Influence of Size of Hematite Powder on Its Reduction Kinetics by H2 at Low Temperature

The reduction kinetics and mechanisms of hematite ore with various particle sizes with hydrogen at low temperature were studied using the thermogravimetric analysis. At the same temperature, after the particle size of powder decreases from 107.5 μm to 2. 0 μm, the surface area of the powder and the contact area between the powder and gas increase, which makes the reduction process of hematite accelerate by about 8 times, and the apparent activation energy of the reduction reaction drops to 36.9 kJ/mol from 78.3 kJ/mol because the activity of ore powder is improved by refining gradually. With the same reaction rate, the reaction temperature of 6. 5 μm powder decreases by about 80 ℃ compared with that of 107.5 μm powder. Thinner diffusion layer can also accelerate the reaction owing to powder refining. The higher the temperature, the greater is the peak of the reduction rates at the same temperature, the greater the particle size, the smaller is the peak value of the reduction rates both inner diffusion and inter-face chemical reaction play an important role in the whole reaction process.     

微尺度氧化铁粉的低温还原机理

用热重分析法研究低温条件下(450、500、550和600℃),氢气还原微尺度氧化铁的还原动力学行为。结果表明:随氧化铁粉粒径减小和反应温度升高,初始反应速率加快,后期反应速率减慢。这是因为反应后期生成大量铁须,铁须之间形成搭桥,导致还原后的粉末严重烧结并致密化,阻碍气体的扩散,致使反应速率减慢。且随着粉体粒径减小,粉体表面吸附能增大,粉体致密程度提高,反应后期的粘结现象更加严重,反应速率相应减慢。采用Hancock-Sharp方法分析微尺度氧化铁粉恒温还原的动力学过程,发现前期阶段Fe2O3→Fe3O4,在500℃以下,相界面化学反应的阻力所占的比例较大,表明此阶段的反应控速环节为界面化学反应,温度超过500℃时,则由界面化学反应机理和相转变机理共同控制,点阵结构由Fe2O3的斜方六面体结构转变为Fe3O4的立方结构;后期阶段Fe3O4→Fe,由于粉体发生粘结,还原反应的控速环节转变为扩散控速。     

低温还原铁矿微粉过程的数值模拟

研究了H2在360～400℃之间还原铁矿微粉的过程，并提出了微空间尺度下瞬时气固反应动力学模型，以模拟氧化铁微粉在低温下的还原过程．这个模型包括了非均相化学反应动力学方程、颗粒内的传热方程和气体扩散方程，采用全隐式有限差分方法对控制方程进行数值求解．通过数值计算得到不同颗粒度矿粉在不同温度下还原率随时间的变化，并根据反应热和气体浓度在反应过程中的行为得知化学反应和气体内扩散在反应速率控制过程中所起的作用。数值模拟结果与试验结果基本吻合．     

MgO对赤铁矿粉矿烧结产物低温还原过程的影响

铁矿粉烧结中添加MgO有助于稳定磁铁矿和抑制二次赤铁矿的形成,可以改善烧结矿低温还原粉化性能,但其对烧结矿低温还原过程的影响尚不是十分明确。通过热重法测定不同MgO含量的烧结产物在550℃还原过程中质量的变化,定量地了解MgO对烧结产物低温还原性的影响。同时结合XRD和光学显微镜跟踪试样在还原前后矿物组成、结构及裂纹的变化,揭示了MgO改善烧结矿低温还原粉化的机理。结果表明,MgO对烧结产物低温还原过程的影响与Al_2O_3含量有关,不含Al_2O_3时,MgO抑制烧结产物的低温还原,添加Al_2O_3后,MgO对烧结产物低温还原的抑制作用减弱。另外,烧结产物还原过程中裂纹的形成与烧结产物的矿物组成及结构有关,适当的孔隙有利于减少裂纹的形成,这很好地解释了实际铁矿石烧结生产中添加少量的MgO可以改善烧结矿的低温还原粉化性。     

低温下氢气还原氧化铁的动力学研究

 用热重分析法研究了低温下不同粒度氧化铁的氢还原动力学,得出在同一温度下,铁矿粉粒度从107.5 μm降到2.0 μm后,由于粉体的表面积大幅度增加,提高了粉气接触面积,从而使得化学反应的速度提高了8倍左右,还原反应的表观活化能从78.3 kJ/mol降低到36.9 kJ/mol;当反应速度相同时, 粒度6.5 μm的粉体的反应温度比107.5 μm的降低了80 ℃左右。同时,通过理论推导和实验结果表明,当反应扩散层厚度相同时,铁矿粉粒度越小,反应扩散层厚度越薄,其还原率越高。     

