Synthesis of nanosized calcium carbonate (aragonite) via a polyacrylamide inducing process

A carbonation route for the synthesis of nanosized calcium carbonate (aragonite) was studied. In the process, polyacrylamide was used as an organic substrate to induce the nucleation and growth of calcium carbonate. The calcium carbonate particles were produced by means of carbonation of the mixture of calcium hydroxide and polyacrylamide by bubbling CO₂/N₂ gas mixture_. The operating parameters such as the concentration of organic substrate and temperature were varied to study their influences on the polymorph and crystal sizes of calcium carbonate particles. The morphology of the calcium carbonate particles was characterized with transmission electron micrograph (TEM). The synthesized calcium carbonate particles in the presence of organic substrate are the mixture of aragonite with needle shape and calcite with cubic shape. Fourier transform infrared spectroscopy (FTIR) analysis reveals the presence of aragonite and calcite. The polymorphs and their crystal sizes were characterized with X-ray diffraction (XRD). The calcium carbonate nucleated in the organic substrate exhibited different endothermic peak in differential thermal gravity (DTG) results compared to one coexisting with the organic substrate.     

氯化镁-氢氧化钠复合添加剂制备文石型碳酸钙

采用碳化法,使用氯化镁和氢氧化钠复合添加剂,制备含文石相的沉淀碳酸钙。采用X射线衍射（XRD）、透射电镜（TEM）对产物的晶型和形貌进行表征。讨论了氢氧化钠添加比例、氢氧化钙浓度以及碳化反应温度等工艺条件对实验结果的影响,并分析了过程的反应机理。结果表明：产物中的文石相来自于碳酸钙在氢氧化镁沉淀上的异相成核和生长,方解石相来自于碳酸钙在溶液中的均相成核和生长;氢氧化钠的加入降低了溶液中碳酸钙的过饱和度,抑制了均相成核过程,降低了氯化镁的添加比例。在碳化反应温度为30 ℃、氢氧化钙浓度为1.7~ 2.0 mol/L、氢氧化钠与氯化镁物质的量比为2.6条件下,对文石型碳酸钙的生成较为有利。     

CHEMICAL ABSORPTION OF CO2 AND SO2 INTO Ca(OH) 2 SLURRY

Chemical absorption of CO₂ and SO₂2 as single gases and as a mixture into slurries of Ca(OH) ₂ was studied in a stirred vessel with a flat gas-liquid interface. In the case of CO₂, the reaction product interrupted the subsequent gas absorption in the absence of a surface active agent. With single gases, the enhancement factor for SO₂ was much larger than that for CO₂, though both were larger than that into saturated solution. With the mixed gases, the enhancement factor for S02 was almost equal to that for the single gas absorption, but for CO₂ it was only slightly larger than that into the saturated solution     

Simultaneous CO2 capture and thermochemical heat storage by modified carbide slag in coupled calcium looping and CaO/Ca(OH)2 cycles

The simultaneous CO_2 capture and heat storage performances of the modified carbide slag with by-product of biodiesel were investigated in the process coupled calcium looping and CaO/Ca(OH)_2 thermochemical heat storage using air as the heat transfer fluid. The modified carbide slag with by-product of biodiesel exhibits superior CO_2 capture and heat storage capacities in the coupled calcium looping and heat storage cycles. The hydration conversion and heat storage density of the modified carbide slag after 30 heat storage cycles are 0.65 mol·mol~(-1) and 1.14 GJ·t~(-1), respectively, which are 1.6 times as high as those of calcined carbide slag. The negative effect of CO_2 in air as the heat storage fluid on the heat storage capacity of the modified carbide slag is overcome by introducing CO_2 capture cycles. In addition, the CO_2 capture reactivity of the modified carbide slag after the multiple calcium looping cycles is enhanced by the introduction of heat storage cycles. By introducing 10 heat storage cycles after the 10 th and 15 th CO_2 capture cycles, the CO_2 capture capacities of the modified carbide slag are subsequently improved by 32%and 43%, respectively. The porous and loose structure of modified carbide slag reduces the diffusion resistances of CO_2 and steam in the material in the coupled process. The formed CaCO_3 in the modified carbide slag as a result of air as the heat transfer fluid in heat storage cycles decomposes to regenerate CaO in calcium looping cycles, which improves heat storage capacity. Therefore, the modified carbide slag with by-product of biodiesel seems promising in the coupled calcium looping and CaO/Ca(OH)_2 heat storage cycles.     

