atmosphere to form MgO·Fe₂O₃ with a ferrite structure [16]. These combination reactions are listed as reactions (23)-(25). The relationships between standard Gibbs energy change and temperature for those reactions are plotted in Fig.5.

MgO+Fe₂O₃=MgO·Fe₂O₃                    (23)

MgO+2/3Fe₃O₄+1/6O₂=MgO·Fe₂O₃            (24)

MgO+2FeO+1/2O₂=MgO·Fe₂O₃               (25)

Fig.5 shows that standard Gibbs free energy changes of reactions (23)-(25) increase with the tem- perature rising, and all of them are negative and become

Fig.5 Relationships between standard Gibbs free energy change of reactions (23)-(25) and temperature

more negative with the valence states of iron decreasing, in the chromite roasting temperature range. This suggests that all the reactions between iron oxides and MgO listed above may be carried out and the lower the valence states of iron, the easier the formation of magnesium ferrite compound.

Magnesium oxide can also react with silicon oxide besides the iron oxide, listed as reactions (26)-(27). The standard Gibbs free energy changes of those reactions are shown in Fig.6.

MgO+SiO₂=MgO·SiO₂                      (26)

2MgO+SiO₂=2MgO·SiO₂                    (27)

Fig.6 Relationships between standard Gibbs free energy change of reactions (26)-(27) and temperature

From Fig.6, the standard Gibbs free energy changes of reactions (26)-(27) are negative and the values have no obvious change with the increase of temperature. So, the MgO and silicon oxide may react to form stable magnesium silicates, 2MgO·SiO₂ and MgO·SiO₂. The standard Gibbs free energy change for forming 2MgO·

SiO₂ is more negative than that of MgO·SiO₂, which indicates that 2MgO·SiO₂ is more stable than MgO·SiO₂.

Fig.7 XRD patterns of chromite ore and chromate sinter prepared for different roasting times: (a) Chromite ore; (b) Sinter roasting for 20 min; (c) Sinter roasting for 120 min

3.5 Experimental results

In order to verify the results of the above mentioned thermodynamic analysis, the chromite oxidative roasting experiment was carried out with different roasting time. The main chemical composition of the chromite ore can be characterized as follows (mass fraction): Cr₂O₃ 41.45%, Al₂O₃ 22.95%, Fe₂O₃ 22.29%, SiO₂ 4.24% and MgO 8.63%. The mixture of this chromite ore and Na₂CO₃ was roasted at 1 323 K under oxidation atmosphere. The X-ray diffraction (XRD) patterns of chromite ore and chromate sinters with different roasting times are displayed in Fig.7.

XRD analysis reveals that the major phases in chromite ore are chromium spinel with a few serpentine (Fig.7(a)). Na₂O·Fe₂O₃, Na₂O·Al₂O₃, Na₂O·SiO₂ and unreacted chromium spinel are observed in the sinter with 20 min roasting time (Fig.7(b)), but the contents of those phases decrease and finally disappear with the increase of roasting time. The final phase compositions of oxidization reaction in the roasting process are Na₂CrO₄, MgO·Fe₂O₃, MgO·SiO₂ and MgO (Fig.7(c)). Combined with thermodynamic analysis, the phase transformation indicates that chromite oxidization reaction may experience two stages. In the first stage, most of Na₂CO₃ reacts with chromite spinel to form Na₂CrO₄ and a part of Na₂CO₃ transforms into Na₂O·Fe₂O₃, Na₂O·Al₂O₃ and Na₂O·SiO₂. In the second stage, Na₂O·Fe₂O₃, Na₂O·Al₂O₃ and Na₂O·SiO₂ may further react with chromite ore to form Na₂CrO₄. In the whole roasting process, magnesium compounds transform into MgO·Fe₂O₃, MgO·SiO₂ and MgO.

The results of experiment are consistent with thermodynamic calculation perfectly. Both of them show that the oxidization of the chromite in lime-free roasting process not only take place between chromite ore and Na₂CO₃, but also take place between chromite and intermediate compounds which are produced by subsidiary reactions. So, we should pay more attention to the intermediate compounds in the roasting process when we want to intensify the chromite oxidizing process.

4 Conclusions

1) The spinel structure of chromite such as FeO·Cr₂O₃, MgO·Cr₂O₃, FeO·Al₂O₃ and MgO·Al₂O₃ cannot be destroyed by thermal decomposition reactions at normal roasting temperature.

2) Part of Na₂CO₃ may transform into intermediate compounds such as Na₂O·Fe₂O₃, Na₂O·Al₂O₃ and Na₂O·SiO₂ by subsidiary reactions in the lime-free roasting process. Those intermediate compounds are less stable than Na₂CrO₄ and they can further react with chromite to form Na₂CrO₄.

3) The magnesium compounds in chromite ore may transform into stable phases of MgO·Fe₂O₃, MgO·SiO₂ and MgO in the lime-free roasting process.

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(Edited by YANG Bing)

Foundation item: Project(2009FJ1009) supported by the Major Science and Technology Program of Hunan Province, China

Received date: 2009-10-28; Accepted date: 2010-03-01

Corresponding author: LI Xiao-bin, Professor, PhD; Tel: +86-731-88830453; E-mail: qtg_csu@163.com