FIG. 4. Water treatment at rehabilitation sites of WISMUT GmbH. Schematic diagram of Pöhla pilot plant.
The two years test period, demonstrated that the wetland can be successfully used for removal of the relevant contaminants at this site. Pilot wetland functioned dependably and showed a stable performance. The average removal rate in 1999 was:
Radium approx. 73%
Iron approx. 97%
Arsenic approx. 83%
The overall goal is to identify the optimal operational conditions in the wetland so that the conventional treatment plant at the site can be phased out and replaced by a passive/biological treatment facility by 2002.
The commissioning of a pilot scale test of passive/biological treatment of neutral, sulphate rich seepage water from waste rock dumps at the Ronneburg site is also planned.
7.2. Application of permeable reactive walls for downstream treatment of the contaminated seepage from a tailings pond
A series of development projects to test the applicability of permeable reactive walls for ground water remediation were initiated and funded by the Federal Ministry of Technology.
We are expecting that these projects will create the necessary conditions for the specific use of this technology for long-term treatment of contaminated water from the remediated tailings ponds.
The seepage from the tailings ponds is contaminated mainly by uranium, radium and arsenic.
Presently, the captured seepage is treated by conventional methods jointly with the supernatant water from the tailings pond. The availability of this treatment method is limited to the period until the remediation of the tailings pond progresses to a stage where no more supernatant water is available. Beyond this stage, a pump and treat concept would be required to deal with the continuously escaping seepage water. In view of the high specific costs of the water treatment at the Helmsdorf site (approximately 8 DM/m³ with the tendency to increase up to 20 DM/m³ in the future) and of the enduring requirement to treat the seepage water (probably longer than 200 years) a cost efficient long term alternative to the Helmsdorf type of water treatment is needed for the diminished volume of the water stream in need of treatment.
A possible solution from the present day point of view is offered by use of permeable reactive walls.
The objectives of the various permeable reactive wall projects (carried out by Wismut and other relevant companies and institutions) are:
— the identification of suitable materials for the in situ removal of contaminants, such as sorption agents and suitable host materials;
— the optimisation of the removal efficiency and active lifetime of the reactive materials in laboratory and in column tests at the site of origin of the ground water;
— the modelling of the retention and transport processes in the reactive walls.
As a result of these studies, two products were developed which meet all the requirements related to the objective.
Product 1 Fine-grained heavy spar bonded in a matrix of expanded Geopolymer£, Product 2 Product from expanding and linking of powdery titanium hydroxide with
vinyl acetate adhesive.
The assembled products were crushed and sieved to produce a material with a hydraulic conductivity coefficient of > 10-4 m/s. These products have proven their long-term stability over a pH range from 4 to 10. Figure 5 illustrates the granularity and surface structure of the assembled materials.
FIG.5. Granulated heavy spar bonded in Geopolymer£
Results of column tests to determine the capability of products 1 and 2 compared to foamed zero valent iron for the selective separation of Uranium, Radium and Arsenic from original waters are shown in Table 5. Before breakthrough, the outlet concentrations were typically 10% of the feed. Feed concentrations were 10 mg U/l, 3 mg As/l, and 1500 mBq 226Ra/l.
Table V. Column test results with regard to identification of appropriate reactive materials (extract) Product Contaminated groundwater Helmsdorf
tailings pond
Mine water Pöhla-Tellerhäuser Uranium Arsenic Radium
mg/kg mg/kg Bq/g
Foamed zero valent iron 4500 20 000
---Product 1 --- --- 851)
Product 2 3200 15 000 1001)
1) after 350 days running
Based on the results of the tests, the specifications for the design of a prototype permeable, reactive wall for remediation of the ground water affected by uranium mining and processing will be developed.
7.3. In-situ ground water remediation by Autochthonous Micro-organisms
The US University of New Mexico was contracted by Wismut to test the applicability of microbiological treatment of contaminated ground water in situ. The tests are aimed at removal and fixation of uranium and other toxic metals directly in the ground.
The basic concept of the method is to initiate and enhance the microbiological processes underground by addition of nutrients which support the reduction of the dissolved uranium to form insoluble uraninite and affect the precipitation of dissolved toxic metals as metal sulphides.
The concept is in the stage of laboratory testing. An essential part of the tests is the demonstration that no remobilization of the fixed contaminants results when nutrients supply is suspended or environmental conditions change.
In follow-up to the laboratory tests, a concept for limited-scale field experiment will be developed.
The available results from both the permeable reactive material tests and tests of in situ use of autochthonous microbiological processes are very promising. We are optimistic that based on these advanced tests, it will be possible to develop cost efficient solutions for long term treatment of contaminated seepage from the remediated waste rock dumps and tailings ponds of Wismut.
8. Summary and Conclusions
Wismut is faced with evaluating technical and economic options for treating large volumes of contaminated water associated with tailings ponds, mine flooding and waste rock dumps.
Each source of contaminated water presents unique problems demanding unique solutions.
Thirty technological options were evaluated for treating tailings pond water, and those options finally selected were verified by pilot scale experiments. The resulting treatment scheme, which involves ion exchange treatment followed by immobilization of the resulting sludges has successfully treated 8 million m3 of tailings water since commissioning of the plant in 1995. Treatment of mine waters is accomplished in a series of chambers that act in series to remove contaminants including uranium, radium, arsenic and manganese. Sludges from these chambers are immobilized and the resulting solids are disposed of in specially engineered mine dumps.
The need to employ longer-term water treatment technology has lead to evaluation of permeable reactive material and in situ use of autochthonous microbiological processes.
Wismut is optimistic that based on these tests, it will be possible to develop cost effective solutions for long term treatment of contaminated seepage from remediated waste rock dumps and tailings ponds.
REFERENCES
[1] Workshop WISMUT’97, Water Treatment and Residues Management – Conventional and Innovative Solutions., Chemnitz, September 24–26, (1997).
[2] Uranerzbergbau GmbH, Wasserbehandlungsanlagen bei der WISMUT GmbH, Wesseling, November, (1997).