Ugapyev ¹A.A., Doshlov ²O.I., Gorokhov A.P.³, Lovtsov S.V.⁴,Gorshkov A.E.⁵

¹ PhD student of Chemical Technology Department at Irkutsk State Technical University, Irkutsk, Russia

² Professor of Chemical Technology Department at Irkutsk State Technical University, Irkutsk, Russia

³ Student of Chemical Technology Department at Irkutsk State Technical University, Irkutsk, Russia

⁴ Student of Chemical Technology Department at Irkutsk State Technical University, Irkutsk, Russia

⁵ Student of Chemical Technology Department at Irkutsk State Technical University, Irkutsk, Russia

Evaluation of Petroleum Pitch as binder for anode production

Carbon materials are widely used for many applications all over the world. Various unique properties along with enormous big number of discovered compounds allow to use these materials in all brunches of industry. One of the most large-capacity and significant application of carbon materials is the producing of aluminium. Aluminium is industrially produced by electrolytic reduction of alumina (Al₂O₃) dissolved in electrolyte which mainly consists of cryolite (Na₃AlF₆) at about 9600 C. Carbon is necessary for the reduction of alumina is supplied by the anode. Today, there are two anode designs, prebaked and Soderberg anodes.The prebaked anodes are made from the mixture of petroleum coke aggregate and coal tar pitch as a binder. The mixture is formed (pressed or vibrated) at a pre-defined temperature and then placed to the anode baking furnace and heated to the temperature in excess of 1100 °C[1].

The Soderberg electrodes are manufactured from small briquettes composed of the petroleum coke and coal tar pitch. The pitch binder content is about 25 wt%, what is considerably higher than the amount used in prebaked anodes (13 – 15 %) to ensure sufficient flow of the carbon paste. Briquettes are placed on top of the Soderberg anode block, where they are softened by the heat generated from the process. The paste passes slowly downwards through a rectangular steel casing and is baked into the solid composite. One of the most problematic issues of Soderberg and prebaked anodes is the environmental aspect. The high amount of pitch in the anodes leads to a release of potentially hazardous polycyclic aromatic compounds (PAH), especially benzo(a)pyrene, during the process.

Benzo(a)pyrene is part of polycyclic aromatic hydrocarbons. Benzo(a)pyrene is on the priority pollutant list published by the U.S. Environmental Protection Agency (EPA) because of it is a probable cancer-causing agent in humans. There is some evidence that it causes skin, lung, and bladder cancer in humans and in animals. The source of majority of benzo(a)pyrene compounds in anodes is coal tar pitch. The content of benzo(a)pyrene compounds in anodes is about 10021-12924 ppm [2].

The available solution of carcinogenic benzo(a)pyrenes releasing problem is using of petroleum-derived materials, with no content of danger PAH, as a binder for anodes production. There are a several ways to produce binders from petroleum, such as vacuum distillation of heavy fractions, thermal polycondesation and others, but all of them have significant disadvantages [3]. For example, the content of quinoline-insoluble components (QI) is clearly insufficient.

1. Methods of petroleum pith improvement

The above-mentioned methods of petroleum pitch producing aimed at try reach the properties required by the chemical transformations or the extracting of the useful substances from others. We propose to use the material prepared with some modifications excluding changing in chemical composition. The petroleum bitumen can be used as binder for anode mass production, but some difficulties were detected. Quinoline insoluble components are solid carbon particles found in coal tars produced during the process of coking. These components are important as they strongly influence the behaviour of pitches during anode production [4]. The petroleum bitumen does not have sufficient content of QI components and it can be solved by the adding of solid carbon outside.

For comparison, coal tar pitch was taken (grade V OKP 24 5351 0150). For petroleum pitch preparation petroleum bitumen and comminuted petroleum coke was taken. Petroleum coke produced at Angarsk petrochemical company by patented method of Irkutsk State Technical University. This petroleum coke has very low content of contaminant and high reactivity.

Preparation

The petroleum bitumen was heated to 175⁰C was followed by the addition of comminuted petroleum coke was performed. The mixture was intensively blended for an hour. After cooling of the sample was analyzed in accordance with requirements of ASTM D2415 - 98(2008), ASTM D7280 - 06(2011), ASTM D95 - 05(2010), ASTM D2318 - 98(2008) and the others. The resulted data are presented in following table:

Table II Parameters of petroleum pitch

Description of parameter	Value of parameter
Appearance	Solid (in the form of granules) pitch
Softening point, ^oC	90-100
Content of matters insoluble in toluene (α-fraction), %	34
Content of matters insoluble in quinoline (α1-fraction), %	18
Volatile content, %	60
Ash content, %	0,08
Water content in solid pitch, %, max.	traces

2. Conclusion and discussion

The Table II demonstrates that the petroleum pitch satisfy all requirements for anode binder and has much more value of main characteristics than coal tar pitch. The ash content is very low; it provides production of high-quality aluminium without contaminant. The content of quinoline insoluble components is more than sufficient for successful baking of anode. The homogeneous distribution of solid carbon particles at large provides to create stable three-dimensional structure. Solid carbon particles are going to be nucleus of crystallization during baking of anode. All these fact is evidence that petroleum pitch is real alternative for coal tar pitch in production of aluminium. There is no material in Russian Federation with similar properties. The plant for petroleum pitch production was designed at the territory of Angarsk Petrochemical Company‘s oil refinery. The capacity of this plant is about 35000 tons.

3. Acknowledgements

We would like to send our special thanks to Riccardo Ceccato, supervisor of study at University of Trento (Italy) for support and for fruitful discussions and help during the study. We also send my thanks to everyone at the Irkutsk State Technical laboratory and oil refinery laboratory in Angarsk(Russia).They did huge work on analyzing of the pitches and we thanks them for their help during the study.

4. References

[1] Z. I . Syunyaev, Production, Processing, and Use of Petroleum Coke [in Russian], Khimiya, Moscow (1973).

[2] European Chemicals Bureau, COAL-TAR PITCH, HIGH TEMPERATURE SUMMARY RISK ASSESSMENT REPORT, 2008

[3] I . R. Khairudinov, R. N. Gimaev, A. A. Khaibullin , et al. , Expirience with the production and Use of petroleum pitches [in Russian], TsNIITE Neftekhim, Moscow (1994).

[4] Brooks, J. D., Taylor, G. H., “The Formation of Graphitizing Carbons from the Liquid Phase”, Carbon, 1965, 3, pp. 185-193.