FeCl2 in Electroplating & Cr
Quick summary: FeCl2 reduces Cr⁶⁺ at a ratio of 3 moles of Fe²⁺ to 1 mole of Cr⁶⁺, at pH 2.0–2.5. The pH is then raised to 8–9 to precipitate Cr(OH)₃. Meets QCVN 40:2011 (Cr⁶⁺ < 0.05 mg/L, Column A) consistently when operated according to the proper procedure.
Wastewater from the electroplating and metal surface treatment industries is among the most hazardous types of industrial wastewater, containing Cr⁶⁺ (hexavalent chromium), cyanide, nickel, and other heavy metals. Among these, Cr⁶⁺ is the most hazardous contaminant, classified by the World Health Organization (WHO) and the U.S. Environmental Protection Agency (EPA) as a Group 1 carcinogen. FeCl2 is currently the most effective and cost-efficient chemical for removing Cr⁶⁺ in Electroplating wastewater treatment in Vietnam.
What Is Cr⁶⁺ and Why Is It Dangerous?
Chromium exists in wastewater in two main oxidation states:
- Cr³⁺ (trivalent chromium): Low toxicity; necessary for glucose metabolism in small doses; easily precipitates in pH 7–9 as green Cr(OH)₃.
- Cr⁶⁺ (hexavalent chromium): Extremely toxic; causes lung cancer, skin ulcers, liver and kidney damage, and genetic mutations. It is completely soluble in water and very difficult to remove using physical methods alone.
Sources of Cr⁶⁺ in the electroplating industry: hard chrome plating baths containing 100–300 g/L of CrO₃; decorative chrome plating baths containing 200–400 g/L of CrO₃; Passivation solutions following zinc and nickel plating; rinse water from post-chromium plating processes. The concentration of Cr⁶⁺ in rinse water typically reaches 50–500 mg/L prior to treatment.
QCVN 40:2011/BTNMT — Limits for Cr⁶⁺ in Industrial Wastewater
According to the National Technical Regulation on Industrial Wastewater QCVN 40:2011/BTNMT:
- Column A (discharge into water sources used for domestic water supply): Cr⁶⁺ ≤ 0.05 mg/L
- Column B (discharge into water sources not used for domestic water supply): Cr⁶⁺ ≤ 0.1 mg/L
- Total Cr: Column A ≤ 0.2 mg/L; Column B ≤ 1.0 mg/L
Untreated electroplating wastewater typically contains Cr⁶⁺ levels ranging from 50–500 mg/L, which exceed the discharge limits.
hundreds to thousands of times. Therefore, a Cr⁶⁺ treatment system is legally required for all Electroplating facilities under the 2020 Environmental Protection Law and Decree No. 08/2022/NĐ-CP. Violations of Cr⁶⁺ discharge standards may result in fines of up to hundreds of millions of dong and mandatory suspension of operations.Cr⁶⁺ Reduction Mechanism Using FeCl2
FeCl2 is a strong reducing agent. Fe²⁺ ions are capable of reducing Cr⁶⁺ (in the form of CrO₄²⁻ or Cr₂O₇²⁻) to Cr³⁺ through a redox reaction in an acidic environment.
General reaction equation:
- 3FeCl2 + K₂Cr₂O₇ + 7H2SO4 → Cr₂(SO₄)₃ + 3FeSO4 + 2KCl + 4FeCl3 + 7H₂O
Ionic (chromate) form:
- Cr₂O₇²⁻ + 6Fe²⁺ + 14H⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H₂O
- CrO₄²⁻ + 3Fe²⁺ + 8H⁺ → Cr³⁺ + 3Fe³⁺ + 4H₂O
According to the equations, the molar ratio of Fe²⁺ to Cr⁶⁺ is 3:1. In terms of mass: to reduce 1 mg of Cr⁶⁺, approximately 3.2 mg of Fe²⁺ is required, equivalent to about 8.5 mg of anhydrous FeCl2 or about 28 mg of FeCl2 solution 30%. In actual operation, the dose of FeCl2 is typically increased by 20–30% compared to the theoretical value to ensure complete reduction of Cr⁶⁺.
To learn more about other applications of FeCl2 in industrial Wastewater Treatment, see the Fenton reaction with FeCl2 or compare FeCl2 with FeSO4 and PAC (Polyaluminium Chloride).
Cr⁶⁺ Removal Process Using FeCl2 — 2-Stage Process
Stage 1: Reduction of Cr⁶⁺ to Cr³⁺ (Reduction Tank, pH < 3)
The reduction of Cr⁶⁺ to Cr³⁺ using FeCl2 occurs most effectively at pH < 3, with optimal conditions at pH 2.0–2.5. In this stage:
- Feed electroplating wastewater containing Cr⁶⁺ into a mechanically agitated reaction tank (reduction tank)
- Pump dilute H₂SO₄ with 10–20% to lower the pH to 2.0–2.5 (do not use HCl because the additional Cl⁻ causes equipment corrosion)
- Pump FeCl2 solution at a concentration of 30–40% at the calculated dosage (typically 3.5–4 mol Fe/mol Cr)
- Stir continuously for 15–30 minutes until the diphenylcarbazide test for Cr⁶⁺ yields a negative result
Stage 2: Precipitation of Cr³⁺ as Cr(OH)₃ (Neutralization Tank, pH 8–9)
After Cr⁶⁺ has been completely reduced to Cr³⁺, transfer the solution to the neutralization tank and raise the pH to 8–9 using NaOH or Ca(OH)₂ quicklime:
- Cr³⁺ + 3OH⁻ → Cr(OH)₃↓ (gray-blue precipitate)
- Fe³⁺ + 3OH⁻ → Fe(OH)₃↓ (reddish-brown precipitate)
Optimal pH for complete precipitation: 8.0–9.0. Below pH 7: Cr³⁺ is not completely precipitated. Above pH 10: Cr(OH)₃ may partially dissolve as CrO₂⁻ (chromite). After precipitation, add Anionic Polymer to increase floc size, then allow to settle for 30–60 minutes or pass through a filter press to separate the Cr(OH)₃ + Fe(OH)₃ sludge.
