The Patagonian kaolins offer a wide set of origin of deposits, composition, mineral processing and ceramic properties, giving a rare opportunity for investigating the interdependence of mineralogy, chemical composition, particle size distribution, texture, and surface activity with their technological behaviour in sanitaryware and porcelain stoneware production. In the Santa Cruz and Chubut provinces, Argentina, kaolin deposits were formed by "in situ" alteration of volcaniclastic rocks such as Bajo Grande, Chon Aike or Marifil Fms or by erosion, transportation, and deposition of residual clays in small basins. The genesis, geology, mineralogy, geochemistry, grain size distribution and specific surface area (SSA) of natural and washed kaolins are appraised in an attempt to understand their behavior in the ceramic process. A supergene origin of primary kaolins is inferred on the basis of palaeoclimatic and geochemical evidence that corroborates stable isotopic data. The mineralogy, grain size, and textural characteristics of clays are controlled by parent rock composition (primary kaolins) or by provenance and proximity to source areas (sedimentary kaolins). Different typologies may be distinguished: 1) the sedimentary clays of Baqueró Fm Lower Member, related to Bajo Grande basement, are kaolinitic-smectitic, very fine-grained, and with a very high SSA; they exhibit excellent plasticity and a noteworthy fusibility, but cause an increase of slip viscosity/thixotropy and drying sensitivity as a reduction of casting rate. 2) The clays related to Chon Aike or Marifil Fms are kaolinitic, showing intermediate values for SSA and a coarse particle size distribution, associated with quite a fine-grained texture. They have good plasticity, inducing a certain drying sensitivity, without compromising their rheological and casting behaviour; their main limits are difficult pressing and refractoriness. 3) The Baqueró Fm Upper Member received a considerable pyroclastic supply, fostering the development of a fine-grained clay in which kaolinite(?halloysite) and higher values of kaolinite crystal order prevailed. This improves drying and firing kinetics, though plasticity is low and slip rheology problematic, due to abundant soluble salts. 4) Primary kaolins - derived from weathering of pyroclastic sequences of Chon Aike and Marifil Fms - are coarse-grained, composed of kaolinite+quartz?halloysite and show a very low SSA, being unsuitable for every ceramic application unless previously washed. 5) Alteration of mostly crystalline pyroclastics yielded ordered kaolinite and illite (?halloysite) with a fine particle size distribution and intermediate values of SSA. Beneficiation of these kaolins allows to get the best rheological, casting and drying behaviour, together with a good fusibility and fast firing kinetics, though plasticity is insufficient for pressing. 6) Alteration of mainly vitreous pyroclastics produced halloysite (+kaolinite) with a fine-grained texture and moderately high values of SSA. Washed kaolins have good drying and casting response, though slip rheology and plasticity are unsatisfactory due to both soluble salts and fibrous halloysite, and a refractory behavior. A great deal of technological parameters seem to be reasonably foreseen by the surface activity of clays and especially the Methylene Blue index, that is the most reliable, simple and economic method to predict and control the ceramic response of kaolins and ball clays.
Dependence of Ceramic Properties on Origin and Composition of Kaolins. Case-history from the Santa Cruz and Chubut Provinces, Patagonia, Argentina
Dondi M;
2006
Abstract
The Patagonian kaolins offer a wide set of origin of deposits, composition, mineral processing and ceramic properties, giving a rare opportunity for investigating the interdependence of mineralogy, chemical composition, particle size distribution, texture, and surface activity with their technological behaviour in sanitaryware and porcelain stoneware production. In the Santa Cruz and Chubut provinces, Argentina, kaolin deposits were formed by "in situ" alteration of volcaniclastic rocks such as Bajo Grande, Chon Aike or Marifil Fms or by erosion, transportation, and deposition of residual clays in small basins. The genesis, geology, mineralogy, geochemistry, grain size distribution and specific surface area (SSA) of natural and washed kaolins are appraised in an attempt to understand their behavior in the ceramic process. A supergene origin of primary kaolins is inferred on the basis of palaeoclimatic and geochemical evidence that corroborates stable isotopic data. The mineralogy, grain size, and textural characteristics of clays are controlled by parent rock composition (primary kaolins) or by provenance and proximity to source areas (sedimentary kaolins). Different typologies may be distinguished: 1) the sedimentary clays of Baqueró Fm Lower Member, related to Bajo Grande basement, are kaolinitic-smectitic, very fine-grained, and with a very high SSA; they exhibit excellent plasticity and a noteworthy fusibility, but cause an increase of slip viscosity/thixotropy and drying sensitivity as a reduction of casting rate. 2) The clays related to Chon Aike or Marifil Fms are kaolinitic, showing intermediate values for SSA and a coarse particle size distribution, associated with quite a fine-grained texture. They have good plasticity, inducing a certain drying sensitivity, without compromising their rheological and casting behaviour; their main limits are difficult pressing and refractoriness. 3) The Baqueró Fm Upper Member received a considerable pyroclastic supply, fostering the development of a fine-grained clay in which kaolinite(?halloysite) and higher values of kaolinite crystal order prevailed. This improves drying and firing kinetics, though plasticity is low and slip rheology problematic, due to abundant soluble salts. 4) Primary kaolins - derived from weathering of pyroclastic sequences of Chon Aike and Marifil Fms - are coarse-grained, composed of kaolinite+quartz?halloysite and show a very low SSA, being unsuitable for every ceramic application unless previously washed. 5) Alteration of mostly crystalline pyroclastics yielded ordered kaolinite and illite (?halloysite) with a fine particle size distribution and intermediate values of SSA. Beneficiation of these kaolins allows to get the best rheological, casting and drying behaviour, together with a good fusibility and fast firing kinetics, though plasticity is insufficient for pressing. 6) Alteration of mainly vitreous pyroclastics produced halloysite (+kaolinite) with a fine-grained texture and moderately high values of SSA. Washed kaolins have good drying and casting response, though slip rheology and plasticity are unsatisfactory due to both soluble salts and fibrous halloysite, and a refractory behavior. A great deal of technological parameters seem to be reasonably foreseen by the surface activity of clays and especially the Methylene Blue index, that is the most reliable, simple and economic method to predict and control the ceramic response of kaolins and ball clays.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
