How Is Phosphoric Acid Made? Wet vs. Thermal Process

Phosphoric acid is made by one of two fundamentally different routes: the wet process, which dissolves phosphate rock in sulfuric acid and accounts for the vast majority of global production, and the thermal process, which reduces phosphate rock in an electric furnace to produce elemental phosphorus that is then burned and hydrated into acid. The wet process is cheaper and feeds the fertilizer industry; the thermal process is far more energy-intensive but yields an exceptionally pure product needed for food, electronics, and pharmaceuticals. Which one gets used depends almost entirely on what the acid is for.

What Goes Into the Reactor

Both processes start with the same raw material: phosphate rock, a broad term for sedimentary or ignite deposits rich in calcium phosphate minerals. The dominant mineral in most commercial deposits is a form of apatite, often carbonate-fluorapatite (sometimes called francolite), along with hydroxyapatite and varying amounts of quartz and clay minerals.¹ The phosphorus content of marketable rock is typically expressed as P₂O₅, and commercial-grade concentrates generally fall in the range of about 28 to 31 percent P₂O₅.²

Rock quality matters enormously. Phosphate deposits carry along a cocktail of impurities: iron, aluminum, magnesium, and silica oxides at percent-level concentrations, plus trace amounts of heavy metals like cadmium, arsenic, chromium, and uranium.² These impurities do not just dilute the product; they complicate filtration, contaminate the acid, and end up concentrated in waste streams. A rock with a low impurity-to-phosphate ratio is worth considerably more than one with the same phosphorus content but high iron or aluminum, because the downstream headaches multiply with every percentage point of contaminant.

Roughly 220 million tonnes of marketable phosphate rock concentrate are mined each year globally, about three-quarters from sedimentary deposits and one-quarter from igneous ones. The sedimentary deposits, found mainly in Morocco, China, the Middle East, and the United States, tend to be higher in organic matter and trace metals. Igneous deposits, common in Russia and parts of Africa, are typically lower in contaminants but harder to access.

The Wet Process

The wet process produces the overwhelming share of the world’s phosphoric acid. The chemistry is straightforward in principle: you dissolve ground phosphate rock in sulfuric acid. The calcium in the rock combines with the sulfate to form calcium sulfate (a form of gypsum), while the phosphate goes into solution as phosphoric acid. The reaction happens in large, agitated reactors where the rock is fed continuously into a bath of recycled acid and fresh sulfuric acid.

The practical challenge is controlling which form of calcium sulfate crystallizes out of that reaction. Depending on temperature and acid concentration, you get one of three crystal forms: dihydrate (ordinary gypsum), hemihydrate, or anhydrite. Each form has different filtration properties and traps different amounts of phosphate in its crystal structure, so the choice of crystal form defines the entire plant design.³

The dihydrate process was essentially the only game in town until the early 1970s. It runs at lower temperatures, is forgiving with different rock types, and has decades of operational track record. The acid it produces comes out relatively dilute, typically around 28 to 32 percent P₂O₅, and needs to be concentrated by evaporation before most downstream uses.³ That evaporation step consumes significant steam and energy, which became a real cost problem after the 1973 oil crisis.

The hemihydrate process emerged as an alternative precisely because it produces acid at a higher concentration, in the range of 40 to 50 percent P₂O₅, often eliminating the need for a separate evaporation stage entirely.⁴ That means lower capital costs and less steam consumption. The tradeoff is that hemihydrate crystallization is fussier to control, and the gypsum crystals that form can trap more unreacted phosphate, reducing the overall yield of phosphorus recovery. Hybrid routes that crystallize hemihydrate first, then recrystallize it into dihydrate in a second step, try to capture the benefits of both worlds: higher acid strength with better phosphorus recovery.

After the reaction, the slurry of acid and gypsum crystals goes to a large vacuum filter. The acid passes through, and the gypsum cake is washed to recover as much phosphate as possible before being sent to waste. Filtration performance depends heavily on crystal size and shape, which in turn depend on how well impurities in the rock are controlled. Ionic impurities at very low levels can actually help filtration by modifying crystal habit, but above about 0.1 percent, they start to interfere, making the filter cake denser and harder to wash.⁵

Why Wet-Process Acid Needs Purification

The acid that comes off the filter is green to dark brown, loaded with dissolved metals, fluoride, sulfate, and organic compounds from the rock. For fertilizer production, this is often acceptable, since the impurities either are not harmful to crops at the concentrations present or are diluted enough in the final product. But for any food-grade, pharmaceutical, or industrial-chemical application, the crude acid must be purified extensively.

