Gold’s Toughest Ores Get a Cyanide-Free Makeover in New Review

Gold is getting harder to come by. The easily milled, free-milling deposits that fueled more than a century of mining are steadily running out, and the industry is turning to so-called refractory ores in which the precious metal is locked inside sulfide minerals such as pyrite, arsenopyrite, and stibnite. According to a new review published in the journal Discover Minerals, these difficult ores already account for roughly two-thirds of known gold resources and about one-third of global production, and that share is expected to keep rising. The problem is that conventional cyanidation, the dominant extraction method for over a hundred years, typically recovers less than half of the gold from ores rich in arsenic and antimony.

In refractory sulfide ores, gold occurs as micro- to nanosized inclusions or is incorporated directly into the crystal lattice of the host mineral. Cyanide simply cannot reach it. Worse, the same ores actively sabotage the leaching process: during cyanidation, cyanide ions and oxygen that should be dissolving gold are consumed by reactions with arsenopyrite and stibnite, producing thiocyanate along with toxic arsenite, arsenate, and antimonite species in solution. Both arsenic and antimony are highly toxic, so any viable process must also prevent their release into the environment.

The authors evaluate five main pretreatment strategies designed to crack open the sulfide matrix before leaching. Ultrafine grinding reduces ore particles to a d80 of roughly two to three microns using stirred mills such as the ISAMill, physically exposing locked gold without chemicals. In one stibnite-bearing ore, this treatment lifted cyanidation recovery from 40 percent without grinding to 66 percent after grinding to about 3.7 microns. But grinding has limits: it liberates only physically encapsulated gold, not gold chemically bound within sulfide lattices, and it is highly energy intensive, which restricts its economics for low-grade ores.

Roasting and pressure oxidation are the industrial workhorses. Oxidizing roasting, typically performed in rotary kilns or fluidized-bed reactors, decomposes arsenopyrite and pyrite into porous iron oxides while volatilizing arsenic as arsenic trioxide around 450 degrees Celsius, with antimony and iron sulfides oxidizing at 600 to 700 degrees. Gold extraction after roasting commonly reaches 80 to 90 percent. Microwave-assisted roasting, a newer variant, removed more than 90 percent of sulfur, arsenic, and carbonaceous phases from a flotation concentrate and yielded gold recoveries exceeding 95 percent. The environmental price is steep, however: sulfur oxide emissions that contribute to acid rain, highly toxic arsenic trioxide gas, and dense iron oxide layers that can encapsulate remaining gold particles and hinder downstream leaching.

Pressure oxidation achieves similar chemistry under elevated temperature and oxygen pressure, converting sulfides into hematite, jarosite, and ferric arsenate phases while dissolving arsenic into the acidic solution. Notably, antimony largely stays in the solid residue, easing downstream separation. In one reported case, reductive leaching of stibnite from gold flotation concentrates using chromium(II) ions removed more than 60 percent of the sulfur and antimony, and subsequent cyanidation of the residue recovered approximately 95 percent of the gold. The drawback is cost: pressure oxidation demands autoclaves, elevated temperatures, and oxygen, making it capital and energy intensive and poorly suited to low-grade ores.

Bio-oxidation offers a gentler, low-energy alternative. Acidophilic bacteria such as Acidithiobacillus and Leptospirillum species, thriving at pH 1.2 to 1.8 and 30 to 35 degrees Celsius, oxidize sulfide minerals and regenerate the ferric iron that attacks arsenopyrite. Prolonged treatment of up to 30 days has delivered gold extraction efficiencies exceeding 90 percent. But the microbes are vulnerable: dissolved arsenic(III) inhibits bacterial activity, and dissolved antimony(III) is even more toxic, disrupting enzyme function and metabolism. Stibnite itself is notoriously resistant to biological attack, making bio-oxidation ineffective for selective antimony removal. Slow kinetics, passivating layers of jarosite, goethite, and elemental sulfur, and the need for large reactors at low pulp densities further constrain the technology. Adding catalytic metal ions helps: five milligrams per liter of silver ions raised arsenic removal from about 30 to over 47 percent while shortening the required bio-oxidation time from 19 to 15 days.

