## 2 Molten Salt Mixtures
### Molten Salt Mixtures' Thermophysical Properties
According to the scientific literature, this reaction can occur through several mechanisms [14; 21; 73; 74], but the result is invariably the formation of metal oxides, followed by hydroxide and carbonate production, if the mixture is in contact with moisture and carbon dioxide [22]. If nitrites are present, they are oxidized into nitrates above a certain temperature threshold. It is interesting to note that alkaline nitrates undergo an equilibrium reaction with oxygen which becomes relevant above 550 \({}^{\circ}\)C [14; 75; 76; 77]:  
\[\text{MNO}_{3}\rightleftharpoons\text{MNO}_{2}+\text{$\
u$}_{2}\,\text{O}_{2} \text{(M=Na, K, Li)} \tag{1}\]  
In general, a limited presence of nitrite is not considered a cause of chemical degradation, given its low impact on the mixture's thermophysical properties [22]. In this context, Table 2 reports the temperature limits under air atmosphere, unless where differently specified. Regarding the solar salt mixture, [PERSON] et al. [78; 79] reported a significant increase in its chemical stability, up to 650 \({}^{\circ}\)C, using, as the cover \begin{table}
\begin{tabular}{l l l} \hline
**Property** & **Value** & **Unit** \\ \hline Chemical composition & \(\text{NaNO}_{3}\text{/KNO}_{3}\) (60/40) & \({}^{\circ}\)wrt. \\ Density & \(\rho=2090-0.63\cdot\text{T}\) & kg m\({}^{-3}\) \\ Dynamic viscosity & \(\mu=71,645\cdot T^{-1.763}\) & Pa s \\ Thermal conductivity (max. operation temperature) & \(k=0.3804+3.452\cdot 10^{-4}\cdot\text{T}\) & W m\({}^{-1}\) K\({}^{-1}\) \\ Heat capacity & \(c_{p}=1.5404+3.0924\cdot 10^{-5}\cdot T\) & kJ K\({}^{-1}\) kg\({}^{-1}\) \\ Thermal stability & 600 & \({}^{\circ}\)C \\ Liquidus temperature & 238 & \({}^{\circ}\)C \\ \hline \multicolumn{3}{c}{**Hitec\({}^{\text{\text{\text@underline{6}}}}\) (Na/K nitrate/nitrite)**} \\ \hline Chemical composition & \(\text{NaNO}_{3}\text{/KNO}_{3}\text{/NaNO}_{2}\) (7/53/40) & \({}^{\circ}\)wrt. \\ Density & \(\rho=-0.9\cdot T+2269.4\) & kg m\({}^{-3}\) \\ Dynamic viscosity & \(\mu=146,452\cdot T^{-1.903}\) & Pa s \\ Thermal conductivity & \(k=0.5843\mp 0.0006\cdot\text{T}\) & W m\({}^{-1}\) K\({}^{-1}\) \\ \multicolumn{3}{c}{\(c_{p}=1.55-0.0001\cdot\text{T}\)} & kJ K\({}^{-1}\) kg\({}^{-1}\) \\ Thermal stability (max. operation temperature) & 450 under air; 530 under inert gas & \({}^{\circ}\)C \\ Liquidus temperature (initial solidification point) & 141 & \({}^{\circ}\)C \\ \hline \multicolumn{3}{c}{**Hitec XL\({}^{\text{\text@underline{6}}}\) (Na/K/Ca nitrate)**} \\ \hline Chemical composition & \(\text{NaNO}_{3}\text{/KNO}_{3}\text{/Ca}\) (NO\({}_{3}\))\({}_{2}\) & \({}^{\circ}\)wrt. \\ Chemical composition & \((15/43/42)\) & kg m\({}^{-3}\) \\ Density & \(\rho=2240-0.827\cdot T\) & kg m\({}^{-3}\) \\ Dynamic viscosity & \(\mu=509,611\cdot T^{-2.072}\) & Pa s \\ Thermal conductivity & \(k\cong 0.519\) (Constant in the operating range) & W m\({}^{-1}\) K\({}^{-1}\) \\ Heat capacity & \(c_{p}=1.542-0.000322\cdot T\) & kJ K\({}^{-1}\) kg\({}^{-1}\) \\ Thermal stability (max. operation temperature) & \(\leq\)425 & \({}^{\circ}\)C \\ Liquidus temperature (initial solidification point) & \(\sim\)125 & \({}^{\circ}\)C \\ \hline \multicolumn{3}{c}{**Na/K/Limitrate**} \\ \hline Chemical composition & \(\text{NaNO}_{3}\text{/KNO}_{3}\text{/LiNO}_{3}\) (18/45/37) & \({}^{\circ}\)wrt. \\ Density & \(\rho=2051-0.6639\cdot T\) & kg m\({}^{-3}\) \\ Dynamic viscosity & \(\mu=58,725\cdot T^{-1.69}\) & Pa s \\ Thermal conductivity & \(k=0.0005\cdot T+0.4\) & W m\({}^{-1}\) K\({}^{-1}\) \\ Heat capacity & \(c_{p}=1.5395+0.0003\cdot T\) & kJ K\({}^{-1}\) kg\({}^{-1}\) \\ Thermal stability (max. operation temperature) & 600 & \({}^{\circ}\)C \\ Liquidus temperature (initial solidification point) & 120 & \({}^{\circ}\)C \\ \hline \end{tabular}
\end{table} 
Table 2: Thermophysical properties of solar salt, Hitec\({}^{\text{\text@underline{6}}}\), Hitec XL\({}^{\text{\text@underline{6}}}\), and lithium-containing ternary MS mixtures [72].