Can Aggregates Recovered from Hazardous Soils Really Be Considered Clean?

There is growing interest across the waste and construction industries in recovering aggregates from hazardous soils, including material contaminated with heavy metals.

Recovering and reusing material rather than simply disposing of it can have clear environmental and commercial benefits. But before we focus on recovery percentages, there is an important question that needs to be answered:

If an aggregate has been recovered from hazardous soil, what evidence demonstrates that it is genuinely suitable for use in construction?

Washing Does Not Automatically Mean “Clean”

Soil washing is, fundamentally, a separation and treatment process. Depending on the contaminants, the material being treated and the system being used, contaminants may be removed from one fraction and concentrated into fines, filter cake, process water or another residual stream.

That can be an effective treatment process. However, it does not automatically mean that every stone or aggregate leaving the plant is free from the contamination associated with the original hazardous material. This is particularly important when dealing with heavy metals and mineralised rock.

Heavy Metals May Not Simply Be Sitting on the Surface

In areas of the country where naturally occurring heavy metals are not a significant concern, the situation may be more straightforward. In Cornwall, however, this question deserves particular attention because our historic metal-mining industry was principally concerned with extracting mineralised rock — not soil.

Metals and associated mineral phases are not necessarily sitting conveniently on the outside of a stone, waiting to be washed away. Depending on the geology and mineralogy, they can occur within the rock itself.

That creates an important distinction: washing the outside of a piece of rock is not necessarily the same as removing constituents contained within the rock.

And what happens if that recovered rock is subsequently crushed to manufacture aggregate? Crushing creates fresh surfaces and can potentially expose mineral phases that were previously enclosed within the rock.

So the important question is not simply:

“Did the aggregate pass a test after washing?”

It should also be:

“What was the original material? What is contained within the rock? What happens when it is crushed? And is the resulting aggregate genuinely suitable for its intended use?”

Hazardous In — Aggregate Out?

If material enters a treatment facility classified as hazardous because of heavy-metal contamination, customers should be able to understand the journey from that hazardous input to any aggregate subsequently sold as a recovered product.

That means asking what contaminants were originally present, whether they were surface-bound, associated with fines or present within the mineral structure of the rock, and which fractions contain those contaminants following treatment.

It also means understanding what testing has been carried out on the recovered aggregate, whether the effect of subsequent crushing and processing has been considered, and what the aggregate is ultimately going to be used for.

Simply achieving a high percentage of aggregate recovery does not, by itself, answer those questions.

Wash Plants Are Not a New Invention

Some operators are promoting this technology as relatively new or groundbreaking. But washing, screening and separating soils and aggregates are established techniques that have been used across the mineral, aggregate, waste and remediation industries for many years.

Modern plants can incorporate sophisticated separation technology, water treatment and process controls, and these developments can improve treatment and recovery considerably.

But the underlying principle remains the same: the material is being separated into different fractions.

Technology can improve that separation, but it does not remove the need to understand the contamination, the geology and the suitability of the recovered materials.

Hydrocarbons and Other Contaminants Matter Too

Heavy metals are a particularly important consideration in Cornwall, but they are not the only issue. Hydrocarbons and other contaminants can behave very differently during treatment, with their concentration, mobility and response to treatment depending on the contaminant, the material being treated and the process being used.

There is therefore no blanket assumption that, simply because material has passed through a wash plant, every resulting aggregate fraction is automatically suitable for unrestricted reuse.

The original material, the treatment process, the testing regime and the intended end use all matter.

What About Concrete and Cementitious Products?

There is another consideration in Cornwall when recovered aggregate is intended for use in concrete, blocks or other cementitious products.

Cornwall has a well-documented history of problems associated with Mundic, where unsuitable mineral constituents within aggregates used historically in concrete contributed to deterioration.

Heavy-metal contamination should not be confused with the specific mineralogical mechanisms responsible for Mundic. However, the presence of unstable sulphide minerals associated with some mining waste can raise additional questions about the suitability and long-term behaviour of recovered material.

The wider lesson is important: the mineralogical composition and provenance of an aggregate matter.

For this reason, we do not believe that rock recovered from hazardous or heavily metal-contaminated material should be incorporated into concrete, blocks or other cementitious products without very careful consideration of its provenance, composition and long-term suitability.

An aggregate meeting a particular chemical specification at one point in the treatment process does not necessarily answer every question about its mineralogical composition or how it may behave after further crushing or incorporation into another product.

The Wheal Jane Approach

At Wheal Jane Waste & Recycling, we take a deliberately different approach.

We do not use hazardous, heavy-metal-impacted soils as feedstock for our recycled aggregates. Our quality recycled aggregates are manufactured from suitable inert soils, stones and construction and demolition waste.

Hazardous heavy-metal-impacted material accepted through our appropriate recovery route has a completely different destination.

Rather than attempting to turn that material into aggregate and return it to the construction supply chain, suitable material is beneficially reused in the restoration and remediation of the Clemows Valley Tailings Dam, in accordance with our permitted recovery operation.

That distinction is important. It allows hazardous material to have a genuine beneficial recovery destination without relying on the sale of aggregates recovered from that hazardous material back into the construction supply chain.

Because of Cornwall’s particular geological considerations, we also do not sell our recycled aggregates for incorporation into concrete, blocks or other cementitious products.

Recovery Should Be About More Than Producing a Saleable Aggregate

There is nothing inherently wrong with soil washing. It is an established and potentially valuable recovery technology, and we are not suggesting that every aggregate recovered from hazardous material is automatically unsuitable for reuse.

The point is much simpler:

The fact that an aggregate can be physically recovered from hazardous soil does not, on its own, demonstrate that it is appropriate for every subsequent use.

Responsible recovery should consider:

• Where did the material originate?
• Why was it classified as hazardous?
• What contaminants were present?
• Where did those contaminants go during treatment?
• Could contaminants or other relevant constituents remain within the recovered rock?
• What happens when that rock is subsequently crushed?
• What testing demonstrates suitability for the proposed end use?
• What happens to the remaining contaminated fractions?

At Wheal Jane, we believe these questions should be answered before material recovered from hazardous soils is put back into the construction supply chain.

Because recovering an aggregate is one thing. Demonstrating that an aggregate recovered from hazardous material is genuinely suitable for its intended use is another.