Recycling Electrochemical Machining for Metal Recovery and Elimination of Waste
Conventional, direct-current ECM typically operates with neutral salt electrolytes, which affords two particular advantages: the sludge formation itself is beneficial as it limits the dissolved metal concentration, in turn effectively eliminating any metal re-deposition onto the tool/counterelectrode; and empirical observation from industrial ECM practice is that neutral electrolytes generally yield superior surface finish as compared to acidic or alkaline electrolytes [[i]]. Faraday has previously demonstrated the capability of carefully tuned, pulsed FARADAYIC® waveforms to achieve extremely high quality surface finish in acidic and alkaline electrolytes, as well as neutral salt solutions. Also, separately tuned waveforms should allow on the one hand minimal re-plating on the tool in the ECM cell, and on the other an efficient electrowinning (EW) recovery process in an integrated electrowinning cell. As will be described, the FARADAYIC®(R)ECM technology successfully exploits both of these capabilities, enabling near-complete internal recycle of electrolyte and recovery of metals in compact, solid form. Figure 1 schematizes the (R)ECM process flow: blanks are machined into finished parts in the ECM cell and the dissolved metal is recovered in the EW cell, while substantially all of the electrolyte is retained within the system.
Faraday initiated (R)ECM development efforts by screening candidate electrolytes for metal solubility limits and facility of metals electrodeposition using Hull cell tests. After initial electrolyte down-selection, ECM of 1″x1″ coupons was performed using various FARADAYIC® waveforms both to test the achievable surface finish and also to generate “live” ECM-derived electrolytes with which to reconfirm their suitability for the metal recovery function. The best-performing electrolyte for each material was then used in a larger “pilot-scale” (R)ECM system, in which high-rate ECM and in situ EW unit operations were tested, first separately and then in an integrated fashion. Beryllium-free nickel-copper (C18000) and Cr-Mo alloy steel (SAE4150) were separately machined in electrolyte to concentrations between 800-3500 ppm and subsequently recovered by electrowinning in solid, metallic form devoid of hydroxides/hydrated oxides, without intermediate electrolyte processing. Metal concentrations were maintained within a target operating window defined by polishing performance exhibited in the coupons studies (1500-2000 ppm) by appropriate adjustments to the electrical parameters of one of the (R)ECM unit operations. Development of FARADAYIC® (R)ECM for other alloys including 316L stainless steel and Inconel®718 is currently underway.
[[i]] Wessel, “Electrochemical machining of Gun barrel Bores and Rifling” Naval Ordnance Station, Louisville, KY, September 1978; http://handle.dtic.mil/100.2/ADA072437.