over the first few weeks of the new year, i have been continuing to experiment with alkaline glycine leaching of base metals as a renewable process for liberating gold from plated printed circuit boards. before i get into the details, i need to reiterate something for my own sake that has cost me many hours on multiple occasions: when trying out novel techniques from a paper, run the calculations on the numbers to determine the scale of the process for a useful result.

first it was realizing i would have to extract multiple hundreds of kilograms of defatted soybean flour into a sub-liter volume of concentrate in order to achieve the effectiveness of a process i'm not at liberty to talk about. this time it was chasing the dream of using alkaline glycine as a lixiviant for gold/silver/other precious metals. had i run the numbers when i was coming up with a workup to try out these processes i would have quickly realized that under idealized laboratory conditions the best case scenario based on what we currently know about the process, you can get a pregnant leach solution of (drumroll please): 0.5mg/L¹

yes you read that right, you can dissolve half of a milligram of gold in one liter of a 2M solution of glycine that is being held at 60C for 24hr with a pH of 12. that means to extract 1g of gold (86.50 USD at time of writing) you would need 2000L of this solution to pass over your material for 24 hours. let's get an idea of the amount of reagents this would take:

• 2000L of distilled water
• 300.268kg of glycine (4000 moles @ 75.067g/mol)
• 800g NaOH (20 moles @ 40g/mol) to adjust pH to 12

immediately we can see this is absurd on any scale besides massive commercial/industrial, and precludes any hobbyist usage of this procedure. not to mention that in terms of thinking about electronic waste as a gold ore, it is a high yield per tonnage "ore" so to speak.

now that that's all out of the way, i can espouse some thoughts and informations about the other use of glycine for hydrometallurgy in the home lab/refinery. one thing that seems to be really useful about alkaline glycine extraction is it's relative safety as a solution, and effectiveness at room temperature. unfortunately without a way to regenerate the solution, it's at a severe disadvantage to the cupric chloride method i mentioned in my last blog post. i had high hopes for the use of electrowinning to recover the copper and regenerate the glycine for re-use, ideally until the amino acid deteriorated beyond function as a ligand.

these hopes were dashed a bit during the last week as i tried a mix of things that all seemed to have their own issues:

• Stainless steel anodes likely contaminate the solution with Cr3+ ions (and possible other alloying metals like Molybdenum)
• Lead anodes cannot be used as they can complex with glycine leading to lead glycinate in solution and degradation of the anodes
• Platinized titanium anodes catalyticly oxidize glycine at the anode to form formaldehyde as well as other hazardous compounds, and render the solution non-recyclable
  • I found no mention of it but I have no reason not to think that more expensive iridium oxide/tantalum interlayer/titanium anodes would behave any differently
• Using an intermediate solvent extraction to move the copper to a sulfate-based electrolyte (allowing for more traditional copper electrowinning processes) is not only more complicated, the extraction solvents are highly toxic, not readily available (only have seen this used/mentioned: LIX-84), and also degrade the glycine content of the leach solution

now all that being said, there are a couple things that i have yet to fully rule out:

1. a paper i found about copper electrowinning (available on that wiki page) mentions the leach solution consisting of 1M glycine and 1M sodium carbonate, which i have yet to try. they then have success with copper electrowinning using two stainless steel electrodes at a low cell potential of -800mV
2. the last non-reactive electrode material i know of, that i have yet to try is carbon, which in theory could be used as an anode. it could be used either in a bag or allowing the deteriorated particles to fall into the container. possibly to act as a fluxing material during the later smelt of the collected copper for electrorefining/inquarting/whathaveyou

as discouraging as it is that it did not work out in reality as it did in my head, i suppose science is nothing more than ruling things out until you have led yourself to the way the world really works. i have a graphite electrode en route to me as i write this, and if that doesn't work satisfactorily then perhaps i'll order more glycine and try out some different leach solutions.

until next time, i hope you see horses on your long drives~