Reduction Monitoring in the Chromium Wastewater Treatment Process

下载 (277 KB)

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 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
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 transmitter (pH/ORP): FLXA202-D-BーD-☐-P1-NN-A-N-LA-☐-NN
4-wire pH/ORP converter: FLXA402-A-B-□-P1-NN-A2-WR-☐-☐-☐-NN

Measurement Conditions

Measurement conditions
Measurement conditions
  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.

行业

  • 水和废水

    横河电机通过开发更节能的技术,帮助减少运营的碳排放,以及构建保护环境免受污染的坚固建筑,为可持续水生产提供控制解决方案。横河凭借先进的技术和广泛的应用专业知识与客户合作,提供可持续水生产的解决方案,促进客户的业务,并让整个工厂在生命周期中增加价值。横河的技术和产品可提高工厂的性能,确保它们在当今的水处理领域中具有竞争力,并降低运营成本。横河支持市政和工业水处理领域的各种水控制方面的应用。

    更多
  • 水处理

    横河电机提供理想的解决方案,以改善水处理厂的运行,使其更加可靠。这些解决方案包括复杂的技术、过硬的产品质量和贯穿生命周期的广泛应用技巧。

    更多
  • 污水处理

    污水处理厂使用大量的电动机、泵、鼓风机等,因此降低其耗电量以降低运营成本是十分重要的。横河电机提供控制解决方案,以提高能源效率。

    更多

相关产品&解决方案

  • 2-Wire Transmitter/Analyzer FLXA202

    The FLEXA™ series analyzers are used for continuous on-line measurements in industrial installations. With an option for single or dual sensor measurement, they are the most flexible two-wire analyzer available.

    更多
  • pH和ORP传感器

    pH电极和传感器是进行pH值测量的传感部分,有伸缩式、流通式、浸入式和直插式等多种安装选项。选择合适的pH电极/传感器对优化测量结果至关重要。

    更多
  • pH和ORP分析仪

    pHORP流量计、分析仪和变送器用于pHORP的连续过程监测,以确保水/产品质量,如监测污水排放、批量中和、浆料存储、洗涤塔,冷却塔、化学品、水/废水处理和许多其他应用。

    更多

置顶
WeChat QR Code
横河电机(中国)有限公司