Two methods have been used for concentration and purification of uranium: ion exchange and solvent extraction. Early operations used ammonium type resins in polystyrene beads for ion exchange, but solvent extraction is now in general use. The mixed uranium oxide concentrate U3O8 received by the refinery is dissolved in nitric acid. The resulting solution of uranium nitrate UO2(NO3)2.6H2O is fed into a countercurrent solvent extraction process, using tributyl phosphate dissolved in kerosene or dodecane. The uranium is collected by the organic extractant, from which it can be washed out by dilute nitric acid solution and then concentrated by evaporation. The solution is then calcined (heated strongly) to produce pure UO3.
Gaseous uranium hexafluoride (UF 6 ) is used as the feed in the gas centrifuge and gaseous diffusion processes, and uranium tetrachloride (UCl 4 ) is used as feed in the electromagnetic isotope separation (EMIS) process. Nearly all uranium enrichment plants utilize UF 6 as their feed.
Uranium ore concentrates, also known as yellowcake, typically contain 60-80 percent uranium and up to 20 percent extraneous impurities. There are two commercial processes used to produce purified UF 6 from yellowcake. The primary difference between the two processes -- solvent extraction/fluorination ("wet process") and fluorination/fractionation ("dry process") -- is whether the uranium is purified by solvent extraction before conversion to UF 6 or by fractional distillation of the UF 6 after conversion.
In the wet process, yellowcake is dissolved in nitric acid (HNO 3 ), and the insoluble residue is removed by filtration or centrifugation. Uranium is separated from the acid solution with liquid-liquid extraction, the uranyl nitrate product is decomposed to uranium trioxide (UO 3 ) via thermal denitration, and the trioxide is reduced to uranium dioxide (UO 2 ) with hydrogen or cracked ammonia (NH 3 ). In most cases, the standard Purex process, using tri-n-butyl phosphate (TBP) in a hydrocarbon diluent, separates uranium from its impurities in the extraction step. In the dry process, the conversion and purification steps occur throughout the process. If the yellowcake was produced by the alkali-leach process (yields Na 2 U 2 O 7 ), the sodium must be removed from the material by partial digestion in sulfuric acid followed by ammonia precipitation of ammonium diuranate [(NH 4 ) 2 U 2 O 7 ]. The ammonium-containing uranium salt is decomposed to UO 3 by heating, and this oxide is reduced to UO 2 with hydrogen or cracked NH 3 .
The remaining steps used to produce UF 6 for both processes are similar in that the UO 2 is converted to UF 4 by hydrofluorination (using hydrogen fluoride gas-HF). The UF 4 (impure in the dry process) is converted to UF 6 using electrolytically generated fluorine gas (F 2 ). In the dry process, the UF 6 is purified in a two-stage distillation step. Direct fluorination of UO 3 to UF 6 has been used, but this procedure is more amenable to relatively small capacity plants. Hydrogen fluoride (HF) gas is used to produce uranium tetrafluoride (green salt), which is subsequently reduced to uranium metal in a reduction vessel. HF is an extremely hazardous material; consequently, HF production is rated as one of the most hazardous operations. Also, this method of producing uranium metal results in some of the largest recycle streams of the chemical recovery operations.
The EMIS uranium-enrichment process uses UCl 4 for its feed material. Uranium tetrachloride is produced by the reaction of carbon tetrachloride (CCl 4 ) with pure UO 2 at 700 °F.
Many countries around the world have extracted uranium from its ores or from yellowcake. The processes for preparing the feedstocks are basic industrial chemistry. The enabling technologies are those which use HF, NH 3 , F 2 , CCL 4 , and precursor uranium compounds to prepare UF 6 and UCL 4 .
Uranium conversion plants and systems may perform one or more transformations from one uranium chemical species to another, including: conversion of uranium ore concentrates to UO3, conversion of UO3 to UO2, conversion of uranium oxides to UF4 or UF6, conversion of UF4 to UF6, conversion of UF6 to UF4, conversion of UF4 to uranium metal, and conversion of uranium fluorides to UO2. Many key equipment items for uranium conversion plants are common to several segments of the chemical process industry, including furnaces, rotary kilns, fluidized bed reactors, flame tower reactors, liquid centrifuges, distillation columns and liquid-liquid extraction columns. However, few of the items are available "off-the-shelf"; most would be prepared according to customer requirements and specifications. Some require special design and construction considerations to address the corrosive properties of the chemicals handled (HF, F2, CLF3, and uranium fluorides). In all of the uranium conversion processes, equipment which individually is not especially designed or prepared for uranium conversion can be assembled into systems which are especially designed or prepared for uranium conversion.
Uranium Conversion Plant Equipment
(1) Especially designed or prepared systems for the conversion of uranium ore concentrates to UO3. Conversion of uranium ore concentrates to UO3 can be performed by first dissolving the ore in nitric acid and extracting purified uranyl nitrate using a solvent such as tributyl phosphate. Next, the uranyl nitrate is converted to UO3 either by concentration and denitration or by neutralization with gaseous ammonia to produce ammonium diuranate with subsequent filtering, drying, and calcining.
(2) Especially designed or prepared systems for the conversion of UO3 to UF6. Conversion of UO3 to UF6 can be performed directly by fluorination. The process requires a source of fluorine gas or chlorine trifluoride.
(3) Especially Designed or Prepared Systems for the conversion of UO3 to UO2. Conversion of UO3 to UO2 can be performed through reduction of UO3 with cracked ammonia gas or hydrogen.
(4) Especially Designed or Prepared Systems for the conversion of UO2 to UF4. Conversion of UO2 to UF4 can be performed by reacting UO2 with hydrogen fluoride gas (HF) at 300-500ºC.
(5) Especially Designed or Prepared Systems for the conversion of UF4 to UF6. Conversion of UF4 to UF6 is performed by exothermic reaction with fluorine in a tower reactor. UF6 is condensed from the hot effluent gases by passing the effluent stream through a cold trap cooled to -10ºC. The process requires a source of fluorine gas.
(6) Especially Designed or Prepared Systems for the conversion of UF4 to U metal. Conversion of UF4 to U metal is performed by reduction with magnesium (large batches) or calcium (small batches). The reaction is carried out at temperatures above the melting point of uranium (1130ºC).
(7) Especially designed or prepared systems for the conversion of UF6 to UO2. Conversion of UF6 to UO2 can be performed by one of three processes. In the first, UF6 is reduced and hydrolyzed to UO2 using hydrogen and steam. In the second, UF6 is hydrolyzed by solution in water, ammonia is added to precipitate ammonium diuranate, and the diuranate is reduced to UO2 with hydrogen at 820ºC. In the third process, gaseous UF6, CO2, and NH3 are combined in water, precipitating ammonium uranyl carbonate. The ammonium uranyl carbonate is combined with steam and hydrogen at 500-600ºC to yield UO2. UF6 to UO2 conversion is often performed as the first stage of a fuel fabrication plant.
(8) Especially Designed or Prepared Systems for the conversion of UF6 to UF4. Conversion of UF6 to UF4 is performed by reduction with hydrogen.
(9) Especially designed or prepared systems for the conversion of UO2 to UCl4 as feed for electromagnetic enrichment.
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