菱镁矿热分解的动力学研究

利用热重(TG)分析法,对不同粒度菱镁矿的热分解过程进行研究.根据Flynn-Wall-Ozawa法,拟合计算得到不同粒度菱镁矿热分解的活化能和指前因子.采用Thermo-kinetics软件对可能性最大的5种动力学机理函数进行拟合,根据相关系数最大的原则确定最佳分解机理.研究结果表明:菱镁矿热分解的活化能随菱镁矿粒度的增大而减小,当菱镁矿的粒度由小增大时,控制其热分解过程的机理由化学反应逐渐向颗粒内部的传热和CO2的扩散传质转变;其热分解过程的最可几机理函数为R3模型,即三级相边界扩散反应,函数方程为G(α)=1-(1-α)1/3.     

碱式碳酸镁的热分解研究

通过TGA、DSC热分析法,研究了碱式碳酸镁的热分解过程。发现碱式碳酸镁分两个阶段分解,并测得每一阶段的特征温度和吸热值。由Kissinger与Ozawa-Doyle两种方法求得两个阶段热分解反应的表观活化能Ea和指前因子lgA。用常见的固体热分解机理函数拟合了ln[G(α)/T2]~1/T曲线,用Coats-Redfern法确定了碱式碳酸镁热分解过程第一阶段的机理是三维扩散机理D3,第二阶段的机理是随机成核和核生长机理F1,获得其动力学方程。采用DSC方法测试了碱式碳酸镁的比热容,用最小二乘法拟合曲线,获得其比热的表达式为Cp=-3.890 2+0.207 2T-0.002 58T2+1.386 3×10-5T3-2.755 4×10-8T4。     

1,1′-二羟基-5,5′-联四唑钛盐的合成和热分解行为

以二氯乙二肟、叠氮化钠、盐酸羟胺和三氯化钛等为原料,合成了1,1′-二羟基-5,5′-联四唑钛盐(Ti-BHT)燃烧催化剂。利用差示扫描量热法和热重法研究了不同升温速率下Ti-BHT金属盐的热分解过程,获得了热分解动力学参数和热分解机理函数;用Ozawa法和Kissinger法计算了热分解动力学参数,进而计算出自加速分解温度、热爆炸临界温度和热力学参数;用微量热法测定了Ti-BHT的比热容。结果表明,Ti-BHT的活化能Ek为143.49kJ/mol,指前因子Ak为1.23×10~(13)s~(-1),热分解属于n=3的随机成核和随后生长机理;自加速分解温度TSADT为466.21K,临界爆炸温度Tbpo为505.42K,热分解活化自由能ΔG~≠为142.74kJ/mol,活化焓ΔH~≠为139.41kJ/mol,活化熵ΔS~≠为-6.78J/(mol·K);Ti-BHT在298.15K的标准摩尔比热容为800.51J/(mol·K);摩擦爆炸概率为20%,特性落高大于125.9cm,说明其机械感度较低,具有较好的安全性能。     

α-萘乙酸的非等温动力学研究

对α-萘乙酸(C_(12)H_(10)O_2)的热分解机理进行了研究,采用TG曲线确定了它的热分解过程,并通过四种方程对其热分解过程的活化能En进行了计算,利用41种不同的机理方程af)((微分机理方程)和G(α)(积分机理方程),对其热分解过程的非等温动力学数据进行了线性回归处理,并推断出其热分解机理为n=1/4的化学反应机理,最可几函数为4/3af-=)1(4)(a,并建立了其动力学方程。     

MnSO4·H2O热解制备四氧化三锰反应动力学

为了确定空气中硫酸锰分解生成四氧化三锰的机理,用TG-DSC结合XRD分析研究了硫酸锰的热分解行为,用非线性等转化率法和普适积分法研究了硫酸锰脱水和分解过程的反应动力学。研究结果表明,200～400℃之间为MnSO4·H2O脱水阶段,积分动力学机理函数为G(α)=α^3/2, 符合Mampel Power法则,平均表观活化能为117.11 kJ·mol-1;750～1050℃之间为MnSO4的分解阶段,积分动力学机理函数为G(α)=1-(1-α)1/2,符合相边界反应的收缩圆柱体模型,平均表观活化能为226.44 kJ·mol-1,MnSO4等温分解结果与TG-DSC分析结果一致。     

