Ind. Eng. Chem. Res. 1994,33,421-427
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Continuous Rotary Kiln Calcination of YBaCuO Precursor Powders Suhas D. Shelukar, H. G. Keshava Sundar, Raphael Semiat: James T. Richardson, and Dan Luss* Department of Chemical Engineering and Texas Center for Superconductivity, University of Houston, Houston, Texas 77204-4792
A bench rotary kiln was used to produce pure YBazCu~06+~ by continuous calcination of sprayroasted precursor powder using a countercurrent air flow to avoid a reaction between evolved COZ and the product. Product agglomeration caused a spread in the velocity distribution, requiring the use of extra calcination for a pure product. High-purity YBa2Cu306.6 was obtained at 940 "C for 0.9vol 7% solid loading, an average residence time of 75 min, andN0 ( N = rotation rate, 8 = inclination) between 3 and 4.5 rpm deg. However, a t a residence time of 77 min, increased product agglomeration at 2.5 rpm deg caused incomplete conversion to YBazCu306.6. Increasing the average residence time above 75 min increased the oxygen content of the product, with pure orthorhombic YBazCu306.9 forming a t 160 min. Due t o reaction and temperature induced changes in particle size and shape, existing correlations cannot predict the dependence of the average solid residence time on the operating conditions.
Introduction High-qualityHTSC (high-temperature superconductor) powders are prepared by calcining precursors containing intimate stoichiometric mixtures of the reacting metal ions. Various chemical routes are used to synthesize YBaCuO powders from a ( Y B ~ ~ C U or ~ O123) ~ +superconducting ~ liquid solution, either by nucleation and growth within the solution, such as coprecipitation (Allen et al., 1992) and sol-gel (Murakami et al., 1990),or by solvent removal techniques, such as spray-drying (Block and Dolhert, 1991), spray-roasting (Zhang et al., 19911, or freeze-drying (Thierauf et al., 1992). Typical air calcination temperatures are in the range of 920-940 "C (Ruckenstein et al., 1989; Shelukar et al., 1993), but a lower calcination temperature of 800 "C may be used at low oxygen partial pressure (Balachandran et al., 1989). Formation of YBazCu306is faster in the absence of oxygen (Gallagher and Fleming, 1989; Formica et al., 19921, but YBazCU306 decomposes in inert atmospheres unless removed immediately after formation (Gallagher and Fleming, 1989; Thomson et al., 1989). Calcination of precursor powders strongly affects the economics of large-scale processing of YBaCuO powders, due to the long processing times required by conventional batch tray/boat processes (Spencer et al., 1990). Many calcination problems are caused by the presence of bariumcontaining salts instead of BaO. For example, in a precursor containing Ba(N03)2, melting at 600 "C causes local inhomogeneity/segregationand an increase in particle size, reducing the calcined product purity (Severin et al., 1988). During calcination at low temperatures, the precursor forms undesirable intermediate phases that inhibit production of pure YBazCUs06+~(Ruckenstein et al., 1989;Formica et al., 1992). For example,when BaCO3 is present, a black shell of YBazCUsO6+, and BaCuOz is Yz03, formed around a grayish core of BaC03, YZCUZOS, and CuO, and limits the rate of COz diffusion and BaC03 decomposition (Formica et al., 1992). Balachandran et al. (1989)and Gallagher and Fleming (1989)suggested that BaC03 decomposition is accelerated at low levels of COz
* Author to whom correspondence should be addressed (email:
[email protected]). + Present address: Department of Chemical Engineering, Technion, Haifa, Israel 32000.
and 02.Grader et al. (1989)found that calcination was accelerated by vacuum processing at 750 "C, at which a reduced phase of CuO reacts withBaC03 to form BaCuzOz. Subsequent oxidation around 800 "C produces pure 123. We studied batch rotary kiln calcination of several sprayroasted precursors (Shelukaret al., 1993). The best results were obtained by 15 min of air calcination at 940 "C, with a loading of 1.5 vol % precursor consisting of YzO3, CuO, and BaC03. Note that the powder was in the kiln also during the heating (70 min) and cool-down (750 min) periods. The presence of nitrate salts always decreased product quality. Increasing the precursor loading required an increase in calcination time, but the rotation speed had no impact on product quality. Product agglomeration, which did not affect product quality, increased with higher solid loadings and lower rotation rates. Batch calcination often leads to product quality variation from one batch to another. Thus, it is highly desirable to develop a continuous calcination process that produces consistently high quality material for economical largescale manufacturing. Balachandran et al. (1992)used continuous rotary calcination of YBaCuO powders but did not study the effect of operating parameters on performance. We report the development of a laboratory process for continuous rotary kiln calcination of a spray-roasted precursor, prepared by Seattle Specialty Ceramics (SSC), Inc. (Woodenville, WA), consisting of YzO3, CuO, and BaCO3. The goal was to examine the impact of operating conditions, such as kiln rotational rate, slope, solid feed rate, and average residence time on product quality. At present, the design and scale-up of rotary kilns are based on operating experience and equipment-specificempirical procedures that rely on superficial insight into underlying principles of operation. In addition, complex interrelationships between chemical reactions, flow and transport processes, and the changes in particle shape and size are very specific to each kiln and therefore difficult to generalize. Hence, the design of a rotary kiln is still considered an art rather than a science (Helmrich and Schugerl, 1980;Baker, 1983). There is a need for a sound rational methodology for the scale-up of rotary kiln reactors. Thus, special attention was given to determining the rate processes that introduce uncertainties into the scale-up of this reactor and their impact.
OSSS-5SS5/94/2633-O421$04.50/0 0 1994 American Chemical Society
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422 Ind. Eng. Chem. Res., Vol. 33, No. 2,1994
1s
22
29
43
36
50
two theta, degrees Figure 3. XRD pattern of the SSC precumor.
Table 1. Propertiea of the SSC Precursor Powder
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Figure I. Schematic of the continuous rotary kiln.
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stoichiometrv densities. g/em3 true density particle density bulk density tap densitp surface area. mYg cumulative particle size, %
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