Composite chelating agent used for treating cadmium-containing waste water and application method thereof

The invention provides a composite chelating agent used for treating cadmium-containing waste water and application thereof. The composite chelating agent is prepared by compounding, by mass, 20 to 65% of trisodium thiocyanate and 35 to 80% of sodium dimethyldithiocarbamate. Application of the composite chelating agent in treatment of cadmium-containing waste water comprises the following steps: adding the composite chelating agent into the cadmium-containing waste water, wherein a mass ratio of the added composite chelating agent to the waste water is 1: 200 to 1: 5000; carrying out stirring for 5 to 30 min and then adding aluminum polychlorid, wherein a mass ratio of the aluminum polychlorid to the waste water is 1: 200 to 1: 5000; carrying out stirring for 5 to 30 min and then carrying out standing at normal temperature for 0.5 to 12 h; and subjecting obtained turbid liquid to filtering with a filter, wherein effluent obtained after filtering has cadmium content lower than cadmium content prescribed in national first-class discharge standard in China. The application method has the advantages of simple process flow, low cost, convenient operation, high efficiency, energy saving, a few sediments, no toxicity and exemption from incurrence of secondary pollution.