Methods of obtaining nuclear-grade metallic zirconium
The main industrial source of zirconium is zircon ZrSiO4 mineral.
Zirconium ores are concentrated by gravity methods with subsequent purification of concentrates by magnetic and electrostatic separation.
For industrial zirconium production zirconium concentrates with the zirconium dioxide content of 60-65% are used.
Production of nuclear-grade metallic zirconium is carried out in several ways:
– by means of fluoride technology of calcium-thermal reduction of potassium fluorozirconate for production of a primary zirconium ingot and its subsequent melting to a commercial ingot;
– using a fluoride technology by means of potassium fluorozirconate electrolysis in molten KCl, NaCl or their mixture with obtaining zirconium crystals and their subsequent purification and melting to commercial ingots;
– using an iodide technology with production of ZrI4, its dissociation on a hot tungsten or zirconium wire to obtain crystals of metallic zirconium and its use in a charge for smelting a nuclear-pure zirconium alloy;
– according to chloride technology by chlorination of zircon raw materials to obtain technical ZrCl4, its subsequent purification by sublimation and magnesium-thermal reduction of purified ZrCl4 with production of zirconium sponge and its remelting to an ingot.
The cleanest zirconium is obtained using iodide and fluoride technologies, but they are more complex and expensive compared to chloride technology. Therefore, many manufacturers of nuclea-pure metal zirconium (USA, France, India, China) use chloride technology.
Process of magnesium-thermal production of zirconium
The magnesium-thermal process of zirconium reduction is carried out using the Kroll method. Interaction of zirconium tetrachloride with liquid chlorine takes place according to the following reaction:
ZrCI4(gas) + Mg(melt)→ Zr(solid) + MgCI2(melt)
The basic process diagram for production of zirconium sponge includes the following process operations:
- salt chlorination of zirconium dioxide;
- purification of the obtained technical zirconium tetrachloride;
- reduction of purified zirconium tetrachloride;
- vacuum separation of zirconium-containing reaction mass;
- preparation of chemical reducing agent.
1. Salt chlorination of zirconium dioxide.
The main materials for carrying out the process of chlorination are:
- rough zirconium dioxide ZrO2;
- pitch coke;
- salts NaСl and КСl;
- evaporated chlorine.
Pitch coke is crushed, dried to a moisture content of <0.1% and grinded to a fraction of -0.16 mm (>90%). The charge components are mixed, fed into a hopper and loaded in doses with a screw feeder onto the melt mirror.
The raw material/coke ratio is approximately (10-7.5) : 1.
Evaporated chlorine is supplied from a cylinder flask.
The ZrO2 chlorination process is carried out in a salt chlorinator, which is equipped with a central chlorine pipeline, heated by an electric furnace and has a drainage device.
The central chlorine pipeline is made of quartz. The lower end of the chlorine line rests against the bottom of the chlorinator, and chlorine passes through the axial channel and bubbles the melt through the distributor from below.
When the melt level in the chlorinator increases to 700 mm or 5-10% СаCl2 accumulates in the melt, it is drained into a receiving metal sheet located under the chlorinator.
Chlorination is carried out with a chlorine flow of 90 dm3/h and a melt temperature of 850о C.
The vapor-gas mixture from the chlorinator is discharged through a gas duct for condensation of ZrCl4 sublimates into a condenser. Zirconium tetrachloride sublimates from a solid to a vapor state at 330°C and condenses from a vapor to a solid state without a liquid phase stage.
The resulting technical ZrCl4 is unloaded into a collecting hopper and fed for reduction-sublimation purification.
2. Purification of the technical zirconium tetrachloride product.
The cleaning process is carried out in a continuous mode, in which technical ZrCl4 from the hopper is fed continuously or in portions into the salt sublimator by a feeder.
The sublimator is protected from atmospheric air by argon supply.
Purification of technical zirconium tetrachloride consists of evaporating ZrCl4 from a bath filled with molten salts KCl + NaCl in a ratio of 1:1, with addition of the reducing agent (charcoal or metallic Mg) and is carried out at a temperature of 450-500о С.
Purified ZrCl4 vapour is deposited in chamber condensers. The purified product is collected in sealed containers and transferred to the reduction stage to obtain sponge zirconium.
The working melt in the sublimator is periodically renewed by draining and adding fresh salts. Charcoal or metal Mg is renewed when required.
As a result of reduction-sublimation purification purified zirconium tetrachloride of satisfactory quality regarding the main impurities is obtained with a zirconium extraction coefficient of 80-85% as a technical product.
3. Reduction of purified zirconium tetrachloride.
Reduction of zirconium tetrachloride takes place stepwise through formation of lower zirconium chlorides. Besides zirconium tri- and dichlorides are unstable at temperatures above the melting point of magnesium dichloride and disproportionate into zirconium tetrachloride and zirconium monochloride.
Interaction of magnesium and zirconium tetrachloride begins at a temperature of 410-470о С. Below 650о С zirconium tetrachloride is reduced only to lower chlorides.
