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

Faraday is developing and installing a β-scale Recycling Electrochemical Machining ((R)ECM) system at the U.S. Army Benet Laboratory. The (R)ECM system is sized to recover up to 0.5 m3 per year of metal from an electrochemical machining operation. The metals of initial interest include: 1) beryllium-free nickel-copper alloy (C18000), 2) Cr-Mo alloy steel (SAE4150), 3) nickel alloy (IN718) and 4) tantalum tungsten (Ta10W) alloy. The (R)ECM system couples an electrochemical machining unit operation with an electrowinning (EW) unit operation for recovery of metals, elimination of waste, and minimization of water usage.  Consequently, the RECM system is consistent with the U.S. Army’s “Vision for Net Zero”[1].

Electrochemical machining (ECM) is 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 whereby the machined material is generally precipitated from the machining electrolyte as 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 is often designated as a hazardous material. Furthermore, the rifling of a gun barrel by ECM generates ~350 gal of centrifuged sludge per ~250 in3 of material removed [2]. This represents greater than 300X volume multiplier of waste sludge relative to the amount of 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 [3].

Faraday has previously demonstrated the ability of pulse current/pulse reverse current (PC/PRC) waveforms electrochemically machining materials while achieving a high quality surface finish (Figure 1) [4],[5]. Smooth surfaces are typically not possible in conventional DC ECM.  The PRC waveforms control the ECM process and consequently ECM with PRC is adaptable to a variety of electrolytes.

To develop a (R)ECM process, we select an electrolyte whereby the machined material is soluble and avoids sludge formation by precipitation. The ECM unit operation is coupled to an EW unit operation where the machined material is collected as metal. The ECM and EW unit operations are adjusted to keep the soluble metal concentration in a range where the ECM process is not adversely effected and the EW operation is efficient and economical. Consequently, metals are recovered, waste is avoided and water usage is minimized.

In the presentation, we will summarize the initial electrolyte selection studies for C18000, SAE4150, IN718 and Ta10W materials. In additionally, we will demonstrate integrated operation of the (R)ECM process where the soluble metal concentration is maintained in a pre-determined range by 1) simultaneous (R)ECM operation by adjusting the ECM unit operation in the presence of constant EW unit operation, 2) simultaneous (R)ECM operation by adjusting the EW unit operation in the presence of constant ECM unit operation, and 3) sequential operation of the ECM and EW unit operations. Finally, the status (R)ECM system design and installation at Benet Laboratories will be reviewed.     

References

[1] “Vision for Net Zero” http://army-energy.hqda.pentagon.mil/programs/netzero.asp

[2] Wessel, “Electrochemical machining of Gun barrel Bores and Rifling” Naval Ordnance Station, Louisville, KY, September 1978; http://handle.dtic.mil/100.2/ADA072437.

[3] K.P Rajurkar, D. Zhu, J.A. McGeough, J. Kozak, A. De Silva “New Developments in Elector-Chemical Machining” Annals of the CIRP Vol 48(2) (1999).

[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 issued June 11, 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 issued May 6, 2003>

Figure 1