Reaction mechanism between Al_(2)O_(3)–MgO refractory materials and rare earth high-carbon heavy rail steel

Submerged entry nozzle(SEN)clogging is a major problem affecting the production quality of rare earth steel,and finding a suitable refractory outlet can significantly reduce production costs.To explore the relationship between refractory composition and interface interaction,unprotected coated Al_(2)O_(3)–MgO refractories and SiO2-coated Al_(2)O_(3)–MgO refractories were added to rare earth high-carbon heavy rail steel under laboratory conditions,and the Al_(2)O_(3)–MgO refractory was found to be more suitable.The results show that,from the epoxy resin side to the refractory side,the contour of the refractory interface reaction layer can be divided into two main layers:an iron-rich reaction layer and an iron-poor reaction layer.Calculations based on the spherical model suggest that the adhesion force is proportional to the size of the refractory particles and inclusions,and the same result applies to the surface tension.Controlling the inclusions at a smaller size has a specific effect on alleviating the erosion of refractories.Combined with the erosion mechanism of Al_(2)O_(3)–MgO refractories,the interface reaction mechanism between Al_(2)O_(3)–MgO refractories and molten steel was proposed,which provides ideas for solving SEN clogging.

Sulfuric acid leaching of low-grade refractory tantalum–niobium and associated rare earths minerals in Panxi area of China

By mineral liberation analyzer （MLA） automated quantitative system, the niobium-tantalum ore in this study was classified as refractory mineral of low-grade which was mainly composed of fergusonite, polycrase, ilmenorutile, and bastnasite. Most of niobium and rare earth metals were dis- seminated in these minerals （particle sizes of 95 % rare earth elements 〈40 μm）, which cannot be concentrated through traditional floatation process and treated by the current hydrofluoric acid leaching in industry application. It is nec- essary to develop a new approach to recovery these valuable metals. In this study, an improved method is proposed to extract these metals, which includes acid roasting and sulfuric acid leaching. The influence of mineralogy, acid concentra- tion, leaching temperature, leaching time, and liquid to solid ratio was experimentally studied. It is observed that after the ground ore （-74 μm） was roasted at 350 ℃ for 2 h with sulfuric acid （18.4 mol.L- 1） in the ore to acid mass ratio of 1：1, and then the roasting residue reacted with 25 % sulfuric acid at 90 ℃ for 2 h, the recovery rate of rare earth elements reaches 〉90 %, and the leaching extent of niobium can reach 84 %.

Difference of Mullite Whiskers between Ytterbia-Doped Aluminum Hydroxide-Silica Gel and Mechanochemical Process

A number of processing routes were developed for the synthesis of mullite whiskers. Mullite whiskers that were derived from ytterbia-doped aluminum hydroxide-silica gel were studied; another process for preparing mullite whiskers from Al2O3-SiO2 oxide mixtures by mechanochemical method was encountered by chance. The method of ytterbia-doped aluminum hydroxide-silica was used to enhance mullite formation from gel. For adding ytterbia, mullite formation can take place at 1000℃and almost single-phase mullite can be achieved at 1150℃, which is a significantly lower temperature than mechanochemical method. This enhancement of mullite formation is also attributed to refinement of aluminum hydroxide-silica by gel. Furthermore, well-developed single-crystal mullite whiskers were produced from the gel powder instead of normal-shaped grains. The presence of YbO might have promoted the formation of nanosized Al2O3 particles, which then acted as a template leading to mullite whiskers at higher sintering temperature.

Comparative study on interaction between rare earth oxide refractories and rare earth treated steel

Refractories have an important effect on the cleanliness of molten steel.The interaction between CeAlO_(3) refractories and Ce treated steel was explored and compared with that between Al_(2)O_(3) refractories and Ce treated steel.The results show that the Ce content in steel decreases sharply after contact with Al_(2)O_(3) refractories and the pick-up of dissolved Al occurs.Compared with the test of Al_(2)O_(3) refractories,the loss of Ce content is less in the test of CeAlO_(3) refractory,and the total oxygen content and dissolved Al content in steel increase less.The dense and continuous Ce_(2)O_(2)S interface layer is observed at the CeAlO_(3) refractories/steel interface,which can serve as the metallurgical isolation layer and hinder the further interaction between refractories and steel.Therefore,the higher steel cleanliness and less penetration of refractories by molten steel are obtained in the test of CeAlO_(3) refractories.The CeAlO_(3) refractories show a promising prospect in the production of clean Ce treated steel.

