482 Transactions. — Chemistry and Physics. 



Also — 



KoZnCN + ZnCla = 2 ZnCyo + 2 KCl. 

 Also, with the copper compound, if any — 



ZnClo. + K2Cu,CN4 = ZnCu2CN4 + 2 KCl. 



Collect the granular precipitate of ZnCya — and in case of 

 copper ZnCu2CN4 — by means of filter press, or any other 

 means of filtration or decantation. Eeject the supernatant 

 liquor, which will be found to be entirely free from either 

 available cvanide, total cyanide, copper, and containing per- 

 haps a doubtful trace of the precious metals. 



The Au and Ag are almost entii-ely precipitated on addi- 

 tion of the ZnCl.j. 



And add to it a concentrated solution of caustic alkali, 

 suflficient to dissolve the cyanide precipitate. Thus — 



ZnCy^ + 4 KHO = Zn(OK), + 2 KCN + 2 H,0. 



Thus, with copper compound — 



ZnCa^CN^ + 4 KHO = Zn(OK)., + KoCa^CN^. 



To the highly concentrated solution of potassium-cyanide 

 and potassic cupro- cyanide (if copper be present in the original 

 mill solution), also containing zincate of potassium, we now 

 add the requisite quantity of alkaline sulphide for the purpose 

 of removing the latter compound, Zn(0K).2, care being taken 

 to use, as previously directed, a slightly less amount of NajS 

 than is actuall)' required to remove the whole of the zinc. 



It is now desired to recover the cyanide now held up by 

 the copper in the solution as C2CU2CN4. 



Case 3. — The highly concentrated solution of KCN, almost 

 entirely freed from zincate of potash, but containing a portion 

 of its cyanide combined in the inert compound K.CuoCN^, is 

 now subjected to the action of a very weak electric current 

 in a precipitation-box similar to that used in the Siemens- 

 Halske process, though from the fact that a highly concen- 

 trated solution is to be operated upon the box may be much 

 smaller than ordinarily ; in practice, 100 tons of foul or waste 

 mill solution can be readily concentrated into 5 tons. A 

 current of 2-5-3 volts is found most suitable, with a volume 

 of 0076 amperes per square foot. The negative pole of the 

 dynamo or batteries, as the case may be, is connected with 

 the cathodes, wiiich are very thin copper plates, not neces- 

 sarily pure. The positive pole of plating dynamo or batteries 

 is connected with the anodes of the box, wiiich are thick zinc 

 plates. 



The electrical action causes the potassic cupro-cyanide to 

 split up, the copper precipitates readily upon the cathodes, 

 along with any bullion whicli may be contained in tiie solu- 



