The hydrothermal dissolution of a Brazilian glauconitic siltstone, known as Verdete, in the presence of calcium hydroxide was studied using conventional and rotary furnaces. The objective was to assess the behaviour of Verdete, a mixture of micas and K-feldspar, in response to reaction with calcium hydroxide to recover potassium-bearing soluble species. The hydrothermal procedure generated Ca-, Si- and Al-bearing insoluble products with defined crystalline structure (hydrogrossular and tobermorite), resulting from the breakdown of K-feldspar. The use of citric acid as extracting agent led to potassium recoveries up to 300% greater than in cases where the extracting agent was water, due to the formation of an insoluble and amorphous layer (containing mostly K, Mg and Fe) between Verdete and the alkaline solution. Altered feldspar, as well as possibly potassium-substituted tobermorites, may have the potential to release potassium (and other elements) in the long term, while the amorphous layer is responsible for faster release in the short term.
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