Separation Using Selective Dissolution

Equal volumes of the six major thermoplastics were used HOPE, LDPE, PET, PP, PS and PVC. Tetrahydrofuran was selected as the first trial solvent due to the data acquired in previous compositional quenching work [Lynch and Nauman, 1989J. Xylene has also been used. Results achieved are shown in Table 3.6. It shows that a four way split between plastic types can be achieved with good separation efficiencies. A lower separation efficiency is expected with an actual commingled waste plastic stream due to variations in polymer properties among manufacturers. However, it is expected that compositional quenching will overcome the problems. Preliminary economics has indicated a 50 million pound per year plant will process waste plastic for around 150/pound [Lynch and Nauman, 1989.  

Fiaure 3.8 Single Solvent Selective Dissolution Process Flow Sheet [Lynch and Nauman 1989] 

The multiple solvent process involves the use of a solvent compatible with a limited number of polymers. It has advantages over the single solvent process in that lower pressures and temperatures are necessary which results in reduced energy requirements. Because a different solvent is used for each polymer, a potentially higher purity product can be obtained. Work in this area has focused on PET bottle flake purification following mechanical cleaning from other soda bottle constituents of HDPE, PP, paper and aluminum. This purification process would result in a high purity PET polymer at an increased cost. 

Stage I - Dried chips from a mechanical, density based, float-sink system (such as the Rutgers BBRP process) are fed in, where they are washed with the process solvent at a temperature sufficient to remove insoluble impurities (up to 130 C), but insufficient to dissolve the intended polymer. For example, in the PET train, this washing will remove any adhesives, PS or PVC which may be present from use in 2 liter bottles or from sortation error. 

Stage II - The intended polymer is dissolved by the process solvent at higher temperatures (around 170°C). Once dissolution is complete, the solution can be purified. 

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In an industrial application dissolution/reprecipitation technology is used to separate and recover nylon from carpet waste [636]. Carpets are generally composed of three primary polymer components, namely polypropylene (backing), SBR latex (binding) and nylon (face fibres), and calcium carbonate filler. The process involves selective dissolution of nylon (typically constituting more than 50wt% of carpet polymer mass) with an 88 wt % liquid formic acid solution and recovery of nylon powder with scCC>2 antisolvent precipitation at high pressure. Papaspyrides and Kartalis [637] used dimethylsulfoxide as a solvent for PA6 and formic acid for PA6.6, and methylethylketone as the nonsolvent for both polymers. 

Selective dissolution of the polymer may be used industrially to separate polymer from additives for recycling purposes. However, separation of PPE from its additives (CB, talc, mica) in integrated circuit board scrap by means of trichloroethylene would not seem to meet industrial requirements (toxicity, cost) [10]. 

A hydrocyclone system has been proposed for the separation of mixed plastics including among others HIPS. Both water and calcium chloride solutions have been used. A procedure for the determination of the HIPS content in a ABS/HIPS material has been proposed. The analysis method relies on the selective dissolution in R-limonene (44). 

Recovery of vanadium with peroxygens involves both oxidation and com-plexation. In solution, conversion of lower oxidation states into vanadium(V) allows separation by solvent extraction (Figure 6.18).269 This chemistry can be used for vanadium by-products in uranium extractions. With hydrogen peroxide, vanadium(IV) is not oxidized in acidic solution, but rather in alkaline conditions, e.g. 60 °C at pH 9 (Figure 6.19).270 Use of excess hydrogen peroxide readily forms peroxo complexes and this is of value in selective dissolution of vanadium from secondary sources. 

It has been reported that use of a 0.01 to 0.1 mol L hydroxylamine hydrochloride solution acidified at pH 2 causes the selective dissolution of manganese oxides with minimum extraction of iron oxides (Shuman, 1985 Kersten and Forstner, 1986 Berti and Cunningham, 1997). Nevertheless, the low pH value may lead to a partial release of TE bound to organic matter. The separated reducible fraction can thus be overestimated. An improved method for the selective dissolution of manganese oxides involving a nonacidified 0.1 mol L hydroxylamine solution at pH 3.6 as a leachant has been proposed by Neaman et al. (2004). Actually, the nonacidified hydroxylamine solution is not expected to dissolve considerable amounts of iron oxides, organic matter, or... 

The selective dissolution technique was used to improve the phase separation, and successfully fabricate core-shell microspheres for controlled delivery of drug with reduced initial burst release. These microspheres showed sustained release of aspirin for at least 456 h with a little burst release (3.49%). 

Selective dissolution can also be used in the recycling of fibre-reinforced composites [10]. Several consecutive washings with a solvent allow the separate recovery of the matrix material and reinforcements [11]. 

Based on the above ideas, it is perhaps apparent that separation schemes can be based on a careful match of solvents to polymers. Far less apparent is the fact that a single solvent or a small number of solvents can be used to separate quite complex mixtures of polymers when the dissolution temperature is used as an operating variable. This approach is the basis for the Rensselaer selective dissolution process [IS]. 

Separation efficiencies. The practical separation of comingled plastics by selective dissolution requires a judicious choice of solvents and temperatures. For the separation of chemically dissimilar materials, it may be possible to obtain a clean separation by using several different solvents. Such separations can be essentially quantitative with respect to the polymers. However, crosscontamination between the solvents would necessitate a fairly elaborate solvent recovery system. 

Another option is dissolving the plastics and later reprecipitating them. Either selective dissolution or selective reprecipitation in an organic solvent or a combination of solvents can be the basis of the separation. However, such systems are complex and seldom economical. Solvent retention is often a problem as well. While these systems have been the subject of research, no commercial systems currently use this approach. 

When MDI was used in the polymer synthesis, irrespective of the synthesis route it obtained homogeneous soluble polymers inseparable in fractions, by selective dissolution in DMF. Certainly, the sequence of ordering on the macromolecular chain differs from a PU synthesis route to another. As mentioned, the reaction rate between phenyl isocyanate (FI) and chain extender, ethylene glycol (EG) was found by us to be 4 times higher than the reaction between FI and PEA when measured separately, and 11.6 higher respectively if they are in a mixture (EG + PEA) (synergetic effect) [45,89]. 

Iron oxides and hydroxides are the most important iron-bearing constituents of soils, sediments and clays. To characterize the samples, i.e. the identification of the different minerals present and the determination of their morphology and chemical composition, a variety of standard techniques are commonly used such as X-ray and electron diffraction, chemical analyses, optical and electron microscopy, infrared spectroscopy and thermal analysis (DTA, DTC,...). Most of these techniques are further applied in conjunction with selective dissolution or other separation methods in order to obtain more specific information about particular components in the complex soil system. In addition to all those characterization methods, MS has proven to be a valuable complementary technique for the study of these kinds of materials and in particular for the characterization of iron oxides and hydroxides which are usually poorly crystallized. 

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