Physical, chemical and microstructural characterization of two problematic soils from the Paraguayan Chaco

It is not uncommon for Geotechnical Engineering works to be carried out under unfavorable conditions that compromise the earth-stability. In this context, the Paraguayan Region of Chaco is notably known owing to the presence of problematic soils that possess dispersive characteristics and/or present high amounts of soluble-sulfates content. Geomaterials of such nature affect mainly the road infrastructure earthworks due to, respectively, their promptness to erosive phenomena when in contact with water and swelling owing to the grown and hydration of expansive minerals such as ettringite and thaumasite, when treated with calcium-based materials. Therefore, present research presents a detailed characterization of a dispersive soil and a sulfate-rich dispersive soil, both collected in the Western Region of Paraguay. Physical, chemical and microstructure tests were carried out in order to verify and explain the deleterious behavior observed in both soils.

Concentrating Pt from a spent alumina carrier catalyst by a roast-leach process

A roast-leach process has been developed for concentrating platinum (Pt) from a spent Pt-Al2O3 catalyst. The effects of (NH4)2SO4 to spent catalyst mass ratio, roasting temperature and roasting time on the Pt enrichment factors were investigated. The results showed that the optimized roasting conditions were (NH4)2SO4 to spent catalyst mass ratio of 7.5, roasting temperature of 350 °C and roasting time of 5 h. The roasted product was mainly comprised of soluble NH4Al(SO4)2 and Al2(SO4)3 according to XRD analysis. The roasted product was leached in sulfuric acid to remove the soluble sulfates, and meanwhile Pt was kept in the residue. Under optimized conditions, a Pt concentrate with Pt content of 73.2% could be obtained from the spent catalyst with 0.27% Pt, and the corresponding Pt enrichment factor was more than 270.

Development and validation of an analytical method for the extraction and quantification of soluble sulfates in red clay

In this work a fast and environmentally friendly method for the extraction and quantification of soluble sulphates in red clay using microwave assisted extraction (MAE) and determination by turbidimetry was developed and validated. The favorable conditions for the extraction of soluble sulphates in red clay by MAE were: 1 g of sample, with particle size of 63μm, dissolved in 50 mL of distilled water, it was extracted using a microwave oven with 70% of power during 5 min, later the sample was centrifuged during 5 min and then filtered. The soluble sulphates in red clays were quantified at 420 nm. The procedure proposed showed linear behaviour in the tested rank (5-7000 mg SO42-/kg of clay) with R2 0.9993. The limits of detection and quantification were 4.30 and 14.33 mg/kg, respectively, with a variation coefficient of 1.41%. The method proposed in this work allows to determine soluble sulphates in red clay with a recovery of 94%.

Ettringite Formation in Lime-Treated Soils

The use of calcium-based stabilizers such as calcium oxide (lime) in sulfate-bearing clay soils has historically led to structural distress because of the formation of a mineral called ettringite and possibly thaumasite. In trying to control the damage associated with such formations, engineers have attempted to determine a threshold level of soluble sulfates–-a quantity that is relatively easy and quick to measure at which significant ettringite growth and, therefore, structural distress occurs. This is indeed a complex problem related to not only soil composition but also construction methods, availability of water, ion migration, and the ability of the void structure to accommodate the expansive mineral growth. Unfortunately, experience alone and rules-of-thumb based on experience are not sufficient to deal with this complex issue. Thermodynamic geochemical models of the lime-treated soil can be used as a first step toward establishing thresholds for problematic levels of soluble sulfates for a specific soil. A foundation for the model development is presented, and two soils are compared to illustrate their sensitivities to ettringite growth on the addition of lime. Because the model predicts ettringite growth on the basis of site-specific properties, the model can be used to assess the potential amelioration effects of soluble silica.