Zircon leaching

This dissociated zircon is amenable to hot aqueous caustic leaching to remove the siHca in the form of soluble sodium siHcate. The remaining skeletal stmcture of zirconia is readily washed to remove residual caustic. Purity of this zirconia is direcdy related to the purity of the starting zircon since only siHca, phosphate, and trace alkaHes and alkaline earth are removed during the leach. This zirconia, and the untreated dissociated zircon, are both proposed for use in ceramic color glazes (36) (see Colorants for ceramics). 

Chemical leaching (1,12) with acids is used to extract metal contamination. High purity zirconia, Zr02, is produced by the caustic fusion of zircon [14475-73-1], ZrSiO, foUowed by the chemical removal of sUica. Chemical leaching is generaUy foUowed by washing. 

Zircon Alkali fusion at 600 °C ZrSi04 + 4 NaOH —> Na2Zr03 + Na2Si03 + 2 H20 Water leaching... 

HAL [Hot acid leaching] A process for purifying silica sand or zircon by leaching out surface iron compounds with hot sulfuric acid. Derived from an earlier process, invented in 1955 by British Industrial Sand, in which silica sand was treated with hot, gaseous hydrogen chloride. The process for cleaning zircon sand was developed jointly by Hepworth Minerals Chemicals, UK, and Metallurgical Services Pty, Australia, in 1991. 

The overall development program included the study of other exchange materials such as niobates, zirconates, and tantalates, some of which had superior ion exchange and leaching properties, but were initially economically unattractive as compared to the titanates. These alternate materials will be briefly discussed along with applications to nuclide stabilization in other areas of nuclear processing. 

Optical microscope observations have shown that zircons often exhibit extremely complex microstructures (on the scale of 1-100 /tm), in which changes of birefringence correlate with the distribution of U and Th (Chakoumakos et al. 1987). However, no attempts appear to have been made to relate these microstructures to other impurities (such as water-related species) and crystal defects, both of which may significantly influence the processes of metamictization and recrystallization. Such a study involving TEM might also provide important information about the diffusion and leaching of radioactive impurities (and the products of their decay), processes that have important implications for ceramic nuclear-waste disposal and for techniques of age determination based on measurements of Pb/U isotopic ratios. 

Zircon is usually accompanied by quartz, ilmenite and rutile, and in lesser amounts by garnets, kyanite and monazite, The treatment processes utilize the differences in density ilmenite and rutile are extracted by magnetic and electrostatic separation respectively. These processes provide technical-grade zircon purer zircon is produced by acid leaching. The product usually contains particles mainly 75 — 150 im. 

Based on the leaching data [40,69,83] as well as data on radiation resistance of fast neutron irradiated [84-85] and actinide-doped Synrocs [86-88], as well as individual synthetic phases and their natural analogues study [14-18,89-91] zirconolite- and pyrochlore-based ceramics have been proposed for immobilization of excess plutonium and the other actinides. Moreover, additional phases, which were not part of the Synroc formulation, have been considered because of their chemical durability and radiation resistance (e.g., murataite, zircon, garnet, monazite, britholite). Of particular interest are the zirconate pyrochlores, many of which are extremely radiation resistant [8,92]. Zirconolite-based ceramics. Zirconolite, ideally CaZrTi207, is a major actinide host phase in the Synroc-type ceramics. Study of natural and synthetic samples... 

The dissolution rate of zircon in water (Soxhlet tests) decreases from 4.1x10 at 250 to 4.6x10 g/(m xday) at 90 °C [184]. Synthetic ceramics doped with of 6 to 10 wt.% Pu were produced [185]. Leach rates of Pu (MCC-1, 90 °C) were found to be 7x10 (sample with 6 wt.% Pu) and 0.2 g/(m xday). In the sample with 10 wt.% Pu, extra PUO2 was also present. Zircon is amorphized at a dose of 10 a-events/g or 0.6 dpa [72,186,187] and exhibits significant volume expansion (by 18-20%). This leads to microcracking [188] and an increase of the leach rate. Zircon has a rather low capacity with respect to trivalent actinides... 