K2CO3-Na2CO3-H2O三元体系288 K相平衡研究

采用等温溶解平衡法研究了三元体系K2 CO3 -Na2 CO3 -H2 O 2 88K时的相平衡及平衡液相的主要物化性质 (密度 ,电导率 ,pH)。研究发现 :该三元体系有复盐NaKCO3 ·6H2 O形成 ,根据溶解度数据绘制了相图 ,相图中有 2个共饱点E、F ,三条单变度曲线AE、BF、EF ;平衡固相分别为K2 CO3 ·3/ 2H2 O、Na2 CO3 ·10H2 O、NaKCO3 ·6H2 O。实验结果表明K2 CO3 对Na2 CO3 有强烈的盐析作用 ;并简要讨论了实验结果。     

Preparation and characterization of CaO nanoparticles from Ca(OH)2 by direct thermal decomposition method

CaO is an important material because of its application as catalyst and effective chemisorbents for toxic gases. In this research CaO nanoparticles were prepared via direct thermal decomposition method using Ca(OH)₂ as a wet chemically synthesized precursor. Nanocrystalline particles of Ca(OH)₂ have been obtained by adding 1 and 2 M NaOH aqueous solutions to 0.5 M CaCl₂·2H₂O aqueous solutions at 80 °C. The precursor [Ca(OH)₂] was calcined in N₂ atmosphere at 650 °C for 1 h. Samples were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), infrared spectrum (IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Brunaure–Emett–Teller (BET). SEM images showed that CaO nano-particles were nearly spherical in morphology. TEM images illustrated that produced CaO nano-particles had the mean particle size of 91 and 94 nm for 1 and 2 M NaOH concentration, respectively. As a result, this method could be used for production of CaO nano-particles on large-scale as a cheap and convenient way, without using any surfactant, organic medium or complicated equipment.     

CO_2浓度对氢氧化钙碳化性能的影响

碳化时间及CO2浓度,显著影响氢氧化钙的碳化特性。本试验中,保持CO2压力为0.2 MPa、用水量为10%,控制CO2浓度变化范围为10%~99%,碳化时间为5 min~24 h。试验结果显示,在同一CO2浓度下,随碳化时间延长,氢氧化钙碳化速率逐渐提高,试块强度也不断增加;而随着气体浓度的不断增加,氢氧化钙的碳化速度也不断增加。研究发现,在试块达到相同碳化程度时,试块在低浓度CO2气体下的碳化强度要高于高浓度气体条件下的碳化强度。对碳化样品的XRD分析表明,碳化产物中仅含有方解石型碳酸钙。     

CO2碳酸化石灰岩酸解产物回收乙酸及副产沉淀碳酸钙

乙酸酸解石灰石造腔是一种建造地下储库同时环保地开采石灰岩制备沉淀碳酸钙的新方法。通过耦合乙酸酸解石灰石及酸解产物乙酸钙CO₂碳酸化的工艺过程,研究了乙酸酸解石灰岩的表面反应动力学和乙酸钙CO₂碳酸化的工艺技术条件。采用正交实验分析法,研究了CO₂碳酸化反应中乙酸钙浓度、反应温度、CO₂压力、反应时间对乙酸钙碳酸化反应制沉淀碳酸钙的影响,并通过正交实验确定了最优化操作条件。实验结果表明,乙酸酸解反应速率主要受乙酸浓度控制。CO₂碳酸化反应在当乙酸钙溶液浓度为0.631 mol·L^-1,CO₂压力为5.0 MPa,温度为80℃,反应时间为50 min时CO₂碳酸化效率达到最高（23.13%）,生成的沉淀碳酸钙产品各项指标均符合中国国标优级要求。     

