Monitoring Fluorine in Metallurgical Fuel

Abstract

The environmental impact of metallurgical fuel, which includes various types of mineral fuel (such as coke, coal, lignite, peat, fuel shales, and their derivatives) is a continuing concern. In the combustion of metallurgical fuel, harmful materials (chlorine, fluorine, sulfur, and arsenic) are released, with consequent environmental degradation. Safety regulations regarding coal production include restrictions on the content of harmful impurities and their limiting permissible concentrations. On account of the broad distribution of fluorine in nature and industry, it is of special importance to monitor the fluorine content in metallurgical fuel. Physical methods based on the excitation of various radiation spectra permit determination of the fluorine content, without decomposition of the initial solid sample. However, such methods have problems in terms of sensitivity, precision, and the complexity of the equipment required. In the ion-chromatographic, ionometric, and other methods, the samples are broken down, and the fluorine content is determined in solution. Usually, high-temperature processes are used for decomposition of the samples: pyrohydrolysis, combustion in oxygen and in a calorimetric bomb, and alkaline fusion. In the present work, a selective method is developed for the ionometric determination of fluorine, by means of a fluorine-selective electrode. Samples of lignite, gas coal, semicoke, and coke nuts are investigated. The samples may effectively be decomposed by two-stage high-temperature melting with KNCO3. Fluorine is transferred to solution in the form of free fluoride ions by means of hydrolytic codeposition of interfering cations with chloride iron(II). The analytical procedure is described. It consists of decomposition of the sample and the ionometric determination of fluorine. The precision and truness of the proposed method is verified by variation in the sample size. In the samples considered, the fluorine content is within the limits typical of commercial coal. That indicates that the use of the samples in fuel is environmentally acceptable. The proposed method is promising for the monitoring of fluorine impurity in metallurgical fuel. It is selective and simple to use.