Model of limestone calcination / sulfation under oxy-fuel fluidized bed combustion

The characteristics of the simultaneous calcination/ sulfation of limestone under oxy-fuel fluidized bed combustion were studied and compared with those of the sulfation of precalcined CaO. During the calcination stage, SO2 can react with product CaO and slow down the CaCO3 decomposition rate by the covering effect of the CaSO4 product. The sulfation rate of simultaneous calcinatiort/sulfation is slower than that of precalcined CaO, but with a long enough sulfation time, the calcium conversion of simultaneous calcination/sulfation is higher than that of the precalcined CaO. A grain-micrograin model is established to describe the simultaneous calcination, sintering and sulfation of limestone. The graln-micrograln model can reflect the true reaction process of the calcination and sulfation of limestone in oxy-fuel fluidized bed combustion.

Evolution of the Surface Area of Limestone during Calcination and Sintering

The calcination reaction of limestone is always companied by sintering of the calcined product. In addition, accelerated sintering rates and a reduced specific surface area are observed in the presence of steam and carbon dioxide. To simulate the change of surface area and the porosity of limestone samples in a simultaneous calcination and sintering process, a combined model based on both a sintering model and a calcination model is established. The calcination model, which predicts calcination conversion as a function of time, is based on the initial properties of the sorbent. The sintering model is according to the German and Munir model in which the main transport mechanism is supposed to be lattice diffusion. In a flow reactor, the surface area value and calcination rate of limestone in the presence of steam and CO2 are also described by the combined model with modified parameters.

THERMAL DECOMPOSITION BEHAVIOUR OF NATURAL BASTNASITE CRYSTAL IN CALCINATION

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Grain Size Effect of Calcite on Calcination: A Case Study from Afyon (Western Turkey)

Limestones have been one of the major economic sources in state’s economy and becoming more and more important in recent years. They are mainly exploited for crushed stone, dimension stone and micronized calcite production including lab-grade CaO with high purity. Although some of the applications require fairly simple value-adding processes such as crushing and/or sizing, some may require rather complicated processes such as calcination and coating.

Emirati Limestones: Impact of Low Temperature Microstructure on the Industrial Applications

The main aim of this work is to study the possibility of using different Emirati limestones for the production of quicklime. Representative limestone samples have been collected to represent the Triassic, Jurassic, Cretaceous, Eocene and Oligocene ages. The limestone samples have been characterized for their microstructure, mineral and chemical composition, physico-mechanical characteristics, thermal behavior using polarizing and scanning microscopes together with X-ray micro-tomography, XRD-IR, XRF, Archimedes and Mercury intrusion methods and DTA-TGA, respectively. Post characterization, the samples were fired in electrical muffle furnace for calcination under the firing conditions (800℃ to 1100℃) for (0.25 – 2 h). Then the lime grains have been characterized for their hydration rate, free lime content, pore-distribution and microfabric.

Dynamic Simulation on Surface Hydration and Dehydration of Monoclinic Zirconia

The commonly used oxide-supported metal catalysts are usually prepared in aqueous phase,which then often need to undergo calcination before usage.Therefore,the surface hydration and dehydration of oxide supports are critical for the realistic modeling of supported metal catalysts.In this work,by ab initio molecular dynamics(AIMD)simulations,the initial anhydrous monoclinic ZrO_(2)(111)surfaces are evaluated within explicit solvents in aqueous phase at mild temperatures.During the simulations,all the two-fold-coordinated O sites will soon be protonated to form the acidic hydroxyls(HO_(L)),remaining the basic hydroxyls(HO^(∗))on Zr.The basic hydroxyls(HO^(∗))can easily diffuse on surfaces via the active proton exchange with the undissociated adsorption water(H_(2)O^(∗)).Within the temperatures ranging from 273 K to 373 K,in aqueous phase a certain representative equilibrium hydrated m-ZrO_(2)(¯111)surface is obtained with the coverage(θ)of 0.75 on surface Zr atoms.Later,free energies on the stepwise surface water desorption are calculated by density functional theory to mimic the surface dehydration under the mild calcination temperatures lower than 800 K.By obtaining the phase diagrams of surface dehydration,the representative partially hydrated m-ZrO_(2)(111)surfaces(0.25≤θ<0.75)at various calcination temperatures are illustrated.These hydrated m-ZrO_(2)(111)surfaces can be crucial and readily applied for more realistic modeling of ZrO_(2) catalysts and ZrO_(2)-supported metal catalysts.

