How a European project is turning a toxic industrial liability into everyday materials, while capturing CO₂ at the same time.
Millions of tonnes of phosphogypsum (internationally known as PG) lie stored in vast open-air stacks across Europe, close to cities and coastlines, some for decades. In the FIC-Fighters project, we are developing a chemical process to transform this waste into useful, marketable materials. This article explains, in accessible language for all audiences, what the NaOH Route consists of, how it works, and why it matters.
FIC-Fighters is developing a chemical process to transform PG into useful, marketable materials. It is called the NaOH Route and has two variants. In both, PG is mixed with an alkaline liquid that triggers a chemical reaction. The first one uses pure caustic soda. The second replaces that soda with wastewater from aluminium factories: a liquid that would normally be discarded, but which already contains the ingredients needed for the reaction to work. The result in both cases is similar: the PG is converted into new materials, while CO₂ is incorporated into solid carbonate products.
Why This Technology Matters
The chemical industry produces large quantities of phosphoric acid, a compound essential to produce fertilisers, among other uses. For every tonne of acid that leaves the factory, between four and five tonnes of a solid by-product are generated: PG. This waste currently has no major commercial outlet, so it accumulates indefinitely in vast open-air stacks. In some cases, when the factory is in coastal areas, PG has been discharged directly into the ocean (as happened for decades on the Portuguese Atlantic coast and in the Gulf of Mexico), causing serious damage to marine ecosystems.
These stacks are not merely a land-use problem. They contain naturally occurring radioactive materials, heavy metals, and sulphates that can leach into the soil and groundwater or be dispersed through the air. Their presence near cities and inhabited areas represents a sustained environmental hazard.
But FIC-Fighters does not only solve a local problem. One of the key stages of the process involves the newly generated materials reacting with carbon dioxide (CO₂) and fixing it in a stable way within their mineral structure. This transforms the process into something more than a waste management solution: it places it in the category of carbon capture and utilisation (CCU) technologies, one of the most urgent areas of research in the fight against the global climate crisis.
Six European locations are at the centre of FIC-Fighters’ work: Barreiro (Portugal), Cartagena (Spain), Kutina (Croatia), Veles (North Macedonia), Prahovo (Serbia), and Turnu Măgurele (Romania). All of them illustrate decades of accumulation without a viable industrial solution.
While for years the only response was containment, FIC-Fighters proposes something different: exploiting the chemical properties of PG as an opportunity, making it react with other substances to generate products that manufacturing industries already purchase. The most remarkable thing is that there is no single way to achieve this: there are two chemical pathways, each with its own reagents, its own reactions, and its own products.
In short, FIC-Fighters simultaneously tackles two of the great environmental challenges of our time: the local pollution generated by decades of PG accumulation near European cities and coastlines, and climate change, whose scale is planetary. One chemical process, two problems solved.
Two Routes Within the NaOH Process: Pure NaOH and Al-waste
The so-called “NaOH Route” is a process concept encompassing two distinct chemical variants. Both share the same equipment and the same general logic, but they use different reagents and produce different intermediate solid materials. In both cases, the first reaction (PG + soda or wastewater from the aluminium industry (Al-waste)) generates two separate fractions: a liquid one (containing sodium sulphate, a by-product with its own commercial value) and a solid one (a mineral capable of capturing CO₂). The key difference lies in the intermediate solid: depending on the variant, its chemistry changes, and therefore the final products vary accordingly.
Pure NaOH Route: The Validation Pathway
In the pure NaOH Route, PG reacts directly with caustic soda dissolved in water. This is the easiest variant to control in the laboratory, and for that reason it is used as the starting point and reference: it allows the process chemistry to be studied precisely, and the behaviour of the equipment to be analysed under perfectly defined conditions, before introducing more complex scenarios.
To understand what happens in this reaction, it helps to know exactly what the “lime” generated actually is. What we commonly call “lime” can refer to several calcium compounds depending on the process they have undergone. It all begins in nature with limestone, the same mineral as chalk. When this rock is subjected to high temperatures, quicklime is produced — a highly reactive and corrosive white powder. If water is added to that quicklime, it “slakes” and transforms into hydrated lime or portlandite: a soft, safe material known since antiquity for its uses in whitewashing walls, disinfecting, and preparing mortars.
In the NaOH Route, portlandite is not obtained by calcining limestone in high-temperature kilns, but directly from PG through its reaction with caustic soda. This is what makes the process particularly interesting: that same white and versatile mineral (with its own market in paper, plastics, and construction) is generated without the need for kilns or high energy consumption.
But the role of portlandite in the process does not end there. Portlandite acts as a CO₂ capturer. It captures and fixes it within its structure, transforming itself into calcium carbonate (CaCO₃), the main component of chalk and one of the most in-demand industrial additives in the world (used in paper, plastics, and construction).
