Alpha Alumina Formation Research Articles

Balancing Sodium Impurities in Alumina for Improved Properties

As there are direct and indirect impacts of feed material purity on the aluminum production process and metal grade, there is a high demand on the so-called pure smelter grade alumina (SGA)—the main feedstock for aluminum production. In this work, impurities within the precursor gibbsite used for SGA production and SGA are studied using NanoSIMS and XPS with a focus on sodium—the most abundant impurity. Although the industry trend is towards minimizing sodium due to the well-known negative impacts on the process, high sodium is also correlated with relatively attrition-resistant calcined products. Here, we show that this relationship is indirect and arises from sodium’s role in inhibiting α-alumina formation. Alpha alumina formation in SGA has previously been demonstrated to induce a macro-porous and therefore attrition-prone microstructure. Sodium distribution within the precursor gibbsite and its migration during the calcination process are proposed to be most likely responsible for the spatial distribution of α-alumina within the calcined product grain. This in turn determines the behavior of the product during its transportation and handling (i.e., attrition). Therefore, tolerance of a certain amount of sodium within the precursor material does demonstrate a net benefit while balancing its negative impacts on the process.

Comparative Study of Influence of Anions on Thermal Behavior of Nanosized Alumina

Nanoalumina precursors homogeneously precipitated from aluminum nitrate and aluminum chloride were investigated for their thermal transitions and structural transformations. From both the monovalent anionic salts under identical reaction conditions, alumina precursors derived were same i.e., pseudoboehmite but their thermal behavior was different. Pseudoboehmite prepared from nitrate was amorphous and prone to dehydroxylation and subsequent calcination while pseudoboehmite prepared from chloride had relatively higher crystallinity and delayed dehydroxylation and calcination. In case of nitrate, life duration of gamma-alumina was brief i.e., up to 350ºC whereas it in case of chloride it showed off at 375 and persisted up to 800ºC. Consequently, the complete formation of alpha alumina occurred at 1150ºC in the former case while alpha phase was in process of its conversion from theta phase in this temperature regime in the latter case. DSC-TG, XRD, SEM, etc. were carried out for the characterization.

Functionalization of gamma-alumina nanofibers by alpha-alumina via solution combustion synthesis

In this study alpha-alumina thin films have been deposited continuously onto novel gamma-alumina nanofibers with 7nm in diameter and 5cm in length. This selective alpha phase deposition on gamma phase was carried out by a one-step solution combustion process using aluminum nitrite as the precursor of aluminum and urea as a fuel. Effect of the fuel amount on the combustion product was studied by adjusting Al/urea ratio in solution. Stoichiometric amount of urea was considered to be optimal for crystallization of alumina nanoparticles. The morphology of the resulting product was found to be insignificantly affected by aluminum–urea ratio. The possible catalytic behavior of urea in formation of alpha-alumina was discussed. Microstructural characterization by XRD, TEM, and FESEM has revealed a uniform deposition of ultra-fine grained alpha-alumina film on the surface of gamma-alumina nanofibers.

Aluminum hydroxide transformations during thermal and vapor heat treatments

We have studied the mechanisms underlying the formation of corundum crystals from hydrargillite and boehmite during thermal and vapor heat treatments and have identified the major stages in the formation of corundum structure, thought of as the evolution of the system toward the most thermodynamically stable state. The results indicate that isothermal autoclaving at temperatures from 390 to 450°C gives rise to structural changes in alumina, which involve the following stages: the formation of microcrystalline boehmite from coarser hydrargillite crystals, accompanied by disintegration of agglomerates, boehmite flocculation and dehydration, conversion of the dehydrated boehmite to alpha-alumina within each floccule, and formation of corundum single crystals. Unflocculated boehmite crystals are absorbed by the growing crystal faces. If presynthesized boehmite is used as the precursor, the process also involves flocculation, dehydration, conversion to alpha-alumina, and formation of corundum single crystals. The structural changes induced in hydrargillite and boehmite crystals by annealing in air and vacuum at temperatures from 1400 to 1500°C have been analyzed. The formation of alpha-alumina from boehmite below <1200°C is accompanied by no changes in crystal habit, whereas annealing in the range 1400–1500°C gives rise to sintering of fine alumina crystals and changes in crystal habit. The degree of alignment of the crystals is shown to influence their structural transformations.

