Spray-roasting

Chapter 8 Other Ceramic Powder Fabrication Processes 

TABLE 5 Decomposition Temperatures for Metal Salts [8], Lower Temperatures Are for Dehydration 

These decomposition temperatures are t3rpically less than 500°C and often follow complex mechanisms for example. 

This complex reaction sequence leads to equally complex decomposition kinetics. As discussed in Chapter 5, the shrinking core model is applicable to simple one-step thermal decompositions of the type 

Fluid-solid reactions can be kinetically limited by several steps  

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Other Processes. Dead Sea Periclase (DSP, Mishor Rotem, Israel) converts magnesium chloride into MgO by spray-roasting, then hydrates the MgO to Mg(OH)2. The Mg(OH)2 is washed and dmm filtered. DSP purchases the brine from Dead Sea Works, which collects and stores enriched brine from the southern margins of the Dead Sea (77). 

Solid state reactant Thermal decompositions of a solid Oxidation or reduction of a solid Precipitation Solution heating or cooling Evaporative salting-out Chemical reaction with insoluble product Hydrothermal synthesis Forced insolubility Dissolution reprecipitation Evaporative Condensation Gas phase reaction with solid product Thermal decompositions Oxidation or reduction reactions Combination reactions with a solid product Solvent removal Spray drying Freeze drying Spray roasting Sol-gel synthesis Melt solidification... 

This chapter is devoted to the other ceramic powder fabrication processes not easily classified in the solid, liquid, and gas phase sjmthe-sis schemes of Chapters 5, 6, and 7. These methods include spray drying, spray roasting, freeze drying, metalorganic decomposition, sol-gel sjmthesis, and melt and fiux solidification. Each of these techniques is described in various levels of detail. 

For different values of n ing(i ) = i2", other kinetic expressions can be developed. Figure 8.10 [18] shows the type of powder produced on spray diydng a solution that consists of metal salts of barium and iron in the ratio 1 12 (i.e., barium ferrite). Here we see the remains of the spherical droplets with a surface that consists of the metal salt precipitates, which form a narrow size distribution of platelet crystals (see Figure 8.10(a) and (b)). This narrow crystal size distribution is predicted by the population balance model if nudeation takes place over a short period of time. When these particles are spray roasted (in a plasma gun), the particles are highly sintered into spherical particles (see Figure 8.10(c)). 

Spray roasting is simply an extension of spray drying to higher temperatures, where thermal decomposition of the sprayed salt decomposes. The following steps are involved ... 

Determine the spray roasted particle diameter resulting from the Rayleigh breakup of a jet 0.1 mm in diameter operating on a 0.01 molar doped iron nitrate solution of Ni-Al—Ga spinel ferrite (i.e., MFe204M = Nio 5-AI0 3—Gao ). Assume that the roasting conditions are sufficient to thermally decompose the hydrated metal nitrates that result from spray drying to their respective individual oxides. 

In an extension of the spray-drying technique called spray roasting , evaporative decomposition of solutions (EDS) , spray pyrolysis , or aerosol pyrolysis , the temperature of the heated chamber is high enough to decompose the dried salts after the solvent has evaporated. Nitrate salts are used because of their low decomposition temperatures. The technique eliminates the problems of handling dried nitrate powders, which can be hydroscopic. These methods are used to prepare chalcogenide powders" and barium titanate . 

An alternative method that has been used to prepare ferrite particles in a single step is the evaporative decomposition of the solutions, or spray-roasting (Ruthner, 1977). The solutions were mixed and atomised, and the droplets fell through a reaction chamber at 900-1050 °C. The solvent evaporated and the salts decomposed to oxides. The process took 3-5 s. By using a roasting furnace for industrial production, agglomerates of 40-200 pm containing 1 pm ferrite particles were obtained. The furnace feedstock was an aqueous suspension of the oxides, carbonates or hydroxides of the desired composition. However, the residence time was insufficient and complete transformation to the desired ferrite was not achieved. 

Ruthner, M. J. (1989). Long term availability of spray roasted iron oxides for the production of ferrites. In Advances in Ferrites Proceedings of the Fifth International Conference on Ferrites, India, 1989, Vol. 1. Eds C. M. Srivastava and M. J. Patni. Oxford IBH Publishing Co. PVT Ltd, Bombay, pp. 23-34. 

If a higher temperature and a reactive (e.g., oxidizing) atmosphere are used in the spray chamber, solutions of metal salts can be not only dried but also decomposed at the same time. This process is known as various names, such as spray pyrolysis, spray roasting, spray reaction and evaporative decomposition of solutions, among which spray pyrolysis is the most commonly used [217-223]. 

A solution containing 0.1 mol/1 of zinc acetate is spray roasted using a nozzle that produces 40-p,m diameter droplets produce ZnO particles. If the particles are only 50% dense, estimate their diameter. 

Processes characterized by the use of an aerosol precursor source are known by a variety of terms—spray pyrolysis, spray roasting, evaporative decomposition, and aerosol decomposition, to name a few. A schematic of a generic thermal decomposition apparatus is presented in Figure 2.1 and the aerosol precursor source is... 

Co-spray Roasting. A process for making ferrites (q.v.) in which a solution of mixed Mn and Fe chlorides is sprayed into a large roaster. A mixture of Mn and Fe oxides is formed, and HCl recovered. Zinc oxide is added, and MnZn ferrites produced. 

Figure 34.6 Left) Surface SEM of polished Lao.sSro.sCoOs-g support prepared by colloidal filtration of untreated commercial powder followed by sintering at 1020°C. This cross section clearly demonstrates the presence of solid inhomogeneity and a hollow sphere that was formed in spray roasting of the powder. Right) Defect in 10-p.m-thick Lao.5Sro.5Co03 8 membrane prepared by pulsed laser deposition on support as shown right. A surface defect has caused a large, nonselective pinhole.

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