Why incorporate CO₂ capture if the products from the first reaction already have commercial value? Technically, carbonation produces a higher added-value product. Economically, by permanently fixing CO₂, the process can generate carbon credits, which can help emitting industrial sectors meet their regulatory obligations. This becomes an additional revenue stream that improves the viability of the business model.
Portlandite can therefore be marketed directly, or take the additional carbonation step, thereby adding the permanent capture of CO₂ to the process balance.
From one waste, up to three products with a real market emerge: portlandite, sodium sulphate, and calcium carbonate.
Scope note: Throughout FIC-Fighters, the carbonation stage operates exclusively with pure CO₂ from commercial cylinders and with synthetic gas mixtures prepared in the laboratory. Integration with real gas streams from industrial stacks is not an objective of the current project: it is the next project. FIC-Fighters is building the scientific knowledge that will make it viable.
| Reagent | Route A: Pure NaOH | Route B: Al-waste (target) |
|---|---|---|
| Alkaline reagent | Pure caustic soda (NaOH) — purchased reagent | Alkaline waste from aluminium factories, rich in sodium and aluminium — reused waste |
| CO₂ capturer | Portlandite (calcium hydroxide) | Katoite (calcium-aluminium compound) |
| Products after carbonation | Precipitated calcium carbonate (CaCO₃) | Precipitated calcium carbonate (CaCO₃) + aluminium hydroxide (Al(OH)₃) |
| Industrial symbiosis | PG [waste from fertiliser industry] + pure NaOH [commercial product] → single valorisation | PG [waste from fertiliser industry] + Al-waste [waste from aluminium industry] → double valorisation |
| Role in the project | Validation and technical reference pathway | Strategic target pathway (maximum sustainability) |
Al-waste Route: The Strategic Target Pathway
The Al-waste Route is FIC-Fighters’ strategic priority, since it delivers an additional benefit: industrial symbiosis. Instead of purchasing pure caustic soda, the process uses waste from aluminium processing, via extrusion and/or anodising. In other words, this is a route entirely based on the use of industrial wastes, giving them a second life.
These waste liquids are rich in sodium and aluminium, which fundamentally changes the outcome of the first reaction. The presence of aluminium means the solid obtained is no longer portlandite, but katoite, a calcium-aluminium compound that also acts as a CO₂ capturer.
And the added value multiplies when katoite captures CO₂; the reaction simultaneously produces CaCO₃ (as in the previous route) and aluminium hydroxide (Al(OH)₃), a raw material for the construction and chemical industry sectors.
Two industrial wastes enter the process and up to four commercially valuable products come out. That is why one speaks of “double industrial symbiosis”: two problems are eliminated and new products are generated at the same time.
It is important to note that the chemistry of the Al-waste NaOH Route, particularly in the CO₂ capture stage, is proving more complex than anticipated. Results vary depending on the specific composition of each aluminium waste, and the process presents fluidity challenges in the resulting mixture. This is precisely one of the active fronts of the project at this stage: in research, one cannot simply expect the anticipated results to be confirmed. FIC-Fighters is working to optimise the conditions that maximise katoite formation and overcome these technical challenges.
What This Means for People and for Europe
For communities living near PG stacks, the NaOH Route offers the prospect of a decades-long problem beginning to be resolved. The most emblematic case in Europe is the largest PG stack on the continent, located just 500 metres from the city of Huelva (Spain, 145,000 inhabitants), covering approximately 1,200 hectares (a surface area similar to the city itself) and adjacent to the Huelva estuary, which increases the risk of leaching towards the estuary and coastline. A stack that shrinks because its material is being valorised is a stack that represents a declining environmental risk. PG ceases to be a “ticking time bomb” and becomes a “controlled open-air mine”.
On the other hand, for the European industry, the NaOH Route creates new domestic sources of materials currently imported or produced from virgin resources. Producing them from waste (with a net positive carbon balance) supports Europe’s strategic material self-sufficiency and its alignment with the European Critical Raw Materials Act, which sets a strategic recycling target of 25% by 2030. Dependence on external imports of these minerals is reduced; Europe no longer needs to extract new resources to obtain what it already has, in the form of waste, within its own territory.
In summary, FIC-Fighters does not only fight against the massive accumulation of industrial waste. It works to consolidate a self-sufficient European industry, aligned with the principles of the circular economy and the fight against climate change, that knows how to intelligently exploit the waste we have already generated, rather than exploiting new natural resources.
Key References
- Cárdenas-Escudero, et al. https://doi.org/10.1016/j.jhazmat.2011.09.045
- Romero-Hermida, et al. https://doi.org/10.1016/j.jcou.2017.01.002
- CapturaCO2, S.L. (2012). Procedure for CO₂ and SO₂ Capture. Spanish patent no. ES2392280. Spanish Patent and Trademark Office.
- European Commission. (2024). European Critical Raw Materials Act. Regulation (EU) 2024/1252. https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=OJ:L_202401252