Ferric Sulphate, CAS 10028-22-5: Grades and Bulk Supply

1. Introduction to Ferric Sulphate

Ferric sulphate, also known as iron(III) sulfate or iron sesquisulfate, is an inorganic compound with the chemical formula Fe₂(SO₄)₃. It is one of the most important salts of trivalent iron and plays a critical role in a variety of industrial sectors, including water and wastewater treatment, pigment manufacturing, electronics, and catalysis. Its high oxidation potential, strong coagulating properties, and compatibility with a broad range of aqueous systems make it a versatile chemical in both industrial and environmental engineering applications.

From a chemical engineering perspective, ferric sulphate represents a mature yet continuously optimized commodity chemical. Its production processes have evolved over decades to improve purity, yield, and environmental efficiency while minimizing waste streams associated with sulfuric acid and iron oxide utilization. In modern industry, ferric sulphate is typically produced in both solid (granular or crystalline) and liquid (aqueous solution) forms, depending on the intended end use.

This article provides an in-depth technical overview of ferric sulphate, focusing on its chemical characteristics, reaction mechanisms, industrial manufacturing technologies, and diverse applications, along with relevant engineering considerations such as process design, material handling, and environmental impact.

2. Chemical and Physical Properties

2.1 Molecular Structure and Formula

Ferric sulphate has the empirical formula Fe₂(SO₄)₃ and a molecular weight of 399.88 g/mol. It consists of two ferric ions (Fe³) and three sulfate ions (SO₄²), forming an ionic compound. The ferric ion is in the +3 oxidation state, exhibiting strong Lewis acidity, which accounts for many of the compound’s reactivity characteristics.

In the solid state, ferric sulphate often exists as hydrated salts, the most common being:

Ferric sulphate nonahydrate (Fe₂(SO₄)₃9H₂O)
Ferric sulphate hydrate with variable hydration states (xH₂O, where x typically ranges from 5 to 12)

The hydration level is dependent on temperature, humidity, and the method of crystallization. Hydration profoundly affects solubility, stability, and handling characteristics.

2.2 Physical Characteristics

Property	Description
Appearance	Yellow-brown to reddish-brown solid or amber liquid
Odor	Odorless
Density (solid, anhydrous)	~3.1 g/cm³
Solubility	Highly soluble in water; hydrolyzes to form acidic solutions
pH (1% solution)	Typically between 1.5 – 3.0
Melting point	Decomposes before melting
Hygroscopicity	Strongly hygroscopic; absorbs moisture readily
Stability	Stable under normal conditions but hydrolyzes slowly in aqueous solutions

2.3 Chemical Reactivity

Ferric sulphate exhibits the reactivity typical of a strong oxidizing salt of trivalent iron. Its most significant reactions include:

Hydrolysis:

Fe2(SO4)3+6H2O→2Fe(OH)3+3H2SO4

The hydrolysis releases sulfuric acid, contributing to solution acidity.

Reduction:
Under reducing conditions, Fe³ can be reduced to Fe²:

2Fe3++Fe→3Fe2+

Precipitation Reactions:
Ferric sulphate reacts with alkaline solutions to form ferric hydroxide:

Fe2(SO4)3+6NaOH→2Fe(OH)3+3Na2SO4

Complexation:
Ferric ions readily form complexes with organic ligands such as citrate, EDTA, or phosphate species, influencing solubility and bioavailability.
Thermal Decomposition:
Upon heating above ~480°C, ferric sulphate decomposes to ferric oxide and sulfur trioxide:

Fe2(SO4)3→Fe2O3+3SO3

These reactions underpin the compound’s role as a coagulant, oxidant, and intermediate in pigment and catalyst manufacturing.

3. Industrial Production Processes

3.1 Raw Materials

The principal feedstocks for ferric sulphate production are:

Iron metal (Fe) or iron scrap
Iron oxides (Fe₂O₃, Fe₃O₄)
Sulfuric acid (H₂SO₄) (usually concentrated, 93–98%)
Oxidizing agents (air, oxygen, or hydrogen peroxide)

Depending on the feedstock and desired purity, the process may be configured as a direct acid dissolution or oxidation-based route.

3.2 Direct Dissolution of Iron in Sulfuric Acid

This is the simplest and most widely used process for liquid ferric sulphate manufacture.

