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Dry grinding is the process where the particle size of a substance is reduced without liquid. This process is a common early step in several raw material processing fields and in the production of ethanol. In many cases, dry grinding is a very simple process, often only requiring 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 to break down ores and other hard raw materials for further processing, dry grinding requires material being placed into a grinder 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 set of ways. 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 motivator, the material is crushed by impacts with large metal hammers. If the other material is the primary grinder, then the machine typically is made of a large rolling drum, like a giant clothes dryer. The material rolls around inside and bangs against the walls and other particles until it breaks down to the required size.
From here, the reduced material either goes straight to sorting or to an air-reduction chamber. Air reduction is an important step in some forms of dry grinding. When the particle size is small enough, air will keep the material trapped in suspension. This keeps the material from being sorted or stored. An air reduction chamber pulls the air from the system, but leaves the suspended material behind to settle.
The last step in the dry grinding process is sorting based on size. Material that is too large is returned to the grinding step to go through the machinery again. Material of the proper size goes on to the next step in the refinement process.
Ethanol production also uses dry grinding, but it has a longer process that has many steps in common with wet grinding. After the corn is broken down, often through machine force in a hammer mill, it goes into a storage chamber where it is kept wet. This allows the material to ferment and eventually become ethanol. Left over material that has the proper structure is allowed to dry and is put through the mill again.
In most circumstances, dry grinding is much cheaper than wet grinding. There are both fewer steps and less pieces of machinery in a dry process. Wet grinding does have its advantages, mostly in the production of food and food byproducts.