Solubility of ferrocyanide compounds. Ferrocyanide Safety Project, Interim report FY1994

Abstract

The solubility of cs,NiFe(CN),(c) as a function of NaOH and temperature was determined to ascertain whether C@iFe(CN),(c) shows retrograde solubility (Le., decreasing solubility with increasing temperature), which would have bearing on the possible formation of "hot spots" in the tanks and thus the safety of the ferrocyanide tanks. The results show that the aqueous concentrations of Cs in equilibrium with Cs,?NiFe(CN),(c) at 25"C, 60"C, 75"C, and 90°C are similar (within the limits of experimental error), indicating that Cs,?NiFe(CN),(c) does not show retrograde solubility. To understand general solubility relationships of Ni,Fe(CN),(c) and to determine the influence on solubility of high electrolyte concentrations (e.g., NaNO,) that are commonly encountered in the ferrocyanide tanks, the solubility of Ni,Fe(CN),(c) as a function of CsNO,, NiCl,, and NaNO, was determined. In general, Ni,Fe(CN),(C) is fairly insoluble and shows slightly increased solubility at high electrolyte concentrations only. For Ni,Fe(CN),(c) in NiCl,, the aqueous Fe concentrations show first a decrease and then an increase with the increase in NiCL, concentrations. The increase in Fe concentrations at high Ni concentrations appears to be the result of replacement of Fe by Ni in the Ni,Fe(CN),(c) structure. For Ni,Fe(CN),(c) in CsNO, and at 0.001 M N%Fe(CN),, the cs is quantitatively removed from solution at low added Cs concentrations and appears to approach the final solid composition of Cs,?NiFe(CN),(c). The solubility of Ni,Fe(CN),(c) in NaNO, and at 0.001 M Na,Fe(CN), shows an increase in Ni concentrations to about 0.5 mg/L at NaNO, concentrations > 1 .O M. These increased Ni concentrations may be the result of substitution of Na for Ni in the solid phase.

Electrolyte Effects on the Stabilization of Prussian Blue Analogue Electrodes in Aqueous Sodium-Ion Batteries

“…However, strongly deviating degradation mechanisms might have to be considered for battery electrodes under aqueous conditions. In the case of PBA materials for ASIBs, research efforts related to their degradation mainly focused on finding and quantifying dissolved transition-metal cations and hexacyanometallate or cyanide anions in the electrolyte postmortem. − Several reasons for the observed instability have been discussed, attributing the loss of active material to the deterioration of the TM–NC–Fe bonds. Although the ligand bindings in Fe­(CN) 6 3/4– themselves are supposed to be stable up to moderately acidic conditions, the interaction of the N-coordinated TM atoms with hydroxide anions is believed to rupture the compound. − Naturally, some OH – species will be present in any aqueous electrolytes and might even be specifically adsorbed at cathode materials with operating potentials close to the oxygen evolution reaction (OER), further promoting detrimental side reactions.…”

Leaching Characteristics of Hanford Ferrocyanide Wastes

“…Previous testing , on ferrocyanide simulants showed that little dissolution of cesium nickel ferrocyanide occurred when contacted with up to 4 M caustic solution. In these same tests, the sodium nickel ferrocyanide (Na 2 NiFe(CN) 6(s) ) resulted in a measurable hydrolysis.…”

“…Aging studies displayed that sodium nickel ferrocyanide reacted with free hydroxide to form nickel hydroxide and ammonia under tank conditions and, when driven to completion, increased the cesium content in solution . Ferrocyanide simulant studies, however, concluded that cesium nickel ferrocyanide was a stable compound in up to 4 M sodium hydroxide. , No studies were done, however, on the stability of the cesium nickel ferrocyanide in aged real waste and under tank conditions.…”