Selection and use of refining agent in aluminum melting

Selection and use of refining agent in aluminum melting

Selection and use of refining agent in aluminum melting

The composition of different aluminum melting refining agents will be adjusted and optimized according to the specific refining requirements and the type of aluminum alloy.

In practical applications, it is necessary to select appropriate refining agents based on factors such as the composition of the aluminum alloy, the smelting process, and the requirements for the refining effect.

The selection of aluminum melting refining agents is an important link in aluminum alloy production.

The following are some key points:

1. Select according to the purpose of refining

Degassing refining

If the main purpose is to remove gases (such as hydrogen) in aluminum alloys, refining agents with chloride salts and fluoride salts as the main components can be selected.

For example, the commonly used hexachloroethane (C₂Cl₆) decomposes at high temperatures to produce chlorine and chlorides.

Chlorine can react with hydrogen to produce hydrogen chloride, thereby removing hydrogen from aluminum alloys.

There are also some fluorine-containing refining agents, such as calcium fluoride (CaF₂) and sodium fluoride (NaF), which can reduce the surface tension of aluminum alloy melts and make it easier for gases to escape.

Slag removal refining

For removing oxide inclusions in aluminum alloys, refining agents containing basic oxides can be used.

For example, refining agents with cryolite (Na₃AlF₆) as the main component can absorb and dissolve inclusions such as aluminum oxide in aluminum alloys, playing a role in purifying the melt.

In addition, some composite refining agents contain magnesium oxide (MgO) or calcium oxide (CaO).

These basic oxides can react with acidic oxide inclusions to form compounds that are easy to float and separate.

2. Consider the composition of aluminum alloys

Magnesium-containing aluminum alloys

For aluminum alloys with high magnesium content, avoid using ingredients that may react violently with magnesium when selecting refining agents.

Because magnesium is a relatively active metal, for example, avoid using refining agents containing a large amount of sodium to prevent magnesium from burning and causing changes in the composition of the aluminum alloy.

For such alloys, refining agents containing rare earth elements can be selected.

Rare earth elements can improve the structure and properties of aluminum alloys, and at the same time play a certain protective role for magnesium during the refining process.

Silicon-containing aluminum alloys

When refining silicon-containing aluminum alloys, it is best for the refining agent to cooperate with the silicon element without producing harmful reactions.

Some refining agents can promote the uniform distribution of silicon and improve the quality of aluminum alloys.

For example, refining agents containing elements such as titanium and boron, which interact with silicon, can refine the grains of aluminum alloys and help remove impurities in the melt during the refining process.

3. Considering from the refining process

Gas blowing refining process

If gas blowing refining is adopted, the refining agent should have good volatility and reactivity.

For example, when chlorine is used as the gas for gas blowing refining, chloride refining agent is used in combination to enhance the refining effect.

At the same time, it is necessary to ensure that the refining agent can be evenly distributed in the aluminum alloy melt during the gas blowing process, so the particle size and fluidity of the refining agent are also important.

The refining agent with moderate particle size and good fluidity is conducive to uniform diffusion in the melt.

Flux Cover Refining Process

When flux cover refining is used, the selected refining agent should be able to form a continuous and stable covering layer on the surface of the aluminum alloy melt.

For example, fluxes based on sodium chloride (NaCl) and potassium chloride (KCl) have low melting points and can form a covering layer on the surface of the melt to prevent the melt from oxidation and gas absorption.

And this flux covering layer should have a certain ability to absorb and dissolve inclusions so that impurities in the melt can be effectively removed during the refining process.

4. Environmental protection and safety factors

Select environmentally friendly refining agents and try to avoid using refining agents containing highly toxic ingredients.

For example, although the traditional arsenic-containing refining agent has a good refining effect, it is rarely used now due to the toxicity of arsenic.

At the same time, the safety of the refining agent during use should be considered.

For example, some refining agents that produce flammable and explosive gases at high temperatures require strict safety measures when used, such as good ventilation conditions.

Determining the amount of aluminum melting refining agent to be added is a process that takes into account multiple factors:

1. According to the weight of the aluminum alloy melt

Refining agents are usually added according to a certain proportion of the weight of the aluminum alloy melt.

This proportion is generally between 0.2% – 1.0%.

For example, for a small aluminum alloy casting operation, if the melt weight is 100 kg, 0.2 – 1 kg of refining agent may need to be added, depending on the type of refining agent and actual conditions.

