# A Review on the Recovery and Separation of Gallium and Indium from Waste
## 3 Methods of Recovering Gallium and Indium from Secondary Sources
### Waste Pretreatment
#### 3.1.2 Mechanical and Physical Processing  
Mechanical treatment mainly involves various types of crushing and grinding processes, the aim of which is to homogenize the mixture and obtain grains of the appropriate size. WEEE is crushed, most often while using shredders and hammer mills. The degree of fragmentation that must be achieved depends strictly on the future fate of the material: hydrometallurgical processes require fine grains (<200 \(\upmu\)m), while pyrometallurgical processes also work well when the substrate is relatively coarse-grained [21]. At this stage, again, there may be many differences between the methods, even if they concern the same electronic items. An example could again be LCD panels. After the previously described preliminary processing, the obtained material is ground in ball mills, because the research has shown that the small grain size has a positive effect on the efficiency of the acid mixture washing process (grains below 5 mm allow for washing of up to 92% of indium). As already mentioned, crushing and grinding always involve some losses, which is why solutions are sought to minimize them. The HEBM method, which requires precise removal of the polarizing foil and liquid crystal, uses a high-energy ball mill. The obtained grains have a size of approximately 1 \(\upmu\)m. The grinding time is about a minute, which is much less than in the case of classic grinding, which usually takes up to 6 h. In addition to saving time, this method also provides increased efficiency: during a test, it allowed 86% of indium to be washed from the material, while traditionally ground material subjected to the same solution released only 76.4% of the indium contained in it [23]. The fragmentation of the material also allows it to be subjected to a physical separation aimed at enriching it with the ingredients (of course, if it is predicted in a specific method). These separations are eagerly used due to their simplicity and low costs, and their effectiveness is often closely related to the level of material fragmentation. They use differences in various properties of the substances included in each mixture, such as relative density, conductivity, magnetic susceptibility, wettability, and brittleness, to separate them [25]. The five most often used operations are the following: flotation, magnetic separation, electrostatic separation, eddy current separation, and gravity separation. Magnetic separation, which is usually performed first, separates the ferrous metals from the rest of the material using magnetic drum separators. The lightest components of the mixture, such as plastic, can be separated in air classifiers. Their operation is based on a simple principle: air is passed into a chamber of the apparatus from the bottom, into which the shredded material is also introduced. The air lifts the lightest ingredients, and the heavier grains sink lower. Nonferrous metals are separated by considering their conductivity-to-density ratio. Metals with a high value of this ratio (e.g., aluminum, copper, and zinc) are separated via eddy current. Other nonferrous metals are separated electrostatically or using permanent magnets [21; 25; 26]. Gravity separation can have various technical solutions, but the main feature used in the process is always the difference between relative densities of the components. It can be performed, for example, on shaking tables or through separation in a heavy medium, which involves the use of a liquid of appropriately selected density for separation, in which some of the ingredients sink to the bottom and some come up to the surface [27; 21]. Floration takes advantage of the differences in wettability. It is conducted in a tank called a floating machine filled with liquid (usually water). Air is passed through the system and the flotation reagents are added to facilitate the flotation process. Hydrophobic ingredients are surrounded by gas bubbles and brought to the surface, where they form a mineral foam.