Clay-isolation of chemical waste in mines

Transcription

1 Clay-isolation of chemical waste in mines R. Pusch Geodevelopment AB, IDEON Research Center, SE Lund, Sweden Abstract Surrounding and mixing of solid waste by low-permeable expansive clay very effectively retards dissolution and migration of toxic elements due to very slow wetting and diffusive ion transport, and very low permeability of the clay. Drifts and rooms of abandoned mines that are filled with hazardous waste surrounded and mixed with smectite clay will not give off toxic elements in many tens or hundreds of thousands of years to the groundwater and by selecting suitable clay types and application techniques it is possible to isolate such waste at a reasonable cost. 1 Clay as isolating medium 1.1 General Clays have a low permeability and sorb cationic elements and some of them are also expansive and can fill up the space in which they are placed, like drifts and rooms in an abandoned mine. They are therefore ideal for embedding or surrounding solid waste. The smectite family, particularly montmorillonite, has the lowest hydraulic conductivity and the highest expandability of all clay types and should be considered for such purposes. The most suitable form of the clay for isolation of solid waste is dried and ground powder obtained by drying, crushing and sieving natural clay of required quality. Bentonite, being smectitic clay formed from volcanish ash in nature, is commercially available in any desired form from many mineral companies. The dry material can be compacted to form blocks in which solid waste objects can be embedded, or applied layerwise by sandwiching clay and waste layers that are compacted on site by ordinary constructors techniques. Using drifts and rooms in deeply located abandoned mines the clay takes up water from the rock and eventually gets fully water saturated. The time for reaching this stage can be many decades or

2 170 Waste Management and the Environment II centuries and not until then chemical reactions between the clay and the waste start yielding release of ions from the waste containers and the waste itself. Cations are sorbed and immobilized by the clay but in a long perspective the porewater becomes saturated with them, which leads to migration to the rock and further to the biosphere. Even at moderate clay density ion migration takes place by diffusion and not by porewater flow. The most important clay properties are 1) the chemical environment provided by the clay matrix since it determines the dissolution rate of the waste, 2) the ion exchange capacity since it determines the sorption ability, 3) the ion diffusivity since it controls the ion migration, and 4) the hydraulic conductivity since it is so low that all ion transport takes place by diffusion. The host rock affects the ion transport to the biosphere by its content of pathways, like conductive fracture zones and the pervious excavationdisturbed zone that surrounds the drifts and rooms, as well as by providing groundwater of various chemical compositions. 1.2 The basic mineral unit the clay particle Figure 1 shows the crystal structure of montmorillonite showing cations and water molecules in the interlamellar space, which can host up to 3 hydrates at full expansion. The number of interlamellar hydrates is related to the density of the clay. Hydrates are also present at the basal surfaces of stacks of lamellae, where electrical double-layers are formed with the negative charge representing the mineral surface and the positive charge being provided by sorbed cations. The interaction of electrical double-layers cause osmotic effects and partly controls the interparticle distance. Figure 1: Montmorillonite particle consisting of two lamellae. Hydrated cations are in the interlamellar space and at the free basal surfaces. The chemical composition of the porewater strongly affects the particle spacing and arrangement and thereby the bulk physical properties. If the clay initially has sodium ions in the exchange positions, which is the case for most commercial bentonites, and the porewater becomes rich in bi- or polyvalent cations the stacks of lamellae contract and the voids between them become larger, which leads to an increase in hydraulic conductivity. This effect is very

