The soda-ash roasting of chromite minerals: Kinetics considerations
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RODUCTION
THE production of chromium chemicals from the chromite mineral has been traditionally achieved via the sodaash roasting process, which is being continuously improved to increase the efficiency of chromate extraction. The residue generated during this process is commonly dumped at specially designed and managed landfill sites. However in recent years, the concern over environmental pollution from hexavalent chromium (Cr6+) from the waste residue has become a major problem for the chromium chemicals industry. In the past, several research groups[1,2,3] have studied the soda-ash roasting process. The main thrust of previous research has been to optimize the process parameters for improving kiln performance. Current literature is particularly lacking in two areas: the detailed kinetics analysis and the reaction mechanism. Various methods such as pellet roasting[4] and duplex pellet technology[5] were investigated. Techniques such as the sodium hydroxide leaching[6,7] and hydrometallurgical extraction of chromate from lean chromite ores[8] were also reported. However, the traditional method of sodium chromate extraction on a commercial scale is based on soda-ash roasting either in a kiln or in a rotary hearth furnace. During the soda-ash roasting reaction, the chromium ion (Cr3+) from the cubic spinel of the chromite mineral reacts with sodium carbonate to form a water-soluble sodium chromate compound. The overall reaction can be represented as follows: FeCr2O4 ⫹ 2Na2CO3 ⫹
7 O2 → 2Na2CrO4 4
1 ⫹ Fe2O3 ⫹ 2CO2 2
[1]
V.D. TATHAVADKAR, Postdoctoral Student, and A. JHA, Professor of Applied Materials, are with the Department of Materials, University of Leeds, Leeds, LS2 9JT, United Kingdom. M.P. ANTONY, formerly Postdoctoral Research Fellow, Department of Materials, University of Leeds, is Scientific Officer, IGCAR, Kalpakkam, India. Manuscript submitted September 5, 2000.
METALLURGICAL AND MATERIALS TRANSACTIONS B
The reaction takes place in two steps. In the first step, as shown in Eq. [2a] the Fe2+ ion in the chromite spinel lattice oxidizes to the Fe3+ ionic state, while Cr3+ ions form sodium chromite, via Reaction [2b]. 1 1 O → Cr2O3 ⫹ Fe2O3 4 2 2
[2a]
Cr2O3 ⫹ Na2CO3 → Na2Cr2O4 ⫹ CO2
[2b]
FeCr2O4 ⫹
Sodium chromite thus formed oxidizes to sodium chromate in the presence of oxygen in a subsequent step, via the chemical reaction given by Eq. [3]: Na2Cr2O4 ⫹ Na2CO3 ⫹
3 O2 → 2Na2CrO4 ⫹ CO2 2
[3]
According to the Le Chatelier principle, the equilibrium in Eq. [3] will shift to the right-hand side in the presence of increasing oxygen partial pressures and decreasing CO2 pressures at the reaction interface between Na2Cr2O4 and Na2CrO4. Since the chromite mineral is a solid solution of MgO⭈Al2O3, FeO⭈Cr2O3, MgO⭈Cr2O3, and MgO⭈Fe2O3 spinels, the chemical potential of Cr2O3 (or the partial molar Gibbs energy of Cr2O3) in Reaction [2b] is strongly dependent on the mineral compositions. These thermodynamic factors determine the presence of a number of vacant cation octahedral and tetrahedral sites and, therefor
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