Recovering More Copper from Low-Grade Sulfide Ore
Miners need an economical way to extract copper from low-grade chalcopyrite ore while using less energy and water and producing less waste. Chalcopyrite, the mineral containing about 70% of remaining copper reserves, forms a surface layer during chemical leaching that blocks the electron transfer needed to dissolve copper; conventional heap leaching typically recovers less than half the copper in primary sulfide ores. Fine grinding or high-temperature pressure treatment can overcome this barrier, but add substantial energy and equipment costs, especially for ore containing just 0.3–0.5% copper. The challenge is to extract copper selectively at ambient temperature and pressure, fit existing mines’ heap-leach and copper-recovery equipment, and scale to more than 100,000 tonnes of copper per year.
Borrowing oilfield scale inhibition, we use tiny doses of growth-blocking chemicals to keep copper-leaching heaps open and preserve the dissolved iron needed to extract copper.
How it works
Acidic liquid trickles through heaps of crushed ore to dissolve copper, but jarosite, an iron-bearing mineral, and silica gel can seal the gaps between rocks and coat the ore. Oilfield operators tackle similar blockages with chemicals that attach to crystal growth sites, so tiny doses suppress scale even when the water chemistry favors it. We propose adding two commodity water-treatment chemicals to ore preparation and irrigation: HEDP to block jarosite growth sites, and polymaleic acid to prevent silica from joining into a gel. At maintenance doses of just 5–15 and 3–10 parts per million respectively, the aim is to keep jarosite below 100 nanometers and silica dispersed, small enough to wash through the heap. Preventing jarosite growth also leaves more iron dissolved in the liquid, where it helps dissolve copper. Operators then adjust aeration to control the iron chemistry. This approach intervenes before deposits require removal and fits existing ore-preparation and irrigation equipment.
Borrowing from microbial iron cycling in wetland soils, we use bacteria with opposing metabolisms to balance the chemistry at buried copper-mineral surfaces and prevent the coating that stops copper dissolving.
How it works
In heaps of chalcopyrite ore, the iron compound used to dissolve copper can also trigger a surface coating that blocks further extraction. Adjusting the incoming leaching solution gives operators little control over individual mineral grains deep inside the rocks. We borrow a local balancing mechanism from wetland microbes: one group converts iron into its oxidizing form, ferric iron; another converts it back. Pulsing the heap’s air supply creates oxygen-rich zones near air channels and oxygen-poor zones inside ore particles. Iron-oxidizing bacteria replenish ferric iron near the air channels, while iron-reducing bacteria consume excess ferric iron beside buried mineral grains. We tune the air pulses and bacterial mix to keep local conditions within the narrow range where copper continues to dissolve. Air-off periods also slow ferric-iron production, limiting exposure to conditions that build the blocking coating. This puts biological control where bulk chemical adjustments cannot reach, using the heap’s aeration and irrigation systems without wiring sensors to individual grains.
Borrowing the electron shuttles that bacteria use inside biofilms, we add a reusable molecule to copper-leaching liquid to carry charge through the mineral’s blocking surface layer and keep copper dissolving.
How it works
When miners dissolve copper from chalcopyrite ore, a thin surface layer forms and blocks the electron transfer needed to keep the reaction going. Bacteria inside biofilms solve a similar problem with small molecules that carry electrons to oxidants outside the cells. We propose adding BQS, a synthetic electron-carrying molecule, to the liquid circulated through heaps of ore. BQS diffuses through tiny gaps between crystals in the surface layer, accepts electrons from the mineral, then returns to the surrounding liquid. There, ferric iron removes those electrons so BQS can repeat the cycle. Copper dissolution can continue without stripping away the layer or forcing electrons through it. Using less ferric iron and less acidic liquid also reduces jarosite deposits and silica gel that clog heaps. Operators add the reagent within the existing leaching and copper-recovery circuit.