3 Waste Management and the Environment II 171 important because it means that the hydraulic conductivity of soft clay increases very much if cations other than Na and Li are released from the waste. 1.3 Porewater chemistry, ion exchange and sorption In Na smectitic clay saturated with low-electrolyte water ph is high, i.e. in the interval 9-10, but drops to 6-8 when the porewater is rich in salt because cations replacing protons in the crystal lattice are released to the porewater. At higher temperatures than 50 o C ph starts dropping. The lattice charge deficit caused by vacancies and replacement of Si by Al and other substitutions, and exposure of structural elements like hydroxyls gives the smectites an ability to sorb and exchange cations and charged inorganic and organic molecules. The anion exchange capacity is usually low since the net surface charge of the crystallites is negative. Cations are not equally replaceable and do not have the same replacing power. In principle, the following law of replacement power applies: Li < Na < K < Ca < Mg < NH 4. However, the replaceability depends on several factors, especially the ion concentration and size as well as the type of anion. The cation exchange capacity of pure smectites is meq/100 g and this is also valid for many organic molecules like benzidine and amines. 1.4 Ion diffusivity Ion diffusion is controlled by the charge and size of the ions and of the accessible space and its tortuosity, which are functions of the bulk density of the clay. For very high densities the anion diffusion capacity is very low because of Donnan exclusion from the interlamellar space while the cation diffusion capacity is high and relatively independent of the density since cations move both in the interlamellar- and extralamellar space. The effective diffusion coefficient is in the range of E-12 to E-9 m 2 /s for both. 1.5 Hydraulic conductivity The hydraulic conductivity is primarily determined by the bulk density of the clay since it determines the void size and interconnectivity. For densities higher than about 2000 kg/m 3 the conductivity of smectite-rich clay is E-14 to E-13 m/s and relatively independent of the chemical composition of the water, while for densities lower than about 1600 kg/m 3 it is higher than about E-10 m/s for electrolyte-poor water and exceeds E-6 m/s for salt water with Ca as dominant cation [1]. The strong density dependence is explained by the salt-induced coagulation of the soft clay gels in low-density clays. It is hence obvious that effective waste isolation requires a high density of the clay.

4 172 Waste Management and the Environment II 1.6 Optimal selection of clay While the low conductivity and high expandability of smectite-rich clays make them most suitable materials for sealing purposes they are very expensive and other expansive soils, like bentonites with a smectite content down to % or mixed-layered clays should therefore be considered. The Friedland Ton in the Neubrandenburg area in Germany, can serve as an example. Samples of clay in dried and crushed form can be compacted to form blocks and pellets with high density for isolating solid and semi-solid waste. Comprehensive laboratory testing has been made for characterizing a number of clay types and the rate of migration of hazardous elements through it can be predicted [1]. 2 Example 2.1 Clay isolation of used batteries The performance of clay consisting of mixed-layer minerals has been modelled with respect to the water saturation of clay powder mixed with batteries. The case considered is illustrated by Figure 2. Figure 2: Example of storage of batteries contained in compacted blocks of mixed dry clay and batteries in a mine Waste Three types of batteries were investigated: 1) uncorroded alkali batteries, 2) strongly corroded Hg batteries (Figure 3), and uncorroded button-type Hg batteries. In mercury dioxide batteries the mercury is in metallic form and is expected to appear as such in the clay and also in ionic or complex forms. In other types of batteries mercury is added to the zinc at the manufacturing of the anodes. It may remain in metallic form or as oxide after the disappearance of the zinc.

5 Waste Management and the Environment II 173 Figure 3: Strongly corroded Hg batteries (Photo: R. Sjöblom) Clay material Friedland Ton was used for the tests. It has the following chemical composition: 18 %, Fe 2 O %, MgO 2 2 %, CaO <1 %, Na 2 O 0.9 %, K 2 O 3.1 %. Na is the dominant adsorbed cation. The grain size distribution was: 2-8 mm granules make up 20 %, 1-2 mm make up 20.4 %, make up 42.4 %, and minus 0.1 mm granules make up 17.2 %. The water content of the powder was 7 percent by weight Test performance In a first series of tests non-corroded alkaline batteries with 50 mm length and 14 mm diameter were placed in Friedland Ton powder and compressed under 50 MPa pressure in 60 mm high oedometers with 100 mm diameter. Saturation and percolation with salt solutions (20 % NaCl, and 3.5 % CaCl 2 ) were made for 10 months. The dry density (ratio of minerals and volume of the minerals, porewater and air-filled voids) of the clay was 1700 kg/m 3. After termination of the tests the cells were opened and the clay analyzed with respect to major cations. In a second test series, designed to find out whether less dense clay represents an option, two strongly corroded Hg batteries with 30 mm diameter and 60 mm length were used. The dry density 1450 kg/m 3 could be reached by applying a dynamic compaction energy comparable to what can be achieved by field compaction methods. Saturation and percolation with distilled water and 3.5 % CaCl 2 solution was made for 8 months in separate tests. A third test series was made with 20 uncorroded button-type Hg batteries with mm diameter and 4 mm height placed in clay with a dry density of 1450 kg/m 3. Saturation and percolation was made with distilled water for 12 months after which percolation for 6 months was made with 3.5 % CaCl 2 solution Hydration of the clay Uptake of water of the clay column that had an initial degree of fluid saturation of 25 to 40 % in the various tests takes place as a diffusion process. For Friedland Ton with a dry density of 1700 kg/m 3 the effective diffusion coefficient is about E-9 m 2 /s. The study showed that it is approximately valid also for 1450 kg/m 3 dry density (Figure 4).

