Preparation And Kinetics Study Of Nanograde Iron Oxide Powders By Solid-state Reaction

Nanomaterials have been widely used in chemical, biological, pharmaceutical, plastic, paint, ceramics, as well as other industries, because of the special structure and properties. Ultrafine nano ferric oxide that is a kind of functional material with merits such as small partial size, large specific surface area, a good dispersibility and a porous structure. A sorbent could be considered as the best desulfurizer which gets ferric oxide as active component. While the dispersion of the active ingredient to get a direct influence on the desulfurization activity.In this paper, ferric nitrate, ferric chloride and oxalic acid were used as raw materials to synthesis the precursor of compound of ferric oxalate with a method of solid phase reaction. Then the ultrafine iron oxide nanoparticles were obtained via thermal decomposition. The precursor and nano iron oxide were characterized by DG-TGA, XRD and IR. Each of the two thermal decomposition processes of precursors was analyzed, as well as the calculation of the thermal decomposition of the apparent activation energy and the thermal decomposition mechanism. At last, the grain growth kinetics of the nano iron oxide has been investigated. The conclusions shown as follows:(1) Ferric oxalate were obtained derive from ferric nitrate via the method of solid phase reaction at ambient temperature, while the ferrous oxalate was observed as soon as the ferric chloride was treated as raw material;(2) The thermal decomposition process of the ferric oxalate precursor made of ferric nitrate includes that ferric oxalate is reduced to ferrous oxalate, thermal decomposition produce γ-Fe2O3at first. The iron oxides has changed in the crystal phase between380℃-400℃, γ-Fe2O3transforms into α-Fe2O3. The apparent activation energy of the ferric oxalate thermal decomposition E is calculated by Ozawa method, E=136.11KJ/mol, The reaction mechanism function is deduced by Coats-Redfern method combined with Ozawa method:G(α)-[－ln(1-α)]1/2,f(α)＝2(1-α)[-ln(1-α)]1/2;(3) The production of the ferrous oxalate precursor thermal decomposition is γ-Fe2O3at first. The iron oxides has no phenomenon of crystal phase transition before400℃. The apparent activation energy of the ferrous oxalate thermal decomposition E is calculated by Ozawa method, E=111.40KJ/mol. The reaction mechanism function is deduced by Coats-Redfern method combined with Ozawa method:G(α)-ln(1-α)]2/3 f (α)=3/2(1-α)[-ln(1-α)]1/3.(4) The principle of both the ferrous oxalate and ferric oxalate precursor thermal decomposition is random nucleation and consequent growth. That means that the decomposition reaction is started in the crystalline imperfections. The decomposition product transforms into new nucleus and the undecomposed precursor, diffusing across the product phase, continue to react in the nucleus. Thus the product gradually formed but the reaction reduced until the precursor is decomposed completely;(5) In the range of300℃～700℃, the dynamic growth index n of the preparation of the grain of nano iron oxide via the roasting of ferric oxalate is18-24, the grain growth activation energy Q is279.46KJ/mol, while, as it came to the ferrous oxalate, n=10.8, Q=50.70KJ/mol, respectively;(6) The calcination temperature, which was considered as the most affective factor with respect to calcination time, is the main influencing factor. Iron oxide nanoparticles having better dispersibility and smaller particle size prepared from iron ferric roasting.