September 2022 – Zirconium Metal

Zircon and Zircon Sand (Zr2SiO4)

Zircon sand, also known as zircon, is a mineral mainly composed of zirconium silicate (Zr2SiO4). Pure zircon is colorless and transparent crystal; depending on the origin, type, and quantity of impurities, zircon sand may appear in yellow, orange, red, brown, and other colors. The uniform Mohs hardness of zircon is 7-8, the refractive index is 1.93-2.01, and the melting point fluctuates within 2190-2420 ℃ with different impurities.

Chemical Composition

The main chemical composition of zircon is ZrO2 & SiO2, and a small amount of impurities such as Fe2O3, CaO, AI2O3, etc. The theoretical composition of zircon sand is ZrO2: 67.1%; SiO2: 32.9%. It is the only compound in the ZrO2-SiO2 system. But natural zircon sand only contains about 57~66% ZrO2.

Properties

Zircon is a mineral composed primarily of zirconium, silicon, and oxygen that crystallized out of magma when igneous rocks formed. It is the most important zirconium-bearing mineral – it is the most widely distributed, the most abundant, and the most types of zirconium minerals. Zircon belongs to the tetragonal crystal system, often in the form of well-developed cone-shaped small square cylinders, and also irregular granular. It is brittle and has a shell-like fracture. It is mostly symbiotic with ilmenite, rutile, monazite, xenotime, etc. in the coastal sand.

Application

Zircon is the main raw material for the preparation of zirconium, hafnium and various zirconium products. It is also a high-quality refractory material with high melting point, low thermal conductivity and small linear expansion coefficient, and is widely used in metallurgy, casting and other industries.

The addition of zircon to other materials can improve their performance. For example, adding zircon sand to synthetic cordierite can broaden the sintering range of cordierite without affecting its thermal shock stability; Adding zircon sand to high-alumina bricks to manufacture anti-stripping high-alumina bricks, the thermal shock stability is greatly improved; It can also be used to extract ZrO2.

More specific applications are as follows:

Refractory: zirconium refractory, such as zirconium corundum brick, zirconium refractory fiber;

Sand for casting mold in the foundry industry: molding sand for precision casting, precision enamel utensils;

Glass, metal: sponge zirconium;

Production of zirconium compounds: such as zirconium dioxide, zirconium oxychloride, sodium zirconate, potassium fluorozirconate, zirconium sulfate, etc.

If you are interested in other zirconium materials, please visit https://www.samaterials.com/.

Separation of Zirconium and Hafnium by Solvent Extraction

Solvent extraction of zirconium and hafnium is one of the common methods for separating zirconium and hafnium. Compared with other zirconium and hafnium separation methods (such as pyrolysis separation and solvent extraction separation), this method has the advantages of large production capacity, simple process and easy to achieve continuously.

Principle

The extraction agents used for the separation of zirconium and hafnium mainly include ketone extractants, neutral phosphorus-containing extractants and amine extractants.

A commonly used ketone extractant is methyl isobutyl ketone (MIBK), which can form a neutral extract with hafnium thiocyanate and is preferentially extracted into the organic phase.

Methyl isobutyl ketone 3D ball. Source: Wikipedia

A typical neutral phosphorus-containing extractant is tributyl phosphate (TBP), which is preferentially extracted into the organic phase through the coordination of oxygen atoms in chemical bonds with zirconium metal atoms to form a neutral extract compound Zr(NO3)4•2TBP.

Ball and stick model of Tributyl phosphate. Source: Wikipedia

The commonly used amine extractant is trioctylamine (TOA). Trioctylamine forms an extract with zirconium ions in an acidic medium, and is preferentially extracted into the organic phase.

Process flow

There are three extraction processes: MIBK, TBP, and N235.

MIBK extraction

It uses ZrCI4 as raw material, adds water and NH4CNS ingredients. MIBK preferentially extracts hafnium, leaving a large amount of zirconium in the aqueous phase. This is the earliest extraction process used to separate zirconium and hafnium, and it is adopted by major producers of zirconium and hafnium such as the United States, France, Germany, and Japan.

TBP extraction

There are two aqueous feed systems for this process: nitric acid, and a mixed acid of nitric & hydrochloric acid. The former is to convert the product of zircon decomposed by alkali fusion method into nitric acid aqueous phase feed liquid, and use TBP to preferentially extract zirconium; the latter use Zr-CI4 as raw material, add water, nitric acid and hydrochloric acid as ingredients, and then use TBP to preferentially extract zirconium.

The separation coefficient of zirconium and hafnium in the TBP extraction process is large, and the number of extraction stages is small, and atomic-level zirconium oxide and hafnium oxide can be obtained at the same time. However, the water-phase feed liquid is highly corrosive, and the emulsification problem in the extraction process has not been completely solved, thus affecting its popularization and application.

N235 extraction

First, the zircon is decomposed by alkali fusion method, and the product is washed with water to remove silicon, and then leached with sulfuric acid to obtain a sulfuric acid solution of zirconium, and then the zirconium is preferentially extracted with N235. After washing, atomic-level zirconia containing hafnium <0.01% can be obtained. The hafnium in the raffinate is enriched to 50% to 70%, and then extracted by P204, and the zirconium and hafnium are further separated to obtain atomic energy level hafnium oxide containing more than 96% of hafnium.

This process has low material toxicity, light equipment corrosion, stable operation, and easy disposal of waste, so it is currently recognized as one of the best extraction processes.

Extraction equipment

There are two main types of extraction equipment, one is the extraction tower, and the other is the box-type mixer-settler. The former is used by the MIBK process, and the latter is used by TBP and N235 extraction process. The extraction tower occupies a small area and has a large production capacity. The box-type mixer-clarifier is simple in structure and stable in operation, and is generally made of acid-resistant materials such as plastic or plexiglass.

