GB759109A - Treatment of cyanide solutions - Google Patents

Abstract

759,109. Automatic control systems. WALLACE & TIERNAN, Inc. Aug. 11, 1954 [Aug. 13, 1953], No. 23357/54. Class 38 (4). [Also in Groups I and XXXVI] The method of eliminating the toxic properties of a cyanide solution by alkaline chlorination, whereby the dissolved cyanides are oxidized to cyanates is characterized in that the extent of the reaction is determined by measuring the potential between a measuring electrode immersed in the solution, and a reference electrode. Using a gold measuring electrode and a silver chloride reference electrode the untreated cyanide solution, at a pH of 8 or more, produces a P.D. of - 200 mv., which voltage increases to +275 mv. when the cyanides have all been converted to cyanates. Alternatively a platinum measuring electrode and a calomel reference electrode are satisfactory. If further chlorine is added the cyanates are broken down to form carbon dioxide and nitrogen, the quantity of chlorine required for the second reaction being proportional to that required for the first reaction. The presence of other impurities in the solution, in particular ammonia even in small quantities, delays the second reaction by combining with some of the excess chlorine to form chloramine. The pH value after the first reaction should be 8.5-10; if it is above 10.5 the second reaction does not take place, even without ammonia. This second reaction is accompanied by a further increase in potential to over 600 mv. Thus to ensurecontinuation of the first reaction the chlorine supply should be such as to maintain a potential above - 200 mv. and below 275 mv. Cell construction (Fig. 2).-As shown, a gold wire measuring electrode 20 is supported in a porous tube 24 containing the test liquid, surrounded by the electrolyte, sodium chloride solution, in a chamber 25 which contains a silver reference electrode 26. Solid sodium chloride 32 is located in the electrolyte to maintain a saturated solution. The test liquid is projected into a chamber 14 and circulates past the electrode 20 to leave at 12. The circulating liquid carries a small quantity of grit 22 which cleans electrode 20 by scouring, and further cleaning is obtained by a cyclic short-circuit between electrodes 20, 26 obtained by a slowlyrotating cam 42. System arrangement (Fig. 5).-The cyanide solution is fed through a pH cell 47 whose output via valve 49 controls a supply of caustic solution to bring the initial pH value to 8.5- 9.0 and is collected in a tank 46. The solution is then pumped into a first reaction tank 52a from which a sample flow is pumped through a measuring cell 10 as described above, whose output adjusts a valve 65 controlling the entry of chlorine and caustic solution in a fixed proportion to the pipe line 70 between the two tanks 46, 52a so that the electrode potential in cell 10 is maintained constant. To obtain the second reaction, the treated solution is pumped into a second reaction tank 84, and is fed with a further quantity of chlorine and caustic as determined by cell 10 and controlled by valve 65. A further measuring cell 95 may measure the reaction stage of a sample flow from tank 84, to ensure complete breakdown of the cyanides to carbon dioxide and nitrogen. Specification 659,789, [Group XXXVI], is referred to.