Dissolution, solvent-based separations

Solvent-based recycling processes can separate plastics from other types of waste by selective dissolution. In particular, polymers of different chemical constitution can be separated and also blends of polymers (129). 

We have considered the common non-solvent-based methods of separating postconsumer plastics. However, except for depolymerization, only selective dissolution is capable of purifying bonded, blended, and fill plastics effectively. Dissolution of the polymer releases the impurities which are then removed by filtration, adsorption, or flotation/sedimentation. This yields polymers of high purity for reuse in original applications. The major drawback of a solvent system is Ae increased expense due to the complexity of equipment and higher energy requirements. 

Four cases are of special Interest, the precipitation of a dissolved polymer melt by addition of a non-solvent, the separation of chemically different polymers by mixtures of several solvents (below their CST), (11) the dissolution of a polymer melt by mixture of two non-solvents, and the swelling of a crosslinked polymer melt by monomeric solvents. The incidence of the first three cases is described in Figures 5 and 6, respectively, for nonspecific Interactions between polymer and solvent(s). Specific interactions are qualitatively just as predictable as in the case of specific interactions between monomeric liquids, say molecular compound formation or acid/base interaction. But quantitative predictions are even more difficult here than in the monomer case. 

Solvent deasphalting. This is an extraction of the heaviest fractions of a vacuum residue or heavy distillate. The extract is used to produce the bitumen. The separation is based on the precipitation of asphaltenes and the dissolution of the oil in an alkane solvent. The solvents employed are butane or propane or a butane-propane mixture. By selecting the proper feedstock and by controlling the deasphalting parameters, notably temperature and pressure, it is possible to obtain different grades of bitumen by this process. 

Acids that are solids can be purified in this way, except that distillation is replaced by repeated crystallisation (preferable from at least two different solvents such as water, alcohol or aqueous alcohol, toluene, toluene/petroleum ether or acetic acid.) Water-insoluble acids can be partially purified by dissolution in N sodium hydroxide solution and precipitation with dilute mineral acid. If the acid is required to be free from sodium ions, then it is better to dissolve the acid in hot N ammonia, heat to ca 80°, adding slightly more than an equal volume of N formic acid and allowing to cool slowly for crystallisation. Any ammonia, formic acid or ammonium formate that adhere to the acid are removed when the acid is dried in a vacuum — they are volatile. The separation and purification of naturally occurring fatty acids, based on distillation, salt solubility and low temperature crystallisation, are described by K.S.Markley (Ed.), Fatty Acids, 2nd Edn, part 3, Chap. 20, Interscience, New York, 1964. 

Separation based on valency change.—The easy oxidation of Ce3 to Ce4+ permits its isolation from other rare earths. The separation of cerium is usually performed by selective leaching with acids, or by complete dissolution [129, 130] followed by hydrolysis. The solvent extraction behaviour of Ce(N03)4 has been extensively studied. Among the various extractants, alcohols, ethers, organic and inorganic acids, ketones etc., TBP proved to be most advantageous in large scale operations [131,132]. 

Reprocessing is based on liquid-liquid extraction for the recovery of uranium and plutonium from used nuclear fuel (PUREX process). The spent fuel is first dissolved in nitric acid. After the dissolution step and the removal of fine insoluble solids, an organic solvent composed of 30% TriButyl Phosphate (TBP) in TetraPropylene Hydrogenated (TPH) or Isopar L is used to recover both uranium and plutonium the great majority of fission products remain in the aqueous nitric acid phase. Once separated from the fission products, back-extraction combined with a reduction of Pu(I V) to Pu(III) allows plutonium to be separated from uranium these two compounds can be recycled.2... 

Nonaqueous liquid electrolyte solutions may be divided into subgroups according to several criteria based on the differences among the various polar aprotic solvents. The first division can be between protic or polar aprotic nonaqueous solvents and nonpolar solvents. In polar aprotic and protic nonaqueous systems, conductivity is achieved by the dissolution of the electrolytes and the appropriate charge separation of the dissolved species, allowing for their free migration under the electrical field. In nonpolar systems the conductance mechanism may be more... 

A thermodynamic method, more fitting to this chapter, has been proposed by Nauman et al. They claim a process for the separation of a physically mixed solid polymers by selective dissolution. They rely on the different polymer solubility characteristics. Tables of this property have been reported and are based on regular solution theory and Hildebrand solubility parameters. The core of the Nauman invention is to find suitable solvents to dissolve particular polymers under defined temperature and pressure conditions. A mixture of polymers is first added to one solvent, at a given temperature, in order to dissolve a particular polymer. The remaining polymer mixture is then treated at a higher temperature with the same solvent or with a different solvent. For clarity, two examples are taken from the patent."... 

---