Dynamics of Iron Oxide Reduction during Heat Treatment of Fuel-Melting Materials

N. G. Antonov, V. P. Zaryuto, S. I. Smolyaninov, G. G. Krinitsyn

(Presented by the scientific-methodological seminar of the organic departments of the Chemical-Technological Faculty)

The use of peat as a technological fuel in processes for producing iron from ores has a number of distinctive features. The most important of these is the release of a significant quantity of gaseous and liquid pyrolysis products, which are good reducing agents. At the same time, the role of particular decomposition products in the reduction process is not the same.

We have investigated the process of reduction of iron oxide in peat-ore moldings by products of the thermal decomposition of peat \((A^c — 4.85\%;\ V^r — 85.93\%;\ C^r — 52.30\%;\ H^r — 6.43\%;\ N^r — 2.43\%)\), at a heating rate of \(5^\circ/\text{min}\) and with an hourly hold at the end of the coking interval. The content of \(\mathrm{Fe_2O_3}\) in the materials is \(33.14\%\) on a dry-substance basis.

Data on the dynamics of gas evolution were obtained on a combined derivatographic installation, the main components of which are a derivatograph and two chromatographs—GACH-21 for analysis of \(\mathrm{CO_2}\) and KhT-2M for \(\mathrm{CH_4}\), \(\mathrm{CO}\), \(\mathrm{H_2}\).

The phase composition of the iron oxides was determined by chemical and X-ray diffraction methods on a DRON-1 unit, with an iron anode; anode voltage \(20\ \text{kV}\), anode current \(15\ \text{mA}\).

Considering the data on the reduction of \(\mathrm{Fe_2O_3}\) in fuel-melting materials (FMM) (Figs. 3, 4), three temperature intervals may be noted:

1. The low-temperature interval up to \(500—550^\circ\text{C}\). The rate of the reduction processes is low.

2. The temperature interval \(500—650^\circ\text{C}\) is characterized by a continuous increase in the rate of the reduction processes and may be called transitional.

3. The elevated-temperature interval (above \(650^\circ\text{C}\)) is the interval of intensive reduction processes.

Data on the dynamics of gas evolution from peat and fuel-melting materials (Figs. 1, 2) indicate that in each of these temperature intervals the reduction of iron oxides is due to different products of the thermal decomposition of the organic mass of peat. Increased rates of evolution of \(\mathrm{CO}\), \(\mathrm{CO_2}\), and \(\mathrm{H_2}\) from peat-ore material in the first temperature interval indicate that liquid products and their radicals actively interact with iron oxides even at comparatively low temperatures, \(250—550^\circ\text{C}\). If one considers the share of participation of liquid, gaseous, and solid produc-

of pyrolysis products in the removal of oxygen from oxides (Figs. 1–4), it should be noted that, in the interval of liquid evolution (up to 550°C), \(Fe_2O_3\) is reduced to \(Fe_3O_4\), and about 20% of the oxygen of iron oxide is removed. Of this 20%, the larger part must be carried away by carbon monoxide, which at lower temperatures has a greater affinity for oxygen than does hydrogen [1, 2, 6], and which is evolved from the resin as a result of the interaction of \(Fe_2O_3\) with the liquid products.

In addition, adsorption of water vapor on iron oxides blocks the reaction surface for hydrogen [1].

With regard to the reduction of iron oxides by methane, it must be said that up to 500°C it is inert and only above this temperature does it become involved in the reduction processes.

Fig. 1. Dynamics of evolution of CO (1, 2) and \(CO_2\) (3, 4) from FMM (1, 3) and peat (2, 4)

In the transition interval 500–650°C, another approximately 10% of the oxygen of \(Fe_2O_3\) is removed. \(Fe_3O_4\) is converted into metallic iron and ferrous oxide. Resin, CO, \(H_2\), and the carbon of the solid residue do not play a major role in this, since evolution of liquid components practically ceases, little CO and \(H_2\) is evolved, and the gasification reaction

\[ C + CO_2 = 2CO \]

at these temperatures has very low rates. Apparently, the principal source of oxygen removal is methane. The rate of the reaction of \(CH_4\) with oxides in this temperature interval is rather high [4, 5], and one molecule of it can remove 4 atoms of oxygen from oxides.

Fig. 2. Dynamics of evolution of \(H_2\) (1, 2) and \(CH_4\) (3, 4) from FMM (1, 3) and peat (2, 4)

The temperature interval 650–900°C is characterized by an intensive process of accumulation of metallic iron. Here about 70% of the initial oxygen of iron oxide is removed.

Methane and hydrogen at these temperatures are active reducing agents, but they are released in insufficient amounts. Therefore, the main role must belong to solid carbon. The increased reactivity of the carbon of the solid residue and the catalytic influence of metallic iron on the reaction \(C + CO_2 = 2CO\) [4, 5]

Fig. 3. Dynamics of iron oxide reduction in FMM

Fig. 4. Degree of reduction of iron oxide during heat treatment of FMM

promote the release of large amounts of CO, which also reduces iron oxides to the metal.

Thus, during heat treatment of FMM at temperatures above 200°C, iron oxide begins to be reduced and by 550°C passes into \(Fe_3O_4\). Traces of metallic iron appear at temperatures above 300°C. The reduction of iron oxides proceeds successively from higher to lower:

\[ Fe_2O_3 \to Fe_3O_4 \to FeO \to Fe. \]

\[ \phantom{Fe_2O_3 \to Fe_3O_4}\searrow Fe \]

The main part of the oxygen of the oxides is removed through the gasification reaction of the carbon of the solid residue.

LITERATURE

1. A. D. Gotlib. The Blast-Furnace Process. Moscow, Metallurgiya, 1966.
2. N. A. Goldshtein. A Short Course in the Theory of Metallurgical Processes. Moscow, Metallurgizdat, 1961.
3. S. G. Rostovtsev, V. K. Simonov, O. A. Kostelov. In: Mechanism and Kinetics of Metal Reduction. Moscow, “Nauka,” 1970, pp. 24–31.
4. V. V. Veselov, V. P. Dorokhovich. In: Mechanism and Kinetics of Metal Reduction. Moscow, “Nauka,” 1970, pp. 108–115.
5. O. L. Kostelov, S. G. Rostovtsev. Izvestiya VUZov, Ferrous Metallurgy, 1966, No. 9, pp. 14–19.
6. O. A. Esin, P. V. Geld. Physical Chemistry of Pyrometallurgical Processes. Part 1. Sverdlovsk, Metallurgizdat, 1962.