Aluminium Purification Process

High-purity aluminum production relies on a three-tiered approach: rotary degassing to strip hydrogen, flux injection to separate alkali metals and oxides, and ceramic foam filtration to physically capture solid inclusions.

Achieving zero-defect casting requires strict adherence to these steps in the casthouse. Without effective purification, the molten melt suffers from porosity and weak mechanical properties. The industry standard involves placing an inline degassing unit between the holding furnace and the casting table. This unit introduces inert gases like nitrogen or argon which bond with hydrogen. Simultaneously, ceramic foam filters (CFF) utilizing deep-bed filtration mechanisms trap micron-sized non-metallic particles. Implementing this integrated system guarantees compliance with ASTM quality standards and significantly lowers scrap rates in downstream processing.

The Critical Need for Purification in Modern Metallurgy

Raw aluminum leaving the smelter or recycling furnace is rarely clean enough for high-end applications like aerospace, automotive, or foil manufacturing. The melt contains enemies that compromise structural integrity.

Operators face three primary contaminants:

1. Hydrogen Gas: The only gas soluble in liquid aluminum. It causes porosity (pinholes) during solidification.

2. Oxide Films (Dross): Aluminum reacts instantly with oxygen, creating films that weaken the metal continuity.

3. Alkali Metals: Elements like Sodium (Na), Lithium (Li), and Calcium (Ca) that reduce ductility.

ADtech focuses on engineering solutions that address these specific contaminants. If you skip purification, you risk product failure. A single oxide inclusion can cause a tear in thin-gauge foil or a crack in an alloy wheel.

Mechanism of Hydrogen Absorption

Moisture is the main source of hydrogen. It comes from humid air, damp tools, or incompletely dried refractories. When water vapor (H2O) touches molten aluminum (Al), a reaction occurs:

The atomic hydrogen diffuses into the melt. When the metal cools, the solubility of hydrogen drops drastically, forcing the gas out of solution and forming bubbles. We must remove this gas before the metal freezes.

Primary Purification Technologies

Refining molten aluminum is not a single step. It is a sequence of operations. We will examine the core technologies used globally and by ADtech to ensure melt cleanliness.

1. Rotary Degassing Systems

The most efficient way to remove hydrogen is rotary degassing. This process involves a spinning rotor and a shaft submerged in the melt. The system injects an inert gas (Argon or Nitrogen) through the shaft.

The spinning rotor shears the large gas bubbles into millions of tiny micro-bubbles. This increases the surface area significantly.

How it works:

• Diffusion: Hydrogen atoms in the melt are drawn to the low partial pressure inside the inert gas bubbles.

• Flotation: The bubbles rise to the surface, carrying the hydrogen out.

• Inclusion Removal: The bubbles also attach to oxide particles, floating them to the dross layer for skimming.

ADtech degassing units utilize optimized rotor designs to prevent vortexing. A vortex can suck surface dross back into the melt, defeating the purpose.

2. Flux Injection (Fluxing)

Fluxing involves adding chemical salts to the melt. While manual addition was common in the past, modern lines use automated injection machines. These machines inject a precise mixture of granulated flux and inert gas deep into the molten bath.

Functions of Flux:

• Wetting Agent: It changes the surface tension, allowing aluminum trapped in the dross to flow back into the bath.

• Chemical Cleaning: Certain fluxes react with Calcium or Sodium to remove them.

• Wall Cleaning: Exothermic fluxes help keep furnace walls free of corundum buildup.

3. Ceramic Foam Filtration (CFF)

Filtration is the final gatekeeper. Positioned just before the casting table, the filter box houses the Ceramic Foam Filter. ADtech specializes in CFF manufacturing, using phosphate-bonded high-alumina or silicon carbide materials.

The filter works through two mechanisms:

• Cake Filtration: Large particles are blocked at the surface of the filter.

• Deep Bed Filtration: Smaller particles are trapped inside the tortuous path of the filter’s internal structure due to gravity, friction, and chemical affinity.

Selecting the Correct Filtration Parameters

Choosing the right filter is engineering, not guessing. The metric used is PPI (Pores Per Inch). A higher PPI means finer filtration but requires higher metal head pressure to start the flow.

Filter Selection Matrix

The following table provides data regarding filter selection based on application.

Application Type	Recommended PPI	Filter Material	Main Goal
Standard Extrusion Billets	30 – 40 PPI	Alumina / SiC	Remove large oxides, prevent die lines.
Automotive Casting (Wheels)	40 – 50 PPI	Silicon Carbide	Eliminate inclusions that cause cracks.
Aerospace Alloys	50 – 60 PPI	High Alumina	Maximum purity for fatigue resistance.
Can Body Stock	50 PPI	Alumina	Prevent tearing during rolling.
Foil Rolling (Thin Gauge)	60+ PPI	High Alumina	Zero pinholes required.

