Powder

A powder (sometimes referred to as powdered material) is a dry bulk solid, composed of a large number of extremely fine particles, that may flow freely or become airborne when shaken or tilted. To be classified as powder, the material must be dry, as in free of any liquid. If the powder contains liquid, then it's classified as gel, paste or solid emulsion.

Powder vs other granular material

The main difference between powders and other granular materials is the size of their particles. The constituent particles that compose granular materials should be large enough such that they are not subject to thermal motion fluctuations. Thus, the lower size limit for grains in granular material is about 1 µm. In powders, the particles are generally small enough that they can become airborne if the powder is agitated or scattered in air, while granular materials cannot become airborne.

Types of powder

Metal

Metal in their powdered form are generally more reactive than in their bulk form. Extremely finely powdered iron is pyrophoric, while bulk iron is less reactive. Osmium and iridium are also a good example, in that they're very inert in their bulk form, but very reactive as powder. However, in case of some metals, such as gold or copper, there is little difference between the reactivity of their powdered and bulk forms.

Soft metals are difficult to turn into powders, as their granules will tend to coalesce over time. However, the powdered form of many metals that are normally thought to be impossible to obtain, like mercury, can be obtained by reducing the salt of said metal (like soluble mercury salts) with a metal like iron.

Metallic powders are commonly used in fireworks and flash powders.

Nonmetal

Powdered nonmetallic materials, such as powdered elements (boron, carbon, phosphorus, sulfur, silicon), nonmetallic compounds (oxides, nitrides, carbides) are somewhat more reactive than in their bulk form, due their higher surface-to-volume ratio.

Organic

Airborne powdered organic materials can pose a serious fire or explosion hazard, such as finely powdered starch (grain flour) sugar suspended in air in confined areas, phenomenon known as dust explosion.

Other

Powdered metallic oxides are useful as catalysts or as building block in most construction materials (cement, clay, ceramic materials).

Insoluble organometallic compounds can only exist as powder, as most will decompose when heated, rather than melt. This is also true for most insoluble coordination complexes.

Availability

Metallic powders are sold by various suppliers. They can be found in many hardware stores as pigment, usually as a suspension with an organic solvent. Metallic powders can also be bought from art stores.

Starch and sugar powder can be purchased from any food store. Starch powder can be extracted directly from potatoes and other starch-rich plants.

Oxide powders can be purchased from many hardware stores and pottery suppliers, as pigments. Many however tend to be calcinated, making them inert to most reagents.

Preparation

Metallic powders

Metallic powders can be prepared by finely grinding metals of small size using ball mills. Spark-free grinding materials, like lead-antimony balls should be used to prevent the creation of sparks which can ignite the very fine powder. This is also mandatory when grinding other combustible materials, like organic compounds.

Fine metal powders can also be produced through chemical methods. Copper powder can be produced by reducing a copper salt with a more reactive metal, like iron, zinc or aluminium. Do note that due to various reasons, this method may not give just powder, but also metal sheets or flake-like copper particles.

Very fine iron powder can be obtained through the pyrolysis of iron(II) oxalate. The resulting iron powder is so fine it will spontaneously burn in air, and this type of iron powder is also known as "pyrophoric iron".

Zinc powder can be easily produced from the electrolysis of sodium zincate (made from sodium hydroxide and zinc/zinc oxide), using nickel and zinc/copper electrodes.

When preparing magnesium or aluminium powders by milling larger pieces of said metals, there's a serious risk of explosion. There are several ways to mill Al and Mg safer:

• Addition of small amounts of charcoal will help coat the highly reactive metallic powder in a think layer of charcoal dust, which severely reduces the possibility of spontaneous ignition in air, both during the milling process and during the removal of the powder from the mill
• Milling the powder under inert gas (like nitrogen or argon), followed by very slow introduction of air in the mill. This is usually done for Mg powder, as magnesium naturally forms a thin protective layer of MgO on the surface of the metal. By carefully and very slowly replacing the inert gas with air, the Mg powder slowly passivates in air, though only enough not to be pyrophoric, and the resulting powder is still highly flammable.

Nonmetallic powders

Charcoal can be made by grinding it in a mill, or a ball mill. Charcoal made from softwood is considered the best as it grinds easier.

Sulfur can be grind to a powder in a similar way. Extremely fine powdered sulfur can also be made via chemical processes, like the reduction of thiols using sulfur dioxide. This process is done in the oil industry on a massive scale, and it's the reason why powdered sulfur is so cheap and readily available.

Red phosphorus powder can be obtained by exposing solutions of white phosphorus to strong UV light. The resulting red P however is highly reactive, and can even react with aq. NaOH, something which normal red P cannot do..

Oxide powders

Copper(II) oxide powder can be relative easy to produce by passing current between a pair of copper rods in a NaCl solution to copper(II) hydroxide, which is filtered from the resulting solution, washed, dried, grounded and then calcinated to CuO. To obtain fine CuO powder, a mortar and pestle or a mill can be used to grind it into a very fine powder.

Fine silica powder can be made by neutralizing water glass (sodium silicate) with an acid, then filter the resulting suspension, which is heated at high temperatures to drive off all the water, leaving behind alumina powder. Alternatively, hydrolysis of silicon tetrachloride will also yield ultra-fine SiO₂ powder.

Aluminium oxide powder can be made by precipitating aluminium hydroxide from an aluminium salt with sodium hydroxide, filtered, then calcinated at high temperatures.

A simpler method involves precipitating a metal salt with aqueous ammonia, then the resulting mixture is calcinated. No filtering is required.

Projects

• Flash powder
• Powder metallurgy
• Sintering
• Make non-newtonian liquids

Handling

Safety

Flammable powders can pose a serious fire and explosion risk.

Storage

Powders should be kept in closed containers, away from draft and fire source.

Disposal

Disposal depends on the type of powder.

See also

• Black powder
• Flash powder

References

Relevant Sciencemadness threads

• Making metal powders
• Making Lithium Powder