Rapid Detection of Zinc in Coal Ash by Laser Induced Breakdown Spectroscopy

The composition of coal ash refers to complete combustion of the minerals in the coal, producing a variety of metals and non-metallic oxides and salts, which is an important parameter when using coal. Coal has been widely used in the production and people's life, as an important energy substance. A large amount of coal dust (coal ash) from coal combustion was released into the atmosphere and interacted with various substances in the atmosphere to form haze. A series of physico-chemical reactions take place between metal oxides in coal ash and small droplets in the air, which result in the formation of haze. In this study, laser induced breakdown spectroscopy (LIBS) was employed to analyze the elements in coal ash. The experimental samples were provided from a steel company, which was divided into seven parts. Distilled water and 0.1 parts per thousand, 0.2 parts per thousand, 0.2%, 0.4%, 0.8%, 1% zinc sulfate solution were added into samples, which were labeled with number 1 similar to 7 respectively. In order to obtain a better LIBS signal, the sample was powdered. The water in the solution was used to thoroughly mix zinc with coal ash. In the experiment, the coal ash was pressed into 10 mm diameter and 10 mm thick coal ash blocks by using a tabletting machine. In order to get accurate elemental results, X-ray fluorescence spectroscopy were also employed for reference, and the original sample did not contain zinc. Due to the uncertainty of spectral analysis and wavelength shift phenomenon, qualitative analysis of element was inaccurate. To solve this problem, four kinds of high-purity elements including iron, calcium, titanium and aluminum were separately selected. Under the same experimental conditions, four measured elemental spectral lines were compared with the corresponding spectra in the NIST atomic spectral database. All the spectra in the experiment were corrected according to the wavelength difference or shift At this point, the elemental spectrum of pure elements can be aligned with the samples' spectrum. The samples then can be identified and confirmed when the characteristic line in the elemental spectrum is aligned with the spectrum in the samples. Because Al has similar chemical and physical property with target element, and Al is one of the major elements in coal ash and the crust, and has moderate spectral intensities. For quantitative analysis, the internal standard calibration method was used to determine the concentration of zinc in the samples. The results of simulating zinc-containing atmospheric aerosols were achieved by adding zinc to coal ash. Some other related metal elements, including iron, calcium, manganese, titanium and aluminum were also used to spike into coal ash to simulate atmospheric aerosols. The relative difference between the two methods is 1.78%, 3.39%, 5.17%, 0.20%. The reason for the difference may be due to the lack of resolution of the spectrometer or the impact of background noise, which could lead to the measurement error. Due to the limitations of laboratory conditions, we can't be sure whether the influence of matrix will affect the experiment results, and which will be further confirmed by future experiments. The linear correlation coefficient of zinc in coal ash was determined to be 0. 995 72, indicating that the rough estimation of zinc content can be achieved by the intensity of zinc in the spectrum The experiment concluded that LIBS technology can be used for the rapid detection of metal elements in coal ash, and this work provides a novel method for the detection of atmospheric environment based on the content of zinc. After establishing the calibration curve of the elements, the LIBS technique can be used to conduct a rapid and accurate quantitative analysis in the future.