Sulfuric Acid For Phosphate Fertilizer What Map And Dap Plants Actually Buy
Sulfuric Acid for Phosphate Fertilizer: What MAP and DAP Plants Actually Buy
Sulfuric acid entering a monoammonium phosphate (MAP) or diammonium phosphate (DAP) complex is procured as a specification-defined acidulation reagent rather than a generic chemical. The fluorapatite acidulation reaction Ca10(PO4)6F2 + 10 H2SO4 + 20 H2O → 6 H3PO4 + 10 CaSO4·2H2O + 2 HF consumes 2.30 t of 100% H2SO4 per tonne of P2O5 on a stoichiometric basis. Commercial phosphate complexes typically consume 2.60–2.80 t of 100% H2SO4 per tonne of P2O5 after carbonate, iron, aluminum, and magnesium gangue reactions and deliberate excess sulfate demand. The product actually purchased is usually merchant-grade virgin sulfuric acid at 93.0–98.5 wt%, priced on a 100% H2SO4 basis under contract limits for iron, chloride, arsenic, suspended solids, and specific gravity.
In a dihydrate phosphoric acid complex rated at 1,000 t/d P2O5, the purchase requirement is approximately 2,700 t/d of 100% H2SO4. At 93.0 wt%, the as-received acid feed is approximately 2,900 t/d; at 98.5 wt%, the as-received flow is approximately 2,740 t/d. Sulfuric acid is not normally added directly to the MAP/DAP pre-neutralizer in standard flowsheets; it is consumed in the phosphoric acid attack section. In sulfur-fortified MAP or DAP grades, a separate small-volume sulfate stream may be introduced later, but this is distinct from the main acidulation acid.
What concentration is actually ordered at the tank farm?
A concentration of 93.0–93.5 wt% is common for outdoor carbon steel storage because its freezing point lies below -30 °C, whereas 98.0–98.5 wt% acid can approach 0 °C and may require insulated or traced railcars and storage in cold climates. Specific gravity at 20 °C is typically specified in the 1.82–1.84 g/cm3 range for 93–98 wt% acid and is converted to concentration by table. Total acidity is determined by sodium hydroxide titration using a mixed indicator; ISO 910 is the recognized method for total acidity and free sulfur trioxide content in commercial oleum and sulfuric acid, while ASTM E223-23 covers routine sulfuric acid and oleum analysis including specific gravity and impurity test methods.
The preference for 93 wt% acid in phosphate service is not primarily economic; it reduces SO3 vapor pressure and acid mist during storage and handling, and it provides water for heat absorption in the rock attack tank. Acid plants may produce 98.5 wt% acid and dilute it before transfer, or the absorption train may be operated to produce lower strength directly. Tank farm titration records and specific gravity checks are used to reject off-spec shipments because concentration differences alter acid metering and sulfate balance. Oleum is not purchased for MAP or DAP acidulation. Its free SO3 content would impose freezing and vapor hazards without process benefit, and the acid plant absorber product is diluted or produced directly to the desired concentration.
Impurity ceilings before the rock attack tank
Merchant acid certificates of analysis set limits on total iron, chloride, arsenic, suspended solids, and sometimes nitrogen oxides. Iron is relevant because iron in the acid reports to phosphoric acid and can form Fe3(PO4)2 or FePO4 sludges, increasing viscosity and phosphate losses in the gypsum cake. Chloride is monitored because free chloride accelerates pitting of stainless steels such as 316L, 904L, and duplex grades in phosphoric acid evaporators and pipe reactors. Arsenic is a regulatory and gypsum disposal issue, particularly when metallurgical acid is blended. Suspended solids can blind flowmeter orifices and spray nozzles.
| Parameter | Sulfur-burning acid | Metallurgical acid | MAP/DAP process effect |
|---|---|---|---|
| Total iron, mg/kg | 10–50 | 50–300 | Sludge formation, higher P2O5 losses in gypsum |
| Chloride, mg/kg | <10 | 10–100 | Pitting in 316L and duplex piping, especially in evaporators |
| Arsenic, mg/kg | <1 | 5–50 | Transfers to acid and gypsum; may affect fertilizer regulatory status |
| Suspended solids, mg/kg | <20 | 20–200 | Flowmeter drift and nozzle fouling in acid distribution |
Published supplier-specific limits vary; the table reflects representative procurement ranges rather than a universal standard. A site that buys virgin sulfur-burning acid may set the chloride ceiling at 10 mg/kg, while a site that blends metallurgical acid may accept 50 mg/kg only after verifying chloride pitting resistance with exposed coupon tests in the phosphoric acid evaporator.
