Calcining

Calcination (also referred to as calcining) is a thermal treatment process in presence of air or oxygen applied to ores and other inorganic solid materials (i.e., minerals, ceramic powders) to bring about a thermal decomposition, phase transition, or removal of a volatile fraction, including chemically bonded water. The calcination process normally takes place at temperatures below the melting point (or fusion point) of the product materials. Calcination is also used to extract metals from ores. Typical process temperatures are between 500°C and 1300°C.

Cement - The most common application of calcination is in the cement industry. After mining, grinding and homogenization of raw materials the first step in cement manufacture is calcination of calcium carbonate (lime) followed by burning the resulting calcium oxide together with silica, alumina, and ferrous oxide at high temperatures to form clinker. The clinker is then ground or milled together with gypsum and other constituents to produce cement. This process is usually done in large rotary kilns, but some requirements may need a batch laboratory kiln for testing purposes.

Aluminum - Generating aluminum from bauxite is another major industrial process requiring calcination. The raw material bauxite reacts with caustic soda to aluminum hydroxide. Rotary kilns are used to convert aluminum hydroxide into aluminum oxide by driving off water. Next in the calcination process, the aluminum oxide undergoes an electrolytic process to convert to aluminum. It is possible that batch lab kilns may be used for testing purposes. The atmosphere in the process is extremely corrosive.

Dyes & Pigments - Various dyes and pigments require calcination to convert oxides and remove water such as in the manufacture of iron oxide pigments and other inorganic ceramic pigments. zirconium-praseodymium yellow pigments - praseodymium oxide is calcined at temperatures in the range from 800 to 1100 C. Mason Color (East Liverpool Ohio www.masoncolor.com) is one of a number of US pigment manufacturers, which typically use large batch kilns for calcining at temperatures up to 2500°F max. Others would be US Pigment (uspigment.com) Elgin Il. And Ferro Corporation (www.ferro.com) Cleveland, OH. Smaller batch kilns are likely used in the Laboratory.

The production of Kaolin clay (Al2O3·2SiO2·2H2O) is another industrial process where calcining is used to initiate phase transformations. Endothermic dehydroxylation begins at 550–600 °C to produce disordered metakaolin, Al2Si2O7, but continuous hydroxyl loss (-OH) is observed up to 900 °C. Usually large rotary calciners are used (similar to what is used in the cement industry) but batch laboratory kilns are likely used for testing purposes.

Soft Ferrites – Manganese-Zinc (Mn-Zn), Nickel-Zinc (Ni-Zn) and Lithium Titanium (Li-Ti or Microwave) ferrites all use calcining 900-1100C (1200C for Garnets) During calcination the carbonates are converted into oxides, the impurities get evaporated and the shrinkage at final product is reduced to 17 to 18% instead of 20 to 21%. This reduces the possibility of cracks formation during cooling of sintered product. Rotary kilns are typically used for production but batch lab furnaces are used for QC.

Electronic Ceramics such as indium tin oxide (ito) is used for sputtering targets for the CVD deposition on glass for HD TVs. The process includes: precipitating indium and tin hydroxides, calcining the hydroxides to produce granulated ITO powder, preparing an aqueous slurry of the ITO powder with additives such as special sintering aids, dispersing agent and binders, milling the slurry to obtain a slip, preparing compacted ITO green bodies by casting the slip using porous molds or drying the slip to yield granulated ITO powder and cold isostatic pressing the powder, and sintering the green body to yield ITO target of high density greater than 99% of theoretical.The calcination may be carried out at a temperature in the range 800 °C - 1200 °C, more preferably 1000 °C.

Rare Earth Oxides – Such as Cerium oxide (one of 17 similar oxides –also praseodymium, lanthanum, neodymium, samarium, and gadolinium.), also known as ceric oxide, ceria, cerium oxide or cerium dioxide, is an oxide of the rare earth metal cerium. It is a pale yellow-white powder with the chemical formula CeO2.Cerium oxide is formed by the calcination of cerium oxalate or cerium hydroxide. Rare Earth Calcination - monazite and bastnaesite - Bastnäsite ore has cerium, lanthanum and yttrium in its generalized formula but officially the mineral is divided into three minerals based on the predominant rare earth element Scandium.

