ANALYSIS OF CARBON MATERIALS USED AS ANODES IN FLUORINE PRODUCTION

Abstract

The relevance. Electrolysis of hydrogen fluoride of potassium difluoride molten KF·nHF system is the only industrial method for obtaining fluorine. In this case, medium-temperature (95–105 °С) electrolyzers with an electrolyte of KF·2HF composition with a current strength of 5 to 40 kA are mainly used. The electrolyzer is a very complex device, but many problems associated with the corrosivity of fluorine, hydrogen fluoride and other fluoride environments have been already solved. At the same time, there is a technological challenge to increase the service life of anodes made of carbon plates. Carbon has various forms depending on the structure of the material: graphite, soot, coke, diamond and others. The most stable carbon form for electrolysis of medium flowing at 95±105 °C was coke, wherein the carbon has a disordered X-ray-amorphous structure, so these plates called coke. Coke plates are obtained mainly from petroleum pyrolysis low-sulfur coke of a certain fractional composition and coal tar pitch. Recently, coke plates have become of extremely poor quality. Some components of the plates were replaced by cheaper ones, for example, oil pitch was replaced by coal tar. The quality of the coke plates produced by Russian manufacturers must meet the requirements of Technical Specifications (TS) 48-12-34-95 «Coke burned plates». TS establish the following indicators and requirements for them: apparent density – not less than 1,64 kg/dm3; compressive strength – not less than 58,8 MPa; porosity – no more than 21 %; ash content is not more than 0,6 %; the specific electrical resistance is (25–40) μOm·m. In the analysis of fluorine production in Russia, it was noted that the life of many coke plates that meet the requirements specifications is not the maximum. This testifies that the indicators set by this specification are not enough to assess the quality of a coke plate. Therefore, research for development of new quality tests of coke plates is relevant. Due to this we carried out a comprehensive analysis of the quality of coke plates produced by foreign companies in order to test the possibility of using them in fluorine production in Russia. The main aim is to analyze physical-mechanical, chemical and physicochemical properties of coke plates made by foreign firms and their quality. Methods: differential thermal analysis, X-ray structural analysis, atomic emission spectroscopy, scanning electron microscopy, physical-mechanical and electrical methods of analysis. Results. The authors have determined density, porosity, ash content, specific resistance, compressive strength of coke-oven plates of foreign manufacturers, which basically meet the requirements of TS 48-12-34-95. However, they differ. The compressive strength of Chinese plates of the «HS» mark exceeds the requirements of TS by almost 1,8 times. The porosity and ash content of Japanese plates are much lower than the requirements of TS. The elemental analysis of plates by atomic-emission spectroscopy method was carried out and it was established that the chemical compositions of coke plates are close. The total content of impurities in the investigated coke plates does not exceed 0,5–0,6 wt. %, the content of the main harmful sulfur impurity in the samples being not more than 0,1–0,3 %. The size of pores and their distribution in the plates were determined by scanning electron microscopy. In general, French and Chinese samples are characterized by pores with dimension in the range of 2,9–117 μm. German samples are characterized by pore size which is less than 5,9 μm. Almost 70 % of pores of Japanese samples correspond to three intervals: 2,9–5,9; 2,0–2,3 and 0,9–2,0 μm. X-ray diffraction analyses show that materials of all manufacturers have similar diffraction patterns, except for the samples of Japanese «GS-R», which are characterized by the presence of a significant amount of carbon with a graphite structure, determined by the position of the reflexes. This is obviously due to the various conditions of crystal formation in production of these coke plates. According to the results of differential thermal analysis, the authors determined the characteristics of the main processes occurring during heating the coke plates in the air. The coke plates we classified as well according to thermal stability.

Keywords

• coke plates
• mechanical data
• porosity
• electrical resistance
• ash content
• chemical composition
• differential thermal analysis
• scanning electron microscopy
• x-ray structural analysis
• atomic emission analysis