钙碳比对CaSO4-CaCO3-(NH4)2CO3-H2O体系反应-结晶过程的影响

在CaSO₄-CaCO₃-（NH₄）₂CO₃-H₂O反应体系中,生成碳酸钙与硫酸铵的控速步骤是石膏溶解。溶解表面被新生的碳酸钙结晶覆盖是重要影响因素。基于反应面模型,通过调整钙离子溶出速率与碳酸根加入速率之比（钙碳比）,将反应-结晶面控制在溶解表面的固液扩散边界层临界距离之外从而避免覆盖。实验采用表面积一定的天然纯净块状石膏试样,并向溶解体系滴加碳酸铵溶液,使钙碳比分别为0.125、0.25、0.5、1、2、4和8,观察碳酸钙在石膏表面的包覆现象,并预测反应-结晶面与溶解表面之间的微距离。结果显示,当钙碳比为0.125、0.25、0.5和1时,试样表面出现碳酸钙包覆但包覆量随λ增大而减小;当钙碳比为2、4和8时,试样表面不出现包覆,碳酸钙结晶生成于液相主体。将钙碳比为1视为临界值,该条件下反应-结晶面与溶解表面的微距离大于18 μm。     

Precipitation of barium carbonate from alkanolamine solution—study of CO2 absorption from landfill gas (LFG)

Addition of barium chloride (BaCl₂.2H₂O) to alknolamine aids formation of barium carbonates when landfill gas (LFG) is passed through the alkanolamine solution. The process is an economical and efficient way of removing large amounts of CO₂. Supersaturation plays an important role in this interesting process. Copyright © 2010 Society of Chemical Industry     

Preparation of Ca(OH)2 nanoparticles by impinging stream reaction precipitation method

In this paper, Ca(OH)₂ nanoparticles are prepared by impinging stream co-precipitation method. The process of preparing nanoparticles by impinging stream is optimized. The influence of process variable, such as dispersant type, dispersant dosage, circulating flow rate, reactant concentration ratio of OH⁻ and Ca²⁺, reaction temperature and circulation time, on the particle size of Ca(OH)₂ nanoparticle are investigated. The results show that the concentration ratio of reactants has a significant effect on the size of Ca(OH)₂ nanoparticles. The optimal process conditions are obtained by single factor experiment, PEG6000, 3.3% dispersant dosage, Q = 1000 L · h⁻¹, C = 1.88, T = 46.68°C and t = 60 min. The average particle size of Ca(OH)₂ nanoparticles prepared under these conditions is 107.67 nm. According to the microstructure analysis, the prepared Ca(OH)₂ nanoparticles samples have high purity and a good crystal structure. The powder dispersed with a marked hexagonal crystal shape and good thermal decomposition.     

Studies of alkali-silica reaction. Part 3. Mechanisms by which NaCl and Ca(OH)2 affect the reaction

Recently it has been observed that a concentrated solution of NaCl accelerates alkali-silica reaction and that the presence of free Ca(OH)₂ is a prerequisite for expansion to occur. This paper reports work done to understand the chemical processes involved.From the results of this investigation following mechanism has been proposed to explain the above observations. In the presence of free Ca(OH)₂, Na⁺ ions from alkali salts and OH⁻ ions from Ca(OH)₂ together with water molecules penetrate reactive grains. SiOSi bonds in reactive grains are broken by penetrating Na⁺ and OH⁻ ions thereby opening the grains for further penetration of materials. At the same time silica ions tend to diffuse out of the reactive grains. Expansion occurs when more materials enter the reactive grains than diffuse out.     

Microstructural and strength improvements through the use of Na2CO3 in a cementless Ca(OH)2-activated Class F fly ash system

This study explores the beneficial effects of Na₂CO₃ as an additive for microstructural and strength improvements in a Ca(OH)₂-activated fly ash system. NaOH-activated fly ash samples were also tested to compare the effect of Na₂CO₃. Compressive strength testing, XRD, SEM/BSE/EDS, ²⁹Si/²⁷Al MAS-NMR, MIP and TGA were performed. The testing results indicate that the use of Na₂CO₃ for Ca(OH)₂-activation led to a noticeable improvement in strength and microstructure, primarily due to (1) more dissolution of raw fly ash at an early age, (2) more formation of C–S–H [or C–S–H(I)], (3) porosity reduction, and (4) pore-size refinement. We also found that (1) an early high alkalinity from the NaOH formation was not a major cause of strength, (2) geopolymer was not formed despite the early NaOH formation, and (3) no visible pore-filling action of CaCO₃ was observed. However, Na₂CO₃ did not produce any improvement in strength for NaOH-activated fly ash.     

