Investigation Of Flotation For Separation Of Mixed Waste Plastics ABS And HIPS

The purpose of plastics flotation is to separate plastics with hydrophobic surfaces fromthose with hydrophilic surfaces. Although most plastics are hydrophilic，such as ABS，itssurface could be selectively modified to become hydrophilic by wetting agent. Materialshaving hydrophilic surfaces settle selectively during air bubbling under certain conditions，forexample process parameters and solutions with specific properties. Plastics flotation is analternative method, which shows advantages such as cost-effective and higher separationefficiency, especially for plastics with similar density. In this paper, ABS and HIPS which arediffcult to separate and have high separation value were selected for the breakthrough. In aespecially designed flotation column made by ourselves, effect of process, wetting agent,properties of solution were investigated. Then, the interface interaction between solid phase,liquid phase and gas phase in plastics flotation system were calculated for discuss theadsorption model of bubbles on the surfaces of ABS and HIPS. Combined flotation test andflotation theory, it is available to provide parameters and theory for the development ofplastics flotation from laboratory to industry.Results of optimization on process parameters by flotation experiment determined thesuitable process conditions. When flotation temperature is25℃, the broken flake plasticswhich size close to5mm×5mm, air flow rate is0.1L/min and tannic acid is20mg/L, ABS canbe separated partly from HIPS. As a result, HIPS particles were floated with78.2%purity and83.2%efficiency while ABS particles were obtained with82.1%purity and76.8%efficiency.Through the comparisons of height and diameter,750ml flotation column is more suitable forthe existing flotation system.The surface free energy of plastics and wetting agents was calculated using theLifshitz–van der Waals/acid–base approach after contact angle measurement. On this basis, Hydrophile Index of waste plastic and Wetting Index of agent were calculated. Combiendeffect of tannic acid, calcium lignosulphonate and sodium carboxymethyl cellulose onflotation with its molecular structure, three selection criteria of wetting agent was concludedpreliminarily. First, wetting agents molecule should contain phenolic hydroxyl which is polargroup, especially polyhydroxy phenolic structure. Then, Lewis base conponents of surfaceenergy parameters of wetting agents is considerable large, and therefore wetting agents will bea strong electron donner. At last, a bigger wetting index is considered to be a significant basisof wetting ability.Based on the extended DLVO theory, the conclusion of the plastics particles float or notin the flotation system determine by adsorptive capacity of bubbles on plastics in the presenceof wetting agents was derived by calculate the interface interaction at intrinsic flotation andwetting agent adjustment flotation. The adsorption of bubbles on ABS contain bubbles onABS through hydration film and bubbles on wetting agents through hydration film, whilebubbles on HIPS contain bubbles on plastics through wetting agents eles. Therefore there is astronger force between HIPS and bubbles than ABS, and ABS settled while HIPS floated.The separation of waste ABS and HIPS by flotation in methanol and acetic acid werecompared. On this basis, effect of pH of flotation system on flotation behavior of ABS andHIPS adjusted by acetic acid, sodium hydroxide and hydrochloric acid were investigatedwhile surface tension adjusted by methanol. Then, flotation mechanism was analyzed. Theexperimentation showed that hydrophilic/hydrophobic of ABS and HIPS will be changed bypH and the optimum pH is in acidic range. Different pH regulator has different influence onhydrophilic/hydrophobic of ABS and HIPS. It can be seen that employing methanol to adjustsurface tension can greatly increase the separation efficiency for original flotation separationof ABS and HIPS. Optimum ABS and HIPS separation was achieved with the tannic acidconcentration at200mg/L, the terpineol concentration at20mg/L and methanol volumeconcentration at24%. The purity of ABS and HIPS was up to91.24%and97.73%,respectively; the recovery of ABS and HIPS was up to97.70%and90.60%, respectively.Through the study on flotation mechanism, the pH adjustment flotability of ABS and HIPS belongs to electrostatic adsorption in chemical control flotation, H+plays the role of a bridgebetween tannic acid and the plastics. Moreover, there is a stronger depression on ABS thanHIPS. While surface tension adjustment flotability of ABS and HIPS attributes to gammaflotation as the wetting critical surface tension ofABS is greater than HIPS and ABS settled.