3,4–双(4'–氨基呋咱基–3')氧化呋咱的热分解动力学、比热容和热爆炸参数研究

采用差示扫描量热仪和微量热仪对3,4–双(4'–氨基呋咱基–3')氧化呋咱(DATF)的热分解动力学、比热容和热分解参数进行研究。结果表明:DATF热分解反应动力学方程为dα/dt=(1013.05/β)(1–α)exp(–1.5×105/(RT)),DATF比热容(单位为J/(g·K))与热力学温度的关系式为cp=0.011 35+0.004 23 T–7.827 8×10–7T2,298.15 K时DATF标准摩尔热容为303.14 J/(mol·K)。根据比热容关系式及DATF热分解参数获得DATF的热爆炸临界温度为560.63 K,绝热至爆时间为32.03 s。     

非等温热重法研究聚苯醚热分解动力学

采用非等温热重法对聚苯醚的热分解动力学进行研究,计算了反应活化能,采用积分法结合36种动力学函数来判断聚苯醚热分解的机理函数,得到了聚苯醚热动力学参数即反应的动力学函数,平均活化能（E_a）为211.64kJ/mol,指前因子（A）的平均值为6.24×10⁷s^-1,也获得了对应的热分解动力学方程。     

阻燃剂聚丙烯酸五溴苄基酯的热分解动力学

用热分析法研究了阻燃剂聚丙烯酸五溴苄基酯在空气和氮气气氛中的热分解动力学。该阻燃剂在空气气氛中为两步分解,在氮气气氛中为一步分解,利用Friedman法求出聚丙烯五溴苄基酯的第一步分解反应的活化能变化趋势,同时利用Satava-Sestak法研究了其热分解机理。结果表明,在0.10至0.90的转化率范围内,聚丙烯酸五溴苄基酯在空气气氛下的活化能为167.35kJ/mol,在氮气气氛下的活化能为171.94kJ/mol,热分解机理均为Avrami-Erofeev方程,随机成核和随后生长,反应级数分别为n=23和n=12。动力学方程分别为G(a)=[-ln(1-a)]23和G(a)=[-ln(1-a)]12。     

2-巯基吡啶镉(Ⅱ)、汞(Ⅱ)配合物的热分解动力学研究

采用TG-DTG技术研究了2-巯基吡啶镉(Ⅱ)、汞(Ⅱ)配合物在氮气气氛中的热分解机理及非等温动力学。采用积分法(Coats-Refern方程,HM方程,MKN方程)和微分法(Achar方程)对非等温动力学数据进行了分析,得到了配合物第一步热分解反应的机理函数、动力学参数和热分解动力学方程。结果表明:其热分解过程属F2(化学反应)机理控制,非等温热分解的动力学方程为dα/dT=A/β.e-E/RT(1-α)2,其中镉(Ⅱ)配合物的表观活化能E=86.35 kJ/mol,指前因子A=4.72×107s-1;汞(Ⅱ)配合物的表观活化能E=189.67 kJ/mol,指前因子A=3.79×1018s-1。     

西藏班戈湖水菱镁的矿热分解特性

采用热重–差示扫描量热法、热重–质谱分析、原位X射线衍射等手段对西藏班戈湖地区水菱镁矿的热分解过程、气体产物、固体产物进行研究,并对热分解机理进行分析。结果表明:水菱镁矿主要在150~650℃温度区间内发生总吸热量约为889.8 J/g多段分解反应,在350℃前脱去结晶水,生成含碳酸根的非晶态镁化合物,随后脱去结构水,并在350~550℃内脱去碳酸根释放出CO_2生成方镁石;水菱镁矿在不同温度区间产生的H_2O和CO_2逸出峰,可能与晶体结构中存在的复杂氢键系统及不同的碳酸根结构有关。     

Non-isothermal kinetic study of high-grade magnesite thermal decomposition and morphological evolution of MgO