The Gboko Limestone, Yandev, Benue State, Nigeria: Geology, Geochemistry and Industrial Potentials

The Cretaceous shallow marine Gboko limestone, Yandev, Nigeria is a component of the sedimentary fill of the 800 km NE-SW trending Benue Trough, Nigeria. The limestone is made up of thin bedded to massive limestone beds interspersed with laminated grey shale having foraminifera as the dominant fossil. The limestone has both mud supported and grain supported texture, and micrites constitute about 75% of the limestone. Bulk chemical composition analysis of the limestone reveals average CaCO3 of 92.41% and a range of 77.50% - 99.00%. Mineralogical impurities include quartz, dolomite, pyrrhotite, fluorapatite etc. Trace elements concentration analysis was carried out using Energy Dispersive X-ray (EDXRF) spectrophotometry and showed the following trace elements: Mn (841.3 ppm), Sr (444.6 ppm), Fe (470 ppm), Zn (114.6 ppm) and Pb (116.4 ppm). Calcining the limestone in a laboratory muffle furnace at 1050°C for 90 minutes produced a compact, soft burnt porous and reactive lime that does not crumble into fines. The lime so produced neither meets the requirements of the Steel Making Shop (SMS) of the Ajaokuta Steel Plant nor could it be used in the growing sugar refining industry in Nigeria. It can however be used in the food and the food by-products industry, environmental, agricultural and petroleum industries etc. The raw stone remains a major source of raw materials for cement manufacture for the ever expanding building industry.

Strength of Limestone-based Non-calcined Cement and its Properties

A new type of cement was prepared with ground limestone powder, blastfumace slag, steel slag and gypsum without calcination. The fraction of ground limestone powder in the cement was as high as 40 wt%-60 wt% without Portland clinker. All of its physical properties can meet the requirements of masonry cement standards. The impact of limestone content on physical properties of the cement and determined its impact on law was investigated. The steel slag can excit the aquation activity of this cement effectively, and the influence of its quantity on the strength of the materials was studied, which shows that the optimum quantity of mixing is 10%. By way of changing the different content of the lime stone by quartzy sample, the law of the compression strength and the PH value was determined, confirming that the lime stone can promote the early aquation of the slag and improve the early strength. The main hydration product of this cement is calcium aluminate hydrate, ettringite and calcium silicate hydrate, as indicated by XRD and SEM analysis.

Hydration Properties of Calcined Clay Pozzolan and Limestone Mineral Admixtures in Binary and Ternary Cements

This paper investigates the properties of hydrated binary and ternary blended cements using limestone and calcined clay pozzolan as supplementary cementitious materials. The blended cements were hydrated and their phase compositions were evaluated by thermogravimetric and powder X-ray diffraction at 28 days. The morphology of the samples was also determined. The water demand, setting time, compressive and flexural strengths of mortar and concrete samples were determined up to 365 days. The study concluded that the portlandite [Ca（OH）2] content was considerably reduced whilst ettfingite formation were enhanced as a result of admixture reactions. The water demand and setting times of blended cements were lower than OPC with 5% admixture content but higher with increasing content. The mechanical test results also showed that Class 42.5N and 32.5R cements can be produced from the binary and ternary blends containing up to 10% and 20% admixtures, respectively.

Production of an Eco-Cement by Clinker Substitution by the Mixture of Calcined Clay and Limestone, Songololo (DR Congo)

