Abstract:Kaolin is important because it has many applications in the paper, paint-coating, functional filler, extender and pigment industries. This material needs to be bleached by iron leaching to be useful for these applications because this element can be the principal cause of its undesirable coloration. In this work, the kinetics of leaching of iron contained in kaolin clays from the community of Agua Blanca Iturbide, Hidalgo, Mexico, was studied. The leaching experiments were carried out using phosphoric acid as the reagent of leaching for the iron dissolution process. Temperature, acid concentration, and particle size were investigated to determine the most important kinetics parameters during iron leaching. It was determined that the rate of iron dissolution increases with increasing of phosphoric acid concentration, the temperature, and decreased particle size of clay. Data obtained showed that iron dissolution from kaolin clay is due to the diffusion of iron through the product layer. The results revealed that the activation energy of the process was 10.18 kJ¨mol´1.

The present work analyzes the effect of small additions of chromium in the final structure of a commercial Austempered Ductile Iron (ADI). The study is made with emphasis on the effect of this element on the processing window. The results indicate that chromium addition decreases the stability of austenite promoting the martensitic transformation during cooling which increases the hardness and reduces the elongation. Likewise the increase in chromium addition delays the rate of transformation to ausferrite (stage I) and accelerates the stage II reaction promoting the bainite formation narrowing the optimal processing window. However, this effect is markedly reduced when the material is subjected to a long process of homogenization, prior to austempering process. The results of this work are discussed in terms of the degree of chromium segregation during solidification.