Alkaline pretreatment takes a more surgical approach, using sodium hydroxide and sodium sulfide to selectively dissolve arsenic and antimony while avoiding the sulfur oxide and arsenic trioxide emissions of thermal routes. A two-stage scheme first dissolves arsenic in sodium hydroxide, then leaches the residue in sodium sulfide medium to extract antimony, which can be recovered by electrowinning. This sequential approach achieves more than 96 percent arsenic and roughly 97 percent antimony removal without significant co-dissolution. Cleaning these elements out of the ore pays off downstream: gold recovery can climb from below 50 percent to between 80 and 95 percent while cutting cyanide, hydroxide, and oxygen consumption. The trade-off is the generation of large volumes of arsenic- and antimony-bearing effluents that must be stabilized and managed.

On the leaching side, the review identifies ammonium thiosulfate as the most promising cyanide alternative. It is far less toxic, highly selective for gold, and works well for carbonaceous, preg-robbing ores where cyanidation fails. Gold is oxidized by the cupric tetraammine complex in the presence of thiosulfate ions, with ammonia stabilizing copper and preventing copper-driven reagent decomposition. Yet sulfide minerals remain a serious threat: they catalyze thiosulfate breakdown, with pyrite the worst offender, and arsenopyrite additionally releases sulfide ions that precipitate as elemental sulfur on gold surfaces, passivating them and slowing dissolution. Additives such as EDTA, carboxymethyl cellulose, phosphate species, and sodium citrate can suppress this decomposition by chelating copper and modifying sulfide surface reactivity.

Even when thiosulfate leaching succeeds, getting the gold back out of solution is hard. Activated carbon, the workhorse of cyanide circuits, has poor affinity for the gold-thiosulfate complex, which carries a negative three charge compared with the single negative charge of the gold-cyanide complex and is far bulkier, causing electrostatic repulsion and steric exclusion from the carbon’s micropores. Emerging solutions include a hybrid system pairing zero-valent aluminum with activated carbon, in which galvanic coupling drives gold cementation from solution; this combination achieved up to 99 percent gold recovery from printed circuit boards and recoveries exceeding 90 percent from pregnant leach solutions containing co-dissolved copper, iron, cobalt, nickel, and zinc. Ion-exchange resins offer another route, with strong-base resins reaching loadings of 10 to 25 kilograms of gold per tonne, though they preferentially adsorb competing copper-thiosulfate complexes, while weak-base resins are more selective but hold only about 2 kilograms per tonne and weaken further at alkaline pH.

The review’s central message is that no single technology solves the problem alone. Thiourea leaches gold rapidly under acidic conditions and can exceed 80 percent recovery after bio-oxidation, or above 90 percent with a post-bioleaching neutralization step that removes ferric ions, but growing evidence of its carcinogenicity has sharply limited industrial adoption. Chloride systems dissolve gold faster than cyanide and can reach about 81 percent extraction from high-arsenic ores without pretreatment, but they are corrosive, require expensive equipment, and often operate at 170 to 200 degrees Celsius with high oxygen pressures. The authors argue that the real bottleneck has shifted from liberating gold to integrating the whole flowsheet: matching pretreatment chemistry to lixiviant stability, recovering gold efficiently from cyanide-free solutions, recycling reagents, and stabilizing arsenic and antimony into durable mineral phases such as scorodite. Future industrial implementation, they conclude, will depend on hybrid flowsheets that optimize sulfide decomposition, reagent stability, residue management, and gold recovery simultaneously, supported by digital tools such as machine learning and real-time speciation monitoring to keep these sensitive chemistries under control.

Subject of Research: Hydrometallurgical pretreatment and cyanide-free leaching strategies for recovering gold from arsenic- and antimony-bearing refractory sulfide ores

Article Title: A review of hydrometallurgical strategies for gold recovery from arsenic- and antimony-bearing refractory ores: mechanisms, challenges, and emerging trends

Article References: Mhandu, T. J., Park, I., Jeon, S., Mufalo, W., Fissha, Y., Arima, T., Furusaki, A., Ito, M., & Hiroyoshi, N. (2026). A review of hydrometallurgical strategies for gold recovery from arsenic- and antimony-bearing refractory ores: mechanisms, challenges, and emerging trends. Discover Minerals, 3(1), Article 7. https://doi.org/10.1007/s44346-026-00019-6

Image Credits: AI Generated

DOI: 10.1007/s44346-026-00019-6

Keywords: gold recovery, refractory ores, hydrometallurgy, arsenic, antimony, thiosulfate leaching, pressure oxidation, bio-oxidation, alkaline pretreatment, ultrafine grinding, cyanide alternatives, sulfide minerals