Charging is the first thing you do with scrap. It is also the first place you lose metal. I have spent 25 years watching operators dump scrap into furnaces without thinking about what happens next. The result is always the same: higher oxidation loss, more dross, lower recovery.
In 2021, I stood in a recycling plant in Hai Phong, Vietnam, watching a crane operator drop a bundle of loose aluminium turnings into a 5-tonne reverb furnace from a height of two metres. The turnings exploded on impact — not a steam explosion, but a mechanical one. Light, fluffy material scattered across the bath surface. Half of it floated. The other half oxidised before it could submerge. The plant was recovering 83% of its charge. They blamed the scrap. They blamed the furnace. They never blamed the charging.
This article is about fixing that. Different scrap types need different charging strategies. Get them right, and you recover more metal from the same furnace, the same energy, and the same labour.
Why Charging Strategy Matters More Than You Think
Charging is not just about getting scrap into the furnace. It is about getting it into the melt — submerged, dissolved, and protected from oxidation. Every second scrap sits on the surface, it oxidises. Every minute it floats, it loses metal.
Research on aluminium scrap melting confirms what I have seen on the shop floor: more non-metallic contaminants, smaller size, and higher surface-area-to-volume ratio generally lead to lower metal recovery. Thin, fluffy scrap oxidises faster than thick, dense scrap. That is physics. You cannot change it. But you can change how you charge it.
The same research found that recycling scraps with lower recyclability normally generates more salt slags, and high slag viscosity entraps more fine aluminium metal. That means poor charging doesn’t just lose metal directly. It creates downstream problems that cost more money to fix.
The Five Scrap Categories (and How They Behave)
Not all scrap is the same. The European standard EN 13920 classifies aluminium scrap by composition and metal yield. But for charging purposes, I use a simpler system based on physical behaviour.
| Scrap Category | Examples | Density | Surface Area | Charging Difficulty |
|---|---|---|---|---|
| Heavy solids | Ingots, sows, thick castings | High | Low | Easy — sinks naturally |
| Medium density | Extrusion offcuts, window frames | Medium | Medium | Moderate — needs guidance |
| Light gauge | UBC, thin sheet, foil | Low | High | Difficult — floats and oxidises |
| Turnings & chips | Machining swarf | Low | Very high | Very difficult — oil, moisture |
| Dross & residues | Black dross, salt cake | Variable | High | Special handling required |
The key variable is surface area to volume ratio. Heavy solids have low surface area — less metal exposed to air. Turnings have enormous surface area — every chip is a potential oxidation site. A study on aluminium process scrap remelting found that metal recovery decreased when remelting thinner scrap, and recovery for machining chips was lower than for sheet scrap of the same thickness due to greater surface roughness.
That is why charging strategy must change with scrap type.
Strategy 1: Heavy Solids — Charge Fast, Charge Deep
Heavy solids are the easiest. They sink. They melt slowly but predictably. The risk is not oxidation — it is thermal shock to the refractory and cold spots in the bath.
The approach: Charge heavy solids first, when the bath is at full temperature. Place them away from the burner to avoid flame impingement. Do not drop from height — the impact damages the lining. Use a charging machine or crane with a controlled descent.
I remember a foundry in Malaysia that dropped 500 kg ingots from a height of 3 metres. The refractory cracked within six months. The repair cost $12,000. The operator was trying to save time. He cost the plant more than he saved.
Strategy 2: Medium Density — Pre-Treat and Submerge
Extrusion offcuts and window frames are the workhorse scrap for most secondary smelters. They are relatively clean, but they can be coated with paint or anodising. The coating adds organic content that creates emissions and dross.
The approach: Pre-treat where possible. De-coating by thermal treatment removes paint and reduces dross formation. If you cannot pre-treat, charge medium-density scrap into the melt pool, not onto the surface. Use a charging well or a submerged lance to push material below the surface.
The research is clear: scrap should be charged into the furnace as quickly as possible to avoid oxidation, which leads to metal losses and lower metal yield. Speed matters, but direction matters more. Fast and deep beats fast and shallow.
Strategy 3: Light Gauge — The Hardest Challenge
Used beverage cans, thin sheet, and foil are the most difficult scrap to charge. They are light. They float. They oxidise almost instantly when exposed to furnace atmosphere.
The physics is unforgiving. A UBC has a surface-area-to-volume ratio hundreds of times higher than a thick casting. When it hits the melt surface, the oxide skin forms before the metal can submerge.
The approach: Mechanical submersion is essential. Research on submersion techniques identifies several methods: stirring, feeding, shooting, compacting, and vortexing. The most effective for light gauge scrap is a vortex created by an impeller in a charging well. The rotation sets up a converging flow that pulls light scrap below the surface.
But there is a critical parameter: rotation speed. Neff recommends keeping impeller speed below 150 rpm to avoid entraining air and dross. Too fast, and you create more problems than you solve. Too slow, and the scrap floats.
