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Online LIBS analysis of ash in coal – Part 1

World Coal,

C.D. Gehlen and J. Makowe, Laser Analytical Systems and Automation GmbH, Germany, discuss the use of laser induced breakdown spectroscopy for online analysis of coal.

Laser induced breakdown spectroscopy (LIBS) is a non-contact analysis method that is used for the analysis of solid, liquid and gaseous samples (Miziolek et al, 2006; Noll, 2012). A lens focuses a pulsed laser beam onto the sample. Typically the duration of the laser pulse is in the range of 6 – 30 nanoseconds (nsec), while the interaction region on the sample is in the range of several hundred µm2. This leads to peak intensities of some GW/cm2 (Aydin et al, 2008). Such high energy density in the interaction region removes a small amount of the sample material within the first few nanoseconds and electronically excites and ionises this material, forming a plasma plume.

The emission of the plasma consists of atomic emission lines, the spectral wavelengths of which are characteristic of chemical elements. The intensities of these spectral lines correlate with the concentration of the elements in the sample. Quantitative and qualitative chemical analysis of the sample is possible by the observation of the plasma emission using an optical spectrometer with a sufficiently high dynamic range and wavelength resolution to separate the lines belonging to different chemical elements. The analysis is performed simultaneously for all chemical elements whose spectral lines lie in the detected spectral range of the spectrometer. One LIBS analysis is finished after some tenths of a microsecond (µsec), corresponding to the lifetime of the plasma plume. Using modern data acquisition electronics, up to 1000 LIBS measurements per second are possible (Bette et al, 2005). In other words, 1000 analyses can be performed within a second.

In this article, the results obtained by a fully automated coal analyser from Laser Analytical Systems & Automation GmbH (LSA) installed in a running coal washing plant are presented. The obtained element concentrations and ash content are compared to conventional routine analysis performed in the plant facility. Through this example, we demonstrate that the LSA analysers are able to measure remarkably low element contents in material streams. Furthermore, short-term variations in the material composition are made visible, in contrast to the performance achievable by routine analyses, performed offline in a laboratory.

Automated online analysis
Today, most routine analysis that occurs during production is performed by automatic or manual sampling in defined time intervals. The samples are sent to a laboratory and carefully prepared for subsequent analysis. In general, the time between the sampling and the availability of the results is in the range of several hours to several days, depending on the application and the distance between the facility and the laboratory. This approach makes continuous monitoring of the material flow quite demanding, as the short-term variations in raw materials or product composition is difficult to observe. Hence, the available data for the optimisation of the production process is not complete. This results in a loss of quality of the products or a loss of money due to the fact that the production is performed at a level much higher than required (e.g. production of coal with an ash content below the limit requested by the customer).

LSA provides fully automated online analysis systems that are based on LIBS. The analysis is performed contactlessly in 24/7 operation. Because of the continuous acquisition of up to 1000 measurements per second, a remarkably good level of accuracy is achieved in analysing and characterising huge mass flows. Typical applications of these systems are in the mining and recycling industry (primary and secondary raw materials) where huge mass flows with an unknown elemental composition are common (Gehlen, 2010; Aydin, 2006). The system can be installed above a conveyor belt that transports the material at several m/sec. The heterogeneities of the raw materials are balanced out by averaging several hundred or thousands of measurements, taken quasi-continuously across the whole material that passes the LIBS analyser.

There are two scenarios that are common for the analysis of material flows by the analyser system (Figure 1).

Figure 1: An example for two different kinds of applications after the characterisation of material flows by laser analysis. In the upper case, the material flow is analysed and the product is transported further. In the lower case, the material is analysed and grouped to divide it into two different grades (A, B).

The first scenario for online analysis of material flows is the continuous characterisation of the product. This is of interest for production processes like the washing of coal. The analysis results are sent to the process control to adapt the production process and to ensure the requested quality of the product. By this, the material can be continuously characterised and the production process can be adapted quickly and very precisely.

A secondary scenario is the characterisation of the material on the conveyor belt for a period of some sec/m. In this scenario, the material on the conveyor belt is divided into sections. An average value can be achieved for each section and sorting can be performed shortly after the analysis. This is an example of an application in the raw materials industry where the material is sorted, depending on the average content of the elements of interest to avoid the transport, crushing and preparation of e.g. dead rock.

The second part of this article, providing a case study analysis of coal at a coal washing plant, can be reached here.

This article was first presented at Coal Prep International 2013 and is presented here by permission of Penton Media. Coal Prep International 2014 will take place in Lexington, Kentucky between 18 April and 1 May 2014.

AYDIN Ü., NOLL R., MAKOWE J. (2006): “Automatic sorting of aluminium alloys by fast LIBS identification”, in ANGELI, J., (Ed.) 7th International Workshop Progress in Analytical Chemistry in the Steel and Metal Industries (Glückauf GmbH, Essen, Germany), pp. 309 – 314.

AYDIN, Ü., ROTH P., GEHLEN C.D. and NOLL, R. (2008): “Spectral line selection for time-resolved investigations of laser-induced plasmas by an iterative Boltzmann plot method” Spectrochimica Acta Part B: Atomic Spectroscopy (Vol. 63; Issue 10), pp. 1060 – 1065.

BETTE H., NOLL R., MÜLLER. G., JANSEN H.-W., NAZIKKOL, C. and MITTELSTÄDT, H. (2005): “Highspeed scanning LIBS at 1000 Hz with single pulse evaluation for the detection of inclusions in steel”, J. Laser Appl. (Vol. 17), pp. 183 – 190.

GEHLEN C. (2010): “Snapshot”, AT Mineral Processing (Vol. 51), pp. 34 – 36.

MIZIOLEK A.W., PALLESCHI, V. and SCHECHTER, I. (2006): Laser-Induced Breakdown Spectroscopy (LIBS): Fundamentals and Applications (Cambridge University Press, Cambridge, UK).

NOLL R. (2012): Laser-Induced Breakdown Spectroscopy: Fundamentals and Applications (Springer Verlag, Berlin and Heidelberg, Germany).

Written by C.D. Gehlen and J. Makowe, Laser Analytical Systems and Automation GmbH.

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