Metallurgist, Vol. 56, Nos. 7–8, November, 2012 (Russian Original Nos. 7–8, July–August, 2012)

PROCESSING PROPERTIES OF OXIDE-CARBON BRIQUETTES WITH A COMPLEX BINDER

I. L. Gonik, O. P. Bondareva, N. A. Novitskiy, and A. G. Tyupina

UDC 669.01:622.788.32

The oxide-carbon briquette (OCB), being a raw material that includes a complex binder and a carbonbearing reducing agent, is a relatively new composite charge material. Its use can help recycle iron-bearing waste products that exist in large amounts at metallurgical plants but often go unused. Studies were made of the mechanical properties of oxide-carbon briquettes. There are currently no standards for testing briquetted charge materials, so the mechanical properties of OCBs were evaluated by using the standard methods employed to test coal briquettes for drop strength, compressive strength, and moisture absorption. Keywords: charge materials, metallurgy, charge, briquette, mechanical properties, reduction of iron, mill scale, waste products.

Technologies for briquetting charge materials in metallurgy began to be developed in the middle of the Twentieth Century, although the available data were and still are insufficient to fully understand the reduction reactions which take place in a briquette. These processes thus need to be studied by experimental investigation and factory testing. There are currently no universal standards either for briquetted charge materials or for the briquetting process itself. Thus, the processing properties of briquettes vary and are controlled by the company that makes the briquettes [1]. Despite this situation, the charge material that is briquetted should have processing characteristics which are at least the equal of the corresponding characteristics of conventional charge materials – sinter, pellets, etc. Cold extrusion has become the most widely used technology for briquette production, thanks to its accessibility. In contrast to sintering, for example, cold extrusion makes it possible to partially or completely do away with high-temperature treatments. The extent to which this becomes possible depends on the final properties required of the briquettes. The energy that is saved compared to sintering is one of the biggest advantages of the cold extrusion. The Materials Engineering Department of Volgograd State Technical University conducted studies of the processing properties of oxide-carbon briquettes (OCBs) obtained by cold extrusion. A technology for production and use of the briquettes has been developed by researchers in the department [2]. The briquette is a new charge material which has processing properties that are comparable to those of the charge materials traditionally used in blast-furnace smelting (pellets and sinter). Those properties should allow for transport of the charge material to the site of use, its storage under different external conditions, and its charging into the furnace in which it will be processed. Among the indices that characterize the quality of a charge material are its compressive strength, drop strength, and moisture absorption. Practical determination of these properties makes it possible to evaluate the ability of a briquetted charge material to withstand abrasive-impact, impact, and crushing loads during storage and use. In fact, these are the very mechanical indices that in large part determine the practical utility of briquettes and their ability to be stored under different atmospheric conditions at different metallurgical plants. Volgograd State Technical University, Volgograd, Russia; e-mail: [email protected]. Translated from Metallurg, No. 8, pp. 35–38, August, 2012. Original article submitted February 20, 2012.

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0026-0894/12/0708-0570 ©2012 Springer Science+Business Media New York

Fig.1. Oxide-carbon briquette (OCB).

TABLE 1. Composition and Processing Properties of the OCBs in the Test Series Component-by-component composition, % carbon

scale

binder

Extrusion pressure, MPa

Drying (200°C)

Tests for drop strength

Firing (800°C)

Amount of fines, %

–

97

7.60

2.56

2

95

7.69

2.62

5

92

8.18

3.00

7

90

8.98

3.49

10

87

11.40

4.53

3

12

Tests for compressive strength

Compressive strength, MPa

–

97

11.3

38.8

2

95

9.2

34.2

5

92

6.2

27

7

90

4.7

22.5

10

87

3.5

16.2

14

71

5

–

13.2

13

72

10

–

28.6

3

12

12 12

73

15

–

32.1

10

70

20

–

38.3

Determination of OCB moisture absorption

10

87

Degree of moisture absorption, % 8

6.8

2.6

10

6.3

2.3

12

5.4

1.7

14

4.8

1.4

16

4.1

1.2

18

3.2

1.0

3

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Fig. 2. Dependence of the amount of fines after the dropping of an OCB on the carbon content of the briquette: 1) after drying at 200°C; 2) after firing at 800°C.

Fig. 3. Dependence of briquette compressive strength on OCB composition: 1) after drying at 200°C; 2) after firing at 800°C.