Investigation on the preparation of Si/mullite/Yb_2Si_2O_7 environmental barrier coatings onto silicon carbide

With the development of aero-engine,gas import temperatures of hot section structural materials are increasingly higher.Metal alloy materials due to the rapidly decreased mechanical properties at relative high temperature are gradually replaced with silicon-based non-oxide silicon carbide ceramics.However,silicon carbide ceramic materials tend to spall and deform in engine combustion environment,need environmental barrier coatings for the protection of the matrix.The preparation of Si/mullite/Yb2Si2O7 envir...

Effect of Y^(3+), Gd^(3+) and La^(3+) dopant ions on structural, optical and electrical properties of o-mullite nanoparticles

Dielectric ceramics of M（x）Al6（1-x）Si2O13 doped mullite were synthesized via co-precipitation technique. The X-ray diffrac-tion profiles revealed that these nanoparticles were crystallized well and the volume of mullite unit cell was increased as a function of the ionic radius of dopant ion. TEM images showed regular orthorhombic crystal morphology for the pure mullite sample. Meanwhile, the doped samples exhibited slightly distorted crystal morphology of larger particle sizes. DSC thermograms evinced that the exo-thermic peak temperature of mullite was shifted to the lower value with M3＋ion insertion. The photoluminescence spectra were stud-ied for mullite samples, and it was found that the intensity of the emission spectra was affected by the M3＋ion type. It was found that, Y3＋doped mullite achieved the minimum dielectric loss value of 0.01 in the radio wave frequency region （1 MHz）. Meanwhile, Gd3＋doped mullite achieved the minimum dielectric loss value of 0.09 in the microwave frequency region （1 GHz）.

Structure evolution and thermal stability of La_2O_3-doped mullite fibers via sol-gel method

The lanthana-doped mullite fibers were prepared via a sol-gel method from the aluminum isopropoxide-aluminum nitrate-tetraethyl orthosilicate system. The structure evolution and thermal stability of the mullite fibers were investigated by means of X-ray diffraction （XRD）, Fourier transform infrared （FT-IR） spectroscopy, differential scanning calorimetry （DSC）, scanning electron microscopy （SEM） and trans-mission electron microscopy （TEM）. The results showed that the fibers obtained after calcination at 1000℃ was in the form of Al2O3-rich mullite, which tended to transform to stoichiometric mullite with the lanthana content increasing. The lattice parameter a exhibited a decline trend with the lanthana content increasing, while b showed an upward tendency. Correspondingly, the redshift of Si-O-Si bond from 1255 to 1245 cm-1 was observed. The grain growth was inhibited for the doped fibers, among which the 5% La2O3-doped sample presented a mini-mum crystallite size （17.2 nm） after calcination at 1000 ℃ for 1 h.

Effect of Er2O3 and Pr6O11 on non-isothermal kinetics of mullite formation from kaolinite

The non-isothermal kinetics of mullite formation from both non-mixed and mixed with different rare earth oxides（Pr6O11 and Er2O3） kaolinite were studied by comprehensive thermal analysis technologies and X-ray diffraction（XRD）. Meanwhile, Kissinger equation, Ozawa equation and JMA modified equation（I） and（II） were employed for analysis of the effects of Pr6O11 and Er2O3 on the kinetics of kaolinite in phase transformation at high temperatures. The results showed that the addition of two kinds of rare earth oxides influenced the crystallization kinetic parameters of kaolinite. Crystallization activation energies and frequency factors of the kaolinite mixed with Pr6O11 were obviously decreased compared with the kaolinite with none, but the kaolinite mixed with Er2O3 weakly decreased. Crystallization behaviors were not changed, belonging to volume crystallization. Mullite formation from kaolinite was suppressed while generation of cristobalite was promoted by Pr6O11, and effect of Er2O3 on mullite formation from kaolinite under high temperature was not evident.

Microstructure and phase formation of mullite-Pr6O11 composite prepared by spark plasma sintering

The present work investigates the effect of high praseodymium oxide(Pr6O11)content on the microstructure and phase formation of mullite(3Al_(2)O_(3)·2SiO_(2))precursor by means of the spark plasma sintering process.30 wt%Pr6O11was added to a mullite precursor consisting of aluminum nitrate nonahydrate and tetraethyl orthosilicate through a high energy mixer mill in ethanol media.The spark plasma sintering was performed at a te mperature of 1200℃under 23 Pa vacuum co nditions by applying initial and final pressure of 10 and 30 MPa,respectively.XRD analysis confirms the existence of mullite,alumina,Pr6O11,Pr2O3and quartz as the only crystalline phases.FESEM images reveal an interesting deposition of hexagonal-shaped Al_(2)O_(3)particles on polished surfaces and complex oxide phases of the fiber network adhering to alumina particles in the form of tails seen on the fracture surfaces.Moreover,the bending strength of 213±21 MPa,Vickers hardness of 9.3±0.1 MPa and fracture toughness of6.21±0.12 MPa·m1/2are obtained for the prepared composite.

