Does the following reaction occur? If so, at what temperature? Would $\pu{640 °C}$ be enough?
$$\ce{2 K4[Fe(CN)6] + 2 K2CO3 + C -> 12 KCN + 2 Fe + 3 CO2}$$
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Disclaimer: This answer is just for education purpose only. DON'T TRY TO MAKE POTASSIUM CYANIDE ON YOUR OWN if you don't have profound laboratory experience. Potassium cyanide is extremely toxic and would even cost your life.
Yes, your reaction of producing potassium cyanide has been discussed in a forum:
...Dry fusion of pure $\ce{K4Fe(CN)6}$ is a bit tricky, if you will try to do this on air contact you will end with with cyanate and cyanogen beeing evolved. To prevent (this) oxidation process, $\ce{K2CO3}$ and carbon works the best - you don't need protecting atmosphere you just must cover reaction vessel with lid to reduce free air flow, reaction products CO and CO2 will form protecting buffer of gas under the cover and prevent oxidation of product to cyanate, remember you don't need completely airtight vessel (reaction gases must evolve freely) just cover crucible with ceramic lid and heat it up in electric furnace. I made $\ce{K4Fe(CN)6 + K2CO3 + C}$ process for many times now and produced a lots of cyanide using this method, temperature program i've used ~30 min to 600C, then 1 hour at this temp, rise to 650 in 30 minutes, and one more hour at 650. If your crucible is high and you sit it long enough cyanide separates from Fe and carbon particles and solidifies as shiny snow white layer at the top layer, however if this has't happened and resulting mass is black and feromagnetic this is fine too, you can separate cyanide by dissolving and filtering.
Other methods are also possible like simply decomposing potassium hexcyanoferrate or with any combination of potassium hexacyanoferrate/ potassium carbonate/carbon reaction.
$$\ce{K4Fe(CN)6 + K2CO3 -> 5KCN + KCNO + CO2 ^ + Fe}$$ $$\ce{K2CO3 + C + NH3 -> 3KCN + H2O}$$
Nilay Ghosh
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A great idea at the end of reaction(1 hour for one mole of KCN should be long enough) is to quickly put the crucible in an 8 ounce metal can modified with gas inlet at bottom and exhaust port near top.The can must have a lid that seals tightly. Now flush the can with nitrogen gas for about 1 minute to flush out oxygen!This will stop oxygen reacting with KCN to make KOCN cyanate.Then reduce nitrogen flow to several bubbles per minute.The crucible is processed when cool. – Bradford L Drake Apr 16 '18 at 03:31
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3I often think that such disclaimers at publicly posted and easily abuseable info do not really release authors from their moral responsibility. One should remember Google does not care about search reasons. – Poutnik Apr 16 '23 at 10:02
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Definitely a +1. One minor point is the reaction may not occur with the stoichiometry given. Heating carbon with CO2 could promote a side reaction forming CO. You would add more carbon to the mixture to compensate. – Oscar Lanzi Apr 16 '23 at 10:11
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Best is the following reaction since there is no fire because there is no fuel to burn!
$$\ce{K4[Fe(CN)6] + K2CO3 + C -> 5KCN + KOCN + FeC + O2}\tag{R1}$$
The black solid is $\ce{FeC},$ which is slightly magnetic. Since $\ce{K4[Fe(CN)6]}$ melts at $\pu{300 °C}$ and decomposes at $\pu{400 °C},$ best is to heat at $\ce{390 °C}$ for 1 hour, then ramp up the temperature to $\pu{700 °C}$ to force the transition of cyanate to cyanide:
$$\ce{KOCN + C -> CO + KCN}\tag{R2}$$
Heat at $\pu{700 °C}$ about 20 minutes and then shut off the oven. With thermal gloves, take stainless steel crucibles out of oven to cool quickly. When about $\pu{120 °C},$ add $\pu{100 ml}$ water and let sit for 20 minutes.
Then use screwdriver to crack the solids off the crucible bottom and walls. Let the liquid settle and decant most of it. Filter the black iron carbide. pH of the liquid is about 12 and the test with iron(II) sulfate and iron(III) chloride forms beautiful Prussian blue!
andselisk
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