Zirconia has good heat resistance and corrosion resistance and has a wide range of applications. The main producing areas of zirconium ore are Australia and South Africa, these two countries account for 65% of the world's output. Zirconium ore has stable chemical properties, strong corrosion resistance to molten steel, low expansion rate and high thermal conductivity, so it is widely used as a refractory material. Zirconium is a relatively large element on the earth, but high-quality zirconium ore is limited, and other materials are difficult to replace zirconium-based raw materials, so recycling should be considered.
1 Introduction
Zirconium (Zr) has good heat resistance and corrosion resistance, and it has a wide range of uses. It is mainly used in nuclear power fuel-coated tubes and other equipment related to nuclear power. It is also used in chemistry, medical equipment, and superconducting materials, and will continue to be developed in the future.
Dry zirconia manufactured by dry refining methods such as arc melting has the largest demand. In melting, most of the stabilizers such as calcium are added to zirconia. Widely used in ceramic tiles, cast steel shell materials, continuous casting nozzles, refractory materials for glass, abrasive materials, ceramic pigments, brake materials, and regulators for firing electronic parts.
Most wet refining methods use zirconium oxychloride as an intermediate raw material. Zirconium oxychloride is manufactured by decomposing zirconium ore in alkaline and removing impurities such as silicon. The aqueous solution of zirconium oxychloride is treated by co-precipitation and hydrolysis to produce wet zirconia. Wet zirconia has a wide range of uses. It can be used in electronic materials, automotive exhaust purification catalysts, industrial catalysts, oxygen sensors, precision ceramics, anti-reflection membranes, electrolytes for solid oxide fuel cells, high-quality paper membrane materials and adsorbents, etc. .
Zircon (ZrSiO4) is used as raw material for refractory materials for ironmaking and glass, additives for building tiles and sanitary ware and abrasive tools.
Zirconium is an element that is used for various purposes and has many possibilities. This article mainly introduces the market trends, characteristics and concerns of zirconium used in refractories.
The world's zirconium ore output in 2016 was 1,460 thousand tons. The producing countries of zirconium ore are mainly Australia and South Africa, which account for 65% of the world's production. The output of other producing countries has increased or decreased somewhat, but is basically stable.
Refractory materials account for a large proportion of zirconium demand. Zirconium ore series and zirconia series are expensive and have limited uses. Zircon-based refractory bricks are used in glass melting furnaces, zirconia-based unshaped refractory materials are used in steel-making ladle, and zirconia-based refractory bricks are used in steel-making continuous casting nozzles. In addition to refractory materials, zirconia demand trends are divided by use: electronic material components are being miniaturized, and the market for smart phones and sensor boards is expanding, so the demand is still maintained; the demand for precision ceramics is stable; the demand for automotive exhaust catalysts is horizontal fluctuation.
3.1 Zircon raw materials
Zirconium has a Clark value of 0.02 (20th place) and is an element widely distributed on the earth's surface. As a resource ore is produced in places such as beaches, river banks, and sand dunes. The deposits are concentrated into sand. Zircon has stable chemical properties, high corrosion resistance to molten steel, low expansion rate and high thermal conductivity, so it is widely used as a refractory material. Figure 1 illustrates the shape of zircon sand as an example. There are some differences in the shape, color and particle size distribution of zircon sand according to different places of origin.
Zircon contains about 1% to 2% of hafnium (Hf) as an impurity. Hafnium and zirconium have similar chemical properties and are not easy to separate. There are many reports that the thermal stability of zircon varies with its heating and impurities, and the solid-phase decomposition temperature ranges from 1600 to 1700°C. Zircon has a tetragonal crystal structure with a density of 4.6 to 4.7 and a Mohs hardness of 7.5. Most of the zirconium in zircon ore is replaced by uranium and thorium. There are also reports that some minerals with uranium and thorium as the main components are mixed in zircon. Zircon is decrystallized due to radioactive damage caused by alpha rays released from the ore (decidualization refers to the destruction of the crystalline structure by radioactive elements), and the density sometimes drops to about 4.0. The color of ore is generally reddish brown, but there are also yellow, green and blue. Heated at high temperature in a reducing atmosphere, the reddish brown will become colorless and transparent, which is very precious as a gem. The radioactivity of zircon will be described in detail in section 4.
The zircon sand commonly used in the market is mixed with silica sand, coexisting with magnetite, ilmenite, rutile and monazite. The largest producing area in the world is Australia. The mines are mainly concentrated on the east and west coasts. At the same time, it is also rich in resource ore such as ilmenite and rutile. Sometimes these impurities also contain radioactive substances. In recent years, the sales of low-radio-energy zircon produced in the United States and West Africa have gradually expanded, but the output of low-radio-energy zircon in the world is on a decreasing trend.
3.2 Zirconia-based raw materials
The raw materials of zirconia used in refractories are mainly oblique zircon, desiliconized zirconia and fused zirconia.
(1) Oblique zircon
The main producing areas of plagioclase are Russia and South Africa, but the mining areas in South Africa have been exhausted, and now only the Kovdorskiy mine in Russia is produced. The deposits in Russia contain a lot of iron and apatite, and the content of oblique zircon is about 0.15%. In the future, it may be mined from deposits with less plagioclase content, which requires attention.
(2) Desiliconized Zirconia
Desiliconized zirconia is produced by adding zircon (ZrSiO4) and a specified amount of carbon (C), using electric fusion. It is melted by arc heat at about 3000℃ in an electric melting furnace, and a reduction reaction occurs. The reaction formula is ZrSiO4+C→ZrO2+SiO↑+CO↑. SiO is a substance that is easy to sublimate. It will become a gas when melted, and it will be recovered by a dust removal device. This reaction is repeated to obtain a high-purity ZrO2 (desiliconized zirconia) melt. The cooling method is generally to tilt the electric furnace, while slowly discharging the molten material, while blowing with high-pressure air. It is also called foamed zirconia because it solidifies into a foamed state by blowing the molten material.
