Thermal dehydration kinetics of phosphogypsum

Abstract

ABSTRACT:Phsophogypsum is a by-product from the processing phosphate rock. Before the use of it in cement industry such as setting regulator is necessary a study of dehydration reaction of phosphogypsum to avoid the false setting during the milling.The aim is to study the thermal behavior of two different phosphogypsum sources (Spain and Tunisia) under non-isothermal conditions in argon atmosphere by using Thermo-Gravimetriy, Differential Thermal Analysis (TG-DTA) and Differential Scanning Calorimetry (DSC).DSC experiments were carried out at temperatures ranging from ambient to 350 °C at different heating rates. The temperatures of conversion from gypsum to hemihydrate and anhydrite states and heat of dehydration were determined. Various methods were used to analyze the DSC data for reaction kinetics determination. The activation energy and frequency factor were calculated for dehydration of phosphogypsum. Activation energy values of the main dehydration reaction of phosphogypsum were calculated to be approximately 61-118 kJ/mol. RESUMEN:Estudio cinético de la deshidratación térmica del fosfoyeso. El fosfoyeso es un subproducto procedente del procesado de la roca fosfato. Una de las posibles vías de reutilización y revalorización es su uso como regulador del fraguado en la industria cementera. Debido a los posibles problemas de falso fraguado asociado a los procesos de deshidratación que tienen lugar durante la molienda del cemento, esta investigación estudió el comportamiento térmico, bajo condiciones no-isotérmicas en atmósfera de argón, de dos fosfoyesos, mediante TG-DTA y DSC.Los ensayos de DSC se realizaron hasta los 350 °C a diferentes velocidades de calentamiento. La temperatura de conversión del yeso a las formas de hemihidrato y anhidrita y el calor de hidratación fueron determinados.Las cinéticas de reacción fueron obtenidas analizando los datos de DSC mediante varios métodos. Se calculó la energía de activación y el factor de frecuencia para las reacciones de deshidratación del subproducto. Los valores de energía de activación de las principales reacciones de deshidratación del fosfoyeso fueron obtenidos, aproximadamente 61-118 kJ/mol.

Statistical approaches in the analysis of in situ thermo‐Raman spectroscopic data for gypsum as a basis for studying dehydration and phase transformations in crystalline hydrates

“…[ 7,50 ] According to Manzello et al, [ 51 ] the transition occurs at temperatures about 623 K and higher. According to López et al, [ 13 ] it occurs at about 673 K, and a weak exothermic reaction is recorded, in which the structure of soluble γ‐anhydrite is irreversibly rearranged into an insoluble β‐state with lower energy; no weight loss is observed. Note that the CaO 8 dodecahedra and sulfate‐ions form alternating chains with common edges parallel to the c axis in the structure of β‐phase [ 52 ] ; the identical chains are also present in the gypsum structure.…”

“…In practice, the study of CaSO 4 ·2H 2 O gypsum—a typical natural crystalline hydrate, multipurpose raw material with a wide range of product varieties, primarily constructive—is of the greatest interest. [ 13–16 ] Gypsum physico‐chemical properties, especially thermal characteristics, have fundamental mineralogical significance for the description of the group of sulfate‐crystalline hydrates in particular, and crystalline hydrates in general. These data are also necessary for practical applications, in particular, to control the technological processes of firing gypsum raw materials.…”

“…Water molecules localized between these layers form hydrogen bonds with the oxygen of sulfate groups, resulting in each Ca cation being coordinated by six oxygen atoms of sulfate groups and two water molecules. [ 4 ] Gypsum thermal transformations have been repeatedly investigated using various methods (see, for example, the literature [ 5–14 ] ). The first endothermic peak on the DTA curves associated with gypsum degradation is observed at 393–413 K [ 29,30 ] or at 413 K. [ 7 ] It should be noted that according to Raman spectroscopic (RS) data, it is fixed at 388 K [ 10,31 ] or at 360–425 K [ 8 ] and at 382–413 K according to SR‐PXD.…”

“…The second peak in the thermal curves, associated with bassanite dehydration, is recorded at 428 K [ 7 ] or at 449–506 K. [ 13 ] According to Raman spectroscopic data, the structural transformation corresponding to this thermal process occurs at 448 K [ 10 ] and at 438–445 K according to SR‐PXD data. [ 1 ] In the indicated temperature range, hemihydrate transforms into soluble γ‐anhydrite, which is accompanied by the transformation of the orthorhombic lattice into a hexagonal one.…”

Research on the Thermal Conductivity and Water Resistance of Foamed Phosphogypsum

“…This is attributed to the fact that the increased foam volume can lead to increasing the internal porosity of phosphogypsum, thus decreasing the mechanical strengths [28,29]. However, the addition of cement can improve the compactness of the microstructures of phosphogypsum, thus improving the mechanical strengths [30,31]. Tables 5 and 6 show the fitting results of flexural and compressive strengths of phosphogypsum and the foam volume, respectively.…”