Underground repositories for chemically toxic waste in German salt and potash mines
Published:January 01, 2008
German potash and salt mines have pioneered underground isolation of chemotoxic wastes. Comprehensive site-specific safety assessments have validated short- and long-term performance for each facility. Adherence to strict site prerequisites and waste-acceptance criteria combined with the reliance on enclosure inside multiple barriers justify confidence in the retention capacity of the total system. Underground waste isolation has been proven to be a success in Germany. Application of the lessons learned here will help solve environmental problems worldwide.
Germany is one of the world's leading suppliers of specialty and standard fertilizers, plant care, and salt products. Several underground mines extract potash and magnesium compounds mixed with mostly salt, as well as rock salt by itself. The vital minerals contained in raw potash ore, i.e., potassium and magnesium chlorides and sulfates, are then refined to produce high-grade mineral fertilizers. Underground waste disposal and reutilization has been a complementary part of German salt and potash mining for ∼35 yr. K+S Group (K+S) and Südwestdeutsche Salzwerke AG (SWS) currently operate a total of eight underground waste isolation facilities.
The commercial business of underground disposal of highly chemotoxic waste started in 1972 with the underground waste disposal mine (repository) of Herfa-Neurode. There, a long-term safe disposal scheme was implemented for the first time, which established the standard for Germany and beyond. Today, there are three underground waste disposal mines in Germany (Fig. 1), at Herfa-Neurode (K+S), Heilbronn (SWS), and Zielitz (K+S).
Underground reutilization (Fig. 2) of chemotoxic and comparatively benign waste as backfill is practiced at, among others, the Werra potash mine (K+S) and the Bernburg (K+S) and Kochendorf (SWS) rock salt mines, and this practice has been validated by long-term safety assessment. Backfilled chambers at these sites provide the same degree of safety as underground waste disposal mines.
Site-Specific Safety Assessment
The key to operating a waste disposal mine or waste reutilization mine is the site-specific safety assessment. Analyses of potential risks during the project stages of installation, operation, and postoperation form the basis of the site-specific safety assessment. A safety concept is generated by using geological, hydrogeological, and waste data and then linking them to technical planning and other site-related data. The combination of geotechnical risk assessment, operational-phase risk assessment, and the safety concept feeds into the long-term safety evidence. This evidence includes the assessment of natural and technical barriers, the assessment of incidents and contingencies, and finally the assessment of the total system: waste–mine workings–rock mass (Fig. 3).
The principal concept of underground placement of wastes is total enclosure, permanent isolation from the biosphere, and avoidance of groundwater contamination. The rock surrounding the waste has to be suitable for encapsulation. Together with the over- and underlying impermeable rock strata, e.g., anhydrite or clay, the geological barrier system is formed.
The underground repositories discussed here are located in rock salt or potash mines; the host rock is salt. This is also the case in potash mines, where salt is the main constituent of potash seams. Rock salt fulfills the requirement to be impermeable to gases and liquids, to encase the waste because of its convergent behavior (creep), and to confine it entirely once there is no void left for the creep to close. Excavations are sufficiently stable to guarantee operational safety. Controlled subsidence of the overburden and other long-term processes are acceptable if it can be demonstrated that only rupture-free deformation will occur, that the integrity of the geological barrier is maintained, and that no pathways are formed through which water might be able to contact wastes or through which waste constituents might migrate to the biosphere.
Long-term safety has been demonstrated for all repositories discussed here.
German underground repositories abide by a list of prerequisites:
Waste disposal uses inactive areas of a mine, remote from current extraction.
It must be possible to isolate waste areas from active mining areas.
Excavations must have been left open, without backfill.
Excavations must remain stable and accessible for a prolonged time.
The mine and repository must be dry and free of water.
Excavations used for waste disposal must be isolated from water-bearing layers, ensuring that the waste is removed from the biosphere for all time.
To avoid any risk to concurrent mining operations and employees, minimum waste-acceptance criteria must be met. Waste cannot
be spontaneously combustible,
release hazardous gases,
undergo further chemical reactions,
undergo adverse reactions with rock salt,
increase in volume, or
emit a pungent smell.
Examples of permissible waste categories are:
residues from incineration plants processing municipal solid waste and hazardous waste;
quenching salt residues;
waste containing heavy metals;
contaminated soil and construction and demolition waste;
evaporation residues from landfill leachate; and
capacitors/transformers containing polychlorinated biphenyls (PCBs).
Multibarrier Isolation System
A multibarrier isolation system contains a combination of technical or artificial barriers and natural barriers. At the underground disposal mine Herfa-Neurode (Fig. 4), the system consists of three natural barriers: the rock salt formation (∼300 m thick), the clay layers above the salt (∼100 m thick), and the Bunter Sandstone (∼500 m thick). The Bunter Sandstone contains several aquicludes.
