Sinterization and hydration of synthesized cement clinker doped with sulfates

This paper aims to evaluate the influence of three kinds of sulfates from the green production of cement on its sintering and hydration. The properties of clinker and hydration were monitored by thermogravimetric and differential thermal analysis (TG–DTA), X-ray diffraction, X-ray fluorescence and isothermal conduction calorimeter. Results indicate that gypsum lowers the decomposition temperature of CaCO₃ and all these Sulfates will enhance the solid-phase reaction but increase melting temperature. Sulfates reduce the content of C₃S, but K₂SO₄ and 2CaSO₄·K₂SO₄ is conducive to the formation of β-C₂S. The hydration induction period is shortened by the sulfates. K₂SO₄ and 2CaSO₄·K₂SO₄ improve the early hydration of clinker, but gypsum may lightly reduce the hydration reactivity of clinker in acceleration period. 2CaSO₄·K₂SO and K₂SO can significantly accelerate the compressive strength development of cement clinker before 3 d; by contrast, gypsum is detrimental for that. The precipitation of hydration products (CH and C–S–H) in clinker with sulfates is more than that of clinker without sulfates at 9 h. K₂SO₄ can accelerate the hydration of clinker without forming ettringite.

     

Production of potassium sulfate from potassium hydrosulfate solutions using alcohols

Potassium sulfate is used to produce multicomponent fertilizers, free of chlorides. The desalting out of potassium sulfate from an aqueous solution of potassium hydrosulfate was conducted using 40 mass %, 45 mass %, or 50 mass % aqueous solutions of either methanol or propan-2-ol. Composition of the resultant precipitate was analyzed using chemical methods and XRD analysis. The results of the XRD analysis revealed that the main precipitate phase is K₂SO₄. Small amounts of K₅H₃(SO₄)₄ were detected when the desalting out was carried out from 2.5 M KHSO₄ solution using 40 mass % and 50 mass % methanol solution. When the amount of potassium bisulfate in the solution increased to 3.5 M and 3.8 M, the main phase consisted of K₃H(SO₄)₂. Generally, the desalting out process using propan-2-ol caused the formation of K₃H(SO₄)₂. Potassium sulfate was obtained only by desalting out the 2.5 M KHSO₄ solution using 50 mass % aqueous propan-2-ol. Presented at the 34th International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare, 21–25 May 2007.     

High temperature thermogravimetry of chlorides and sulphates : A study of the application to soils

Quantitative volatilization of NaCl and KCI occurs between 900 and 1200°. CaCl₂ and MgCl₂ are converted to the oxides at lower temperatures. CaSO₄, Na₂SO₄ and K₂SO₄ require the admixture of quartz to catalyse their decomposition with a total loss of SO₃ between 1150 and 1335°. MgSO₄ does not require quartz for its decomposition. The catalytic effects of Al₂O₃ and Fe₂O₃ on sulphate decomposition were also examined. The findings were applied to the analysis of saline soils. The thermogravimetric determination of chlorides in soils is subject to several interferences, but the conditions are more favourable for sulphates.     

Heat of solution of polyhalite and its analogues at T = 298.15 K

Calorimetric measurements have been performed to determine the heat of dissolution of polyhalite K₂SO₄ · MgSO₄ · 2CaSO₄ · 2H₂O and its analogues K₂SO₄ · MSO₄ · 2CaSO₄ · 2H₂O (M = Mn, Co, Ni, Cu, and Zn) at T = 298.15 K. The dissolution experiments were carried out in NaClO₄ solution with varying concentrations (0.5 to 2.0) mol kg^?1. All polyhalites dissolve exothermically. Exothermicity increases with concentration of NaClO₄. An extrapolation to infinite dilution was done using the SIT model.Within the limits of experimental uncertainty, the enthalpies of dissolution for the triple salts K₂MgCa₂(SO₄)₄ · 2H₂O with M = Mg, Mn, Ni, and Zn coincide. The value for the cobalt salt is noticeably less exothermic. Dissolution enthalpy of leightonite K₂CuCa₂(SO₄)₄ · 2H₂O, which does not crystallize in the polyhalite structure, deviates considerably within the series.     