Solid Oxide Fuel Cells - Most notable uses for these rare earths are in solid oxide fuel cell anode and cathodes. - Lanthanum strontium manganate. The most widely used electrolyte material for SOFCs is yttria-stabilized zirconia (YSZ) with a typical composition of (ZrO2)0.92(Y2O3)0.08 or 8YSZ. Figure 2.7 illustrates a flow chart for the synthesis of ultrafine YSZ powders through a sol-gel process. The precursors were zirconium propoxide (Zr(OPr)4) and yttrium nitrate hexa-hydrate in 1-propanol. After the gelation step at 50_C, samples were dried at 80_C for a minimum of 24 h to obtain a xerogel. Then, they were calcined at 950_C for 2 h. Some applications for yttria-stabilized zirconia require calcination temperatures at 1300–15001C for 3 hours.

Nano ceramics such as Ba0.5Sr0.5Co0.2Fe0.8O3 (BSCF) powders have been synthesized by Sol-Gel process using nitrate-based chemicals for SOFC applications since these powders are considered to be promising cathode materials for SOFC. Glycine was used as a chelant agent and ethylene glycol as a dispersant. The powders were calcined at 850ºC /3 hr in the air using a Bench Top Muffle Furnaces furnace.

Doped ceria has also been extensively studied as electrolytes in reduced-temperature SOFCs. Gadolinium doped ceria (GDC, Ce0.9Gd0.1O1.95) is considered to be one of the most promising electrolytes for SOFCs to be operated below 650_C. Further, doped cerias have also been successfully used as part of anodes for SOFCs, especially those using hydrocarbon fuels. Nano-crystalline GDC powder has been prepared by a sol-gel thermolysis method. After the GDC gel precursors are calcined at 400_C, the powders showed cubic fluorite structure.

Zirconium tetrachloride (ZrCl4), yttrium nitrate hexahydrate (Y(NO3)36H2O), and nickel nitrate hexahydrate (Ni(NO3)26H2O) were used as precursors. The obtained xerogel is then calcined to obtain NiO/YSZ powder. During calcination this polymeric metal ion complex is decomposed into CO2 and H2O, and their escape from the reaction mixture prevents agglomeration by ensuring that the mixture remains porous.

Solid Alkaline Inorganic Fuel Cell - CH3COONa and (CH3COO) 2Co•4H2O were dissolved in water. This solution was dried at 80ºC with stirring, and kept in an oven overnight at 80ºC. The dried powders were milled and calcined at 750ºC for 5 h. The calcined sample was crushed and pelleted. The pellets were calcined at 790ºC and crushed, The NaCo2O4 powders were obtained and used to prepare a NaCo2O4 disk for a fuel cell device.

PEM (proton exchange membrane) Fuel Cells - Ceramic oxides are widely used in industry as catalysts, paint pigments, medical supplies, chemical sorbents, and magnetic products. FeOOH Nanoparticles (Ferroxanes) and Ferroxane-Derived Ceramics have been developed for membranes used in fuel cells.

Catalyst - Initial studies with Pd@ceria/alumina demonstrated that the activity of this catalyst is a strong function of calcination temperature, with catalysts showing greatly improved activity when heated to above 1073 K. To understand this, we have investigated the properties of a Pd@CeO2/Si-Al2O3 catalyst after calcination at 773 K and after calcination at 1073 K.

Calcining to higher temperatures increased rates for the methane-oxidation reaction over Pd@CeO2/Si-Al2O3 by a factor of more than three for measurements in 0.5% CH4 and 5% O2. The effect of increasing calcination temperature on this catalyst was even more dramatic for methane-steam reforming (MSR).

Nuclear Fuel Disposal - The behavior of spent nuclear fuel under geological conditions is a major issue underpinning the safety case for long-term disposal. The aim of this work was to produce non-radioactive UO2 fuel analogues to be used to investigate spent fuel dissolution under realistic repository conditions. This report is divided in two parts: Part A concerns CeO2 and ThO2; Part B concerns CaF2. The densification behavior of several cerium dioxide powders, derived from cerium oxalate, and ThO2 were investigated to aid the selection of a suitable powder for fabrication of fuel analogues for powder dissolution tests. CeO2 powders prepared by calcination of cerium oxalate at 800°C and sintering at 1700°C, and ThO2 powders sintered at 1750 °C gave samples with similar microstructure to UO2 fuel and SIMFUEL

Other Applications - Calcined (or alpha) alumina is made by calcining a source alumina powder at 1200-1300C to convert it to pure Al2O3. Carbon/graphite (continuous operation), silicas, titanates, tungsten, cobalt, iron, zinc, quartz, and molybdenum, are sometimes calcined. Synthesis of NiO & ZrO2 Powders – calcined 600-1000C. TiO2 powder formed at low temperature by sol-gel method using Ti(OC4H9)4 and HNO3 and obtained mean particle size of about 50 nm after calcination at 600˚C.

Kilns

Many of our larger kilns have been used in calcining.

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