Novel micro-granulates for porcelain stoneware tiles: preliminary data on powder rheology and compaction

Using spray-dried bodies for feeding tile production plants is nowadays widely consolidated. The industrial production is moving towards increasingly larger sizes, especially for porcelain stoneware tiles and slabs. This trend implies a demand of high-quality ceramic powders, particularly performing all along the production process, as achievable by spray-dried bodies. However, spray-drying requires a significant consumption of both water and energy. An interesting alternative appears to be the dry granulation process, because of remarkable water and energy savings. In the past decades, it has always been penalized by a worse rheology and compaction behavior, with respect to spray-dried powders, that reflected on microstructure and characteristics of fired products. Recently, a novel micro-granulation hybrid process has been developed for porcelain stoneware bodies. It is based on wetting-agglomeration-drying operations that are able to provide granules with technological characteristics adjustable on request and behavior apparently close to that of spray-dried powders. Aim of our work is the evaluation of these novel micro-granulates in the industrial production of porcelain stoneware tiles and large slabs. Comparison is made with conventional technologies used for dry-granulated and spray-dried powders. For this purpose, dry micro-granulates manufactured on the Migratech 4.0 pilot line were investigated for their characteristics (grain size, shape and moisture distribution), flowability (static and dynamic angles of repose, poured and tapped density, mass flow rate, Hausner ratio) and compaction behavior (bulk density of green and dried bodies for increasing applied load, green and dry modulus of rupture, microstructure by optical and scanning electron microscopy, pore size distribution by mercury intrusion porosimetry). The new dry micro-granulates exhibit improved rheological and compaction performances, approaching those of spray-dried bodies. Some differences in compaction behavior are discussed in relation to the distinct microstructures and textural properties of powders obtained by spray-drying and dry micro-granulation, respectively.