Market and product

Compiled by Bao Hien

Steel slag, sewage sludge ash, spent batteries and wastewater can all serve as sources of phosphorus for fertilizer production or battery materials. However, the amount recovered is not the only factor determining economic viability. Chemical consumption, purification costs and the quality of the final product can fundamentally change the economics of a recovery process.

Phosphorus is essential to all living organisms and is one of the three primary macronutrients required by crops. Around 50% of global food production depends on phosphate fertilizers, while nearly 80% of mined phosphate rock is used by the fertilizer industry.

Unlike nitrogen, which can be synthesized from atmospheric nitrogen through the Haber-Bosch process, industrial phosphorus supplies still come primarily from phosphate rock. Global phosphate rock reserves are estimated at around 74 billion tonnes, with annual production of approximately 240 million tonnes. Supply is concentrated in a small number of countries, while high-grade deposits that can be mined economically are becoming increasingly difficult to access.

Meanwhile, large quantities of phosphorus are lost through wastewater, agricultural runoff and solid waste. When phosphorus concentrations in water exceed appropriate thresholds, the nutrient can stimulate excessive algal growth, deplete dissolved oxygen and damage aquatic ecosystems. Recovering phosphorus can therefore help conserve resources while reducing environmental pollution.

Untapped phosphorus reserves in waste

Phosphorus is present in numerous industrial and municipal waste streams, but its concentration and chemical form vary considerably.

Phosphogypsum, a by-product of phosphoric acid production, is one of the largest industrial waste streams containing phosphorus. Global stockpiles exceed 6 billion tonnes, with nearly 300 million tonnes generated annually. China alone produces approximately 80 million tonnes a year, while the global recycling rate remains at just 10–15%.

The steel industry is another significant source. More than 200 million tonnes of steel slag are generated worldwide each year, and approximately 90% of the phosphorus entering the steelmaking process is transferred into the slag. In Japan, this amounts to around 97,000 tonnes of phosphorus annually, equivalent to nearly 30% of the country's phosphorus demand for fertilizer production. Basic oxygen furnace (BOF) slag and dephosphorization slag typically contain 3–10% P₂O₅.

Other waste streams also offer considerable potential. Phosphate mine tailings still contain approximately 2.18–5.24% phosphorus in the form of residual apatite. Incinerated sewage sludge ash (ISSA) contains 5–26% P2O5, comparable to some low-grade phosphate ores. Meanwhile, spent lithium iron phosphate (LFP) batteries contain approximately 18.2–40% phosphorus, alongside lithium and iron.

In wastewater treatment, much of the incoming phosphorus can be retained in sludge. Activated sludge worldwide is estimated to capture around 1.3 million tonnes of phosphorus annually. If recovered efficiently, phosphorus from municipal and industrial wastewater could meet approximately 20% of global fertilizer demand.

Why chemical form determines the recovery method

The main challenge is not simply how much phosphorus a waste stream contains, but how the element is bound to other components. In many cases, phosphorus is associated with calcium, iron or aluminum, requiring different treatment conditions for effective separation.

In steel slag, approximately 70–90% of the phosphorus is concentrated in a dicalcium silicate–tricalcium phosphate solid-solution phase, while iron is mainly present in other phases. This separation creates an opportunity for selective extraction. One study found that hydrochloric acid leaching at pH 3 recovered 86.6% of the phosphorus while dissolving only 0.19% of the iron.

However, using more acid does not necessarily improve recovery. Excessively strong acid can dissolve more calcium, potentially triggering secondary precipitation and reducing the amount of phosphorus ultimately recovered.

Results can be even more promising with certain other materials. For aluminum phosphate-rich residues from lithium processing, sodium hydroxide solution at 70°C recovered 99.45% of the phosphorus in just five minutes. For spent LFP battery residues, combining hydrochloric acid with hydrogen peroxide achieved a recovery rate of nearly 99.9%.

Nevertheless, high extraction efficiency at the leaching stage does not reflect the performance of the entire process. Sewage sludge ash illustrates this limitation. Acid leaching can extract 97.8% of the phosphorus, but it also dissolves 91.5% of the calcium, 39.7% of the aluminum and 48.8% of the iron. These impurities make subsequent purification more complex and expensive.

The final product determines the value of recovery

Once separated from the waste, phosphorus must undergo further processing to produce a usable material.