Ordinary Portland Cement (OPC) is by mass the largest manufactured product on Earth, responsible for approximately 6% - 8% of global anthropogenic carbon dioxide emissions (CO2) and 35% of industrial CO2 emissions. On average 0.8 to 0.9 ton of CO2 is emitted to produce one ton of OPC. In this paper, partial substitution of clinker (30% - 35%) by the calcined clay-limestone mixture was investigated in order to produce an eco-cement (LC3). Analyzes by XRF, XRD and ATG/ATD have characterized different components, determined the calcination temperature and selected the right clay which can act as effective Supplementary Cementitious Material (SCM). Mechanical tests on mortar carried out over a period of 90 days. The WBCSD/WRI “Greenhouse Gas Protocol” methodology then allowed the calculation of CO2 emissions into the atmosphere. Three types of clay are available in the Songololo Region. The kaolinite is the principal clay mineral and its content varies from 27% to 34%. The sum of kaolinite and amorphous phase which enable clay to react with cementitious material ranges from 57% to 60%. The SiO2 content ranges from 33% to 76%, the Alumina content from 12% to 20% so that the ratio Al2O3/SiO2 is on the higher side (0.17 - 0.53). The calcination window is between 750°C and 850°C and the best clay which can act as SCM identified. The clinker’s substitution reduced CO2 emissions from 0.824 ton of CO2 for one ton of OPC to 0.640 ton of CO2 for one ton of LC3, means 22% less emissions. The compressive strengths developed by LC3 vary from 8.91 to 57.6 MPa (Day 1 to Day 90), exceed those of references 32.5 cement and are close to 42.5 cement. In view of the results, LC3 cement can be considered for industrial trials.

Influence of water-to-binder ratios on the performance of limestone calcined clay cement-based paste for mining applications

This study aims to investigate the impact of various water-to-binder(w/b)ratios by mass(0.3–2.0)on the performance of limestone calcined clay cement(LC^(3))pastes.The flowability,setting time,density,unconfined compressive strength,hydration,and microstructure of LC^(3) pastes under different w/b ratios were thoroughly investigated.The results show that increasing the w/b ratio extends the flowability and setting time while significantly reducing the unconfined compressive strength of LC^(3) pastes.LC^(3) pastes with w/b ratios above 0.6 exhibited a final flow diameter surpassing 167 mm,indicating good flowability.The 28-d unconfined compressive strength decreased from 82.3 to 1.3 MPa as the w/b ratio increased from 0.3 to 2.0.This study confirmed that the relationship between unconfined compressive strength and the w/b ratio of LC^(3) follows Abram’s law.Furthermore,a modified gel/space ratio was proposed to reflect the concentration of solid products,effectively explaining the influence of the w/b ratio on strength,with an R2of 0.96.

Development of limestone calcined clay cement concrete in South China and its bond behavior with steel reinforcement

Limestone calcined clay cement(LC^3),consisting of ordinary Portland cement(OPC)clinker,calcined clay,limestone powder,and gypsum,has been considered a promising solution to current challenges in the cement and concrete industry,such as high carbon emissions,high energy consumption,and resource shortages.This study carries out a series of experimental investigations of LC^3-based paste,mortar,and concrete,including microstructural analyses(e.g.hydration product characterization and pore structure analysis)and macro-scale testing(e.g.workability and mechanical properties),using raw materials from south China.The results show that,in LC^3 paste,the replacement of clinker by calcined clay and limestone leads to an increased volume of small pores but decreased total volume of pores.The workability of LC^3 mortar and concrete can be readily tailored using conventional superplasticizers.When designed for comparable 28-d compressive strength,the LC^3 mortar and concrete tend to have lower early-age compressive strength,but comparable compressive strength and higher flexural strength than those of the OPC counterparts at late ages.This study also examines the bond-slip behavior between LC^3 concrete and steel bars and finds that the bond strength is comparable to that of OPC concrete with the same 28-d compressive strength,but that the LC^3 concrete-rebar interface exhibits higher bond-slip stiffness.These findings on LC^3 concrete provide fundamental information and guidance for furthering the application of LC^3 binder in structural concrete in the near future.

Geo-environmental properties and microstructural characteristics of sustainable limestone calcined clay cement(LC3)binder treated Zn-contaminated soils

