Zircon sand

The heavy mineral sand concentrates are scmbbed to remove any surface coatings, dried, and separated into magnetic and nonmagnetic fractions (see Separation, magnetic). Each of these fractions is further spHt into conducting and nonconducting fractions in an electrostatic separator to yield individual concentrates of ilmenite, leucoxene, monazite, mtile, xenotime, and zircon. Commercially pure zircon sand typically contains 64% zirconium oxide, 34% siUcon oxide, 1.2% hafnium oxide, and 0.8% other oxides including aluminum, iron, titanium, yttrium, lanthanides, uranium, thorium, phosphoms, scandium, and calcium. 

Decomposition of Zircon. Zircon sand is inert and refractory. Therefore the first extractive step is to convert the zirconium and hafnium portions into active forms amenable to the subsequent processing scheme. For the production of hafnium, this is done in the United States by carbochlorination as shown in Figure 1. In the Ukraine, fluorosiUcate fusion is used. Caustic fusion is the usual starting procedure for the production of aqueous zirconium chemicals, which usually does not involve hafnium separation. Other methods of decomposing zircon such as plasma dissociation or lime fusions are used for production of some grades of zirconium oxide. 

Chlorination. Historically, the production of zirconium tetrachloride from zircon sand involved first a reduction to carbide nitride (see above) followed by the very exothermic reaction of the cmshed carbide nitride with chlorine gas in a water-cooled vertical shaft furnace ... 

Normally, zircon sand is readily available as a by-product of mtile and ilmenite mining at ca 150 per metric ton. However, zircon and baddeleyite are obtained as by-products of their operations, and therefore, the supply is limited by the demand for other minerals. In 1974, when a use for zircon in tundish nozzles developed in the Japanese steel industry, a resulting surge in demand and stockpiling raised zircon prices to 500/t. Worldwide production by country is given in Reference 80. 

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. 

Zirconocenium cations, 76 95 Zircon pigments, 79 404-405 Zircon refractories, ASTM classifications and specifications for, 27 509-510 Zircon sands, 73 81 analysis of, 26 623t Zirpro Process, 26 404 Ziziphin, 24 246 Zn-Ag cartridges... 

Hafnium is obtained commercially from mineral zircon, which is zirconium orthosilicate [14940-68-2]. Zircon usually contains hafnium oxide, Hf02, in an amount that ranges between 1 to 2%. Zircon sand is separated from heavy... 

Zirconia. Zircon (zirconium silicate), the most widely occurring zirconium-bearing mineral, is dispersed in various igneous rocks and in zircon sands. The main deposits are in New South Wales, Australia Travancore, India and Florida in the United States. Zircon can be used as such in zircon refractories or as a raw material to produce zirconia. The zircon structure becomes unstable after about 1650°C, depending on its purity, and decomposes into Zr02 and Si02 rather than melting (see Zirconiumand zirconium compounds). 

Zirconia Refractories. The most common zirconia-containing refractories are made from zircon sand and are used mosdy for container glass-tank subpaver brick. Refractory blocks made from a composition of zircon and alumina, used to contain glass melts, are generally electromelted and then cast. These exhibit excellent corrosion resistance but are subject to thermal shock. Refractories made from pure Zr02 are extremely expensive and are... 

Zirconia. Zircon (zirconium silicate), the most widely occurring zirconium-bearing mineral, is dispersed in various igneous rocks and in zircon sands, Zircon can be used as such in zircon refractories or as a raw material to produce zirconia, ZrC>2. 

In the TBP process, which was developed in the USA283 and in England,286 zirconium(IV) hydroxide (produced, for example, by the hydrolysis of the material obtained from the caustic fusion of zircon sand) is dissolved in nitric acid to give a solution containing 30-100 g of zirconium (plus hafnium) per litre and 5-8 M free nitric acid. The zirconium is extracted into a 50-60% solution of TBP in a suitable hydrocarbon diluent, the loaded organic phase is washed in 5 M nitric... 

NRC has issued an assessment of potential doses to the public associated with the distribution, use, and disposal of exempt products or materials containing low levels of source or byproduct material (Schneider et al., 2001) (see Section 4.1.2.5.2). In a case involving disposal of large volumes of zircon sand produced in processing of zirconium-bearing minerals, the estimated annual dose to a future on-site resident at a disposal site was 100 p.Sv, due to the elevated levels of thorium and uranium. In all other cases, however, the estimated annual dose was substantially less than 10 xSv. Since the volumes of exempt material were large in many cases, this analysis indicates that substantial volumes of waste that contains low levels of radionuclides are potentially exemptible. 

Foundries using zircon sands Sandblasting with zircon sands... 

Separation of hafnium from zirconium. In a zirconium-hafnium separation process developed by Eldorado Nuclear, a mixture of sodium zir-conate and hafnate can be obtained by fusing zircon sand with NaOH and dissolving the product in water. Acidification with nitric acid then gives aqueous zirconyl (ZrO +, or hydrolyzed Zr + ) and hafnyl (HfO " ") nitrates. Extraction with TBP then gives an extract containing mainly [Hf0(N03)2(TBP) ], but most of the ZrO " " remains in the aqueous phase and can be recovered on evaporation as essentially Hf-free Zr0(N03)2(s). The effectiveness of this process can be ascribed to compounding of factors... 

The alkali-fuaon process was developed by the Ames Laboratory of the U.S. Atomic Energy Commission [B2] to provide a method for producing zirconium salts that did not need the high temperature of an electric furnace. A flow sheet for this process is shown in Fig. 7.5. In this process, zircon sand is nuxed vrith from 1.0 to 1.5 times its wei t of sodium hydroxide, and the mixture is heated in a furnace at 565°C. The sodium hydroxide melts at 318°C, and as its temperature rises it reacts with the zircon ... 

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