How to simulate phosphoric acid plant fluorine recovery: fluosilicic acid from two scrubbers
A wet-process phosphoric acid plant releases the phosphate rock's fluorine as gas in two places: the attack tank, as silicon tetrafluoride, and the vacuum concentrator, where the acid's fluosilicic acid breaks down to SiF₄ and HF about 1:2 (EPA AP-42 §8.9 puts 20-40% of the rock's fluorine into the concentrator vapour). Both streams are scrubbed with water to fluosilicic acid, H₂SiF₆, sold to make aluminium fluoride and cryolite, as EFMA/Fertilizers Europe BAT Booklet No. 4 (2000) §2.4 describes them. The fume scrubber takes the attack-tank gas of the wet-process-phosphoric-acid-dihydrate example (72.6 mol/s CO₂ and 1.50 mol/s SiF₄) through two absorbers in series at 90% each, the booklet's figure for one atmospheric absorber: 99.0% of the fluorine is removed, the booklet's 'better than 99%', and 1.14 g/s of fluorine leaves with the stack gas. With no HF in that gas the SiF₄ hydrolyses, 3 SiF₄ + 2 H₂O → 2 H₂SiF₆ + SiO₂, so a third of the silicon drops out as 107 kg/h of silica, and the acid leaves at the booklet's 22% H₂SiF₆, 2.33 t/h of it. The hot humid gas condenses 11.56 mol/s of water into the liquor on its way to 320 K, and the 128 kW reaction heat is part of the 678 kW the liquor cooler removes. The vacuum scrubber takes the concentrator vapour at 0.12 bar in a single stage at 85%, inside the booklet's 83-86% recovery for a vacuum system: SiF₄ + 2 HF → H₂SiF₆ uses all the SiF₄ with the HF, so no silica forms, and the acid leaves at 20% (the booklet's 17-23%), 7.72 t/h of it; 59.8 g/s of fluorine goes on with the water vapour to the barometric condenser. Representative inputs, labelled: the concentrator vapour (21 mol/s of fluorine, half the DH example's acid fluorine, with the 30 kg/s of water a 28 to 54% P₂O₅ concentration evaporates), the scrub water rates and the scrubber temperatures.
- 1Open the ready-made model
Open the "Phosphoric acid plant fluorine recovery: fluosilicic acid from two scrubbers" model in the MaximaLabs workspace — no install, no license. It loads live on the canvas, ready to edit and run.
- 2Confirm the thermodynamics
This process is modeled with the BRINE property package over water, co2, silicon_tetrafluoride, hydrogen_fluoride, fluorosilicic_acid, silica — already selected, so the phase equilibrium and enthalpy are physically consistent from the first run.
- 3Review the flowsheet
The flowsheet chains 2× Vacuum Scrubber. Every block is a real, solvable unit op you can reconfigure on the canvas.
- 4Run the simulation
Click Run. The deterministic solver converges the material and energy balances (recycles included) and fills the live stream table — the AI never invents a number.
- 5Read the results and iterate
Inspect the converged streams, tweak a spec, and re-run — or ask the AI copilot to explain a result or diagnose a failed solve in plain English.
- Thermodynamics
- BRINE
- Components
- water, co2, silicon_tetrafluoride, hydrogen_fluoride, fluorosilicic_acid, silica
- Unit operations
- 2× Vacuum Scrubber
Opens live on the canvas — free, no install.
Explore the model & flowsheetModeling assumptions & limitations
What this model captures, and what it deliberately does not — from the engineers who built it.
- 1Removal per absorber is an input, not predicted from the contactor; the H₂SiF₆ vapour pressure (Munter et al. 1947) that limits product strength could not be obtained, so no equilibrium cap is applied
- 2The gas leaves water-saturated at the scrubber temperature, the liquor's water activity taken as 1
- 3The energy balance is on formation enthalpies, with no sensible heat for the aqueous H₂SiF₆ and HF and no heat of dilution
- 4The reactor's own fluorine split is an input of that example, and this flowsheet takes its off-gas as fixed numbers, not as a live link.
Frequently asked questions
- What does the Phosphoric acid plant fluorine recovery: fluosilicic acid from two scrubbers model simulate?
