Calcination oxidative atmospheres

Thermal treatment of a material in a gas oxidizing atmosphere is the simplest concept. This can be done in air, air diluted in N2, dry air, or in ultrahigh purity O2. In the laboratory practice, calcination is done in flowthrough beds, aided by fluidization, or in static box furnaces. Important aspects are the bed geometry, the removal of the generated gases, and temperature gradients. 

Solid-State Reactions of Iron Compounds. Black iron oxides obtained from the Laux process (see below) or other processes may be calcined in rotary kilns with an oxidizing atmosphere under countercurrent flow to produce a wide range of different red colors, depending on the starting material. The pigments are ground to the desired particle size in pendular mills, pin mills, or jet mills, depending on their hardness and intended use. 

The SC-155 material calcined in oxidizing atmosphere at 1000 "C and until constant weight produced a self supported porous silica network (the carbon was burnt out) which was called S-155 this silica structure maintained the same shape and volume as the original composite. On the other hand, the SC-155 treated with 20 % HF aqueous solution until the elimination of Si02 produced a self supported carbon network called C-155, and this carbon structure maintained the same shape and volume as the original composite. 

It was mentioned that when a piece of SC-155 was calcined at 1000 "C in oxidizing atmosphere, the obtained product was a white porous amorphous silica material with the same shape and external volume as the starting one, and when a piece of the same material was submitted to acid attack by 20 % HF aqueous solution, a self supported carbon network was obtained, also with the same shape and external volume as the original These experiences show that the silica network and carbon network are self supported structures and independent one from the other. 

The NiO/NiAl30< catalyst was prepared in our laboratory by coprecipitation of nickel nitrate hexahy drate and aluminium nitrate nonahydr te following a method described elsewhere [3]. The samples were calcined in an oxidant atmosphere at 1023 K for a period of time of 3 hours. 

Calcination is a heat treatment carried out in an oxidizing atmosphere at a temperature slightly above the projected operating temperature of the catalyst. The object of calcination is to stabilize the physical, chemical, and catalytic properties of the catalyst. During calcination several reactions and processes can occur ... 

The oxidizing atmosphere during the pretreatment of the ore helps to break up chemically the naturally occurring stable bonds in the solid, resulting in better dissolution rates. In a process developed for the recovery of scandium from uranium plant iron sludges (R15), the calcination of the sludge at 250°C was found to be very effective in the removal of organic materials and appreciably decreased the consumption of acid. 

Although extensively studied, the Phillips catalyst is still poorly known [3]. Much uncertainty concerns the mechanism and kinetic control of polymerization. A strong point of debate concerns the molecular structiue of Cr(VI) species which, during the activation step, can be anchored on the support as monomeric or polymeric chromate [4-6], The chemical state of Cr depends on parameters such as the nature and properties of the oxide support, the preparation step (precursor, loading), the activation step (temperature, time, calcination rate, atmosphere), the chemical treatments (reduction by CO, Hz, dehydration by CS2) and the reaction progress [7]. 

The development of the rotary kiln, now predominantly used for the production of Portland cement, started around the late 1870s but the technology was not patented until 1885 (Ransome [B.69]). A rotary kiln (Fig. 6.7-35) is a long refractory-lined steel cylinder that is inclined at about 3-6° to the horizontal. At the lower end is a burner (coal, oil, or gas fired) and the material to be calcined enters on the other end. While passing down the kiln, chemical and physical reactions take place in the oxidizing atmosphere. Hot clinker emerges at the burner end and must be suitably cooled before being milled to yield cement. The rotary kiln process may be carried out with wet (slurry), semi-dry, or dry feed. 

Figure 3 Preparation procedure for bulk Y-Ba-Cu-O superconductors. Calcination must be done in an oxidizing atmosphere.

A complete range of metastable cerium-zirconium mixed metal oxide powders (CexZr(i.x)Oy, 0 < X < 1) were prepared through a similar hydroxide precipitation technique reported by Hori, et al. [11]. Cerium (IV) ammonium nitrate and zirconium oxynitrate precursors are completely dissolved in de-ionized water with mild heat and precipitated through the addition of excess ammonium hydroxide (-100 vol%). The ceria-zirconia is thoroughly washed with excess distilled water and allowed to evaporate to dryness overnight. The ceria-zirconia system is calcined in atmosphere for 1 hour at 773 K and subsequently milled into a fine powder. The model ceria-zirconia catalysts are prepared from the ground cerium-zirconium oxide powders using a 13 mm diameter pellet die and hydraulic press. 

Depending on the conditions of preparation, Th(N03)4 crystallizes from solutions of thorium hydroxide (or from HNO3 solutions of moderately calcined oxide) with varying contents of water of crystallization. When a not too acid solution is concentrated by evaporation, Th(N03)4 crystallizes in the cold with 12 moles of HgO. A solution evaporated at IS C yields the penta-hydrate, which is stable to 80°C if heated in an atmosphere free of COg. At higher temperatures, it converts to the trihydrate, and between 125 and 150°C, to the hemihydrate. Above 150°C the remaining water is split off, together with nitrogen oxides. 

Calcination is heat treatment in an oxidizing atmosphere at a temperatme slightly higher than the intended operating temperature of the catalyst. In calcina-... 

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