Conversion of phosphogypsum to potassium sulfate

Summary Solubility of calcium sulfate in concentrated aqueous chloride solutions is of particular significance in chloride hydrometallurgy and various crystallization processes, such as the production of potassium sulfate from phosphogypsum and potassium chloride. This paper examines an example of the second type of application in which gypsum and potassium chloride are reacted to form K₂SO₄. The solubility of phosphogypsum in aqueous solutions of KCl, HCl, and mixtures of both has first been measured at various temperatures and concentrations. The parameters investigated are HCl concentration up to 6M, KCl concentration up to 180 g L^-1 and temperature from 25 to 80°C. In addition, the influence of co-existing chloride salts, such as (HCl+KCl), on the solubility of calcium sulfate is estimated from 25 to 80°C. The solubility increases obviously with the temperature increment as it does initially with acid concentration, reaching a maximum of about 3M HCl, 130 g L^-1 KCl and then drops. At the same time, the solubility of CaSO₄·2H₂O decreases with increasing KCl concentration.     

Crystallization and Phase Stability of CaSO<Subscript>4</Subscript> and CaSO<Subscript>4</Subscript> – Based Salts

Summary.  Calcium sulfate occurs in nature in form of three different minerals distinguished by the degree of hydration: gypsum (CaSO₄·2H₂O), hemihydrate (CaSO₄·0.5H₂O) and anhydrite (CaSO₄). On the one hand the conversion of these phases into each other takes place in nature and on the other hand it represents the basis of gypsum-based building materials. The present paper reviews available phase diagram and crystallization kinetics information on the formation of calcium sulfate phases, including CaSO₄-based double salts and solid solutions. Uncertainties in the solubility diagram CaSO₄–H₂O due to slow crystallization kinetics particularly of anhydrite cause uncertainties in the stable branch of crystallization. Despite several attempts to fix the transition temperatures of gypsum–anhydrite and gypsum–hemihydrate by especially designed experiments or thermodynamic data analysis, they still vary within a range from 42–60°C and 80–110°C. Electrolyte solutions decrease the transition temperatures in dependence on water activity. Dry or wet dehydration of gypsum yields hemihydrates (α-, β-) with different thermal and re-hydration behaviour, the reason of which is still unclear. However, crystal morphology has a strong influence. Gypsum forms solid solutions by incorporating the ions HPO₄ ²⁻, HAsO₄ ²⁻, SeO₄ ²⁻, CrO₄ ²⁻, as well as ion combinations Na⁺(H₂PO₄)⁻ and Ln³⁺(PO₄)³⁻. The channel structure of calcium sulfate hemihydrate allows for more flexible ion substitutions. Its ion substituted phases and certain double salts of calcium sulfate seem to play an important role as intermediates in the conversion kinetics of gypsum into anhydrite or other anhydrous double salts in aqueous solutions. The same is true for the opposite process of anhydrite hydration to gypsum. Knowledge about stability ranges (temperature, composition) of double salts with alkaline and alkaline earth sulfates (esp. Na₂SO₄, K₂SO₄, MgSO₄, SrSO₄) under anhydrous and aqueous conditions is still very incomplete, despite some progress made for the systems Na₂SO₄–CaSO₄ and K₂SO₄–CaSO₄–H₂O. Corresponding author. E-mail: daniela.freyer@chemie.tu-freiberg.de Received December 17, 2002; accepted January 10, 2003 Published online April 3, 2003     

Polyhalite and its analogous triple salts

Polyhalite (K₂SO₄ · MgSO₄ · 2CaSO₄ · 2H₂O) and analogue triple salts, where Mg²⁺ is substituted by Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺, have been synthesized. The salts were characterized by thermal analysis, Raman spectroscopy and X-ray powder diffraction. Diffraction patterns and Raman spectra resemble those of natural polyhalite, except K₂SO₄ · CuSO₄ · 2CaSO₄ · 2H₂O. The latter corresponds to the mineral leightonite, which is structurally different.     

Zur Darstellung und Charakterisierung von Calciumhydrogensulfaten

Preparation and Characterization of Calcium Hydrogen Sulfate CaSO₄ · H₂SO₄ was identified as calcium hydrogen sulfate whereas CaSO₄ · 3 H₂SO₄ is an adduct of CaSO₄ with H₂SO₄. Depending on the excessive amount of H₂SO₄ both compounds exist side by side up to a temperature of 343 K, whereas above this temperature only Ca(HSO₄)₂ is stable. The DTA curve of Ca(HSO₄)₂ shows two maxima at 488 K and 523 K, according to the separation of H₂O under formation of pyrosulfate and decomposition of this compound under elimination of SO₃. In comparison with other hydrogen sulfates Ca(HSO₄)₂ shows a considerable increased O? H distances. The d-values of Ca(HSO₄)₂ are calculated and represented.     

