Industrial chemicals manufacturer & supplier — nationwide delivery
Hotline: 0979 891 929
Request a quote

Process for Reducing Chromium VI (Cr6+) in Electroplating Wastewater Using FeCl2 Solution | Lộc Thiên

Tóm tắt nội dung
  1. 1. Toxicity of Chromium VI and Stringent Discharge Requirements for the Electroplating Industry
  2. 2. Chemical Equation & Reaction Mechanism of Cr6+ Reduction with FeCl2
  3. 3. Technical Specifications of Chemicals Used in the Cr6+ Reduction Process
  4. 4. SOP 4-Step Operating Procedure for Cr6+ Reduction in Electroplating
  5. 5. FAQ - Frequently Asked Questions about Cr6+ Reduction with FeCl2

Reducing Chromium VI (Cr6+) in electroplating wastewater using FeCl2 is a standard industrial reduction-oxidation physicochemical method that converts highly toxic hexavalent chromium ions (Cr6+, Dichromate group Cr2O7²⁻, CAS 1333-82-0) into the less toxic trivalent chromium form (Cr3+) prior to precipitation in an alkaline environment. Using FeCl2 15–30% (Iron(II) Chloride, formula FeCl2, MW 126.75 g/mol, CAS 7758-94-3) solution provides instantaneous reaction rates at pH 2.0–3.0, low consumption ratios (actual 8–10 kg pure FeCl2 per 1 kg Cr6+), and savings of up to 35–45% in chemical costs compared to methods using iron alum FeSO4 or Sodium Bisulfite.

  • Complete reaction at acidic pH (pH 2.0 – 3.0): Fe²⁺ ions donate electrons reducing Cr⁶⁺ to Cr³⁺ in under 5 minutes.
  • Thorough dual precipitation: Raising pH to 8.5–9.5 with NaOH 50% simultaneously forms Cr(OH)3 and Fe(OH)3 precipitates that settle extremely quickly.
  • B2B-grade FeCl2 solution from Lộc Thiên: Stable concentration, no sediment, supplied via 24-ton tanker trucks or 200L HDPE drums.

1. Toxicity of Chromium VI and Stringent Discharge Requirements for the Electroplating Industry

Electroplating wastewater (decorative plating, hard chrome plating, surface passivation) contains very high levels of hexavalent chromium (Cr6+), typically ranging from 50 to 500 mg/L. Cr6+ is a potent carcinogen, causes skin cell corrosion, and induces genetic mutations.

According to environmental standards QCVN 40:2011/BTNMT (Column A) and QCVN 07:2009/BTNMT, the allowable concentration of Cr6+ discharged into receiving water bodies must not exceed 0.1 mg/L, and total chromium (Total Cr) must be ≤ 0.5 mg/L. Therefore, the reduction of Cr6+ to Cr3+ is the most critical step in an electroplating ETP (Effluent Treatment Plant).

2. Chemical Equation & Reaction Mechanism of Cr6+ Reduction with FeCl2

In an acidic environment, the reaction between Iron(II) Chloride (FeCl2) solution and Chromic Acid proceeds according to the net ionic equation:

$$\text{Cr}_2\text{O}_7^{2-} + 6\text{Fe}^{2+} + 14\text{H}^+ \rightarrow 2\text{Cr}^{3+} + 6\text{Fe}^{3+} + 7\text{H}_2\text{O}$$

Molecular equation with HCl Acid:

$$\text{H}_2\text{Cr}_2\text{O}_7 + 6\text{FeCl}_2 + 12\text{HCl} \rightarrow 2\text{CrCl}_3 + 6\text{FeCl}_3 + 7\text{H}_2\text{O}$$

Quantitative Ratio Analysis (Stoichiometry):

  • Theoretically: To reduce 1 mole of Cr6+ ($52\text{ g}$), 3 moles of Fe2+ ($3 \times 55.85 = 167.55\text{ g Fe2+}$) are required, equivalent to $380.25\text{ g anhydrous FeCl2}$. The theoretical mass ratio is 7.31 g pure FeCl2 / 1 g Cr6+.
  • In actual ETP operation: Due to dissolved oxygen in the water consuming a portion of the Fe2+, a safe dosage excess is 8.5 – 10.0 g anhydrous FeCl2 / 1 g Cr6+ (approximately $30 – 35\text{ ml}$ of FeCl2 25% solution per $1\text{ g Cr6+}$).