6 174 Waste Management and the Environment II Degree of water saturation, % Distance from wet filter, cm Figure 4: Diffusion-controlled fluid saturation for 1450 kg/m 3 dry density for D=E-9 m 2 /s. Upper: 10 months. Lower:4 months Chemical reactions and release of toxic species It is observed that the strong compression of the mixture of clay and noncorroded alkaline batteries had squeezed out electrolytes from the batteries and that Ca, Zn and Ni had entered the surrounding clay but only to a very small distance. Taking the migration of Zn that had migrated farthest after 10 months, as caused by diffusion the D-coefficient is E-13 m 2 /s, which is much lower than what is typical for polyvalent cations (Figures 5 and 6). The reason for the discrepancy is believed to be complexation and fixation of the reaction products. Figure 5: Alkaline battery from which electrolytes were squeezed out forming halos. In the test with strongly corroded Hg batteries the cells were opened 10 months after start. A strong smell of ammonium gas was noticed and the rather

7 Waste Management and the Environment II 175 low degree of water saturation % is explained by gas in the voids. As in the first experiment Zn was the only element that had migrated to a significant extent from the batteries, the diffusion coefficient of the migration being estimated at about E-10 m 2 /s. The last test series, i.e. the 18 months long experiments with uncorroded Hg batteries in clay with moderate density showed that no ion release at all from the batteries had taken place. Figure 7 illustrates the typical appearance of the sectioned clay in conjunction with sampling for analyses. The clay was perfectly uniform with no colour changes or other indications of chemical reactions. The originally shiny batteries had retained their appearance except for some slight greyness indicating very slight chemical interaction with the clay minerals and porewater. The corrosion depth cannot have exceeded 10 µm as concluded from light microscopy. As expected, the analysis showed that massive ion exchange from initially sorbed sodium to calcium had taken place at the saturation and percolation of calcium chloride. The concentrations of Ni and Sr and, naturally, Hg were very low indicating that the batteries have not undergone measurable dissolution. Zn diffusion in Friedland Ton w ith 1950 kg/m3 dry density, 250 days Concentration, ppm Distance from battery, cm Figure 6: Zn concentration in the clay. The curve to ppm is the diffusion profile for D=E-13 m 2 /s and coincides with measured data Summary of experimental results The main results of the experiments can be summarized as follows: 1. The water saturation rate of Friedland clay with an ultimate density of 1950 kg/m 3 is not significantly quicker than of the same clay with a density at water saturation of 2100 kg/m 3. This is of fundamental importance since it means that the time for wetting, which determines the moment when chemical reactions between waste and clay can start, is not very different for the much cheaper and simpler field compaction technique than of highly compacted clay blocks.

8 176 Waste Management and the Environment II 2. Uncorroded Hg batteries embedded in Friedland clay with a density at water saturation of 1950 kg/m 3 do not undergo dissolution in low-electrolyte porewater and 3.5 % CaCl 2 solution in 1.5 years. The reason for this is the high ph, which was found to reach a constant value of slightly less than 8.1 in the study. 3. The corrosion rate is estimated to be about 1 µm per year. For a battery with 500 µm thickness of the metal coating, through-corrosion will hence take at least 500 years. 4. The study strongly supports the basic idea of the proposed technique to embed batteries in smectitic clay of sufficient density. A dry density of 1450 kg/m 3, which is suitable for use of the investigated type of clay, can easily be reached by using block compaction techniques and also in the field by compaction of slightly wetted powder or granulates using suitable rollers, hence representing a cheap alternative. Figure 7: Sectioned clay sample. The shiny surface of the batteries imply insignificant corrosion after percolation with distilled water for 12 months and subsequent percolation for 6 months with 3.5 % CaCl 2 solution. 3 Conclusions and suggestions 3.1 The isolating function of clay The fluid saturation rate of the clay embedment of the waste is a key issue. Using typical data the first 20 cm of a 100 cm zone of pure smectite clay will be fluid-saturated after about 2 years and it will take 20 years to saturate the first 30 cm. The process slows down successively and saturation to 100 cm depth of clay surrounding a clay/waste mixture will require about 4000 years (Figure 8), in which time no elements will be released from the waste. For less smectite-rich clays and with a water pressure of a few MPa superimposed, fluid saturation will be somewhat but not dramatically quicker.