- A wet-process phosphoric acid plant releases the phosphate rock's fluorine as gas in two places: the attack tank, as silicon tetrafluoride, and the vacuum concentrator, where the acid's fluosilicic acid breaks down to SiF₄ and HF about 1:2 (EPA AP-42 §8.9 puts 20-40% of the rock's fluorine into the concentrator vapour). Both streams are scrubbed with water to fluosilicic acid, H₂SiF₆, sold to make aluminium fluoride and cryolite, as EFMA/Fertilizers Europe BAT Booklet No. 4 (2000) §2.4 describes them. The fume scrubber takes the attack-tank gas of the wet-process-phosphoric-acid-dihydrate example (72.6 mol/s CO₂ and 1.50 mol/s SiF₄) through two absorbers in series at 90% each, the booklet's figure for one atmospheric absorber: 99.0% of the fluorine is removed, the booklet's 'better than 99%', and 1.14 g/s of fluorine leaves with the stack gas. With no HF in that gas the SiF₄ hydrolyses, 3 SiF₄ + 2 H₂O → 2 H₂SiF₆ + SiO₂, so a third of the silicon drops out as 107 kg/h of silica, and the acid leaves at the booklet's 22% H₂SiF₆, 2.33 t/h of it. The hot humid gas condenses 11.56 mol/s of water into the liquor on its way to 320 K, and the 128 kW reaction heat is part of the 678 kW the liquor cooler removes. The vacuum scrubber takes the concentrator vapour at 0.12 bar in a single stage at 85%, inside the booklet's 83-86% recovery for a vacuum system: SiF₄ + 2 HF → H₂SiF₆ uses all the SiF₄ with the HF, so no silica forms, and the acid leaves at 20% (the booklet's 17-23%), 7.72 t/h of it; 59.8 g/s of fluorine goes on with the water vapour to the barometric condenser. Representative inputs, labelled: the concentrator vapour (21 mol/s of fluorine, half the DH example's acid fluorine, with the 30 kg/s of water a 28 to 54% P₂O₅ concentration evaporates), the scrub water rates and the scrubber temperatures.
- Which thermodynamic method does it use?
- The BRINE property package, over water, co2, silicon_tetrafluoride, hydrogen_fluoride, fluorosilicic_acid, silica — already selected. You can switch the method on the canvas before running.
- Which unit operations are in the flowsheet?
- It chains 2× Vacuum Scrubber. Every block is a real, solvable unit operation you can reconfigure, add to, or remove.
- Do I need to install software or buy a license?
- No. Phosphoric acid plant fluorine recovery: fluosilicic acid from two scrubbers runs entirely in your browser on MaximaLabs — free, no install, no license. Open the model to load it live and run the deterministic solver.
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Sour-gas sweetening — six-category footprint
A fuel-fired feed-gas heater, acid-gas removal, and a VOC purge — vented to atmosphere — so the Sustainability panel shows all six impact categories at once: carbon, water, acidification (vented H₂S/NH₃ + combustion NOx), eutrophication (NH₃ + NOx), photochemical ozone (vented benzene) and cumulative energy demand. Illustrative screening vent compositions — a real plant Claus/incinerates the H₂S and controls the VOC rather than venting; the point is to exercise the multi-category footprint on a process that genuinely carries these species. Spec-based separators, so it converges fast and conserves mass.
Phosphoric acid concentration: steam evaporator baseline (29 → 54% P2O5)
The conventional wet-process step, at the scale of one Jorf Lasfar concentration line: 138 t/h of 29% P₂O₅ acid, heated by 3.5 bar LP steam in a graphite exchanger and flashed under vacuum (0.12 bar) to 54% P₂O₅, evaporating 64.0 t/h of water. The steam feed is tuned so the concentrate lands on 54.0 wt% P₂O₅ (the user variable p2o5_wt_product), which takes 73.6 t/h of steam — 1.84 t steam per tonne of P₂O₅, 44.5 MW of condensing duty. That is the single-effect ideal; the published plant benchmark of ~2.8 t/t carries exchanger approach, losses and off-design operation on top. The point of the number here is what it costs the site: at Jorf Lasfar this steam is a co-product of the exothermic sulfuric-acid plants, which is the fact any electric alternative has to beat. Compare the two sibling examples, phosphoric-acid-concentration-microwave and -hybrid. Model scope, stated up front. The acid runs on the brine package, which carries phosphoric acid as a non-volatile molecular solute with its CRC liquid heat capacity (145 J/mol/K) — the streams show the real H₃PO₄/water compositions and the acid's sensible heat is in the balance — and a water activity fitted to measured data: binary boiling points (Li 2017, NIST TRC ThermoML Archive) and 20 °C vapour pressures up to 85 wt% (Kirk-Othmer), with a held-out error of 2% in water activity. The boiling-point elevation is therefore the fitted one, 23.5 K at 54% P₂O₅ and 0.12 bar (the ideal, Raoult's-law activity this replaced gave 9 K). Checked against a boiling point the fit never saw, it puts 85 wt% acid at 164 °C at atmospheric pressure against a measured 158 °C. Above 18 mol% acid the temperature dependence rests on one coefficient and that single check, so read the concentrate temperature to within a few kelvin. No heat of dilution is carried (about 1.5% of the latent duty at 54% P₂O₅). The 320 K feed stands in for a hotter plant feed. The acid heated at 1 atm here partly boils in the exchanger (the model's fitted boiling point at 1 atm); the real acid is kept liquid under static head and flashes in the chamber.
