Gold primarily exists as native metal, often alloyed with silver, copper, and mercury to varying degrees. Natural gold deposits vary greatly in particle size and mineral composition. Gold deposits can be broadly categorized into two types: placer deposits, where gold has been released from its host rock through natural weathering (typically water erosion), and hard rock (or vein) deposits, where gold remains locked within the host rock.
Types of Gold Deposits
- Placer Deposits: Placer deposits contain gold that has been released and pre-concentrated by natural weathering. Placer deposits are typically composed of gravel, sand, or clay and can be easily mined without the need for blasting. Gold in placer deposits is often coarse, fully liberated, and can be easily recovered using traditional gravity concentration techniques without the need for crushing or grinding.
- Hard Rock Deposits: Hard rock or vein deposits share a common characteristic: gold is either locked in or enclosed within surrounding rock matrices, requiring crushing and grinding to release. Hard rock gold deposits can be further classified into three subclasses: free-milling, sulfide-associated, and refractory gold ores.
- Free-Milling Gold: Gold ore is considered free-milling if over 90% of the gold can be recovered using traditional gravity cyanidation methods, with low reagent consumption, and if 80% of the grinding circuit passes 75 µm. Examples of free-milling gold ores include quartz vein deposits and oxide ores.
- Sulfide-Associated Gold: Gold is often closely associated with sulfide minerals such as pyrite, arsenopyrite, and chalcopyrite. Gold associated with sulfides is usually very finely disseminated and difficult to separate from the sulfide.
Turn Gold Ore into Gold
- Crush and mill the ore: Use a crusher and a stamp mill or ball mill to crush the rock ore to a fine powder.
- Chemical extraction process (optional): Some ores require a chemical process to extract the gold. Common methods include cyanide leaching, where cyanide solution is used to dissolve the gold, or flotation, where chemicals such as frothers and collectors are added to create a froth that separates gold-bearing minerals from other materials.
- Smelting: Once the ore is crushed and the gold is separated, it needs to be melted to remove impurities and solidify the gold into bars or ingots. This process is called smelting.
- Refining: The gold obtained from smelting is often further purified through refining processes such as electrolysis or chemical purification to achieve higher purity levels.
Gold Recovery
Gold recovery typically refers to the processes and techniques used to extract gold from ore, electronic waste, or other sources where gold is mixed with other materials. Here are some common methods of gold recovery:
- Gravity Concentration: Gold is heavier than most other particles, so using gravity to separate it from other minerals can be effective. Techniques like panning, sluicing, and shaking tables are used for this purpose.
- Cyanidation: This method involves dissolving gold from its ore using cyanide solutions. The gold forms a soluble complex which can then be separated from the rest of the ore.
- Flotation: This technique is used to separate gold-bearing sulfide minerals from other materials. The ore is ground and mixed with chemicals that cause gold-bearing minerals to float to the surface, where they can be collected.
- Heap Leaching: Crushed ore is piled onto a pad and sprayed with a cyanide solution. The solution percolates through the ore, leaching out the gold, which is then recovered from the solution.
- Electrolytic Recovery: In this method, gold is extracted from solutions containing gold ions by passing an electric current through them, causing the gold to deposit onto electrodes.
- Smelting: This process involves heating the gold-containing material to a high temperature, which melts the gold and separates it from impurities.
- Recovery from Electronic Waste: Gold can be recovered from electronic waste such as circuit boards and computer parts through processes like acid treatment, burning, or mechanical separation.