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Dry milling is a process that reduces particle size without liquid and is used in feedstock processing and ethanol production. It involves placing material in a millstone or grate mill where it is pulverized by physical force. The reduced material is then sorted by size, with material that is too large being returned to the grinding stage. Dry grinding is generally cheaper than wet grinding but has limitations. Ethanol production also uses dry milling, but with a longer process that involves fermentation. Wet grinding is advantageous in food production.
Dry milling is the process where the particle size of a substance is reduced without liquid. This process is a common initial step in several fields of feedstock processing and ethanol production. In many cases, dry grinding is a very simple process, often requiring only a few steps. Dry grinding is generally much less expensive than wet grinding, the other popular grinding method, but it doesn’t work as well for some tasks.
A common way of breaking down ores and other hard raw materials for further processing, dry milling requires that the material be placed in a millstone or grate mill for reduction where it is pulverized by physical force. There are specialized machines used for different substances, but they all work in a similar way. Depending on the type of machine used, the primary force may come from the machine or from the other material: when the machine is the engine, the material is crushed by impacts with large metal hammers. If the other material is the primary grinder, the machine typically consists of a large rotating drum, like a giant dryer. The material rolls in and slams against walls and other particles until it breaks to the required size.
From here, the reduced material goes directly to sorting or an air reduction chamber. Air reduction is an important step in some forms of dry milling. When the particle size is small enough, the air will keep the trapped material in suspension. This prevents the material from being sorted or stored. An air reducing chamber draws air out of the system, but allows suspended material to settle.
The last stage of the dry milling process is sorting by size. Material that is too large is returned to the grinding stage to pass through the machinery again. The correct size material moves on to the next stage of the refinement process.
Ethanol production also uses dry milling, but has a longer process that has many steps in common with wet milling. After the corn is broken down, often through machine force in a hammer mill, it enters a storage chamber where it is kept moist. This allows the material to ferment and eventually become ethanol. The leftover material that has the right structure is left to dry and re-milled.
In most cases, dry grinding is much cheaper than wet grinding. There are fewer steps and less machinery in a dry process. Wet grinding has its advantages, especially in the production of food and food by-products.
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