Case Study: Treating Electroplating Wastewater with FeCl₂ — 95% Reduction in Cr⁶⁺

Date: · Author: Hóa chất Lộc Thiên

1. Background

The Bien Hoa Industrial Park (Dong Nai) is one of the major electroplating hubs in the Southeast region. Here, dozens of electroplating plants operate daily, discharging wastewater containing significant amounts of heavy metals—among which Cr6+ (hexavalent chromium) is the most serious pollutant. Cr⁶⁺ is an extremely toxic substance capable of causing cancer, genetic mutations, and liver and kidney damage even at very low concentrations. For this reason, QCVN 40:2011/BTNMT sets very strict limits: the concentration of Cr⁶⁺ in treated industrial wastewater must not exceed 0.1 mg/L (Column B) and 0.05 mg/L (Column A).

An electroplating plant in the Bien Hoa Industrial Park has a treatment capacity of 200 m³ of wastewater per day. The influent wastewater has a Cr⁶⁺ concentration ranging from 15 to 25 mg/L, far exceeding the permissible limit set by QCVN 40:2011/BTNMT (Column B: 0.1 mg/L) —which is 150–250 times the safety threshold. This plant specializes in decorative plating for automotive and motorcycle components; the chrome plating line accounts for 60% of its output, resulting in consistently high levels of Cr⁶⁺ in the wastewater. Given this situation, the plant faces the risk of having its operations suspended if it cannot find an effective and stable long-term treatment solution.

2. The Problem

The traditional method the plant had previously used involved using NaHSO₃ (sodium bisulfite) to reduce Cr⁶⁺ to Cr³⁺, followed by adding NaOH to raise the pH to 8–9 to precipitate Cr(OH)₃. However, this approach has several limitations:

With chemical and sludge treatment costs reaching 4,200,000 VND/day, the plant needs a solution that ensures compliant effluent while optimizing operating costs.

3. Solution

After conducting a site survey and a two-week pilot test, the technical team at Lộc Thiên Chemicals proposed switching to FeCl₂ (Iron(II) Chloride) as a replacement for NaHSO₃. Reaction mechanism:

3Fe²⁺ + Cr⁶⁺ → 3Fe³⁺ + Cr³⁺

FeCl₂ reduces Cr⁶⁺ to Cr³⁺ in an acidic environment. Subsequently, 50% NaOH is added to raise the pH to 8.5, simultaneously precipitating Cr(OH)₃ and Fe(OH)₃.

The new process is established as follows:

1. Reaction Tank 1: Add FeCl₂ (38% solution) to the wastewater; maintain pH at 3–4; agitate for 15 minutes
2. Reaction Tank 2: Add 50% NaOH to raise the pH to 8.5, forming a precipitate; agitate for 10 minutes
3. Settling Tank: Use anionic polymer to aid settling; retention time is 2 hours
4. Neutralization Tank: Adjust the pH to 6–8 before discharging the effluent

The key point is that Lộc Thiên Chemicals supplies FeCl₂ that meets industrial standards, with regular deliveries twice a week from the Đồng Nai warehouse, eliminating the need for the plant to stockpile large quantities of chemicals.

4. Results

After 3 months of stable operation with the new process, the results exceeded expectations:

• The outlet Cr⁶⁺ concentration decreased by 95%, stabilizing below 0.05 mg/L—significantly lower than the QCVN threshold (0.1 mg/L)
• Chemical costs decreased by 31%, equivalent to savings of ~39 million VND/month
• Sludge volume decreased by 25%, resulting in additional savings on sludge dewatering and transportation costs
• Reaction time was reduced by nearly half, increasing the system’s treatment capacity

