Selective dissolution-precipitation: the lab method that recovers near-pure PE from cartons

Most industrial PolyAl recycling today is mechanical: shred, wash, extrude. A smaller share is thermal (pyrolysis). But a third route — well documented in the peer-reviewed literature — uses chemistry to do what physics struggles with: produce a near-pure polyethylene fraction from used beverage cartons. This is selective dissolution-precipitation (SDP).

The principle

Polyethylene dissolves in certain hydrocarbon solvents at elevated temperature. Aluminium does not. If you can find a solvent that selectively dissolves the PE in PolyAl reject — without dissolving anything else — you can:

1. 1Filter the dissolved polymer away from the aluminium foil and any contaminants.
2. 2Cool the filtered solution. The PE precipitates out as a clean solid.
3. 3Recover the solvent for reuse.

The result is a polyethylene fraction with a purity that mechanical processes cannot reach, plus an aluminium fraction nearly free of polymer residue.

How it works in practice

The reference process is described in *Resources, Conservation and Recycling* (Pappa et al., 2020), which characterised the recycling of post-consumer multilayer food and beverage cartons using selective dissolution-precipitation. The pilot-scale workflow is:

• Feed preparation: PolyAl reject is shredded to a few millimetres.
• Dissolution: The shredded reject is contacted with a hydrocarbon solvent at controlled temperature (typically 70–90 °C). Polyethylene dissolves; aluminium and any residual cellulose do not.
• Solid-liquid separation: The remaining solids — predominantly aluminium foil — are filtered out and recovered.
• Precipitation: The polymer-rich solvent is cooled. The PE precipitates as a clean powder or fine flake.
• Solvent recovery: The solvent is distilled off and recycled back into the process.

The output is a recycled PE with a purity comparable to mechanically recycled HDPE — with much lower aluminium contamination than the composite pellets produced by purely mechanical routes.

Why isn't it everywhere?

Three reasons SDP remains pilot-scale rather than dominant:

1. 1Solvent capex and safety. Hydrocarbon solvents at 70–90 °C require closed systems, vapour recovery and explosion-proof equipment. The capital cost per tonne of capacity is significantly higher than for mechanical recycling lines.
2. 2Energy intensity. Distilling solvent for reuse consumes thermal energy. Unless the heat is sourced from low-carbon electricity or waste streams, the climate footprint can erode the recycling benefit.
3. 3Market for composite pellets exists. As long as industrial buyers (pallet makers, profile extruders, urban-furniture manufacturers) are happy with composite PolyAl pellets containing aluminium as a functional filler, the cheaper mechanical route remains attractive.

Where SDP genuinely wins

There are use cases where the higher purity of SDP-recovered PE pays back the extra capex:

• Applications requiring tight ash limits, e.g. blown film grades or compounding feedstock for specialty masterbatches.
• Closed-loop schemes where a single packaging brand wants its recovered PE back as new packaging-grade material.
• Aluminium-credit revenue: the recovered aluminium fraction from SDP is purer than that from mechanical separation, so it commands a higher resale price.

How SDP relates to mechanical PolyAl recycling

SDP and mechanical recycling are not competitors so much as complementary stages of the PolyAl value chain. Mechanical recycling is the workhorse — the right technology for the bulk of pallet, profile and crate applications. SDP is a precision tool, suitable for the fraction of demand that genuinely needs near-virgin PE purity.

For most industrial buyers reading this site, mechanical PolyAl pellets (from Palurec, Plastigram, Recon Polymers or GetPolyal) are the right answer. SDP is one to keep an eye on as it scales beyond pilot — but it is not, today, the volume play.