Thermal decomposition was the prerequisite and basis for the utilization of magnesite resources. However, the calcination of magnesite was usually accompanied by high energy consumption, which not only caused serious waste of magnesite resources, but also restricted its high value-added utilization. Therefore, calcination conditions were the key to controlling thermal decomposition process of magnesite. The kinetics of high-grade magnesite thermal decomposition was elaborated by non-isothermal thermogravimetric analysis, and meanwhile the effect of heating rate on the magnesite thermal decomposition reaction and morphology of MgO particles were characterized. Both Doyle and Gorbatchev approximate functions were used to simulate the magnesite thermal decomposition process, where the experimental data (correlation coefficient) fitted by the latter could obtain more acceptable kinetic parameters of the magnesite thermal decomposition. The good linear relationship between the activation energy and the pre-finger factor allowed for a kinetic compensation of the thermal decomposition of magnesite. Furthermore, higher heating rate induced the formation of terraced grains, grain network group, cubic grains, and spherical grains for the samples sintered at 1200°C. The heating rate largely affected the magnesite particle morphology, grain size distribution and activity, and also provided important technical indicators in the actual production of magnesia refractory raw materials.     

Use of differential scanning calorimetry in measuring the thermal decomposition of mineral carbonates occurring in coal

Differential scanning calorimetry (d.s.c.) was used to measure in nitrogen the enthalpies of decomposition of mineral carbonates occurring in coal, namely magnesite, siderite, calcite, dolomite and ankerite. Measured values were compared with calculated values, and reasonable agreement was obtained provided that heat losses from the sample were minimized. In air, magnesite, dolomite, ankerite and calcite afforded large negative contributions to the calorific value of coals, whereas siderite produced a small positive calorific contribution.     

用粉煤灰和菱镁矿低温制备堇青石质多孔材料

在粉煤灰中添加不同比例菱镁矿,于1 200、1 250℃和1 300℃烧成温度下制备出堇青石质多孔耐火材料。研究表明：菱镁矿在烧成过程中于700℃分解产生CO2,起到造孔剂和发泡剂的作用,分解生成的MgO将与粉煤灰中的Al2O3、SiO2反应生成堇青石;随着菱镁矿添加量的增加,粉煤灰中Al2O3和SiO2与MgO的反应更完全,主要生成堇青石和少量镁橄榄石;当菱镁矿添加量过多时,新生成的MgO与粉煤灰中SiO2反应,产生液相过多,造成部分气孔封闭,使得烧成后试样的显气孔率随菱镁矿添加量的增加呈先增加后减小的趋势。优化烧成工艺条件为1250℃并保温3h,当菱镁矿添加量为15%（质量分数）时,制备出显气孔率达48%、孔径为20～40μm、体积密度为1.27g/cm3、抗压强度为38MPa、抗折强度为29MPa的堇青石质多孔材料。     

Characteristics of reaction and product microstructure during light calcination of magnesite in transport bed

A gas-heating laboratory transport bed was adopted to simulate the industrial transportbed reactor for flash calcination of magnesite, and a method based on TG analysis of a reacted sample was further developed to calculate the decomposition rate or conversion of its containing MgCO₃. The study investigated how the conversion and product reactivity as well as microstructure vary with reaction conditions including temperature, particle size and times of re-calcination for powder magnesite. Magnesite powder (<150 μm) calcination is a quick reaction that reaches 98% decomposition of its containing MgCO₃ in 1—2 s,corroborating the feasibility of magnesite flash-calcination in transport bed reactors. The coloration time given by the citric acid chromogenic method was 17—55 s and 294 s for the obtained products from transport and fixed beds, respectively. This proves the obviously higher activity and thus improved microstructure of the product from transport bed. During the calcination process, the MgO grain size of the product gradually increases, and the surface structure changes from loose and porous to dense and smooth. This structural change can be completed within a few seconds.     

