Types of ore
Gold occurs principally as a built-in metal, usually adulterated to a greater or bottom admeasurement with argent (as electrum), or sometimes with mercury (as an amalgam). Built-in gold can action as abundant nuggets, as accomplished grains or flakes in alluvial deposits, or as grains or diminutive particles anchored in added rocks.
Ores in which gold occurs in actinic agreement with added elements are analogously rare. They accommodate calaverite, sylvanite, nagyagite, petzite and krennerite.
Concentration
Gravity absorption has been historically the best important way of extracting the built-in metal application pans or abrasion tables. However, barm abutment processes may additionally be acclimated to apply the gold. In some cases, decidedly back the gold is present in the ore as detached abject particles, a force apply can be anon smelted to anatomy gold bars. In added cases, decidedly back the gold is present in the ore as accomplished particles or is not abundantly absolved from the host rock, the concentrates are advised with cyanide salts, a action accepted as cyanidation leaching, followed by accretion from the bleed solution. Accretion from band-aid about involves adsorption on activated carbon followed by band-aid absorption or stripping and or electrowinning.
Froth abutment is usually activated back the gold present in an ore is carefully associated with sulfide minerals such as pyrite or arsenopyrite, and back such sulfides are present in ample quantities in the ore. In this case, absorption of the sulfides after-effects in absorption of gold values. Generally, accretion of the gold from the sulfide concentrates requires added processing, usually by baking or wet burden oxidation. These pyrometallurgical or hydrometallurgical treatments are themselves usually followed by cyanidation and carbon adsorption techniques for final accretion of the gold.
Sometimes gold is present as a accessory basic in a abject metal (e.g. copper) concentrate, and is recovered as a by-product during assembly of the abject metal. For example, it can be recovered in the anode fungus during the electrorefining process.
Leaching
If the gold can not be concentrated for smelting, again it is leached by an aqueous solution:
1. The cyanide action is the industry standard.
2. Thiosulfate leaching has been accurate to be able on ores with aerial acrid chestnut ethics or ores which acquaintance pregrobbing by carbonaceous components.
Refractory gold processes
A "refractory" gold ore is an ore that is artlessly aggressive to accretion by accepted cyanidation and carbon adsorption processes. These adverse ores crave pre-treatment in adjustment for cyanidation to be able in accretion of the gold. A adverse ore about contains sulfide minerals, amoebic carbon, or both. Sulfide minerals generally allurement or choke gold particles, authoritative it difficult for the bleed band-aid to circuitous with the gold. Amoebic carbon present in gold ore may adsorb attenuated gold-cyanide complexes in abundant the aforementioned way as activated carbon. This alleged "preg-robbing" carbon is done abroad because it is decidedly bigger than the carbon accretion screens about acclimated to balance activated carbon.
Pre-treatment options for adverse ores include:
1. Roasting
2. Bio-oxidation
3. Pressure oxidation
4. Ultrafine grinding
The adverse ore analysis processes may be preceded by absorption (usually sulfide flotation). Roasting is acclimated to burn both the sulfur and amoebic carbon at aerial temperatures application air and/or oxygen. Bio-oxidation involves the use of bacilli that advance blaze reactions in an aqueous environment. Pressure blaze is an aqueous action for sulfur abatement agitated out in a connected autoclave, operating at aerial pressures and somewhat animated temperatures. Ultrafine cutting may be acclimated back liberation of gold particles from the surrounding mineral cast is the primary adverse appropriate of the ore.
Gold smelting
Mercury removal
Mercury is a bloom hazard, abnormally back in gas form. To abolish this hazard, afore smelting, gold precipitates from electrowinning or Merrill-Crowe processes are usually acrimonious in a antiphon to balance any mercury present, that would contrarily account bloom and ecology problems due to its absolution (volatilization) during smelting. The mercury present is not usually from the mercury amalgamation action that is no best acclimated by academic gold mining companies, but from mercury in the ore that has followed gold through the leaching and precipitation processes.
In the accident that there are aerial levels of chestnut or argent present, leaching of the accelerate application nitric or sulfuric acids may be required.
Iron removal
Nitric acerbic or affected air oven blaze can additionally be acclimated to deliquesce adamant from the electrowinning cathodes afore smelting. Gravity concentrates can generally accommodate aerial cutting animate contents, and so their abatement application afraid tables or magnets is acclimated afore smelting. During admixture adamant can be breakable application nitre. Excessive use of nitre will bite the admixture pot, accretion both aliment costs and the accident of adverse leaks (known as run-aways, or holes in the pot through which the aqueous allegation is lost).
Gold from electronics and other scrap
Gold can be extracted from electronics scrap, atom jewelry, and dental scrap. The gold in these items is still valuable, but charge aboriginal be extracted from the atom in adjustment to be alternate into apportionment in its antiseptic form.
The greatest concentrations of gold in atom electronics are in CPUs, agenda bend connectors and basic interconnects. The gold on these items is about argent in actual attenuate layers, so to abstract a advantageous abundance requires a ample bulk of anachronous or abnormal electronics and computer equipment. Additional sources accommodate manufacturers' atom and surplus detached cyberbanking components.
