Magnetic Density Separation Research Articles

An Innovative Magnetic Density Separation Process for Sorting Granular Solid Wastes

Solid waste sorting is an important pre-treatment in recycling to improve the efficiency of material recovery and reduce costs. Motivated by the PEACOC project on metal recovery from solid wastes, an innovative magnetic density separation (MDS) process has been developed for solid waste sorting. It has intrinsic advantages over conventional gravity separation technologies and the previously industrialized MDS process. The new MDS process applies an inclined planar magnet and a horizontal basin containing a static magnetic fluid as the separation medium. A particle sliding phenomenon is identified as a feature that could help the separation. Experiments have been carried out to demonstrate the role of the MDS in concentrating valuable metals in shredded PCBAs and reducing metallic contaminants in plastic fractions of shredded wires. A pilot scale facility is introduced to show the design to achieve continuous production and to reduce the consumption of ferrofluid.

Magnetic density separation of particles in honeycomb-generated wake turbulence

Magnetic density separation (MDS) is a technique for separating granular materials based on mass density. MDS uses magnetically responsive fluids and engineered magnetic fields to create a vertical gradient of apparent mass density inside the fluid. We present a numerical study to investigate the effect of honeycomb wake turbulence on the motion of almost neutrally buoyant spherical particles in a magnetized fluid. A four-way coupled Euler-Lagrange approach is used where particles are effectively treated as finite-size point particles. It is shown that the honeycomb-generated wake turbulence increases the levitation time of the particles. The presence of particles results in an earlier break-up of the individual velocity profiles stemming from the honeycomb cells. The effects of honeycomb geometry and cell Reynolds number on the collective motion of particles and turbulence properties are investigated, where the choice for the best honeycomb is a trade-off between the separation error and the particle flux.

Thermal and electrical design of superconducting demonstrator for magnetic density separation

In this paper the focus is on thermal and electrical design aspects of a NbTi-based demonstrator magnet for magnetic density separation (MDS) that is being constructed at the University of Twente. MDS is a recycling technology that allows the separation of non-magnetic particles based on their mass density, using a vertical magnetic field gradient and a ferrofluid. To minimize the distance between the planar array of racetrack coils and the ferrofluid bath, the system is conduction-cooled. First the thermal design is presented, which shows that the coils can operate below 4.5 K with sufficient margin using a single cryocooler. High-purity aluminium heat drains enable a low thermal gradient across the cold mass. The current path is introduced, as well as the adopted protection scheme. The magnet’s stored energy can safely be dumped in the coils. Diodes are placed (anti-)parallel to the coils in the cold to prevent high terminal voltages. In the case of a quench in the superconducting part of the current leads or an external anomaly, a switch is opened and the current is forced through a resistor in series with the diodes, causing a deliberate transition of the coils to the normal state and thus a fast ramp-down.

Mechanical design of a superconducting demonstrator for magnetic density separation

The focus of this paper is on the mechanical design of a NbTi-based demonstrator magnet for magnetic density separation (MDS) that is being constructed at the University of Twente. MDS is a new recycling technology that allows the separation of non-magnetic particles based on their mass density, using a vertical magnetic field gradient and a ferrofluid. The unique mechanical design challenge for this type of magnet is the desired minimization of the distance between a sim1 m2 planar array of cryogenic racetrack coils and the ambient-temperature ferrofluid bath. The optimization of the magnet geometry results in a distance between the coils and ferrofluid of 50 mm. This is made possible by opting for conduction-cooling, for the inclusion of room-temperature rods that pass through the cold mass to support the cryostat, and for the geometry of the cassette that reacts to the Lorentz force.

