I'm looking for a suitable substitute for a $\ce{NaNO2/KNO3}$ salt bath (for quite obvious reasons due to the other applications of the latter chemical). The salt mixture needs to have around 400 to 550 °C of molten working temperature (the lower ranges are not interesting), and preferably be reasonably safe to work with (as safe as the aforementioned salt bath, so totally harmless isn't the goal). The purpose is precise annealing of brass alloys in a home shop environment (so no fume hoods and blast cabinets available).
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$\begingroup$ I don't really understand the "quite obvious reasons" for not using NaNO2/KNO3 molten salt bath. A saltpeter bath is a #1 choice for annealing of brass alloys. Could you probably add more details why it doesn't suit you? $\endgroup$– andselisk ♦Mar 3, 2019 at 12:21
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2$\begingroup$ KNO3 is used in various explosives and pyrotechnics, and as such is quite cumbersome to purchase and ship. But yes, it's the obvious choice, which is why I'm having such a hard time finding an alternative (other option is obviously that an alternative isn't available). $\endgroup$– StuggiMar 3, 2019 at 12:24
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$\begingroup$ Well, it's also used as common fertilizer, or even in food preparation... $\endgroup$– MithoronMar 3, 2019 at 22:36
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1 Answer
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I wasn't sure what other conditions you needed for your salt bath, yet I listed few for your convenience together with the salt bath of $\ce{KNO3/NaNO2}$ (bath 5) for comparison: $$ \begin{array}{ccc} \text{Bath#} &\text{Composition} & \text{Approx. melting poing} & \text{Working temp. range} \\\hline 1 & \ce{NaCl}, \ce{KCl}, \ce{BaCl2}, \ce{CaCl2} & \pu{400 ^\circ C} & \pu{500-800 ^\circ C} \\ & (10\!-\!15 \!:\!20\!-\!30 \!:\!40\!-\!50 \!:\!15\!-\!20) & & \\ 2 & \ce{NaCO3}\, (45\!-\!55), \ce{KCl}\, (55\!-\! 45) & \pu{450 ^\circ C} & \pu{550-900 ^\circ C} \\ 3 & \ce{NaCl}\, (20), \ce{KCl}\, (30), \ce{BaCl2}\, (50) & \pu{540 ^\circ C} & \pu{570-900 ^\circ C} \\ 4 & \ce{NaNO3}\, (100) & \pu{370 ^\circ C} & \pu{400-600 ^\circ C} \\ 5 & \ce{KNO3}\, (50\!-\!60), \ce{NaNO2}\, (50\!-\!40) & \pu{135 ^\circ C} & \pu{160-550 ^\circ C} \\ \end{array} $$
I also listed $\ce{NaNO3}$ bath in case you won't mind using it since it is not potassium salt and working range is ideal for your requirement. As indicated, you can change composition as much as you wanted to to get right melting point and the working temperature range you wanted to.
answered Mar 3, 2019 at 22:13
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2$\begingroup$ Use caution with soluble Ba salts because of toxicity. $\endgroup$ Mar 3, 2019 at 22:25
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$\begingroup$ Cheers mate, I'll research a pure NaNO2 salt bath since it's closest to the working range. I read somewhere that temperatures over 590 C can decompose the standard KNO3/NaNO2 mix and release nitrogen oxide, which I understand is something you definitely want to avoid. I gather this is also true for just NaNO2? $\endgroup$– StuggiMar 4, 2019 at 20:39
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$\begingroup$ Sorry to inform you that I made a typo on entry 4 of the chart. It is $\ce{NaNO3}$ instead of $\ce{NaNO2}$. By the way, melting temperature of $\ce{NaNO2}$ is $\pu{271 ^{\circ}C}$, but it decompose at $\pu{320 ^{\circ}C}$, well below the temperature you want. $\endgroup$ Mar 6, 2019 at 22:05