Resins, limitations from corrosion

Fig. 1. Schematic flowsheet of uranium processing (acid leach and ion exchange) operation. Numbers refer to the numbers that appear in the boxes on the flowsheet. Operations (3), (6), (9), and (11) may be done by thickening or filtration. Most often, thickeners are used, followed by filters. The pH of the leach slurry <4) is elevated to reduce its corrosive effect and to improve the ion-exchange operation on the uranium liquor subsequently separated, In tile ion exchange operation (7), resin contained in closed columns is alternately loaded with uranium and then eluted. The resin adsorbs the complex anions, such as UC fSO 4-. in which the uranium is present in the leach solution. Ammonium nitrate is nsed for elution, obtained by recycling the uranium filtrate liquor after pH adjustment. Iron adsoibed with the uranium is eluted with it. Iron separation operation (8) is needed inasmuch as the iron hydroxide slurry is heavily contaminated with calcium sulfate and coprecipitated uranium salts. Therefore, the slurry is recycled to the watering stage (3). Washed solids from 1,6). the waste barren liquor from (7), and the uranium filtrate from (11) are combined. The pH is elevated to 7.5 by adding lime slurry before the mixture is pumped to the tailings disposal area. (Rio Algom Mines Limited, Toronto)...

Resins are available, having average n valnes from 1.3 to 16. Viscosity increases with molecular weight. Above an average n value of 2, the resins are amorphous solids with increasing Tg. Because of side reactions, commercial resins have an /n less than 2, commonly about 1.9. BPA epoxy resins are used in coatings in which excellent adhesion and corrosion resistance are required. A limitation of their use is poor exterior durability. Epoxy resins are also prepared by reaction of ECH with novolak phenolic resins. The resulting novolak epoxy resins have /n of 2.2-5.5 (see Epoxy Resins). 

Temperature control is critical because of the sensitivity of viscosity to temperature. Thermowells (encasements to place a thermocouple) can be built into the barrel wall to measure the temperature. Melt temperature should be measured directly to avoid dependence on the barrel temperature, which could significantly differ from the melt. A thermocouple is placed in the barrel in a manner that its tip is immersed in the melt, extending to the middle of the flow chatmel. The thermocouple should be made from a corrosion-resistant material such as Hastelloy-C. Temperature controllers should be equipped with limit switches or other mechanisms to detect the failure of thermocouples, the heater, or the controller itself. An ammeter can detect a full-on failure that could lead to degradation of the resin. In cases where more than one band heater is controlled by a controller, all units should have the same power flux. 

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