Optimum Coil-System Layout for Magnet-Driven Superconducting Magnetic Density Separation

This article discusses the optimum layout of coils of a superconducting magnet system for magnetic density separation (MDS). MDS is a novel separation technology that combines a vertical magnetic field gradient with a ferrofluid to separate mixtures of non-magnetic particles based on their mass density. The MDS process can separate more than two types of particles in a single process step, thereby distinguishing it from other separation techniques using a ferrofluid. The authors are currently constructing a superconducting MDS demonstrator. Ideally, the gradient of the magnetic field magnitude should change only with the distance above the magnet but remain constant in a horizontal plane. In principle, such an ideal field profile can be generated with an infinite harmonic sheet current. In practice, edge effects appear due to the necessity of using a finite number of coils. These cause a horizontal component in the field gradient and also change the vertical component. We compare the vertical magnetic field gradient of various coil layouts to see which configuration performs best. To facilitate ease of production, the analysis is restricted to flat racetrack coils. The main result is that the specific shape of a racetrack coil has a larger influence on the vertical gradient than the number of coils. The feed particles need to be pushed through the separation chamber from the insertion to the collection point. One option to realize this is to use an MDS setup in which the magnet is inclined with respect to the horizontal plane. This tilting results in a horizontal magnetic force component, which drives feed particles through the fluid bed. We show that a three-coil layout provides the largest usable fluid bed depth for a wide range of tilt angles.

Fundamental Electromagnetic Configuration for Generating One-Directional Magnetic Field Gradients

In this article, electromagnet layouts are presented, which generate a magnetic field with a magnitude gradient that does not vary significantly in a horizontal plane but decreases monotonically with the vertical height above the magnet. Such a one-direction magnetic field gradient is a specific requirement for magnetic density separation (MDS), a novel recycling technology that combines a vertical magnetic field gradient with a ferrofluid to separate a mixture of non-magnetic materials based on their mass density. We are assembling the first superconducting magnet to be used for this application. In contrast to other separation technologies that use ferrofluid, multiple products can be separated in a single process step. First, the idealized current distribution is introduced that produces such a magnetic field with a magnitude that decays only in one direction. This ideal field can be approximated with practical coil configurations, which are evaluated with a Fourier analysis to derive an optimal cross-sectional layout based on flat racetrack coils. The analysis concludes with a discussion of the effect of winding pack thickness on the value of the magnetic field above the magnet system and the peak field inside the winding pack. The conclusions of this study are applicable not just for MDS but for any application that requires a magnetic field gradient that changes only in one direction.

Direct numerical simulation of magneto-Archimedes separation of spherical particles

Abstract

Colloidal Stability of Aqueous Ferrofluids at 10T.

Magnetic density separation is an emerging recycling technology by which several different waste materials—from plastic products, electronics, or other—can be sorted in a single continuous processing step. Larger-scale installations will require ferrofluids that remain stable at several teslas, high magnetic fields at which colloidal stability was not investigated before. Here we optically monitor the concentration profile of iron oxide nanoparticles in aqueous ferrofluids at a field of 10 T and a gradient of 100 T/m. The sedimentation velocities and equilibrium concentration profiles inform on maintenance or breakdown of colloidal stability, which depends on the concentration and magnetic coupling energy of the nanoparticles. Comparison with results obtained with a small neodymium magnet indicate that stability at moderate fields is predictive of stability at much higher fields, which facilitates the development of new ferrofluids dedicated to magnetic density separation.

An innovative recycling process to obtain pure polyethylene and polypropylene from household waste

An innovative recycling process, based on magnetic density separation (MDS) and hyperspectral imaging (HSI), to obtain high quality polypropylene and polyethylene as secondary raw materials, is presented. More in details, MDS was applied to two different polyolefin mixtures coming from household waste. The quality of the two separated PP and PE streams, in terms of purity, was evaluated by a classification procedure based on HSI working in the near infrared range (1000–1700nm). The classification model was built using known PE and PP samples as training set. The results obtained by HSI were compared with those obtained by classical density analysis carried in laboratory on the same polymers. The results obtained by MDS and the quality assessment of the plastic products by HSI showed that the combined action of these two technologies is a valid solution that can be implemented at industrial level.

