Solidification of wastewater sledge from Nuclear Power Station Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech republic Dept. Of Chemistry of Geopolymers Laboratory of raw material treatment Hanzlicek T., Steinerova M., Perná I., Straka P. Report on Project No. FI –IM/128 reported 10/2005

Wastewater sledge resource Sedimentation tanks gather radioactive, generally inorganic sledge accompanying wastewater treatment and contains mixture of different materials. The inorganic sledge is highly watered from 30 – 60 wt.%. In certain cases sledge contains also small quantities – up to 5 % of Spent Ion Exchangers and may content some textile or rubber fibers and rests. Wastewater sledge is considered as radioactive material and must be treated according to the regulation.

Geopolymer matrix The studied geopolymer matrix was prepared from the waste clay materials (Eximos A.S., Sklopísek Střeleč – East Bohemia Region) lowering the price of binder as much as it could be. Advantage of used clay is content of 48 – 52 % of fine sand grains (smaller than 100 m), which finally help to reach higher compressive strength resistence (over 10 MPa). Geopolymer matrix is activated by alkali aqueous solution and than water content in solidificated sledge could be recalculated according to its necessary quantity.

The sledge with 30 wt. % of water The samples were prepared by mixing the geopolymer matrix with watered sledge reducing the water content in first step of preparation. In that case the inorganic sledge with small amount of spent ion exchangers ( up to 2 – 3 volume %) was not retaining the water content. During the mixing of the pretreated matrix with inorganic matter sledge could be on the beginning observed the act of added water – the prepared mixture slightly lose its viscosity. After homogenization all content of fluid mass ( during aprox. 10 minutes) could be easily filled into the mould. The content of dry sledge reaches 35 wt.% of the final mass weight.

The sledge with 60 wt. % of water The highly watered sledge was of the same origin (inorganic mixture of sediments with low quantities of spent ion exchangers). In that case was necessary to prepare the geopolymer matrix doped with ash gathered in coal burning power plant in textile filters because water excess in sledge could misbalance the geopolymer reaction. Obtained mixture was a viscous matter and difficult to mould but with slight vibration the samples were satisfactory prepared. By the ash additive and use of very watered sledge the final content is much lower than in above mentioned series of sample. The maximum content is about 15 wt.%.

The qualities of solidificated sledges According to the previous works using geopolymer structure to immobilize toxic metals and radioactive tracers we could suppose that all dangerous elements are to be captured in geopolymer network. In both cases solidificated matters were insoluble in water and if submerged any cracks or fissures could be observed. Detected compression pressure resistance in both cases overpass the limit of the appropriate norms – results show the resistance higher than 10 MPa. According to the economic point of view must be mentioned the volume of toxic waste which in first case is 3 times bigger and in highly watered sledge is more than 6 times bigger.

Solidification versus final volume of the dangerous waste Because of escalating volume 3, respectively 6 times the experiments were leaded by the idea to use only very cheap and easily obtained raw materials. This was a reason to choose waste clay material (yearly more than 20,000 tons and in case of highly watered sledge the use of daily increasing, immeasurable quantities of ashes. The only problem could be seen in large areas of storage but the advantages of geopolymer binder are to be taken into consideration: -long term stability, -high resistance to the rising temperature up to 1000 °C, -geopolymer does not burns and not release smoke.

Conclusion Geopolymer matrix could fix different impurities which contains water sledge and encapsulate them into the siloxo-sial network. Geopolymer matrix could immobilize radioactive metals in the formatted polymer like network. Geopolymer matrix is an economic way how to solidify partially dewatered radioactive sledge (30 wt.% of water) incidents as sediment in wastewater tanks of nuclear power plants.