Added to the natural barriers are the technical barriers of waste packaging, separation walls, dams, and shaft seals. After a chamber is full, it is closed with brick walls. Waste and mining areas are separated by dams, which are constructed to withstand the calculated hydrostatic pressure at the depth of the mine and repository. Once mining is finished and disposal operations are complete, the shafts—the only temporary connections between underground and surface—are sealed with watertight materials. K+S participated in a federal research project that demonstrated the long-term stable and water-tight sealing of a shaft.
The geological situation is shown in Figure 4. The Zechstein salt deposit was formed more than 250 m.y. ago by the evaporation of seawater. The deposit is 300–400 m thick. The potash seams are the upper seam, Hessen, and the lower seam, Thüringen. The seams are between 2 and 8 m thick.
It is noteworthy that 14–25 m.y. ago, the salt was penetrated in several places by basaltic dikes and pipes. Despite the tremendous thermal and tectonic stresses, the salt layers remained nearly unchanged; only in the vicinity of the basaltic dikes did carbon dioxide gas (CO2) invade the salt. The gas, liquefied under the high petrostatic pressure, is still present in the rock salt today. This is proof that the salt is so impermeable that even gases, which were incorporated as tiny microscopic inclusions under very high pressure, have not been able to escape in millions of years.
Mining uses the room-and-pillar system; the excavated rooms are used for disposal of waste. Waste is delivered by truck or rail. Before loading the hoisting cage, waste containers are taken to the acceptance control room (Fig. 5A). Checks include representative sampling and verification of the accompanying documentation, the quantity, the labeling of each packaging unit, and the adherence to any special conditions agreed to beforehand. A composite sample is taken from each waste lot. All samples are stored long term in a separate room underground (Fig. 5B).
Location and time of emplacement are documented in comprehensive detail. A map of the disposal mine provides a complete record of waste chambers, walls, and dams (Fig. 6). The underground waste disposal mine is thus comparable to a large warehouse in which the locations of all goods have to be well documented. That way, it is possible to return waste from the underground if recycling should prove viable in the future.
The waste is transported to the shaft once the checks are complete. At the underground shaft station, forklifts load containers onto trucks, which then travel several kilometers to the disposal areas. Forklifts with special attachments for big bags or drums on pallets unload and stack the waste containers. After a chamber has been filled, it is closed with a wall (Fig. 7).
The rock salt deposit that hosts the Heilbronn underground repository was formed during the Triassic, it is horizontally bedded, and it lies 150–230 m below the surface. The deposit is protected by an anhydrite layer that is impervious to water and is more than 50 m thick (Fig. 8). The salt bed is more than 40 m thick, but only the lower 10–20 m is mined because of the higher purity of that layer. The mining method is room-and-pillar. The rooms are 15 m wide, 10–20 m high, and 200 m long; they are driven parallel to each other and separated by 15–18-m-wide support pillars.
Waste can be delivered by truck or rail. Big bags, drums, and other containers are handled on the surface and at the shaft bottom by forklifts. Trucks haul the containers to the repository area, 4 km away. In the disposal room, a mobile crane is used to stack the containers (Fig. 9). All waste-receiving and waste-handling elements are similar to those of Herfa-Neurode.
The Zechstein salt deposit was formed more than 250 m.y. ago and is part of the so-called “Calvörde Block” (Fig. 10). The salt is up to 500 m thick and is isolated by impermeable clay layers from the overlying groundwater horizons. Potash is extracted by the room-and-pillar system. Rooms suitable for waste disposal are up to 16 m high.
In principle, the Zielitz disposal system is similar to Herfa-Neurode. However, because of steep inclines and ramps on the way to the disposal chambers, the waste containers are transported in enclosed trailers with fixed wheels. Containers have to be loaded into a trailer at the surface, and the whole trailer is then lowered down the shaft and pulled by a tractor to the disposal chamber. A forklift stacks the containers several tiers high (Fig. 11).
As part of the process of exploiting deposits of different sizes and shapes with a host of mining methods, some mines employ various technical means to use or reuse (as opposed to dispose of) waste material in selected chambers or areas as backfill for ground support or for other safety-related purposes.
The Bernburg mine extracts rock salt for industrial applications, table salt, and for de-icing. Chambers with a total volume of ∼100,000 m3 are available for backfilling with suitable wastes. Waste can be delivered by truck or rail. Typical wastes are contaminated soil, building rubble, sandblasting residues, slag, etc.
At the mine surface, the delivered waste is conditioned in a mixing and preparation plant to produce an authorized backfill mixture. This dry material is then poured underground through a pipe and transported by truck to the chamber to be secured, where it is dumped and piled up (Fig. 12).
This reutilization mine is operating in the same rock salt deposit as the disposal mine Heilbronn, but it uses a different part of the mine.
