Process Optimization In Treatment Of The Spent Electroless Nickel Plating Baths

The spent electroless nickel plating baths is a kind of surface active waste（hazardous waste code:HW17）which is composed of a small amount of aging plating solution and a large number of rinsing wastewater.Its composition is complex.It is difficult to treat nickel complex and hypophosphite directly.It needs to be diluted with other wastewater or treated by combined process.The ammonium chloride evaporation wastewater is the byproduct of resource utilization of copper containing waste（hazardous waste code:HW22）,with relatively single composition and easy treatment.In this paper,the ammonium chloride evaporation wastewater and the electroless nickel plating baths were comprehensively treated.Firstly,the ammonium chloride evaporation wastewater was pretreated by the struvite method as a diluent of the electroless nickel plating baths,and then the Fenton method was used to remove most of the pollutants,and finally the heavy metal chelating agent was used for deep removal of nickel.Single factor experiment and cross experiment were carried out to determine the optimal reaction conditions,conclusions listed as follows:1.The optimal reaction conditions of the struvite method for the ammonium chloride evaporation wastewater pretreatment were as follows:initial reaction p H value was 10.5,stirring time was 15 min,stirring speed is 150 r/min,and dosing ratio M:A:P=1.25:1.0:1.2.Under the optimal reaction conditions,NH₃-N decreased from 336 mg/L to 4.3 mg/L,the removal rate reached 98.7%,and the residual TP was 34.0 mg/L.2.The optimum reaction conditions for the treatment of the spent electroless nickel plating baths by Fenton method were as follows:dilution ratio was 100 times,Fe²⁺/H₂O₂molar ratio was 1:1,H₂O₂/COD_Crmass ratio was 2:1.Under the optimal reaction conditions,the water quality was as follows:Ni content was 0.57 mg/L,Zn content was 0.024 mg/L,p H value was 8.0,COD_Crwas 41.1 mg/L,NH₃-N was 3.93 mg/L,TP was 0.02 mg/L.3.DDTC（sodium diethyldithiocarbamate）was selected for deep removal of nickel as heavy metal chelating agent.The optimum reaction conditions were as follows:p H value was8.5,DDTC/Ni mass ratio was 80:1.Under the optimum reaction conditions,the Ni content was reduced to 0.081 mg/L,COD_Crincreased to 44.6 mg/L.4.The cost of reagents for the ammonium chloride evaporation wastewater pretreatment was 47.88 yuan/m³,and the cost of reagents for the spent electroless nickel plating baths was471.15 yuan/m³.After the spent electroless nickel plating baths were treated by this comprehensive treatment process,the water quality indices of Ni,Zn,COD_Cr,NH₃-N,TP met the discharge requirements of new projects in the Pearl River Delta region of Guangdong provincial local standard discharge standard for pollutants in electroplating water（DB 44/1597-2015）.For the relatively high income of hazardous waste disposal enterprises,the cost of reagents was acceptable.