Properties of Zirconium—- a Rare Metal

image_pdfimage_print

Zirconium is a lustrous metal with silver gray, with density of 6.49, melting point of 1,852°C and boiling point of 4,377°C. Zirconium is not chemically active, but dense zirconium metal is relatively stable in air, and loses its metallic luster because the surface forms oxide coating on heating. Zirconium metal powder is flammable in air, so it can be used as detonators and smokeless powder. Thin zirconium wire can be lit by matches. With a strong affinity for oxygen, zirconium can take away oxygen of magnesium oxide, beryllium oxide and thorium oxide, and make itself into zirconium oxide. With a strong hydrogen absorption property, zirconium can be used as hydrogen storage materials. At high temperatures, zirconium can react with not only nitrogen, but also with non-metallic elements and many metallic elements to generate solid solution compounds. With the corrosion resistance, zirconium can’t react with dilute hydrochloric acid, dilute sulfuric acid and alkali solution, but it easily dissolves in hydrofluoric acid and aqua regia.

Zirconium content is 0.025% in the earth crust, ranking the 20th. Although there are a dozen minerals containing ZrO2 more than 20%, only zircon (ZrSiO4) and baddeleyite (ZrO2) are used for industrial production. Zircon has a symbiosis with ilmenite, rutile and monazite, and it can be found in the beach sands too. All zircons contain hafnium oxide (HfO2) and radioactive substances, generally with radiation intensity of magnitude of 10-7mCi/g. Zircon’s radioactivity increases with its HfO2 content.

Zircon and baddeleyite are the main source of zirconium. Zircon can be reduced to zirconium metal by firstly adding to appropriate amount of petroleum coke, and then injecting chlorine gas at a temperature of 1,000°C to obtain zirconium tetrachloride (ZrCl4), finally making its vapor in contact with the molten magnesium metal. High-purity zirconium metal can be made by the thermal decomposition of iodides.

ZrCl4 is solid at room temperature, and sublimates at the temperature of 437°C. Therefore, ZrCl4 obtained from the condenser results of the gaseous solidification, and high-density products can be got so long as people can control heat transfer rate and other conditions. ZrCl4 can be reduced to ZrCl3 and ZrCl2, which are major components of molten salt in the production of metal zirconium by the electrolysis. Preparation for general industrial zirconium is to make fine ZrCl4 through the sublimation and purification, and process the reduction by magnesium, thereby zirconium sponge can be obtained without separation of hafnium.

Zirconium is used primarily as cladding materials of elements for nuclear reactor fuels, so there is a hafnium-zirconium separation process in the zirconium smelting process. NH4CNS-MIBK solvent extraction is the most common industrial separation method, with methyl isobutyl ketone (MIBK) as the extraction agent. Disadvantages of this method are: a. low separation coefficient and many series required; b. NH4CNS is easy to decompose and produce CN-, which causes waste water with toxicity and has to be handled in-house.

In recent years, there are the HNO3 series TBP (tributyl phosphate) extraction and HCl-HNO3 series TBP extraction. For the former, its ore decomposes by NaOH melting method, bringing a range of difficulties including three-phase in the extraction. The latter uses ZrCl4 as the raw material which avoids the above difficulties. But there is a defect that the solution is of strong corrosion. Then the ZrO2 obtained is chlorinated once again to ZrCl4, which is called secondary chlorination in the industry. ZrCl4 is made into the crude zirconium by the sublimation, purification, and then metal thermal reduction (magnesium reduction or sodium reduction), finally removing MgCl2 by the vacuum distillation and recycling the excess magnesium (watering during the sodium reduction). This process is similar to the reduction of titanium, with the only difference that magnesium requires for the pre-treated purification. Chemical reaction formula of magnesium reduction is: ZrCl4 + 2Mg → Zr + 2MgCl2, reduction temperature is about 850°C. Temperatures of the vacuum distillation are from 950°C to 1,000°C. Zirconium itself has the suction effect, so the final vacuum level is generally 10-5 torr.

Preparation of high-purity zirconium is to make ZrI4 decompose on the hot wire, known as crystalline rods in the industry. ZrI2 and ZrI3 play roles in this process. Zirconium and zirconium alloys adopt the vacuum consumable electro-arc remelting furnace for the melting and ingot casting, with pipes as the most commonly used section bar and with molding methods including forging, extrusion, drawing, the same as method of machining titanium tubes.

Zirconium and zirconium alloys are mainly used in water-cooled atomic reactors. In the nuclear reactor, the uranium rod does not directly contact with water. Because hot water erodes uranium rods which make water stained with radioactivity, it will harm to human health. Zirconium as sheath for uranium rods meets the following four requirements: a. with strong corrosion resistance, it will not react with nuclear fuels and the heat transfer medium (e.g. water); b. it has enough strength, heat resistance, corrosion resistance; c. it rarely absorbs neutrons so as to ensure the process of the “chain reaction” of the fission; d. it is easy to mold.

Zirconium powder is also used as an additive of special steel. Stainless steel containing zirconium and heat-resistant steel are important materials for the production of armored vehicles, tanks, artillery and armor plates, etc. Zirconium can not only strengthen the strength and hardness of steel, but also improve the mechanical processing properties, hardenability and weldability of steel. It can also shred sulfides of steels so as to refine its grain components. Adding zirconium can enhance steel’s oxidation resistance, and significantly increase the corrosion resistance as well. With a high melting point of 2,675°C and good chemical stability, zirconium dioxide is used as an advanced refractory material.