## 2 Materials and Methods
### Emulsification-Separation of RAP
#### 2.2.2 RAP Emulsification-Separation Experiments  
The emulsification-separation equipment used in this study was a mixing device modified from a drilling rig, where the drill bit of an ordinary drilling machine was replaced with a dispersion disk. For the emulsification-separation experimental process, the pretreated RAP was first preheated in an oven for about 30 min, during which time the emulsion was prepared. A certain amount of surfactant was weighed, poured into the tap water, stirred, and dispersed. After the RAP was preheated, it was gradually scooped into the container and stirred to disperse. During this process, the aggregates were placed near the container wall and were flipped and stirred every 10 min or so to sufficiently mix them. The asphalt foam and mineral foam in the upper layer of the emulsion surface were collected to prevent them from adhering again to the RAP. At the conclusion of the experiment, the aggregate was separated from the emulsion by sieving. After standing for 30 min, the emulsion was separated from the silt to obtain the recoverable residual emulsion. A flowchart of the emulsification-separation process is illustrated in Figure 3.  
Figure 1: Appearance and molecular structure formulas of surfactants.  
The main function of the surfactants is to soften the asphalt. If their oil removal rate is the highest under identical conditions, their dispersing effect is the best, the agglomeration is easily dispersed, and their grading passing rate is the highest. In preliminary experiments, we chose 1800 RPM, 80 \({}^{\circ}\)C, and a 60 min rotation time; three different anionic surfactants, SDBS, SDS, and MES, were selected to study the effect of different concentrations (0-6 wt%) on the asphalt content. We then changed the emulsion/RAP to obtain the ratio with the highest oil removal efficiency.  
The effects of different process parameters on RAP clusters varied, with speed (i.e., 280, 700, and 1800 RPM), temperature (i.e., 60, 80, and 90 \({}^{\circ}\)C), and time (i.e., 30, 60, and 90 min) all affecting the dispersion efficiency of agglomeration. Therefore, it was impossible to consider only changes in the asphalt content. The fineness modulus ratio (FMR) and degree of agglomeration (PL) were used as evaluation criteria to determine the best process parameters for lotion separation. The dispersion effect of RAP under the three process parameters was investigated using a three-factor, three-level orthogonal experiment, and the gray correlation method was used to determine the influence of the different process parameters on the dispersion effect. The three factors and three levels of the orthogonal experiment are shown in Table 2.  
Figure 3: Flowchart of RAP emulsification–separation.  
Figure 2: Pretreatment processing of the RAP material.  
Owing to the significant influence of shear and collision on the dispersion of RAP particles of different sizes during mixing, this effect was refined after determining the relevant experimental parameters. Different sizes (13.2-16, 9.5-13.2, 4.75-9.5, and 2.36-4.75 mm) of pretreated RAP were selected to adjust the dispersing effect. First, RAPs of different sizes were handled separately; then, the sizes were adjusted, and RAPs of different sizes were gradually mixed in a 1:1 proportion to achieve the best size range for the dispersing effect.