Separation of plastics by flotation

Sammanfattning: This thesis analyses and summarises the importance of plastics waste recycling and plastics waste separation, presents an innovative principle for plastics flotation, and provides various methods for the separation of plastics mixtures. The objective is to understand the mechanisms for selective flotation separation of plastics, get plastics mixtures separated and enable more plastics wastes to be recycled. From image and sieve analysis, it is shown that plastics particles in the cutting products can be classified by a specific size distribution and a shape parameter. The density measurement shows that the densities of seven plastics change from 1.039 to 1.427 g/cm3 with a density order PVC>POM>PET>PC>PMMA>ABS>PS. But some plastics are quite similar in density, such as POM nearly equal to PVC, PMMA to PC, and ABS to PS. Based on the research on flotation behaviours of plastics, a theoretical equation was derived in the present study. It establishes the relation of floatability of plastics with surface chemical-related factors and gravity factors. From the equation, it can be seen that plastics flotation is dominated not only by surface chemical-related factors, but also by gravity factors. In fact, it is a combination of froth flotation and gravity separation. According to the equation, particle size control idea was applied for separation of plastics mixture. The results show that this method can greatly improve the separation efficiency for plastics flotation. From the flotation separation of plastics by using an alkyl ethoxylated nonionic surfactant (S-15-7), it is found that the Gamma flotation method not only can be used to separate plastics mixtures with different densities, such as the separation of POM and PVC from PC, POM and PVC from PS and ABS, PET and PMMA from PS and ABS, but also can be used to separate plastics mixture with similar densities, such as the separation of PMMA from PC. Products with grade higher than 99% and recovery higher than 97% can be obtained for the separation of some plastics mixtures. According to the investigation on surface chemical factors (wettability of the plastics and liquid surface tension) and gravity factors (particle density and shape parameter), it is shown that the depressing effect of the surfactant on the plastics is mainly a result of the reduced liquid surface tension, and the flotation selectivity for the plastics with identical particle size is not only dominated by wettability of plastics, but also by particle density and particle shape. Finally, methyl cellulose (MC) was used for separation of plastics. The results show that seven plastics (POM, PVC, PET, PMMA, PC, PS and ABS) can be separated into three groups. Group one includes POM and PVC. They are depressed at very low MC concentrations. Group two, including PET, PMMA and PC, has an intermediate floatability. Group three (ABS and PS) has a high floatability. They are almost not depressed within the given MC concentration range. A similar investigation on surface chemical factors (wettability of the plastics and liquid surface tension) and gravity factors (particle density and shape) was carried out. It is suggested that the depressing effect of MC on plastics is ascribed mainly to its adsorption on plastics surface. The MC molecules absorbed on plastics expose some of their polar groups oriented towards the aqueous phase, hence making the plastics surface hydrophilic. van-der-Waals force interactions are the main interactions between MC molecules and plastics surfaces. The flotation selectivity in this process is also influenced by wettability of the plastics, particle density and shape provided the particle size is identical.