Phosphoric acid concentration: microwave heating (29 → 54% P2O5)
The same duty with the steam exchanger replaced by volumetric microwave heating: 44.5 MW absorbed in the acid, then the same vacuum flash to 54.0 wt% P₂O₅ and the same 64.0 t/h of water removed — the microwave does not remove water, it supplies the latent heat. At a 65% wall-plug magnetron efficiency (915 MHz industrial units reach ~85%) that is 68.5 MW of electricity, 1,710 kWh per tonne of P₂O₅ (1,310 at 85%). The comparison that decides it is the energy carrier, not the heating mechanism: at Jorf Lasfar the baseline's 1.84 t/t of steam is a co-product of the sulfuric-acid plants, and the site turns that steam into electricity at roughly 30%. Sending it through a turbine, a magnetron and back into the acid delivers about a fifth of the heat the exchanger delivers directly (0.30 × 0.65 ≈ 0.2), so on this site the microwave route needs roughly five times the primary steam energy of the baseline. Where microwaves do earn their place is not here but at the superphosphoric end (>70% P₂O₅, viscous, fouling, where the recent literature targets them), and an electric route for the 29-54 step should be compared against mechanical vapour recompression, which reuses the latent heat at a coefficient of performance of 10-30. Sweep the absorbed duty to see the product concentration and the electricity follow it. Model scope, stated up front. The acid runs on the brine package, which carries phosphoric acid as a non-volatile molecular solute with its CRC liquid heat capacity (145 J/mol/K) — the streams show the real H₃PO₄/water compositions and the acid's sensible heat is in the balance — and a water activity fitted to measured data: binary boiling points (Li 2017, NIST TRC ThermoML Archive) and 20 °C vapour pressures up to 85 wt% (Kirk-Othmer), with a held-out error of 2% in water activity. The boiling-point elevation is therefore the fitted one, 23.5 K at 54% P₂O₅ and 0.12 bar (the ideal, Raoult's-law activity this replaced gave 9 K). Checked against a boiling point the fit never saw, it puts 85 wt% acid at 164 °C at atmospheric pressure against a measured 158 °C. Above 18 mol% acid the temperature dependence rests on one coefficient and that single check, so read the concentrate temperature to within a few kelvin. No heat of dilution is carried (about 1.5% of the latent duty at 54% P₂O₅). The 320 K feed stands in for a hotter plant feed. Microwave penetration depth (about a centimetre in a conductive acid at 2.45 GHz) and the HF attack on quartz windows are equipment questions this model does not carry.
Phosphoric acid concentration: microwave-then-steam hybrid (29 → 42 → 54% P2O5)
The transition path the concept proposes: microwaves take the acid from 29 to 42% P₂O₅ in a first vacuum flash (29.6 MW absorbed), a pump returns the 42% acid to atmospheric pressure, and the existing steam exchanger and second flash finish it to 54.0 wt% — 24.2 t/h of steam (0.61 t/t) plus 1,140 kWh of electricity per tonne of P₂O₅ at 65% magnetron efficiency, the same 64.0 t/h of water removed in total. The split is not free: two-thirds of the latent duty moved from a co-product steam to purchased electricity, and nothing about the second stage got easier. What a hybrid does buy is a pilot that can be run beside the existing evaporator without betting the line on it, and the freedom to put the microwave stage where it has a case — the viscous, fouling end — rather than on the dilute feed. Compare phosphoric-acid-concentration-steam and -microwave. Model scope, stated up front. The acid runs on the brine package, which carries phosphoric acid as a non-volatile molecular solute with its CRC liquid heat capacity (145 J/mol/K) — the streams show the real H₃PO₄/water compositions and the acid's sensible heat is in the balance — and a water activity fitted to measured data: binary boiling points (Li 2017, NIST TRC ThermoML Archive) and 20 °C vapour pressures up to 85 wt% (Kirk-Othmer), with a held-out error of 2% in water activity. The boiling-point elevation is therefore the fitted one, 23.5 K at 54% P₂O₅ and 0.12 bar (the ideal, Raoult's-law activity this replaced gave 9 K). Checked against a boiling point the fit never saw, it puts 85 wt% acid at 164 °C at atmospheric pressure against a measured 158 °C. Above 18 mol% acid the temperature dependence rests on one coefficient and that single check, so read the concentrate temperature to within a few kelvin. No heat of dilution is carried (about 1.5% of the latent duty at 54% P₂O₅). The 320 K feed stands in for a hotter plant feed.
