v.— CHEMISTRY AND PHYSICS. 



Art. XLII. — On the Regeneration of Cyanide- solutions. 



By Godfrey Doveton, M.A.I.M.E. 



Communicated by Professor James Park. 



[^Read before the Otago Institute, I3th October, 1903.] 



Case 1. — The recovery of the cyanide from the inert com- 

 pound K2ZnCN4, which is formed during the precipitation of 

 the metals upon zinc-shaving, fihform zinc, &c. — zinc fume, 

 emulsion, or dust. Thus — 



Zn + 4 KCN + 2 H^O = ZnK.CN, + 2 KOH + H^. 



To the solution containing the double cyanide of zinc a 

 certain amount of a caustic alkali is added. Thus, with caustic 

 potash — 



ZnK,Cy, + 4 KHO = Zn(CK,) + 4 KCN + 2 H^O. 



The requisite amount of caustic alkali is readily found by 

 a simple laboratory experiment. 



To the solution containing potassium-cyanide and zincate 

 of potash is now added a certain quantity of alkaline sulphide, 

 slightly less than actually required to throw down all the zinc 

 as sulphide. Thus — 



Zn(CK)o 4- 4 KCN + 2 H^O -f Na^S = ZnS + 4 KCN 

 + 2 KHO + 2 NaOH. 



The requisite amount of alkaline sulphide is regularly 

 found by simple laboratory experiment. 



Case 2. — Eecovery of total cyanide, where so desired, 

 either from foul solutions or weak waste solutions. 



A. Determine amount of available KCN. 



B. Determine amount of total KCN. 



C. Determine amount of protective alkali. If much of C 

 is present it will be found of advantage to partially neutralise, 

 to prevent high consumption of zinc salt in the following opera- 

 tion. 



D. To the tank of solution add sufficient zinc salt to pre- 

 cipitate all the cyanides as zinc-cyanide. Thus, with zinc- 

 chloride — 



2 KCN + ZnCl., = ZnCy^ -f 2 KCl. 



31— Trans. 



