Separation of plastics: The importance of kinetics knowledge in the evaluation of froth flotation

Separation of plastic wastes using froth flotation - An overview

Despite various initiatives and efforts, plastic solid waste (PSW) has become a major global problem due to decades of relentless use of plastics. Since non-biodegradable plastics can persist in the environment for hundreds of years, threatening animal and human life, discarding them into the environment is not a viable option. Plastic recycling is a critical research area that requires urgent attention since less than 10% of the seven billion tons of globally generated plastic waste has been recycled so far. With recent technological developments, it is now possible to recycle many types of PSW using a variety of methods. This review provides an overview of the froth flotation technology that is currently being researched for PSW recycling. Fundamental working principles, the current state of the development, and limitations of this technique are reviewed. It is suggested that froth flotation with continuous development has tremendous potential to result in a more efficient and environmentally friendly approach to PSW recycling.

Effects of Hydration on the Adsorption of Benzohydroxamic Acid on the Lead-Ion-Activated Cassiterite Surface: A DFT Study

The strategy of enhancing the surface activity by preadsorption of metal ions (surface activation) is an effective way to promote the adsorption of surfactant on surfaces, which is very important in surface process engineering. However, the adsorption mechanism of surfactant (collector) on the surface preadsorbed by metal ions in the explicit solution phase is still poorly understood. Herein, the effects of hydration on the adsorption of benzohydroxamic acid (BHA) onto the oxide mineral surface before and after lead-ion activation are investigated by first-principles calculations, owing to its importance in the field of flotation. The results show that the direct adsorption of BHA on the hydrated surface is not thermodynamically allowed in the absence of metal ions. However, the adsorption of BHA onto the lead-ion-activated surface possesses a very low barrier and a very negative reaction energy difference, indicating that the adsorption of BHA on hydrated Pb²⁺ at cassiterite surface is very favorable in both thermodynamics and kinetics. In addition, the adsorption of BHA results in the dehydration of hydrated Pb²⁺. More interestingly, the surface hydroxyl groups could participate in and may promote the coordination adsorption through proton transfer. This work sheds some new lights on understanding the roles of interfacial water and the mechanisms of metal-ion surface activation.

The Differences in Evaluation of Flotation Kinetics of Talc Ore Using Statistical Analysis and Response Surface Methodology

This paper investigates the effect of sodium hexametaphosphate (SHMP) depressant/dispersant in the presence of methyl isobutyl carbinol (MIBC) frother and soluble starch (SS) depressant on the flotation kinetics of talc ore. Emphasis is on a comparison between the evaluation of a custom design of experiment (DoE) using the multilinear regression analysis (MRA) and response surface methodology (RSM) approach. Although analysis of variance (ANOVA) is a good first step in the evaluation of the effect of factors on froth flotation processing, it nonetheless only reveals the effects that are the same under all conditions. In the case of SHMP, its effect on separation efficiency is positive; however, if it is used along with SS, the effect is negative. Moreover, if a higher frother dosage is used, the effect of SHMP on separation efficiency is negligible.

Flotation dynamics of metal and non-metal components in waste printed circuit boards

Flotation is an effective and clean separation technology to realize the recovery of metal in waste printed circuit boards (WPCBs). The flotation kinetic of metal and non-metal components was concerned in this study. In addition, the loading of bubbles, the collision and shedding of particles and bubbles were used to assist in proving the particle dynamics results. By analyzing the force on the particles, the load of bubbles on particles was analyzed, and the appropriate volume ratio of bubbles to particles was 1.5-8.0, depending on the particle density. Moreover, Clift model and Schiller-Naumann model has high fitting accuracy for the final bubble velocity. In addition, metal particles have greater settling velocity, which results in shorter collision time with bubbles. In the process of bubble-particle rising, the shedding probability gradually decreases, and the shedding probability of metal particles is much higher than that of non-metal particles. The results of flotation kinetics show that the removal of impurity particles represented by silicon mainly occurs in the initial stage of flotation process. The loss of copper increases with flotation time and collector dosage. This study reveals the flotation kinetics of particles from the perspectives of bubble loading, bubble-particle collision and shedding.

