Electrochemical Machining (ECM) has been utilized in many industries for several years for applications ranging from finish machining of airplane engine turbine fins to small caliber barrels. Unlike conventional machining processes, ECM is not affected by material strength, hardness, or toughness which can cause significant processing time and increase defect rate in high strength materials. ECM removes metal via anodic dissolution of the workpiece. A metal hydroxide sludge is produced when the dissolved metal ions bond with hydroxide ions in solution.
The objective of this project is to demonstrate a combined Electrochemical Machining (ECM) process with an Electrowinning process to yield a fully recyclable, sustainable manufacturing process that reduces hazardous waste streams, up to 90%, recovers 100% of wasted salts and water, and improves surface finish as compared to conventional machining processes.
Electrowinning (EW) or Reverse ECM ((R)ECM), was first developed to extract non-ferrous metals from ore (e.g., bauxite) by driving the ore into an electrolytic solution. The metal (e.g., aluminum) could then be electroplated out of solution. In this work, electrowinning will be used to purify the ECM electrolyte by electroplating the dissolved metals onto a recycling cathode. The metal removed by ECM is thus “electrowon” by the recycling cathode, eliminating the generation of metallic hydroxides that are typically filtered, pressed, and shipped as dewatered sludge for third party metallic recovery. The purified electrolyte is then being returned to the machining process.
ECM is a non-conventional machining method that uses anodic dissolution for controlled metal removal in an electrolyte cell. An electrolyte flows between the cathode and the work piece, while a high current at low voltage waveform is passed between them. The (R)ECM process provides a solution to a common problem with all ECM processes. It is a process for preventing the waste metal hydroxide, the solid precipitant, also referred to as the sludge. The combined process will recover the anodically dissolved metals from the workpiece by electroplating them back out of solution onto a recycling cathode.
This work focused on machining 4150 steel using a closed loop ECM/(R)ECM cell while simultaneously plating the waste metals in the EW cell. This steel was chosen to address armament applications for the Army with a focus on rifling, boring, and honing of tubes. This demonstration used 40mm tubes to show the machining and recycling process of the FARADAYIC® closed loop ECM/(R)ECM process.
Performance objective to reduce 90% of waste generated compared to baseline characterization was successful. No waste or sludge was generated during this demonstration. The electrolyte was tested for composition at baseline and throughout this demonstration to determine iron and other detectable trace elements. Conductivity, pH, and sulfate concentrations were monitored to determine if the electrolyte would need to be replenished or disposed and replaced with new. This demonstration resulted in 100% reduction of electrolyte and sludge waste generated compared to baseline.
A 100% reduction of salt loss was successful as no additional salts were required to replenish or replace the electrolyte. A 100% reduction of water loss was also demonstrated. The electrolyte did not lose water to disposal and there was no need to replenish during this demonstration. No liquid effluent was released meeting the zero liquid emission metric.
Implementation issues identified during this work include management of electrowon metals and electrolyte monitoring. Metals electrowon onto plates need to be scraped and disposed. This is a laborious task, and has been factored into the process time and cost. Copper tooling is typically used for these machining tools; unfortunately, due to the corrosiveness of the ammonium sulfate based electrolyte, titanium tooling is necessary for all tools, fixtures, or components that come into contact with the electrolyte. Titanium is more expensive than copper and increases the initial cost of the tooling head as compared to ECM/(R)ECM process using a less corrosive electrolyte. The pH of the electrolyte used in this demonstration also poses a risk to the health and safety of personnel. Proper personal protective equipment (PPE) is necessary for those coming into contact with this process.