Questions and answers submitted during the webinar can be found below.
Question/Comment | Response |
| 1Q: There is knowledge that leaching Hg-contaminated tailings with cyanide is a worst-practice. Is there any research about the potential dangers of using these alternative lixiviants on Hg contaminated tailings? | 1A: Very little. The only direct test on mercury-bearing ASGM tailings (Mohamed et al. 2025, Cameroon) found glycine mobilised mercury much as cyanide did, about 30% in both cases. Deng et al. 2026 saw Jinchan dissolve less mercury than cyanide from one ore, but it contains cyanide anyway. Thiosulfate is known to complex and co-load mercury in large-mine circuits; halides and thiourea would be expected to leach it, but nobody has tested them on ASGM tailings. So, no alternative has been shown to be safe on mercury-contaminated tailings, and the rule is unchanged for every reagent: remove the mercury first. |
| 2Q: I recall that Thiosulphate at Goldstrike required high tech recovery (basically advanced water treatment with membranes - from water treatment colleagues involved). | 2A: Correct in substance. Goldstrike leached double-refractory ore with calcium thiosulfate after pressure oxidation, recovered gold on strong-base ion-exchange resin rather than carbon, and depended on advanced water treatment, including membranes, to control thiosulfate degradation products (tetrathionate, trithionate, sulfate) and recycle water. The reagent is cheap and low in acute toxicity; the circuit around it is not. Barrick converted back to cyanide CIL in 2023 for higher recovery and lower cost. None of that flowsheet exists in an ASGM hub, which is why the deck rates thiosulfate as instrumented-plant only. |
| 3Q: What is your opinion of the Canadian product called RZOLV, marketed as a water-based metal extraction solution that offers an effective and safe alternative to cyanidation and mercury? | 3A: For ASGM the same caveats as the rest of the sulfur family apply, with one added: a confidential formulation means the five-sample assay comes first, and "non-toxic" is a claim until an SDS with full composition and independent ecotoxicology are published. |
| 4Q: If an ASGM site uses the cyanide process, who performs the electrowinning step? How is that done in the field? | 4A: Mostly nobody, because most ASGM cyanide sites never produce a pregnant eluate. The usual flowsheet is vat or tank leaching with carbon in columns; the loaded carbon is then burned to ash and smelted or sold to a buyer or a licensed processing centre with an elution and electrowinning circuit. Burning carbon is common and the worse option: it wastes carbon, loses gold to the ash and, with cyanide-bearing carbon, releases HCN and cyanate fumes. Zinc precipitation is the other field route: zinc shavings or dust drop a gold-zinc sludge that is acid-washed and smelted; cheap and needs no power, but the zinc-cyanide effluent still has to be destroyed. Electrowinning happens at the centralised plants and toll processors, and it is not exotic: a hot caustic-cyanide elution column, a rectifier and a steel-wool cathode cell, run by a plant operator or an itinerant metallurgical technician. A 50 to 200 t/day processing centre in Ghana, Peru or Tanzania often has one, and it is exactly what planetGOLD hubs provide. |
| 5Q: I noticed that Borax was not included in your overview - any particular reasons? | 5A: Borax is not a lixiviant, so it does not belong in a review of leaching agents. It dissolves nothing: sodium tetraborate is a flux used in the final smelting step, where it lowers the melting point of the gangue and takes silica, iron and other oxides into a glassy slag so that a gravity concentrate can be melted directly to doré. The "borax method" promoted in the Philippines and elsewhere is therefore a mercury-free finishing route for a high-grade gravity concentrate, an alternative to amalgamation, not to cyanidation. It works only where gravity has already produced a rich concentrate (roughly several hundred g/t or better), does nothing for fine, refractory or low-grade gold, and carries its own hazards (fume, burns, borate in slag). In the deck's framework it sits on rung 1 of the transition ladder, gravity concentration and direct smelting, and in the preferred-model flowsheet under gravity-first recovery, upstream of any leach. If it is worth naming, the place is slide 16 or the glossary (not shown!) |
| 6Q: This is more of a comment than a question, but I think it is also important to note that recovering the gold from these alternative lixiviant systems is often also more complicated than gold-cyanide complexes. | 6A: Thanks, and agreed, it deserved more than the one line it got. Aurocyanide is unusually convenient: it loads on plain carbon and strips with hot caustic cyanide, and that circuit already sits in every processing centre. Thiosulfate won't load on carbon and needs resin; glycine loads slowly with copper competing; the halides need corrosion-resistant kit; and the proprietary agents load on carbon precisely because they are cyanide. That is why "recovered from solution" is its own column in the evidence table, and why so many cases stop at "gold dissolved". Happy to pick this up after the session, it is exactly the practitioner detail Report 1 needs. |