Upgrading of PVC rich wastes by magnetic density separation and hyperspectral imaging quality control

Polyvinylchloride (PVC) is one of the most produced polymers in Europe, with a share of 11% in terms of mass (8 milliontons) of total polymer consumption, but in 2010 only 5% of the total PVC production came from recycled materials, where other polymer recycling achieves a level of 15% on average. In order to find an innovative process to extract PVC from window frames waste, a combination of two innovative technologies was tested: magnetic density separation (MDS) and hyperspectral imaging (HSI). By its nature, MDS is a flexible high precision density separation technology that is applicable to any mixture of polymers and contaminants with non-overlapping densities. As PVC has a very distinctive high density, this technology was tested to obtain high-grade PVC pre-concentrates from window frame waste. HSI was used to perform a quality control of the products obtained by MDS showing that PVC was clearly discriminated from unwanted rubber particles of different colors. The results showed that the combined application of MDS and HSI techniques allowed to separate and to check the purity of PVC from window frame waste.

Method for Assessing Mineral Composition-Dependent Patterns in Microbial Diversity Using Magnetic and Density Separation

This study introduces a new method for characterizing mineral-associated microbial diversity in sedimentary environments, a habitat that has been intrinsically challenging to study in regard to microbe-mineral interactions. Mineral components were enriched from bulk environmental samples by magnetic susceptibility or density separation techniques and used in subsequent molecular and microscopic analyses. Testing and optimization of the method was performed on geochemically-distinct sediment horizons from Eel River Basin methane seeps and pyrite and sphalerite-rich hydrothermal vent samples from the Lau Basin. Initial results show reproducible variations in microbial diversity between mineral fractions from marine sedimentary environments enriched in authigenic pyrite and/or transition metal-bearing clay minerals. Specifically, different archaeal clades associated with the anaerobic oxidation of methane and putative sulfate-reducing deltaproteobbacteria show preferential colonization patterns, suggesting potential ecophysiological differences between closely-related taxa. These results indicate that mineral colonization may influence the extent and distribution of microbial diversity throughout unconsolidated sediments of the marine subsurface. The combination of mineral separation and molecular analyses introduced here provide a new approach for revealing previously concealed patterns of mineral-associated microbial diversity across a wide range of environments, from hard rock habitats to fine-grained lithologies.

Classification of polyolefins from building and construction waste using NIR hyperspectral imaging system

This work was carried out to develop a hyperspectral imaging system in the near infrared (NIR) range (1000–1700nm) to classify polyolefin particles from complex waste streams in order to improve their recovery, producing high purity polypropylene (PP) and polyethylene (PE) granulates, according to market requirements. In particular, hyperspectral images were acquired for polyolefins coming from building & construction waste (B&CW), divided into 9 different density fractions, ranging from <0.88g/cm3 up to 0.96g/cm3 and in different color classes. Spectral data were analyzed using principal component analysis (PCA) to reduce the high dimensionality of data and for selecting some effective wavelengths. Results showed that it was possible to recognize PP and PE waste particles and to define the “real cut density” between PP and PE from B&CW, to be utilized in the recycling process based on magnetic density separation (MDS). The results revealed the potentiality of NIR hyperspectral imaging as an objective and non-destructive method for classification and quality control purposes in the recycling chain of polyolefins.

Analysis of car shredder polymer waste with Raman mapping and chemometrics

Abstract. A novel evaluation method was developed for Raman microscopic quantitative characterization of polymerwaste. Car shredder polymer waste was divided into different density fractions by magnetic density separation (MDS) tech -nique, and each fraction was investigated by Raman mapping, which is capable of detecting the components being presenteven in low concentration. The only method available for evaluation of the mapping results was earlier to assign each pixelto a component visually and to count the number of different polymers on the Raman map. An automated method is pro-posed here for pixel classification, which helps to detect the different polymers present and enables rapid assignment ofeach pixel to the appropriate polymer. Six chemometric methods were tested to provide a basis for the pixel classification,among which multivariate curve resolution-alternating least squares (MCR-ALS) provided the best results. The MCR-ALSbased pixel identification method was then used for the quantitative characterization of each waste density fraction, whereit was found that the automated method yields accurate results in a very short time, as opposed to manual pixel countingmethod which may take hours of human work per dataset.

