By Rebekah Smith | Updated Mar 24, 2022
Gold can be extracted from stone using an alkaline or acidic leaching process that incorporates halogens such as chlorine, iodine, or bromine. Chlorine is the most cost‑effective and widely available option. When sodium chloride (common salt) and hydrochloric acid (HCl) are combined, the resulting solution produces a mild electrochemical reaction that releases gold from the surrounding rock and soil.
Grind the ore until it reaches a 200‑mesh size (≥200 particles per square inch). Place the crushed material in a plastic‑tub cement mixer, add 10 lb of river rock, and mix until the ore is uniformly 200 mesh or finer. Remove the larger river rock, which will remain largely inert during leaching.
Transfer the ore to a large plastic bucket or garbage can. Add hot water, sodium hypochlorite (bleach), and HCl to create a solution that is 15 % sodium hypochlorite and 33 % HCl by volume. Ensure the mixture completely covers the ore; add more solution if necessary.
Insert an ORP meter probe into the mixture. ORP measures the oxidizing power of the solution, which directly correlates with gold leaching efficiency (Mine‑Engineer.com). Add HCl incrementally until the ORP reaches 1,000 mV. Add only small amounts at a time to prevent excessive bubbling or gas release.
Use the cement mixer to agitate the bucket slowly. Aeration introduces oxygen, accelerating the leaching process. Continue mixing until the ORP drops to 400 mV.
Keep the ORP near 1,000 mV while avoiding values below 400 mV. HCl acts as an electron‑acceptor; its effectiveness peaks at higher ORP, while the reaction ceases once ORP falls to 400 mV.
Repeat steps 3 and 4 until the ORP stabilizes when HCl is added. A stable ORP indicates that all soluble gold has been released. Allow the solids to settle to the bottom of the bucket.
Filter the liquid using a coffee filter in a funnel or a canvas strainer. The gold remains dissolved in the HCl solution. Add sodium metabisulphite to precipitate metallic gold by lowering the pH and reducing the solution. Filter again with a fine gold filter, and collect the water in a separate bucket for safe disposal.
Gradually add sodium bicarbonate to the remaining HCl mixture. Stir until the fizzing stops and the pH reaches 7.0. A neutral solution is safe for drainage per guidelines from the Massachusetts Department of Environmental Protection.
Perform the entire process outdoors and face into the wind to avoid inhaling chlorine gas, which can irritate lung tissue. The mixture is corrosive; avoid skin contact and wear protective gloves, goggles, and a face mask at all times.