菱镁矿输送床轻烧过程反应与产物微观结构特性

利用高温烟气加热实验室规模输送床模拟菱镁矿闪速轻烧工业输送床反应器，建立基于热重分析计算菱镁矿中MgCO₃分解率方法，研究了煅烧条件和原料粒径对菱镁矿细粉输送床分解的转化率和产品活性的影响，揭示了煅烧过程中产物微观结构变化特性。菱镁矿粉（<150 μm）轻烧为秒级快速反应，仅需1~2 s菱镁矿中MgCO₃分解率即可达98%以上，验证了输送床闪速轻烧技术的可行性。输送床煅烧产物的柠檬酸显色时间17~55 s，活性显著高于固定床煅烧产物（显色时间294 s），且产物活性由菱镁矿分解率和微观结构共同决定；煅烧过程中产物MgO晶粒尺寸逐渐增大，表面结构由疏松多孔变为致密光滑，该结构变化可在数秒内完成。     

Use of Genetic Algorithm to Determine the Kinetic Model of Solid-State Reactions

Solid-state reactions take place by different rate-controlling heterogeneous processes. To find the appropriate kinetic model for a particular solid-state reaction, a genetic algorithm-based simulation technique was carried out using DTA data with a fitness function, and a computer program was developed for the same. The process was applied to the decomposition reactions of limestone and magnesite samples. It was observed that both the decomposition reactions mostly followed the Avrami–Erofeev kinetics model.     

Microstructures and strengths of microporous MgO-Al2O3 refractory aggregates using two types of magnesite

Six microporous MgO-Al₂O₃ refractory aggregates were prepared with Al(OH)₃ and two different types of magnesite powders via an in-situ decomposition pore-forming technique. One magnesite powder contained more CaO (Magnesite C), the other one contained more SiO₂ and Al₂O₃ impurities (Magnesite SA). Effects of magnesite powder type and content (11 wt.%, 36 wt.% and 77 wt.%) on the phase compositions, microstructures and mechanical strengths of the prepared microporous aggregates were investigated by X-ray diffractometry (XRD), mercury porosimetry measurements, and scanning electron microscopy (SEM), etc. With the addition of 11 wt.% and 77 wt.% magnesite, the type of magnesite had only a small effect on the microporous corundum-spinel and periclase-spinel aggregates, while great influence on the microporous spinel aggregates was observed with 36 wt.% addition. This was mainly because of the sintering process with liquid phase. The best microporous MgO-Al₂O₃ refractory aggregates, which had an apparent porosity of 39.1%, a median pore size of 3.38 μm and a compressive strength of 66.3 MPa, were prepared by using 36 wt.% Magnesite C. This work has practical significance for the efficient utilization of magnesite and the development of energy-saving lightweight MgO-Al₂O₃ refractories.     

轻烧菱镁矿制备高纯纳米氧化镁

以轻烧菱镁矿为原料,通过酸浸去除杂质得到纯净的含镁溶液,以草酸为沉淀剂,采用直接沉淀法制备纳米氧化镁粉体。考察了草酸和氧化镁的摩尔比、反应温度、前驱体草酸镁热分解温度及时间对纳米氧化镁粒径大小的影响。采用热重-差热分析仪、激光粒度仪、X射线衍射仪、扫描电子显微镜等对前驱体的热分解情况、产品的粒径及晶型结构进行检测。结果表明:合成的氧化镁粉体的粒径分布较窄,分散性良好,平均粒径在80nm左右,纯度达到99.1%。     

菱镁矿混合氯化镁焙烧制备高纯氧化镁

采用菱镁矿添加无水氯化镁混合焙烧的方式,在较低焙烧温度下制备了高纯度的氧化镁。通过热重-差热（TG-DTA）法分析了菱镁矿的焙烧反应过程,并对焙烧产品进行了X射线衍射（XRD）、扫描电镜（SEM）及纯度分析。与传统焙烧方法制备氧化镁相比,添加氯化镁的焙烧工艺可有效地达到除钙效果。通过机理探讨表明,氯化镁渗透到菱镁矿中起隔断作用,有效增加了菱镁矿的比表面积,降低了焙烧温度,达到了节能降耗的效果。在焙烧温度为650 ℃、保温时间为1 h条件下,所得氧化镁的纯度可达99.3%,钙杂质质量分数可降至0.07%。