High-Temperature Mineral Phases Generated in Natural Clinkers by Spontaneous Combustion of Coal

Elisa Laita,Alfonso Yuste,Blanca Bauluz

doi:10.3390/min9040213

Abstract

The aim of this study is to analyze natural clinkers (= calcined clays by coal combustion) from a lower Cretaceous coal outcrop in Ariño (Teruel, NE Spain) in order to describe mineral and textural transformations produced during the spontaneous combustion of coal. To achieve this aim, samples were analyzed using X-ray diffraction and optical and electron microscopy. Spontaneous combustion resulted in the melting of the surrounding clays, with the generation of an Al–Si-rich vitreous phase. Subsequently, high-temperature phases crystallized from this vitreous phase. These new minerals are interesting due to their similarity with those formed during ceramic processes, used in the manufacture of stoneware and ceramic tiles, as well as in refractory ceramics, and with natural events such as metamorphic and igneous processes. The studied natural clinkers are composed of vitreous phase mullite, hematite, hercynite, cristobalite, quartz, pyroxenes, cordierite, gypsum, pyrite, and calcium oxides. A trend from hematite to hercynite composition indicates compositional variations at sample scale, which evidence d-spacing differences in hercynite and may be related to the Al and Fe content in hercynite depending on its texture. The mullite shows higher Si/Al ratio (1.21) than the theoretical composition (0.35), indicating that this mullite is more Si-rich. Three pyroxene-type compositions (diopside-type, ferrosilite-type, and a Ca–Al-rich pyroxene) were found. Both the mullite and the pyroxenes are nonstoichiometric.

Highlights

Spontaneous coal combustion is a frequent phenomenon that can take place in coal deposits when high quantities of these materials are stored for large periods [1,2]
Spontaneous combustion occurs in coal seams, stockpiles, and coal wastes when the rate of heat generation from the oxidation of organic matter exceeds the rate of heat dissipation [3]
The natural clinkers analyzed in this study present a scoriaceous appearance, with light and versicolor tones; they include clay and carbonaceous levels

Summary

Introduction

Spontaneous coal combustion is a frequent phenomenon that can take place in coal deposits when high quantities of these materials are stored for large periods [1,2]. Spontaneous combustion occurs in coal seams, stockpiles, and coal wastes when the rate of heat generation from the oxidation of organic matter exceeds the rate of heat dissipation [3]. Other factors could favor or hinder spontaneous combustion [4]. According to these authors, heat from the oxidation of inorganic coal-bearing phases, such as pyrite, could be the key factor in attaining the necessary heat for the self-ignition of coal. Several parameters influence the process of spontaneous heating and combustion in coal. These parameters relate to mining (panel dimension, ratio of coal, extraction, etc.), the geological parameters (faults, joints, etc.), and the coal chemical composition [10]

Objectives

Methods

Findings

Discussion

Conclusion