Download 291 SEPARATION TECHNIQUES FOR - Ars Separatoria

XXV ARS SEPARATORIA – Toruń, Poland 2010
291
SEPARATION TECHNIQUES FOR PRETREATMENT OF
INSTUTIONAL RADIOACTIVE WASTES
Marian HARASIMOWICZ, Paweł BIEŁUSZKA,
Grażyna ZAKRZEWSKA-TRZNADEL
Institute of Nuclear Chemistry and Technology, ul Dorodna 16, 03-195 Warsaw
Abstract
The pretreatment techniques for radioactive wastes by filtration with various
depth filters, precipitation, sorption of radioactive ions on hexaferrocyanides,
polyacrylic acid (PAA) and zeolite 4A was investigated. The aim of the experiments
was removal of the radioisotopes of 137Cs, 60Co and 85Sr already in the beginning
of decontamination process - before sorption or membrane separation. The value of
decontamination factor Df was determined for some radioactive model solutions
and filters used as the barrier for the impurities which could damage the UF, NF
and RO membranes in next stages of decontamination process.
1. INTRODUCTION
The nuclear industry and all users of radioactive materials produce a
wide range of liquid wastes. Many of these wastes need to reduce the
quantities of radioactive and non-radioactive contaminants to levels, which
allow them to be safely discharged according to international and national
regulations.
The conventional technologies for removal of various radioisotopes
from wastewater include chemical precipitation, ion exchange, adsorption,
membrane processes and evaporation. The wastes from storage tanks are
pumped to the filtering unit. Battery of various filters is used to separate fine
particulates, that were not settled in sedimentation process – rest of the
sludge, sand slurries and solid particles.
Radioactive wastes in storage tanks of the Institute of Atomic Energy at
Świerk/Otwock near Warsaw can be treated as low-saline solutions with
low-level of radioactivity emitted by beta + gamma radioisotopes: their total
salt concentration is 0.1-2.0 g/L and specific activity is 103-105 Bq//L.
Large part of radioactivity (60-70%) is emitted by 3 radioisotopes:
137
Cs, 60Co and 85Sr - therefore it is favourable to remove them already in the
initial stage of decontamination process.
292
XXV ARS SEPARATORIA – Toruń, Poland 2010
2. EXPERIMENTAL
The solutions used in the experiments were prepared by use following
salts: CsCl, Co(NO3)2·6H2O, SrCl2·6H2O, and CuSO4.
The ion concentration in the feed solutions and the filtrates obtained as
a result of applied decontamination process, were analysed by measuring of
electrical conductivity by means of a conductivity-meter WTW Multiline P4
and spectrophotometer DR/2000 HACH with the filter RC-45/25.
The results for 6 type of hollow filters are shown in Table 1. Following
agents were applied: zeolite A4 Na12[(AlO2)12(SiO2)12]·xH2O pulver 5 µm,
activated (Aldrich, 1301-78-9), and K4[Fe(CN6)]·6H2O.
For preparation of radioactive solutions the following radioisotopes
were used: 137CsCl in 0.1% HCl solution, 137Cs: T1/2 = 30,2 years,
60
Co(NO3)2·6H2O in water solution, 60Co: T1/2 = 5.24 years
85
SrCl2·6H2O in water solution, 85Sr: T1/2 = 65 days.
The radioactivity of the feed and filtrate samples were determined using
A-22p STANDARD-70 Analyzer (made by POLON, Warsaw) connected to
SSU spectrometric photo-multiplayer with SKG crystal NaJ/Tl activated
(Koch Leb. Ltd, England). The calibration of this measuring system using
standard-samples of 137Cs, 60Co and 85Sr makes possible the recalculation of
the pulses/sec for 20-mL sample to specific activity in Bq/L. The
radioactivity of the feed was as follows: 137Cs - 8.6 Bq/L, 60Co -15.2 Bq/L
and for 85Sr - 6.2 Bq/L. Activity of the filtrates was 10- to 200-times lower.
*) 137Cs removal
As the sorbent of Cs+ ions the slurry of Cu-ferrocyanide was used. The
slurry can be precipitated in situ from K4[Fe(CN6)]·6H2O and CuSO4 to
obtain a optimal conditions for the sorption of radioactive ions. After 24h
seasoning the solution was filtrated and a specific activity of the filtrate was
measured. The same experiment was carried out with Ti-ferrocyanide slurry.
**) 60Co removal
First experiment was carried out by use of poly(acrylic acid) PAA 35%
sodium salt water solution (Mw 250 000 D). For the second experiment the
portion of zeolite A4, activated 4h at 400 0C, was added. The saturation level
was reached, when the proportion of zeolite concentration to Co2+ ions was
20 : 1.
