Mineral purification, often known as mineral processing or beneficiation, transforms raw, extracted rock into a marketable product. The earth’s crust contains valuable minerals mixed with unwanted material called gangue. To make mining economically viable, the concentration of the desired mineral must be increased significantly, moving it from low-grade ore to a high-grade concentrate. This initial separation relies on exploiting the differences in physical and chemical properties between the target mineral and the surrounding waste rock, allowing for final refining steps that yield pure metal or industrial products.
Preparation of Raw Ore
The purification process begins with comminution, a mechanical stage involving size reduction of the large rocks extracted from the mine. Run-of-mine ore can be several feet in diameter, so the first step is crushing. Machines like jaw and gyratory crushers break the material down into manageable pieces, typically a few inches in size.
The primary purpose of size reduction is liberation, physically separating the valuable mineral particles from the surrounding gangue. Crushed material then moves to grinding, often done in large rotating ball or rod mills. These mills use steel media to pulverize the ore into a fine powder or slurry, which is necessary to fully expose the individual mineral grains.
Following grinding, the material undergoes sizing, which separates the particles into distinct size ranges using screens or hydrocyclones. The efficiency of subsequent separation methods, such as gravity or flotation, is highly dependent on a uniform particle size. Particles that are too large or too small are often sent back for further grinding to maximize the exposure and recovery of the valuable mineral.
Physical Concentration Methods
Once the mineral is liberated and sized, concentration methods exploit physical differences between the valuable particles and the waste. Gravity separation relies on the difference in specific gravity between the minerals. Since most metal-bearing minerals are significantly denser than gangue, they are separated when moved through a fluid medium like water.
Gravity separation techniques include:
- Jigging, which uses pulsating water currents to stratify particles, causing heavier material to settle quickly.
- Shaking tables, which employ an inclined, riffled surface that vibrates, causing denser particles to migrate while lighter particles are washed away.
- Spiral concentrators, where a slurry flows down a helical trough, pushing lighter material to the outer edge while denser particles remain closer to the center.
Magnetic separation targets minerals with inherent magnetic properties, such as iron ore containing magnetite or hematite. This process involves passing the crushed ore through a strong magnetic field generated by drums or rollers. Ferromagnetic minerals are pulled away from the non-magnetic gangue. High-intensity separators are used to recover even weakly magnetic, or paramagnetic, minerals, ensuring clean separation from the waste material.
Chemical and Surface Concentration Methods
When physical differences are insufficient for separation, methods exploiting surface chemistry are required. Flotation is the most widely used concentration method for finely ground ores, particularly sulfides of copper, lead, and zinc. This process involves adding chemical reagents to the mineral slurry in tanks, which selectively alter the surface of the target mineral to make it water-repellent.
Chemicals known as collectors bond to the surface of the valuable mineral, facilitating its adherence to air bubbles introduced into the slurry. Frothers are also added to stabilize the bubbles and create a persistent, mineral-laden foam on the surface of the flotation cell. This froth, which contains the concentrated target mineral, is then skimmed off for further processing, while the hydrophilic gangue remains below.
Leaching is a chemical concentration technique that uses a solvent to dissolve the target mineral, typically used for precious metals like gold and some base metals like copper. In the common cyanidation process for gold, a dilute alkaline cyanide solution is percolated through crushed ore or mixed in agitated tanks. The cyanide reacts with the gold to form a soluble gold-cyanide complex, dissolving the metal into a pregnant solution. This process is increasingly being supplemented or replaced by less toxic lixiviants like glycine in some modern operations.
Final Refining for High Purity
The output of the concentration stages is a mineral concentrate, which must undergo final refining to achieve purity. Smelting is a high-temperature process that converts the concentrate into a molten metal and a separate waste slag. This method involves heating the concentrate in a furnace with a chemical reducing agent, often carbon, to chemically strip oxygen or sulfur from the metal compound.
The intense heat causes a chemical reaction where carbon monoxide reduces the metal oxide, leaving behind a layer of molten, impure metal and a lighter layer of slag containing the remaining impurities. The product of smelting is often an intermediate blister metal that is not yet pure enough for industrial use. This material then proceeds to electro-refining, a process that achieves the highest purity levels.
Electro-refining is an electrochemical process where the impure metal is cast into an anode and immersed in an electrolytic solution with a pure metal cathode. When an electric current is applied, the metal dissolves from the anode and is selectively deposited as a highly pure metal onto the cathode. Impurities, including precious metals like gold and silver, often drop to the bottom of the cell as an anode slime, which is then recovered as a byproduct.