Wet-process phosphoric acid: electrochemical removal of copper and cadmium
Wet-process phosphoric acid (about 42 wt% H₃PO₄, 10 kg/s) carries the metals its phosphate rock brought in: iron, zinc, nickel, copper and the cadmium that limits how the acid may be used in fertilizer. Two flow-through electrochemical cells remove the two noble enough to plate from strong acid. The first holds its cathode at -0.05 V vs SHE: copper's Nernst potential sits far above that, so copper plates at its mass-transfer limit (plug-flow law, 99% removed) while zinc, nickel, iron and cadmium, whose potentials lie below the cathode, stay dissolved. The second cell runs at -0.55 V and is told that only cadmium plates: mass transfer alone would take 99.4% of it, but at that potential and 323 K the Nernst floor holds about 2.6e-5 mol/L in solution, so the solver reports the equilibrium-limited 91% instead. Each cell's current comes from the metal's partial current and the current efficiency you give it; the copper and cadmium currents regenerate phosphoric acid (one proton per dihydrogen-phosphate counter-ion), and the rest of the charge evolves hydrogen beside the anode's oxygen, which the cell gas reports, with a flammability warning.
Wet-process phosphoric acid: dihydrate reactor-filter, 1500 t/d P2O5
A dihydrate (DH) phosphoric acid line at the scale of one large Moroccan train: 4,830 t/d of a 33% P₂O₅ phosphate rock (78 wt% fluorapatite, 13% calcite, 9% silica) attacked at 75 C with 98 wt% sulfuric acid at 2.5% sulfate excess, the gypsum filtered off and washed in three countercurrent stages. The phosphoric_acid_reactor block makes 1,500 t/d of P₂O₅ as a 28.4 wt% P₂O₅ filter acid at 94.2% recovery; the three losses close it exactly: 1.0% unattacked rock, 3.0% co-crystallized in the gypsum lattice, 1.9% water-soluble P₂O₅ left in the washed cake by the countercurrent washing equation. It throws 4.79 t of dry gypsum and uses 2.80 t of sulfuric acid per t P₂O₅. Gypsum is the stable hydrate here, and that is computed, not assumed: the acid's water activity (0.838, from the H₃PO₄/water model fitted to measured data) sits above the gypsum/bassanite transition activity (0.565, from LLNL log K(T)). The attack releases 50.0 MW on formation enthalpies (-464 kJ per mol of apatite) plus the acids' heats of solution: the 98 wt% acid enters already partly diluted and takes 1.1 MW back, while the phosphoric acid made dissolves at its own strength and adds 3.5 MW (NBS 1982). The flash cooler removes 25.6 MW by evaporating 11.0 kg/s of water, which leaves through the off-gas with the CO₂ and SiF₄. At the 6 h digestion residence EPA's 1976 phosphoric acid report cites, the attack system holds about 4,060 t of slurry (48 wt% solids). Fluorine is split 7% to the gas, 43% to the cake as CaF₂ and 50% dissolved in the acid, the typical dihydrate distribution in the EFMA BAT booklet on phosphoric acid (most of the dissolved share leaves later, in the concentrator). The tilting-pan gypsum filter is sized by Ruth's constant-pressure law: 230 m2 at 6.5 t P₂O₅/m2/day, limited by cake formation (the cake forms in 18 s of its 18 s sector; the three-stage wash needs 20 s of its 81 s). The vacuum (14.5 inHg), the 3 min cycle and the 18 s formation time are within the ranges in Pillai et al., US 5,185,135; the filtrate viscosity, 3.95 mPa s, is the middle of the 3.3-4.6 mPa s Anouar et al. (Heliyon 2024) measured on wet-process acid, at a temperature they do not state; the wash liquor's is the pure-acid fit at the first wash filtrate's strength. The cake's specific resistance, 1.44e8 m/kg, is back-calculated to the middle of the 5.5-7.8 t P₂O₅/m2/day Anouar et al. measured on dihydrate slurries. The cloth resistance (1e9 1/m) and the wash/dry sector split are representative, not sourced; at 1e10 1/m no cake reaches 5.5. Scope, stated rather than implied: (1) one block for the attack tanks, flash cooler and filter, so there is no tank-by-tank residence time or crystal size, and the filter's cake resistance is an input, not a prediction from crystal size; (2) the attack efficiency, co-crystallized fraction, fluorine split and cake liquor are plant inputs, not predictions (the 4% insoluble loss, mostly co-crystallized, follows the EFMA booklet's 4-6% DH losses "most of them co-crystallised"); (3) the heats of solution are 298.15 K values, one strength per liquid stream; (4) the water activity reads the P₂O₅ strength only, not the sulfate, fluoride or impurities that also lower it; and (5) anhydrite, the truly stable phase above about 42 C, is not considered, as in the real process, where gypsum forms metastably.