MgO对铁矿石烧结过程中二次赤铁矿形成的影响

前人研究表明,二次赤铁矿是导致烧结矿低温还原粉化的主要因素,它的生成与烧结原料中MgO含量有关,但其影响机理尚不明确。通过热重法测定不同MgO和Fe_2O_3比例试样在升温-恒温-降温过程中质量变化,定量地了解MgO对Fe_2O_3向Fe_3O_4转变的影响。同时结合XRD分析和光学显微镜观察,揭示了矿物和相结构变化,研究了烧结过程中MgO对二次赤铁矿生成的抑制机理。结果表明,加入MgO后,它与Fe_2O_3反应形成了铁酸镁和含镁磁铁矿,前者促进了升温过程含镁磁铁矿生成,而后者低温下比磁铁矿更稳定,抑制了降温过程磁铁矿氧化,减少了二次赤铁矿的生成。因此,明确了加入少量MgO改善烧结矿低温还原粉化性能的主要原因。     

H_2-CO混合气体还原氧化亚铁粉体的动力学

通过热重实验获得了873、973和1173 K三个温度条件下不同组成的H_2-CO混合气体还原氧化亚铁的动力学曲线,发现在873和973 K时由于析碳反应的发生,动力学曲线较为混乱,没有规律,而在1 173 K时,还原曲线则随还原气体中H_2含量(体积分数)的增加表现出明显的规律性.通过H_2-CO与H_2-Ar气体还原氧化亚铁动力学的比较,混合气体中CO参与反应的速率与其含量(体积分数)基本符合线性关系.还原产物形貌观察的结果表明,随着反应温度的升高,还原产物孔隙增大,铁相充分发育长大并逐渐有明显的烧结现象.     

Al-Ti-Mg复合脱氧对钢中夹杂物及组织的影响

通过采用Fe-50%Ti、Ni-16.7%Mg和Fe-70.74%Si等合金对工业纯铁脱氧,研究了Al-Ti-Mg复合脱氧低碳钢中夹杂物的类型和尺寸分布规律以及钢的铸态组织.实验结果表明:Ti处理比未加Ti处理试样夹杂物总量增加400mm^-2,而且夹杂物明显细化;Ti-Mg复合脱氧,钢中夹杂物的总量相对于Mg单独处理增加200 mm^-2;Ti处理后,奥氏体晶粒内出现大量针状铁素体.对比而言,Al-Ti-Mg复合处理钢中针状铁素体分布最为均匀,无块状铁素体出现.     

氧化物杂质对铁氧化物还原动力学的影响

系统分析了氧化物杂质对铁氧化物还原动力学的影响及其机理。发现：各种杂 特点与杂质浓度、存在开矿和／或加入方法,样品的初始化学组成和物理性状,还原剂的种类、反应温度、还原分数等多种因素有关。     

低温非平衡条件下氧化铁还原顺序研究

研究了低温条件下(＜570℃)CO和H2还原氧化铁的动力学机理.结果表明:实际还原过程属于非平衡态过程,它的还原机理与还原气体的成分相关.当还原气体中CO(或H2)的含量不能满足Fe3O4 CO(H2)→3FeO CO2(H2O)反应进行的要求时,氧化铁的还原顺序为Fe2O3→Fe3O4→Fe;如果还原气体成分满足此要求,Fe2O3→Fe3O4→Fe和Fe2O3→Fe3O4→FeO→Fe两种还原顺序将同时存在.     

Kinetics of hexavalent chromium reduction by grey cast iron powder

The kinetics of Cr (VI) reduction by grey cast iron powder (HT250 as per Chinese standard GBT 9439-2010) was investigated in this paper. Factors such as pH, initial Cr (VI) concentration, particle size, stirring speed and temperature on Cr (VI) reduction were first examined at a moderate reducing agent dosage. It was found that Cr (VI) reduction could only occur at an initial pH of 2.0 and above this value, the reaction nearly stopped. Additionally, the obvious dependence of Cr (VI) reducibility on other parameters was also revealed. The changes of pH, ORP and the concentration of Fe ions during the reaction were investigated. The correlations between the experimental data and reaction models involving homogeneous and heterogeneous reactions were studied. The results indicated first-order kinetics for the Cr (VI) reduction, which was a homogeneous reaction with an apparent activation energy of 20.718 kJ/mol. A semi-empirical equation was proposed to describe the kinetics of Cr (VI) reduction by HT250 powder. It was concluded that HT250 powder was a promising zero-valency iron (ZVI) material offering excellent reducibility and its use can be extended to the pre-treatment of wastewater containing other heavy metal ions such as Cu(II) and Pb(II).     

