Characterization of Kef Shfeir phosphate sludge (Gafsa, Tunisia) and optimization of its dewatering

Water separation and recovery through thickening require adapted flocculants and densification processes. This study aimed to maximize water recovery from phosphate sludge (PS) at Kef Shfeir mine operation, Gafsa Phosphate Company, Tunisia. Representative samples of PS, PS treated with flocculant (F-PS), raw water, and recycled water were collected on the mine site. Solid samples (PS and F-PS) were characterized physically, chemically and mineralogically. To maximize water recovery, thickening tests were performed on the PS using different flocculants to optimize flocculant concentration, the agitation speed and the settling time. Results showed that PS had positive surface charge since its paste pH (7.3) was lower than pH_PZC (8.0), whereas the tested flocculant (Slim Floc used by the company) showed negative surface charge. Solid samples contained coarse medium and fine particles of carbonates, silicates and residual hydroxyapatite. The cumulative fractions +32 μm of PS contained a promising residual potential of fluorapatite (up to 39.2%). Water recovery was about 58.1%, when the anionic Slim Floc was used, for a consumption rate of 1200 g/t of dry solids. Best efficiency (84%) of water recovery was obtained with the anionic flocculant E24 for a consumption rate of 360 g/t of dry solids, which is 3 times lower than actual flocculant consumption.

The role of hardpan formation on the reactivity of sulfidic mine tailings: A case study at Joutel mine (Québec)

The former Eagle and Telbel mine site (hereafter referred to as Joutel mine), located near the town of Joutel in the Nord-du-Québec (Canada) houses a tailings storage facility (TSF) that has been inactive since 1996. Fresh, unweathered tailings (beneath 10-30 cm of oxidized horizon) are characterized by an average sulfide content of 6-7 wt% and an average Fe-Mn-carbonate content of 20-40 wt%. The oxidation of Joutel's tailings under atmospheric conditions resulted in the precipitation of secondary phases such as ferric oxyhydroxides and gypsum. Accumulation of these secondary phases throughout the TSF caused cementation and agglomeration of grains, which decreased the porosity of the material in a horizon below the surface. This horizon, which is referred to as hardpan, is frequently encountered within fine, reactive tailings. Characterizations showed that hardpans have a highly compact texture. The formation of hardpans limits vertical water infiltration and oxygen diffusion and these layers greatly affect the global geochemical behavior of underlying tailings in the Joutel TSF by protecting the unweathered material from oxidation. As a result, the water quality of the TSF is largely controlled by the reactivity of the upper oxidized tailings horizon. Joutel's oxidized tailings showed an acidic behavior early during laboratory kinetic leaching tests despite the near absence of sulfides and neutralizing minerals. However, when unweathered tailings were added under oxidized tailings, the water became neutral and metal leaching rates were reduced. After over a year of laboratory leaching tests, hardpans formed within the columns and the natural phenomenon was reproduced under laboratory conditions.

Geochemical behavior and environmental risks related to the use of abandoned base-metal tailings as construction material in the upper-Moulouya district, Morocco

In some developing countries, base-metal residues that were abandoned in tailing ponds or impoundments are increasingly used as construction material without any control, engineering basis, or environmental concern. This uncontrolled reuse of mine tailings may constitute a new form of pollution risks for humans and ecosystems through metal leaching. Therefore, the aim of the current study is to assess mine drainage, metal mobility, and geochemical behavior of two abandoned mine tailings commonly used in the upper-Moulouya region (eastern Morocco) as fine aggregates for mortar preparation. Their detailed physical, chemical, and mineralogical properties were subsequently evaluated in the context of developing appropriate alternative reuses to replace their conventional disposal and limit their weathering exposure. The obtained results showed that both tailings contain relatively high quantities of residual metals and metalloids with lead (ranging between 3610 and 5940 mg/kg) being the major pollutant. However, the mineralogical investigations revealed the presence of abundant neutralizing minerals and low sulfide content which influence mine drainage geochemistry and subsequently lower metals mobility. In fact, leachate analyses from weathering cell kinetic tests showed neutral conditions and low sulfide oxidation rates. According to these results, the tailings used as construction material in the upper-Moulouya region have very low generating potential of contaminated effluents and their reuse as aggregates may constitute a sustainable alternative method for efficient tailing management.