Reaction:

2Fe+6H2SO4+O2→2Fe2(SO4)3+6H2O

Process Steps:

Feed Preparation:
Cleaned and sized iron scrap or sponge iron is fed into a corrosion-resistant reactor (commonly glass-lined steel or acid-resistant polymeric reactors).
Acid Charging:
Concentrated sulfuric acid is added to the reactor, and the mixture is agitated to enhance mass transfer.
Oxidation Stage:
Air or pure oxygen is bubbled through the mixture to oxidize Fe² (formed initially as ferrous sulfate) into Fe³, yielding ferric sulphate.
Filtration and Purification:
The resulting solution is filtered to remove unreacted solids and impurities such as silica, carbon, or metallic residues.
Concentration or Crystallization:
Depending on product specification, the filtrate may be concentrated under vacuum to achieve target Fe₂(SO₄)₃ concentration or cooled for crystallization of hydrates.
Quality Control:
Analytical monitoring ensures proper Fe³/Fe² ratios, sulfate concentration, and low impurity levels.

This route is favored for its operational simplicity and ability to utilize low-cost scrap iron. However, it produces acidic effluent streams that require neutralization.

3.3 Oxidation of Ferrous Sulphate (Roasting or Liquid-Phase Oxidation)

Another industrial method starts from ferrous sulfate, a common byproduct of titanium dioxide production or pickling processes.

Reaction (liquid-phase oxidation):

2FeSO4+H2SO4+O2→Fe2(SO4)3+H2O

In this process:

Aqueous ferrous sulfate is acidified with sulfuric acid.
Air or oxygen is introduced to oxidize Fe² to Fe³.
The reaction is catalyzed by ferric ions and typically carried out at 80–100°C.

Advantages:

Utilizes industrial waste (ferrous sulfate heptahydrate) as raw material.
Environmentally favorable, as it converts waste into a valuable coagulant.

Disadvantages:

Requires careful control of oxidation conditions to avoid incomplete conversion.
Impurities in ferrous sulfate feed can affect product color and stability.

3.4 Roasting of Pyrite or Iron Sulfide

Although less common today, ferric sulphate can also be obtained by roasting pyrite (FeS₂) in air and absorbing the sulfur oxides in water to form sulfuric acid and ferric sulphate:

4FeS2+11O2→2Fe2O3+8SO2

SO2+O2→SO3

Fe2O3+3SO3→Fe2(SO4)3

This method is more energy-intensive and is mainly of historical interest or used in integrated sulfuric acid plants.

3.5 Process Design Considerations

From a chemical engineering perspective, several factors are critical in ferric sulphate plant design:

Reaction Kinetics: Controlled oxidation rates prevent ferrous sulfate accumulation.
Heat Management: Exothermic acid dissolution requires cooling to avoid boiling or decomposition.
Material Selection: Corrosion-resistant materials (e.g., rubber-lined carbon steel, FRP, or PTFE) are essential due to high acidity.
Crystallization Dynamics: Proper temperature profiles and residence times are needed to control hydration states.
Waste Minimization: Acid recovery and effluent treatment systems are integral to sustainable operation.

4. Product Forms and Specifications

Ferric sulphate is available in several commercial grades:

Liquid Ferric Sulphate (LFS):
Typically 40–60% Fe₂(SO₄)₃ by weight; dark brown solution used directly in water treatment.
Solid Ferric Sulphate (Hydrated Crystals):
80–90% Fe₂(SO₄)₃xH₂O; less common due to hygroscopicity but preferred in applications requiring dry chemicals.
High-Purity Grades:
Produced for electronic or analytical applications, requiring ultra-low impurity levels (<50 ppm heavy metals).

Typical quality parameters include:

Fe³ content: 12–13% (liquid), 20–23% (solid)
Fe² content: <1%
Insoluble matter: <0.1%
pH (1% solution): 1.5–3.0

5. Industrial Applications

5.1 Water and Wastewater Treatment

Ferric sulphate is a premier coagulant used globally for clarification, phosphorus removal, and color reduction in water treatment systems.

Mechanism:
Upon dosing into water, ferric sulphate hydrolyzes to form ferric hydroxide, which acts as a flocculant:

Fe2(SO4)3+6H2O→2Fe(OH)3+3H2SO4

The gelatinous Fe(OH)₃ flocs adsorb suspended particles, organic matter, and phosphate ions, enabling efficient sedimentation or filtration.

Advantages in treatment:

High phosphorus removal efficiency (≥95%)
Works effectively over a broad pH range (4–9)
Produces denser, faster-settling flocs compared to alum
Reduces hydrogen sulfide odor and color

Ferric sulphate is widely used in:

Municipal drinking water plants
Industrial wastewater treatment
Sludge conditioning
Odor control in sewage systems

5.2 Mining and Metallurgical Processes

In hydrometallurgy, ferric sulphate functions as an oxidizing agent for leaching operations, particularly in the extraction of:

Copper from chalcopyrite (CuFeS₂)
Uranium from ores
Zinc and nickel from sulfides

The ferric ion oxidizes metal sulfides, while being reduced to ferrous ion, which is subsequently re-oxidized by aeration or bacterial action (bioleaching):

Fe2(SO4)3+CuFeS2→CuSO4+2FeSO4+S0

5.3 Pigments and Inorganic Chemicals

Ferric sulphate serves as a precursor for the synthesis of iron oxide pigments (Fe₂O₃) via thermal decomposition. The controlled heating of ferric sulphate produces finely divided oxides with desirable pigmentary properties.

