SolidificationStabilization of Power Plants Wastes Potential Water Pollutants Slobodanka Marinkovic,

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Solidification/Stabilization of Power Plants
Wastes- Potential Water PollutantsSlobodanka
Marinkovic, Aleksandra Kostic-Pulek,Svetlana
PopovFaculty of Mining and Geology, the
University of Belgrade,Belgrade, Serbia and
Montenegro
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• Introduction
• Disposal of solid wastes from coal power plants
  (bottom ash, fly ash and flue gas
  desulphurization gypsum) is becoming a major
  issue because of their potential to contaminate
  surface and groundwater with arsenic, boron,
  heavy metals, sulphate anions, etc.
• What is fly ash?
• Fly ash is composed of oxides of iron, silicon,
  aluminium, magnesium, calcium, sodium and
  potassium. Along with oxides, fly ash contains
  trace elements (As, Sb, Cd, Cu, Cr, Ni, Zn, Mn,
  Hg, Pb, etc.). Laboratory studies reveal the
  possibility that trace elements can be easily
  mobilized and pollute the surrounding waters.

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• What is FGD gypsum?
• Flue gas desulphurization (FGD) gypsum is a waste
  from power plants, obtained in desulphurization
  process based on SO2 absorption and reaction with
  Ca(OH)2. FGD-gypsum disposal is a source of
  contamination of the surrounding waters by
  sulphate ions.
• The lignite power plant Nikola Tesla is the
  biggest power plant in Serbia. It produces about
  5 6 million tones of coal ashes per annum. The
  disposal sites of these wastes are located in an
  area rich in ground and surface waters. Hence
  contamination of the surrounding waters and the
  water of the river Sava (which is the final
  recipient of waters from the power plant disposal
  sites) is predictable.
• Moreover, the concentration of SO2 from the
  Nikola Tesla power plant exceeds the
  recommended air limit of the Europe Union. Hence,
  a flue gas desulphurisation system will have to
  be built at the Nikola Tesla power plant.
  There, an additional solid waste FGD gypsum
  will be produced.

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• Lets find a solution
• Solidification/stabilization (S/S) is a technique
  presently widely practiced
• for remediation of coal wastes containing harmful
  constituents. This treatment inhibits the
  migration of hazardous constituent into the
  surrounding environment. Solidification refers
  to changes in the physical properties of wastes.
  They include an increase in the compressive
  strength, a decrease in permeability, and the
  encapsulation of hazardous constituents.
  Stabilization refers to chemical changes of the
  hazardous constituents in a waste, including
  converting the constituents into a less soluble,
  mobile, or toxic form.
• Many lignite coals produce a low calcium fly ash.
  This fly ash reacts with lime forming calcium
  silicate and calcium aluminate hyrates (C-S-H and
  C-A-H). When sulphates are present (from
  FGD-gypsum, for example), they may combine with
  lime, alumina from the fly ash and water to form
  calcium-aluminate-sulphate hydrat
  ettringite(?Ca6?Al(OH)6?2?24H2O?(SO4)3?2H2O).

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• The structure of ettringite consists of columns
  of ?Ca6?Al(OH)6?2?24H2O?6 with the intercolumn
  space (channels) occupied by anions
  ?(SO4)3?2H2O?6?.
• Structural Ca2 and Al3 can be replaced by other
  metal cations (Zn2, Cd2, Cu2, Ni2, Pb2,
  Cr3, Fe3, Si4, Ti4, etc.). Furthermore, SO42?
  and H2O can be replaced by anions (CrO42?,
  AsO43?, ZnO22?, CO32?, B(OH)4?, etc.).
• The aim of the present study was to test the
  possibility of solidification/stabilization of
  fly ash from the Serbian lignite power plant
  Nikola Tesla and FGD gypsum from the Bohemian
  lignite power plant Hvaletice, in the presence
  of lime.

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• Experimental
• Fly ash from the Serbian lignite power plant
  Nikola Tesla, FGD gypsum from the Bohemian
  lignite power plant Hvaletice (no Serbian plant
  has a FGD system installed yet) and lime from a
  mineral source (Serbia) were used in this study.
• Calcined gypsum (??CaSO4?0.5H2O), used in this
  work, was prepared by heating FGD gypsum
  (CaSO4?2H2O) in dryer at 135 oC.
• Two mixtures were prepared at room temperature)
• 1. fly ash-FGD gypsum-lime-water (the mass ratio
  of components was 163116, respectively), and
• 2. fly ash-calcined FGD gypsum-lime-water (the
  mass ratio of components was 7215,
  respectively).
• The samples prepared in this way were placed in
  cylindrical moulds and cured in ambient air for
  30 and 180 days. After these periods the
  specimens were examined by means of DT, TG and
  XRD analysis. In addition, all the specimens were
  tested for their compressive strength.

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• Results and discussion
• The DTA curves of the specimens formed in the two
  mentioned mixture
• showed two endothermic peaks which corresponded
  to ettringite and gypsum. The exothermic peaks in
  the DTA curves resulted from ignition of
  residual particles of coal, or carbon particles
  from the fly ash.
• The results of thermogravimetric analysis showed
  that the total content of formed hydrates was
  greater in the system fly ash-calcined FGD
  gypsum-limewater, than in the system fly ash-FGD
  gypsum-lime-water.
• It must be to emphasized that gypsum is a
  reactant in the system fly ash-FGD
  gypsum-lime-water, but it is a product of the
  hydration reaction in the system fly ash-calcined
  FGD gypsum-lime-water.
• The values of the compressive strength of the
  specimens from the system fly ash-calcined FGD
  gypsum-lime-water were greater than those of
  specimens from the system fly ash-FGD
  gypsum-lime-water. This is in accordance with
  literature data that the compressive strength of
  hardened specimens from the system fly
  ash-CaO-CaSO4-H2O is directly related to the
  number of new phases and their content (the
  number and the total content of formed new phases
  are greater in the system fly ash-calcined FGD
  gypsum-lime-water than in the system fly ash-FGD
  gypsum-lime-water).

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• Conclusion
• The present study showed that in the system
• 1. fly ash-FGD gypsum-lime-water (with a mass
  ratio of the components 163116, respectively)
  and
• 2. fly ash-calcined FGD gypsum-lime-water (with a
  mass ratio of the components 7215,
  respectively) solidification/stabilization
  processes can be performed.
• These systems satisfied the compressive strength
  (0.34 MPa) stipulated for solidification/stabiliza
  tion processes, especially the system involving
  calcined FGD gypsum.
• The formation of ettringite, in both systems, is
  very important because it can result in the
  incorporation of trace constituents (cations and
  oxyanions) into the ettringite structure, with
  the result of loss leaching of them in ground and
  surface water.
• Acknowledgements
• This work was financially supported by the
  Serbian Ministry of Science and Environmental
  Protection and the Serbien power plant
  NikolaTesla.