For zirconium oxide

The mineral baddeleyite (Zr02) is found in nature only in small quantities. The main raw material for zirconium oxide ceramics is thus zircon (ZrSi04), from which pure Zr02 is produced by fusing with lime and coke to reduce the Si02 and the impurities. 

The natural raw material for zirconium oxide ceramics is the mineral zircon (ZrSi04)... 

Equilibrium Constants for Zirconium Oxidation-Reduction Reactions in 1 1 Mole KCl NaCl... 

The main production method for zirconium oxide is electric arc melting of zircon between 2100 and 2300°C. Dissociation still occurs at these lower temperatures, but solid zirconia is produced along with liquid silica. The purity of the Zr02 produced is about 99%. 

Table 7. Chemical etching recipes for zirconium oxide materials ...

Table 16. Preparation procedure II for zirconium oxide ZrOa...

Table 41. Recommended preparation procedure I for zirconium oxide ZrOj...

Table 42. Recommended preparation procedure II for zirconium oxide Z1O2...

Prepared in accordance with recommend procedure III for zirconium oxide (see Table 43). 

To form a ceramic object with a complex three-dimensional shape, the finely divided ceramic powder, possibly mixed with other powders, is compacted under pressure and then sintered at high temperature. The temperatures required are about 1650°C for alumina, 1700 C for zirconium oxide, and 2050°C for silicon carbide. During sintering the ceramic particles coalesce without actually melting (compare the sintering temperatures with the melting points listed in Table 12.4). 

An important application for zirconium oxide is as an alloying element in aluminum oxide ceramics. Sintered aluminum oxide is brittle. It has been found that the toughness can be considerably improved by mixing aluminum oxide with 4-15% zirconium oxide and sintering the two together. Then a white ceramic , tough enough for many applications, e.g. tools, is obtained. 

Another important binary ceramic system is that for zirconium oxide (zirconia) and calcium oxide (calcia) a portion of this phase diagram is shown in Figure 12.24. The horizontal axis extends to only about 31 wt% CaO (50 mol% CaO), at which composition the compound CaZr03 forms. It is worth noting that one eutectic (2250°C and 23 wt% CaO) and two eutectoid (1000°C and 2.5 wt% CaO, and 850°C and 7.5 wt% CaO) reactions are fonnd for this system. 

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. 

Hafnium dioxide is formed by ignition of hafnium metal, carbide, tetrachloride, sulfide, boride, nitride, or hydrous oxide. Commercial hafnium oxide, the product of the separation process for zirconium and hafnium, contains 97—99% hafnium oxide. Purer forms, up to 99.99%, are available. 

Tubes for dynamic membranes ate usually smaller (ca 6-mm ID). Typically, the tubes ate porous carbon or stainless steel with inorganic membranes (sihca, zirconium oxide, etc) formed in place. 

The Phalaborwa complex ia the northeastern Transvaal is a complex volcanic orebody. Different sections are mined to recover magnetite, apatite, a copper concentrate, vermicuhte, and baddeleyite, Hsted in order of aimual quantities mined. The baddeleyite is contained in the foskorite ore zone at a zirconium oxide concentration of 0.2%, and at a lesser concentration in the carbonatite orebody. Although baddeleyite is recovered from the process tailings to meet market demand, the maximum output could be limited by the requirements for the magnetite and apatite. The baddeleyite concentrate contains ca 96% zirconium oxide with a hafnium content of 2% Hf/Zr + Hf. A comminuted, chemically beneficiated concentrate containing ca 99% zirconium oxide is produced also. 

Zirconium oxide is fused with alurnina in electric-arc furnaces to make alumina—zirconia abrasive grains for use in grinding wheels, coated-abrasive disks, and belts (104) (see Abrasives). The addition of zirconia improves the shock resistance of brittle alurnina and toughens the abrasive. Most of the baddeleyite imported is used for this appHcation, as is zirconia produced by burning zirconium carbide nitride. 

Zirconium oxide is used in the production of ceramic colors or stains for ceramic tile and sanitary wares. Zirconia and siHca are fired together to form zircon in the presence of small amounts of other elements which are trapped in the zircon lattice to form colors such as tin—vanadium yellow, praseodymium—zircon yellow [68187-15-5] vanadium—zircon blue [12067-91 -3] iron—zircon pink [68412-79-3] indium—vanadium orange (105—108). 

Zirconium oxide increases the refractive index of some optical glasses, and is used for dispersion hardening of platinum and mthenium. Very fine zirconium oxide has been used for polishing glass but ceria seems to be preferred. 

Zirconium tetrafluoride [7783-64-4] is used in some fluoride-based glasses. These glasses are the first chemically and mechanically stable bulk glasses to have continuous high transparency from the near uv to the mid-k (0.3—6 -lm) (117—118). Zirconium oxide and tetrachloride have use as catalysts (119), and zirconium sulfate is used in preparing a nickel catalyst for the hydrogenation of vegetable oil. Zirconium 2-ethyIhexanoate [22464-99-9] is used with cobalt driers to replace lead compounds as driers in oil-based and alkyd paints (see Driers and metallic soaps). 

Hafnium-free zirconium is particularly weU-suited for these appHcations because of its ductiHty, excellent oxidation resistance in pure water at 300°C, low thermal neutron absorption, and low susceptibiHty to radiation. Nuclear fuel cladding and reactor core stmctural components are the principal uses for zirconium metal. 

Carbide. Zirconium carbide [12020-14-3] nominally ZrC, is a dark gray brittle soHd. It is made typically by a carbothermic reduction of zirconium oxide in a induction-heated vacuum furnace. Alternative production methods, especially for deposition on a substrate, consist of vapor-phase reaction of a volatile zirconium haHde, usually ZrCl, with a hydrocarbon in a hydrogen atmosphere at 900—1400°C. 

Zircon is synthesized by heating a mixture of zirconium oxide and silicon oxide to 1500°C for several hours (163). The corresponding hafnium silicate, hafnon, has been synthesized also. Zircon can be dissociated into the respective oxides by heating above 1540°C and rapidly quenching to prevent recombination. Commercially, this is done bypassing closely sized zircon through a streaming arc plasma (38). 

Zirconium tetrachloride is instantly hydrolyzed in water to zirconium oxide dichloride octahydrate [13520-92-8]. Zirconium tetrachloride exchanges chlorine for 0x0 bonds in the reaction with hydroxylic ligands, forming alkoxides from alcohols (see Alkoxides, METAl). Zirconium tetrachloride combines with many Lewis bases such as dimethyl sulfoxide, phosphoms oxychloride and amines including ammonia, ethers, and ketones. The zirconium organometalLic compounds ate all derived from zirconium tetrachloride. 

Another type of membrane is the dynamic membrane, formed by dynamically coating a selective membrane layer on a finely porous support. Advantages for these membranes are high water flux, generation and regeneration in situ abiUty to withstand elevated temperatures and corrosive feeds, and relatively low capital and operating costs. Several membrane materials are available, but most of the work has been done with composites of hydrous zirconium oxide and poly(acryhc acid) on porous stainless steel or ceramic tubes. 

Ceramic Membranes Alumina-based microfiltration membranes and porous carbon substrates are tightened for use as UF membranes usually by depositing a layer of zirconium oxide on the surface. 

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