Physico-Chemical Separation Of Recyclable Resources Interface Interactions In Plastics Flotation System

Natural resources can be divided into three types of immutable, recyclable and non-recyclable resources, in which recyclable resources means reusable resources, including land, forest, water, atmosphere and industrial solid waste resources etc. Recycled materials are part from recyclable resources, which can be recycled as materials, and then exclude resources such as land, water and atmosphere, moreover, some disposal methods like burn, dunghill and landfill can not be classified to the category of recyclable materials.Recycling of materials found in recyclable resources, based on the difference of physico-chemical properties of various useful components in recyclable resources, the effective separation process using physical or chemical methods is necessary, this process called Physico-Chemical Separation of Recyclable Resources. The research emphases include physico-chemical properties of recyclable resources and physico-chemical principles of every separation method. This thesis study on plastics flotation, involves physico-chemical separation of plastic wastes.Plastics flotation is a new research system compared to the traditional minerals flotation. The thesis deals with the application of flotation technology in the plastics separation, using methods of the surface preparation, surface energy detection and the calculation of the interface interaction free energy. From the electrostatic force, the Lifshitz-van der Waals and the Lewis acid-base interaction force (hydrophobic attraction and hydrate repulsion), hydrogen bond aspects, the thesis analyses systematically one of the key problems in plastics flotation - "The interface interaction in plastics flotation system", including the interaction between low-energy surface and water, low-energy surface and the flotation reagents, the interaction among plastic particles, the interaction between plastic particles and bubbles. Using the free energy of interactions and their potential curves, we can study three kinds of flotation or separation behavior of plastics, including intrinsic floatability, frother adjustment floatability and wetting agent adjustment floatability. Based on the above information, we can develop the theory of plastics flotation which with solid/liquid/gas three phase interface interaction for core, include wetting and adsorption behavior of low energy surface, aggregation and dispersion behavior between plastic particles, collision and conglutination behavior between plastic particles and bubbles, etc. This theory can be used for the selection or design of plastics flotation reagents, and help to solve the technical problems dealing with the waste plastics separation in complicated system. It is available to the development of plastics flotation from laboratory to industry.Via the research about the wetting behavior of low-energy surface and the intrinsic floatability of plastics, the thesis obtained the important parameter which can determine whether the particles can float or not in the flotation system - Hydrophile Index, the necessary condition of the flotation of solid particle is the Hydrophile Index less than 5. After systematically analyzed the micro-mechanics factors in the form of hydration layer on the particles surface in the aqueous medium and the form of the hydration film under the circumstance of the existence of bubbles, it obtained the relationship between the factors such as the Hydration Film Thickness (H₀, nm), the Adhesion Energy (G_ad, mJ/m²), the particle size and surface character. It examines the influence of the existence of additive to the wetting character and flotation behavior and obtained the quantification relationship between various factors. The intrinsic floatability of plastics is that under the circumstance of not adding every flotation reagent, the flotation only under the influences of bubbles. Using the two factors of H₀ and G_ad between bubbles and particle surface to investigate the intrinsic floatability, we can gain the graph of F-H₀-G_ad, in which the F is the floatability.In aqueous medium, weak hydrophilic substances (such as the frother) are repulsive to the high-energy surface or the high-polarity low-energy surface, in contrast, it has high appetency with the low-energy surface which has the relatively low surface energy. After compared the magnitude of the Lewis acid-base interaction free energy between low-energy surface and water, low-energy surface and less hydrophilic substances, we can divide the low-energy surface into three sorts. After systematically research, we can obtain the adhesion model of "hydration liquid film" of frother molecule on the low-energy surface. Under the effect of hydration liquid film, the polar group of frother molecule can be localized in the liquid film, the non-polar group has the function of connecting low-energy surface and the bubbles, the hydration liquid film is the intermediate phase of the solid/gas unity. The frother adjustment floatability of plastics is formed on the base of intrinsic floatability, the addition of frother increases the floatability of the first kind low-energy plastic particles, reduces the floatability of the third kind low-energy plastic particles. Different kinds of frother have great influences on the floatability of the second kind low-energy plastic particles.Four kinds of wetting agent of lignin sulfonate, tannic acid, methylcellulose and Triton X-100 are studied. After systematically research, we can confirm the adsorption mechanism of the wetting agent on the low-energy surface. The adhesion between wetting agent molecule and polymer material surface is accomplished by the hydration film in the form of intermediate phase. Wetting index of the agent and the intensity of Lewis A-B interaction free energy between wetting agent and water are the important characteristic of determining wetting capacity. The sum of the magnitude of Lewis A-B interaction free energy between wetting agent and water and Lewis A-B interaction free energy between polymer materials and water are the index of determining the intensity of physical adsorption of wetting agent. The same wetting agent will have firmer physical adsorption on the material surface of high hydrophile index.Under the effect of bubbles, wetting agent will have desorption from the surface, the lower is the hydrophile index of solid surface, the higher is the tendency of the desorption. The existence of additive will not only improve the physical adhesion of wetting agent molecule on the solid surface, but also the chemical adhesion between wetting agent and additive will determine that some polymer materials which with additive will have easier tendency to be depressed by the wetting agent. The wetting agent adjustment floatability of plastics is that under the combined actions of wetting agent and frother (sometimes there is no frother), the floatability of the plastic particles will have. Under the action of wetting agent, the selectivity of plastics flotation is the physico-chemical progress of wetting agent on the polymer materials surface.The flotation separation of plastics associates with the interaction among particles. In aqueous medium, the van der Waals interaction and the Lewis acid-base interaction among polymer materials particles are all attraction, and the hydrophobic attraction has the key influences. In flotation progress, after the plastic particles adding into the aqueous medium, no matter they are the same particles or the different particles, the results of attraction effect will tend to have all the particles to aggregation. After the addition of wetting agent, the interaction pattern among particles will have significant changes and the attraction will gradually disappear and then transform to more powerful repulsion. In the air flotation, the hydration film on polymer materials surface which has relatively low hydrophile index will being influenced, the wetting agent will have desorption from particle surface under the effect of bubbles, the floatability of particles and the attraction among particles will be recovered and the particles will float with bubbles in the form of aggregation. The wetting agent which is adhesive to the surface of relatively high hydrophile index will not be influenced, the repulsion will remain among particles.The experimental results of plastics flotation show that, If we could use the difference of floatability of plastic particles in different flotation system, under the base of surface layer flotation, intrinsic floatability flotation, frother adjustment floatability flotation and wetting agent adjustment floatability flotation, we could have satisfactory separation results in the flotation of multi-components plastic wastes.