Precipitation systems promote the phenomenon that occurs when a substance held in solution passes out of solution into a solid form. Precipitation systems are useful in many industrial and scientific applications, where solids can be retrieved from solutions by methods of filtration, decanting or centrifuging. They do not produce acid precipitation or acid rain, precipitation that contains relatively high concentrations of acid-forming chemicals that have been released into the atmosphere.
Precipitation systems depend on the nature of a precipitation reaction which results in the desired solid. Many solids precipitate or fall out of a solution when another component or chemical is introduced to the solution, causing a chemical reaction. Other solids form when a solution is supersaturated by a particular compound, or if heat is introduced. For example, carbide precipitation happens when stainless steel materials are heated, usually by welding.
Precipitation systems are used for heavy metal removal, especially toxic metals such as cyanide and mercury. For example, to remove mercury from water, an organometallic precipitation reaction is used, by adding a compound containing at least one atom of a metal bonded to a carbon atom. The resulting reaction causes the mercury to precipitate out of the water into sludge. Precipitation systems are also used to strengthen particular metal alloys. Precipitation hardening involves heating a solution and then rapidly cooling or quenching it to form a supersaturated solution. Over time, the solid alloy precipitates out of the solution forming a strong lattice of molecules. Some precipitations systems occur naturally, such as gold precipitation, which happens when quartz is broken up under pressure and dissolved in hot liquids. Over time as the mixture cools, gold particles precipitate out into larger nodes.
Some precipitation systems are used in biological applications. Biologists use ammonium sulfate precipitation to separate and purify proteins. By using excess salt in the solution, the protein molecules precipitate out of the solution due to a lack of water molecules to bond with.