Reference request on potassium cyanide production method
I did some literature research and I wasn't able to find any methods involving steam (as water in the gas phase) for the synthesis.
This in fact makes sense as alkali metal cyanides are prone to hydrolysis and tend to decompose to formate $\ce{K(HCOO)}$ over time or in hot water:

$$\ce{KCN(aq) + 2 H2O(l) -&gt; K(HCOO)(aq) + NH3(g)}$$

which further decomposes to potash $\ce{K2CO3}$ upon further heating, so presence of water, especially at elevated temperatures, appears to be highly undesirable for the synthesis of cyanides.

I suspect the aforementioned article contains a typo.
Probably it should read "stream of nitrogen" — then it would make more sense as there are several industrial processes utilizing atmospheric nitrogen fixation for making cyanides with coal proposed in late XIX – beginning of XX centuries.

First, there is a German patent by Caro issued around 1900 (I couldn't find the full text or the exact date) mentioning synthesis of the cyanides of group 1 and 2 elements from carbonates using nitrogen and coal [1].

Second, there is Bucher process [2], which, I think, http://potassium.atomistry.com might refer to.
There is a comprehensive review of Bucher process [3]:


  The Bucher process, as known, consists in heating a mixture of sodium carbonate, charcoal, and iron powder (catalyst) in a stream of nitrogen at a temperature above 900°. The reaction is written as follows:
  
  $$\ce{Na2CO3 + 4 C + N2 -&gt; 2 NaCN + 3 CO}$$
  
  […] A stream of nitrogen was passed  through the tube while a certain temperature between 900° and 1000° was maintained for several hours.
  The reaction  product was rapidly cooled by lifting the tube out of the furnace without interrupting the flow of nitrogen and was then examined microscopically.
  In all experiments sodium cyanide, together with some of the ingredients of the original mixture, proved to be present, which confirms the reaction given by Bucher.


There is also a brief overview of industrial methods used for manufacturing alkali metals cyanides throughout the past 170 years in Ullmann’s Encyclopedia of Industrial Chemistry [4]:

1850s–1900s: semi-industrial process (aka "the blood process") of fusion of potassium carbonate with potassium hexacyanoferrate(II) (obtained from animal blood and waste) initially proposed by F. and E. Rodgers [5].

1900s–1960s: Castner–Keller process; primarily employed at Degussa AG [6]:

$$\ce{Na + C + NH3 -&gt;[\pu{600 °C}] NaCN + 3/2 H2}$$

1960s–present sodium and potassium cyanides are produced exclusively by the neutralization process, between a solution of sodium or potassium hydroxide with liquid or gaseous hydrogen cyanide:

$$\ce{MOH + HCN -&gt; MCN + H2O}$$


  Gaseous or liquid $\ce{HCN}$ is first introduced continuously into concentrated solutions of $\ce{NaOH}$ or $\ce{KOH}$ at elevated temperature. 
  The heat of neutralization is used for the subsequent vaporization of water. The use of special mixing devices and the addition of $0.2–3\%$ excess $\ce{NaOH}$ prevents polymerization.
  Simultaneously or in a second step, water is vaporized under reduced pressure and the alkali metal cyanide is precipitated below $\pu{100 °C}.$
  The shorter the residence time of the cyanide in solution, the cleaner the product and the lower the content of hydrolysis products and iron impurities.
  
  
  Figure 6. Flow chart of the main production steps for cyanide
  production ($\ce{NaCN}$ as example)


References


Caro, Nikodem. German pat. anmeld. C15812 (8-4-09). Verfahren zur Darstellung von Cyaniden aus Alkali oder Erdalkalioxyden, bezw. Carbonaten, Kohle und Stickstoff.
Bucher, J. E. The Fixation of Nitrogen. J. Ind. Eng. Chem. 1917, 9 (3), 233–253. DOI: 10.1021/ie50087a011.
Posnjak, E.; Merwin, H. E. Note on the Bucher Cyanide Process for the Fixation of Nitrogen. Journal of the Washington Academy of Sciences 1919, 9 (2), 28–30. https://www.jstor.org/stable/24521552
Gail, E.; Gos, S.; Kulzer, R.; Lorösch, J.; Rubo, A.; Sauer, M.; Kellens, R.; Reddy, J.; Steier, N.; Hasenpusch, W. Cyano Compounds, Inorganic. In Ullmann’s Encyclopedia of Industrial Chemistry; Wiley, 2011. DOI: 10.1002/14356007.a08_159.pub3.
F. and E. Rodgers; Ueber Darstellung und Anwendung des Cyankaliums. Ann. Chem. Pharm. 1842, 41 (3), 285–294. DOI: 10.1002/jlac.18420410303.
Degussa, DE 124 977, 1900; DE 126 241, 1900; DE 148 045, 1901; DE 148 046, 1901; DE 149 678, 1901; GB 265 639, 1926 (H. Freudenberg); DE 851
056, 1943 (W. Stephan).