Mill
Introduction
A mill is a device that breaks solid materials into smaller pieces by grinding, crushing, or cutting. Such comminution is an important unit operation in many processes. There are many different types of mills and many types of materials processed in them. Historically mills were powered by hand (e.g., via a hand crank), working animal (e.g., horse mill), wind (wind) or water (water).
History
The general idea behind the mill is an ancient one, dating back to at least 70,000 BC. A saddle quern was a simple device, generally a flat stone base and a rounded stone to be rolled on it, used for grinding grain in the prehistoric era. The efficiency of these early developments was limited, and it wasn't until the industrial revolution and the invention of steam power that an effective ball milling machine could be built. It is reported that the old method of producing milling stones was in use in flour mills as late as the 20th century.
Types of grinding mills
Windmill, watermill, horse mill
Windmills, watermills and horse mills were used for grinding grains and other tough materials. Early mills were almost always powered by water or wind. The majority of modern windmills take the form of wind turbines used to generate electricity, while watermills are usually used for grinding grains into flour.
Roller mill
A roller mill uses cylindrical rollers that crush and grind the grain when it goes through them. The distance between the rollers can adjusted based on the size of the grain. Roller mills use the process of stress (which is applied by the rotating wheels) and attrition in milling of solids in suspensions, pastes or ointments, and some solid materials. The rollers rotate at different speeds and the material is sheared as it passes through the gap.
Ball mill
A typical type of fine grinder is the ball mill. A slightly inclined or horizontal rotating cylinder is partially filled with balls, usually stone or metal, which grind material to the necessary fineness by friction and impact with the tumbling balls. Ball mills normally operate with an approximate ball charge of 30%.
Hammer mill
A hammer mill is a mill whose purpose is to shred or crush aggregate material into smaller pieces by the repeated blows of little hammers. These machines have many sorts of applications in many industries, including ethanol plants (grains), farm machine, which mills grain into coarse flour to be fed to livestock, fluff pulp defiberizing, fruit juice production, grinding used shipping pallets for mulch, milling grain, sawmills, size reduction of trim scrap and planer shavings into boiler fuel or mulch.
Jet mill
A jet mill grinds materials by using a high speed jet of compressed air or inert gas to impact particles into each other. Jet mills can be designed to output particles below a certain size, while continue milling particles above that size, resulting in a narrow size distribution of the resulting product.
Applications
Mills are used in a wide variety of applications, both for processing raw materials and for producing finished goods. They are a key piece of equipment in many industrial processes, including the production of cement, the mining industry, the food industry, and the pharmaceutical industry.