Jet milling is a process of using highly compressed air or other gasses, usually in a vortex motion, to impact fine particles against each other in a chamber. This gradually reduces them in size, resulting in powders that have particle size dimensions as small as one micron across, or 50-100 times smaller than the width of a human hair. Fluid energy milling is another common term for the process, and it is often employed in the propellant and explosives creation industries, with some systems able to grind rocket propellant fuel down to a two micron particle size at a rate of 500 pounds (227 kilograms) per hour.
Milling machines that employ jet milling processes are also widely used to create polymer powders for the plastics industry, adhesives, cosmetics, paint and ink compounds, pharmaceuticals, and more. Due to the fine nature of size control and variety of output products produced, jet milling equipment can be expensive. Commercial units sold in India can be set to produce anywhere from 22 pounds (10 kilograms) to 1,587 pounds (720 kilograms) per hour.
The basic function of a jet milling machine is rather easy to understand. A feed funnel is used to pour unprocessed material into a compressed intake gas tube that channels it into a cylindrical grinding chamber. The air or compressed gas in the chamber is accelerated into a vortex, with centrifugal force driving smaller particles to the center as impacts occur, and larger ones to the perimeter. As the particles reach a chosen size, they are removed from the center of the vortex through an exit pipe.
The benefits of jet milling are broad, covering advanced materials processing across a range of industries from metallurgy to synthetic chemicals and food processing. There are, however, also challenges to be overcome in the control of particle size and energy efficiency of jet milling machines. Output particle size can affect everything from how a chocolate tastes that contains jet milling products, to drug absorption, toner quality for printing, the durability of tools with ceramic edges, and more.
Milling materials by jet milling continues to grow in industry, however, as it is the primary method of producing materials with particle sizes ranging from 1-10 microns. It also offers advantages over ball or hammer mills in that it results in zero product contamination, and the heating effect of milled products in typical ball and hammer mills is avoided. The Joules Thompson cooling effect of air exiting a jet mill keeps the product at around 200° Fahrenheit (93° Celsius), which is the same temperature as that of the interior grinding chamber.