Crystal modification of calcium sulfate dihydrate in the presence of some surface-active agents

The effect of surface-active agents (surfactants), as additives, on the crystallization of gypsum was studied under conditions of the simulated dihydrate process of phosphoric acid production. Calcium hydrogen phosphate and sulfuric acid were mixed with dilute phosphoric acid at 80 degrees C, and the turbidity of the reaction mixture was measured at different time periods to determine the induction time of gypsum crystal formation. Two types of surfactants, namely, cetyltrimethylammonium bromide (CTAB) as a cationic surfactant and sodium dodecyl sulfate (SDS) as an anionic surfactant were added to investigate their effects on the crystallization of gypsum. Addition of CTAB decreased the induction time and increased the growth efficiency, while addition of SDS increased the induction time and decreased the growth efficiency compared with the baseline (without additives). The surface energy increased with CTAB and decreased with SDS compared with the baseline. The percentage of fine crystals decreased in the presence of CTAB and increased in the presence of SDS compared with the baseline. Gypsum morphology changed from needle-like in the absence of additives to tabular in the presence of CTAB.     

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.     

Thermal decomposition of hydroxylammonium neodymium sulfate dihydrate

The thermal decomposition of hydroxylammonium neodymium sulfate dihydrate has been investigated by simultaneous thermogravimetry and differential thermal analysis. Chemical analysis, X-ray powder spectra and infrared spectroscopy have been employed to characterize the intermediates and the final product. The thermal decomposition can be described by the sequence (NH₃OH)Nd(SO₄)₂·2H₂O(NH₃OH)Nd(SO₄)₂ NH₄Nd₃(SO₄)₅Nd₂(SO₄)3. The first and the second reactions overlap, but the last one is well separated from the first two.     

Novel dihydrate formation in p-nitrophenylglyoxal

It is novel that the reaction of p-nitrophenyl-glyoxal with biochemically relevant compounds has a long lag time before the reaction commences. While utilizing NMR spectroscopy to investigate factors that may affect the lag, it has been discovered that a dihydrate form of pNPG is in equilibrium with a monohydrate while in the presence of water. This behavior is not present in the related phenylglyoxal molecule. Quantum chemical computations on the nonhydrated, hydrate, and dihydrate forms of pNPG, as well as phenylglyoxal and its hydrated forms, using both the B3LYP and MP2 levels of theory indicate that nitro group creates the necessary environment for hydration of pNPG whereas this is not present in standard phenylglyoxal. The equilibrium constants and their relationship with temperature as well as the heat of reaction and change in entropy have been determined experimentally. This data should provide new insights into the behavior of this and related systems in biochemical processes.     

The study of thermal decomposition kinetics of zinc oxide formation from zinc oxalate dihydrate

This study is devoted to the thermal decomposition of ZnC₂O₄·2H₂O, which was synthesized by solid-state reaction using C₂H₂O₄·2H₂O and Zn(CH₃COO)₂·2H₂O as raw materials. The initial samples and the final solid thermal decomposition products were characterized by Fourier transform infrared and X-ray diffraction. The particle size of the products was observed by transmission electron microscopy. The thermal decomposition behavior was investigated by thermogravimetry, derivative thermogravimetric and differential thermal analysis. Experimental results show that the thermal decomposition reaction includes two stages: dehydration and decomposition, with nanostructured ZnO as the final solid product. The Ozawa integral method along with Coats–Redfern integral method was used to determine the kinetic model and kinetic parameters of the second thermal decomposition stage of ZnC₂O₄·2H₂O. After calculation and comparison, the decomposition conforms to the nucleation and growth model and the physical interpretation is summarized. The activation energy and the kinetic mechanism function are determined to be 119.7 kJ mol^?1 and G(α) = ?ln(1 – α)^1/2, respectively.     