Measurement of the relaxation transitions of nitrocellulose based gunpowder

The formation of a new sulfate compound K₄H₂(SO₄)₃ is obtained by evaporation at 25°C of an aqueous solution, which was formed by a mixture of K₂SO₄ and H₂SO₄. The characterization of this solid is carried out by X-ray diffraction, thermal and infrared analyzes. The heat treatment was carried out in interval 25–700°C; the end product of the thermal evolution is K₂SO₄. The vibration bands relating to SO₄ and OH groups were highlighted by the infrared spectroscopy.Moreover, one study of ionic conductivity on this solid compound was carried out according to the temperature in interval 25–80°C. Its activation energy is 0.47 eV. The X-ray intensities collection obtained on a monocrystal of K₄H₂(SO₄)₃ gives the following cell parameters: a=7.035(5), b=19.751(4), c=23.466(2) Å, β=95.25(1)°.     

In situ synchrotron PXRD investigation of fresh and aged Equisetum ramosissimum Desf. (horsetail grass)

As one of the oldest species, horsetail grass (Equisetum ramosissimum Desf.) is known as a living fossil plant, dating back to the Mesozoic era. Horsetail grass is also considered one of the most important sources of bio-silica due to its ability to accumulate high amounts of silica in all parts of the plant; various minerals can also be isolated by heat treatment. Fresh and aged horsetail grass stored for 2 years under ambient conditions was investigated by synchrotron powder X-ray diffraction (PXRD). Clear crystallites were not observed in a fresh sample stored at room temperature; surprisingly, high amounts of gypsum (CaSO₄·2H₂O) and syngenite (K₂Ca[SO₄]₂·H₂O) were observed in the 2-day dried and 2-year aged samples, respectively. However, crystalline silica materials were not observed. In addition, in situ thermal treatment of up to 700°C was applied to investigate the crystals and phase transitions by focusing the X-ray beam onto a single stem. In situ synchrotron PXRD revealed that dehydration occurred in gypsum in the 2-day dried sample with an increase in temperature to hemihydrate (CaSO₄·xH₂O, 0.5 ≤ x ≤ 0.8) and anhydrite (CaSO₄). On the other hand, syngenite was transformed to calciolangbeinite (K₂Ca₂[SO₄]₃) at high temperatures in 2-year aged horsetail grass.     

Thermodynamics of aqueous mixtures of sodium chloride,potassium chloride,sodium sulfate,and potassium sulfate at 25°C

Aqueous solutions of sodium chloride, potassium chloride, sodium sulfate, and potassium sulfate can be mixed in six ways to give ternary mixtures. Two of these have already been studied and results are now presented for the remaining four systems: H₂O–NaCl–K₂SO₄, H₂O–Na₂SO₄–K₂SO₄, H₂O–KCl–Na₂SO₄, and H₂O–KCl–K₂SO₄.     

Extraordinary high potassium ion conducting polycrystalline solids based on gadolinium oxide–potassium nitrite solid solution

A new type of potassium ion conducting polycrystalline solid electrolyte was prepared by forming the Gd₂O₃–KNO₂ solid solution. Since KNO₂ is dissolved in the Gd₂O₃ crystal lattice scale, a considerable enhancement in K⁺ ion conductivity was successfully realized. The K⁺ ion conductivity is more than three orders of magnitude higher than that of the well-known K⁺ ion conducting K₂SO₄ polycrystal solid and the value also exceeds that of K⁺–β^″-alumina single crystal, demonstrating the highest ion conductivity among the K⁺ ion conducting solid series.     

Formation of calcium sulfate whiskers from CaCO3-bearing desulfurization gypsum

Calcium sulfate whiskers can be used as the reinforcing agents in many composites, such as polymers, ceramics, cements, and papers, etc. This paper investigated the feasibility of preparing calcium sulfate whiskers using desulfurization gypsum as the raw material. The desulfurization gypsum composed mainly of CaSO₄·2H₂O (93.45 wt%) and CaCO₃ (1.76 wt%) were treated with dilute H₂SO₄ at room temperature to convert CaCO₃ to CaSO₄; the latter was then treated at 110?C150 °C to form CaSO₄·0.5H₂O whiskers. The removal of the CaCO₃ impurity from the desulfurization gypsum favored the formation of CaSO₄·0.5H₂O whiskers with high aspect ratios.     

