Removal Of Silver From Silver Ammonia Complex Wastewater By Decomplexation-Precipitation-Microfiltration And Design Of Pilot Device For Cesium Removal

Element silver mainly existed in the form of[Ag?NH₃?₂]⁺in the wastewater from electroless silver plating for the production of mirror.It was unable to settle from the wastewater by directly dosing precipitant,thus,decomplexation pretreatment is very necessary before the removal of silver by chemical precipitation.The study found that dilute sulfuric acid as decomplexation agent was better than dilute hydrochloric acid.In the research,a new hybrid process of precipitation-microfiltration was developed to remove the silver ions contained in the silver ammonia compex wastewater.Jar tests were conducted to explore the influence of decomplexation and precipitant dosage on the removal rate of silver and the adsorption behavior of powdered activated carbon?PAC?on trace silver were carried out,through which the basic parameters and operation conditions were determined.Furthermore,treatment efficiency was verified by the bench-scale test.Additionlly,based on the previous results from the cesium removal test,the calculation of the pilot device for the cesium removal by the adsorption/coagulation microfiltration process and adsorption/coagulation filtration process was conducted,respectively,and the preparation of development of emergency equipment was complted.The results of jar tests for the treatment of silver ammonia complex wastewater by decomplexation-precipitation-microfiltration process,showed that the lower the pH,the better the effluent,as well as the dosage of sodium sulfide has a significant effect on the removal rate of silver,and excessive dosage of sulfide leads to the decrease of treatment effect,unexpectedly.The optimum operation parameters were as follows:the pH value was 6 after decomplexation,the dosage of sodium sulfide calculated?the molar ration of sulfide to silver was 0.6?was 20 mg/L,the mixing time was 20 min,the velocity gradient was 182 s^-1,and the sedimentation time was 5 min,the silver concentration in the effluent ranged from 0.4 to 0.7 mg/L.Besides,the adsorption process of PAC on trace silver fitted better with Langmuir adsorption isotherm,and the adsorption amount was 7.4 mg/g when the initial silver concentration was approximately 0.73 mg/L under the temperature of 20?.The test results indicated that the average silver concentration in the effluent was0.7 mg/L in the bench-scale test without PAC addition,while that was only 0.3 mg/L in the bench-sale test with PAC addition.Obviously,the removal efficiency was increased by PAC adsorption.In both two tests,the average pH values in the effluent form the agitator were 6.22 and 6.30,and the turbidity were 6.3 and 3.0 NTU,repectively.The measured concentrations of SO₄^2-and NO₃^-in the membrane effluent were stable at 600 mg/L and 8 mg/L,respectly,which were consistent with the theoretical value.The turbidity in the membrane effluent was all below 0.2 NTU.The quality of effluent water is good,also,the membrane flux decreaeded slowly in the tests.Both schemes of the two divices with treatment capacity of 20 m³/d were determined,which would be made of stainless steel.The adsorption reactor with a total height of 1.85 m and a diameter of 0.90 m is cylindrical vessel with a row of mud bucket at bottom.Distinctionly,the membrane reactor combined upper part with a diameter of 1.20 m and lower half with a diameter of 0.80 m using a reducing cylinder.The sand filter was designed as a cylindrical tank with a diameter of 0.32 m and a total height of 2.95 m equipped with small resistance water distribution system,whose filtering velocity was designed as 10.3 m/h.