Spent sulfuric acid from refinery alkylation or titanium dioxide production is generally not purchased for MAP/DAP acidulation unless regenerated. Alkylation acid contains dispersed hydrocarbons and water; sulfate-route TiO2 waste acid carries iron, titanium, and sulfate-salt solids that can disturb gypsum crystal growth and filter cake permeability. Regenerated acid can be considered when the chemical oxygen demand is below a site-specific limit and the metal profile meets merchant acid ceilings.
Acidulation performance in MAP and DAP phosphoric acid production is governed less by total acidity than by the free sulfate balance in the attack slurry. The filter acid sulfate concentration is routinely held between 1.5 wt% and 3.5 wt% SO3. Sulfate depletion below the lower threshold promotes recrystallization of gypsum to fine hemihydrate or anhydrite phases that blind the filter cloth, while excess sulfate above the upper threshold co-precipitates calcium sulfate on the rock particles and reduces P2O5 recovery. The purchased acid is therefore metered against online free sulfate analyzers and rock feed mass, not simply against a fixed rock-to-acid ratio. On a tilting-pan or horizontal belt filter, a filter cake permeability drop of 10–15% is typically enough to reduce wash efficiency and elevate moisture in the gypsum discharge.
Carbonate gangue in the rock consumes additional sulfuric acid. For a rock containing 30 wt% P2O5 and 5 wt% CO2, carbonate attack can add roughly 0.4 t of 100% H2SO4 per tonne of P2O5, because each mole of CaCO3 consumes one mole of H2SO4. The actual purchase requirement therefore escalates for sedimentary phosphate ores with high carbonate content, and the acid contract must accommodate feed rock variability.
MAP and DAP plants do not differ significantly in sulfuric acid purchase specification; both consume acid in the upstream phosphoric acid unit. The downstream pre-neutralizer controls the NH3:H3PO4 mole ratio to approximately 1.0 for MAP and 2.0 for DAP, but this ratio is adjusted with ammonia and phosphoric acid, not raw sulfuric acid. The acid requirement is a function of P2O5 production from the rock attack section, not the final N:P grade.
When a phosphate complex substitutes metallurgical acid for sulfur-burning acid
Metallurgical acid from copper or zinc smelters is often lower cost and can be used in phosphate acidulation when impurity limits are maintained. The substitution is typically introduced as a blend rather than a full replacement. The chloride concentration in the blended acid is the first process constraint because chloride concentrates in phosphoric acid and can initiate pitting in 316L and duplex stainless steel at evaporator temperatures above 80 °C. Arsenic in metallurgical acid transfers to the phosphoric acid and subsequently to gypsum or fertilizer; receiving terminals may require arsenic below 20–50 mg/kg depending on jurisdiction and gypsum use.
Filterability testing is performed before approving a substitution. A leaf test or vacuum filter test on the plant phosphoric acid slurry measures specific cake resistance and wash curve; if specific resistance increases by more than 15–20% relative to baseline, the blend ratio is reduced. Some plants also monitor dissolved organic carbon because smelter acid from sulfide roasters may carry traces of organic flotation reagents or sulfurous species that affect acid color and redox potential. The decision to accept metallurgical acid therefore depends on both the certificate of analysis and a site-specific filterability result.
Acid unloading, storage, and the railcar-to-reactor boundary
Concentrated sulfuric acid is transported under UN 1830, Class 8, packing group II in carbon steel railcars or tank trucks; the exact lining and temperature controls depend on acid concentration. Carbon steel is passivated by concentrated sulfuric acid at ambient temperature, but the protective iron sulfate film is velocity-sensitive. Transfer piping for 93–98 wt% acid is designed for flow velocities below 1 m/s to avoid film erosion, and pumps use materials such as alloy 20, silicon iron, or PTFE-lined casings. Storage tanks are often carbon steel with a corrosion allowance of 3–6 mm and are vented through desiccant or scrubber devices to prevent atmospheric moisture ingress.
Purchasing contracts specify a certificate of analysis per railcar or truck, with total acidity, specific gravity, iron, chloride, arsenic, suspended solids, and appearance. Off-spec acid is held in a quarantine tank and blended back under controlled ratio if the deviation is limited to concentration or suspended solids. Acid containing chloride above the contract limit is generally returned, not blended, because chloride is not removed in the dihydrate filter and remains in the phosphoric acid loop.
Verification of delivered acid at the laboratory follows ASTM E223-23 for specific gravity and total acidity; metals are determined by inductively coupled plasma atomic emission spectroscopy after dilution. The laboratory also measures suspended solids by filtration through a 0.45 μm membrane and reports residue on ignition. These measurements are used to approve unloading, not to adjust the purchase price, because pricing is fixed on the 100% H2SO4 quantity calculated from the net weight and the certified concentration.