A new precipitation pathway for calcium sulfate dihydrate (gypsum) via amorphous and hemihydrate intermediates

This work investigates the early stages of precipitation of calcium sulfate from aqueous solution at room temperature and shows for the first time that amorphous calcium sulfate (ACS) and calcium sulfate hemihydrate are sequentially precipitated prior to calcium sulfate dihydrate (gypsum).     

Enthalpy of formation of zinc acetate dihydrate

The enthalpy of formation of zinc acetate dihydrate (Zn(CH₃COO)₂ · 2H₂O) was measured with respect to crystalline zinc oxide (ZnO), glacial acetic acid (CH₃COOH) and liquid water by room temperature solution calorimetry. The enthalpy of formation was verified by utilizing two independent thermodynamic cycles, using enthalpy of solution measurements in 5 mol · L^?1 sodium hydroxide (NaOH) and in 5 mol · L^?1 hydrochloric acid (HCl) solutions. The enthalpy of the reaction ZnO (cr) + 2CH₃COOH (l) + H₂O (l) to form Zn(CH₃COO)₂ · 2H₂O (cr) is –(65.78 ± 0.36) kJ · mol^?1 for measurements in 5 mol · L^?1 NaOH and –(66.25 ± 0.17) kJ · mol^?1 for measurements in 5 mol · L^?1 HCl. The standard enthalpy of formation of Zn(CH₃COO)₂ · 2H₂O from the elements is –(1669.35 ± 1.30) kJ · mol^?1. This work provides the first calorimetric measurement of the enthalpy of formation of Zn(CH₃COO)₂ · 2H₂O.     

二水草酸钙的形成及其对尿路结石的防治

二水草酸钙(COD)是泌尿系结石的主要成分之一,其成核、生长和聚集过程与尿石的形成密切相关。本文结合我们近年来的工作,综述了生物膜及其模拟膜对COD的调控作用,尿大分子、焦磷酸盐、多磷酸盐、柠檬酸、酒石酸及表面活性剂等尿小分子的诱导作用,过饱和度、化学计量条件、pH值、离子强度、温度等体系参数对COD形成的影响;讨论了COD的形貌、晶面电荷及其与细胞膜粘附的研究进展。从临床上预防和治疗草酸钙结石的角度,综述了有利于COD形成的因素。     

水溶性聚合物和两亲聚合物

研究了晶种存在下，丙烯酸／乙酸乙烯酯无规共聚物对二水合硫酸钙从过饱和水溶液中结晶动力学的影响。发现该共聚物存在下ＣａＳＯ４·２Ｈ２Ｏ的结晶过程存在诱导期。诱导期随共聚物浓度或共聚物中乙酸乙烯酯含量的增加而增加，随温度、晶种含量或初始钙离子浓度的降低而增加。诱导期后的结晶生长速度与共聚物浓度、共聚物组成和晶种用量几乎无关。     