Test specimens of briquettes were prepared to determine the processing properties of this charge material. The main components were mill scale from the rolled-product shops at the Volgograd Metallurgical Plant Krasnyi Oktyabr and a carbon-bearing reducing agent – pieces of depleted graphite electrodes. The chemical composition of the mill scale, wt.%: Fetot = 72.49; FeO – 56.32; Fe2O3 – 41.0; S – 0.008; P – 0.009. We used the multi-component oxide system SiO2–B2O3–CaO–K2O as the binder. This system ensures that the briquettes will have the necessary strength within the range 600–1400°C (the briquettes’ melting point) [3]. The mill scale, reducing agent, and binder were crushed to the 0.3–1.0 mm fraction. After mixing of the crushed materials and the addition of an aqueous solution of sodium silicate, we prepared cylindrical briquettes with a diameter of 45 mm and a height of 25 mm. The specimens were obtained by semi-dry extrusion with an extrusion pressure of 12 MPa. Figure 1 shows the OCBs studied here and Table 1 shows their composition. We therefore prepared two series of specimens having the same weight but different amounts of carbon-bearing reducing agent. The first batch of specimens was dried in air inside a drying cabinet over 1.5 h at 200°C. The second batch was fired at 800°C for 1 h to obtain the necessary strength and then cooled in open air. The mechanical properties of the OCBs were determined by using a method employed to test coal briquettes [4]. Mechanical strength was evaluated by dropping specimens onto a metal plate from a height of 1.5 m and screening the resulting material to determine the amount of fines that was formed. Table 1 shows the results of the drop tests. Drop strength 572

TABLE 2. Results of Physico-Mechanical Tests of Briquettes Made by the Ekomashgeo Strength, MPa Type of treatment design

actual

TU requirements

4.2

8

Drying (no lower than 20°C)

3.83

7.4

Hygrothermal (60°C, moisture content 98%)

6.0

6.9

Natural curing, 1 day/5 days

2.3/5.5

11.1/5.8

Fig. 4. Dependence of OCB moisture absorption on extrusion pressure.

depends on the composition of the briquette – especially the carbon content of the OCB – and on the type of heat treatment administered to the briquettes after extrusion. Figure 2 shows the effect of the carbon content of the briquettes on the quantity of fines formed after they were dropped. The compressive strength of the briquettes was determined by testing them in accordance with a method used for coal briquettes [4]. This entailed preparing a series of specimens of briquettes with the same composition as the OCBs used in drop tests. Another series of specimens with a different binder content was also prepared for testing (see Table 1). These tests were conducted on a hydraulic press until the specimens fractured. Figure 3 shows the dependence of the briquettes’ compressive strength on the composition of the OCBs. The compressive strength of the OCBS was determined in relation to their binder content. The amount of binder in the briquettes was varied from 5 to 20%, and their strength increased accordingly from 13.2 to 38.3 MPa. This shows the strong effect of the complex oxide binder on the strength of the OCBs after firing (see Table 1). For comparison, Table 2 shows the strength characteristics of briquetted charge material obtained by the technology used at the Ekomashgeo in Tula [1]. Moisture absorption by briquetted charge materials is an important index of their service properties. This index determines the conditions under which the materials will be stored and transported, in addition to their frost resistance. Moisture absorption by the OCBs was evaluated by the gravimetric method (see Table 1) in accordance with the approach recommended for coal briquettes [5]. The main factors that affect the degree of moisture absorption by OCBs are extrusion pressure (Fig. 4) and the briquettes’ binder content, since extrusion pressure determines the density of the composite and the binder keeps out moisture that could fill the pores of an OCB. Conclusions 1. Results obtained from determination of mechanical properties in drop tests showed that the average quantity of fines formed in such tests is four times smaller for fired briquettes than for briquettes after drying. 573

2. Compressive tests showed that the average strength of the OCBs after firing (more than 20 MPa) is significantly greater than after drying (from 4 to 11 MPa). A increase in the content of carbon-bearing reducing agent from 0 to 10% in OCBs with a binder content of 3% was accompanied by a decrease in the strength indices from 11 to 3.5 MPa after drying and from 38 to 16 MPa after firing. 3. The low values of moisture absorption (1–3%) obtained for the OCBs confirms that moisture content has a negligible effect on the service properties of the fired briquettes. 4. The mechanical properties of OCBs meet the requirements established for briquetted charge materials destined for metallurgical use [6].

REFERENCES 1. 2. 3. 4. 5. 6.

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V. I. Kotenov, E. Yu. Barsukova, S. G. Murat, et al., “Production and use of metallurgical briquettes at the Tulachermet,” Metallurg, Special Issue, 3–36 (2005). I. A. Gonik, V. P. Lemyakin, and N. A. Novitskii, “Features of the use of briquetted iron-bearing wastes,” Metallurg, No. 6, 36–38 (2011). E. E. Ageev, Yu. A. Bondarev, V. G. Bulgakov, et al., Russian Federation Patent No. 2102494, MPK6 C21B11/00, 13/00, “Method of producing cast iron and steel in metallurgical furnaces,” subm. 02.27.1995, publ. 01.20.1998. GOST 21289-75, Coal Briquettes. Method of Determining Mechanical Properties. GOST 21290-75, Coal Briquettes. Method of Determining Moisture Absorption. B. M. Ravitch, Briquetting in Nonferrous and Ferrous Metallurgy [in Russian], Metallurgiya, Moscow (1975).