Spent recycling

Liquid Effluents. Recycling of acid, soda, and zinc have long been necessary economically, and the acid—soda reaction product, sodium sulfate, is extracted and sold into other sectors of the chemical industry. Acid recovery usually involves the degassing, filtering, and evaporative concentration of the spent acid leaving the spinning machines. Excess sodium sulfate is removed by crystallization and then dehydrated before sale. Traces of zinc that escape recovery are removable from the main Hquid effluent stream to the extent that practically all the zinc can now be retained in the process. 

The spent hydrogen fluoride layer, which contains water and sodium bifluoride, from this process is treated with sulfur trioxide or 65% oleum, and hydrogen fluoride is distilled for recycle to the next batch (176,177). 

Used oil disposal trends include waste minimisation such as by reclaiming used fluid on site, as well as recycling of mineral oil lubricants instead of disposing by incineration. The recycling effort involves a system where spent mineral oils are collected then shipped to specialty refineries where the materials are distilled, hydrofinished, and re-refined into fresh base stocks. These re-refined materials are virtually identical to virgin feedstocks. 

The reducing gas is distributed in reactor 4 by an ahoy grid, passes through the fluid bed, then exits the reactor via cyclones. The gas passes through reactors 3 and 2 so that a counter flow between gas and soHds is estabUshed. The spent reducing gas is scmbbed to remove dust and water vapor. Part of the cleaned top gas is recycled and the remainder is used as fuel. 

For the process step involving handling of spent sulfuric acid, several patents have been issued in which improvements in this step were a main claim. The azeotropic nitration of benzene essentially eliminates the need to reconcentrate sulfuric acid (10,11). The nitration step is carried out at higher than usual temperatures (120—160°C). Because excess benzene is used, the higher temperature allows water to be removed as a water—benzene azeotrope. The water is separated and the benzene phase, containing approximately 8% nitrobenzene, is recycled back into the reactor. The dry sulfuric acid is then reused continuously. 

Uranium-239 [13982-01 -9] has a half-life of 23.5 min neptunium-239 [13968-59-7] has a half-life of 2.355 d. Recycling or reprocessing of spent fuel involves separation of plutonium from uranium and from bulk fission product isotopes (see Nuclearreactors, chemical reprocessing). 

Supply Projections. Additional supphes are expected to be necessary to meet the projected production shortfall. A significant contribution is likely to come from uranium production centers such as Eastern Europe and Asia, which are not included in the capabihty projections (27). The remaining shortfall between fresh production and reactor requirements is expected to be filled by several alternative sources, including excess inventory drawdown. These shortfalls could also be met by the utili2ation of low cost resources that could become available as a result of technical developments or pohcy changes, production from either low or higher cost resources not identified in production capabihty projections, recycled material such as spent fuel, and low enriched uranium converted from the high enriched uranium (HEU) found in warheads (28). 

If the economics of recycling were improved, that option would become preferable for spent fuel because the permanent repository issues of the residual fission products would be simpler. The economic value of the energy generated from the recycled plutonium and uranium would substantially allay the costs of the repository as compared to the spent fuel throwaway option. 

Oxidation of cumene to cumene hydroperoxide is usually achieved in three to four oxidizers in series, where the fractional conversion is about the same for each reactor. Fresh cumene and recycled cumene are fed to the first reactor. Air is bubbled in at the bottom of the reactor and leaves at the top of each reactor. The oxidizers are operated at low to moderate pressure. Due to the exothermic nature of the oxidation reaction, heat is generated and must be removed by external cooling. A portion of cumene reacts to form dimethylbenzyl alcohol and acetophenone. Methanol is formed in the acetophenone reaction and is further oxidized to formaldehyde and formic acid. A small amount of water is also formed by the various reactions. The selectivity of the oxidation reaction is a function of oxidation conditions temperature, conversion level, residence time, and oxygen partial pressure. Typical commercial yield of cumene hydroperoxide is about 95 mol % in the oxidizers. The reaction effluent is stripped off unreacted cumene which is then recycled as feedstock. Spent air from the oxidizers is treated to recover 99.99% of the cumene and other volatile organic compounds. 

In the latter twentieth century, spent automotive catalysts have emerged as a significant potential source of secondary Pt, Pd, and Rh. In North America, it has been estimated that 15.5 metric tons per year of PGM from automotive catalysts are available for recycling (22). However, the low PGM loading on such catalysts and the nature of the ceramic monoliths used have required the development of specialized recovery techniques as well as the estabhshment of an infrastmcture of collection centers. These factors have slowed the development of an automotive catalyst recycling iadustry. 

Pollution Control in the Bleachery. The quantity of water necessary for bleaching, and consequently the volume of effluents, has been decreased significantly by various schemes for recycle of Hquors, eg, pulp washing using dilute spent Hquors and countercurrent flow. Effort is underway to close bleach plants and further reduce water consumption. 

Although more often associated with household and commercial waste, recycling has proven to be very successhil in the industrial arena. Industrial recycling is the recovery for reuse or sale of materials from what otherwise would be wastes destined for disposal (5). Typically, the reclaimable materials employed in industrial recycling may consist of obsolete products, spent materials, industrial by-products or residues, or pollution control products. The recycling of many of these products is so well estabHshed that under standard commercial practices such materials are destined only for recovery, not for disposal. 

Because sulfur suppHes, either as elemental sulfur or by-product sulfuric acid, have grown owiag to iacreased environmental awareness, demand for sulfur has decreased ia some consuming iadustries for the same reason. Industries such as titanium dioxide productions, which traditionally utilized sulfuric acid, have concerted to more environmentally friendly processes. In addition, many consumers who contiaue to use sulfuric acid are puttiag an emphasis on regenerating or recycling spent acid. 

Human toxicity, aquatic toxicity, and the environmental impact of engine coolants and deicing fluids ate typically measured on the fresh fluid only. Spent fluids contain varied contaminants that can drastically affect the toxicity and environmental impact of the fluid. Most pronounced is the impact of heavy-metal contaminants in spent antifreeze. Data on spent and recycled antifreeze, compiled by the ASTM Committee on Engine Coolants, show an average lead level 11 ppm, as weU as various other metal contaminants (iron, copper, zinc) (18). The presence of these contaminants in a used fluid may require special disposal techniques for the fluids. 

Ideally, a system for recycling spent antifreeze consists first of the removal of the deleterious contaminants such as the corrosion products, corrosive ions, degradation products, and remaining inhibitors. Then the clean fluid could be reinhibited to a known concentration of both inhibitors and glycol. 

Catalyst lifetime for contemporary ethylene oxide catalysts is 1—2 years, depending on the severity of service, ie, ethylene oxide production rate and absence of feed poisons, primarily sulfur compounds. A large percentage (>95%) of the silver in spent catalysts can be recovered and recycled the other components are usually discarded because of thek low values. 

Dichloroethane is produced commercially from hydrogen chloride and vinyl chloride at 20—55°C ia the presence of an aluminum, ferric, or 2iac chloride catalyst (8,9). Selectivity is nearly stoichiometric to 1,1-dichloroethane. Small amounts of 1,1,3-tfichlorobutane may be produced. Unreacted vinyl chloride and HCl exit the top of the reactor, and can be recycled or sent to vent recovery systems. The reactor product contains the Lewis acid catalyst and must be separated before distillation. Spent catalyst may be removed from the reaction mixture by contacting with a hydrocarbon or paraffin oil, which precipitates the metal chloride catalyst iato the oil (10). Other iaert Hquids such as sdoxanes and perfluorohydrocarbons have also been used (11). 

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