A plant in Guangdong installed a vortex charging well with a variable-speed impeller. They ran it at 120 rpm. Light-gauge scrap recovery improved by 8 percentage points. The investment was $18,000. It paid back in four months.
Compacting is another option. Compacting UBCs into dense briquettes reduces surface area and increases density. The scrap sinks faster and oxidises less. A briquetting machine costs $25,000–$50,000 depending on capacity. For a plant processing more than 5 tonnes of UBCs per day, the payback is under a year.
Strategy 4: Turnings and Chips — Deal with the Oil First
Machining chips are the hardest scrap to charge well. They are small, light, and covered in cutting fluid. The oil creates three problems: it adds organic content that increases emissions and dross, it creates smoke and flame when it hits the hot melt, and it can cause dangerous flare-ups.
The approach: Pre-treatment is not optional. Centrifuging removes bulk oil. Thermal de-oiling removes the rest. The Best Available Techniques guidance is explicit: scrap material should be cleaned of oils, paints and plastics during pretreatment.
If you cannot pre-treat, charge chips through a submerged lance or feed tube. The patent literature describes feeding chips through a pipe whose end is submerged in the melt, with the feeding mechanism either a roll feeder or an auger. This prevents air from contacting the chips before they melt. The vapours from the oil are combusted above the bath, providing a non-oxidising atmosphere at the surface.
A foundry in Italy that installed a submerged chip feeder reduced metal loss on turnings from 12% to 5%. The key was not just the feeder — it was the discipline of keeping the feed tube submerged.
Strategy 5: Dross and Residues — Special Handling
Dross is not really “scrap” in the conventional sense. It is a byproduct of previous melting. But many secondary smelters charge dross back into rotary furnaces to recover entrained metal.
The approach: Dross requires salt flux to recover metal effectively. Rotary furnaces are the standard technology for this application. The salt protects the metal from oxidation and helps coalesce fine droplets.
The charging strategy for dross is different: charge in smaller batches, maintain higher salt factor, and rotate the furnace to promote contact between salt and metal. The tilting rotary furnace design uses less salt flux than fixed-axis designs.
A Real Case from Vietnam
In 2021, I worked with a recycling plant in Hai Phong that was melting a mix of extrusion offcuts, UBC, and turnings. Their metal recovery was 83%. They had no charging strategy — the crane operator dropped whatever was next into the furnace.
We made four changes:
-
Separated scrap by type. UBC, turnings, and offcuts were stored in separate bays.
-
Installed a briquetting machine for UBC. Compacting reduced volume and surface area. The briquettes sank faster.
-
Built a submerged feed system for turnings. A simple tube with an auger mechanism fed chips below the surface.
-
Changed charging sequence. Heavy offcuts went in first, then UBC briquettes, then turnings through the submerged feeder.
Results after two months:
| Metric | Before | After | Improvement |
|---|---|---|---|
| Metal recovery | 83% | 91% | 8% improvement |
| Dross metal content | 42% | 28% | 14% reduction |
| Charging time | 45 min/batch | 32 min/batch | 29% faster |
| Energy consumption | 890 kWh/t | 780 kWh/t | 12% reduction |
The plant manager told me: “We thought the problem was the furnace. It was the way we fed it.”
A Simple Charging Checklist
-
Separate scrap by type. Do not mix heavy solids with light gauge.
-
Pre-treat where possible. De-oil turnings. De-coat painted scrap. Dry wet scrap.
-
Compact light gauge. Briquetting UBC and thin sheet reduces oxidation.
-
Charge deep, not high. Use charging wells, submerged feeders, or vortex systems.
-
Match charge rate to melt rate. Do not overwhelm the furnace.
-
Sequence correctly. Heavy first, light last. Hot bath for difficult scrap.
-
Control atmosphere. Keep the furnace door closed except when charging.
-
Train operators. They need to understand why charging matters.
-
Measure recovery. You cannot improve what you do not measure.
-
Review and adjust. Every scrap mix is different.
When to Call an Expert
You can improve charging yourself. But sometimes an experienced eye sees what you miss. The plant in Vietnam had a competent team. They had kept the furnace running for years. But they had never been trained on scrap-specific charging strategies.
My fee for the three-day audit and commissioning support was $8,500. The annual saving was over $600,000. That is a 70x return.
If your metal recovery is below 88%, and you have not changed your charging practice in years, call someone. Or use the checklists on this blog. Either way, do not wait.
Final Thoughts
Optimizing furnace charging for different scrap types is not about buying a new furnace. It is about understanding how different materials behave and adapting your practice to match.
In my 25 years, I have seen plants recover millions by changing how they charge. The furnace did not change. The scrap did not change. The people did.
If you run a recycling plant, start by separating your scrap. Then pre-treat what you can. Then charge deep, not high. You will see the difference in your recovery rate.
Internal links: How to Reduce Metal Loss in Secondary Aluminium Melting for Ingot Production, How to Choose a Secondary Aluminium Melting Furnace for Ingot Production,Furnace Maintenance Checklist.