FeCl2 In Electroplating Technology
Used as a Surface Activator (Pickling)
In electroplating lines, FeCl2 is used in pickling solutions to remove the iron oxide layer from the steel surface prior to plating. A dilute FeCl2 solution (5–10%) at pH 1–2 effectively dissolves FeO, Fe₂O₃, and Fe₃O₄ effectively while causing less attack on the base steel compared to the concentrated HCl, thereby reducing hydrogen embrittlement—a serious issue in hard chrome plating for load-bearing components.
Contaminant Control in Nickel Plating Baths
>In a nickel plating bath (Watts bath), a Fe²⁺ concentration of less than 10 ppm is a critical quality requirement. FeCl2 is sometimes added to precisely control the Fe ion concentration in the bath in order to adjust the internal stress of the nickel plating layer. However, this application requires strict control and is not common in Vietnam.
Case Study: Electroplating Plant in Binh Duong
An electroplating plant in the Sóng Thần Industrial Park, Binh Duong (with a wastewater flow rate of 200 m³/day) implemented a Cr⁶⁺ treatment system using FeCl2, achieving the following results:
- Input parameters: Cr⁶⁺ = 120–180 mg/L; pH = 2.8–3.5; total Fe = 5 mg/L
- Actual FeCl2 dosage: 450 mg/L (30% solution), reaction time of 20 minutes at pH 2.2
- Post-treatment output parameters: Cr⁶⁺ < 0.05 mg/L; Total Cr < 0.1 mg/L; Total Fe < 0.5 mg/L
- Chemical costs: Approximately 8,500–11,000 VND/m³ of wastewater (including H₂SO₄, FeCl2, NaOH, and polymer)
- Monitoring results: Compliant with QCVN 40:2011 Column B continuously for 18 months of operation
Cr(OH)₃ + Fe(OH)₃ sludge is separated using a filter press; moisture content < 70%; disposal is contracted to a hazardous waste treatment facility licensed under Circular 02/2022/TT-BTNMT.
Notes on Operating the Cr⁶⁺ Removal System
- Check the FeCl2 input: If FeCl2 is partially oxidized to FeCl3, it will reduce the system’s ability to reduce Cr⁶⁺. Verify using an Fe²⁺ test strip before use.
- Do not allow electroplating wastewater to be exposed to air for extended periods: At low pH, Cr⁶⁺ oxidizes Fe²⁺ to Fe³⁺ right in the transport pipes.
- Control temperature: Temperatures > 40°C accelerate the reaction but also accelerate the oxidation of FeCl2; this should be taken into consideration.
- Install an online Cr⁶⁺ monitoring system: An optical sensor measuring Cr⁶⁺ using the diphenylcarbazide method enables automatic control of the FeCl2 dosage.
- Use H₂SO₄ to adjust the pH: Use H₂SO₄ industrial-grade solution diluted 10–20% to lower the pH to 2.0–2.5; do not use HCl.
Frequently Asked Questions About Cr⁶⁺ Removal Using FeCl2
Why use H₂SO₄ instead of HCl to lower the pH?
HCl adds Cl⁻ ions to the wastewater, increasing corrosion of stainless steel equipment and complicating the analysis of Cr⁶⁺ using colorimetric methods. H₂SO₄ does not have this issue and is cheaper than HCl.
If FeCl2 is oxidized to FeCl3, can it be used to reduce Cr⁶⁺?
It is not effective. Fe³⁺ has no reducing properties and cannot reduce Cr⁶⁺. You must test FeCl2 with an Fe²⁺ test strip before use. A sign that FeCl2 has oxidized is when the solution changes from light blue to brownish-yellow.
Is Cr(OH)₃ sludge from filtration considered hazardous waste?
Yes. Sludge containing Cr(OH)₃ and Fe(OH)₃ from the Electroplating wastewater treatment process is classified as hazardous waste under Circular 02/2022/TT-BTNMT. You must contract with a licensed hazardous waste treatment facility.
Purchase FeCl2 Specialized Electroplating Chemicals at Loc Thien
Loc Thien Chemical supplies FeCl2 industrial-grade solution with a concentration of 30–40%, which is widely used in electroplating and electro-deposition plants, and
Metal surface treatment throughout Ho Chi Minh City and neighboring provinces such as Binh Duong, Dong Nai, and Long An. The product meets quality standards with stable Fe²⁺ content / purity and low impurity levels, making it suitable for both Cr⁶⁺ removal and Fenton process applications.- FeCl2 solution 30–40%, available in 30L drums, 1000L IBCs, and tank trucks
- COA (Certificate of Analysis) included with each shipment
- Free technical support for dosage calculations and Cr⁶⁺ removal tank design
- Same-day delivery in Ho Chi Minh City and industrial parks in Binh Duong and Dong Nai
- Competitive pricing, discounts available for long-term contracts
Contact us now for a quote:
Loc Thien Chemical
452/6B Provincial Road 10, Binh Tri Dong Ward, Ho
Chi Minh City
Hotline: 0979 891 929
Website: hoachatlocthien.com
Supply / Quote: FeCl2 (Ferrous Chloride) at Loc Thien — hotline 0979 891 929 (on request, no retail sales).