温度对氢气还原氧化铁影响及电化学性能研究

采用纯的氧化铁进行直接还原的实验研究,利用H2作为还原气体在高温管式炉中进行焙烧还原.结果表明,随着反应时间的增加,氧化铁块还原率相应的增大.反应温度的不同,氧化铁块还原速率及电导率也相应的不同.600~900℃下经过一定时间的氢还原,还原产物的还原速率随温度的增加而增加,均可达到95%以上.通过交流阻抗法对还原产物的电导率的测试,得到了还原产物的电导率随时间和还原度的变化曲线.随着温度及时间的不同,电导率也相应变化,电导率的不同,可间接反映还原过程中产生了导电性不同的物质.     

低温取向硅钢初次再结晶动力学模型及织构演变

研究了低温取向硅钢在625~850℃的等温退火过程,根据Avrami方程建立了其初次再结晶动力学模型,并利用EBSD技术对初次再结晶织构演变过程进行了分析。结果表明:当退火温度为850℃时保温5s即可发生完全再结晶;而在625℃下保温1.7h以上再结晶才能进行完全。当退火温度在625~700℃时,Avrami指数n在0.82~0.89之间,当退火温度在750~850℃时,n值在1.25~1.27之间,相对应的再结晶激活能QRX分别为239.3、160.4kJ/mol。初次再结晶早期主要发生在高储存能的γ纤维织构上,而中后期则主要发生在低储存能的α纤维织构上。初次再结晶完成后织构类型主要是以{111}112组分为主的γ织构、{114}418织构及少量分布在次表层的Goss织构。     

MgCl2溶液对烧结矿低温还原粉化率的影响

试验测定了 Mg Cl2 溶液不同浓度和喷洒量对济钢现有原料条件下烧结矿低温还原粉化率 (RDI)和还原度 (RI)的影响。结果表明 :每 5 0 0 g烧结矿喷洒 10～ 2 0 m L浓度为 1.0 %～ 3 .5 %的 Mg Cl2 溶液 ,能显著降低烧结矿的 RDI,且还原度不受影响 ,以 5 0 0 g烧结矿喷洒浓度为 2 .5 %的 Mg Cl2 溶液 10 m L综合效果最佳 ,可使烧结矿的 RDI从 3 6%降至 4.4%。     

硅铁配比对氧化镍矿低温金属化还原的影响

以三氧化二铁、氧化亚镍和硅酸镁为原料,依照云南代表性氧化镍矿原矿中铁、镍和硅质量比进行配矿,研究硅铁比对镍铁金属化以及镍铁合金聚合行为的影响。结果表明,当Fe/Ni=13/1,Si/Fe=1.2/1,煤粉加入理论量2倍,1 280℃反应60 min后,经磁选分离获得的精矿中镍、铁品位分别为4.95%、50.93%,镍、铁回收率分别为99.32%、80.12%。还原产物主要相为MgSiO_3和[Fe,Ni],镍铁合金聚集状态良好,精矿主要相为[Fe,Ni],仅有少量SiO_2残留,分离效果较好。云南代表性氧化镍矿原矿的验证试验取得了相似结果。     

Volume Changes of Iron Oxide Compacts under Isothermal Reduction Conditions

Compacts made from chemically grade Fe₂O₃ were fired at 1473K for 6 hrs. The fired compacts were isothermally reduced either by hydrogen or carbon monoxide at 1073–1373K. The O₂ weight‐loss resulting from the reduction process was continuously recorded as a function of time using TGA technique, whereas the volume change at different reduction conditions was measured by displacement method. Porosity measurements, microscopic examination and X‐ray diffraction analysis were used to characterize the fired and reduced products. The rate of reduction at both the initial and final stages was increased with temperature. The reduction mechanism deduced from the correlations between apparent activation energy values, structure of partially reduced compacts and application of gas‐solid reaction models revealed the reduction rate (dr/dt) at both the initial and final stages. At early stages, the reduction was controlled by a combined effect of gaseous diffusion and interfacial chemical reaction mechanism, while at the final stages the interfacial chemical reaction was the rate determining step. In H₂ reduction, maximum swelling (80%) was obtained at 1373K, which was attributed to the formation of metallic iron plates. In CO reduction, catastrophic swelling (255%) was obtained at 1198K due to the formation of metallic iron plates and whiskers.     

