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.
Industrias
-
Agua y agua residual
Yokogawa ha estado suministrando soluciones de control para la producción hídrica sostenible desarrollando tecnología con mayor eficiencia energética, ayudando a reducir la huella de carbono de las operaciones y fabricando productos de gran solidez que protegen el ambiente contra los contaminantes. Con nuestra tecnología de vanguardia y amplios conocimientos de las aplicaciones, trabajamos con usted para proveer soluciones hídricas sostenibles que impulsen su negocio y agreguen alto valor a lo largo del ciclo de vida de la planta. Nuestra tecnología y nuestros productos mejoran el desempeño de las plantas y garantiza que puedan operar competitivamente en los mercados del agua de hoy, así como reducir sus costos operativos. Yokogawa brinda apoyo en una amplia gama de aplicaciones para el control del agua en los mercados del agua tanto públicos como privados.
-
Tratamiento de aguas
Yokogawa ofrece soluciones óptimas para mejorar el funcionamiento confiable de las plantas de tratamiento de agua. Estas soluciones incluyen tecnologías sofisticadas, una calidad del producto sin igual y la extensiva aplicación de conocimientos a lo largo de todo el ciclo de vida.
-
Tratamiento de aguas residuales
Las plantas de tratamiento de aguas residuales utilizan muchos motores, bombas, ventiladores, etc., y es importante reducir su consumo de electricidad con el fin de reducir los costos de operación. Yokogawa ofrece soluciones óptimas de control para mejorar la eficiencia energética.
Productos y Soluciones Relacionadas
-
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.
-
Digital SMART SENCOM™ Adapter, SA11
Reusable SMART adapter, requiring only the analog sensor to be disposed of when it reaches the end of its lifetime. With the SENCOM 4.0 platform, Yokogawa delivers reduced costs and waste while contributing to its long-term business goals of a sustainable future for all.
-
Flow/NPT Fittings FF20/FS20
FF20/FS20 Fittings for pH and ORP
-
Multi Channel 4-Wire Analyzer FLXA402
Available in single or multi-sensor measurement
-
pH and ORP Analyzers
Optimize field maintenance, calibration, and system configuration
-
pH and ORP Sensors
Ensure fluid process operations
¿Tiene Dudas?
Contacte a un Experto de Yokogawa y conozca como podemos ayudarle a resolver sus retos.