Note: ADtech engineers recommend pre-heating the filter box to avoid thermal shock which can crack the ceramic.

The Interaction of Degassing and Filtration

Operators often ask if they can perform just one of these steps. The answer is no. They function in tandem.

If you filter without degassing, the metal will still have hydrogen porosity. If you degas without filtering, inclusions will pass through. In fact, the degassing unit often agglomerates small inclusions into larger clumps. The downstream filter then captures these clumps easily.

Placement Strategy:

The standard layout for a casting line is:

1. Furnace.

2. Launder.

3. ADtech Degassing Unit.

4. ADtech Filter Box (CFF).

5. Casting Table.

This order is vital. Degassing creates turbulence. You want that turbulence to happen before the filter so the flow is laminar (smooth) when it enters the mold.

Optimizing the Process: Flow Rates and Rotor Speeds

Success lies in the details. Running a degassing unit at maximum speed is not always better. Excessive speed creates surface turbulence, exposing fresh metal to atmospheric oxygen.

Recommended Operational Parameters

Parameter	Standard Range	Impact of Deviation
Rotor Speed (RPM)	350 – 550 RPM	Too high creates dross; too low reduces bubble dispersion.
Gas Flow Rate (L/min)	15 – 25 L/min	Too high creates “boiling”; too low fails to remove H2.
Metal Temperature	700°C – 750°C	Low temp clogs filters; high temp increases H2 absorption.
Filter Pre-heat	15 – 20 mins	Lack of pre-heat leads to frozen metal blocking the filter.

Operators must calibrate these settings based on the alloy type. 5000-series alloys (high Magnesium) behave differently than 6000-series (Magnesium-Silicon).

Case Study: Improving Yield in Michigan, USA

Location: Automotive Die-Casting Facility, Michigan

Time: February 2023

Client Challenge:

A Tier-1 supplier for electric vehicle battery trays faced a rejection rate of 8.5%. The defects were identified as oxide inclusions and hydrogen porosity. The parts failed pressure tightness tests. The facility used only simple mesh filters and manual fluxing.

The ADtech Solution:

We deployed a holistic purification upgrade.

1. Installation: Replaced the launder section to accommodate an ADtech Online Degassing Unit (compact dual-rotor design).

2. Filtration: Installed a dual-stage filter box using 40 PPI followed by 50 PPI ceramic foam filters.

3. Process Change: Switched from manual fluxing to automated injection within the degassing chamber.

The Results:

After a 30-day trial period, the data showed significant improvement.

• Scrap Reduction: The rejection rate dropped from 8.5% to 1.2%.

• Density Index: Improved from 8% to under 1.5% (indicating low hydrogen).

• ROI: The reduction in remelting costs paid for the equipment upgrade within four months.

Advanced Flux Chemistries

Flux is not just salt. It is a sophisticated chemical tool.

1. Covering Flux:

Used in the furnace. It floats on top of the melt to create a barrier against air. This prevents oxidation.

2. Drossing Flux:

Added before skimming. It creates an exothermic reaction (heat) that separates valuable aluminum from the oxide skin (dross). This ensures you throw away only waste, not good metal.

3. Cleaning Flux:

Injected into the melt. It helps lift inclusions to the top. ADtech provides granular fluxes that are free of sodium for alloys where sodium is a contaminant (like high-magnesium alloys).

Flux Injection vs. Manual Addition

Manual addition is inconsistent. One operator might throw in too much, another too little. Injection guarantees a homogeneous distribution. The carrier gas (Nitrogen) helps disperse the flux throughout the entire bath depth.

Maintenance and Longevity of Purification Equipment

Equipment performance degrades without care. The rotor in a degassing unit is a consumable. It is usually made of graphite or silicon nitride.

Common Maintenance Checks:

• Rotor Erosion: Graphite oxidizes over time. An eroded rotor head loses its geometry, failing to shear bubbles effectively. Check dimensions weekly.

• Heater Elements: In the filter box, electric heaters maintain temperature. If one fails, the metal may freeze.

• Seal Integrity: Gaskets around the filter plate must be tight. If the seal fails, metal bypasses the filter, rendering the process useless.

Life Expectancy Table

Component	Material	Estimated Lifespan
Degassing Rotor	Silicon Nitride	6 – 12 Months
Degassing Rotor	Graphite	3 – 6 weeks
Heater Protection Tube	SiC	6 – 12 Months
Ceramic Foam Filter	Alumina	Single Use (Disposable)
Launders/Linings	Refractory	1 – 2 Years

Note: Silicon Nitride is more expensive upfront but offers superior longevity compared to graphite.

The Role of Grain Refiners

While purification removes bad things, grain refinement adds good things. Often added right after the degassing unit, grain refiners (typically Titanium-Boron rods) dictate the microstructure of the aluminum.