稀有金属元素在硬质合金中的应用研究进展

综述了国内外对稀有金属元素在硬质合金中的应用研究进展,分析了四类元素稀有难熔金属、稀土、稀贵金属和稀散金属对硬质合金微观组织与性能的影响,并展望了稀有金属元素在硬质合金中的应用前景。

掺杂氧化钇和富镧钐渣对连铸水口MgO-C耐火材料性能的影响

本研究分别考察了氧化钇(Y_(2)O_(3) )和富镧钐渣掺杂量对MgO-C耐火材料微观结构、烧结性能、力学性能和抗稀土钢侵蚀性能的影响。结果表明,Y_(2)O_(3) 和富镧钐渣通过与SiO_(2)和Al_(2)O_(3)反应而生成稀土硅酸盐和稀土铝酸盐,促进材料烧结,提高材料致密性。随着Y_(2)O_(3) 和富镧钐渣掺杂量增大,MgO-C耐火材料的体积密度、抗折强度和耐压强度均先增大后减小,而显气孔率的变化趋势相反。当Y_(2)O_(3) 和富镧钐渣的掺杂量分别为0.5%时,MgO-C耐火材料的体积密度分别为2.87 g/cm^(3)和2.85 g/cm^(3),显气孔率分别为9.6%和10.2%,抗折强度分别为7.6 MPa和6.8 MPa,抗压强度分别为46.4 MPa和41.2 MPa,致密性和常温力学性能提高的幅度最大,抗Ce基稀土钢侵蚀性能相比未掺杂稀土氧化物的试样分别提高了7.6%和10.2%。

贵州织金某细粒难选钛矿石赋存状态研究

为高效开发利用贵州织金地区某红土型细粒钛矿石原生矿,确定合适的选矿工艺,采用现代测试手段对该矿石的赋存状态进行研究。结果表明:钛矿石中TiO_(2)的品位为4.08%~5.71%,钛主要以钛铁矿的形式存在,其次为锐钛矿。试样中Ba、Cr、Ga、Sr、V、Zr等微量元素及稀土含量较高,Ba的含量为(264~719)×10^(-6)、V(431~568)×10^(-6)、Zr(363~500)×10^(-6)。稀土元素总量∑REE为(301.11~801.36)×10^(-6),LREE/HREE值为3.08~3.68,δCe值为0.36~0.89,δCe值均小于1,具有较低的Ce负异常特征,表明研究区的钛矿石主要形成于还原的海水沉积环境,形成过程主要受陆源碎屑影响。δEu值分布在0.98~1.12,除了样品BQ-T1显示弱负异常外,其余样品显示出很弱的正异常。矿床稀土分布模式呈明显右倾曲线,为轻稀土相对富集型。该矿石具有重要的开发利用价值,但钛矿的嵌布粒度很细,且矿物组成复杂,属难选钛矿石。钛铁矿原矿呈灰黑色,风化后的钛矿资源呈松散的红土型。造成TiO2回收率低的主要原因是钛铁矿、锐钛矿与斜长石、石英、方解石紧密连生或被黏土矿物包裹,该矿石适合采用阶段磨矿、浮选、化学选矿等联合工艺来综合回收多种有用矿物。稀土元素与TiO2关系紧密,且富集趋势呈正相关性,在回收TiO2的同时可以考虑对稀土元素进行综合利用。研究结果为细粒钛矿资源的综合利用提供选别依据。

混合稀土氧化物对镁质耐火材料结构与性能的影响

研究了添加Y_2O_3的混合稀土氧化物对镁质耐火材料的烧结性、力学性能和显微结构的影响。结果表明:添加含Y_2O_3的混合稀土氧化物可以促进镁质耐火材料的烧结,提高材料的力学性能,同时改善材料的组织结构。含Y_2O_3的混合稀土氧化物的添加,使镁质耐火材料的结合方式由硅酸盐相结合转化为高熔点的稀土硅酸盐相结合,从而导致镁质耐火材料的高温强度得到显著地提高。