Hyperspectral Imaging for Process and Quality Control in Recycling Plants of Polyolefin Flakes

Hyperspectral imaging in the near infrared range (1000–1700 nm) was evaluated to identify different polyolefin flakes for quality assessment of recycled products. According to market requirements, the output of the recycling process of polyolefins must be high purity secondary polypropylene and polyethylene granulates. Hyperspectral images were acquired for selected plastic flakes coming from household waste, classified according to their typology. Spectra were analysed using principal component analysis to reduce the high dimensionality of data and for selecting effective wavelengths. Partial least square discriminant analysis was applied for classification of the two polyolefin typologies. Prediction results showed that it is possible to recognise the different plastic flakes with sensitivity and specificity ranging from 0.90 to 0.99. The developed procedures based on hyperspectral imaging can be utilised for quality control of the two pure polypropylene and polyethylene flow streams obtained by the innovative recycling process based on magnetic density separation.

Sorting of vegetable seeds by magnetic density separation in comparison with liquid density separation

Sorting of vegetable seeds by magnetic density separation in comparison with liquid density separation

Characterization of post-consumer polyolefin wastes by hyperspectral imaging for quality control in recycling processes

In this paper new analytical inspection strategies, based on hyperspectral imaging (HSI) in the VIS–NIR and NIR wavelength ranges (400–1000 and 1000–1700 nm, respectively), have been investigated and set up in order to define quality control logics that could be applied at industrial plant level for polyolefins recycling. The research was developed inside the European FP7 Project W2Plastics “Magnetic Sorting and Ultrasound Sensor Technologies for Production of High Purity Secondary Polyolefins from Waste”. The main aim of the project is the separation of pure polyethylene and polypropylene adopting an innovative process, the magnetic density separation (MDS). Spectra of plastic particles and contaminants resulting from post-consumer complex wastes and of virgin polyolefins have been acquired by HSI and by Raman spectroscopy. The classification results obtained applying principal component analysis (PCA) on HSI data have been compared with those obtained by Raman spectroscopy, in order to validate the proposed innovative methodology. Results showed that HSI sensing techniques allow to identify both polyolefins and contaminants. Results also demonstrated that HSI has a great potentiality as a tool for quality control of feed (identification of contaminants in the plastic waste) and of the two different pure polypropylene and polyethylene flow streams resulting from the MDS-based recycling process.

Turning Magnetic Density Separation into Green Business Using the Cyclic Innovation Model

The industrial implementation of Magnetic Density Separation (MDS) in the recycling of raw materials requires more than just the science of understanding why it works ('know-why') and the technology of how that can be accomplished ('know-how'). In addition, detailed knowledge of the market for streams of end-of-life products (supply side) and the market for recycled raw materials (demand side) are paramount to optimize the practical implementation and the design of an industrial plant. Indeed, in such a plant MDS is used to separate mixed streams of recycled materials into product streams which comply with market demands on grade and purity of the product compared with virgin streams. A recently developed non-linear innovation model is used to connect four fundamental cycles which continuously interact in the process of turning MDS technology into green business. The strength of this Cyclic Innovation Model lies in connecting technical capabilities with societal market needs. Only when a good match between the two is found a new technology such as, MDS develop into an innovation. The separation of polyolefins from end-of-life product is selected as one of the first industrial-sized applications of MDS in recycling. CIM is used to identify opportunities and challenges which need to be addressed to turn the MDS technology into an innovation that builds an economically attractive business in the context of a green society.

Capabilities of Ultrasound for Monitoring and Quantitative Analysis of Polyolefin Waste Particles in Magnetic Density Separation (MDS)

