EFFECT OF THE DEGREE OF GRINDING OF ORE AND FLUXES ON THE QUALITY OF FUEL-SMELTING MATERIALS BASED ON PEAT

I. V. GEBLER, S. I. SMOLYANINOV, A. G. NEZNANOV

In connection with the problem of metallurgical fuel for the iron ores of the West Siberian basin, the Department of Chemical Technology of Fuel of the Tomsk Polytechnic Institute is conducting work to investigate the possibility of using peat for these purposes, reserves of which are large in the area of the iron-ore deposit. The authors primarily envisaged the possibility of using air-dry peat in the form of fuel-smelting materials in low-shaft blast furnaces with oxygen-enriched blast. The task of obtaining fuel-smelting materials is connected both with the need for close contact between the ore and the reducing agent for smelting in low-shaft furnaces, and with the need to agglomerate the dusty ores of the West Siberian basin. Previously published works [1, 2] dealt with the influence of forming pressure, the initial and final moisture content of peat, and the influence of additions of iron ore and fluxes to raw peat at the moment of forming on the mechanical strength of peat moldings at different stages of pyrolysis.

It should be said that the task of agglomerating dusty ores concerns not only the ores of the West Siberian basin. Modern ferrous metallurgy is impossible without the use of artificially agglomerated charge components, the share of which is continuously increasing. The following methods of agglomerating ore components of the charge are known: a) briquetting by molding and pressing, with binders and without them, and b) various methods of sintering (agglomeration). The sintering methods have become most widespread. The addition of sinter to the blast-furnace charge up to 30–35% has a positive effect on the economy of the process both as a result of coke saving due to better heat exchange and due to a reduction in the carryover of ore with flue dust. Operation on a charge with a sinter content above the indicated figure is considered economically disadvantageous, since the costs of sintering are not covered by coke savings [3].

The production of ferro-coke may also be included among the methods of agglomerating ore fines; successful experiments in obtaining it have been carried out both abroad and in the Soviet Union. This method requires the presence of well-caking coals, and the presence of iron oxides in the coking charge adversely affects the yield of chemical coking products.

Previous experiments were carried out with the ore and fluxes added separately. It was of interest to study the effect of these additions when present together, in order to obtain fuel-smelting materials with the required ratios of fuel, ore, and fluxes. In addition, in the experiments described above, the mechanical strength of the specimens was sufficient only for low-shaft furnaces.

In experiments on the addition of fluxes it was noted that the adopted coarseness of grinding of the additions, 1–0 mm, has an unfavorable effect on the shrinkage of the specimens during coking, and that the best strength results are achieved with finer grinding of the additions.

The degree of comminution of the fillers—ore and fluxes—can evidently have an effect not only through improvement of the physical homogeneity of the mixture, which causes more uniform shrinkage. Iron oxides and limestone are not inert additions in the chemical sense. On the contrary, we may expect active interaction of these components at least with the humic acids of the peat. In this case, increasing the degree of comminution of the ore and fluxes will lead to a large increase in the contact surface of the reacting substances and, undoubtedly, will affect the properties of peat briquettes.

Fig. 1. Abrasion of fuel-smelting materials (percentage yield of the class below 3 mm) in the air-dry (a/d) state at various final heating temperatures (figures by the curves), as a function of the particle size of the ore and flux.

Experiments were carried out in which ore was added to raw peat at the moment of forming in amounts of 3.6 and 10%, with the corresponding calculation of

quantity of fluxes [4]. Ore and fluxes of different degrees of grinding were used: 1.00–0.0 mm, 0.25–0.0 mm, 0.15–0.0 mm, and 0.088–0.0 mm.

The general procedure was similar to that described above [1, 2]. In this series of experiments, the porosity of the specimens and the percentage reduction of iron according to N. P. Chizhevskii [5] were additionally determined.

The results of the experiments show that changing the degree of grinding of the ore and fluxes has an extremely strong effect on the mechanical strength of air-dry peat and especially of coked specimens.

With increasing grinding of the additions from 1 to 0.088 mm, the abrasion of air-dry specimens with additions of 6 and 10% ore (Fig. 1) decreases from 1.8–2.4% to 1.0–1.4%, respectively. For specimens heated to 400°, this index decreases from 87.5 to 5.3–6.7%, and for specimens coked at 800°C—from 100% to 2.5–9.0%, respectively.

The resistance to crushing changes in the same order (Fig. 2). For air-dry specimens, the resistance to crushing

Fig. 2. Resistance to crushing of fuel-smelting materials in the air-dry state (a/d) and at various final heating temperatures (figures by the curves), depending on the particle size of the ore and flux.

varies within the range 200–400 kg/cm². For specimens heated to temperatures of 400 and 800°, the resistance to crushing increases sharply from values below 20 kg/cm² to 100–150 kg/cm². The change in the strength index follows the same pattern.

When testing specimens of fuel-smelting materials for reducibility of iron, it was found that in the coke from these materials, fired at 800°, the percentage of reduced iron is 60–70% (with the addition of 6–10% ore to raw peat). The degree of reduction decreases with increasing grinding of the ore (Fig. 3) from 70 to 60%, which is directly related to porosity and, evidently, also to the conditions of indirect reduction.

