Oxidation Monitoring in the Cyanide Wastewater Treatment Process

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Oxidation Monitoring in the Cyanide Wastewater Treatment Process

Cyanide decomposition process
Cyanide decomposition process

Cyanide-bearing wastewater from mining and electroplating facilities and certain types of chemical plants is toxic and must be treated by oxidation with chlorine or chloride to bring the cyanide concentration within regulatory limits. The waste materials contain alkaline, rare earth metals, and other heavy metals such as iron, nickel, zinc, cadmium, copper, silver and gold. As well as sometimes can contain the deadly poison, cyanide. As the speed of the oxidation reaction is closely tied to the pH value, a pH analyzer is used together with an ORP (oxidation-reduction potential) analyzer to monitor the completion of the reaction. The use of these analyzers also ensures that excessive amounts of chemicals (e.g., chlorine) are not used to produce the reaction. Both 2-wire and 4-wire analyzers are suitable for this measurement system. A gold electrode should be used for ORP measurement of cyanidecontaining solutions.

Expected Benefits

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

Process Overview

Measurement of ORP

Cyanide decomposition takes place in two stages. In the primary reaction, cyanide is oxidized to cyanate under high alkaline conditions (high pH and ORP). In the first reaction tank, the pH of the waste is measured and caustic (NaOH at 50% strength) is injected to raise the pH to 10 or higher. The oxidation reduction potential (ORP) of the waste is measured, and chlorine gas (Cl2) is automatically injected to raise the ORP to 400 mV or higher. The reaction normally occurs within 5 to 10 minutes.

In the secondary reaction, the cyanate is further oxidized under near-neutral conditions and converted to harmless carbon dioxide and nitrogen gases. In the second reaction tank, the pH of the waste is measured, and acid is injected to lower pH to 7-8. This process takes 2 to 5 minutes. In the third reaction tank, the ORP of the waste is measured and chlorine gas (Cl2) is automatically injected to raise the ORP to 600 mV or higher (Meanwhile the pH controller maintains the set-point at 7-8, correcting for any acidity created by the addition of the chlorine gas). This reaction occurs, taking 10 to 15 minutes, then the cyanide is eventually converted to harmless materials by the above reaction and the waste can be discharged.



Solution Details

Measurement system
2-wire/4-wire ORP measurement system
2-wire/4-wire pH measurement system
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)
2-wire transmitter (pH/ORP): PH202G-E-E
4-wire pH/ORP converter: PH450G-A-☐
Distributor (for 2-wire system): PH201G- A☐*B

Measurement Conditions

A gold electrode should be used to ensure accurate measurement of ORP in cyanide solutions. When the potential is near or under 200 mV during the cyanide oxidation process, a platinum electrode produces a catalytic reaction that causes hydrogen to form on the electrode surface. This generates potential and affects measured values.

pH and ORP levels in this area can be simultaneously monitored with the PH450 pH/ORP Analyzer. The model PH450 pH/ORP Analyzer performs continuous sensor diagnostics to notify the user when the sensor is coated or the glass electrode has broken.

The PH20 "Tempress" sensor with has a 3/4" NPT threads is convenient to install and use in the reaction tanks. Adapters enable it to be converted into a 1" MNPT thread, as well as a flow thru assembly or direct insertion assembly with a cleaning system. This sensor features four separate elements (pH, reference, temperature, ORP) in a rugged PVDF body. The integral number-coded cable and integral NPT threads allow for easy and convenient installation. When used with PH4050 or PH202 pH/ORP Analyzers, it can give simultaneous measurements of pH and ORP is possible with one sensor.


Power supply: 17 to 40 V DC (from distributor)
Power supply: 96 to 264 V AC, 50/60 Hz
Power consumption: approx. 15 VA
PH201G distributor
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


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Yokogawa의 pH와 ORP 분석계는 높은 신뢰성을 제공합니다. 이 제품은 고급 기능을 갖추고 있으며 광범위한 생산 공정과 중규모 폐수 처리 시설의 수질관리를 비롯하여 일반적인 pH 및 ORP 제어시스템과 같은 다양한 어플리케이션에 유용합니다.

pH and ORP Analyzers

pH와 ORP 미터, 분석계 및 전송기는 수질/제품 품질을 보장하고 유출물 배출량, 배치 중화, 펄프 스톡, 세정기, 냉각 타워, 화학, 폐수 처리 및 기타 여러 어플리케이션의 연속 공정 모니터링에 사용합니다.

pH and ORP Sensors

pH 전극 및 센서는 pH 측정의 감지 부분입니다. Retractable, flow-though, 침적 및 직접 삽입을 포함한 다양한 설치 옵션이 있습니다.

적절한 pH 전극 / 센서 선택은 최적의 측정 결과를 위해 중요합니다.


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