Advances in the characterization of industrial minerals

Edited by George E. Christidis


The use of minerals by man is as old as the human race. In fact the advancement of human civilization has been intimately associated with the exploitation of raw materials. It is not by chance that the distinction of the main historical eras is based on the type of raw materials used. Hence the passage from the Paleolithic and Neolithic Age to the Bronze Age is characterized by the introduction of basic metals, mainly copper, zinc and tin, to human activities and the Iron Age was marked by the introduction of iron. Since then the use of metals has increased and culminated in the industrial revolution in the mid-eighteenth century which marked the onset of the industrial age in the western world. However, during the past 50 years, although metals were equally important to western economies as they had been previously, the amount of metals extracted annually in western countries has decreased significantly and metal mining activity shifted mainly to third world countries (in Africa, South America, Asia) and Australia, due to economic and environmental constraints. At the same time the role of industrial minerals has become increasingly important for the western economies and today, in developed EU countries, the production of industrial minerals has surpassed by far the production of metals. In some EU countries, metal mining activities have stopped completely. The importance of industrial minerals is expected to increase further in the future.

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    Industrial minerals and rocks are Earth materials utilized because of their characteristic physical and/or chemical properties and not because of their metal content and which are not energy sources. According to this definition they cover a broad spectrum of minerals and rocks which form at all geological environments. The relative importance of industrial minerals to the economy of the various countries reflects the economic maturity of that country and today they constitute the most important raw materials exploited in the developed industrialized countries. The unit value of many industrial minerals is small compared to that of metals and depends on the geographic site from which they are extracted, i.e. they have a large place value. The small unit value also dictates the extent of processing and beneficiation. As they are used by the industry because of their physical and chemical properties, different industrial minerals may often compete for the same applications. In some cases the industrial practice requires production of synthetic industrial minerals, such as zeolites and diamonds, with tailored properties and therefore high added value. Due to increasing environmental awareness, there is need for utilization of waste materials from mining activities, which are also in the mineral form and can thus be considered as industrial minerals. The economic significance of industrial minerals is expected to increase further in the future.

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    Industrial minerals include both common and uncommon minerals and some rocks. Construction raw materials such as aggregates are included. Industrial minerals can be categorized according to their market characteristics as bulk minerals, either within or outside a vertically integrated industry, national or regional commodities, internationally traded commodities, dual-purpose metal ores, or very high-value minerals. Bulk minerals are almost entirely used for construction. National and regional commodities in general supply the needs of basic industries in a country. Internationally traded commodities are less widespread. They are often marketed through an industrial minerals trader, with no direct relationship between the producer and consumer.

    A broad correlation can be established between the industrial mineral categories and the terrain in which they can be found. A passive continental margin with a sequence of sedimentary rocks can contain many of the industrial minerals used as national or regional commodities. Many bulk minerals are also found here. Internationally traded commodities tend to be found in either basement strata, active continental margins or in rift-valley terrains. Weathering processes lead to further industrial minerals found either as residual or alluvial and placer deposits.

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    Powder X-ray diffraction is the method of choice for characterizing the nature of crystalline solids and it can also be applied to non-crystalline solids. The method is ideal for analysing crystalline phases (e.g. minerals) because diffracted X-rays are direct probes of the repeating atomic units in solids. Qualitative analysis is based on the fact that each crystalline structure has a certain distribution of repeat distances which results in a diffraction pattern that is much like a fingerprint. The particular distribution and intensity of diffraction peaks is uniquely characteristic of each material. Quantitative analysis, i.e. determination of the amounts of more than one phase in a mixture, can be done because the diffraction intensities are directly related to crystal structure and the amounts of each phase. Quantitative analysis methods range from those using one or a few reflections to those using the entire diffraction pattern. The latter can employ either measured standard patterns or patterns calculated based on the crystal structures of the component phases, known as the Rietveld method. These full-pattern methods have important advantages as they use all intensity data in a pattern rather than one or a few of the most intense reflections. Some of the most troublesome systematic errors, including sample displacement, zero-point shift, and preferred orientation, can be refined, and the method yields unit-cell parameters of accuracy comparable to that obtained when using an internal d-spacing standard. The method finds wide application in industry, including modal analysis and compositional determinations of individual components using unit-cell parameter systematics. in addition, modern quantitative analysis methods can often be sensitive to amounts of <0.1 wt.%.

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    In the minerals industries, there is a frequent requirement to work with fine particulate matter, in the forms of powders, suspensions and granulates. The analysis and description of these particulates is an essential part of their processing and end-use; in particular, the characterization of their size distribution and morphology is useful in predicting behaviour in key mineral processes, such as comminution, sedimentation, filtration, flotation, calcination or granulation. For many industrial mineral applications, particle size and shape are also key to the end-function, such as in abrasives (such as sandpaper) or paint additives (such as matting aids). In this chapter, a particle’s size and the size distributions of a particle population are defined, and the prevalent methods and mechanisms of measuring size are discussed. The strengths and weaknesses of inferring distributions from images, light scattering patterns, sedimentation rates, and cytometric counts are weighed, and advice given on which methods may best suit one’s circumstances. In addition, image-analysis methods, which give size but also shape indicators, are described briefly.