Limestone calcined clay cement(LC3)is an environment-friendly and sustainable cementitious material.It has recently gained considerable attention for the stabilization/solidification(S/S)of soils contaminated by heavy metals.However,the existing studies on S/S of Zn-contaminated soils using LC3 in terms of hydraulic conductivity and microstructural properties as compared to ordinary Portland cement(OPC)are limited.This study focuses on the evaluation of the mechanical,leaching,and microstructural characteristics of Zn-contaminated soils treated with different contents(0%,4%,6%,8%,and 10%)of low-carbon LC3.The engineering performance of the treated Zn-contaminated soils is assessed over time using unconfined compressive strength(UCS),hydraulic conductivity(k),toxicity characteristic leaching procedure(TCLP),and synthetic precipitation leaching procedure(SPLP)tests.Experimental results show that the UCS of Zn-contaminated soils treated with LC3 ranged from 1.47 to 2.49 MPa,which is higher than 1.63%–13.07%for those treated with OPC.The k of Zn-contaminated soils treated with LC3 ranged from 1.16×10^(−8)to 5.18×10^(−8)cm/s as compared to the OPC treated samples.For the leaching properties,the leached Zn from TCLP and SPLP is 1.58–321.10 mg/L and 0.52–284.65 mg/L as the LC3 contents ranged from 4%to 10%.Further,the corresponding pH modeling results indicate that LC3 promotes a relatively suitable dynamic equilibrium condition to immobilize the higher-level Zn contamination.In addition,microscopic analyses demonstrate that the formations of hydration products,i.e.,Zn(OH)_(2),Zn_(2)SiO_(4),calcium silicate hydrate(C–S–H),calcium silicate aluminate hydrate(C–A–S–H)gel,ettringite,and CaZn(SiO_(4))(H_(2)O),are the primary mechanisms for the immobilization of Zn.This study also provides an empirical formula between the UCS and k to support the application of LC3-solidified Zn-contaminated soils in practical engineering in the field.

Degradation of limestone calcined clay cement(LC^(3))mortars under sulfate attack

Limestone Calcined Clay Cement(LC^(3)) is a newly proposed low-carbon cement,which can effectively reduce energy consumption and carbon emissions of the traditional cement industry without changing the basic mechanical properties of cement-based materials.In this study,the degradation process of mortar samples of limestone and calcined clay cementitious material under sulfate attack is studied by both macroscopic and microscopic analysis.The results show that compared with pure Portland cement,the addition of calcined clay and limestone can significantly reduce the expansion rate,loss of dynamic modulus and mass loss of mortar specimens under sulfate attack.The addition of calcined clay and limestone will refine the pore size distribution of mortar specimens,then inhibiting the diffusion of sulfate and formation of corrosive products,therefore leading to a significant improvement of the sulfate resistance.

钼酸铵生产中杂质钾的浸出特征分析研究

在采用焙烧―氨浸法生产钼酸铵过程中,通过ICP、XPS、SEM等手段研究了钼焙砂氨浸过程中钾元素的浸出规律,并借助MLA和Factsage热力学软件考察了氨浸过程中钾元素的赋存形态变化。采用未反应核模型、抛物线扩散方程、双常数方程、Elovich方程及一级动力学方程等动力学模型探讨了钾释放的动力学规律。结果表明:钼焙砂氨浸过程中钾的浸出可分为2个阶段:前期为离子钾的浸出过程,主要发生KCl、K 2SiF 6的离子交换反应,浸出活化能较小,仅为4.79 kJ/mol,Elovich模型对该阶段的拟合效果最好;后期主要为矿物钾中钾的浸出过程,浸出活化能较大,为34.55 kJ/mol,最优动力学模型为双常数模型。浸出后期,钾的释放速率较慢,4种含钾矿物的释钾能力由弱到强顺序为:伊利石<钡铁云母<云母<正长石。

石灰石-煅烧黏土-水泥(LC^(3))体系的水化动力学模型

石灰石-煅烧黏土-水泥(LC^(3))是一种新型复合建筑材料,在满足水泥可持续生产和节能减排方面具有优良的应用前景。本工作通过考虑煅烧黏土和石灰石矿物掺合料的稀释效应、成核作用和火山灰反应等影响作用,提出了一种评估LC^(3)混凝土化学和力学性能的水化动力学模型。根据动力学模型,分析计算了不同掺量情况下LC^(3)胶凝体系的累计水化热、氢氧化钙含量和结合水总量。通过将模型分析结果与试验结果相比较,证明了所建立的模型可较好地模拟LC^(3)水泥胶凝体系的水化进程。结果表明,在一定掺量范围内,LC^(3)水泥胶凝体系的水化程度与掺量成正比,而氢氧化钙含量、结合水总量和累计水化热与之成反比,LC^(3)材料用于水泥辅助胶凝材料时的推荐掺量为25%~35%。

