Cyanide plating processes

The chemical supplier can also identify any regulated pollutants in the facility s treatment chemicals and offer available substitutes. The federally regulated pollutants are cyanide, chrome, copper, nickel, zinc, lead, cadmium, and silver. Local and/or state authorities may regulate other substances, such as tin, ammonia, and phosphate. The current status of cyanide and noncyanide substitute plating processes is shown in Table 9.11... 

Several binary alloys of technological importance are known to form by way of an underpotential co-deposition mechanism. The abnormal composition-potential relationship observed in Cu-Zn alloys deposited from cyanide-based electrolytes, one of the most widely used commercial alloy plating processes, is attributed to the underpotential co-deposition of Zn [64]. The UPD of Zn is also known to occur on Co and Fe and has been included in treatments focusing on the anomalous co-deposition of Co-Zn [65] and Ni-Zn alloys [66-68]. Alloys of Cu-Cd have been shown to incorporate Cd at underpotentials when deposited from ethylene diamine solution [69-71]. 

It may be possible to improve the operation of zinc cyanide process solutions and effect enormous reductions in waste generation.[12][13] After improving operations, it may be further possible to capture and return "escaped" process solution from the rinsing system. However, even if cyanide plating systems could be "close-looped" and the process solutions successfully maintained using standard purification techniques, the trend is definitely towards the substitutes. It may be necessary to operate... 

A large number of commercially important plating processes occur from complex ion baths in which the metal is a constituent of an anionic complex, e.g. copper, zinc, cadmium, silver and gold are all commonly plated from cyanide baths, and tin plates from a stannate bath in which [SnIV(OH)6]2 is present. Chromium is commonly plated from a chromate bath although in this case the background medium is acid rather than alkaline. Thus the mechanism of deposition of metals from anionic complexes is of particular interest. It will be instructive to comment on two situations, one occurring in alkaline baths, the other in acidic baths. 

While cathodic processes may produce positive payoffs by recovering metals from waste streams, anodic processes are essentially destructive in nature. Positive payoffs may occur if it is possible to carry out an anodic process which destroys toxic species at the same time that the cathode is recovering metals. For example, during the treatment of a cyanide plating bath, heavy metals are recovered at the cathode while cyanide is destroyed at the anode along with any organic additives and brighteners. 

C. Okumoto, M. Nagashima, S. Mizoiri, M. Kazama, and K. Akiyama, Flow Injection Analysis of Cyanide in Wastewater from Metal Plating Processes [in Japanese]. Eisei Kagaku, 30 (1984), 7. 

Sodium cyanide is also used in the electroplating industry. Cadmium is usually plated from cyanide solutions from a still plating process (90 to 120 g/L NaCN), or barrel plating... 

There are several reasons for performing speciation. Of course there are the academic studies of various types of trace metals and their conversion and equilibria of various forms. Industrial speciation is important. The effectiveness of chromium plating baths depends on the amounts of Cr(III) and Cr(VI) that are present. Gold plating processes involve the use of Au(I) and Au(III) cyanide complexes. Vanadium speciation is important in the recovery of sulfur from geothermal water. 

A comparison of cyanide and sulphite gold plating processes, D. R. Mason and A. Blair, Trans. Inst. Met. Finish., 1977, 55, 141. 

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