Study On Separation Of Multicomponent Waste Plastics By Froth Flotation And Functional Mechanisms Of Flotation Reagents

In recent years,China's plastics industry developed rapidly.Large quantities of waste plastics behind huge production and consumption will bring a series of negative environmental problems if not handled properly.Therefore,its beneficial reuse has been proved to be a long-term solution.Currently the primary cause for low efficiency in beneficial reuse of waste plastics has been found no correlation with its recycling methods in the last stage,but rather in the middle stage of the lax separation.As a kind of separation technology based on the floatability differences between fine particles,flotation is expected to make up for inadequacies of existing separation methods.In this paper,multicomponent waste plastics system was selected as the target to be separated,with the aid of modern testing and orthogonal experimental methods,the main influencing factors and optimum separation parameters in the flotation process were studied systematically.The functional mechanisms of selected reagents on three phases(gas-liquid-solid)were discussed in detail,and the results were validated from the viewpoint of interfacial free energy.The results indicated that:(1)The differences of floatability for plastic samples were closely related to the the differences of density first,followed by the differences of hydrophobicity.The influence of flotation time,impeller speed and gas flow rate on single component sample recovery is obvious compared with particle size.The addition of foam reagents(TP,MIBC and PEG)increased the flotation recoveries of other plastic samples except HDPE.The wetting reagents SL and CMC were used together with foam reagent(TP)for the separation of two PP and PET samples,respectively.The wetting reagent TS was used alone for the separation of HDPE samples.The pH and ionic strength were found to have an adverse impact on the anionic wetting reagents SL and CMC,while have less impact on non-ionic TS.(2)The optimal conditions for separation of HDPE were flotation time of 4min,impeller speed of 1000 rpm,air flow rate of 150L/h,TS concentration of 60mg/L and adjustment time of 12 min.TS concentration was the significant influencing factor,the recovery and purity in validation experiments were up to 93.99% and 91.08%,respectively.The optimal conditions for separation of PP were flotation time of 4min,impeller speed of 1000 rpm,air flow rate of 100L/h,TP concentration of 25mg/L,SL concentration of 35mg/L and adjustment time of 10 min.SL concentration was the significant influencing factor,the recovery and purity in validation experiments were up to 57.83% and 92.70%,respectively.The optimal conditions for separation of PET were flotation time of 4min,impeller speed of 2200 rpm,air flow rate of 300L/h,TP concentration of 30mg/L,CMC concentration of 400mg/L and adjustment time of 11 min.Impeller speed was the significant influencing factor,the recovery and purity in validation experiments were up to 52.63% and 81.65%,respectively.(3)When the optimal conditions of HDPE,PP and PET were run in tandem,the HDPE was first collected as foam product with recovery and purity of 94.33% and 91.31%,respectively.And the PP was then collected as foam product with recovery and purity of 50.98% and 88.95%,respectively.In addition,the PET was last collected as sink product with recovery and purity of 49.66% and 72.90%,respectively.The complete separation process was green and cost-effective.(4)The foam reagent PEG was hardly adsorbed on the surface of plastic samples,while the wetting reagent SL was only difficult to be adsorbed on the surface of HDPE.Beside this,the adsorption curves of SL on other six samples got equilibrium at about 10 min and fitted well with the pseudo-first-order kinetic model.Both Langmuir model and Freundlich model could be used to describe the isothermal process of SL adsorption,however,the Langmuir model was more suitable due to smaller values of four error functions,which further indicated that the adsorption configuration of SL was single-layer structure and spontaneous.The thermodynamic parameters showed that the adsorption process of SL has the characteristics of endothermic and entropy increase,combined with the infrared characterization of adsorbed samples further indicated that the hydrogen bond was essence of the adsorption process.(5)It was inferred that the foaming reagents could not be adsorbed on the plastic surfaces as they could not compress the aqueous layer effectively.The interpolymer was formed by the floating foam and plastic particles,combined with the optimization of gas phase resulted in an obvious increase of plastic recovery.The wetting reagents could compress the aqueous layer to rupture by themselves,and then adsorbed on the plastic surfaces through the way of hydrogen bond forming.Due to the presence of polar groups on wetting reagents,the plastic surfaces became more hydrophilic and could not form the interpolymer with the foam,thus,resulted in an inhibited performance of flotation recovery.The above mentioned assumptions have been verified through the calculation of interface free energy,especially for the core contents of aqueous layer.(6)The selection index systems of reagents used in the process of waste plastics flotation were established.The majority of reagents used in this study were evaluated through the index systems,and results were in good agreement with experimental data,which further proved their high reference values in actual production process.