Thorium recovery

Origins. Most of the radioactive waste at SRP originates in the two separations plants, although some waste is produced in the reactor areas, laboratories, and peripheral installations. The principal processes used in the separations plants have been the Purex and the HM processes, but others have been used to process a variety of fuel and target elements. The Purex process recovers and purifies uranium and plutonium from neutron-irradiated natural uranium. The HM process recovers enriched uranium from uranium—aluminum alloys used as fuel in SRP reactors. Other processes that have been used include recovery of and thorium (from neutron-irradiated thorium), recovery of Np and Pu, separation of higher actinide elements from irradiated plutonium, and recovery of enriched uranium from stainless-steel-clad fuel elements from power reactors. Each of these processes produces a characteristic waste. 

The alkyl amines offer greater selectivity than organophosphorus compounds in many applications, particularly in uranium hydrometallurgy. Amine extraction is typified by the Amex process, which uses a tertiary or branched secondary amine to extract uranium from sulfate leach liquors (11). A similar process based on the use of a primary or straight-chain secondary amine (sometimes modified with an organic-soluble alcohol) has given good results in thorium recovery (12). 

J. D. Chilton, et al., Separation of Uranium from Thorium by Liquid Metal Extraction, Thorium Recovery, and Fission Product Distribution, NAA-SR-6666, AI(1962). 

There are two breeder reactor fuel cycles. One involves the irradiation of U/ Pu oxide fuel with fast neutrons and is at the prototype stage of development. The other involves the irradiation of Th/ U oxide fuel with thermal neutrons and is at the experimental stage. Fuel from the U/ Pu cycle may be reprocessed using Purex technology adapted to accommodate the significant proportion of plutonium present in the fuel. Increased americium and neptunium levels will also arise compared with thermal reactor fuel. The Th/ U fuel may also be reprocessed using solvent extraction with TBP in the Thorex (Thorium Recovery by Extraction) process. In this case the extraction chemistry must also take account of the presence of Pa arising as shown in Scheme 2. 

The thorium concentration thus builds up to a fairly high level in solution but a proportion of soluble thorium remains physically adhering to the large bulk of zirconium residue. This is conveniently removed by a simple washing treatment with water, preferably in a counter-current manner. In order to avoid too high a total volume of solution, the thorium is precipitated at pH 7 to 8 by means of sodium hydroxide and the resulting product dissolved in the acid leach liquor. The final thorium concentration can in this way reach 100 to 200 g/1. with an overall thorium recovery of 93 per cent. 

There are also thorium recovery processes based on extraction from sulfuric acid solutions, e.g., with primary, secondary, or tertiary amines or alkyl phosphorous acids such as bis-2-ethylhexyl phosphoric acid (HDEHP) or dibutylbutyl phosphonate (DBBP). Thorium is then stripped into a nitric add solution. The alkyl phosphorous acid processes are often employed when recovering thorium as a by-product in uranium production. 

Thorium, uranium, and plutonium are well known for their role as the basic fuels (or sources of fuel) for the release of nuclear energy (5). The importance of the remainder of the actinide group Hes at present, for the most part, in the realm of pure research, but a number of practical appHcations are also known (6). The actinides present a storage-life problem in nuclear waste disposal and consideration is being given to separation methods for their recovery prior to disposal (see Waste treati nt, hazardous waste Nuclear reactors, waste managet nt). 

There are a number of minerals in which thorium is found. Thus a number of basic process flow sheets exist for the recovery of thorium from ores (10). The extraction of mona ite from sands is accompHshed via the digestion of sand using hot base, which converts the oxide to the hydroxide form. The hydroxide is then dissolved in hydrochloric acid and the pH adjusted to between 5 and 6, affording the separation of thorium from the less acidic lanthanides. Thorium hydroxide is dissolved in nitric acid and extracted using methyl isobutyl ketone or tributyl phosphate in kerosene to yield Th(N02)4,... 