(3) Fused Zirconia
Fused zirconia is a high melting point material with good heat resistance, corrosion resistance and thermal shock resistance. These characteristics can be controlled by the type and amount of stabilizer. In addition, because of its low thermal conductivity, it is widely used in the nozzles of continuous casting equipment for iron and steel and regulators for firing electronic parts, as well as heat insulation coatings and various thermal spray materials.
The production method of fused zirconia raw material is the dry refining method described above. Add the specified stabilizer [mainly calcium oxide (CaO)] to the desiliconized zirconia or the oblique zircon, and melt it together with an electric arc furnace. After cooling and solidifying the molten material, it is pulverized into powder and a whole-grain fused zirconia raw material is obtained.
Pure zirconia without stabilizer is the three crystal phases of monoclinic phase, square phase and cubic phase in the order of phase stability at low temperature. In terms of equilibrium, the low temperature below 1170°C is a monoclinic phase, 1170-2370°C is a square phase, and 2370°C to a melting point (about 2700°C) is a cubic phase. The phase transition temperature changes according to factors such as kinetics. There have been many studies on the application of the high melting point of zirconia to refractory materials. The monoclinic phase of zirconia transfers to the square phase at high temperature, accompanied by a volume shrinkage of about 4.6%. When cooling to normal temperature, thermal expansion hysteresis can be seen, but it shifts to monoclinic phase, and there is volume expansion. If this volume expansion is not controlled, it cannot be used as a practical material. The solution is to add a stabilizer to make it solid-dissolve in zirconia to control the crystalline phase and suppress the expansion and contraction caused by phase transition. The crystalline phase controlled in this way becomes a square phase and a cubic phase, which are also called stable phases. Table 6 lists the physical properties of zirconia and zircon.
As a stabilizer for stabilizing zirconia, calcium oxide is generally used, but for special purposes, yttrium oxide (Y2O3) and magnesium oxide (MgO) are used. Yttrium is expensive, but yttrium-stabilized zirconia is not easy to produce destabilization, and has strong resistance to slag containing silica (SiO2). The destabilization mentioned here means that due to the elution of the stabilizer and the influence of mechanical stress and thermal shock, the stable phase transfers to the monoclinic phase. The stabilizer can be seen to dissolve in contact with the slag. In addition, stabilized zirconia is used for continuous casting nozzles, which is one of the causes of damage. The phase transition caused by mechanical stress is called stress-induced phase transition, which is derived from the strength and toughness of zirconia.
The proportion of the stable phase is called stability. The calculation method of stability is described later.
Secondly, list the thermal expansion properties and destabilization properties data in the typical composition. Regarding the thermal expansion properties, data of calcium stabilized zirconia (CSZ) is shown. The calcium oxide addition amount is 4%, 6% and 8%. After the CSZ powder is crushed to about 1 μm, it is press-formed into a predetermined size. The obtained molded body was sintered at 1500°C and used to measure thermal expansion.
Regarding the destabilization properties, data on CSZ, yttrium-stabilized zirconia (YSZ), and magnesium-stabilized zirconia (MSZ) are shown. The original sample of CSZ is the sample for measuring thermal expansion; the original sample of YSZ is the sample with the stabilizer addition amount of 6%, 8% and 16%; the original sample of MSZ is the sample with the stabilizer addition amount of 3% and 6% of the sample. These powders were pulverized to 1μm and kept at 1450°C for 300h. First take samples after 30h, and take samples every 60h after 60h. Figure 6 shows the destabilization properties of fused zirconia. Since fused zirconia undergoes a quenching process after melting, the heat easily becomes non-equilibrium state. As the holding time increases, the heat becomes an equilibrium state. In addition, more stabilizers are added to improve the stability, but the mechanical strength tends to decrease. It is necessary to select the type and amount of stabilizer according to the application.
4About the radioactivity of zircon
The products used in daily life, continuously inhaled more than 3.5mg/m3 of inhalable dust, within 2m of the zircon powder, and continuously exposed to this environment for 2000h every year. Because of the low radiation energy, it is transported as a non-dangerous product.
As U-238, zircon has a radioactivity of 10Bq/g; as Ra-226, it has a radioactivity of 3 to 4Bq/g; as Th-232, it has a radioactivity of 10Bq/g. Therefore, attention should be paid to radioactivity. The content of uranium and thorium in zircon varies according to the place of origin, and zircon with higher radioactive energy cannot be traded in the market. Zircons with high radioactivity (containing about 3% Th, etc.) are rare, but these need to be treated as low specific radioactive materials. In order to avoid dust inhalation, it is necessary to avoid prolonged work near storage places. As mentioned earlier, in the desiliconized zirconia using zircon as a raw material, the silica gas contains almost no radioactive elements, and all remain in the desiliconized zirconia. In short, the radioactive elements of zirconium are concentrated about 1.5 times (radioactivity is about 1.5 times), so be careful.
5 Conclusion
The market trends and characteristics of zircon and zirconia raw materials are briefly introduced. It is generally believed that each raw material is developed for various purposes, and the demand will increase in the future. The zircon mining area in South Africa has been exhausted, and the supply trend of Russian mines needs attention. In addition, high-quality zircon with low content of radioactive isotopes is on a declining trend, and the supply trend is the same as that of plagioclase.
Zirconium is a more abundant element on the earth, but high-quality zircon is very limited. As the characteristics of zirconium determine that other materials are difficult to replace, it is necessary to consider the issue of recycling.
Wechat Us 
