SUITMA 2005 Cairo - Confinement of highly reactive waste sulfides into mortar matrix
SUITMA 2005 Cairo
| This abstract is about one of the papers of the Cycling of city fluids and solid refuse theme of the SUITMA 2005 symposium. |
| Inna V. Filippova,i | Lev O. Filippov,i |
| Jacques Yvon.i |
- i - Laboratoire environnement et minéralurgie (LEM), Université de Lorraine - CNRS, Nancy, France
- ii - Departamento de Edafología e Química Agrícola, Universidad de Santiago de Compostela, Santiago de Compostela, Spain.
The ecological management of mineral processing wastes not only deals with the development of stabilization methods to prevent the possible pollution, but also with the rehabilitation of the occupying areas and the improvement of the visual aspect of landscape. One of conventional uses of such wastes in a hardened mortar matrix allows its disposal in artificial spaces resulting from mining activities.
The pyrite containing samples with low acid neutralization potential from an old French waste dump revealed the capacity of sands and clayey wastes to produce acid waters (pH 1.8-2.4) and release heavy metals. The choice of an adapted method for recovering and/or the detoxifying wastes by limiting the mobility of toxicants depends on many factors. Some of these factors deal with the content of wastes, the presence of chemically unstable products resulting from oxidation of sulfides (sulp lev.filippov@ensg.inpl-nancy.frhates), other factors can be the granular, mechanical, physical-chemical or geological and hydrogeological characteristics, they generate specific risks, and expenses. All these factors affect the waste recovery and stabilization and/or waste consolidation by mortars aiming at preventing their oxidation.
Continuous leaching tests with deionized water, sulfuric acid solution and “real acid water” show that the direct consolidation of the finely dispersed clay wastes (size fraction is 91-95% less than 20 µm with content of pyrite up to 9 %) with mortar is accompanied by a destruction of the mortar matrix due to the pyrite oxidation and subsequent sulfate corrosion.
As revealed by leaching with deionized water, the stabilization of highly reactive sulfide-containing wastes in a concrete matrix is more efficient than the direct confinement. The values of pH stabilize around 10 to 12 for a 5-6 months leaching duration independently of the type of cement used. The contact of cement matrix with the sulfuric acid (pH 1.4) leads to a decrease of pH values to 8.6-9.6. The amounts of iron and other extracted metals (Cu, Al) were negligible and did not depend on the initial acidity of the medium. However, the kinetics of the pH rise for the based on Portland cement CEM II/B-M mortar is faster after 144 hours leaching compared to mortar based on blast furnace cement CEM III/B containing 20 to 34 % of Portland clinker. The attack of the mortars by sulfuric acid solution does not affect its integrity and does not promote the calcium sulfate formation.
A 1920 hours leaching of the clayey sample, treated by a based on Portland cement CEM I mortar with the “real acid water” resulting itself from the leaching of a pyrite-containing clay [pH: 1.90, E: 580 mV, conductivity: 3.33 mS/cm, sulfate concentration: 2.81 g/L; metal ions; (Fe3+=544.0 mg/l, Cu=151.8 mg/l, Al=111.2 mg/l)] leads to pH 6.2. Thus, the resistance of the binding matrix to the sulfate attack depends on the combined effect of the metal and sulfate ions, and of the bacteria action that strongly influence the degradation of the cementing envelope. Formation of the calcium sulfate in the surface layers indicates the initiation of degradations of the cementing matrix under the action of "real acid water". Analysis of the leaching behavior of non protected samples and covering concretes allow suggesting the covering of wastes by Portland cement mortar as a solution for the stabilization of highly reactive sulfide-containing clayey fractions directly on site, in natural or artificial cavities.