Solvent extraction is the dominant purification method. The crude acid is mixed with an organic solvent (or mixture of solvents) that preferentially dissolves the phosphoric acid while leaving most impurities behind. The loaded solvent is then stripped with water to recover a much cleaner acid. Researchers have studied various solvent combinations for this purpose; one approach using mixtures of N-octanol and tributylphosphate achieved about 91 percent P₂O₅ extraction efficiency in a multi-stage counter-current setup.⁶

A persistent problem with solvent extraction is what to do with the leftover raffinate acid, the fraction that the solvent didn’t pick up. It’s high in impurities, viscous, and hard to use productively. Newer processes aim to extract the metal impurities from this raffinate rather than discarding it, recovering additional phosphorus that would otherwise be wasted.⁷ Some of these approaches also recover valuable metals: recent work has demonstrated selective extraction of rare earth elements like yttrium, holmium, and ytterbium alongside removal of zinc, copper, and cadmium from industrial phosphoric acid.⁸ Turning a waste-treatment step into a source of rare earths is an appealing idea, though it remains at the research stage for most plants.

The Thermal Process

The thermal process takes a completely different approach. Instead of dissolving phosphate rock in acid, it smelts a mixture of phosphate rock, coke (as a carbon reductant), and silica in an electric arc furnace at very high temperatures. The carbon strips the oxygen from the phosphate, producing elemental phosphorus vapor (P₄), which is condensed to a liquid and stored underwater. The silica combines with the calcium to form a molite slag, and carbon monoxide exits as a furnace gas.

To make phosphoric acid, the elemental phosphorus is burned in air. It combusts vigorously to form phosphorus pentoxide (P₄O₁₀), which is then hydrated with water or dilute acid to produce phosphoric acid.⁹ Because the phosphorus was distilled and condensed before burning, virtually all the impurities from the original rock were left behind in the slag. The result is an extremely pure acid, sometimes called “furnace-grade” or “thermal” phosphoric acid, suitable for food additives, semiconductor etching, and metal surface treatment without the extensive solvent-extraction purification the wet process requires.

The catch is energy. The electric-furnace route is one of the most energy- and carbon-intensive chemical processes still in industrial use.¹⁰ Running an arc furnace hot enough to reduce phosphate ore consumes enormous amounts of electricity and metallurgical-grade coke. This makes thermal acid far more expensive per tonne than wet-process acid. The process survives commercially only because certain industries require purity levels that are either impossible or uneconomical to achieve by purifying wet-process acid. Semiconductor fabrication, for instance, needs phosphoric acid with parts-per-billion-level trace metals, which is most reliably achieved starting from elemental phosphorus.

The Fluorine Problem

One issue common to both processes, though handled very differently, is fluorine. Phosphate rock naturally contains significant fluoride, bound up in the apatite crystal lattice. During the wet process, the sulfuric acid liberates this fluoride as hydrogen fluoride (HF) gas and hexafluorosilicic acid (H₂SiF₆). Some of these volatile fluorine compounds escape into the gas above the reactors, requiring scrubbing systems like wet cyclonic scrubbers or venturi scrubbers to capture them before they reach the atmosphere. A substantial fraction of the fluoride remains dissolved in the product acid, typically at concentrations between 1.3 and 2.6 percent by weight depending on the rock source.

In the thermal process, fluorine is largely retained in the furnace slag along with other impurities, which is one reason the product acid is so much cleaner. Captured fluorine from wet-process scrubbers is not always wasted; hexafluorosilicic acid is the primary feedstock for producing sodium fluoride used in municipal water fluoridation, and it has applications in aluminum smelting. But managing fluorine emissions remains one of the major environmental and safety concerns at any wet-process phosphoric acid plant.

The Phosphogypsum Problem

For every tonne of phosphoric acid (as P₂O₅) produced by the wet process, somewhere between 3.3 and 7 tonnes of calcium sulfate waste are generated, depending on the rock composition. This waste, called phosphogypsum, is one of the largest-volume industrial by-products on Earth. Most of it is stockpiled in massive open-air stacks, often near coastlines close to the acid plants.¹¹

Phosphogypsum is mostly just gypsum, but it is not ordinary gypsum. It carries elevated concentrations of heavy metals and naturally occurring radioactive materials, primarily radium-226, polonium-210, and lead-210.¹² The U.S. Environmental Protection Agency has classified phosphogypsum as a “Technologically Enhanced Naturally Occurring Radioactive Material” (TENORM), and its use in construction, agriculture, or cement is banned or restricted in many countries.¹¹

Only about 15 percent of global phosphogypsum production finds any reuse at all, mainly in construction materials, as a soil amendment, or as a set controller in Portland cement.¹¹ Research into using phosphogypsum as a soil amendment has shown that while dose rates to non-human biota remain below international safety benchmarks, the application does alter how uranium and radium associate with different soil fractions, raising long-term questions about mobility into the food chain.¹³ The sheer volume of the waste, combined with the low reuse rate, means that phosphogypsum stacks continue to grow at virtually every wet-process plant.

The thermal process sidesteps this problem entirely. Its waste product is a vitrified slag that is chemically stable and can be used as a construction aggregate. No phosphogypsum is generated. But the thermal process’s enormous energy consumption creates its own carbon footprint, so neither route is free of environmental cost.