Phase equilibria in the Na,K,Mg,Ca∥SO<Subscript>4</Subscript>,Cl–H<Subscript>2</Subscript>O system at 50°С in the polyhalite crystallization region

Phase equilibria of the Na,K,Mg,Ca||SO₄,Cl–H₂O system at 50°С in the polyhalite (K₂SO₄ · MgSO₄ · 2CaSO₄ · 2H₂O) crystallization region were studied using the translation method. Polyhalite was found to be involved, as an equilibrium phase of the title system at 50°С, in 17 invariant points, 36 monovariant curves, and 24 divariant fields. A fragment of equilibrium phase diagram of the title system in the polyhalite crystallization region was constructed.     

Metastable phase equilibria for the ternary aqueous system of lithium sulfate and potassium sulfate at <Emphasis Type="Italic">T</Emphasis> = 308.15 K: Experimental data and prediction using Pitzer model

The metastable solubilities and the physicochemical properties including density, refractive index, pH and conductivity in the ternary system (Li₂SO₄ + K₂SO₄ + H₂O) at T = 308.15 K were determined experimentally using the isothermal evaporation method, and the metastable phase diagram and the physicochemical properties versus composition diagram were plotted. In the metastable phase diagram, there are two invariant points, three univariant curves and three crystallization regions corresponding to lithium sulfate monohydrate (Li₂SO₄ · H₂O), double salt (K₂SO₄ · Li₂SO₄) and arcanite (K₂SO₄). It was found that the double salt of K₂SO₄·Li₂SO₄ belongs to the incongruent double salt, and the hydrate of Li₂SO₄ · H₂O belongs to hydrate type I. On the basis of Pitzer model of the electrolyte solution theory, the mixing-ion parameter of θ_Li,K, \({\Psi _{Li,K,S{O_4}}}\) and the metastable equilibrium constants of the solid phases K₂SO₄, Li₂SO₄ · K2SO4 and Li₂-SO₄ · H₂O at 308.15 K were obtained for the first time. The calculated metastable solubility data for this ternary system at 308.15 K agree well with the experimental values, and this result indicates that the mixing-ion parameters and the metastable equilibrium constants obtained in this work are reliable.     

(Solid + liquid) metastable equilibria in quaternary system (Li2SO4 + K2SO4 + Li2B4O7 + K2B4O7 + H2O) at T = 288 K

Metastable equilibrium solubilities and properties such as densities, conductivity, pH, refractive index, and viscosity of the solution were determined experimentally. According to the experimental data, the metastable equilibrium phase diagram was plotted. In the phase diagram, there are three invariant points, seven univariant curves, five fields of crystallization: Li₂SO₄ · H₂O, K₂SO₄, Li₂B₄O₇ · 3H₂O, K₂B₄O₇ · 4H₂O, and K₂SO₄ · Li₂SO₄. The double salt K₂SO₄ · Li₂SO₄ was found in the quaternary system metastable equilibria. Lithium sulfate (Li₂SO₄) has the highest concentration and strong salting-out effects on other salts.Also, the relationship diagram between the properties and the ion concentration of solution was constructed. It can be seen from the relationship diagram that the equilibrium solution density values, viscosity values, and refractive index values are increased apparently with the rise of sulfate ion concentration, reaching the maximum values at eutonic point F3. Electrical conductivity values and pH values, however, fall down with the rise of ion concentration on the whole.     

Study on phase diagrams and properties of solutions in ternary systems Li+,K+(Mg2+)/SO42--H2O at 25°C

Solubilities of ternary systems Li⁺,K⁺/SO^2-₄-H₂O (1) and Li⁺,Mg²⁺/SO₄^2--H₂O (2) were investigated by isothermal method at 25°C. Physico-chemical properties of solutions, such as density, refractive index, viscosity, conductivity and pH, were determined. Phase diagram of the system (1) consists of three solubility branches and three crystallization fields corresponding to K₂SO₄, Li₂SO₄·H₂O and LiKSO₄. LiKSO₄ is an incongruent compound, and its transition point is estimated graphically to be 45.5–46.0°C. No solid solution of LiKSO₄ with Li₂SO₄·H₂O was found in the system. The system (2) is a simple eutonic type. Pitzer model of electrolyte solution was used to check the obtained solubilities. Data comparison gives good agreement. Two equations were used to correlate density, refractive index of the solution with its composition. Differences between measured and calculated values are less than 0.6% for density, 0.15% for the latter.     

