Kinetics of carbochlorination of chromium (III) oxide

TLDR: In this article, the shrinking sphere model is used to describe the carbochlorination mechanism of chromium oxide and that is controlled by the chemical reaction, and the reaction order is about 1.34 and is independent of temperature.

Abstract: Kinetics of the carbochlorination of Cr2O3 has been studied with Cl2+CO gas mixtures between 500 °C to 900 °C using thermogravimetric analysis. The apparent activation energy is about 100 kJ/mol. Mathematical fitting of the experimental data suggests that the shrinking sphere model is the most adequate to describe the carbochlorination mechanism of chromium oxide and that is controlled by the chemical reaction. In the temperature range of 550 °C to 800 °C, the reaction order is about 1.34 and is independent of temperature. Changing the Cl2+CO content from 15 to 100 pct increases the reaction rate and does not affect the reaction mechanism. Similarly, changing the ratio of Cl2/(Cl2+CO) from 0.125 to 0.857 does not modify the carbochlorination mechanism of Cr2O3. In these conditions, the reaction rate passes through a maximum when using a chlorinating gas mixture having a Cl2/(Cl2+CO) ratio of about 0.5.

Figures

Fig. 1- Results of (a) SEM and (b) XRD analysis of Cr2O3 sample.

Fig. 2 - Standard free energy change of Cr2O3 carbochlorination reactions.

Fig. 10- Mathematical fitting of carbochlorination data of Cr2O3 with Cl2+CO for (a) T ≤ 700 °C and (b) T ≥ 725 °C using equation 7.

Fig. 9- Mathematical fitting of carbochlorination data of Cr2O3 at 700 °C for different model equations.

Fig. 11- Isotherms of carbochlorination of Cr2O3 at 800 °C with Cl2+CO for (a) 12.5 ≤ Cl2 ≤ 50.0 pct and (b) 55.0 ≤ Cl2 ≤ 100.0 pct.

Fig. 12- Effect of Cl2/(Cl2+CO) ratio on the Cr2O3 carbochlorination rate at 800 °C.

Frequently asked questions

Q1. What are the contributions in "Kinetics of carbochlorination of chromium (iii) oxide" ?

In this paper, Ndue Kanari, B.R. Reddy, and I. Gaballah have shown that the reaction products of the carbochlorination of Cr2O3 will be CrCl2 and CrCl3 at temperatures lower and higher than 650 °C. 

Q2. How much carbon is needed for the chlorination of the above mentioned oxides?

They emphasized that for a reaction time of one hour, temperatures higher than 800 °C, a chlorine flow rate of about 3 L/g/h and three times the stoiciometric amount of carbon are necessary for the chlorination of the above mentioned oxides. 

Q3. What temperature was the reaction rate at which the gas mixture was diluated?

To determine the effect of diluting the carbochlorinating gas mixture with nitrogen on the reaction rate, a series of experiments was performed at 550 °C, 650 °C and 800 °C with gas mixture having a Cl2+CO content that varied from 15 pct to 100 pct. 

Q4. What temperature is the reaction rate of Cr2O3?

The thermodynamic study suggests that the reaction products of the carbochlorination of Cr2O3 will be CrCl2 and CrCl3 at temperatures lower and higher than 650 °C. 

Q5. What is the reaction extent of the cl2+CO gas mixture?

The chlorine content in the Cl2+CO gas mixture was varied from 0 to 100 pct that corresponds to a Cl2/(Cl2+CO) ratio of 0 to 1, while the total gas flow rate was kept constant at 80 L/h. 

Q6. What is the effect of increasing the Cl2+CO content in the chlorinating gas mixture?

Increasing the Cl2+CO content in the chlorinating gas mixture from 15 pct to 100 pct does not affect the carbochlorination mechanism of Cr2O3. 

Q7. What is the kinetics of the carbochlorination of chromite?

Carbochlorination at low temperatures of the chromite concentrate allowsthe selective removal of iron leading to higher chromium content and Cr/Fe ratio. 

Q8. How did they study the chlorination of Cr2O3?

Sano and Belton [5] investigated the chlorination of Cr2O3 with Cl2-O2-Ar atmosphere using transpiration technique in the temperature range of 627 °C to 977 °C. 

Q9. What is the effect of gas velocity on the chlorination reaction rate?

The effect of Cl2+CO gas velocity on the chlorination reaction rate was studied to determine the conditions where gas starvation and mass transfer limitations could be minimized. 

Q10. What is the mechanism of the carbochlorination of Cr2O3?

It seems that the carbochlorination mechanism is again independent of the partial pressure of reactive gases in the chlorinating gas mixture. 

Q11. Why is the reaction rate of Cr2O3 almost equal to zero?

The reaction rate of Cr2O3 with 100 pct CO is almost equal to zero and rises with increasing the chlorine content in the gas mixture. 

Q12. How did they convert the chromium oxide to chromium?

Saeki et al. [2] investigated chlorination of Cr2O3 by Cl2 using samples obtained by heating a hydrated chromium oxide either at 440 °C or 1000 °C. 

Q13. What temperature is the reaction rate of chromium trichloride?

During the carbochlorination of Cr2O3, the rate of formation of chromium trichloride was systematically lower than its rate of volatilization between 500 °C and 900 °C. 

Q14. What is the spectra of the sample?

Spectra of EPMA - SEM (electron probe microanalysis of the scanning electron microscope) and that of XRD (X-rays diffraction) of the sample indicate only the presence of Cr2O3 phase (Fig. 1).