Effect of kaolinite and sulfate on the formation of hydroxy-aluminum compounds

OH-Al solutions were prepared by adding appropriate amounts of NaOH to AlCl 3 to obtain OH/Al mole ratios of 2.0, 2.5, 2.7, 3.0, and 3.3 in the final s uspension. Solid Na2SO4 and Georgia kaolinite (KGa-2) were added individual ly and jointly to the OH-Al solutions. All samples were aged for 30, 70, an d 180 d. X-ray diffraction, infrared spectroscopy, scanning electron micros copy, and energy dispersive X-ray spectrometry were used to characterize pr ecipitates. Bayerite, gibbsite, and nordstrandite crystallized at mole rati os of 3.0 and 3.3, with bayerite being the most abundant. A morphology of c lusters of triangular pyramids is described for bayerite. Despite the aging duration, only noncrystalline Al compounds were obtained in mole ratios of 2.0, 2.5, or 2.7. The addition of sulfate to OH-Al solutions in mole ratio s of 2.0 and 2.5 produced crystalline basic aluminum sulfates of variable m orphology, but: with similar chemical compositions. These phases lost cryst allinity with aging. The product from a 2.7 OH-Al solution was X-ray amorph ous hydroxysulfate. In contrast, products obtained at mole ratios of 3.0 an d 3.3 contained no sulfate ion, which restricted the formation of gibbsite, bayerite, and nordstrandite. The addition of kaolinite to the solutions in OH/Al mole ratios of 3.0 and 3.3 favored the formation of nordstrandite. T he simultaneous addition of sulfate and kaolinite to the OH-Al solutions in mole ratios of 2.0 and 2.5 produced prevalent sulfate over kaolinite, wher eas the opposite occurred at mole ratios 3.0 and 3.3.