A variation of the Rotary Breaker that is used in the aluminum industry is the Gravity Discharge Autogenous Mill, or GDAM™. These units, which are similar to a heavy-duty Trommel Screen, generate the least fines and dust of crushed material when compared to high-speed Hammermills and air swept mills.
The crushing tumbling action reduces spent broken carbon anodes and pot lining crust to free the cryolite bath and crushable carbon from tramp metal that may be iron, steel or bits of aluminum. Any uncrushed materials and tramp metal are rejected from the reduced bath, which is used as a cover material for newly lined pots that are to be put back into service for the alumina reduction process.
Typically, the product size requirement is smaller than the product from a Rotary Coal Breaker. Thus, the screen plates in a GDAM™ have smaller openings that are outwardly tapered to help alleviate plugging. Given the nature of cyrolite to fracture into small product sizes, however, the aluminum tramp material from which the cryolite is being separated tends to produce thin shards that could pass between the edges of the screen plates. Therefore, to eliminate the risk of unwanted aluminum pieces passing into the product stream, McLanahan has incorporated a closure bar design to fill gaps between the screen plates. The GDAM™ is designed with tapered lifters that allow material to flow in either direction and operates on a timed, forward and reversed batch process.
How GDAMs™ Work
The GDAM™ consists of a large chain-driven rotating drum that is driven by an electric motor, coupling and gear reducer. The chain drive's segmented sprocket is bolted to the cylinder, allowing rotation. The cylinder is equipped with replaceable locomotive steel tires, which ride on a pair of trunnion rollers at each end of the cylinder. Thrust rollers are positioned at each end to prevent excessive axial float. When the cylinder is properly aligned, the drum will float between the thrust rollers, not ride hard to one side or the other.
The GDAM™ operates in a batch process. The cylinder is operated in a retarding non-advancing rotation. As feed material is introduced at the front end of the cylinder, it is continually lifted and dropped onto a series of perforated screen plates. This batching process continues lifting and dropping material until a signal in the control room indicates that most all of the cyrolite has been crushed. At this point, the cylinder rotation is reversed, thus discharging the accumulated refuse. Once the refuse is discharged, the process begins again.
Why McLanahan GDAMs™
McLanahan Corporation built the first Rotary Breaker in 1893. Since then, our design is an evolution of more than 100 years of rotary equipment manufacture and servicing of our customer’s units. An overlapping screen plate design is bolted to the inside of the steel cylinder beams to further strengthen and add rigidity to the cylinder, while the addition of our closure bar design eliminates the possibility of thin slabs of refuse passing through the cylinder and into the product stream.
Designed to be durable and low-maintenance machines, GDAMs™ are both safe and simple to operate. The low operating speed and horsepower requirement make GDAMs™ economical to operate and easy to maintain. When maintenance is required, inspection doors in the fabricated steel and sectionalised dust housing, which covers the cylinder, allow easy access. An optional inching drive offers a safer means of rotating the cylinder for maintenance procedures. Centrally banked lubrication lines allow an operator to grease all bearings from one location.
All critical cylinder surfaces are machined during the manufacturing process in a single large lathe that allows for a concentric drum that runs vibration free and extends the overall life of the equipment. This eliminates any chances of wobble or vibration during operation. Screen plates are offered in non-plugging designs, which are installed in an overlapping fashion to protect the cylinder beams and provide additional structural support. Chute work at the feed and discharge ends ensures a smooth transition of material in and out of the cylinder.