Thorium is widely but rather sparsely distributed and its only commercial sources are monazite sands (see p. 1229) and the mineral conglomerates of Ontario. The former are found in India, South Africa, Brazil, Australia and Malaysia, and in exceptional cases may contain up to 20% Th02 but more usually contain less than 10%. In the Canadian ores the thorium is present as uranothorite, a mixed Th,U silicate, which is accompanied by pitchblende. Even though present as only 0.4% Th02, the recovery of Th, as a co-product of the recovery of uranium, is viable. 

Chabaux F, Cohen AS, O Nions RK, Hein JR (1995) 2 U- " U- °Th chronometiy of Fe-Mn crasts Growth processes and recovery of thorium isotopic ratios of seawater. Geochim Cosmochim Acta 59 633-638... 

Fig. 12.4 Simplified flow sheet used in the recovery of thorium from its ores.

Radium, thorium, and other radionuclides accumulate in uranium mill tailings. The potential environmental effects of these radionuclides has become of increasing concern to the public. In the future, it may be necessary to modify existing uranium recovery processes to accommodate removal of radium and perhaps other radioactive decay products of uranium. 

Some of the methods commonly used for the determination of thorium in biological materials are given in Table 6-1. The colorimetric methods are not capable of isotope-specific determination of thorium isotopes. Alpha spectrometric and neutron activation analysis are useful in the quantification of isotope-specific thorium and thorium-232, respectively, and have better sensitivities than colorimetric methods. Alpha spectrometry is the commonly used isotope-specific analysis for the determination of thorium-232 and the thorium-230 derived from the decay of uranium-238 (Wrenn et al. 1981). Standard reference materials (SRMs) containing thorium in human liver (SRM-4352) and human lung (SRM-4351) necessary for the determination of absolute recovery in a given sample are available from the National Institute of Standards and Technology (Inn 1987). 

Methods for Determining Biomarkers of Exposure and Effect. A few authors have found elevated levels of thorium in tissues of thorium workers and these studies have been discussed in Sections 2.6 and 5.4.4. However, there are no data in the literature that correlate the concentrations of thorium in any human tissue or body fluid with its level of exposure. If a biomarker for thorium in human tissue or fluid were available, the level of the biomarker in a tissue could be used as an indicator of exposure to thorium. Analytical methods with satisfactory sensitivity are available to determine the levels of thorium in most human tissues and body fluids of exposed and background population, but the recovery of thorium by these methods needs further refinement. 

Lanthanum is most commonly obtained from the two naturally occurring rate-earth minerals, monazite and bastnasite. Monazite is a rare earth-thorium phosphate that typically contains lanthanum between 15 to 25%. Bastnasite is a rare earth-fluocarbonate-type mineral in which lanthanum content may vary, usually between 8 to 38%. The recovery of the metal from either of its ores involves three major steps (i) extraction of all rare-earths combined together from the non-rare-earth components of the mineral, (ii) separation or isolation of lanthanum from other lanthanide elements present... 

There are several processes for commercial thorium production from monazite sand. They are mostly modifications of the acid or caustic digestion process. Such processes involve converting monazite to salts of different anions by combination of various chemical treatments, recovery of the thorium salt by solvent extraction, fractional crystallization, or precipitation methods. Finally, metalhc thorium is prepared by chemical reduction or electrolysis. Two such industrial processes are outlined briefly below. 

The Advanced Recovery Systems, Inc. (ARS) developed the patented, ex situ DeCaF hydrometallurgical technology to decontaminate fluoride by-products and to recover recyclable metals. The technology uses a proprietary acid mixture to digest the fluoride matrix, freeing radioactive contaminants (e.g., uranium, thorium, or radium) and hazardous contaminants (e.g., lead, arsenic, or chromium). Radioactive elements are recycled or disposed. Metals are also recycled, and fluoride is recovered as a high-value salt for aluminum smelting. 

---