Hydrothermal processes in industry
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LECTURE TEXT
Hydrothermal processes in industry Norbert Mertzsch1 Received: 26 March 2020 / Accepted: 27 July 2020 © Springer Nature Switzerland AG 2020
Abstract Hydrothermal processes played a substantial role in ore and mineral formation. They have been used industrially since the nineteenth century. Examples include crystal synthesis, digestion of raw materials, hydrometallurgy under pressure, hardening and synthesis of building materials and execution of chemical processes. Undesired hydrothermal processes play a role in the corrosion of thermal power plants and the aging of catalysts. Keywords Hydrothermal process · Spinels · Gahnite · Crystal growth · Quartz · Barium titanate · Lithium iron phosphate · Bayer process · Pressure leaching · Limestone bricks · Building materials · Zeolites · Corrosion · Aging of catalysts
Introduction
Selected properties of water
Geologic hydrothermal processes have been known for a long time as being responsible for the formation of many ore deposits, e.g., in the Ore Mountains (“Erzgebirge” in southeast Germany) [1] and mineral formations. In recent years, the origin of life has also been associated with hydrothermal processes [2]. Since the middle of the nineteenth century, hydrothermal processes have been investigated experimentally, while hydrothermal syntheses took center stage. A plethora of review articles were published on hydrothermal synthesis [3–5], giving also an account of historical developments. Rabenau’s general definition of hydrothermal syntheses as “heterogeneous reactions in aqueous medium above 100 °C and 1 bar” has not been unanimously accepted [6, 7], and a general agreement is desirable [8]. Schlegel gave a more comprehensive definition of hydrothermal processes [9], according to which the chemical and physical processes have to take place on solid substances and in fluid media under hydrothermal and humid conditions. In technical terms, hydrothermal processes generally take place at temperatures > 100 °C.
Prior discussing the details of individual hydrothermal processes in industry, selected properties of water—relevant to the topic—will be discussed (cf., e.g., [3, 10]). The water molecule H 2O is characterized by strongly polarized bonds between the hydrogen and oxygen atoms; the H–O–H angle amounts to 104.45°. The binding and non-binding electron pairs of oxygen exhibit a weakly distorted tetrahedron. As a result, H 2O possesses a strong dipole moment, which leads to the three-dimensional linking of individual water molecules via hydrogen bonds, which dominate the properties of water, such as high heat capacity, surface tension and evaporation enthalpy. Of course, this linking is highly dynamic. The p–T diagram of water (Fig. 1) shows two distinct points: (1) the triple point in which all three states of aggregation are coexisting. This point, however, is not important for hydrothermal processes. (2) The critical point 374.15 °C/22.124 MPa (221 bar), at which the difference between liquid and vapor phase ceases. The enthalpy of vaporization
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