A study was conducted into the potential of ultrasound as a qualitative and quantitative tool for the Magnetic Density Separator (MDS), where an ultrasound sensor array will operate from inside the MDS ferrofluid. The desired applications of the ultrasound are monitoring of polyolefin particles in flow during the separation process, and performing throughput measurements and quality inspection. The possibility to adapt off-the-shelf medical imaging technology was investigated with an eye towards the real-time requirements for industrial application. It is shown that a medical imager can be adapted for monitoring the flowing particles at speeds up to 30 cm/s, and the imaging quality can be very good under optimum viewing angle conditions. Ultrasound imaging may also allow for quantitative through-put analysis provided the flowing particles are sufficiently spaced, which puts a limit to the maximum measurable throughput. If viewing conditions are not optimum the medical imager relies on the human operator to improve image quality. This situation is undesired for industrial operation where there may be little or no time for operator intervention. To anticipate these occurrences a second imaging method is proposed that could act in conjunction, which is effective since it is based on a different physical principle. Ultrasound quality inspection is a very different technique, and its feasibility depends first of all on whether different types of polyolefin are acoustically distinctive. Using experiments and 3D acoustic computer modelling to determine wave speed and material attenuation, the potential of ultrasound for polyolefin identification under immersed conditions was investigated. With a calibrated measurement setup it is shown that polyolefins may potentially be classified into acoustically distinctive groups. The possible errors in throughput measurement and quality inspection and the requirements for these techniques to perform under operational MDS conditions are also addressed.

The W2Plastics Project: Exploring the Limits of Polymer Separation

The efficient large-scale recycling of plastic waste is of increasing interest from an ecological and economic point of view but it represents a goal that has yet to be achieved by the recycling industry. The W2Plastics project aims at a fundamental change of the present status of plastics recycling by creating a breakthrough technology for the recycling of polyolefin's from complex wastes, i.e., wastes such as Waste from Electric and Electronic Equipment (WEEE), Construction and Demolition Waste (CDW), household waste and Automotive Shredder Residue (ASR). Polyolefin's are a very important family of polymers, constituting more than a third of the total plastics consumption in Europe (EU) and complex wastes provide the vastest, presently unused potential resource of secondary polyolefin's. In spite of that, Polyolefin's (PP, LDPE, HDPE) are the least recycled plastics materials. Only one million ton out of 14 million tons yearly sold in EU is being recycled. Nowadays, Polyolefin recyclers focus mainly on the relatively pure post-industrial or single-product wastes, since these wastes can be made into high-purity product materials by existing and cost-effective process technology. Post-industrial wastes are increasingly exported outside the EU, however, and so the Polyolefin recycling industry and their end-users are forced to look for alternative resources. In principle, post-consumer wastes, such as WEEE, CDW, household waste and ASR provide such a resource. They are a five to ten times larger reservoir of polyolefin's than do post-industrial wastes, but these wastes are also much more complex mixtures of materials and hence much more difficult to recycle. Technologies that are to address these resources need to be extremely powerful, since they must be relatively simple to be cost-effective, but also accurate enough to create high-purity products and able to valorise a substantial fraction of the materials that are present in the waste into useful products of consistent quality in order to be economical. The European FP7 Project: W2Plastics is developing a number of novel concepts, in particular Magnetic Density Separation (MDS) and Ultrasound process and quality control by HyperSpectral Imaging (HSI), into a new technology to recover high-purity polyolefin's from complex wastes at low cost. The unique promise of this new concept derives from its ability to accurately separate many different materials in a single process step, resulting in an environmentally friendly and cheap process.

Wetting Technologies for High-Accuracy Sink-Float Separations in Water- Based Media

The accuracy of sink-float separations in water-based media such as Magnetic Density Separation is compromised by tiny air-bubbles attached to the surface of hydrophobic materials in the feed. Separations of plastics mixtures with accuracy better than 10 kg/m 3 require a total elimination of air-bubbles by pre-wetting the surface of the feed particles with a thin film of water. Theoretical and experimental results are compared for pre-wetting processes based on condensation from the gas phase and direct liquid-solid contact for both shredded waste and virgin polymers. The tests show that the wettability of most polymers improve slightly by steaming for about 1 minute, but the best wetting results are obtained with immersion in boiling tap water. The success of wetting in boiling water can be explained by the deposition of a thin layer of calcium carbonate. Shredded plastics are easier to be wetted than virgin polymers, probably because a relatively rougher surface is caused by the shredder process. A new wetting process to eliminate air bubbles was developed on the basis of these results, which carries a minimal amount of heat and water with the feed to the MDS process liquid.