This is because too little refining agent may not effectively remove gases and impurities in the melt, while too much refining agent will increase costs and may also introduce new impurities.

2. Consider the quality requirements of aluminum alloys

High purity requirements

If the purity requirements of aluminum alloys are very high, such as aluminum alloys used in aerospace and other fields, the amount of refining agent added may be close to the upper limit of the above range.

Because these fields require aluminum alloys to have extremely low gas content and impurity levels, sufficient refining agents are required to ensure the refining effect.

General industrial use

For aluminum alloys for general industrial use, such as aluminum alloy profiles for construction, the amount of refining agent added can be appropriately reduced as long as the basic quality standards are met, generally around 0.2% – 0.5%.

3. Combined with the original state of the aluminum alloy

High impurity and gas content

If the aluminum alloy raw material is of poor quality and contains more oxide inclusions and gases, it is necessary to increase the amount of refining agent appropriately.

For example, the recycled scrap aluminum alloy may contain more impurities due to multiple processing and use.

At this time, it may be necessary to increase the amount of refining agent to 0.8% – 1.0% to ensure effective removal of these impurities and gases.

Relatively pure aluminum alloy

For aluminum alloy raw materials with higher purity, the amount of refining agent added can be relatively reduced.

For example, when using high-purity aluminum ingots as raw materials to cast aluminum alloys, the amount of refining agent added may only be 0.2% – 0.3%.

4. Reference refining process and equipment

Complexity of refining process

If a complex refining process is used, such as a multi-stage refining process, including a combination of gas blowing refining, flux covering refining and other methods, the total amount of refining agent added can be appropriately reduced.

Because multiple refining methods work together to more efficiently remove impurities and gases, the amount of each refining agent can be reduced within their respective appropriate ranges.

Equipment refining capacity

Advanced refining equipment has better stirring and mixing functions, which can make the refining agent more evenly distributed in the aluminum alloy melt and achieve better refining effect.

In this case, the amount of refining agent added can be appropriately adjusted according to the performance of the equipment.

For example, refining equipment with a strong electromagnetic stirring device can make the refining agent better contact with the melt, and the amount of refining agent added may be slightly lower than the amount required for ordinary equipment.

The specific components of aluminum alloy refining agents mainly include the following categories:

1. Chlorides:

*Sodium chloride (NaCl):

It can reduce the surface tension of aluminum alloy melt in the refining agent, making it easier for gases and inclusions generated during the refining process to escape from the melt, and helping other components to be evenly dispersed in the melt.

*Potassium chloride (KCl):

Similar to the effect of sodium chloride, it can improve the refining effect. Its addition can improve the thermal stability of the refining agent and maintain good refining performance during high-temperature smelting.

*Hexachloroethane (C₂Cl₆):

It is a strong degassing component. It decomposes at high temperature to produce chlorine.

Chlorine can react with hydrogen in the aluminum alloy melt to produce hydrogen chloride, thereby achieving the purpose of removing hydrogen.

It is often used in the refining of aluminum alloys.

2. Fluorides:

*Calcium fluoride (CaF₂):

It can absorb and dissolve inclusions such as alumina in aluminum alloys, reduce the viscosity of the melt, and make the inclusions more easily float and separate, thereby purifying the melt.

It can also work synergistically with other components to enhance the slag removal ability of the refining agent.

*Sodium fluoride (NaF):

It can reduce the surface tension of the aluminum alloy melt, promote the escape of gas, and improve the fluidity of the refining agent, making it easier to diffuse in the melt and improve the refining effect.

3. Alkali metal compounds:

*Potassium oxide (K₂O):

It plays a role in regulating the pH of the melt in the refining agent, which helps to remove impurities in the aluminum alloy.

An appropriate amount of potassium oxide can improve the activity of the refining agent and enhance its degassing and slag removal capabilities.

*Sodium oxide (Na₂O):

It has strong water absorption and can absorb moisture in the aluminum alloy melt, thereby reducing the hydrogen generated by the decomposition of moisture and reducing the hydrogen content in the melt.

At the same time, sodium oxide can also interact with other components to improve the refining effect of the refining agent.

4. Carbonates:

During the refining process, carbonates will decompose to produce carbon dioxide gas.