Combination of sodium hypochlorite pretreatment and flotation towards separation of polycarbonate from waste plastic mixtures

This study developed a novel method, surface pretreatment using sodium hypochlorite along with flotation, to facilitate separation of waste polycarbonate from plastic mixtures for recycling. Surface pretreatment was observed that has an obviously negative effect on the floating ratio of polycarbonate and the floating ratio of poly-methyl-methacrylate, polystyrene, and polyvinylchloride was not affected in flotation, and this difference in floating ratio can be expected to separate polycarbonate from plastic mixtures. The optimum conditions obtained included sodium hypochlorite concentration of 0.05 M, pretreatment temperature of 70.0 °C, pretreatment time of 60.0 min, frother dosage of 10.8 mg/L, and flotation time of 4.0 min. Under optimum conditions, polycarbonate was separated effectively from multiple plastic mixtures, and the purity and recovery were 99.8% and 100.0%, respectively. The major mechanism of surface pretreatment was ascertained by the aid of Fourier transform infrared, scanning electron microscope, energy dispersive spectrometer, and X-ray photoelectron spectroscopy, and the hydrophilic groups, pitting, and protuberances introduced on polycarbonate surface caused the reduced floating ratio of polycarbonate. Accordingly, this method can be expected to improve the recycling quality of waste plastics, and provides technological insights in the environmentally friendly disposal of waste plastics.

MIR spectral characterization of plastic to enable discrimination in an industrial recycling context: I. Specific case of styrenic polymers

One of the major limitations in polymer recycling is their sorting as they are collected in mixes. The majority of polymers are highly incompatible without compatibilizers. For sorting of polymers, high-speed online Near-Infrared (NIR) spectroscopy is nowadays relatively widespread. It is however limited by the use of carbon black as a pigment and UV-stabilizer, which strongly absorbs near-infrared signals. Mid-Infrared (MIR) hyperspectral cameras were recently put on the market. However, their wavelength ranges are smaller and their resolutions are poorer, in comparison with laboratory equipment based on Fourier-Transform Infrared (FTIR). The identification of specific signals of end-of-life polymers for recycling purposes is becoming an important stake since they are very diverse, highly formulated, and more and more used in copolymers and blends, leading to complex waste stocks mainly as WEEE (Waste Electrical and Electronic Equipment). Dark colored plastics are the major part of WEEE, which also contains mainly styrenics (ABS, HIPS and their blends). In addition, styrenics are especially concerned by the need of identification. In this framework, spectral characterizations of ten types of polymers were scrutinized through about eighty pristine and real waste samples. Polymer characteristic signals were aggregated in charts to help rapid and automatized distinction through specific signals, even in limited resolution and frequency ranges.

The High Density Polyethylene Composite with Recycled Radiation Cross-Linked Filler of rHDPEx

This article discusses the possibilities of using radiation cross-linked high density polyethylene (HDPEx) acting as a filler in the original high density polyethylene (HDPE) matrix. The newly created composite is one of the possible answers to questions relating to the processing of radiation cross-linked thermoplastics. Radiation cross-linked networking is-nowadays, a commonly used technology that can significantly modify the properties of many types of thermoplastics. This paper describes the influence of the concentration of filler, in the form of grit or powder obtained by the grinding/milling of products/industrial waste from radiation cross-linked high density polyethylene (rHDPEx) on the mechanical and processing properties and the composite structure. It was determined that, by varying the concentration of the filler, it is possible to influence the mechanical behaviour of the composite. The mechanical properties of the new composite-measured at room temperature, are generally comparable or better than the same properties of the original thermoplastic. This creates very good assumptions for the effective and economically acceptable, processing of high density polyethylene (rHDPEx) waste. Its processability however, is limited; it can be processed by injection moulding up to 60 wt %.

Use of Irradiated Polymers after Their Lifetime Period

This article deals with the study of the utilisation of irradiated HDPE products after their end-of-life cycle. Today, polymer waste processing is a matter of evermore intensive discussion. Common thermoplastic waste recycling-especially in the case of wastes with a defined composition-is generally well-known-and frequently used. On the contrary, processing cross-linked plastics is impossible to do in the same way as with virgin thermoplastics-mainly due to the impossibility of remelting them. The possibility of using waste in the form of grit or a powder, made from cross-linked High Density PolyEthylene (rHDPEx) products, after their end-of-life cycle, as a filler for virgin Low Density PolyEthylene (LDPE) was tested in a matrix. It was found that both the mechanical behaviour and processability of new composites with an LDPE matrix, with rHDPEx as a filler, depend-to a high degree-on the amount of the filler. The composite can be processed up to 60% of the filler content. The Polymer Mixture Fluidity dropped significantly, in line with the amount of filler, while the mechanical properties, on the other hand, predominantly grew with the increasing amount of rHDPEx.

