Presence and impact of silica and organics in uranium in-situ leaching solutions

Abstract

All uranium deposits of Uzbekistan mined by in-situ leaching (ISL) method belong to the so-called sandstone type and are formed on the redox barrier, with the Loyliken deposit, belonging to the so-called basal type, formed on the gley barrier, and all the rest, belonging to the roll type - on the complex gley-sulfur-hydrogen barrier. In all cases, the initial reducing agent is carbonized plant detritus (CPD), which is charred plant flakes scattered in sandstone, usually not exceeding 1 mm in cross-section - with the difference that at the gley barrier it is the direct uranium reducing agent, and at the gley-sulfur barrier a significant part of uranium is reduced by hydrogen sulfide, which is a product of biochemical reduction of sulfate ions contained in formation water by the same CPD (the so-called sulfate-reduction reaction). Among other conditions necessary for the formation of conditioned uranium mineralization, it is required that the CPD content in the formation is not less than 0.05% per organic carbon (Сorg). At the same time, uranium concentration in almost all sandstone deposits of Uzbekistan is comparable to Сorg concentration.

English abstract from the paper. The authors’ abstract, reproduced under CC BY 4.0.

In plain English

All uranium deposits in Uzbekistan mined by in-situ leaching are of the sandstone type and formed at a redox barrier. The Loyliken deposit is of the basal type and formed at a gley barrier; all the others are of the roll type and formed at a complex gley–hydrogen sulphide barrier. In every case the original reducing agent is carbonised plant detritus, charred plant flakes usually less than 1 mm across, which at depths of up to 1000 m has the composition of lignite (subbituminous) coal. At the gley–sulphide barrier much of the uranium is reduced by hydrogen sulphide, formed when the same detritus drives the biochemical reduction of sulphate in the formation water. Ore-grade uranium needs at least 0.05% organic carbon in the layer, and even in the roll deposits about a third of the uranium, from 19% at Beshkok to 48% at Loyliken, is bound to organic matter rather than held in minerals.

Silica in natural waters stays mostly between the solubility over quartz, about 6 mg/L, and over partly hydrated amorphous silica, 100–130 mg/L at 25 °C. In most roll deposits of Uzbekistan, orthosilicic acid in the groundwater is 13–30 mg/L. The productive solution at Loyliken is only 1.64 times supersaturated with silica, so its silica is in the form of orthosilicic and disilicic acid: a 60-fold supersaturated solution begins to polymerise after 2–3 hours, and a threefold one only after a month. Sulphuric acid dissolves silicates of the ore during leaching; the silica in the solutions stays low because the ore contains few of the minerals that dissolve with gel formation.

Silica slows the leaching of coffinite, while pitchblende is not affected, and does most harm to ion-exchange resins: 10% silica in a resin lowers its uranium capacity by 10%, and 19.6% by 32%. The organic matter in the solutions is a mixture of humic, fulvic and sulfohumic acids and their salts, which dissolve at the pH of 1.5–3.0 used in leaching. Studies by the university for NMMC found that fulvic acids and lignosulfonates speed up leaching by catalysing the oxidation of uranium and by raising the filtration rate through clay layers; the optimal filtration rate for in-situ leaching is 1–10 m/day.

How to cite

U. Sharafutdinov, I. Karimov, I. Rajabboev, S. Yuldashev, M. Khamidova and D. Khurramova (2025). Presence and impact of silica and organics in uranium in-situ leaching solutions. E3S Web of Conferences, vol. 627, 01011. https://doi.org/10.1051/e3sconf/202562701011