Ca(OH)2-CaCO3复合粉体粉磨过程中的机械化学效应

在CaCO3原料中,利用机械化学法对CaCO3进行Ca(OH)2包覆和活化,借助SEM,TEM,XRD,IR和BET及TG-DSC等手段,研究机械活化时间对复合粉体表面性质及晶体结构的影响.结果表明,机械活化可导致复合粉体颗粒细化,比表面积增大,晶粒变小,晶格畸变,并趋于无定形化,但活化时间过长,Ca(OH)2脱羟反应温度提前,粉体机械化学过程中累积的能量部分释放,使晶粒长大.     

Optimization of the Coating Process of a Ca(OH)2-Based Thermochemical Energy Storage Material

A promising candidate for a thermochemical storage system implemented into district heating is a previously developed core-shell material based on CaO/Ca(OH)₂. Proceeding from the preliminary manufacturing process, this work aims to optimize relevant material properties as mechanical stability and shell uniformity, by varying defined parameters during a reproduceable coating procedure, ready for commercial upscaling.     

Effect of Zn(OH)2 on properties of corundum based castables bonded with calcium aluminate cement

In this work, the effect of Zn(OH)₂ on properties of corundum based castables bonded with calcium aluminate cement (CAC) was investigated. The phase composition and microstructure of castable matrixes containing Zn(OH)₂ after firing 800 °C, 1100 °C and 1550 °C were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectra, respectively. The results indicate that a small amount of Zn(OH)₂ can dramatically improve the medium temperature strength of castables because the generation of zinc aluminate spinel increases the ceramic bonding of castables. In addition, the addition of Zn(OH)₂ also improves the volume stability of CAC-bonded castables due to the enhanced formation of pores from Zn(OH)₂ decomposition in castables.     

Synthesis of nanocrystalline hydroxyapatite from Ca(OH)2 and H3PO4 assisted by ultrasonic irradiation

This work is focused on the synthesis of hydroxyapatite powders by precipitation assisted by a tip ultrasonic irradiation (UI) from Ca(OH)₂ and H₃PO₄. The effect of UI time, applied UI power during the synthesis, initial Ca²⁺ concentration and ageing time of the mixture on phase composition, morphology and specific surface area of the obtained powders was analysed. Particles obtained from the precipitation method using UI or mechanical agitation were compared with a nanosized hydroxyapatite of reference in terms of crystal size, phase composition and thermal stability. This study revealed the feasibility to produce hydroxyapatite powders with controlled crystal size between 31.7 and 70.6 nm by ultrasonic assisted precipitation method. Hydroxyapatite powder with similar surface area to the reference material can be obtained adjusting experimental conditions. The used method allowed us to synthesise hydroxyapatite powders with thermal stability up to 1100 °C.     

3D Nitride Frameworks with Variable Channel Sizes; Synthesis and Powder Neutron Diffraction Study of the Nitride Carbodiimdes, Ca4N2(CN2) and Ca11N6(CN2)2

The synthesis of powders of the calcium nitride carbodiimides, Ca₄N₂(CN₂) (1) and Ca₁₁N₆(CN₂)₂ (2) has been studied and the products characterised initially by powder X-ray diffraction. The latter phase (2) is unobtainable in pure bulk form and is always accompanied by the former compound. Nevertheless, definitive structures of the compounds were obtained from Rietveld refinements against time-of-flight powder neutron diffraction data. Each of the nitride carbodiimides are composed of Ca–N three dimensional networks built from edge- and vertex-sharing NCa₆ octahedra arranged to create one-dimensional channels. The size of the channels is dependent on the Ca:C (host:guest) ratio and the larger channels in (1) are occupied by twice as many guest species as those in (2). Importantly, neutron diffraction data confirm that the identity of these guest species is carbodiimide (as opposed to isoelectronic azide), that neither the framework nor the channels therefore contain excess electrons and that the compounds are hence not “nitride electrides” as per the parent subnitride, Ca₂N.     

聚乙烯醇模板合成链状纳米氢氧化钙机理研究

纳米材料的形貌与团聚控制正成为研究的热点。以聚乙烯醇(PVA124)作为模板、采用液相沉淀法在常温下控制合成了特殊的链状纳米氢氧化钙。通过透射电镜、粒度分析和红外光谱表征表明该链状纳米氢氧化钙能以相对稳定的聚焦态存在,且PVA124与所制备的氢氧化钙间存在较强的作用,而并非简单的吸咐;电导率、紫外可见光谱、TG-DSC等表征表明,钙离子与PVA侧链羟基间存在络合作用,并通过示意图模拟以PVA为模板聚集形成纳米链状沉淀的过程。