UKRAINIAN BALL CLAYS: MINERALOGICAL CHARACTERIZATIONAS A KEY TO EXPLAIN THEIR OUTSTANDING TECHNOLOGICAL PROPERTIES

ZANELLI CH; DONDI M; RAIMONDO M; GUARINI, G; IGLESIAS C; DOMINGUEZ E; ECHEVERRY R

Italia

Conferencia; XIV International Clay Conference; 2009

AIPEA

Chiara Zanelli1*, Michele Dondi1, Mariarosa Raimondo1, Guia Guarini1, Claudio Iglesias2, Eduardo Dominguez3, Ricardo Echeverry4 1CNR-ISTEC, Istituto di Scienza e Tecnologia dei Materiali Ceramici, Faenza, 48018, Italy 2Piedra Grande SA, Trelew, Chubut, 9100, Argentina 3Departamento de Geologia, Universidad Nacional del Sur, Bahia Blanca, 8000, Argentina 4Universidad Nacional de la Plata, La Plata, 1900, Argentina *chiara.zanelli@istec.cnr.it   Ball clay is defined as “a fine-grained, highly plastic, mainly kaolinitic, sedimentary clay, the higher grades of which fire to a white or near white colour in an oxidising atmosphere”. Ball clays are traditionally used in ceramic production, i.e. tableware, sanitaryware and extensively in tilemaking. In particular, they are the most important component of porcelain stoneware bodies, used to manufacture big slabs up to 3 m2. Technological requirements of ball clays are ever stricter in terms of plasticity, rheological behaviour, workability in the green state, refractoriness and firing colour. The best performance achieved in the industrial production of porcelain stoneware tiles is with ball clays coming from Ukraine, which turned to be a world because wide benchmark of their peculiar mineralogical, granulometric and technological properties. This work is aimed at understanding the reasons the outstanding ceramic properties of these raw materials. Seven Ukrainian ball clays, currently used in industrial ceramic production, were selected and their chemical analysis, particle size distribution, rheological behaviour, specific surface, plasticity and methylene blue index were determined. The mineralogical composition was investigated by SEM-TEM and XRPD performed on both randomly oriented bulk samples and oriented <2mm and <0.2mm fractions. Compared with conventional ball clays, the Ukrainian raw materials are characterized by abundant clay minerals, mostly kaolinite (poorly ordered) and randomly interstratified illite/smectite (expandable component <15%), and low content of quartz. The kaolinite-to-I/S ratio is low and I/S terms turn to be predominant in the colloidal fraction. This mineralogy, together with fine particle size, implies plasticity, specific surface and methylene blue index higher than classic ball clays; moreover, rheological properties are suitable  easily dispersible in water and much less active than artificial blends added with bentonite in order to improve plasticity. Firing behaviour is satisfactory for easy sintering and light colour. Overall, the Ukrainian ball clays unrivalled performances are due to the occurrence of moderately active clay minerals, such as disordered kaolinite and illite-rich I/S interstratified, with fine particle distribution: a mix hard to be reproduced by mineralogical treatments.     Keywords: ball clay, technological properties, ceramic behaviour.