Tandem Pulse/Pulse-Reverse Electrochemical Machining and Electrowinning for Metal Recovery, Elimination of Waste, and Minimization of Water Usage
Authors: Skinn Brian , Taylor E. J. , Hall Timothy D , Lucatero Savidra , Snyder Stephen , McCrabb Heather , Garich Holly , Inman Maria 
Published: February 2020

Electrochemical machining (ECM) is well suited for machining parts fabricated from “difficult to cut” materials and/or parts with complicated and intricate geometries. Conventional, direct-current (DC) ECM typically operates with neutral salt electrolytes, in which the machined material is precipitated from the machining electrolyte as hydrated metal oxide/hydroxide sludge. This sludge is generally transported off-site and disposed in a land-fill. Depending on the alloying components in the machined metal, the sludge must often be designated as a hazardous material. Furthermore, the volume of sludge generated by ECM of many common alloys in pH-neutral solutions is more than 300 times greater than the amount of metallic material removed. As recently noted, the generation and disposal of the sludge is a major impediment to the widespread use and implementation of ECM operations [[1]].
In this talk, Faraday will present recent work on development of “Recycling ECM” [(R)ECM], a novel, patent pending [[2]] form of ECM processing coupled with electrowinning (EW), where the machined material remains in soluble form, typically through the use of acidic or buffered-acid solutions, and is directly recovered from the electrolyte without intermediate processing. Literature reports significant challenges with direct current ECM in such electrolytes [[3]], including poor surface finish and adventitious metal deposition on the ECM tooling. Faraday has previously demonstrated the ability of pulse current/pulse reverse current (PC/PRC) waveforms to electrochemically machine materials while achieving a high quality surface finish [[4],[5]], and our prior experience also indicates that proper selection of an ECM waveform should simultaneously avoid the tool deposition observed in the literature. The accompanying figure presents a schematic of a pulse-reverse waveform and enumerates some of the mechanisms of control afforded by the pulse-reverse processing paradigm. The PRC waveforms enable control of the ECM process and consequently ECM with PRC is adaptable to a variety of electrolytes. As well, PC/PRC electrowinning enables metal recovery from electrolytes which are not feasible to process with direct current approaches.
In developing the (R)ECM process for a particular material, we first select an electrolyte in which the machined material is soluble, to avoid sludge formation by precipitation, and from which we are able to deposit it in compact, metallic form. The ECM and EW unit operations are coupled into a complete electrolyte circulation loop and their operating parameters are adjusted to keep the soluble metal concentration in a range where the ECM process is not adversely affected and the EW operation is efficient and economical. Consequently, metals are recovered, sludge waste is avoided and water usage is minimized since losses are limited only to evaporation, adventitious electrolysis, and panel drag-out.
In the presentation, after introducing the (R)ECM concept and its motivation, we will summarize the initial electrolyte selection studies for C18000, SAE4150, IN718 and SS316L materials. In addition, we will present data from integrated operation of the (R)ECM process, where the soluble metal concentration is maintained in a pre-determined range by suitable adjustments of the operating parameters of the ECM and/or EW unit operations. Finally, we will provide a brief overview of a β-scale electrowinning system Faraday has developed and installed at the U.S. Army Benét Laboratories to enable (R)ECM operations there. This system is sized to recover up to 0.5 m3 per year of metal from an existing ECM operation, and will assist Benét Laboratories in their efforts toward meeting the U.S. Army’s “Vision for Net Zero” [[6]].
The authors acknowledge the financial support of U.S. Army Contract Nos. W15QKN-12-C-0010 and W15QKN-12-C-0116, and US EPA Contract No. EP-D-13-040.

References

[[1]] K.P Rajurkar, D. Zhu, J.A. McGeough, J. Kozak, A. De Silva “New Developments in Electro-Chemical Machining” Annals of the CIRP Vol 48(2) (1999).
[[2]] E.J. Taylor, M.E. Inman, B.T. Skinn, T.D. Hall, S.T. Snyder, S.C. Lucatero, E.L. Kathe. “Apparatus and Method for Recovery of Material Generated During Electrochemical Material Removal in Acidic Electrolytes.” U.S. Patent Application No. 2016/0230303 A1, 11 Aug 2016.
[[3]] Wessel, “Electrochemical Machining of Gun Barrel Bores and Rifling,” Naval Ordnance Station, Louisville KY, September 1978. http://handle.dtic.mil/100.2/ADA072437.
[[4]] C. Zhou, E.J. Taylor, J.J. Sun, L.E. Gebhart, R.P. Renz. “Electrochemical Machining Using Modulated Reverse Electric Fields.” U.S. Patent No. 6,402,931, 11 June 2002.
[[5]] E.J. Taylor. “Sequential Electromachining and Electropolishing of Metals and the Like Using Modulated Electric Fields.” U.S. Patent No. 6,558,231, 6 May 2003.
[[6]] “Vision for Net Zero” http://army-energy.hqda.pentagon.mil/programs/netzero.asp.

Figure 1