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    The present chapter describes the application of thermal analysis (TA) in the characterization and processing of industrial minerals. An advantage of TA is its sensitivity to short-ranged ‘X-ray amorphous’ materials and (turbostratic) disordered minerals. In addition, it is more sensitive than X-ray diffraction at detecting small amounts of minerals in the case of decomposing minerals that evolve distinctive gases during thermal treatment.

    Minerals and rocks undergo several thermal reactions (dehydration, dehydroxylation, decomposition, melting, phase transition, oxidation or recrystallization) which are diagnostic of the substance. Unfortunately, the reactions of the individual components in mineral mixtures often superimpose and the results of TA are strongly influenced by several factors, such as sample preparation, selection of experimental parameters, instrument arrangement, etc.

    The present chapter begins with a short theoretical introduction on the principles and methods of TA while the factors that influence TA data and curves are described in more detail as their understanding is most important for interpretation of any measured TA data with respect to mineral structures, material characteristics and behaviour of industrial minerals in technical processes. Note that standardized conditions are essential for reasonable TA data.

    Description of ongoing developments of coupled devices for simultaneous thermal analysis (STA) and their application for quantitative analysis is followed by detailed information on diagnostic thermal reactions for important industrial minerals with focus on clay minerals.

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    This chapter shows how infrared (IR) and Raman spectroscopies contribute to better understanding of industrial minerals. These non-destructive techniques provide information on the chemical composition, structure, bonding and reactivity of molecules and/or minerals. The basis of vibrational spectroscopy theory including the modelling of the vibrational properties and spectra of minerals from ‘ab initio’ or ‘first-principles’ calculations appear in the first part of the chapter. A brief review of the IR and Raman instrumentations and sampling techniques is introduced as well. In the following sections, the spectra of selected minerals are presented and their interpretation is discussed. Raman spectroscopy is less often used for industrial minerals characterization, therefore the emphasis is on the interpretation of the IR spectra of most common industrial minerals in the middle IR (MIR, 4000–400 cm–1) and near-infrared IR (NIR, 8000–4000 cm–1) regions. The MIR spectra of layered silicates (phyllosilicates), zeolites, carbonates, sulphates and phosphates show well defined absorption bands corresponding to fundamental stretching (v) and bending (δ) vibrations of the structural units, e.g. OH, SiO4, CO3, SO4 or PO4 groups. Most of the bands present in the NIR spectra are related to the first stretching overtones (2v) and combination (v + δ) modes of the fundamental OH vibrations. The NIR region has been found to be useful at providing information on the crystal chemistry of clay minerals and their modifications upon various treatments as the OH-stretching overtones and combination vibrations are sensitively affected by the variations in the mineral structure. The last part of the chapter is devoted to the utilization of Raman spectroscopy in selected mineralogical applications, such as determination of polymorphs not discriminated by their chemical composition, e.g. TiO2 polymorphs.

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    Electron microbeam techniques such as Scanning Electron Microscopy (SEM), Electron Probe Microanalysis (EPMA) and Transmission Electron Microscopy (TEM) are commonly used in the characterization of industrial minerals providing morphological, chemical and structural information down to the atomic scale. The principal advantage of the electron microscope over the light microscope is the much improved resolution, due to the very low wavelength of the energetic electron (<1 Å) compared to the visible light, which is employed in the optical microscope. A key advantage of microbeam instruments is the generation of X-rays from the interaction of electron with the sample thereby allowing both the identification of elements present through observation of the KLM X-ray lines and determination of the elemental composition when matrix affects are taken into consideration.

    In this chapter an overview of the two main classes of electron microscopy, scanning and transmission, is presented, including a description of the instrumentation required and a discussion of their similarities and differences. The chapter also includes detailed information regarding the generation, detection and measurement of the various signals within the SEM and EPMA, the two instrument techniques most common to mineralogists. Finally, several case studies highlighting the use of these two electron microbeam techniques in the characterization of industrial minerals are presented. The examples were chosen to both illustrate traditional areas of use and emerging areas of application and include; automated SEM techniques, electron backscattered diffraction, charge contrast and in situ SEM imaging, EPMA mapping techniques, the determination of chemical states in minerals and materials using changes in X-ray peak shape, hyperspectral EPMA, and trace-element speciation using quantitative cathodoluminescence.

