GRSG Conference 2022: Orbit to Outcrop

Poster Title: Fourier-Transformed Infrared Spectroscopy for coal mine waste characterization

Author: Fardad Maghsoudi Moud

Abstract:

Fourier-Transformed Infrared (FTIR) is a technique to identify inorganic and organic materials based on their structure, chemical composition, and molecular bonding (e.g., O-H, Al-OH, Fe, Ti-O, C≡C, N, C=O, C-H). The FTIR has been used to identify clay (e.g., kaolinite, illite, muscovite) and rock-forming minerals (e.g., quartz), as well as organic materials (e.g., lignite).

Also, the FTIR has been used to identify the coal type, and predict the product quality range. However, less attention has been paid to the potential of the FTIR technique in the classification, and quantification of the coal content, Rare Earth Elements (REEs), acidification via pyrite, and the radioactivity of radioelements. As the reflectance values of FTIR spectra have a direct relationship with their mineral composition abundance, they could be used to explore different relationships between the FTIR spectra with associated elements and minerals.

In this study, the ability of the FTIR for quantification and classification of coal content, REEs, sulphuric minerals such as pyrite, and sphalerite, and radioactive elements such as Th is assessed. Pyrite could be seen within the coal seams and its destruction could lead to acid mine drainage and corrosion of pipelines. Since the mine should be reclaimed for agricultural usage, the presence of radioactive elements above a certain level would be potentially dangerous to agricultural products. Many studies reported the concentration of the REEs within the coal seams which could be seen as a second recovery product.

The study area is located at the Profen lignite mine, Germany. The sub-tropical plants were developed within catchment basins, during the Miocene. The decreased plants were covered by soil and glacial during the ice age. Then, the decreased plants sunk into swamps and decomposed into peats. The ground heat and pressure led to the hydrogen, oxygen, and nitrogen removal and formation of coal seams at the end of the Tertiary. The Profen area consists of different lithological units including Aquifer layers with gravel, sand, silt, clay layers with kaolinite, and muscovite in which the coal seam was formed.

Some of thirteen samples were collected from different lithological units of the waste dumps in the study area. The samples were used to collect the Diffuse Reflectance Infrared Fourier–Transformed Spectroscopy (DRIFTS-FTIR) spectra, X-ray Fluorescence (XRF), and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) multi-element values, and X-ray Diffractograms (XRD).

The FTIR spectra were interpreted and validated with the XRD outputs. A matrix consisting of FTIR wavenumbers and their corresponding reflectance value, and ICP and XRF elemental content were created. The principal component analysis (PCA) was used to cluster each ICP- and XRF- derived element with specific FTIR wavenumbers. Afterward, the clusters were interpreted and used to create a set of stepwise multiple linear regression (SMLR) models. The input data for the models were split into 70% training and 30% testing.

Then, the wavenumber range for each element for the training data was used to create predictive SMLR models of that element. The correlation coefficient (R²), and root-mean-square error (RMSE) were computed and used to evaluate and optimize the models before testing them. The optimization was done by checking the selected wavenumbers with their corresponding assigned molecular bands. Finally, the models were used for the training data to assess their performances.

The FTIR spectra interpretation resulted in the identification of lignite, kaolinite, muscovite, and quartz which were validated with the XRD. The ICP and XRF data demonstrated that the S, Fe, and As are correlated (approximately 0.65) with the clay layers containing the lignite and aquifers. This shows that pyrite has been formed within the lignite and due to the underground water table variation it has been dissolved and transported to the aquifer layers.

The presence of pyrite was validated via the XRD. Th, Ti, and Y were highly correlated (approximately 0.7) with each other and were present within the lignite and aquifers. Similar to the pyrite, sphalerite, and wassonite (TiS) was formed within the lignite seam and were destructed by water and transported to the aquifer layers. The presence of Ti was confirmed by the XRD as titanite (CaTiSiO₅) within both aquifer and lignite seam. The FTIR results showed the presence of quartz (780 cm^-1), kaolinite (940, 3696 cm^-1), muscovite (3600 cm^-1), pyrite (880 cm^-1), and lignite (1680-1750 cm^-1) which were validated by the XRD. The clustering results showed that the FTIR spectra could be used for the classification of different minerals/materials such as clay (Kaolinite and muscovite), coal/lignite, high S content units, and high Th content units. The models showed that the FTIR is a powerful tool to quantitatively predict pyrite, coal, REEs and Th contents with an acceptable R² and RMSE.