ISSN 1067-8212, Russian Journal of Non-Ferrous Metals, 2009, Vol. 50, No. 1, pp. 30–32. © Allerton Press, Inc., 2009. Original Russian Text © E.M. Gil’debrandt, V.K. Frizorger, E.P. Vershinina, 2009, published in Izvestiya VUZ. Tsvetnaya Metallurgiya, 2009, No. 1, pp. 27–29.

METALLURGY OF NON-FERROUS METALS

The Effect of the Granulometric Composition and Content of a Coke Charge on the Viscosity of Pitch–Coke Compounds E. M. Gil’debrandta,*, V. K. Frizorgerb,**, and E. P. Vershininaa,* a

Institute of Non-Ferrous Metals and Materials Science, Siberian Federal University (ITsMiM SFU), pr. Krasnoyarskii Rabochii 95, Krasnoyarsk, 660025 Russia * e-mail: [email protected] b OOO Russkaya Inzhiniringovaya Kompaniya, ul. Pogranichnikov 16, Krasnoyarsk, Russia ** e-mail: [email protected]

Abstract—The viscosity of pitch–dust compositions containing 30 and 50% of the coke dust with a particle size of –0.124 mm is measured in a temperature range of 480–750 K. It is found that the viscosity increases as the coke content and dispersity of the coke dust particles increase. The obtained results make it possible to predict the behavior of an anode mass with a variation in the temperature and composition of a coke charge. DOI: 10.3103/S1067821209010088

The anode of an aluminum electrolyzer is the composite carbon material containing the coal–tar pitch and petroleum coke. In recent years the ratio between these components has varied. In a “dry” Soderberg anode, the pitch content decreases from 32 to 28%, and this tendency remains. The coke charge consists of various fractions: coarse, mean, and dust fractions. The last fraction in various formulations of the charge amounts to 40–60% and is checked by the separate masses, depending on the possibilities of the equipment the anode mass workshop. The pitch–coke composition (PCC) is in the liquidphase state during the two most important operating stages, namely, in mixing the coke charge with the liquid pitch while producing the anode mass and in obtaining the body of the Soderberg anode. Here, the main physicochemical processes forming the service properties of the anode occur: the coke particles with the pitch, the filling of pores, the formation of intergranular strata, and the precipitation of large-sized coke particles. All these phenomena proceed in a medium with a certain viscosity. Investigations of the viscosity µ of PCC are reported in [1, 2]. Measurements are performed on a compressive viscometer at T = 348–473 K for the “fat” anode mass and pitch–dust compositions (PDC) containing 10, 25, 40, 50, 60, and 70% of the mill’s petroleum coke dust, with 80% of the content involving the particles – 0.074 mm in size. It is shown that the temperature relationships of the value µ are described by the Frenkel equation. It is of interest to determine the effect of the size of coke particles on the viscosity of compounds. The last is measured with an oscillatory viscometer designed by the Institute of Metallurgy, Ural Division, Russian

Academy of Sciences [3] in the range T = 480–750 K. In the present article, the results of investigating the viscosity of the PDC based on the high-temperature pitch with the softening temperature Ts = 386 K (by Metler) and the petroleum coke are presented. The dust fraction is presented by three particle sizes of –0.125 + 0.08, –0.08 + 0.044, and –0.044 mm. The content of the coke dust (CD) in the PDC is CCD = 30 and 50 wt %; therefore, the coke-to-liquid pitch ratio is 3/7 and 1/1. Compositions of the system under investigation are given in the table, and the results of viscosity investigations are given in Fig. 1. A wide temperature minimum (550–700) K is noted for compositions with 30% coke. The least values of µ are 10 mPa s for large fractions of the coke and 15– 20 mPa s for small fractions (–0.044 mm). It can be concluded from the slope parts of dependences that the viscosity at the given temperature increases as the dispersity increases. The compositions with 50% coke dust have a narrower minimum of viscosity. For them, the effect of the coke particle sizes is clearly pronounced. Their Formulations of pitch–dust compositions

30

Composition

CCD , wt %

1 2 3 4 5 6

30

50

Coke granulometric composition, mm –0.125 + 0.08 –0.08 + 0.044 –0.044 –0.125 + 0.08 –0.08 + 0.044 –0.044

THE EFFECT OF THE GRANULOMETRIC COMPOSITION AND CONTENT µ, mPa s 600

µmin, mPa s 1800 1600 1400 1200 1000 800 600 400 200 0 30 35

(a) 500

3

400 300

1

200

2

100 0 450 900 800 700 600 500 400 300 200 100 0 450

500

550

6

600

650

700

750

800

I

II HTP

MTP

40

45

50

55 60 CCD, wt %

Fig. 2. Viscosity versus the content of the coke dust of the – 0.074 mm fraction.

(b)

coefficients R2 = 0.99 and 0.98, respectively. In the second part, this dependence is described by the logarithmic equation for compounds based on the MTP as ln µII = –13.9 + 0.38CCD and on the HTP as ln µII = −9.05 + 0.30CCD, with high approximation coefficients R2 = 0.98 and 0.96, respectively.