Thus, the porosity of coke (800°) from the indicated materials (Fig. 3) decreased from 55 to 45% (for a specimen with a 6% ore addition) when the ore grinding size was changed from 1 to 0.088 mm. For specimens with a 10% ore addition, this index is 63 and 58% at ore sizes of 0.25 and 0.088 mm, respectively.

Thus, increasing the fineness of grinding of the ore and fluxes added to raw peat at the moment of molding very strongly improves the mechanical strength of fuel-smelting materials at all stages of pyrolysis.

Fig. 3. Reducibility of iron and porosity of samples of fuel-smelting materials heated to 800°, as a function of the particle size of the ore and flux, mm.

For additions of ore not exceeding 6% of the raw mass of peat (with the corresponding amount of fluxes), the degree of grinding may be taken as 0.15 mm, since further grinding has little effect on strength. For larger additions of ore, the degree of its grinding should apparently be taken as greater; however, the final choice in this case must be made together with an economic assessment.

For a comparative evaluation of the results obtained, under equivalent conditions we determined the mechanical strength of coke from the Kemerovo Coke-Chemical Plant and of coke produced at the Kuznetsk Metallurgical Combine. In addition, the strength of charcoal (birch, retort) and the strength of peat samples without additives were determined. Thus, the abrasion of KMK coke proved to be 2.8–3.0%, that of coke from the Kemerovo Coke-Chemical Plant 3.2–3.3%, and that of charcoal 18–19%. The crushing resistance of KMK coke is 250–300 kg/cm². The abrasion of peat samples without additives in the air-dry state is 1.2%, at 400° it is 12%, and at 800° it is 4.2%; the crushing resistance is 300, 26, and 70 kg/cm², respectively.

Fuel-smelting material with an addition of 10% ore to the raw mass of peat has, after removal of volatiles at 800°, a carbon-to-iron ratio of approximately 0.9 : 1, and with an ore addition of 6%, 1.6 : 1. In the first case the indicated ratios are normal for the modern blast-furnace process; in the second, they are quite permissible, since in the smelting of some special grades of cast iron the coke consumption even exceeds this value.

As can be seen from Figs. 1 and 2 and from the data presented, in the air-dry state the fuel-smelting materials are equal to good metallurgical coke in crushing resistance, and surpass it in abrasion resistance. This circumstance is of particular importance when air-dry materials are charged into a blast furnace, since a considerable share of the destructive forces is applied to the fuel during its transport and charging onto the grate.

For the gas permeability of the blast-furnace burden, the abrasion resistance of the burden components is of great importance. This index for the fuel-smelting materials noticeably exceeds the abrasion resistance of coal coke only for specimens with a 10% ore addition heated to 800°. However, this does not give grounds to believe that such materials cannot be used in large blast furnaces. The gas permeability of a column of peat fuel-smelting materials, even those having somewhat higher abrasion than coal coke, may prove, owing to the homogeneity of the material, to be much higher than that of a multicomponent burden in the ordinary blast-furnace process.

All the more certain is the successful use of peat fuel-smelting materials in furnaces of smaller volume, as is clearly evident from a comparison of the mechanical strength of the indicated materials and charcoal, which, as is known, is still used up to the present time.

It is interesting to note that fuel-smelting materials have, in comparison with pure peat, increased mechanical strength in the temperature interval of 400°, where the latter, as a rule, has a minimum of strength. This is important both when these materials are used in the blast furnace in the air-dry state and when they are coked in continuous furnaces.

In specimens heated to a temperature of 800°, the percentage of iron reduced to metal is 60–70%, and the porosity is 45–60%.

Since such an amount of ore can be added to raw peat during forming as will ensure normal ratios between ore and fuel in the blast-furnace process, the above-described method of obtaining fuel-smelting materials can also be considered as a method of agglomerating dusty ores and ore fines.

The production of such fuel-smelting materials can apparently be carried out on ordinary peat-forming machines, with additional costs being minimal.

An indisputable advantage of the proposed fuel-smelting materials is also the fact that already at 800° more than half of the iron is reduced to the metallic state.

Conclusions

1. Fine grinding of the ore and flux, when they are added to raw peat at the moment of forming, greatly increases the mechanical strength of the formed peat at all stages of pyrolysis.

2. Fuel-smelting materials possess mechanical strength that permits their use in low-shaft furnaces and also, apparently, in modern large-volume blast furnaces.

References

1. Gebler I. V., Smolyaninov S. I., Martynov A. F., and Severin B. M. Influence of pressure and moisture on the properties of peat as metallurgical fuel. Peat Industry, No. 8, pp. 16–20, 1959.

2. Gebler I. V., Smolyaninov S. I., Potapenko V. E., Kosolapov V. I. Influence of additions of ore and fluxes on the properties of peat as metallurgical fuel. Proceedings of TPI, vol. 111, 1961.

3. Luyken. Preparation of raw materials for blast-furnace smelting. Metallurgizdat, 1959.

4. Pavlov M. A. Calculation of blast-furnace burdens. Metallurgizdat, 1948.

5. Chizhevsky N. P. Selected Works, vol. 1, Publishing House of the Academy of Sciences of the USSR, 1958.