Overview
A coagulant is aluminium plus acid. Bauxite residue holds the aluminium — and the iron. Phosphogypsum holds the acid. Both are hazardous inventories that their owners pay to store, and together they make a finished water-treatment reagent in a single contact step.
The route has been obvious for decades and has never shipped, for one reason: bauxite residue is alkaline at pH 11–13, its free alkali consumes acid before any metal dissolves, and at market prices for sulfuric acid the neutralisation alone exceeds the value of the metals recovered. ARBOK does not buy the acid. It is produced from phosphogypsum by deep-vacuum phase separation at 2–4 kWh per tonne of feed, which puts the marginal cost of the acid at the price of electricity.
The product is not aluminium sulfate. Acid takes iron along with aluminium, and since ferric sulfate and ferric chloride are coagulants in their own right, the output is a mixed aluminium-ferric sulfate coagulant in which iron is the principal active. The metal ratio is set by the residue, not chosen.
Applications
Primary market: municipal and industrial wastewater — phosphorus precipitation, colour and turbidity removal, hydrogen sulfide control, sludge conditioning. Ferric coagulants are preferred there for a wider working pH band than alum and for doing phosphorus and sulfide in one dosing step.
Secondary: sale of recovered sulfuric acid to third-party coagulant manufacturers, who buy merchant acid today.
Not in scope: drinking-water grade from this feedstock. Acid attack mobilises arsenic, cadmium and chromium along with the actives; potable-grade product requires a clean aluminium source and is a separate line.
Operating Principle
Acid side. Phosphogypsum enters the vacuum section at ambient temperature — no heat supplied, no separate heating circuit, no process temperature setpoint. It separates into fractions: 70 % clean water, 18.5 % sulfuric acid, 7.8 % hydrogen fluoride, 2.5 % phosphoric acid, ~1 % other acids, 0.2 % inert residue. Stoichiometric ceiling 0.57 t H₂SO₄ per tonne of phosphogypsum. No membranes, filters, sorbents or reagents.
Coagulant side. The recovered acid contacts bauxite residue and does two jobs at once — neutralises the free alkali the pond owner is already paying to handle, and takes both metals into solution:
- Al₂O₃ + 3 H₂SO₄ → Al₂(SO₄)₃ + 3 H₂O
- Fe₂O₃ + 3 H₂SO₄ → Fe₂(SO₄)₃ + 3 H₂O
- free alkali first: 2 NaOH + H₂SO₄ → Na₂SO₄ + 2 H₂O
Both metal reactions are exothermic. The product leaves as a sulfate solution, which is the form in which coagulant is delivered to utilities and dosed by pump — so no drying and no crystallisation stage exists in the chain. That removes the most energy-intensive step of any salt plant.
Key Parameters
| Parameter | Value |
|—|—|
| Phosphogypsum operating pressure | deep vacuum within a defined operating window |
| Process temperature | ambient — equal to the feed and the surroundings, no heat supplied, no setpoint |
| Specific energy, phosphogypsum | 2–4 kWh/t of feed |
| Acid yield from phosphogypsum | 18.5 % of feed; stoichiometric ceiling 0.57 t/t |
| Bauxite residue pH | 11–13 |
| Bauxite residue actives | Al₂O₃ 15–20 %, iron up to 20 % |
| Acid demand | 1,438 t H₂SO₄ per 1,500 t/day of residue at 15 % Al₂O₃ and 20 % Fe, before free-alkali neutralisation |
| Feed ratio per tonne of product | 0.82 t bauxite residue + 4.25 t phosphogypsum |
| Product actives | 12.3 % Al₂O₃ + 23.5 % Fe₂O₃ |
| Fe₂O₃ : Al₂O₃ in product | 1.4–1.9 : 1 depending on residue grade |
| Reference plant output | 1,829 t/day dry coagulant, 603,570 t/year at 330 operating days |
| Module count, reference plant | 47 modules — 39 on phosphogypsum, 8 on residue |
| Electricity, reference plant | 9.2 GWh/year — 0.4 % of coagulant revenue at $0.08/kWh |
| Product form | sulfate solution, no drying or crystallisation stage |
| Consumables | none — no membranes, filters, sorbents or reagents |
| Form factor | containerised modules, outdoors on concrete |
| Construction | Hastelloy, PTFE, acid-resistant ceramic on the acid section |
| Service life | 15–20 years |
Architecture and Components
Two coupled sections. The phosphogypsum section is the standard vacuum phase-separation train with acid-resistant internals, discharging separated acid fractions and condensate. The contact section takes recovered sulfuric acid against slurried bauxite residue, discharging the coagulant solution and an inert dewatered solid. Both are containerised and scaled by module count.
Advantages
The acid bill disappears. One tonne of sulfuric acid needs 5.41 t of phosphogypsum, i.e. 16.2 kWh. At $0.10/kWh that is $1.62 per tonne of acid against a market price of $100–150 — 62 to 93 times cheaper. This is the single fact that makes the whole route work where every previous attempt failed.
Neutralisation becomes production. Lime neutralisation of bauxite residue recovers nothing, reduces no mass and emits ~0.08 t CO₂ per tonne treated. Acid neutralisation yields a saleable reagent from the same operation.
Iron is an asset, not a contaminant. Reframing the product as a mixed Al-Fe coagulant removes the separation problem that defeated the classical route entirely.
Unmatchable price floor. A competitor's feedstock has a market — chlorine, steel scrap, bauxite, merchant acid. This feedstock carries a disposal fee. The product can be priced below any competitor's production cost.
No consumables and no drying stage. Electricity is the only input, and the product ships in the form it leaves the reactor in.