The salt bed is thinner in Kochendorf than in Heilbronn. When mining ceased, only an 8-m-thick salt cover remained above the excavations. In a number of places, the overlying anhydrite was exposed, and roof collapses occurred. To minimize subsidence of the surface, the state mine inspector ordered the mined-out areas to be backfilled before the mine could be abandoned.
After stacking a few layers of big bags filled with incinerator residues, a ramp is built with tailings from salt processing, and the big bags are covered with tailings. On top of this new platform, additional layers of big bags are stacked until the mined-out chamber is filled almost to the roof. The remaining space is filled with tailings. The tailings are transported to the chambers by belt conveyors from the Heilbronn mine (the two mines are connected) and slung into place (Fig. 13).
In 1996, Kochendorf started to emplace not only packaged wastes but also loose wastes: slag from incineration plants and foundry sand. They are delivered by train or truck, transferred into a bunker, skipped down the shaft, and transported to the chambers by belt conveyors.
The formerly independent mines Hattorf, Wintershall, and Unterbreizbach were consolidated into the Werra Mine in 1997, so that, now, these underground reutilization mines and the disposal mine Herfa-Neurode occupy different areas of the same mine, all situated in the Werra potash district.
In Hattorf, most waste is delivered in powdered form by tanker truck and conveyed pneumatically into upright silos. Some dry waste is delivered in big bags that are emptied into the silos by a separate system. From the silos, the waste is fed to a conditioning facility. Blending and the addition of suitable liquids turn the waste into a backfill product that is packaged into big bags.
Following hardening and removal of residual gas from the waste, the big bags are transported underground, where they are stacked as backfill. The hollow spaces between and above the big bags are filled with dampened fine salt.
Types of wastes are ash from the incineration of sewage sludge, filter dust from flue gas cleaning in incineration plants for household waste and hazardous waste, ash from other incinerator plants, and other suitable fine-grained or powdered waste.
At Wintershall, powdered waste is also first conveyed pneumatically to upright silos, but then it is filled directly into big bags. The material is then precompressed in special vibrating plate compactors.
Types of wastes are mainly ash from the incineration of sewage sludge, solid reaction products from flue gas cleaning, and other solid waste from gas cleaning, salts, and residues from the evaporation of landfill leachate.
Transport and emplacement of the waste underground are accomplished in the same way as in Hattorf.
Unterbreizbach is also part of the Werra district, which is a subbasin of the European Zechstein Basin. This potash mine lies in the deepest part of the subbasin. Because of localized tectonic strain, thick carnallite accumulations formed as a result of the high mobility of this potash mineral. Accumulations can reach thicknesses of up to 80 m and are surrounded by thin fringes. For the extraction of these accumulations, a new mining method was developed that took into account the special long-term geome-chanical issues; however, some of the accumulations had been mined earlier by the room-and-pillar system. This older technique subjected the pillars to increased geomechanical stress. Backfilling of the openings with wastes suitable for reutilization reduces the strain in the surrounding formation, which in turn prevents spalling of the excavation walls. Backfilling also reduces long-term surface subsidence.
Waste is first conveyed pneumatically to upright silos and then fed through a pipe system into underground intermediate silos. From there, the material is passed to a mixing station, where it is processed into a high-density sludge and subsequently pumped into the backfill galleries. There, the material hardens while completely binding all liquid (Fig. 14).
Loose powdered waste is delivered by tanker truck or train car. Types of wastes are ash from the incineration of sewage sludge, filter dust from flue gas cleaning in incinerators for household and hazardous waste, and from other incinerators.
Summary and Future Prospects
Underground disposal of hazardous waste at the highest standard of safety is possible if the geological situation ensures the long-term isolation of the waste from the biosphere, and if a multibarrier system helps to reduce risks.
Compared to landfill disposal on the surface, underground waste disposal in intact salt mines has real advantages (Fig. 15). No additional surface area is used—the void space created by mining operations already exists. Salt mining leaves behind a growing volume of underground void space where the waste can be permanently isolated from the biosphere. Underground disposal and reutilization of hazardous waste help to solve environmental problems wherever the geological and operational mining conditions are satisfied.
Figures & Tables
Deep Geologic Repositories
Deep Geologic Repositories reviews the success stories of underground waste isolation. It focuses on repositories that did, do, and will permanently and safely isolate dangerous materials from the near-surface biosphere. Complementary topics address the isolation capability of average crustal rock, investigations at one representative underground research laboratory, and the geologic preservation of fission products from Precambrian nuclear reactors. An international cast of contributors presents proven practical solutions to a formerly confounding issue in environmental and engineering geology: What do we do with wastes that retain their dangerous characteristics in human terms forever? The principal answer: Recycling into the lithosphere by “reverse” mining.
- Central Europe
- chemically precipitated rocks
- disposal barriers
- engineering properties
- risk assessment
- sedimentary rocks
- site exploration
- toxic materials
- underground installations
- underground storage
- waste disposal
- Herfa-Neurode Mine