For more information, please visit https://www.samaterials.com/.

3 Methods to Separate Zirconium & Hafnium

The two elements of zirconium and hafnium are symbiotic resources, which means that zirconium generally contains 0.5% to 2% of hafnium. However, the application of zirconium products in various industries requires high purity. For example, zirconium, which is a structural and cladding material for nuclear reactors, must contain less than 0.01% hafnium. In general, the metallurgical process of separating zirconium and hafnium is an important part of the zirconium metallurgical process.

The separation methods of zirconium and hafnium include pyrolysis separation, solvent extraction separation, and ion exchange separation. This article will briefly introduce these three separation methods.

Zirconium and Hafnium Pyrolysis Separation

Pyro separation is a method of separating zirconium and hafnium at high temperature or high pressure by using the difference in vapor pressure of zirconium and hafnium chloride. Zirconium and hafnium pyrolysis can replace the three production stages of extraction, calcination and chlorination in common separation methods. It has the characteristics of a short production process, high efficiency, low reagent cost and light pollution to the environment, and is a promising method for separating zirconium and hafnium.

The pyrolysis method is mainly realized by high-pressure rectification and molten salt rectification. High-pressure rectification is a process of directly separating zirconium and hafnium by using the difference in vapor pressure of Zrcl4 and HfCl4. Molten salt rectification is a process of separating zirconium and hafnium in a rectifying tower by using the difference in saturated vapor pressure of ZrCl4 and HfCl4 in KAlCl4 molten salt.

Zirconium and Hafnium Solvent Extraction

This is a method for the separation of zirconium and hafnium using solvent leather. Compared with other separation methods of zirconium and hafnium, this method has the advantages of large production capacity, simple process, and easy to achieve continuously. It is the most important method for the separation of zirconium and hafnium.

The reagents used in this method mainly include ketone extractant, neutral phosphorus-containing extractant and amine extractant. There are three extraction processes of MIBK, TBP and N235. There are two main types of extraction equipment, one is the extraction tower, and the other is the box-type mixer-settler. The former is used by the MIBK process, and the latter is used by TBP and N235 extraction process. The extraction tower occupies a small area and has a large production capacity. The box-type mixer-clarifier is simple in structure and stable in operation, and is generally made of acid-resistant materials such as plastic or plexiglass.

Further Reading: Separation of Zirconium and Hafnium by Solvent Extraction

Zirconium and Hafnium Ion Exchange Separation

As the name suggests, this is a method for the separation of zirconium and hafnium by ion exchange. The production volume of this method is small. Only the former Soviet Union has used it to further separate zirconium and hafnium from the hafnium-rich material separated by the zirconium-hafnium recrystallization method to obtain hafnium oxide, which is used as the raw material for the production of atomic-level sponge hafnium.

For more information, please visit https://www.samaterials.com/.

2 Methods of Refining Zirconium

Zone melting and purification

This method does not use crucibles, so it can largely protect the product from contamination by the crucible.

The process of zirconium zone smelting and purification generally includes three stages: raw material preparation, zone smelting and product quality inspection.

Stage 1 Prepare raw materials

The sponge zirconium or zirconium iodide is made into rods by electron beam melting, extrusion, machining and other steps.

Stage 2 Zone smelting

Fix the zirconium rod vertically in the vacuum chamber, use the high-frequency induction coil (or electron gun) as the heat source to melt one end of the rod into a melting zone, and then slowly move the melting zone from one end to the other.

Zirconium is a high melting point metal with strong chemical activity. The vapor pressure at its melting point of 2125K is low, and zone smelting is easier to carry out at this time.

The main factors affecting this stage are the purity of raw materials, vacuum pressure, zone melting speed and zone melting times.

Schematic-representation-of-the-zone-melting-process-with-single-heater — Schematic representation of the zone melting process with single heater. Curtolo, Danilo & Friedrich, Semiramis & Friedrich, Bernd. (2017). High Purity Germanium, a Review on Principle Theories and Technical Production Methodologies. Journal of Crystallization Process and Technology. 07. 65-84. 10.4236/jcpt.2017.74005.

Stage 3 Inspect Product Quality

The head and tail sections of the zirconium rod obtained in the previous stage were excised for sampling. Its various parameters are measured by mass spectrometry, hardness determination or specific resistance.

E-beam melting and purification

Zirconium has a great affinity for oxygen and nitrogen, and 0.2% nitrogen or oxygen in zirconium becomes brittle, making it difficult to process into materials. In this method, the metal to be purified is bombarded with electrons in a vacuum to melt it. In the purification process, in addition to avoiding the oxidation and nitridation of zirconium, the gas dissolved in the zirconium can also escape, the mixed oxide can be decomposed or evaporated, and metal impurities (such as copper, aluminum, gold, iron, manganese, chromium, etc.). The processing properties, electrical properties, magnetic properties and mechanical properties of the zirconium ingot purified by electron beam smelting are obviously better than the raw zirconium.

Scheme of Electron beam melting and refining (EBMR) process. Vutova, Katia & Donchev, Veliko. (2013). Electron Beam Melting and Refining of Metals: Computational Modeling and Optimization. Materials. 6. 4626-4640. 10.3390/ma6104626.

An electron beam melting furnace is composed of an electron gun system, furnace body, vacuum system, raw material adding system, mold mechanical device, operation control system, and high voltage power supply device. The larger the scale is, the lower the product smelting cost is. At present, the smelting process is developing into a semi-continuous or continuous production mode to improve production efficiency and production capacity.

The quality of zirconium ingot purified by electron beam melting depends on the raw material, the melting speed, and the power of the melting furnace.

For more information, please visit www.samaterials.com.