It is also used in the production of:

Ferric chloride (by double decomposition with sodium chloride)
Ferric hydroxide gels for adsorption applications
Ferric alum and other specialty coagulants

5.4 Catalyst and Reagent Applications

Ferric sulphate functions as a Lewis acid catalyst in several organic transformations, such as:

Nitration and sulfonation reactions
Oxidation of organic sulfides to sulfoxides
Dehydration and polymerization processes

It also acts as a Fenton-type reagent in advanced oxidation processes (AOPs), where Fe³/Fe² cycles generate hydroxyl radicals for the degradation of refractory organic pollutants.

5.5 Electronics and Photographic Industries

High-purity ferric sulphate is used as an etchant for copper and other metals in printed circuit board (PCB) fabrication. Its strong oxidizing power allows controlled removal of metallic films without excessive undercutting.

In older photographic processes, ferric sulphate was used in cyanotype and photochemical bleaching formulations, though this use is now largely obsolete.

5.6 Soil Conditioning and Fertilizers

In agriculture, ferric sulphate is used as a soil amendment for alkaline soils. By lowering pH and supplying iron, it corrects chlorosis in plants. However, its use has declined due to the availability of chelated iron fertilizers (e.g., Fe-EDTA).

6. Environmental, Safety, and Handling Considerations

6.1 Safety and Toxicology

Ferric sulphate is not acutely toxic but is strongly acidic and corrosive. Direct contact may cause burns, while inhalation of dust or mist can irritate the respiratory tract. Appropriate personal protective equipment (PPE) is required.

Safety Data Summary:

Hazard classification: Corrosive to metals; causes skin and eye irritation
LD₅₀ (oral, rat): >2000 mg/kg
NFPA ratings: Health 2, Flammability 0, Reactivity 1

6.2 Storage and Transport

Store in acid-resistant tanks or containers.
Avoid contact with metals and bases.
Maintain ambient temperature to prevent crystallization (for liquid grades).
Classified as non-flammable and generally non-hazardous under transport regulations (UN3264, Class 8 for liquid acidic mixtures).

6.3 Environmental Impact

Ferric sulphate is environmentally beneficial in most applications, especially in wastewater treatment. However, improper disposal can lead to localized acidification. Iron precipitates are generally inert and safe for landfill disposal after neutralization.

7. Process Optimization and Emerging Technologies

Modern production of ferric sulphate is being refined through several innovations:

Continuous Oxidation Reactors:
Replacing batch systems with continuous stirred-tank reactors (CSTRs) improves conversion efficiency and product uniformity.
Air-to-Oxygen Enrichment:
Using pure oxygen instead of air reduces residence times and enhances Fe² oxidation.
Waste Heat Recovery:
Exothermic reaction heat is recovered to preheat feeds or generate steam.
Zero-Liquid-Discharge (ZLD) Systems:
Integrating acid recovery and crystallization minimizes effluent discharge.
Byproduct Valorization:
Utilizing ferrous sulfate waste from titanium dioxide plants improves circular economy practices.

8. Economic and Market Aspects

Ferric sulphate is a commodity chemical with stable global demand. The major drivers include:

Expansion of wastewater treatment infrastructure.
Stricter environmental regulations on phosphorus discharge.
Replacement of aluminum-based coagulants due to health and sludge-handling advantages.

Production costs are influenced by:

Iron raw material prices
Sulfuric acid availability
Energy and transportation costs
Regional waste recovery initiatives

Global consumption is dominated by the water treatment sector, accounting for more than 70% of total ferric sulphate use.

9. Conclusion

Ferric sulphate (Fe₂(SO₄)₃, CAS: 10028-22-5) is a cornerstone inorganic chemical in modern industry, offering a blend of strong oxidizing power, effective coagulation performance, and economic viability. Its chemistry is rooted in the behavior of trivalent iron ions, which undergo hydrolysis, oxidation, and complexation to yield diverse functional outcomes.

From a chemical engineering standpoint, ferric sulphate production exemplifies the balance between reaction kinetics, corrosion control, purification, and sustainability. The shift toward circular manufacturing — utilizing ferrous sulfate byproducts and closed-loop acid systems — reflects the industry’s move toward greener chemical processes. Its broad applications — spanning water purification, metallurgy, pigments, catalysis, and electronics — underscore ferric sulphate’s indispensable role in both environmental management and industrial manufacturing. As technological and regulatory landscapes evolve, ferric sulphate will remain a critical tool in the global effort to improve water quality, resource recovery, and environmental sustainability.

