Research and development
COMPLEXATION OF NICKEL IONS BY BORIC ACID AND (POLY)BORATES Anaïs Graff, Martin Bachet EDF Research and Development, Depatment of Materials and Mechanics of Components, Ecuelles, 77818 Moret-sur-Loing, France INTRODUCTION
PWR: Primary circuit contamination
Corrosion product of interest: Nickel
Step 1: Uniform corrosion and releasing in the primary fluid of particles named “corrosion products”
Hypothesis of this study:
Why do we study the solubility of nickel?
Complexation of nickel ions by boric acid or (poly)borate can enhance the solubility of NiO
Ni comes from steam generator tubes alloys , which are present in the primary circuit of the reactor (PWR)
Step 2: Transport and activation of the particles into the core
Check the existence of a 58
Step 3: Transport in the whole
Dissolution Precipitation
primary circuit of activated particles
Bellefleur (2012)
Understanding of the corrosion products solubility in the primary circuit media
: Strong emission of radiations
Objectives
Previous experimental studies have focused on the solubility of NiO and Ni at high temperatures and pressures, in different media, but results are controversial!
Step 4: Contamination Chemical conditions of the PC: - Boric acid - 300° °C - Lithium hydroxyde - 150 bar - H2
Ni + n→ Co + p 58
nickel/boron complex at low temperatures
-Quantify that complex by calculating values of the equilibrium constant as a function of temperature - extrapolate at higher T (300° (300°C)
equilibrium state may not reached Solubility higher than thermodynamically predicted
Find a way to reduce the contamination!
EXPERIMENTAL DETAILS
2) pH monitoring with increasing nickel ions concentration
1) Proposition of a triborate complex 1) Bibliography study
2) Importance of polyborates
• Shchigol (1961) studied the solubility of the solid orthoborate of
Polyborates formation conditions:
nickel in boric acid media.
-[H3BO3] > 0.01 Mol/L (= 0.6 g/L) (Mesmer and Al.)
•The solubility measured by Shchigol was found to be increased upon addition of boric acid.. He supposed the existence of a nickel/boron complex:
electrochemistry pH-monitor
- Electrolyte = boric acid (0,5 mol/L)
Formation of the triborate:
Ni( BO2 )2 ,4H 2O(s) + H3 BO3 (aq) → HNi( BO2 )3 (aq) + 5H 2O • Experimental interpretations of Shchigol controversial: equilibrium state non reached, no solid characterization, importance of polyborates. • Few studies are available in literature, but only Shchigol gave experimental data which lead to a value of log K : -11,2 (NEA review)
- Respect of PWR conditions (no impurities)
1) 2B(OH )3 + OH − → B2O(OH )5− + H 2O
Ar or N2
Zhou and al. (2011)
4) Modeling JCHESS: effect of the triborate complex on pH
At 0,5 mol/L of H3BO3 , the triborate is one of the major species in solution and could complex Ni2+
pH variations has been checked by JCHESS modeling by using a log K close to Shchigol conditions:
2+
B3O3 (OH ) + Ni ↔ NiB3O4 (OH )3 + H
- Reaction followed by pH-monitoring
2) B(OH )3 + B2O(OH )5− ↔ B3O3 (OH )4− + 2H 2O
3) Proposition of a complex triborate/nickel
− 4
- Formation of nickel ions by
Potensiostat
-[Ni2+]tot measured by ICP-MS
Cathode reaction (reduction) 2H+ + 2e- = H2 Anode reaction (oxidation) Ni = Ni2+ + 2e-
Evolution of pH with increasing nickel ions concentration (JChess) in H3BO3 = 0,5mol/L
+
Nickel/boron Complex pH
6,2
H+ releasing can be followed by pHmonitoring: pH could increase slower with increasing nickel ions concentration if a complex is formed
Ni2+ + H3BO3= triborate complex + H+
Purpose
6,7 Without complex
5,7 With complex: log K=-11,2
0
0,002
0,004
0,006
[Ni2+] (mol/l)
0,008
0,01
Agreement between experimental and modeling methods
Study of pH variations with increasing nickel ions concentration at 25° 25°C, 50° 50°C and 70° 70°C
RESULTS AND DISCUSSION
Experimental results pH variations with increasing nickel ions concentration in boric acid 0,5mol/L media 6,6
Determination of Log K Temperature dependance of log K
Evidence of complexation at 25,50 and 70°C
-11,5
6,4
-11
6 modeling without complex 25°C
5,8
Modeling 25°C log K = -11,5
pH
5,6
Modeling without complex 70°C
Good agreement of experimental data with modeling
log K
6,2
y = -3441,4x + 0,0736
Modeling without complex 50°C
5,4
-10,5
-10
exp 25°C
5,2
Modeling 50°C log K= -10,5 5
exp 50°C
0,0029
0,00295
Modeling 70°C log k=-10
4,8
4,4 4,2 4 0
0,002
0,004
0,006
0,008
0,01
0,00305
0,0031
0,00315
0,0032
0,00325
0,0033
0,00335
0,0034
1/T (K-1)
exp 70°C 4,6
0,003
Determination of equilibrium constants as a function of temperature (JCHESS)
Reaction used by JCHESS:
2+
+
3H3 BO3 + Ni ↔ NiB3O4 (OH )3 + 2H + 2H 2O
3441.4 + 0.0736 Conclusion: LogK = − T
[Ni2+] total (mol/L)
CONCLUSIONS AND PERSPECTIVES
Conclusions A triborate nickel complex is proposed Equilibrium constants for 3 temperatures have been determined by pH monitoring and modeling An equation of log K as a function of temperature has been calculated. Contact EDF R&D : Anaïs Graff:
[email protected] / +331 60 73 64 97
Perspectives Characterization of the complex by spectrophotometry studies and modeling (ab-inito studies) Measurement of the effect of boric acid on the high temperature solubility of NiO
