## 1 Introduction  
### Background  
In view of the prominent problems of environmental protection, economy, resources, landfill site and safety, it is urgent to recycle solid waste with high quality, high proportion and high efficiency, especially for those such as scrapped vehicles, waste household appliances, waste electronic products, waste building materials and domestic waste. Consequently, whether in developed or developing countries, solid waste recycling is becoming a common concern. As an important step of pretreatment in the process of solid waste recovery, sorting is of great significance to energy conservation, environmental impact, secondary resource market balance, and the improvement of efficiency and quality. As far as present sorting is concerned, there are manual sorting and automatic sorting methods. Manual sorting still exists in developing countries with low labor costs. However, with the gradual increase in labor cost and concerns for health, automatic sorting is becoming popular [1, 2, 3, 4, 5, 6]. Automated waste sorting technology can be classified into two types: direct sorting and indirect sorting [1]. Direct sorting includes: air separation, the dense medium method, eddy current separation (ECS), magnetic separation, the electrostatic method, magnetic levitation, etc. [7, 8, 9, 10, 11, 2] These methods rely on the different physical properties of the materials and directly separate solid waste by buoyancy, electromagnetic force, electric field force, etc. Indirect sorting involves two steps, the first step is identification, dependent on various sensors such as color or grayscale machine vision, X-ray transmission (XRT), X-ray fluorescence (XRF), laser-induced breakdown spectroscopy (LIBS), electromagnetic sensor (EMS), spectroscopy, etc. [4; 13; 14; 15; 16]. The second step is separation based on the identification results. There is another classification with respect to sorting, that is dry sorting and wet sorting [1]. Wet sorting includes the medium density method, the chemical extraction method, etc. Wet sorting usually consumes much liquid and its media. What is more troublesome is that the amount of liquid mixture from wet sorting is difficult to handle and causes secondary pollution. Compared with wet sorting, dry sorting is more environmentally friendly. No matter whether direct or indirect, dry or wet sorting, the physical properties mostly used include density, atomic number, conductivity, permeability, morphology, mechanical, thermal, and optical properties, etc. With respect to scrapped vehicles and household appliances, a series of dry sorting methods can be used and the general sorting process is as follows [3; 6; 15; 17; 18]: (i) disassemble reusable parts by hand and remove lubricating oil, fuel oil, batteries and other liquids or dangerous goods from them. (ii) Shred by machine and obtain shredded solid mixtures. Light floats such as plastic foam, plastic film, etc., can be separated from the mixtures by winnowing. After the removal of dust and fine particles, most of the remaining mixtures are independently bulk (generally they are not covered or adhered to each other) and sometimes graded by screen size. (iii) Sort out ferromagnetic materials from these bulk mixtures by magnetic separation. (iv) The remaining bulk mixtures are sorted by ECS to obtain non-ferrous metal mixtures and non-metallic mixtures, which can be further sorted by one or more indirect methods based on sensors. The finer the classification is, the better the recovery quality. Among indirect sorting methods, color-sorting is the most widely used [4]. This method can identify materials by obtained material characteristics such as color, texture, shape, spatial relationship, etc. However, the method has its weakness in that it is vulnerable to surface pollution and light source changes. Likewise, when it comes to materials with similar apparent color, such as SS (stainless steel), Al (aluminum) and its alloy, Mg (magnesium) and its alloy, etc., the sorting for them becomes more difficult.