An unusual precipitation uses a molten salt as solvent. For the preparation of lead zirconate titanate, Ti02, Zr02, and excess PbO are mixed with a NaCl-KCl mixture and reacted at 1000 C. Lead zirconate titanate is less soluble in the molten salts than is Ti02 or Zr02 and precipitates. Finally, the salt and excess PbO are leached out using an acetic acid solution. 

Steam is evolved, the mix becomes viscous, and finally is converted to a fragile, porous solid ( frit ) when the temperature reaches 530°C. After cooling, this solid is ground and leached with water, which extracts the NajSiOa. The residue then is leached with acid, which dissolves the NajZr03. The final residue consists of unreacted zircon, which may be recycled. Any desired ziiconyl salt can be made by using the appropriate acid in the final leaching step. 

Other impurities can enter the slurry by being leached from the refractories themselves. Calcium and Magnesium are impurities that can have very deleterious effects on the stability of colloidal silica. Some refractories like Zircon are acidic in their reaction with colloidal silica and tend to reduce the pH over time. The stability of alkali stabilized colloidal silicas depend both on minimizing the presence of impurities (especially polyvalent metal ions) as... 

The sodium silicate is removed by leaching in water, which at the same time hydrolyzes the zirconates to complex hydrated hydroxides of zirconia. These can be directly calcined to yield impure oxides, or further purified for example by treatment with sulfuric acid. The zirconyl sulfates formed are precipitated with a solution of ammonia to form the basic zirconium sulfate Zr50g(S04)2-JC H2O, and then calcined. 

The cold frit is crushed in equipment which is preferably totally enclosed to prevent personal contact with the irritant alkaline dust. Leaching is then carried out with water to remove sodium silicate and excess sodium hydroxide. About 7 tons of water per ton of zircon are appropriate, this quantity being divided between several successive leaching operations. Solid-liquid separation is by decantation between each leach. The sodium zirconate is finally filtered off in a fairly dry condition. It is advisable not to use a larger excess of water or peptization of colloidal zirconium com-... 

In a similar process used in the U.S.S.R. and described by Orgarev et a/. advantages are claimed for a potassium chloride-potassium fluoro-zirconate melt, the principal improvement being the increased solubility of the salts at the leaching stage. This is presumably due to the absence of sodium fiuoride. 

Preparation of zirconia by alkaline leaching. Zircon is roasted with sodium hydroxide and calcia at 600 to WOO C. During the process sihca reacts with calcium and sodium to yield calcium and sodium metasihcates. After acid leaching, the product is dyed and calcined to yield pure zirconia with less than 0.10 wt.% residual sihca. 

Semiindustrial tests were carried out on zirconium and hafnium extraction by TVEX-65% TBP from nitric silica-containing pulps with the following composition Zr 17.0-44.0 g/L, Hf 0.2-0.5 g/L, A10.2-0.37 g/L, Fe 1.7-4.0 g/L, Si 0.8-8.2 g/L, HNO3 260.0-420.0 g/L. The pulp was received as a result of HNO3 leaching of cake of zircon with soda. 

Leach of sphene concentrate 28.49 16.70 50.48 Spene is more concordant than zircon OOSTHUYZEN and Burger (1965)... 

Little laboratory experimental work has yet been done on sphene. Oosthuyzen and Burger (1965) did analyze the isotopic composition of the acid leach solution of their sphene (Table 5) and found it to be somewhat radiogenic however, no more so than is found in studies of acid teachings of zircon. (See section on zircon.) Burger et al. (1965) also reported values on teachings of sphene from 600-m.y.-old rocks which may be as radiogenic as the sphene ( Pb/ " Pb of 138.2 of leach LI from sphene X the sphene has a value of 114.3). The effect of this leaching on the sphene is not yet completely evaluated. 

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