Separation of solid wastes for recycling and utilization

Abstract: With depletion of the earth´s natural resources and growth in solid wastes, increasing emphasis must be placed on recycling. Recycling and utilization of solid wastes has become an active subject. The present thesis was focused on this area. Two different types of solid wastes, namely plastics waste and stainless steel slag, were studied for recycling and utilization. The thesis consists of two parts. In the first part, separation of plastics by flotation was presented. The characteristics and flotation behaviours of seven plastics (PS, ABS, PET, PVC, POM, PC and PMMA) were investigated. On this basis, a theoretical equation was derived and obtained for the first time 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 in separation of plastics mixture. The separation results showed that this method can greatly improve the separation efficiency for plastics flotation. Finally, selective flotation separation of plastics by using an alkyl ethoxylated nonionic surfactant (S-15-7) and a wetting agent methyl cellulose (MC) was studied respectively. In the flotation with S-15-7, it was 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 purity higher than 99% and recovery higher than 97% can be obtained for the separation of some plastics mixtures. In the flotation with MC, the results showed that seven plastics 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. According to the investigation on surface chemical factors (wettability of the plastics and liquid surface tension) and gravity factors (particle size, density and shape parameter), it was shown that the depressing effect of the surfactant on the plastics is mainly a result of the reduced liquid surface tension, while the depressing effect of MC on plastics is ascribed mainly to its adsorption on plastics surface. In both cases, the flotation selectivity for the plastics is not only dominated by wettability of plastics, but also by particle size, density and particle shape. In the second part of this thesis, recovery of metals from stainless steel slag was described. The tests were conducted on two types of slag, namely EAF slag (Electric Arc Furnace) and AOD slag (Argon Oxygen Decarburisation), which were collected from the tailings of a slag processing plant operated by Bergslagens Stålservice AB in Sweden. Firstly, physicochemical and mineralogical properties of stainless steel slags (EAF and AOD) oriented to metal recovery were studied. It was shown that although the coarse alloy and metal oxide particles had been separated and recovered in the processing plant at Bergslagen in Sweden, the EAF and AOD samples (tailings) still contain a certain amount of Cr and Ni. Particularly in AOD, the Ni content is as high as 0.2%. It is desirable to further recover Cr and Ni from the tailings to enhance the total recovery of Ni and Cr for the existing processing plant. Then, investigations were placed on gravity separation and magnetic separation of the slags, including the fundamental and separation mechanisms, comparison of different gravity separation methods and magnetic separation methods, optimization of treating process and appraisal of the separation processes. From the gravity separation of stainless steel slag with an optimized gravity-based flowsheet, i.e Jones magnetic separator- Grinding-Shaking table, it was shown that a concentrate containing 11.1% Cr and 5.49% Ni can be obtained from AOD with 2.37% yield, and with 16.51% Cr and 61.74% Ni recovery. For EAF, the concentrate contains 7.07% Cr and 2.10% Ni with 2.28% yield, and with 4.92% Cr and 53.17% Ni recovery. Better separation results can be achieved if using an optimized magnetic flowsheet, namely Jones magnetic separator (or medium intensity magnetic separation device)-Grinding-Drum magnetic separator. A concentrate containing 12.74% Cr and 6.09% Ni can be obtained from AOD with 2.01% yield, and with 15.55% Cr and 62.62% Ni recovery. For EAF, the concentrate contains 11.36% Cr and 4.64% Ni with 1.13% yield, and with 3.95% Cr and 58.19% Ni recovery. Other advantages of the magnetic separation flowsheet over the gravity-based separation flowsheet are that wet drum magnetic machine is easy to operate and occupies smaller area than shaking table for the same throughput.