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Ferric Sulphate: Identity, Forms and Specifications

Ferric sulphate (Fe₂(SO₄)₃, CAS 10028-22-5) is a soluble iron(III) salt and one of the most widely used inorganic coagulants in water and wastewater treatment. It is supplied in several forms – anhydrous, hydrated, liquid solution and granular – and is valued for its ability to coagulate suspended solids, remove colour and precipitate phosphate.

Property	Value
Chemical name	Iron(III) sulfate / ferric sulphate
CAS Registry Number	10028-22-5
Formula	Fe₂(SO₄)₃
Molecular weight	399.88 g/mol (anhydrous)
Appearance	Grey-white to pale-yellow powder (anhydrous); reddish-brown when hydrated
Solubility	Soluble in water; solutions are acidic
Forms supplied	Anhydrous powder, hydrated, granular, and aqueous solution

The hydrated and solution forms are what most water plants buy, because they dissolve and dose easily. The anhydrous and granular grades are used where a dry, storable product is wanted.

Why Ferric Salts Are Used as Coagulants

Ferric sulphate works by the same mechanism as other iron and aluminium coagulants: when dosed into water it hydrolyses to form positively charged iron hydroxide species, which neutralise the negative charge on suspended particles and sweep them into settleable flocs. Compared with ferrous (iron(II)) salts, the ferric form has two practical advantages:

Feature	Ferric sulphate	Ferrous sulphate (for comparison)
Oxidation state	Fe³⁺ (already oxidised)	Fe²⁺ (needs oxidation)
Coagulation	Effective without pre-oxidation	Usually needs oxidation (e.g. chlorination) first
Phosphate removal	Strong phosphate precipitation	Also used, but requires oxidation
Working pH range	Typically acidic to near-neutral	Narrower

Because it is already in the ferric state, ferric sulphate coagulates effectively without a separate oxidation step – which is why it is favoured in drinking-water and industrial-water treatment for turbidity, colour and phosphate removal.

How Ferric Sulphate Is Produced

Route	Feedstock	Note
Direct reaction	Iron / iron oxide + sulfuric acid	Gives ferric sulphate directly
Oxidation of ferrous sulphate	Ferrous sulphate + oxidising agent	Converts Fe(II) to Fe(III)

The product is standardised to the required iron content and form (solution, granular or powder), since the application dictates whether a liquid coagulant or a dry grade is wanted.

Applications of Ferric Sulphate

Sector	Use
Municipal water treatment	Coagulant for turbidity and colour removal
Wastewater treatment	Coagulant; phosphorus removal; sludge conditioning
Industrial effluent	Treatment of textile, paper, and process wastewater
Phosphate removal	Precipitation of phosphate in municipal and industrial streams
Other industrial	Etching, pigment and specialty uses

In municipal and industrial treatment it is often used in a dual-coagulant programme with an organic coagulant such as PolyDADMAC, which can reduce overall dose and improve floc quality.

Handling, Safety and Sourcing

Consideration	What to confirm
Form and iron content	Solution concentration or solid Fe content, as stated on the COA
Insolubles and impurities	Within limits for the water-treatment grade
Handling	Acidic; use appropriate PPE and acid-resistant equipment
Documentation	COA and SDS; drinking-water-grade documents where required

Because it is used in drinking-water treatment, ferric sulphate is normally supplied to a defined grade with documentation that municipal and industrial buyers can audit.

Frequently Asked Questions about Ferric Sulphate

What is ferric sulphate used for?

Mainly as a coagulant in water and wastewater treatment – removing turbidity and colour and precipitating phosphate – and in industrial effluent treatment.

What is the difference between ferric and ferrous sulphate?

Ferric sulphate contains iron in the Fe³⁺ state; ferrous sulphate contains Fe²⁺. Ferric sulphate coagulates effectively without a separate oxidation step, which is why it is often preferred in water treatment.

Is ferric sulphate the same as ferric sulfate?

Yes. “Sulphate” and “sulfate” are the same compound (Fe₂(SO₄)₃); the spelling differs by region. The CAS number is 10028-22-5 either way.

What form is ferric sulphate supplied in?

As anhydrous powder, hydrated or granular solid, or as an aqueous solution – the solution and granular forms are the most common for water treatment.

How is ferric sulphate dosed?

As a liquid or dissolved solid dosed into the coagulation stage; the optimum dose is set by jar testing, because it depends on the water’s turbidity, colour and phosphate load.

How do I buy ferric sulphate?

PolyBlueChem supplies ferric sulphate (Fe₂(SO₄)₃, CAS 10028-22-5) in solution and solid grades. Contact our team for specifications, COA and a quotation.

Ferric Sulphate (Fe₂(SO₄)₃, CAS: 10028-22-5): Chemistry, Production Processes, and Industrial Applications