A fine, equiaxed grain structure allows the casting to flow better and resist cracking. ADtech feeding systems can automate the addition of AlTiB wire to ensure consistent grain size throughout the cast.

Environmental Considerations in Purification

Modern aluminum purification must be green. Old methods using Chlorine gas for degassing are hazardous and illegal in many jurisdictions.

The ADtech Approach to Sustainability:

• Chlorine-Free: We utilize Nitrogen or Argon, which are safe and abundant.

• Reduced Dross: Efficient fluxing reduces the amount of aluminum sent to landfill as dross.

• Energy Efficiency: Our filter boxes use high-efficiency insulation, reducing the energy needed to keep the metal hot.

Reducing scrap is the biggest environmental contribution. Every ton of scrap that must be remelted consumes massive amounts of natural gas and electricity. Getting it right the first time is the ultimate green strategy.

Technical Nuances

To truly understand the aluminium purification process, one must look at the specific defects.

Inclusions: These are non-metallic particles. They can be oxides (films), carbides, or borides. Hard inclusions damage machining tools later in the process.

Alkali Removal: removing Calcium is vital for foil production. Even trace amounts cause foil to break during rolling.

Metallurgical Quality: This refers to the overall cleanliness. Tools like the PoDFA (Porous Disc Filtration Apparatus) or Prefil measurements allow engineers to quantify cleanliness. ADtech equipment helps facilities achieve superior Prefil curves.

When searching for casthouse solutions, managers should prioritize modular systems. A modular degassing unit can be retrofitted into existing lines without major civil works.

Troubleshooting Common Purification Issues

Even with the best gear, things go wrong. Here is a troubleshooting matrix for casthouse operators.

Problem: High Hydrogen Levels after Degassing

• Cause: High humidity day?

• Cause: Rotor speed too low?

• Cause: Carrier gas leakage?

• Fix: Increase gas flow slightly, check gas lines for leaks, ensure rotor is not worn down.

Problem: Filter Blockage (Freezing)

• Cause: Metal temperature too low.

• Cause: Insufficient pre-heating of the filter box.

• Fix: Check furnace temperature. Ensure filter box heaters are active 30 minutes before casting.

Problem: Inclusions found in Final Product

• Cause: Filter bypass (bad seal).

• Cause: Dirty launder after the filter.

• Fix: Inspect expanding gasket on the CFF. Clean launders strictly.

Frequently Asked Questions (FAQs)

Here are the top questions asked by casthouse managers and metallurgists regarding the purification process.

1. What is the most effective method for hydrogen removal in aluminum?

Rotary degassing using an inert gas like Argon or Nitrogen is the industry standard. It physically carries hydrogen out of the melt via bubble flotation.

2. Why is flux injected instead of thrown on top?

Injection ensures the flux contacts the entire volume of metal, not just the surface. This improves reaction efficiency and reduces the amount of flux needed.

3. How often should I change the Ceramic Foam Filter?

CFFs are single-use items. You must change them after every cast or whenever the pressure drop becomes too high, indicating a blockage.

4. Can I use Nitrogen instead of Argon for degassing?

Yes, Nitrogen is cheaper and widely used. However, for certain high-tech alloys or when using Lithium, Argon is required because Nitrogen can react to form nitrides.

5. What is the difference between PPI and mesh size?

PPI stands for Pores Per Inch in a foam filter. Mesh refers to wire screens. 30 PPI is a standard coarseness; 60 PPI is fine. Higher PPI captures smaller particles.

6. How does ADtech ensure the filter box doesn’t cool the metal?

ADtech filter boxes are lined with advanced refractory materials and often equipped with lid heaters to maintain thermal equilibrium.

7. What creates dross in the furnace?

Dross forms when aluminum reacts with oxygen in the air. Turbulence and high temperatures accelerate this reaction.

8. Is chlorine gas still used for purification?

It is rare due to toxicity and environmental regulations. Some facilities use a mix of 90% Nitrogen / 10% Chlorine, but purely inert gases are preferred for safety.

9. What happens if the rotor speed is too high?

It creates a vortex that sucks surface dross and air into the melt, actually making the quality worse. It also wears out the rotor faster.

10. Can purification remove iron from aluminum?

No. Iron is dissolved chemically in the aluminum. Filtration and degassing cannot remove dissolved elements like iron or manganese; they only remove solids and gases.

Final Thoughts on Melt Quality

The aluminium purification process defines the value of your final product. Whether you are casting billets, slabs, or ingots, the presence of hydrogen and inclusions is non-negotiable—they must go.

ADtech provides the hardware and expertise to secure this quality. By combining efficient rotary degassing with precise flux injection and deep-bed ceramic filtration, we help foundries achieve global standards. The cost of purification is a fraction of the cost of a rejected shipment.

Investing in ADtech technologies ensures your metal is clean, your customers are satisfied, and your scrap bins remain empty.