These gases form tiny bubbles in the melt, which will absorb hydrogen and inclusions in the melt during the floating process and take them out of the melt, thereby achieving the purpose of degassing and deslagging.

In addition, the decomposition of carbonates is an exothermic reaction, which can quickly melt the mixed chlorides and help the refining agent to work better.

5. Oxides:

*Silicon dioxide (SiO₂):

It can increase the viscosity of the refining agent, forming a uniform coating on the surface of the melt, preventing the melt from contacting the air and reducing oxidation.

At the same time, silicon dioxide can also react with some metal impurities in the aluminum alloy to form easily separated compounds, thereby playing a role in removing impurities.

*Alumina (Al₂O₃):

It has a small content in the refining agent. Its main function is to act as a filler to adjust the physical properties of the refining agent, such as particle size, density, etc., so that the refining agent is easier to disperse in the melt.

Different proportions of aluminum alloy refining agents will have different effects on the refining result:

1. Influence of the ratio of chloride components

*Degassing effect

When the ratio of chloride (such as sodium chloride, potassium chloride, hexachloroethane) is high, the degassing effect is usually enhanced.

For example, the chlorine produced by the decomposition of hexachloroethane can effectively remove hydrogen from the aluminum alloy melt.

The appropriate chloride ratio can ensure sufficient chlorine generation, so that hydrogen and chlorine can fully react to generate hydrogen chloride for discharge.

However, if the chloride ratio is too high, it may cause over-chlorination of the aluminum alloy melt, introduce too much chlorine impurities, and increase the refining cost.

*Slag removal ability

Chloride can reduce the surface tension of the melt and help the floating separation of inclusions.

Properly increasing the chloride ratio is conducive to improving the slag removal effect, but too much chloride may make the melt too active, which will lead to the formation of new inclusions or cause the originally stable inclusions to be mixed into the melt again.

2. Influence of fluoride component ratio

*Leading role in slag removal

Fluoride (such as calcium fluoride, sodium fluoride) is mainly used for slag removal.

 A higher fluoride ratio will significantly enhance the refining agent’s ability to adsorb and dissolve inclusions such as alumina.

Calcium fluoride, for example, reacts with inclusions, converting them into easily separated compounds. However, too high a fluoride ratio may corrode smelting equipment and increase production costs.

*Synergistic effect on gas removal

To a certain extent, fluoride can also cooperate with chloride to reduce the surface tension of the melt and promote gas escape.

However, if the fluoride ratio is inappropriate, it may interfere with the degassing function of chloride and affect the overall refining effect.

3.Influence of the proportion of alkali metal compounds

*PH adjustment and impurity removal

Changes in the proportion of alkali metal compounds (such as potassium oxide and sodium oxide) mainly affect the pH of the melt.

The appropriate proportion can effectively adjust the pH of the melt and enhance the ability to remove impurities.

For example, an appropriate amount of sodium oxide can absorb moisture in the melt and reduce hydrogen production.

However, if the proportion of alkali metal compounds is too high, the melt may be too alkaline, causing adverse reactions to certain alloying elements in the aluminum alloy, affecting the quality of the aluminum alloy.

4. Effect of carbonate composition ratio

*Degassing and slag removal

Carbonate decomposes to produce carbon dioxide gas during the refining process, which is used to absorb hydrogen and inclusions.

When the carbonate ratio is appropriate, it can effectively remove the gas and inclusions in the melt.

However, if the carbonate ratio is too high, the carbon dioxide gas produced will be too violent, which may cause the melt to splash and increase unnecessary costs.

*Effect on refining agent performance

The decomposition reaction of carbonate generates heat, which can help the chloride melt.

The appropriate carbonate ratio can ensure that the chloride melts in time and works, but an improper ratio may affect the melting speed and uniformity of the refining agent, thereby affecting the refining effect.

5. Influence of oxide composition ratio

*Anti-oxidation and impurity removal

Oxides (such as silicon dioxide and aluminum oxide) are used to form a covering layer on the surface of the melt to prevent oxidation.

The appropriate oxide ratio can effectively reduce the contact between the aluminum alloy melt and the air and reduce the degree of oxidation.

At the same time, silicon dioxide can react with metal impurities to remove impurities.

However, too high an oxide ratio may make the refining agent too viscous, affect its dispersibility in the melt, and reduce the refining efficiency.

December 3, 2024