Factors Affecting Strength of Agglomerates Formed During Spray Drying of Nanophase Powders
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INTRODUCTION Agglomeration is a major obstacle for the processing and use of nanosize particles. It is a problem in a number of powder processing industries including plastics, resins, ceramic materials, dyestuff, pigments, paints, etc. Spray drying is by definition a one step continuous suspended particle processing operation. In the ceramics industry, traditional operations of wet preparation require filter pressing, drying, crushing, re-wetting and classifying, while modern operations with spray drying reduce all these to a single-stage operation. Agglomeration during spray drying is an inherent problem, and the aim of the current study is to determine the factors that affect the strength of agglomerates formed during spray diying. Formation of hard agglomerates may occur during various powder-processing stages when the dispersed particles are subject to large hydrostatic, or capillary, forces which overcome the repulsive inter-particle forces. During drying of a powder slurry, a number of reaction pathways can lead to the formation of metal- oxygen -metal bonds between individual, primary particles. For small sized particles, this can lead to a dissolution -reprecipitation process that removes material from the particle surface and deposits it in the toroidal region between particles. Capillary forces during drying are a strong function of the drying rate and the drying temperature. The maximum capillary tension occurs at the critical point, when the meniscus recedes into the pores of the agglomerate. At this stage, the tension at the drying surface is given by the Laplace equation [1]. 2 ycos(e)
-mx
Where Yiv is the liquid/vapor interfacial energy, 0 is the contact angle between the solvent and the solid surface and rp is the radius of the pore. The pore radius (or the hydraulic radius in case of the particles) is much smaller for nanosize particles and this results in a higher capillary pressure. For close-packed (e = 0.3) 10 nm primary particles
637 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
dried from water, the capillary pressure may approach 14,000 psia (assuming contact angle, 0 =-0). The solubility of the particle also varies inversely with particle radius. One of the most important properties of an agglomerate is its "strength", a term which is not often well defined, but is usually taken to include its resistance to breakage (of any kind) during formation, handling and subsequent processing. It is difficult to assign a quantitative value to "agglomerate strength", and it is usually referred to in relative units, either as "soft" or "hard". Factors such as surface tension, viscosity, solids content, solubility and pH are temperature dependent and hence temperature can be one of the most important factors controlling the agglomeration process. Spray drying allows us to control the drying temperature and conditions over a wide range; and therefore, it is an excellent technique to study the agglomeration process. Numerous techniques have been used to control agglomeration i
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