3. Technical Specifications of Chemicals Used in the Cr6+ Reduction Process

Technical Specification FeCl2 Solution (Iron(II) Chloride) HCl 32% (Acid Adjustment) NaOH 50% (Hydroxide Precipitation)
Chemical Formula FeCl2 HCl NaOH
Molecular Weight (MW) 126.75 g/mol 36.46 g/mol 39.997 g/mol
CAS Number 7758-94-3 7647-01-0 1310-73-2
Commercial Concentration 15% – 30% w/w 31% – 33% w/w 48% – 50% w/w
Specific Gravity (25°C) 1.25 – 1.35 g/mL 1.16 – 1.18 g/mL 1.50 – 1.53 g/mL
Role in Electroplating ETP Primary reducing agent donating e⁻ Lower pH to 2.0-2.5 for reaction Raise pH to 8.5-9.0 for precipitation

4. SOP 4-Step Operating Procedure for Cr6+ Reduction in Electroplating

  1. Step 1: Separate collection of Chromium-containing wastewater & Acidic pH adjustment

    Rinse water containing chromium from the plating line is directed to a dedicated Cr6+ Reaction Tank (not mixed with cyanide or alkaline wastewater streams). Turn on the agitator, and dose HCl 32% automatically via a pH controller to maintain a stable pH of 2.0 – 2.5.

  2. Step 2: FeCl2 solution dosing & Reaction control via ORP monitoring

    A metering pump doses FeCl2 25% solution into the reaction tank. The dosage is controlled using an Oxidation-Reduction Potential (ORP) meter. When Cr6+ ions are completely reduced to Cr3+, the ORP value drops from $+500\text{ mV}$ to the range of $+250\text{ mV} \div +300\text{ mV}$. Maintain a retention time of 15–20 minutes.

  3. Step 3: Neutralization & pH elevation for Metal Hydroxide Precipitation

    Wastewater flows by gravity to the Neutralization Tank. A metering pump doses NaOH 50% solution (or lime milk Ca(OH)2) to raise the pH to 8.5 – 9.0. At this pH range, metal ions convert to insoluble hydroxide precipitates:

    $$\text{Cr}^{3+} + 3\text{OH}^- \rightarrow \text{Cr}(\text{OH})_3 \downarrow \quad (\text{Gray-blue precipitate})$$

    $$\text{Fe}^{3+} + 3\text{OH}^- \rightarrow \text{Fe}(\text{OH})_3 \downarrow \quad (\text{Red-brown precipitate})$$

  4. Step 4: Coagulation, Flocculation & Sludge settling

    Dose 1–2 ppm of Anionic Polymer at the flocculation tank. The Cr(OH)3 and Fe(OH)3 flocs bind together and settle extremely quickly in the Metal Settling Tank. The clear water meets discharge standards and flows to an intermediate tank; the sludge at the bottom is pumped to a filter press for dewatering.

5. FAQ - Frequently Asked Questions about Cr6+ Reduction with FeCl2

1. Why is using FeCl2 solution more advantageous than crystalline FeSO4 salt?

Liquid FeCl2 25% solution is dosed directly via metering pump, eliminating the labor for handling and dissolving dry FeSO4.7H2O crystals. Additionally, the Cl⁻ ion does not cause pipe scaling like the sulfate ion SO4²⁻.

2. What ORP value ensures that Cr6+ has been completely reduced?

For the reaction using FeCl2 at pH 2.0–2.5, when the ORP value reaches +250 mV to +300 mV (vs. Ag/AgCl reference electrode), the Cr6+ content in the water has dropped below 0.05 mg/L, ensuring absolute safety before entering the neutralization tank.

3. How does Lộc Thiên package and deliver FeCl2 solution for Cr6+ reduction?

Lộc Thiên supplies FeCl2 solution at 15–30% concentration in 200L HDPE drums (250 kg), 1000L IBC tanks (1.3 tons), or delivers directly via 10–24 ton tanker trucks equipped with pump-off systems for storage at textile dyeing and electroplating plants.

For more in-depth articles on electroplating applications: FeCl2 in electroplating for Cr6+ reduction and chemical selection solutions: comparison of FeCl2 vs FeSO4 vs PAC in ETP.