The reduction of zirconium tetrachloride is a complex heterogeneous process involving several solid phases, in which the physical phenomena of wetting, evaporation, mass and heat transfer determine the nature of the chemical reaction.
The reduction plant includes:
- an electric pit-type heating furnace with a power rating of 30 kW with an air cooling system for the reductor;
- a reductor proper;
- a water cooling system for the reductor flanges;
- a system for supplying argon to the reductor;
- a vacuum system;
- a system for monitoring and regulating process parameters;
- a power supply system with a control cabinet;
- a device for feeding purified powdered zirconium tetrachloride into the device;
- a container for draining magnesium chloride from the reductor;
- a gas venting system.
In this reduction method magnesium is in a liquid state. The temperature in the reactor is maintained at the level of 800-850о С, and zirconium tetrachloride is supplied in the form of powder or steam from an evaporator, where the temperature is controlled with consideration of steam pressure.
A block of sponge zirconium is formed due to the parallel processes of crystallization, sintering and condensation of reaction products. At first the reaction takes place in the gas phase and on the surface of liquid magnesium. Gaseous intermediate reduction products condense on the cold parts of the reduction device (the cover and the walls of the retort), resulting in formation of a skull sponge. After the reduction process is completed the MgCl2 melt is poured from the reductor into a heated box, the device itself is cooled in the furnace to a temperature of 600° C, removed from the oven and placed in the refrigerator.
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4. Vacuum separation of zirconium-containing reaction mass.
In order to separate magnesium and magnesium chloride from sponge zirconium, vacuum distillation is used, based on high vapor pressure of magnesium and magnesium dichloride at a temperature of 850-950о С.
The temperature of vacuum separation process in production of zirconium is 920-930о С.
For vacuum separation an electric furnace is turned on and vacuum-thermal cleaning of sponge zirconium is carried out under continuous vacuum treatment and heating.
A vacuum separation plant includes:
- an electric pit-type heating furnace with a power rating of 40 kW;
- a vacuum separator;
- a water cooling system for retort and condenser flanges, the cylindrical shell and the outlet vacuum pipe of the of the separator condenser;
- an argon supply system to the separator;
- a vacuum system consisting of an АВЗ-20Д oil forepump, a БН-8 booster oil-vapor pump, vacuum pipelines, vacuum valves and a U-shaped trap;
- a system for process parameters monitoring and control;
- a power supply system with a control cabinet.
The process of separation ends when the specified time is over and a residual pressure of 0,013 Pa in the vacuum separation device is reached.
After the vacuum separation process is completed the apparatus is cooled down in the furnace to a temperature of 600° C, taken out of the furnace and placed in the refrigerator.
After dismantling the separation device the block of sponge zirconium is taken out of the retort and placed on a crushing stand equipped with fire-fighting means (stand cover, argon supply, the presence of dry ground carnallite, sodium chloride, basalt fabric). In the stand a block of zirconium sponge is crushed with a pneumatic hammer to a fraction of less than 25 mm.
The sponge zirconium product is charged into a container with an enclosed plastic bag and sent to form a product batch. Commercial batches of sponges are stored in a dry, unheated room.
5. Preparation of magnesium reducing agent.
To further reduce the amount of impurities in the final sponge zirconium magnesium is cleaned from iron and silicon impurities using a zirconium-containing melt.
The magnesium purification process is carried out as follows: magnesium is put into a reactor, sprinkled with barium flux, the reactor is closed with a cover and installed in a retort placed in an electric furnace.
The reactor with metal is heated until the magnesium melts, then to 700° C. After that a stirrer driven by an electric drive is introduced into the melt through the reactor cover and turned on to mix the melt. The melt is stirred for 10 minutes.
Then a zirconium-containing melt of chloride salts is introduced in an amount of 6.8% from the magnesium weight and stirring of the melt continues for 10 minutes .
After the mixer is turned off the magnesium is being settled at 700-710о С for 30 minutes.
When magnesium is treated with a zirconium-containing melt of chloride salts, magnesium is quite effectively cleaned from iron and silicon impurities and the source of contamination of sponge zirconium is eliminated.
Thus, as the result of the described technology sponge zirconium is obtained, the purity of which with regard to most metal and gas impurities meets the requirements for metal for production of special alloys and products that are used in manufacturing fuel for nuclear reactors.
CONCLUSION
- For production of zirconium zirconium concentrates with the content of 60-65% zirconium dioxide are used.
- Production of nuclear-pure metal zirconium is carried out in several ways: using fluoride, iodide and chloride technologies.
- The magnesium-thermal process of zirconium reduction is carried out by the Kroll method.
- The basic process scheme for production of sponge zirconium is carried out using salt chlorination of ZrO2, reduction-sublimation purification with obtaining cleaned ZrCl2, magnesium thermal reduction of zirconium tetrachloride with obtaining sponge zirconium; vacuum separation of zirconium sponge and preparation of magnesium reducing agent.
- Special technological methods make it possible to obtain metal zirconium (zirconium sponge) of the required quality for subsequent production of zirconium alloys, used for producing fuel for nuclear reactors.
