The Solubility of NiO in Binary Mixtures of Molten Carbonates

Abstract

The solubility of NiO in mixtures of Li2CO3-K2CO3 and Na2CO~-K2CO3 was measured as a function of the calculated basicity of the solvent mixture and as a function of the temperature. Acid and basic dissolution were found in all mixtures of the Na2CO3-K2CO3 system and in the K2CO3-rich mixtures of the Li2CO3-K2CO3 system. The slopes of the acid solubility lines were markedly different from those in the single salts. The solubilities in mixtures were not quantitatively related in a simple manner to the solubilities in the pure salts of which the mixtures were composed. However, useful qualitative trends may be discerned.Materials degradation owing to corrosion is an important issue for virtually all energy-producing systems, including the advanced molten carbonate fuel cells now under development. Current collectors and other metallic components of the fuel cell may be rapidly degraded at high temperatures if molten salt contacts the surface. This form of attack, known as hot corrosion, is thought to be due to "fluxing," wherein the normally protective oxide scale on an alloy is destroyed by a dissolution reprecipitation process. To help identify hot corrosion mechanisms and to evaluate fluxing quantitatively, data on the chemistry of oxide dissolution are needed.Although the solubility of transition metal oxides in pure molten carbonates has been previously reported (1), in practical terms the solubilities in mixtures are often more important. Baumgartner (2), Kaun (3), and Doyon et al. (4) have reported solubilities of I~iO in mixtures of K2CO3 and Li2CO3. The purpose of the present paper is to investigate the effects of basicity, composition, and temperature on the solubility of NiO in binary mixtures of Na2CO3-K2CO3 and Li2CO~-K2CO3. Theoretical BackgroundWhen NiO dissolves in a single molten alkali metal carbonate the solubility is a function of the basicity of the solvent. This basicity is determined experimentally by the cation of the melt and the pCO2 in the gas phase above the melt (1). In a mixture of two or more salts the basicity of the melt is function of these same variables but since two or more cations are present the basicity is also a function of the mole fractions of the components. For example, in a binary mixture of Na2CO3 and K2CO~ the basicity will be a function of the pCO2 above the melt and of the mole fractions of Na2CO3 and K2CO3. If the pCO2 is varied for a mixture of fixed composition the basicity is directly proportional to log (pCO2), just as for the single salts. However, when the composition of the solvent is varied at a fixed pCO2, the basicity is a nonlinear function of composition, expressed as activities of the components. This relationship was explored in a Previous paper (5) and is summarized in Fig. 1 and 2 for mixtures of Li2CO3-K2CO3 and Na2CO3-K2CO3. The M20 described in these figures is the neutral counterpart of a common oxide ion activity calculated from the free energies of dissociation of both salts at the indicated pCO3, and the activity of each component in the mixtu...

Complex Formation during Dissolution of Metal Oxides in Molten Alkali Carbonates

“…Several measurements 6,7,10 have shown that the solubility of transition metal oxides (e.g., NiO) is independent of the partial pressure of oxygen. Therefore the involvement of gaseous oxygen, being either produced or consumed during the dissolution of metal oxides, is not considered.…”

“…As Orfield and Shores 3,6 proposed, the formation of complex ions might be one of the explanations. This paper is devoted to an investigation of the solubility of metal oxides from the viewpoint of the complex formation.…”

“…If the partial pressure of CO 2 is fixed, the basicity is, however, a nonlinear function of the mole fractions of the melt components. 6 In binary mixtures of Li-K, Li-Na, and Na-K carbonates, both acidic and basic dissolution of NiO has been observed. Orfield and Shores 6 reported that the Li 2 CO 3 -rich mixtures (higher than 50% mol Li 2 CO 3 at 910ЊC) showed only the basic dissolution, while the results from Ota et al 7 and Doyon et al 10 for the Li/K ϭ 62/38 melt showed a well-defined acidic dissolution in the temperature range from 650 to 750ЊC.…”

“…At different compositions of the Li/K binary carbonates, the NiO solubility was found to decrease with an increase in the content of lithium salt. The similarity of the lithium effect to that of MgO and SrO led some workers 6,7,10 to conclude that Li 2 CO 3 is the most basic salt among the three alkali carbonates (basic Li 2 CO 3 > Na 2 CO 3 > K 2 CO 3 acidic).…”

“…It is, however, unlikely to attain a lifetime more than 40,000 h because of its solubility in the carbonate melt. Several investigations have been carried out to measure the solubility of NiO in individual alkali carbonates, 3 binary mixtures, [4][5][6][7] ternary mixtures, 8,9 and alkali/alkaline earth carbonate mixtures. 10,11 In order to obtain other materials as possible substitutes for the lithiated NiO cathode, extensive research has been conducted.…”