Separation of mixed waste plastics via magnetic levitation

Separation becomes a bottleneck of dealing with the enormous stream of waste plastics, as most of the extant methods can only handle binary mixtures. In this paper, a novel method that based on magnetic levitation was proposed for separating multiple mixed plastics. Six types of plastics, i.e., polypropylene (PP), acrylonitrile butadiene styrene (ABS), polyamide 6 (PA6), polycarbonate (PC), polyethylene terephthalate (PET), and polytetrafluoroethylene (PTFE), were used to simulate the mixed waste plastics. The samples were mixed and immersed into paramagnetic medium that placed into a magnetic levitation configuration with two identical NdFeB magnets with like-poles facing each other, and Fourier transform infrared (FTIR) spectroscopy was employed to verify the separation outputs. Unlike any conventional separation methods such as froth flotation and hydrocyclone, this method is not limited by particle sizes, as mixtures of different size fractions reached their respective equilibrium positions in the initial tests. The two-stage separation tests demonstrated that the plastics can be completely separated with purities reached 100%. The method has the potential to be industrialised into an economically-viable and environmentally-friendly mass production procedure, since quantitative correlations are determined, and the paramagnetic medium can be reused indefinitely.

Recycling of Virgin and Post-Consumer Polypropylene and High Density Polyethylene

Recycling of plastics is becoming more important nowadays due to the increasing amount of waste that is generated worldwide. Post-consumer plastics have a large volume and their harmful effects must be considered. From this perspective, the option of recycling post-consumer plastics has become more and more necessary. The main objective of this research is the selective separation of polypropylene (PP) and high density polyethylene (HDPE). On this basis, various flotation parameters were tested on virgin plastics and optimized conditions were applied to post-consumer plastics. PP particles with a purity of 96.30% were obtained with a recovery rate of 97.4% in virgin polymer separation, while HDPE particles were obtained as the sunken product with 97.38% purity and a recovery rate of 96.3%. Besides, almost all PP and HDPE particles were selectively separated with the highest recovery rates when using 500 g/t calcium lignosulfonate (CLs) as a plasticizer in post-consumer polymer separation. Moreover, water reuse was investigated to determine the recovery and economy of plasticizer without adding extra water during the experiment. Therefore, it can be concluded that this process is environmentally friendly in terms of PP and HDPE recycling and reuse of process water.

Using factorial experimental design to evaluate the separation of plastics by froth flotation

This paper proposes the use of factorial experimental design as a standard experimental method in the application of froth flotation to plastic separation instead of the commonly used OVAT method (manipulation of one variable at a time). Furthermore, as is common practice in minerals flotation, the parameters of the kinetic model were used as process responses rather than the recovery of plastics in the separation products. To explain and illustrate the proposed methodology, a set of 32 experimental tests was performed using mixtures of two polymers with approximately the same density, PVC and PS (with mineral charges), with particle size ranging from 2 to 4 mm. The manipulated variables were frother concentration, air flow rate and pH. A three-level full factorial design was conducted. The models establishing the relationships between the manipulated variables and their interactions with the responses (first order kinetic model parameters) were built. The Corrected Akaike Information Criterion was used to select the best fit model and an analysis of variance (ANOVA) was conducted to identify the statistically significant terms of the model. It was shown that froth flotation can be used to efficiently separate PVC from PS with mineral charges by reducing the floatability of PVC, which largely depends on the action of pH. Within the tested interval, this is the factor that most affects the flotation rate constants. The results obtained show that the pure error may be of the same magnitude as the sum of squares of the errors, suggesting that there is significant variability within the same experimental conditions. Thus, special care is needed when evaluating and generalizing the process.

Mechanical and chemical recycling of solid plastic waste

This review presents a comprehensive description of the current pathways for recycling of polymers, via both mechanical and chemical recycling. The principles of these recycling pathways are framed against current-day industrial reality, by discussing predominant industrial technologies, design strategies and recycling examples of specific waste streams. Starting with an overview on types of solid plastic waste (SPW) and their origins, the manuscript continues with a discussion on the different valorisation options for SPW. The section on mechanical recycling contains an overview of current sorting technologies, specific challenges for mechanical recycling such as thermo-mechanical or lifetime degradation and the immiscibility of polymer blends. It also includes some industrial examples such as polyethylene terephthalate (PET) recycling, and SPW from post-consumer packaging, end-of-life vehicles or electr(on)ic devices. A separate section is dedicated to the relationship between design and recycling, emphasizing the role of concepts such as Design from Recycling. The section on chemical recycling collects a state-of-the-art on techniques such as chemolysis, pyrolysis, fluid catalytic cracking, hydrogen techniques and gasification. Additionally, this review discusses the main challenges (and some potential remedies) to these recycling strategies and ground them in the relevant polymer science, thus providing an academic angle as well as an applied one.