Zircon chlorination

In France, Compagnie Europnene du Zirconium (CEZUS) now owned jointly by Pechiney, Eramatome, and Cogema, uses a separation (14) based on the extractive distillation of zirconium—hafnium tetrachlorides in a molten potassium chloride—aluminum trichloride solvent at atmospheric pressure at 350°C. Eor feed, the impure zirconium—hafnium tetrachlorides from the zircon chlorination are first purified by sublimation. The purified tetrachlorides are again sublimed to vapor feed the distillation column containing the solvent salt. Hafnium tetrachloride is recovered in an enriched overhead fraction which is accumulated and reprocessed to pure hafnium tetrachloride. 

In the early zircon chlorination plants such as used by W. J. Kroll [K3] at Albany, Oregon, zircon was first converted to zirconium carbide by reaction with graphite in a graphite-lined arc furnace at 1800 C ... 

Carbochlorination. Milled zircon and coke are reacted with hot chlorine gas in a fluidized bed using chlorine as the fluidizing medium ... 

The chlorination is mostly carried out in fluidized-bed reactors. Whereas the reaction is slightly exothermic, the heat generated during the reaction is not sufficient to maintain it. Thus, a small amount of oxygen is added to the mixture to react with the coke and to create the necessary amount of heat. To prevent any formation of HCl, all reactants entering the reactor must be completely dry. At the bottom of the chlorination furnace, chlorides of metal impurities present in the titanium source, such as magnesium, calcium, and zircon, accumulate. 

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 ... 

An extension of the reduction-chlorination technique described so far, wherein reduction and chlorination occur simultaneously, is a process in which the oxide is first reduced and then chlorinated. This technique is particularly useful for chlorinating minerals which contain silica. The chlorination of silica (Si02) by chlorine, in the presence of carbon, occurs above about 1200 °C. However, the silica present in the silicate minerals readily undergoes chlorination at 800 °C. This reaction is undesirable because large amounts of chlorine are wasted to remove silica as silicon tetrachloride. Silica is, therefore, removed by other methods, as described below, before chlorination. Zircon, a typical silicate mineral, is heated with carbon in an electric furnace to form crude zirconium carbide or carbonitride. During this treatment, the silicon in the mineral escapes as the volatile oxide, silicon monoxide. This vapor, on contact with air, oxidizes to silica, which collects as a fine powder in the furnace off-gas handling system ... 

The metal is most often recovered from its principal ore, zircon. The ore is mined, crushed and preliminary segregation is by gravity, electrostatic, and magnetic separation. Separated ore mixed with carbon is charged into an arc furnace and heated to about 3,500°C. This forms zirconium carbide and silicon monoxide, and the monoxide is driven off as vapor. Zirconium carbide is then placed in a chlorinator and heated with chlorine gas at high temperatures. The carbide is converted to zirconium tetrachloride, ZrCfl. Also, small amounts of hafnium that is always associated with zirconium converts to its tetrachloride, HfCfl. 

The principle Zr ore, zircon (Zr silicate) is processed by caustic fusion or by direct chlorination of milled coke and zircon mixts. Washing of the Na fusion cake leave an acid soluble hydrated Zr oxide, whereas chlorination yields mixed Si and Zr tetrachlorides which are separated by distillation. Removal of the Hf from the Zr takes place through counter current liq-liq extraction (Ref 33), For this purpose the oxide or the tetrachloride is dissolved in dil hydrochloric acid to which ammonium thiocyanate is added as a complexing agent. The organic extracting phase is methyl isobutylketone... 

Zirconium(IV) oxide reacts with phosgene at 400 C to give zirconium(lV) chloride [360], and zircon (ZrSiO ) reacts with phosgene between 1000 and 1250 C to give ZiCl [120]. Phosgene has been postulated as an intermediate in the chlorination of zircon [1943], anatase or rutile [1767]. 

Zirconium carbide, ZrC, is made by reacting Zr02 or zircon with graphite in an electric furnace. It has a very high melting point, 3420°C. It reacts with chlorine at SOO C to produce ZrCU. 

Chlorination of zircon has been the process mainly used in the United States because it produces ZrCU, which is used in the Kroll process for making zirconium metal (Sec. 8.3), and because ZrCU was the feed material for the first process developed for separating hafnium from zirconium, using thiocyanate extraction (Sec. 7.3). 

In the newer plants, such as the Wah Chang plant, a mixture of zircon and carbon is chlorinated at 1200°C, to produce ZrCU in a single step ... 

The principal steps in this direct chlorination process for converting zircon to ZrCU are shown in Fig. 7.4. Gases from the chlorinating furnace are cooled to around 100°C to condense crude solid ZrCU and FeCU, then cooled further to condense SiCU, TiCU, and AICI3. The crude ZrCU is purified by sublimation with hydrogen in a stainless steel retort. Hydrogen reduces volatile FeCU to nonvolatile FeCU, which remains in the retort with ZrOj and other nonvolatile impurities. This process removes most of the metals associated with zirconium in zircon except hafnium. 

Industrial by-products that contain zirconium are mainly zircon and zirconia, both of which are insoluble in water, largely inert, and of low toxicity. Water-soluble zirconium compounds are converted at pH 4-9.5 into insoluble zirconia. The only possible atmospheric emission of other zirconium compounds is that of chlorinated and/or hydrolyzed oxychlorides from the processing of sponge zirconium using the Kroll process (reduction of ZrCl4). Analytical data corresponding to the extent of these emissions do not exist (Smith and Carson 1978), but there is no doubt that the exposure of the general population to zirconium compounds is small. 

Pure zirconia is obtained via the chlorination and thermal decomposition of zirconia ores, their decomposition with alkali oxides, and lime fusion. The initial stage of the process is based on the chlorination of zircon in the presence of carbon at a temperature of 800-1200 °C in a shaft furnace ... 

Zirconium is extracted mainly from the siUcate mineral, zircon (ZrSi04), which is found as a beach sand, usually in association with other minerals of commercial value, in Australia, India, Madagascar, Malaya, Ceylon, parts of Africa and the United States. The mineral is obtained in a relatively pure condition by ore dressing methods, except that it is usually associated with about 2 per cent of the isomorphous, and chemically very similar, hafnium silicate. The sihcate has been used directly, in admixture with charcoal, for chlorination to zirconium tetrachloride. The reaction can be represented as,... 

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