One common approach is to crystallize struvite, a compound containing magnesium, ammonium and phosphate that can be used as a slow-release fertilizer. The process typically operates at pH 8–9.5, with phosphorus recovery rates of approximately 80–95%.

Phosphorus extracted from steel slag can also be precipitated as calcium phosphate, with reported efficiencies exceeding 98–99%. Further processing can produce hydroxyapatite (HAP), containing up to 38.5% P2O5 with less than 1% impurities.

A potentially higher-value option is battery-grade iron phosphate (FePO4). For certain feedstocks, including battery residues and lithium-extraction solutions, controlled precipitation can achieve recovery rates above 99% and product purity exceeding 99%. The resulting material can potentially serve as a precursor for lithium iron phosphate cathode materials.

However, battery-grade products must meet stringent compositional and purity requirements, such as those specified in China's HG/T 4701–2021 standard. This means the process must do more than recover large quantities of phosphorus: it must also control impurities and deliver consistent product quality.

Removing phosphorus is not the same as recovering it

Not every technology that reduces phosphorus concentrations in water produces a reusable product.

Adsorption is suitable for dilute wastewater, where direct precipitation may be inefficient. Materials containing lanthanum or zirconium can bind phosphate effectively, but the strength of these bonds also makes desorption and phosphorus recovery difficult. In such cases, phosphorus is transferred from the water into a solid phase rather than converted into a valuable product.

An ion-exchange system incorporating iron oxide operated at a scale of 10 cubic meters per day for 2.5 years, completing 63 cycles. It maintained phosphorus concentrations below 0.1 mg/L in the treated water while concentrating phosphorus to 800 mg/L for subsequent calcium phosphate precipitation.

By comparison, an electrochemical system using iron and air removed 93–99% of the phosphate but recovered only 44.6–47.5% of the phosphorus as the mineral vivianite.

The difference highlights the need to distinguish among three indicators: the proportion of phosphorus removed from a waste stream, the proportion actually recovered, and the proportion converted into a product that meets end-use requirements. A high removal rate does not necessarily translate into equally high recovery or product yield.

Chemicals can determine economic viability

Cost remains one of the biggest barriers to phosphorus recovery. Beyond energy and equipment, processes consume acids, alkalis, precipitating agents and purification chemicals.

Life cycle assessments indicate that, in some sewage sludge ash treatment processes, the production of input chemicals accounts for 55–98% of total environmental impacts. A process that reduces waste or saves resources during treatment may therefore fail to deliver an overall environmental benefit if it requires excessive chemical consumption.

Economically, phosphorus recovery from phosphogypsum and sewage sludge ash has been estimated to cost around €3 per kilogram of phosphorus, compared with €1.1/kg for phosphorus from phosphate rock and €2.2/kg for conventional phosphate fertilizer.

Across the systems assessed, recovery costs range from approximately US$1–4/kg of phosphorus for optimized processes to more than US$70/kg for chemical-intensive ion-exchange systems.

The final product can significantly change the economics. For example, a process producing battery-grade iron phosphate from a rare-earth processing solution has been estimated to generate a net profit of around US$3.97 per kilogram of phosphorus recovered. However, the result depends on process conditions, feedstock quality and market demand, and cannot be generalized to every waste stream.

Policy can create a market for recycled phosphorus

Beyond technology and costs, policy directly influences the commercialization of recovered products.

In the European Union, fertilizer regulations have opened the market to certain recycled materials, including struvite, ash and biochar, subject to requirements and limits for contaminants such as cadmium. Germany and Switzerland have introduced mandatory phosphorus recovery requirements for certain waste streams. China has set a target of achieving a 65% comprehensive utilization rate for phosphogypsum by the end of 2026.

In the United States, one analysis valued phosphorus in conventional fertilizer at US$1.23/kg, while estimating the environmental and social costs of phosphorus losses at as much as US$74.5/kg. According to the study, a phosphorus recovery credit of US$22/kg could substantially improve profitability, although the outcome would still depend on facility scale.

Policies of this kind help account for environmental costs that are not fully reflected in market prices while creating incentives for investment in recycling technologies.

From steel slag and sewage sludge ash to spent batteries, phosphorus can become a secondary raw material for fertilizers and industrial materials. However, no single technology is optimal for every type of waste. Effective recovery depends on the chemical form of the phosphorus, the purity requirements of the final product and the cost of the entire process. Turning waste into a resource requires balancing recovery efficiency, product value and environmental impact from the outset, rather than focusing solely on achieving a high recovery rate.