***) 85Sr removal
The experiments on removal of strontium ions were carried out by use
of the same sorbents, as for caesium ions removal (Cu-ferrocyanide) and
cobalt ions removal (zeolite A4, Cl3Fe·6H2O). The results of the experiments
are collected in Table 2.
XXV ARS SEPARATORIA – Toruń, Poland 2010
293
3. RESULTS AND DISCUSSION
All cylindrical depth filters used as a first step of radioactive waste pretreatment play very important role: they should be a barrier for the impurities
which can damage the next stage elements – the membranes or ion-exchange
sorption columns. As shown in Table 1., PS-filters eliminate sand, sludge,
rust, small solid particles, coal dust and micro-organisms. They can work at
very small operating pressure (< 0.1 bar), therefore by their assembling at
the input of the pressure pump delivering the wastes to UF or NF/RO
membrane modules is possible. Mostly effective filter battery can be built
from 3 types of the filters: PS + (BL, IR or GAC) + CERAM.
Table 1. Characteristics of cylindrical depth filters
Filter type
Pore size
Qf [L/min]
Material
Substances retained
PS-50
> 50 µm,
20
Polypropylene
foam
PS-10
> 10 µm
8
Polypropylene
foam
BL
10-5 µm
8
Sintered
active coal
GAC
50-10 µm
8
Granulated
activated coal
IR
100-50 µm
5
Mixed sorption
bed
3
Sintered
ceramic pulver
Sand, sludges,
rust, coal dust,
bacteria
Sand, rust,
coal dust,
other slurries
Chlorine, organic
compounds, Pb,
heavy metals
Chlorine, organic
compounds, heavy
metals
Ferrous,
ferric and manganese
hydroxides
All solid
impurities
> 0.3 µm
CERAM
0,3 µm
Although the role of these filters in decontamination process is small,
some of them showed sorption property: specific activity of the solution
obtained from BL, GAC and IR filters was lower than for feed stream,
because 1-7 % of ions was absorbed in the filtering bed (Df = 1.01-1.7).
Filtration of the solutions obtained in precipitation and adsorption
processes was carried out by Whatman NY 0.2 µm syringe filters.
The best results for 137Cs+ ions removal was obtained using sorption
column with polymeric-Ti-ferrocyanide spherical granules: value of Df =173
is about 4-times higher than for experiment with Ti-ferroceanide slurry and
5-times for Cu-ferrocyanide slurry. This last reagent was used for radioactive
294
XXV ARS SEPARATORIA – Toruń, Poland 2010
strontium removal: the value of Df = 23 shows, that the sorption of
ions was by 40% lower than 137Cs+ ions.
85
Sr2+
Table 2. Decontamination of model solutions in adsorption/precipitation
Solution
CsCl
Co(NO3)2·6H2O
SrCl2·6H2O
Feed
Filtrate
Feed
Filtrate
Feed
Filtrate
Reagent
Cu-ferrocyanide
PAA-Na/35
Cu-ferrocyanide
A [kBq/L]
8.68
0.235
15.2
1.05
5.87
0.25
Df
37
14
23
Reagent
Ti-ferrocyanide
Zeolite A4
Zeolite A4
A
8.68
0.195
15.2
0.20
5.87
0.16
[kBq/L}
Df
45
75
37
Reagent
*) column with TFC
**) Cl3Fe·6H2O
**) Cl3Fe·6H2O
A
8.68
0.050
15.2
1,15
5.87
0.84
Df
173
13
7
*) polymeric-Ti-ferrocyanide granules for sorption column, **) after 2h, pH=8.5
reRr
Zeolite A4 can be used for cobalt ions removal – its sorption capacity is
5-times higher than PAA-Na/35 or Cl3Fe·6H2O.
4. CONCLUSIONS
The pretreatment processes tested in the experiments presented above
decrease the concentration of radioactive ions of cesium, cobalt and
strontium 7 to 75-times. In the beginning of the radioactive waste
decontamination system the filter battery can be assembled to prevent the
next stages from various damages caused by particulate impurities.
Acknowledgement
This research has been supported by the National Center for Research and
Development (NCBiR). Research Grant No. R05-058 06/2009.
REFERENCES
[1] R. G. Gutman et al., Active Liquid Treatment by a Combination of Precipitation and
Membrane Processes, Rep.AERG-G-3777, UKAEA, Harwell, UK, 1986.
[2] IAEA, Technical Report No.337. Chem Precipitation of ARW, Vienna, 1992.