Among the aluminum devices used in grain machinery, those that can separate based on density can be used to separate impurities that are similar in size and shape but have different weights, such as common metal lumps, mud clumps, stones, ceramics, and coal dust, etc. There are two main methods of separation: wet method and dry method. In the wet method, a typical aluminum device for grain machinery is the wheat washing and drying machine. 

It consists of some relatively heavy impurities such as those from the spiral washing tank and the vertical spin-drying section. The impurities settle relatively quickly in the washing tank and tend to accumulate at the bottom of the tank. Then, they are discharged outside through the small spiral conveyor. Meanwhile, the wheat grains are suspended to the drying part through the large spiral conveyor to remove the moisture on their surface. In the steel equipment, there are a weight classification platform and a weight stone removal machine. At that time, the internal part is evenly tilted on a reciprocating moving perforated plate. Then, with the help of a controllable airflow, the grains become fluidized and enter a semi-suspended state. The grains flow downward through the air flow and are discharged at the lower end. Some heavier mud clumps and stones move upwards along the perforated plate during the reciprocating vibration process and are discharged through the outlet. 
There are two types of methods for supplying gasoline: the suction type and the blowing type. The working principle and structure of the specific gravity classification tower are similar to those of the specific gravity stone remover. They both first separate the grain impurities into two parts according to their weight, then clean them separately, and then reduce the number of cleaning equipment or replace them with smaller equipment. The usage method of the equipment can save water more effectively, prevent wastewater from polluting the environment, and its maintenance method is also very convenient. Therefore, it gradually replaced the wet-type equipment. 
If classified according to the shape of the aluminum devices used in grain machinery, the separated impurities, the materials and the grains are all relatively similar, but their shapes are often different. In this equipment, there is a spiral separator. The spiral separator has no moving parts and does not require a power device. It also contains a large outer spiral housing with several smaller inner spiral surfaces. When the grain is fed into its inlet from the top, the round seeds roll fast and are thrown out of the device under the centrifugal force and are discharged from the outlet. The grain particles flow along each spiral surface to the outlet. The production efficiency of this equipment is relatively low. Currently, it can only be used as an auxiliary machine. The drum selection machine has its working part with a slightly inclined rotating drum. Its inner surface contains multiple grooves and a position that can be adjusted for collection. Any particles remaining inside will be lifted along with the rotation.