Peculiar kinetics of the complex formation in the iron(III)–sulfate system

The results of comprehensive equilibrium and kinetic studies of the iron(III)–sulfate system in aqueous solutions at I = 1.0 M (NaClO₄), in the concentration ranges of T = 0.15–0.3 mM, and at pH 0.7–2.5 are presented. The iron(III)–containing species detected are FeOH²⁺ (=FeH_?1), (FeOH) (=Fe₂H_?2), FeSO, and Fe(SO₄) with formation constants of log β = ?2.84, log β = ?2.88, log β = 2.32, and log β = 3.83. The formation rate constants of the stepwise formation of the sulfate complexes are k_1a = 4.4 × 10³ M^?1 s^?1 for the ${\rm Fe}^{3+} + {\rm SO}_4^{2-}\,\stackrel{k_{1a}}{\rightleftharpoons}\, {\rm FeSO}_4^+The results of comprehensive equilibrium and kinetic studies of the iron(III)–sulfate system in aqueous solutions at I = 1.0 M (NaClO₄), in the concentration ranges of T = 0.15–0.3 mM, and at pH 0.7–2.5 are presented. The iron(III)–containing species detected are FeOH²⁺ (=FeH_?1), (FeOH) (=Fe₂H_?2), FeSO, and Fe(SO₄) with formation constants of log β = ?2.84, log β = ?2.88, log β = 2.32, and log β = 3.83. The formation rate constants of the stepwise formation of the sulfate complexes are k_1a = 4.4 × 10³ M^?1 s^?1 for the ${\rm Fe}^{3+} + {\rm SO}_4^{2-}\,\stackrel{k_{1a}}{\rightleftharpoons}\, {\rm FeSO}_4^+$ step and k₂ = 1.1 × 10³ M^?1 s^?1 for the ${\rm FeSO}_4^+ + {\rm SO}_4^{2-} \stackrel{k_2}{\rightleftharpoons}\, {\rm Fe}({\rm SO}_4)_2^-$ step. The mono‐sulfate complex is also formed in the ${\rm Fe}({\rm OH})^{2+} + {\rm SO}_4^{2-} \stackrel{k_{1b}}{\longrightarrow} {\rm FeSO}_4^+$ reaction with the k_1b = 2.7 × 10⁵ M^?1 s^?1 rate constant. The most surprising result is, however, that the 2 FeSO? Fe³⁺ + Fe(SO₄) equilibrium is established well before the system as a whole reaches its equilibrium state, and the main path of the formation of Fe(SO₄) is the above fast (on the stopped flow scale) equilibrium process. The use and advantages of our recently elaborated programs for the evaluation of equilibrium and kinetic experiments are briefly outlined. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 114–124, 2008     

Crystallization and thermal characterization of calcium hydrogen phosphate dihydrate crystals

Calcium hydrogen phosphate dihydrate (CHPD) is found quite frequently in urinary calculi (stones). The CHPD crystals were grown by the single diffusion gel growth technique in sodium metasilicate gel. The crystals were found to be having platelet and broad needle type morphologies. The crystals were analyzed by FTIR spectroscopy. The thermal properties were studied by employing the thermogravimetric analysis (TG). Various kinetic and thermodynamic parameters for dehydration were estimated from the TG curve. The selected platelet was studied by scanning electron microscope (SEM) for the growth morphologies indicating that the crystals grew in the form of leaflets having prominent (010) faces. This was in agreement with earlier reported studies.     

Effect of microwave treatment on the kinetics of formation and morphology of calcium hydrosilicates

It was shown that microwave treatment improves kinetics of formation and morphology of calcium hydrosilicates obtained from the waste of boric acid production, borogypsum.     

Influence of borax and citric acid on the hydration of calcium sulfoaluminate cement

The hydration of calcium sulfoaluminate cement in the presence of borax and citric acid has been studied using isothermal calorimetry, X-ray diffraction, thermogravimetric analysis, Fourier transfer infrared spectrometry and mercury intrusion porosimetry. The results suggest that the formation of complex between calcium and citrate in the solution adsorbed on the solid surface delayed the C₄A₃ \( \bar{S} \) and anhydrite dissolution and AFt nucleation. At the same time, the presence of borate replaced sulfate anion in ettringite (AFt) to form B-AFt. These reactions have an effect on the setting times and the mechanical strength values. The setting times of the calcium sulfoaluminate cement increased when the mineral admixtures increased in the cement. The effect of borax and citric acid on the strength development at different hydration stages presented different results, namely a decreased strength in an early stage and an increased strength at 3 and 28 days.     

Thermal decomposition kinetics of aluminum sulfate hydrate

The kinetics of individual stages of thermal decomposition of Al₂(SO₄)₃·18H₂O were studied by TG method. It is found that Al₂(SO₄)₃·18H₂O decomposes to Al₂O₃ in four major stages, all of endothermic. Some of these major stages are formed by sub-stages. The first three major stages are dehydration reactions in which two, ten and six moles water are lost, respectively. The last major stage is sulfate decomposition. In this study the kinetic parameter values of these major and sub-stages were calculated by integral and differential methods. The alterations of activation energies with respect to the decomposition ratio and to the method were investigated.