| 7Q: I understand these alternatives are most appropriate for central plants, however ASGM especially in Ghana where I research into is mostly dominated by individuals or small groups that mostly operate informally. How do these methods target these groups as well? | 7A: Fair challenge, and the right one for Ghana. Small cyanide units already exist: vat kits, 1 to 10 t batch tanks, containerized leach-and-carbon units, and Report 1 recommends assessing them. Cyanide's lesson is that the unit is never the problem; what is missing is measured dosing, a lined circuit, carbon recovery, effluent destruction and a trained operator. An alternative packaged the same way inherits the same gaps and is less forgiving: thiosulfate needs resin and control, glycine is slow with half the recovery, the "cyanide-free" agents are cyanide. None is safer in a micro-unit than cyanide today. That is why hub-and-spoke is the model, and the Ministry's push is the right direction. The spoke is what small groups already do without chemicals: mining, crushing, gravity concentration, direct smelting. The hub takes concentrate and tailings, runs the leach (responsible cyanidation now, an alternative once proven on Ghanaian ore), recovers the gold, destroys the effluent and pays on assay. |
| 8Q: How do ASGM operators obtain mercury, and are there restrictions on, or penalties for, the sale and marketing of it? | 8A: Mercury use is still allowed in small scale mining in a number of countries. In countries with restrictions, mercury is still traded via informal/illegal routes.
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| 9Q: How do you balance technical efficiency with environmental sustainability in small‑scale mining operations? | 9A: They are the same! Efficiency is the way to make the most of the resource and save money, get more gold with less mercury… |
| 10Q: We know CN will evaporate in pH lower than ~11, creating hazard. Is this a problem when applying these techniques for Hg removal? How can the operations deal with the challenge of controlling the pH during these processes to prevent CN-related hazards, but also keeping conditions for Hg removal? | 10A: It's not really a problem because sulphide precipitation requires an alkaline solution, though not as alkaline as for cyanide, but within a manageable range that’s easy to adjust for different parts of the process. I think its fairly trivial to adjust pH though, if you need a more acidic solution for a removal process that then needs to become alkaline for the cyanidation step. Additives like CaS are used to increase selectivity without impacting gold recovery. There are many chemical options for controlling mercury lixiviation and precipitation |
| 11Q: Has the gold lixiviant been looked at / offer a synergistic process to also recovery mercury with the gold? | 11A: I reckon they all have pretty good capacity to recover both, I think the more important question is how easy is it to control the selectivity for mercury vs gold in dissolution or stripping. |
| 12Q: Perhaps its necessary to flip the question around - how clean does it need to be? How much is “good enough” for health and environment protection? | Right. Considering discharge and final tailings disposal, the EPA and other entities will tell you how much is allowed in soil in various contexts and in waste disposal of varying levels of hazard management. The more subtle question is how clean does it need to be to treat it with lixiviants. Imagine case 1 where someone takes 1000 g/T tails and reduces the concentration to 100 g/T and someone else who takes 210 g/T tails and reduces it to 200 g/T. Clearly one is better, but what if no more mercury can be removed from the second case with any feasible technique? Should instead we aim for a minimum recovery throughout the entire process, including cyanidation? I think you would have to look at what is practically achievable and estimate a target concentration and update it as more info becomes available. But it might be impossible and unproductive to choose a number. |
| 13:Q: Is it possible to eliminate mercury from contaminated tailings (maybe by planting or adding chemicals ) | 13A: Yes its possible, see my whole presentation. Plants are unlikely to be a good solution, especially as you don’t want to mix organics with metal extraction… root materials seriously degrade gold recovery. |
| 14Q: The Convention provides, under Article 11, for the environmentally sound management of wastes containing mercury (Hg) above a concentration threshold established by the Conference of the Parties (COP), with support from the Basel Convention. At COP-5 in 2023, this threshold was set at 15 mg/kg. | 14A: A high bar for tailings!! That could be possible, not sure that’s a realistic number to expect broadly. |
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