低铁高硅赤铁精矿对氧化球团性能的影响

为合理利用国内低铁高硅铁精矿、降低球团生产成本,研究了低铁高硅赤铁精矿对生球、预热球和焙烧球团性能的影响。结果表明,典型的低铁高硅赤铁精矿A较磁铁精矿有更好的润磨性能。赤铁精矿A的亲水性较磁铁精矿强,在保持生球水分不变且赤铁精矿配比较高的条件下(>10%),生球水分不足,生球质量随着赤铁矿配比的提高而变差。随着赤铁精矿A的配比由0提高到50%,预热球强度由588降低到196 N/个,焙烧球团抗压强度由3 425降低到1 368 N/个,赤铁精矿A配比不宜高于30%,适当提高焙烧温度有利于球团抗压强度的提高。配加低铁高硅赤铁精矿A的球团还原膨胀性能和还原性能均有一定程度改善。     

白云鄂博铁矿球团富氢还原动力学研究

针对气固直接还原工艺中存在着气体利用率低和还原供热不足等问题,利用恒温热重分析(TG)法,研究了氢/碳比率对白云鄂博铁精矿还原速度的影响。结果表明,在还原试验开始后40 min内,还原速率随wH2/wCO比增加而增大,使得Fe2O3→Fe3O4反应时间缩短。基于气相内扩散和界面反应的球团还原速度方程均能较好地处理本研究的数据,得到了反应速度常数与wH2/wCO的关系为:k界面=-0.1975+0.3 575wH2/wCO,k扩散=0.171 01+0.269 7wH2/wCO。根据Arrhenius方程计算出界面反应和气相内扩散活化能分别为26 k J/mol和44 k J/mol,因此本研究条件下限制性环节为气体内扩散控制。     

Rare earth oxide modified Cu-Mn compounds supported on TiO₂ catalysts for low temperature selective catalytic oxidation of ammonia and in lean oxygen

Selective catalytic oxidation(SCO) of ammonia was carried out over Cu-Mn compounds catalysts modified with trivalent rare earth oxide Ce2O3 and La2O3 respectively.TiO2 was used as support and different ratio of O2 were tested in order to find an appropriate O2 concentration(vol.%),and the results showed that 1%O2(vol.%) was propitious to SCO of ammonia.The effects of the two rare earth oxides modified catalysts Ce2O3-Cu-Mn/TiO2 and La2O3-Cu-Mn/TiO2 on the catalytic activity and selectivity of ammonia oxidation were investigated under the reaction condition of 500 ppm ammonia,1%O2(vol.%),at the temperature from 125 to 250 oC.The results revealed the beneficial role of Ce2O3 and La2O3 in catalytic activity at low temperature and lean oxygen concentration,while the modification with Ce2O3 and La2O3 led to the negative influence on N2 selectivity.For the catalysts modified with Ce showed lower NO and N2O selectivity than the catalysts modified with La,then the effects of different Ce loadings on catalytic activity and selectivity were also considered,in combination with catalysts preparation methods,which include incipient wet impregnation,sol-gel method and co-precipitation.Results revealed that the catalysts prepared by sol-gel method obtained preferable catalytic activity compared with the others,reaching 99% ammonia at 200 oC,whereas 96% NO was detected.It also indicated that different catalyst preparation method significantly determined production distribution.     

二氧化锰低温分解动力学研究

采用热重分析仪在5种升温速率下对二氧化锰矿进行热重试验,研究预处理过程中二氧化锰的低温分解行为,并通过物相分析验证热重试验结果及反应推论,应用非模型等转化率法及Popscu法对两步反应动力学求解,得到各自的反应机理函数。结果表明,二氧化锰升温过程发生了两步反应,第一步反应产物为Mn2O3,反应机理为dα/dt=(3.96×109/β)(1-α)e-1.86×105/RT,为一级简单反应,符合Mample单行法则动力学机理特征;第二步产物为Mn3O4,反应机理为dα/dt=(3.28×1024/β)e-2.81×105/RT×(3/2)(1-α)[-ln(1-α)]1/3,符合Avarami-Erofeev方程动力学机理特征。两步反应的反应机理均为随机成核和随后生长。