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    This chapter is dedicated to the science of extracting quantitative information from digital images representing minerals and rocks. Because of the extraordinary complexity of natural textures, but also due to the wide diversity of mineral species, such analysis is still regularly performed by geologists using manual point-counting methods and basic stereological principles. If one aims to automate the process, it is essential to realise that images have to be acquired wherein individual minerals are contrasted as much as possible. This depends heavily on the quality of the imaging instrument and the attention dedicated to sample preparation.

    After reviewing a range of modern mineral-imaging modes using electrons, X-rays, photons and even nanosized probes, the chapter focuses on the tools and techniques most commonly used to archive and process digital images. Special emphasis is given to image segmentation techniques that allow the user to classify pixels and map homogeneous domains that might correspond to specific minerals or single crystals.

    The image analysis part sensu stricto addresses the quantitative description of mineral abundance before suggesting different techniques to analyse size and shape distributions of particles and grains. A brief introduction to network description is also given with special attention paid to the powerful concept of intercepts.

    Finally, images are presented as support for physical simulations that bring new insight to behaviour of geomaterials with respect to processes such as diffusion and percolation.

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    Clays have been used by man since prehistoric times. Initially they were used almost exclusively in the fabrication of ceramics; nowadays they find numerous industrial and technological applications including the production of materials with large added value such as nanocomposites, cosmetics or pharmaceuticals. The term clay should not be considered as a synonym for clay mineral, because clays consist of more than one mineral. The versatile nature of clays is attributed to the presence of clay minerals, which impart significant physical properties to the raw materials, such as particle size and shape, ion exchange, hydration and swelling, plasticity, rheological properties, colour properties and reactions with organic and inorganic compounds. Four types of industrial clay raw materials are examined in this contribution, kaolins, bentonites, fibrous clays (palygorskite and sepiolite) and common clays and shales. The latter are used in the production of structural ceramics, bricks tiles and pipes. The industrial clay deposits are classified as primary (residual formed from in situ alteration of various precursors or hydrothermal) and secondary, formed from deposition of clastic clay materials which were transported from their sources. Assessment of industrial clay deposits comprises determination of physical properties and direct comparison with international or regional standards, which include industrial specifications for particular applications. These specifications are often dictated by the end industrial users. Exploitation of the clay deposits is usually by means of traditional open-cast methods and processing can involve anything from simple crushing, screening and tempering, to elaborate mineral beneficiation techniques such as alkali or acid activation, delamination, magnetic separation, selective flocculation, flotation and leaching. The method used and the extent of beneficiation are dictated by the final industrial application of the clay.

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    This chapter examines the use of industrial clay minerals as nanomaterials. In the first part, clay minerals are introduced and a survey is given of those properties which are relevant for clay-polymer nanocomposites (CPN). These properties are: morphology of the clay-mineral particles, sizes and shapes of the layers, cation exchange, and hydro-philic and hydrophobic properties. Then, the methods of CPN preparation are reviewed briefly, followed by the factors which influence the properties of CPN. Special attention is given to clay-mineral-based bionanocomposites and functional clay-minerals films. The latter can be prepared by spin coating, layer-by-layer assemblage and the Lang-muir-Blodgett technique. A broad range of properties can be introduced in films prepared with appropriate molecules such as polymers, chiral molecules, molecules with non-linear optical properties and magnetic molecules. The area of CPN is broad and open for fundamental and applied research.

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    Portland cement has become a cornerstone of modern society and the present-day cement and concrete industry is one of the largest consumers of industrial minerals. The historical evolution of early calcareous hydraulic binders into the present standard Portland cement is a typical example of a product-performance-optimization process driven by the gradual accumulation of empirical know-how and fundamental process understanding. The early observation that a hydraulic binder may be formed when impure limestone is burnt at temperatures above the decomposition temperature of limestone led to the development of a wide range of early hydraulic binders throughout the 18th and 19th centuries. Initially, burning at relatively low temperatures (1000– 11008C) of impure limestone resulted in the production of fast-setting natural cements and hydraulic limes. Eventually, over the course of the 19th and 20th centuries, sintering at increasingly higher burning temperatures of natural impure limestones and artificial mixes of ground limestone and clay was introduced to produce slow-setting natural cement and finally (proto-)Portland cement with superior strength development.

    Today, the Portland cement-production process consists of an energy-intensive, high-temperature sintering phase (14508C) of the raw materials, followed by fast cooling and fine intergrinding of the clinker product with gypsum to produce the Portland cement. The mineralogy of the clinker phases is relatively complex. C3S and C2S show several high- and low-temperature polymorphs, whereas C3A and C4AF allow considerable compositional solid solution. The addition of water to Portland cement initiates a complex scheme of hydration reactions to form a hardened cement paste. The advent of novel analytical techniques prompted recent advances in the understanding of the structures of the hydration products and the hydration mechanism. Nevertheless many aspects of the hydration reactions remain unsolved. The necessary future developments towards less energy intensive, low-CO2 cements may take advantage of the historical knowledge acquired in the production of a wide range of alternative hydraulic binders.

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