5. Lessons Learned

1. FeCl₂ is the optimal choice for treating Cr⁶⁺ in electroplating wastewater — it is both technically effective and more cost-efficient than NaHSO₃. For the same amount of Cr⁶⁺ to be reduced, the amount of FeCl₂ required (in moles) is less than that of NaHSO₃, and the cost per kilogram is also significantly lower
2. pH control is a critical factor — FeCl₂ performs optimally at a pH of 3–4, requiring an automatic NaOH dosing system to precisely raise the pH after the reduction reaction. Investing in an automatic pH control system is a worthwhile expense, helping to prevent pH fluctuations that reduce precipitation efficiency.
3. A stable supply is key — High-quality chemicals and on-time delivery ensure continuous, uninterrupted plant operations. With six warehouses stretching from Bac Ninh to Can Tho, Loc Thien Chemicals guarantees delivery within 24 hours to all key regions
4. The combination of FeCl₂ and NaOH creates a complete solution — FeCl₂ reduces Cr⁶⁺ and NaOH precipitates Cr³⁺; they are an inseparable pair in electroplating wastewater treatment. Purchasing both chemicals from the same supplier helps optimize logistics and pricing

FAQ — Quick Questions, Concise Answers

Q: Does FeCl₂ corrode equipment? I’m concerned about the durability of the reaction tanks and pipes.

A: FeCl₂ is mildly acidic (the pH of a 38% solution is approximately 1–2) and contains Cl⁻ ions, so it has the potential to corrode common metals. However, this issue is fully manageable: (1) reaction tanks should be made of FRP composite or acid-resistant epoxy-coated concrete; (2) pipes and pumps should be made of PP/PVC-U plastic or 316L stainless steel; (3) the contact time in the reaction tank is only 15–20 minutes, which is not long enough to cause significant corrosion if the correct materials are selected. Loc Thien Chemicals will provide specific advice on compatible materials when you implement the system.

Q: Why should FeCl₂ be used instead of NaHSO₃ to reduce Cr⁶⁺?

A: There are three main reasons: (1) Lower cost — FeCl₂ is about 30% cheaper than NaHSO₃ per kilogram, and the required amount (in moles) is also lower; (2) Faster reaction rate — the reduction process using FeCl₂ is completed in 15 minutes compared to 30–45 minutes for NaHSO₃, helping to increase system capacity; (3) Less sludge and easier settling — Fe(OH)₃ has natural flocculating properties, which support better settling and reduce sludge volume by 25% compared to the NaHSO₃ method. The only drawback: pH must be controlled more strictly during the reduction stage (pH 3–4).

Q: What is the required dosage of FeCl₂ for reducing Cr⁶⁺?

A: Theoretically, the molar ratio of Fe²⁺ to Cr⁶⁺ is 3:1. In actual operation, a 10–20% excess is needed to ensure complete reaction. Empirical formula: For every 1 mg/L of Cr⁶⁺ in the wastewater, approximately 5–7 mg/L of FeCl₂ (38% solution) is required. Example: Wastewater with Cr⁶⁺ = 20 mg/L → FeCl₂ (38%) dosage of approximately 100–140 mg/L. The exact dosage must be determined through a jar test using a sample of your facility’s actual wastewater—Lộc Thiên Chemicals offers free jar testing.

Q: I’d like to purchase a small quantity of FeCl₂ for preliminary testing—is that possible?

A: Absolutely. Loc Thien Chemicals supplies FeCl₂ in quantities ranging from small (30-kg cans, 200-kg drums) to large (5–10-metric-ton tanker trucks). You can order 1–2 drums to run a pilot test for 1–2 weeks before signing a long-term supply contract. We also provide technical staff to visit your plant and provide operational guidance during the trial period.

Q: Does FeCl₂ affect other parameters in the effluent?

A: After the reaction, FeCl₂ forms Fe³⁺, which precipitates with Cr³⁺ at a pH of 8.5 to form Fe(OH)₃. The residual Fe concentration in the water after sedimentation is very low (<1 mg/L if sedimentation is effective). However, please note: (1) FeCl₂ does not treat COD or other metals—separate biological and coagulation tanks are required for these parameters; (2) Cl⁻ from FeCl₂ will remain in the effluent; if local regulations limit Cl⁻ levels, this factor must be taken into account.

🔰 Are you operating a plating wastewater treatment system and looking to optimize costs?
Contact Lộc Thiên Chemicals right away — we supply industrial-grade FeCl₂, 50% NaOH, and comprehensive wastewater treatment solutions.
📞 Hotline: 0979 891 929
🏭 Warehouses: Ho Chi Minh City, Ba Ria-Vung Tau, Dong Nai, Da Nang, Can Tho, Bac Ninh