Back to Basics: Understanding Density in Shredder Residue Separation

In the world of scrap processing and recycling, "Shredder Residue" (often called ASR or Automotive Shredder Residue) is the complex mix of materials left over after a vehicle or appliance has been shredded and the large steel pieces have been magnetically removed.

For years, this residue was considered waste. Today, it is a goldmine of non-ferrous metals and recoverable plastics. But how do you separate a jumbled mess of rubber, glass, plastic, aluminum, copper, and zinc?

The answer lies in physics—specifically, Heavy or Dense Media Separation (HMS or DMS). By manipulating the specific gravity of a fluid bath, processors can sort materials based strictly on weight-per-volume.

Here is a back-to-basics look at how we use three specific density thresholds—1.4, 2.0, and 3.0 kg/l—to turn trash into treasure.

The Lightweights: Recovering Plastics (1.4 kg/l)

The first challenge in processing shredder residue is separating the non-metallics from the metals, and some of them can be valuable, such as plastics.

By separating at a specific gravity of 1.4 kg/l, we create a distinct cut line. Most commodity plastics found in vehicles and appliances—including Polypropylene (PP), Polyethylene (PE), Polystyrene (PS), and ABS—have a density lower than 1.4. Even heavier plastics like PET often hover or float near this threshold.

• The Float: When the residue hits this bath, the concentrated plastic stream floats to the top. This results in a "plastics-rich" feedstock that is free from heavier contaminants like rocks, glass, and metals. This stream is perfectly prepped for advanced plastics recycling processes.
• The Sink: Rubber, glass, stones, and all metals sink to the bottom, moving on to the next stage of separation.

The Mid-Range: Splitting Magnesium and Aluminum (2.0 kg/l)

Once the plastics are removed, we are left with a heavier mix containing rubber, rocks, glass, and various metals. A critical separation required here is distinguishing between Magnesium and Aluminum. While they look similar to the naked eye (both are silvery-white light metals), their densities are different.

• Magnesium (Density ~1.7 kg/l): By setting the media density to 2.0 kg/l, Magnesium becomes lighter than the fluid. It floats to the surface, allowing for the recovery of a high-grade Magnesium product. A few non-metallics float too, such as rubber, but eddy current separation is quite effective at recovering the magnesium from the non-metallics.
• Aluminum (Density ~2.7 kg/l): Aluminum is denser than the 2.0 fluid, so it sinks, along with rocks, glass and heavy metals.

This step effectively scrubs the Magnesium out of the metal stream, ensuring the aluminum recovered later is not contaminated by it.

The Heavyweights: Harvesting Clean Aluminum (3.0 kg/l)

Now we deal with the material that sank in the previous step. This stream contains Aluminum, heavy non-ferrous metals (Copper, Zinc, Stainless Steel), and inert heavy waste (rocks, heavy glass). These materials comprise only ~10% of the volume of the ASR we started with, so the separation equipment going forward is smaller.

To isolate the Aluminum, we raise the density of the media to 3.0 kg/l.

• The Float (Aluminum + Glass/Rocks): Since Aluminum (~2.7 kg/l) is lighter than 3.0, it floats. Because we already removed the Magnesium in the previous step, and the heavy metals (Zinc/Copper) sink in this step, this floating aluminum stream is chemically clean—containing no free Magnesium, Zinc, or Copper.
• The Final Polish (Eddy Current): While the aluminum floats, so do rocks and glass (which generally have densities between 2.4 and 2.8 kg/l). This is where the Eddy Current Separator comes in. Because aluminum is highly conductive and rocks/glass are not, the Eddy Current easily "ejects" the aluminum away from the inert stone and glass, particularly given that these materials have similar densities. The result is a market-ready aluminum product (often called "Twitch") that can either be further separated into Vesper or specific aluminum alloys, or sold “as is”.

The Sinks: The "Red Metal" Treasure Chest (3.0 kg/l)

What happens to the material that is too heavy to float even in the 3.0 kg/l bath?

Anything sinking at this stage is significantly denser than aluminum. This "Sink" fraction is a 100% heavy metals mix.

• Composition: This mix includes Copper (~8.9 kg/l), Zinc (~7.1 kg/l), Brass, Lead, and Stainless Steel. Your high-grade insulated copper wire also ends up here.
• Value: Because all the light materials (plastics, aluminum, magnesium, rocks, glass) have been stripped away in previous steps, this heavy fraction is incredibly valuable. It is typically sent to copper refineries or can be further separated into pure copper, zinc, and stainless steel streams to realize even more value.

Summary

By stepping the density from 1.4 to 2.0 and finally to 3.0, a processor can deconstruct a complex waste stream into pure commodities easily separated by dry separation equipment:

1. <1.4: Clean Plastics Feedstock
2. 1.4 – 2.0: Magnesium (cleaned of rubber via Eddy Current)
3. 2.0 – 3.0: Aluminum (cleaned of rocks via Eddy Current)
4. >3.0: Heavy Metals (Copper/Zinc/Stainless)

It is a process of subtraction by gravity—simple physics delivers high-purity recycling.