Aluminas by Thermal Activation of Fibrillar Pseudoboehmite Powders

The aluminum monohydroxide pseudoboehmite is a common precursor for catalysts and ad/absorbents. Fibrillar pseudoboehmite powders were synthesized by the same preparation method from aqueous sols but aged in different times: 24, 48 and 168 hours. The sol particles were from fibrillar pseudoboehmite; the average length of the microfibrils changed sample to sample, the sizes increasing with the thermal aging time. The sols were dried by different methods: at room temperatures, forming a solid membrane; spray dried; dried at 60°C and 110°C from a gel from the sol aged 168 hours; all were fired between 200°C and 1000°C in oxidative atmosphere to produce activated transition aluminas. The phases formed was identified by XRD. The aim of the present paper is to present data showing that method of drying the sol has an effect on the temperature of formation of alpha-alumina due to the anisodiametric shape of the pseudoboehmite crystals.

Low-temperature formation of alpha alumina powders from carboxylate and mixed carboxylate precursors

Formation of alpha alumina fine powders from aluminum carboxylate and mixed carboxylate precursors have been studied using thermoanalytical and powder X-ray diffraction techniques. Among these carboxylates, aluminum acetate and formoacetate produce alpha alumina below 1000°C, whereas, the other precursors require about 1200°C. The nature of the precursor has been found to influence the course of thermal decomposition and the sequence of phase evolution. The resultant alpha alumina powders have been have been characterised employing surface area and porosity measurements.

SOLID‐SOLID INTERACTIONS IN PURE AND Li2O‐DOPED V2O5/AI2O3 SYSTEM

AbstractThe effects of calcination temperature (500‐1000°C) and lithium oxide‐doping (4 and 10 mole %) on solid‐solid interactions between AI2O3 and V2O5 were investigated using DTA, TG and their derivatives together with XRD techniques.The results obtained revealed that no solid‐solid interaction between vanadium and aluminium oxides occured at 500°C. These oxides interacted in part, at 650°C to produce AIVO4, the completion of this reaction requires a prolonged heating at this temperature. Li2O‐doping much enhanced the interaction yielding AIVO4, the presence of 4 mole % was found sufficient to stimulate the complete conversion of V2O5 into aluminium vanadate. Aluminium vanadate produced in the case of pure mixed solids decomposed entirely by heating at 750°C giving a well crystallized V2O5 and an amorphous alumina. On the other hand, lithium oxide‐doping much increased the thermal stability of AIVO4 at 750°C and led also to the formation of AIVO3 and LiV10O25 phases.The heating of pure mixed oxides in air at 1000°C resulted in the formation of alpha‐alumina (corrundum) together with V2O5. This treatment, in the case of doped mixed solids, effected the formation of alpha‐Al2O3, V2O5 and LiV10O25 phases. The last compound was formed at 750°C and remained stable by heating at 1000°C. The promotion effect of Li2O in AIVO4 formation was tentatively attributed to dissolution of a small portion of Li2O in V2O5 lattice with subsequent increase in the mobility of V+5 ions.

Thermal stabilization of high surface area alumina

The surface area loss of gamma and theta alumina at high temperatures (800–1100°C) was investigated. The cause is shown to be sintering, neck formation between crystallites via surface diffusion. Addition of lanthanum oxide decreases the rate of surface diffusion by the formation of lanthanum aluminate surface layers. In this way the thermal stability of these high surface area aluminas is increased. The phase transformation to alpha alumina, which also occurs in this temperature region, was investigated separately. Kinetic data from DTA experiments demonstrate that alpha alumina formation does not contribute significantly to the surface area loss. A new model for the nucleation of alpha alumina is proposed.

ChemInform Abstract: ALPHA ALUMINA FORMATION IN ALUM‐DERIVED GAMMA ALUMINA

AbstractDie Bildung von α‐Al2O3 in 7‐AI2O3, das aus NH4AI(SO4),° 12 H2O bei 1000°C an der Luft hergestellt wurde, erfolgt bei 1 150°C durch einen Keimbildungs‐ und Wachstumsprozeß, dem eine Inkubationszeit vorausgeht.

Alpha Alumina Formation in Alum‐Derived Gamma Alumina

The formation of alpha alumina in gamma alumina obtained from ammonium alum was studied at 1150°C. The transformation occurs by a nucleation and growth process, preceded by an incubation time. Mechanical treatments of the gamma alumina markedly increase the density of nuclei and hence the transformation rate. The alpha alumina forms in porous colonies consisting of a single crystal with an interconnected wormy morphology.