Reduction Monitoring in the Chromium Wastewater Treatment Process

Introduction

Wastewater from electroplating facilities and certain types of chemical plants contains toxic forms of hexavalent chromium such as chromate and dichromate. The hexavalent chromium in this wastewater must be reduced before the water can be discharged. This requires a two-step process: hexavalent chromium (CR6) is reduced to trivalent chromium (CR3); and CR3 is precipitated as chromium hydroxide. As the oxidation reduction potential (ORP) and the speed of the reduction reaction are closely tied to the pH value, ORP and pH analyzers are used to ensure proper control of the reduction process through such means as the injection of reducing agents.

pH and ORP measurements in these reaction tanks are complicated by the presence of chrome and sulfur dioxide, which can poison the sensor reference under certain circumstances. pH and ORP control in these applications may be conducted using alarm relays and a simple on/off control scheme such as available in the FLXA402 Analyzer. Alternatively, the 4-20mA current output can be used directly for proportional control, or sent to a distributed control system as needed. The FLXA21/FLXA202/FLXA402 can measure both ORP and pH. Both 2-wire and 4-wire analyzers are suitable for this measurement system. A gold electrode should be used for ORP measurement of chromium wastewater.

Reducing trivalent chromium process

Reducing trivalent chromium process

Expected Benefits

  • Measures pH/ORP of chromate wastewater continuously
  • Reduces operating costs

Process Overview

First, hexavalent chromium, either in the form of chromate or dichromate, is reduced to trivalent chromium. Wastewater flows into the first reaction tank, where the pH is measured and sulfuric acid is automatically brought into the process until a pH set point value in the acidic range is achieved. The reaction time is just a few minutes, and a lower pH for an even faster reaction would require considerably more acid. At the same time, the oxidation reduction potential (ORP) of the solution is measured, and sulfur dioxide (SO2), sodium sulfite, or sodium metabisulfite is automatically injected until an ORP value of approximately 280 mV is achieved.

Then in the second tank, the pH is raised to 8.5 by the addition of an alkaline solution such as ammonia or caustic (NaOH), where it is converted to chromium hydroxide. The precipitate, although heavier than the water, does not drop to the bottom due to agitation in the tank. The mixed slurry flows to a settling tank, where the trivalent (Cr3+) chrome settles to the bottom and the clear chromium-free water flows over the tank for further treatment. Chemicals known as coagulants are sometimes added to the second reaction tank to help form larger particles and aid in sludge removal. The reducing agent may be a substance such as ferrous sulfate, sodium metabisulfite, or sulfur dioxide. Chemicals known as coagulants are sometimes added to the second reaction tank to help form larger particles and aid in sludge removal.

Solution Details

Measurement system

2-wire/4-wire pH/ORP measurement system

Sensor/Holder
KCl refillable ORP sensor: OR8EFG-AU-☐☐-TT1-☐*A
KCl refillable pH sensor: PH8EFP-☐☐-TN-TT1-N-☐*A
Submersion type holder: PH8HS-PP-☐☐-T-NN-NN*A
Terminal box (when needed)
WTB10-PH1 (for 2-wire system)
WTB10-PH3 (for 4-wire system)
Converter/transmitter:
2-wire pH/ORP transmitter (pH+ORP 2 input): FLXA21-D-P-S-☐☐-P1-P1-A-N-LA-N
Distributor (for 2-wire system):
PH201G- A☐*C
4-Wire Converter (pH+ORP 2 input): FLXA402-A-B-□□-P1-P1-□□-NR-N-N-N-NN
  1. Check solutions for ORP analyzer
    Oxidation-reduction potential of check solutions (quinhydrone/iron solutions)
    A check solution is used to determine whether the ORP sensor is operating correctly. There are two types of check solutions: quinhydrone and iron.

    Quinhydrone solution: approx. 200 to 300 mV
    Iron solution: approx. 420 to 520 mV
    Unlike a pH standard solution, an ORP check solution does not always indicate a constant reference value. Whether the sensor is operating normally can be determined by seeing whether its readings are within the acceptable range.

    Preparation of a check solution (250 mL)
    Quinhydrone solution: Dissolve the following reagent in pure water and dilute to 250 mL.
    Quinhydrone salts: P/N K9024EC
    Ferrous & ferric: dissolve the following reagent in a 2-mol/L sulfuric acid solution and dilute to 250 mL.
    Ferrous & ferric salts: P/N K9024ED

     

  2. Use of a gold electrode for ORP measurement
    For ORP measurement of chromate wastewater, the use of a gold electrode is recommended.

Utilities

  • FLXA21/FLXA202
    Power supply: 23 to 40 V DC (from distributor)
  • PH201G distributor (for 2-wire system)
    Power supply:
    100 V: 20 to130 V DC/80 to 138 V AC, 47 to 63 Hz
    220 V: 120 to 340 V DC/138 to 264 V AC, 47 to 63 Hz
    Power consumption:
    24 V DC: approx. 200 mA
    100 V AC: approx. 7 VA
    220 V AC: approx. 11 VA
  • FLXA402
    Power supply: 90 to 264 V AC, 50/60 Hz
    Power consumption: approx. 35 VA

Utilities

 

Przemysł

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    Yokogawa has been providing control solutions for sustainable water production by developing more energy-efficient technology, helping to reduce the carbon footprint of operations, and building rock-solid products that protect the environment from contaminants. With our leading-edge technology and extensive application know-how, we work with you to provide sustainable water solutions that boost your business and add high value throughout the plant lifecycle. Our technology and products improve the performance of plants and ensure that they can operate competitively in today's water markets, and also reduce their running costs. Yokogawa supports a wide range of water control applications in both the municipal and industrial water markets.

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    Wastewater treatment plants involve many pieces of equipment such as motors, pumps and blowers, which makes it important to reduce their electricity consumption and save running costs. Yokogawa provides optimum control solutions with digital technologies to improve energy efficiency.

     

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