氟氧化钕耐火材料的研制与应用

为给大型稀土金属电解槽提供一种抗氟盐侵蚀和高温抗氧化性好 ,且对稀土金属产品无污染的内衬材料 ,以氧化钕、氟化钕及钕电解槽炉底的结瘤物为主要原料 ,将氧化钕和氟化钕或炉底料按一定比例混合、压制成块和烧成来制备熟料 ,再将熟料破粉碎 ,配料成型 ,最后烧成 ,制备了以氟氧化钕为主要组成的新型耐火材料。通过实际应用表明 ,该材料具有一定的抗氟盐侵蚀能力和高温下的抗氧化能力 ,应用于大型稀土氟化物熔盐电解槽 。

喷丸与电泳沉积稀土薄膜复合处理提高T91钢抗水蒸气氧化性能的研究

采用喷丸与电泳沉积稀土薄膜复合处理工艺对T91钢进行表面处理 .在管式高温水蒸气氧化实验装置中 ,以 6 0 0℃脱氧水蒸气氧化 30 0h的试验条件评价其抗水蒸汽氧化性能 .结果表明 ,该处理方法可以减小氧化抛物线速度常数 ,有效改善抗氧化性能 ,较单一沉积稀土薄膜效果更为显著 .与原材料相比 ,氧化物晶粒直径可由 3～ 5 μm细化至约 0 .5 μm .分析了喷丸与沉积稀土薄膜对氧化机理的影响 ,并讨论了复合处理工艺对氧化过程中扩散机理的影响以及氧化膜物相组成的变化 .

稀土精矿在耐火材料中的应用研究

探讨了稀土精矿在耐火材料中的应用,研究发现:在不同材质的耐火材料制品中加入稀土矿后,耐火材料的性能均有改变,性能改变比较突出的是酸性耐火材料和偏中性耐火材料,特别是制品的强度提高幅度很大,气孔率的降低非常明显,而且晶相微区分析表明稀土已参与到主晶相的成长发育过程中。这为提高偏酸性和偏中性耐火材料的某些性能提供一条有效的途径。

稀土和难熔金属元素对铂的高温蠕变激活能的影响

研究微量稀土元素（Ｙ、Ｎｄ、Ｓｍ、Ｅｕ、Ｇｄ、Ｅｒ和Ｙｂ等）和少量的难熔金属（Ｗ、Ｔａ、Ｒｅ等）对铂的高温蠕变激活能的影响，结果表明在１２００～１３００℃，稀土元素提高铂的蠕变激活能；难熔金属显著降低铂的蠕变激活能；Ｐｔ－０．５Ｇｄ合金在１３５０～１４００℃具有最高的蠕变激活能值。

碳热还原条件下SiO_2-Al_2O_3系耐火材料的腐蚀行为

以粘土质和高铝质耐火砖为对象 ,采用计算机控制的热重分析系统 ,研究耐火材料在碳热还原反应条件下的腐蚀行为 ,碱金属添加剂对耐火材料腐蚀过程的影响 ,寻求耐火材料的防腐蚀措施。实验结果表明 ,碳热还原过程中 ,SiO2 -Al2 O3 系耐火材料会受到轻度腐蚀 ,相同条件下 ,HB2 0 -0 9#粘土砖侵蚀最严重 ,高铝砖的耐蚀效果最好 ;碱金属添加剂加重耐火材料受侵蚀的程度 ,添加剂用量愈大影响愈大 ;稀土氧化物有抑制和减缓耐火材料腐蚀的作用 ,尤以CeO2 的效果显著。

稀土难熔金属阴极材料的研究进展

介绍了3种阴极,镧钼阴极、稀土钼金属陶瓷阴极及钪钨基扩散阴极的制备方法及发射性能。针对电子管用镧钼阴极发射不稳定的问题,采用原位光电子能谱法研究高温下La2O3(4%,质量分数)-Mo阴极表面La的价态,确定了适合La2O3-Mo阴极的碳化、激活等关键工艺制度,使实用型La2O3-Mo电子管的使用寿命超过了应用要求的水平。为满足大功率磁控管的发展要求,研究了新型稀土钼金属陶瓷阴极。经过高温激活处理后,材料的最大次级发射系数达5.24,实用管型测试结果表明,稀土钼金属陶瓷阴极的性能优于钡钨阴极,显示了良好的应用前景。亚微米结构的钪钨基扩散阴极具有优异的热发射性能,850℃下阴极的发射电流密度可达42A/cm2,激活后Ba,Sc,O等元素形成的活性多层,均匀覆盖在阴极表面,促进了阴极的发射。

稀土锆莫来石耐火材料的显微结构研究

本文运用ＸＲＤ、ＳＥＭ等方法对稀土锆莫来石耐火材料的显微结构，物相变化，以及稀土的作用机制进行了研究。