Coupled Pulse/Pulse-Reverse Electrochemical Machining and Electrowinning for Metal Recovery, Elimination of Waste, and Minimization of Water Usage

Abstract

Electrochemical machining (ECM) is well suited for machining “difficult to cut” materials and/or parts with complicated and intricate geometries difficult to produce by mechanical or electric-discharge fabrication methods. Conventional ECM processes typically operate with neutral salt electrolytes, in which the machined material is precipitated from the ECM electrolyte as hydrated metal oxide/hydroxide sludge. This sludge is generally transported off-site and disposed in a landfill. Depending on the alloying components in the machined metal, it may be necessary to designate the sludge as a hazardous material and landfill it according to much stricter requirements. Furthermore, the volume of sludge generated by ECM of many common alloys in pH-neutral solutions is typically more than 300 times greater than the amount of metallic material removed, requiring the use of a high-CAPEX and -OPEX sludge management equipment train. As recently noted, the generation and disposal of the sludge is a major impediment to the widespread use and implementation of ECM operations [1], especially for smaller fabrication operations seeking to add or expand ECM process capability.This talk will present work on development of “Recycling ECM” [(R)ECM], a novel, patented [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 via EW without intermediate processing (see Figure 1, left). Significant challenges have been reported in literature in attempts to use direct current ECM in such electrolytes [3], including degraded workpiece surface finish and problematic deposition of dissolved metals 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 suggests that selection of a suitable ECM waveform should avoid the adventitious electrodeposition challenges observed in the literature. Figure 1 (right) presents a notional pulse-reverse waveform annotated with key timing and electric fiend intensity parameters, and enumerates some of the mechanisms of control afforded by the pulse-reverse processing paradigm. The enhanced control of the ECM process enabled by the PRC waveform enables operation with a variety of electrolytes, and also enables metal recovery via electrowinning 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, and from which electrodeposition of compact metallic films is feasible. The ECM and EW unit operations are coupled into a complete electrolyte circulation loop and their operating parameters adjusted to maintain soluble metal concentration(s) in a range where ECM is not adversely affected and EW 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.After introducing the (R)ECM concept and motivation, this talk will summarize data gathered from operation of the (R)ECM process, wherein soluble metal concentration(s) were maintained in a pre-determined range by adjustment of ECM/EW operating parameters. Additionally, a brief overview will be provided of a β-scale electrowinning system Faraday 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 financial support from U.S. Army Contract Nos. W15QKN-12-C-0010 and W15QKN-12-C-0116, and US EPA Contract No. EP-D-13-040.Figure 1. (Left) Schematic of a possible (R)ECM implementation for an alloy machined primarily to divalent metal cations. (Right) Schematic of pulse-reverse ECM waveform and process factors addressed by the various waveform components. [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] U.S. Patent No. 9,938,632, granted 10 Apr 2018. U.S. Patent No. 10,214,832, granted 26 Feb 2019. [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