5 4

500

31

550

600 650 T, K

700

750

800

Fig. 1. Viscosity versus temperature for the PDC containing 30% (a) and 50% (b) of the coke dust. (1–6) correspond to composition of Table.

decrease leads to a sequential increase in viscosity from 50 mPa s (for a fraction of –0.125 + 0.08 mm) up to 350 mPa s (–0.044 mm). As the coke content increases from 30 to 50%, the value of µmin increases significantly (by a factor of five for the compound based on a fraction of −0.125+0.08 mm and by a factor of 17 for a fraction of –0.044 mm). To obtain the dependence of the viscosity on the CCD content of the coke dust, additional investigations were carried out. For them, we prepared compositions based on two types of pitch: the mean-temperature (MTP) and high-temperature (HTP) pitches, with Ts = 361 and 386 K, respectively. Coke dust of a fraction of –0.074 mm with a content of 30, 40, and 50%, and, in some cases, over 50%, was used. The measurements were performed at T = 600 K in a region of minimum viscosity. The results are presented in Fig. 2. To practically apply the obtained data, it is easiest to divide the general dependences of viscosity on the CD content into two parts: CCD = 30–50% (I) and CCD > 50% (II). In the first one, the dependence µ = f (CCD) can be described by the linear equations for compounds based on the MTP as µ = –168.7 + 5.9CCD and on the HTP as µ = –511.7 + 17.2CCD, with high approximation RUSSIAN JOURNAL OF NON-FERROUS METALS

It is known that an increase in the part of the dust fraction in the coke charge of the anode mass of the Soderberg anode is conditioned by an increase in the pitch content. This is related first and foremost to the requirements imposed upon the flowability of the mass for filling an anode casing. The obtained results demonstrate that a decrease in the pitch content in the “dry” and, especially, “colloidal” [4] anode should be accompanied by a variation in the granulometric composition of the charge and mixing temperature in order to provide the necessary flowability of the anode mass. In our opinion, it is useful to consider the pitch–dust compositions as one specific class of the structured dispersed systems. Their flowability is determined by the presence of the liquid dispersed medium: the coal pitch. In its turn, the value of µ for pitches with a high softening temperature of T < 500–550 K is greater than that of materials with lower Ts [5]. However, at a higher temperature (up to 650–700 K), both of them have the approximately equal and minimum viscosity. The coke fraction is the solid dispersed phase of the pitch–dust composition. Its total content and granulometric composition affect the viscosity of the system. The form of dependences, which is described for pitches, remains for pitch–coke compositions. At low temperatures and equal coke content, the system based on the pitch with high Ts and a larger dispersity of the particles has a higher value of µ. At CCD < 30%, the value of µmin is virtually independent of the particle size if their size is –0.124 + 0.08 mm. Greater viscosity is noted for the system with the –0.044 mm coke particles. It is evident that the content and granulometric composition of the CD start to significantly affect µ with an increase in CCD > 30%.

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GIL’DEBRANDT et al.

In the practice of anode production, varying the pitch and coke content and its granulometric composition is possible. We believe that the flowability of this system is associated with the presence of the dust fraction of the coke; i.e., the presence and composition of coarse-dispersed fractions have no effect on the flowability of the composition. To predict a variation in viscosity, we assume that it is useful to use certain methodic approaches. First, the coke content should be evaluated by the value of the solid-to-liquid ratio, which is accepted for dispersed systems or, in our case, the ratio of dust fraction to pitch. In this case, with an increase in this ratio by more than 2/3 (40% of the coke and 60% of the pitch), a significant increase in the viscosity should be expected. Secondly, if the ratio of the content of the –0.044 mm fraction to the pitch is more than 2/3, the viscosity of the system can also significantly increase. CONCLUSIONS (i) In a wide range of compositions of the coke charge and temperatures, the viscosity of the pitch–dust

compositions as the host substance of the anode mass of the Soderberg anode is determined. (ii) The obtained results made it possible to predict the behavior of the anode mass with the variation in the temperature and composition of the coke charge. REFERENCES 1. Torklep, K., Light Metals, 1998, p. 455. 2. Gil’debrandt, E.M., Frizorger, V.K., Vershinina, E.P., and Kravtsova, E.D., Izv. Vyssh. Uchebn. Zaved., Tsvetn. Metall., 2008, no. 6, p. 27. 3. Arsent’ev, P.P., Yakovlev, V.V., Krasheninnikov, M.G., et al., Fiziko-khimicheskie metody issledovaniya metallurgicheskikh protsessov (Physicochemical Methods of Investigation of Metallurgical Processes), Moscow: Metallurgiya, 1988. 4. Frizorger, V.K., Khramenko, S.A., and Anushenkov, A.N., Tsvetn. Met., 2007, no. 12, p. 57. 5. Yanko, E.A., Anody alyuminievykh elektrolizerov (Anodes of Aluminum Electrolyzers), Moscow: Ruda i Metally, 2001.

RUSSIAN JOURNAL OF NON-FERROUS METALS

Vol. 50

No. 1

2009