Pore Development during Selective Leaching

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Abstract
The leaching kinetics of a Cu‐50 weight percent Al alloy in aqueous have been studied. The product of the reaction, known as Raney copper, an important industrial catalyst, consists almost entirely of highly porous copper. In forming this material, the Al is selectively dissolved from the alloy, the rate of the leaching reaction being controlled by liquid‐phase diffusion within the pores of the leach residue. At constant temperature, the spacing of these pores increases as the leaching rate decreases. As the temperature decreases, so does the pore spacing. The conversion of the original alloy phase, , to porous copper is considered as an example of a phase transformation, and the pore spacing is taken as a measure of the distance over which Cu‐Al segregation must occur within the alloy. Models based on volume diffusion adjacent to the reaction front and on boundary diffusion at the front are used to describe the relationship between pore spacing and reaction rate. Application of these models to the experimental data, for a temperature of 20°C, yields estimates of with an activation energy of for volume diffusion and with an activation energy of for boundary diffusion. It is concluded that alloy segregation takes place at the leach reaction front by a mechanism of boundary diffusion.

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