Liberação do potássio contido em siltito glauconítico por processo térmico

Abstract

The agribusiness in Brazil is one of the most important sectors of Brazilian economy, being currently responsible for about 23% of the GNP (Gross National Product). To provide an increase in the agriculture productivity, fertilizers NPK (Nitrogen, Phosphorus and Potassium) play an essential role. However, Brazil currently presents a deficit in the production of fertilizers aiming to supply the growing demand. This is mainly concerning to potassium, since its current importation is 94% of the internal consumption. Based on this scenario, deposits of K-bearing silicates with high content of K₂O are main alternative that requires evaluation. This work has as the objective to study, in a laboratory scale, the thermal process of the reaction of glauconitic siltstone, a silicate rock that occurs in São Gotardo region-MG, with limestone, evaluating the chemical and mineralogical modifications that take place in the process, which aims to release potassium, making it more soluble in soil conditions and available for the crops. A mixture of glauconitic siltstone and calcitic limestone ground below 74 µm, as well as the thermal treated products, were characterized by XRF, XRD, FTIR, SEM/EDS, TGA and DSC. The weight loss of the mixture in the reaction was measured as 21.35% and the relative net enthalpy of the process varied between 720 to 910J/g, an endothermic process. The chemical and structural transformations in materials caused by the thermal process were evidenced by thermal analysis (TGA/DSC). Alterations in the potassic minerals composition (glauconite, muscovite and microcline) were observed, which were converted to feldspathoids (leucite and kaliophilite), minerals which bear potassium in an less packed crystalline structure, when compared to Kfeldspar and micas, what increases its availability, determined according to classic methodology. This transformation was responsible to increase the potassium extraction by citric acid solution 2% w/w (1:500 solid-liquid ratio) from 4.2% in the crude mixture to 95% in the calcined product at the highest temperature of the treatment, where the liquid formation was observed due to a high fusion in the material. In this condition, the material presents a good potential to be used in agriculture, providing not only available potassium, but also calcium and magnesium, which are secondary macronutrients.