Supply-chain independence. Removes four purchased inputs at once: chlorine, scrap, ore and merchant acid. Relevant against 45–50 % of world PAC output sitting in one country and against the 2022 European hydrochloric acid failure.
Integrations
ARBOK-SA — the acid source. ARBOK-Aluminium · ARBOK RED MUD · ARBOK-JUMBO — the residue side. ARBOK-Phosphate — adjacent phosphogypsum configuration. ARBOK-VC (Vacuum Cracking) — platform. Arbok-Acidion — alternative acid and caustic route from desalination brine, membrane-based, kept separate. ARBOK-Purification and ARBOK-CRYSTALLIZER — the reagent-free line this product deliberately does not compete with.
Deployment & Operation
Sited where both feedstocks fall within a reasonable haul. Phosphogypsum is the mass-dominant input at roughly 5 to 1, so it dictates the location. Containerised modules on concrete in the open air, 2–3 operators per shift, continuous operation. Commercial model: BOOM on an off-take agreement — the pile owner invests nothing, pays for hazardous-waste handling and buys back reagent, or a straight per-cubic-metre tariff. Payback 5–7 years.
TRL
The two halves carry different readiness. Acid recovery from phosphogypsum has running pilots in Florida, Idaho, Utah, Murmansk, the Urals, Rajasthan, Jordan, Tunisia and Morocco. Bauxite residue processing is TRL 9 at full line scale with a Latin American reference and the documented RUSAL engagement of 2016. The combined coagulant configuration has no field reference and no assigned TRL — to be confirmed by Michael.
Market Potential
World inorganic coagulant demand: aluminium sulfate ~4.6 Mt/year, solid polyaluminium chloride 2.2–2.5 Mt, ferric chloride ~15 Mt implied by a $7.65 bn market at $515/t — roughly 22 Mt/year, about $6.6 bn. Ferric chloride alone is forecast at $9.88 bn by 2030 at 5.24 % CAGR, with municipal wastewater at 42.1 % of revenue.
Covering world demand entirely = 36 plants of the reference size, drawing 18 Mt/year of bauxite residue (12 % of annual arisings) and 93.5 Mt/year of phosphogypsum (37.4 % of arisings). Standing inventories at that draw last 222 years for residue and 64 years for phosphogypsum.
Feedstock base: >4 bn t of bauxite residue behind >500 ponds, +150 Mt/year, 2–5 % used; >6 bn t of phosphogypsum, +200–300 Mt/year at 4–6 t per tonne of phosphoric acid.
Geography with both materials: China (~500 Mt/year new residue, 75–80 Mt/year phosphogypsum), India (~50 Mt/year residue), Brazil (~30 Mt/year), United States. Tightest overlap: Florida's 1 bn t of phosphogypsum in 24 stacks plus 30 Mt/year against Gramercy, Louisiana — ~34 Mt of residue at the country's last operating alumina refinery, both on the Gulf, barge-connected on the Mississippi. At a 100 mg/L dose one plant covers 12.8 % of the dosing of the entire US municipal sewer flow. Australia holds ~600 Mt of residue with no phosphate base; Morocco and Tunisia are the reverse.
Typical Project Economics
Reference plant, 1,500 t/day residue at 15 % Al₂O₃, 330 days:
- Output 603,570 t/year of dry coagulant. At $300/t — against US benchmarks of $515/t for ferric chloride (December 2025) and $346/t for aluminium sulfate (Q1 2026) — ~$181 M/year.
- Feedstock draw 495,000 t/year of residue and 2.57 Mt/year of phosphogypsum, both paid for by the owner.
- Electricity 9.2 GWh/year = $0.74 M at $0.08/kWh, 0.4 % of revenue. No other significant operating cost.
- Payback 5–7 years, then clean profit over the remaining service life.
World scale, 36 plants at 22 Mt/year: ~$6.6 bn/year of coagulant, 65.4 Mt/year of technical water returned, 1.44 Mt/year of CO₂ avoided on lime substitution, 335 GWh/year of electricity — one thousandth of a percent of world generation.
Incumbent benchmark from the client side: at one alumina refinery thermal evaporation of pond liquor is costed by the plant itself at $18.299/m³, ~311 kWh/m³ thermal-equivalent, against 0.72 kWh/m³ net — 432× on energy and 102× on cost.
Risk Factors
Free-alkali acid demand sits on top of the stoichiometric requirement at pH 11–13. The acid is nearly free, which is why this is an engineering line rather than an economic barrier.
Not all residue alumina is acid-available. The Bayer process already took the readily soluble fraction; the balance sits partly in desilication product. Acid attack is less selective than caustic re-digestion and will mobilise more of it, and the recoverable fraction is a property of the specific residue.
Product classification. Iron-dominant output is a different commercial class from alum, sold into wastewater rather than potable treatment. EN 878 does not apply. Arsenic, cadmium, chromium and natural uranium and thorium in the residue will partly report to the product solution — specification and permitting follow the wastewater route, not the drinking-water one.
Phosphogypsum is the mass-dominant input at roughly 5 to 1, which constrains siting more tightly than residue availability suggests.
Co-product balance. Recovering acid from phosphogypsum also produces hydrogen fluoride at 7.8 % of feed. At the scale needed to supply world coagulant demand that output would be several times the world HF market, so HF offtake — not feedstock or coagulant demand — caps how fast the route can scale. HF offtake therefore governs the pace of scale-up.
Related Technologies
ARBOK-SA · ARBOK-Aluminium · ARBOK RED MUD · ARBOK-Phosphate · ARBOK-VC (Vacuum Cracking) · Arbok-Acidion · ARBOK-SODA · ARBOK-Purification · ARBOK-CRYSTALLIZER · ARBOK-JUMBO