Effect of compaction and acid treatment on the surface area and porosity of kaolinite, alumina and silica gel

X-ray analysis, surface area and porosity of kaolinite, silica gel and alumina as powder or compacted at 50 and 100 kN cm(-2), before and after treatment with 4 N HCl, have been studied. All samples of kaolinite exhibit the diffraction pattern of aluminium silicate hydrate (Al2Si2O5 (OH)(4)); for alumina, the diffraction pattern of alpha basic aluminium oxide (alpha-Al2O(OH)(2)) and for silica gel all the X-ray diffraction patterns are of an amorphous phase. A slight decrease in the intensity of the diffraction lines for the compacted solids are due to enhancement of solid-solid interfaces. For kaolinite and alumina, generally compaction is associated with a loss in surface area due the adhesion of neighbouring particles and blocking of a significant fraction of micropores. Acid treatment is also accompanied by a slight change in surface area. For silica gel there is no appreciable change in the texture parameters between powder and compacted solids. Analysis of the nitrogen adsorption isotherms using the alpha(s) method of Sing indicated the presence of narrow pores and mesopores in the original kaolinite powder. For all other samples of kaolinite and alumina, both powder and compacted forms before and after acid treatment, mesopores only are present. For silica gel, all samples essentially contained micropores. A correlation between the amount of N-2 adsorbed for compacted samples relative to the uncompacted samples at the same relative pressure (F-plot) was established to follow changes in the shape of the isotherm consequent to compaction and/or acid treatment. These changes show up clearly as deviations from the horizontal line, providing information on the surface area and any capillary condensation that occurs.