基于响应面法的石灰石煅烧黏土复合胶凝材料体系优化设计和试验研究

为探索石灰石煅烧黏土复合胶凝材料体系中石灰石粉和煅烧黏土的最优配合比,本文采用响应面法进行优化试验研究,以普通硅酸盐水泥、煅烧黏土和石灰石粉掺量作为控制变量,砂浆的28 d抗压强度及28 d抗折强度为响应变量进行力学性能测试,建立矿物掺合料复配比例与力学性能之间的回归模型。在矿物掺合料为15%、30%、45%(质量分数,下同)的条件下,采用回归模型对石灰石煅烧黏土复合胶凝材料体系中石灰石粉与煅烧黏土的配合比进行优化,优化结果分别为:煅烧黏土占比15%,不含石灰石粉;煅烧黏土占比25%,石灰石粉占比5%;煅烧黏土占比30.5%,石灰石粉占比14.5%。上述体系的28 d抗压强度预测值分别为63.26、58.57、48.10 MPa,试验实测值分别为63.74、57.21、47.02 MPa,预测值与实测值之间的相对误差均小于5%。本研究为石灰石煅烧黏土复合胶凝材料体系最优配合比问题提供了新的解决思路和试验参考。

陶砂对石灰石-煅烧黏土-水泥砂浆性能的影响

采用轻质陶砂、水泥、偏高岭土和石灰石粉制备了石灰石-煅烧黏土-水泥(LC3)砂浆,研究了陶砂掺量及预湿状态对LC3砂浆干密度、力学性能、抗渗性能及抗氯离子侵蚀能力的影响,分析了其强度形成和微观结构机理.结果表明:随着砂胶比的增大,LC3砂浆的干密度和抗压强度逐渐降低,累积吸水量逐渐增大,抗氯离子侵蚀能力逐渐增强;与掺未预湿陶砂的LC3砂浆相比,掺预湿陶砂的LC3砂浆试件28 d抗压强度增大3.4%~10.8%,28 d电通量降低59.0%~80.0%,累积吸水量降低更为显著. LC3体系中石灰石粉的成核作用能够促进水泥早期水化,偏高岭土的火山灰反应有利于细化孔结构,同时预湿陶砂的内养护效应能够维持基体内部湿度继续促进水泥水化,使得掺预湿陶砂的LC3砂浆微观结构更为密实,从而提高了其力学性能、抗渗性能和抗氯离子侵蚀能力.

FeO@C/MAX复合材料的制备及电化学动力学性能

将α-Fe_(2)O_(3)@C与钛粉和铝粉一同进行高温煅烧,制备了Fe O@C/MAX(FCM)复合材料。通过XRD、SEM、TEM表征了FCM复合材料在不同Ti/C与Al/C物质的量比下的结构、组成及形貌变化,采用电化学动力学分析方法定量计算了FCM复合材料的赝电容占比,推测可能的电荷储存机理。结果表明,随着Ti/C与Al/C物质的量比的增大,FCM复合材料中MAX相(Ti_(2)Al C和Ti_(3)Al C_(2))的含量随之变化,而α-Fe_(2)O_(3)转变为不稳定的Fe O。当n(Ti)∶n(Al)∶n(C)=3∶1∶2时,制得的FCM-312样品在1 m V/s扫描速率下的比电容最大,为125.09 F/g,约为α-Fe_(2)O_(3)@C的4.76倍。FCM复合材料中部分MAX相在电化学过程中发生氧化还原反应,为离子间电子快速输运提供了条件,增加了FCM复合材料的赝电容占比。其中,FCM-312样品在10 m V/s扫描速率下的赝电容占比为22.12%。

粉石灰悬浮煅烧技术及下游应用

在现有可回收钙资源中,小粒径石灰石和电石渣无论是从储量还是从氧化钙含量来说都是生产石灰的极佳原料。本文详细介绍采用悬浮煅烧技术回收利用小粒径石灰石和电石渣的工艺流程,并提供了湿电石渣悬浮煅烧的生产实例。结合粉石灰指标,分析了在建筑行业较为合适的应用场景,为粉石灰生产企业提供参考。