9 Waste Management and the Environment II 177 Once the confining clay and the clay component of the batteries have become largely fluid-saturated, which will take several additional thousands of years for a drift or room with 5-50 m diameter, dissolution of the waste will start and migration of elements emanating from the batteries be initiated. Considering the practical case in Figure 9, i.e. with the clay-mixed batteries placed in a drift with a very pervious excavation disturbance zone (EDZ) the basis for predicting contamination of the groundwater off the mine is the element concentration at the contact between the EDZ and the clay/waste fill, i.e. the source term. 120 Depth of 100 % water saturation, cm Time, years Figure 8: Movement of the wetting front assuming only diffusion with D= E-9 m 2 /s. EDZ Clay/waste Figure 9: Room surrounded by the EDZ and filled with clay-embedded waste. Evolution of groundwater contamination will be controlled by the rate of the battery dissolution and by the diffusion rate of released elements in the fill and through the clay zone surrounding the fill. Assuming conservatively that dissolution takes place instantaneously when the clay component of the fill has become fluid-saturated and that the diffusion coefficient is E-13 m 2 /s, it will take

10 178 Waste Management and the Environment II millions of years for the only readily released element Zn to reach through very dense clay of the investigated type to the EDZ, and tens of thousands to hundreds of thousands of years for moderately dense clay with a diffusion coefficient of E- 9 m 2 /s. The ultimately contaminated water in the EDZ will flow along the drift driven by regional hydraulic gradients. 3.2 The form of other toxic elements entering the EDZ While Zn, Ni and Cu may migrate through the clay in simple ionic form as found in the experiments Hg will behave differently. Under the oxidant laboratory test conditions as well as in mine repositories, saturation with low-electrolyte water will make Hg appear as neutral hydroxide complexes, while in salt water mercury chloride complexes are expected. They are neutral at low and moderate salinities but anionic in very salt water of ocean type. Since the anionic transport capacity is very much lower than that of cations the latter form of complexes will be immobile in the clay. In the presence of elementary sulphur or pyrite mercury sulphide of low solubility are formed, meaning that mercury will be largely immobilized when disposed in sulphide ore mines [2,3]. The matter of possible movement of Hg and other complexes through the clay driven by water flow is eliminated by the fact that hydraulic gradients across the clay/waste fill will be lower than those driving water through the EDZ for geohydrological reasons and by the fact that the hydraulic conductivity of the investigated type of clay is at least two orders of magnitude lower than for the EDZ [3]. 3.3 Potential of the clay embedment technique to serve for other waste types Naturally, any other solid waste that gives off heavy metals in ionic form can be effectively isolated by use of clays in the described form provided that ph does not drop below 6 or raised above 10, in which cases the chemical stability of the clay is jeopardized. The study referred to in this paper also comprised examination of whether and how liquid organic waste can be isolated by use of expanding clay. The investigated waste was the pesticide Dichlorvos, the organic molecules of which were found to be sorbed in the interlamellar space like Ca, suggesting that migration of both these species takes place according to the same diffusion process. A method for solidification of this liquid by mixing it with clay to a suitable ratio has been developed. Acknowledgements The author is indebted to the European Commission, which supported financially the project Low Risk Deposition Technology (LowRiskDT) Contract No EVG , in the framework of which the present study was performed.

11 Waste Management and the Environment II 179 References [1] Pusch R, Waste Disposal in Rock. Developments in Geotechnical Engineering, 76. Elsevier Publ. Company, ISBN: (1994). [2] de Souza, C.C.B.M., de Oliveira, D.C., Tenorio, J.A.S. Characterization of used alkaline batteries powder and analysis of zinc recovery by acid leaching. Journal of Power Sources, 103, (2001). [3] Morel, F.M.M., Hering, J.G. Principles and Applications of Aquatic Chemistry, Wiley (1993).