Oxidation Monitoring in the Cyanide Wastewater Treatment 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 cyanide-containing solutions.
Expected Benefits
- Measures pH/ORP of cyanide wastewater continuously
- Reduces operating costs
Cyanide decomposition process
Process Overview
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 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) | |
| Analyzer/converter: | |
| 2-wire pH/ORP analyzer (pH+ORP2 input): | FLXA21-D-P-S-☐☐-P1-P1-A-N-LA-N FLXA202-D-B-D-☐☐-P1-P1-A-N-LA-N-NN |
| Distributor (for 2-wire system): | PH201G- A☐*C |
| 4-wire pH/ORP converter (pH+ORP 2 input): | FLXA402-☐-B-☐☐-P1-P1-A2-WR-☐-☐-N-NN |
Utilities
| FLXA21/FLXA202 | |
|---|---|
| Power supply: | 23 to 40 V DC (from distributor) |
| FLXA402-A | |
| Power supply: | 90 to 264 V AC, 50/60 Hz |
| Power consumption: | approx. 35 VA |
| FLXA402-D | |
| Power supply: | 10.8-26.4 V DC |
| Power consumption: | approx. 15 W |
Measurement Conditions
- Cyanide curve measured with chlorine [Measurement of water with a ph value of 9 using a gold electrode (as the indicating electrode) and an Ag/AgCl electrode with 4 mol (as the reference electrode)] Oxidation reaction of CN- with Cl2 exhibits a characteristic curve that is not ideal (see the figure on the right). This is because firstly an excessive amount of chlorine increased the plateau potential of the intermediate, cyanogen chloride to 500 to 600 mV, and secondly oxidation reaction started in the secondary reaction tank to form CNO-, resulting in a decrease in CIO- and stopping the potential from increasing.
- Using a gold electrode as the indicating electrode for ORP measurement 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.
Industries
-
Wastewater Treatment
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.
-
Water & Wastewater
Water resources are finite, and therefore contributing to a sustainable water cycle is one of the Sustainable Development Goals (SDGs). Yokogawa has been providing advanced digital control solutions for the stable supply of clean and safe water, wastewater treatment for protecting the water environment, water loss management and optimization of plant operation for reducing CO2 emissions and running costs. With our leading-edge technologies, dependable products and extensive expertise and experience of diverse water projects around the world, we work with you to provide sustainable water solutions that boost your business and add value throughout the plant lifecycle.
Yokogawa supports a wide range of water control applications in both the municipal and industrial water markets.
-
Water Treatment
Yokogawa provides optimum solutions to improve the reliable operation of water treatment plants. These solutions include sophisticated technologies, unsurpassed product quality, and extensive application know-how throughout the lifecycle.
Related Products & Solutions
-
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.
-
2-Wire Transmitter/Analyzer FLXA21
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.
-
All-in-One pH/ORP Sensor Series FU20 and FU24
The FU20 and FU24, all-in-one pH and ORP, sensors show how Yokogawa applies the motto "Simple is best" to sensor technology.
-
Flow/NPT Fittings FF20/FS20
Yokogawa has invested considerable design and development time in producing a full range of fittings with particular emphasis on designs that reduce installation and maintenance time and consequently save operation costs.
-
pH/ORP Sensors PH8EFP, PH8ERP, OR8EFG, OR8ERG
Yokogawa's pH and ORP analyzers are highly reliable. They feature advanced functions and are useful for a wide variety of applications including water quality management in a broad range of production processes and in medium-sized wastewater treatment plants, or as general pH and ORP control systems.
-
pH and ORP Analyzers
pH and ORP meters, analyzers and transmitters are used for continuous process monitoring of pH and ORP to ensure water/product quality, monitor effluent discharge, batch neutralization, pulp stock, scrubbers, cooling towers, chemical, water/wastewater treatment and many other applications.
-
pH and ORP Sensors
pH electrodes and sensors are the sensing portions of a pH measurement. Various installation options including retractable, flow thru, immersion, and direct insertion. Proper pH electrode/sensor selection is critical for optimal measurement results.