K<Subscript>3</Subscript>VO<Subscript>2</Subscript>(SO<Subscript>4</Subscript>)<Subscript>2</Subscript>: Formation conditions,crystal structure,and physicochemical properties

Potassium oxosulfatovanadate(V) K₃VO₂(SO₄)₂ has been obtained by solid-phase synthesis from K₂SO₄, K₂S₂O₇, and V₂O₅ (2: 1: 1), and its formation conditions, crystal structure, and physiochemical properties have been studied. The conversions of K₃VO₂(SO₄)₂ in contact with potassium vanadates and other potassium oxosulfatovanadates(V) are considered in terms of phase relations in the K₂O-V₂O₅-SO₃ system, which models the active component of vanadium catalysts for sulfur dioxide oxidation into sulfur trioxide. The X-ray diffraction pattern of K₃VO₂(SO₄)₂ is indexed in the monoclinic system (space group P2₁) with unit cell parameters of a = 10.0408(1) Å, b = 7.2312(1) Å, c = 7.3821(1) Å, β = 104.457(1)°, Z = 2, and V = 519.02 ų. The crystal structure of K₃VO₂(SO₄)₂ is built from [VO₂(SO₄)₂]^3? complex anions, in which the vanadium atom is in an octahedral oxygen environment formed by two terminal oxygen atoms (V-O(6) = 1.605(7) Å, V-O(10) = 1.619(7) Å and four oxygen atoms of the two chelating sulfate anions. The vibrational spectra of K₃VO₂(SO₄)₂ are analyzed using these structural data.     

Polyhalite and its analogous triple salts

Polyhalite (K₂SO₄ · MgSO₄ · 2CaSO₄ · 2H₂O) and analogue triple salts, where Mg²⁺ is substituted by Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺, have been synthesized. The salts were characterized by thermal analysis, Raman spectroscopy and X-ray powder diffraction. Diffraction patterns and Raman spectra resemble those of natural polyhalite, except K₂SO₄ · CuSO₄ · 2CaSO₄ · 2H₂O. The latter corresponds to the mineral leightonite, which is structurally different. For polyhalite analogues the cell parameters of the triclinic unit cell have been determined from the powder diffraction patterns. The length of the unit cell vectors varies regularly with the ionic radius of the substituted ion M ²⁺ and is explained by changes in the extension of the coordination octahedron of M ²⁺. Thereby increasing distances of the coordinated water molecules at M ²⁺ parallel with decreasing dehydration temperatures of the corresponding polyhalite. Correspondence: Daniela Freyer, Institut für Anorganische Chemie, TU Bergakademie Freiberg, 09599 Freiberg, Germany.     

Measurement and modelling of solubility for calcium sulfate dihydrate and calcium hydroxide in NaOH/KOH solutions

The solubility of calcium sulfate dihydrate (CaSO₄·2H₂O) and calcium hydroxide (Ca(OH)₂) in alkali solutions is essential to understand their desilication behavior from Bayer liquor. In this work, solubilities of calcium sulfate dihydrate and calcium hydroxide for the ternary systems of CaSO₄·2H₂O–NaOH–H₂O, CaSO₄·2H₂O–KOH–H₂O, and Ca(OH)₂–NaOH–H₂O were measured by using the classic isothermal dissolution method over the temperature range of 25–75 °C. The Pitzer model embedded in Aspen Plus platform was used to model the experimental solubility data for these systems. The experimental solubility data was employed to obtain the new binary interaction parameters for Ca(OH)⁺–OH⁻, Ca(OH)⁺–Ca²⁺ and Ca(OH)⁺–K⁺, suggesting that the species Ca(OH)⁺ is a dominant species in simulated solubility for alkali systems. Validation of the parameters was performed by predicting the solubility for the ternary systems of Ca(OH)₂–NaOH–H₂O, CaSO₄·2H₂O–NaOH–H₂O and CaSO₄·2H₂O–KOH–H₂O with the overall average relatively deviation (ARD) of 2.12%, 0.75% and 1.63%, respectively.