Why the Wet Process Dominates

The economics are stark. The wet process requires sulfuric acid, which is cheap and abundantly available as a by-product of natural gas and oil refining. Roughly half of all sulfuric acid produced worldwide goes to fertilizer manufacturing, and about 95 percent of the world’s phosphate fertilizers are made using sulfuric acid in the wet process.¹⁰ That tight coupling between fossil-fuel refining and fertilizer production has kept wet-process acid costs low for decades.

The thermal process, by contrast, requires massive electricity input and high-quality coke, both of which are expensive. It makes economic sense only when the end product commands a premium price, which limits it to specialty applications. The result is that the global phosphoric acid market is essentially a two-tier system: a huge, low-cost tier of wet-process acid flowing into fertilizers, and a small, high-cost tier of thermal or highly purified wet-process acid serving food, pharmaceutical, and electronics markets.

This arrangement has a vulnerability, though. The wet process’s dependence on sulfuric acid ties phosphate fertilizer production to the fossil-fuel supply chain. If oil and gas production declines or sulfuric acid becomes scarcer for other reasons, the cost structure of conventional fertilizer production shifts. That coupling is one of the drivers behind research into alternative routes.

Emerging Alternatives

A handful of newer processes aim to break the dependence on sulfuric acid while avoiding the massive energy penalty of traditional thermal production. One is the PARFORCE process (Phosphoric Acid Recovery from Organic Residues and Chemicals by Electrochemistry), which uses electrical energy to dissolve phosphate-containing feedstocks without sulfuric acid. It was originally developed to recover phosphorus from sewage sludge ash, but it has been demonstrated on phosphate rock and struvite as well. Because it runs on electricity rather than chemical reagents, it can in principle be powered entirely by renewable energy, and it produces no phosphogypsum.¹⁰

Another approach is the Improved Hard Process (IHP) developed by NovaPhos. This is technically a thermal process, but it differs substantially from the classical electric-furnace route. It skips expensive ore beneficiation steps like flotation, making the overall process more cost-effective and less energy-intensive than traditional thermal production. Its primary product is phosphorus pentoxide rather than elemental phosphorus, which simplifies the path to phosphoric acid. Proponents claim it produces no phosphogypsum, utilizes all by-products, and can run on “green” petroleum coke derived from biomass, making its net CO₂ emissions much lower than either the classical thermal or wet process.¹⁰ A demonstration plant operates in Australia.

Neither of these alternatives has reached the scale needed to displace the wet process in fertilizer markets, and it is worth being realistic about how slowly chemical-process industries change. Plants costing hundreds of millions of dollars run for decades, and proven technology beats elegant alternatives in boardroom decisions. But the combination of phosphogypsum disposal costs, tightening environmental regulations, and potential disruptions to sulfuric acid supply gives these newer routes a plausibility they would not have had twenty years ago.

Supply-Chain Risks and Why They Matter

Phosphate rock deposits are concentrated in a few countries, with Morocco holding the largest reserves by a wide margin. China, Russia, and a handful of Middle Eastern and North African nations account for most of the rest. This geographic concentration makes phosphorus fertilizer supply vulnerable to geopolitical instability, trade disputes, shipping disruptions, and commodity-market shocks.¹⁴

Phosphorus has no substitute in agriculture. Plants need it to grow, and there is no synthetic alternative or workaround. When phosphate rock prices spike, as they did in 2008 and again in 2022, the cost ripples through fertilizer prices and ultimately into food prices worldwide. The wet process’s additional dependence on sulfuric acid adds a second supply-chain pressure point, since sulfuric acid availability is tied to oil and gas refining volumes. If the global energy transition reduces fossil-fuel processing, the sulfuric acid surplus that currently keeps wet-process costs low could shrink, creating cost pressures from a direction the fertilizer industry has not historically had to worry about.

From Acid to Fertilizer and Beyond

Most wet-process phosphoric acid never reaches a consumer as acid. It is reacted with ammonia to produce ammonium phosphate fertilizers, primarily monoammonium phosphate (MAP) and diammonium phosphate (DAP). These are the workhorses of the global fertilizer supply, delivering both nitrogen and phosphorus in a single granule. More specialized products include water-soluble MAP for fertigation systems and ammonium polyphosphate for liquid fertilizers. The development of slurry MAP technology has been particularly significant in countries like China, where it allowed production of high-concentration phosphate fertilizer from medium- and low-grade ore that would not have been economical to process otherwise.

On the non-fertilizer side, purified phosphoric acid goes into soft drinks (it provides the tart bite in colas), food additives, metal surface treatment (phosphating of steel), flame retardants, water treatment chemicals, lithium iron phosphate battery cathodes, and semiconductor etching solutions. The purity requirements vary enormously: food-grade acid needs to meet limits on fluoride, heavy metals, and arsenic, while semiconductor-grade acid requires trace-metal levels measured in parts per billion. The thermal process, or deep solvent-extraction purification of wet-process acid, supplies these higher-purity markets. As demand for lithium iron phosphate batteries has surged in recent years, the competition for purified phosphoric acid between the traditional food-and-beverage sector and the battery industry has added a new dimension to phosphoric acid markets that barely existed a decade ago.

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