Treatment Of Rare Earth Extraction Precipitated Wastewater By Modified Fly Ash And Flocculation Precipitation Method

The mining,extraction and smelting of rare earth resources produce a large amount of rare earth wastewater,which is harmful to the ecological environment and human health.The high ammonia nitrogen,high COD,lead ions and other pollutants in the rare earth wastewater become the main problem of rare earth wastewater treatment.Fly ash has a large specific surface area and has several adsorption points making it have a certain adsorption effect,but the adsorption capacity of a single fly ash is low and it is difficult to have a high removal rate of pollutants,Therefore,in order to better remove pollutants from rare earth wastewater and increase the adsorption capacity of fly ash,the combined treatment of multiple methods is imperative.This paper focuses on the treatment of rare-earth extraction precipitation wastewater using a modified fly ash-flocculation-precipitation combination,by using different modified additives and different modification methods for fly ash,so as to select the best modification method for rare earth extraction precipitation wastewater removal.Main research methods:1 Ammonia nitrogen treatment:Combined treatment by blow-off-adsorption,firstly by adjusting external factors such as the blow-off drum volume and p H to maximise pre-treatment of ammonia nitrogen.Next,different modified fly ash was used for the adsorption of ammonia nitrogen.2.COD treatment:using different modified fly ash for adsorption treatment.3.Lead ion treatment:Flocculation precipitation method and modified fly ash were used for combined treatment,firstly using PAC alone,modified fly ash alone and selecting the best dosage,next using PAC+modified fly ash treatment,and optimised to select the best treatment solution.4 Rare-earth extraction precipitation wastewater treatment:The best treatment solution is selected to achieve the removal of pollutants from rare-earth extraction precipitation wastewater,taking into account the removal effect of modified fly ash on various pollutants,as well as the cost of modification.The results are as follows:（1）Removal of ammonia nitrogen,Considering various factors,ammonia nitrogen blowing off the best treatment conditions for p H=11,blowing off time of48 h,blowing off the drum volume of 2.5 L·min-1.At this time,the ammonia nitrogen ion in the water turns into ammonia gas overflow.Using a 2 mol-L-1 sodium hydroxide solution,the fly ash was modified and adsorbed for 5 h.The ammonia nitrogen would then fill the adsorption sites.At this time,the efficiency and cost reached the maximum benefit,and the adsorption capacity increased by 0.5 mg·g-1,increasing by 61%.The whole adsorption process conforms to the second-order kinetic model of adsorption,which is monolayer adsorption and spontaneous adsorption.（2）COD removal:The adsorption capacity of different modified fly ashes was different.The adsorption capacity of the modified fly ash made from 2 mol-L^-1sodium hydroxide solution was 1.63 mg-g^-1for 3 h,which was more than double the adsorption capacity compared to the unmodified fly ash.The adsorption of COD from the fly ash modified by heating and stirring of the salt solution was more effective than that of the fly ash modified by pyrocautery,and the concentration of the treated solution was reduced by 20 mg-L^-1in comparison.the adsorption kinetics of the alkali-modified fly ash was in accordance with the secondary adsorption kinetic model,and the whole adsorption process was single-molecule layer adsorption,and the adsorption process was spontaneously generated.（3）Removal of lead ions:PAC has a certain effect on the removal of lead ions,but the efficiency is not high;the removal effect of fly ash made from modified fly ash modified by sodium hydroxide solution with the concentration of 1 mol-L^-1and 2mol-L^-1is the best,and the adsorption efficiency reaches 15.43 mg·g^-1The removal efficiency of fly ash modified by acid solution,alkali solution and scorch modification decreased with the addition of PAC,while the removal efficiency of fly ash modified under other modification conditions increased with the combined treatment of PAC.The removal rate of lead ions from the fly ash modified by heating and stirring with sodium carbonate solution was 98.9%,the same as that of the fly ash modified with sodium hydroxide.When comparing the different modification methods,the fly ash modified by heating and stirring of the salt solution was more effective in adsorption of lead ions than the fly ash modified by fire burning,with a0.3%increase in removal rate.The alkali-modified fly ash was able to be fitted with the secondary adsorption kinetic equation;it was a single molecular layer adsorption,the whole adsorption process was a heat absorption reaction and the adsorption was spontaneous.（4）Rare earth extraction precipitation wastewater treatment:the removal rate of lead ions by a single alkali modified fly ash was 89.4%,the removal rate of COD was 31.1%and the removal rate of ammonia nitrogen was 11.9%,all of which were lower than the removal rates tested in the laboratory.When comparing the overall adsorption efficiencies,the fly ash produced by the 2 mol-L^-1sodium hydroxide concentration modification was the most efficient in removing pollutants from the rare earth extraction precipitated wastewater,therefore the 2 mol-L^-1sodium hydroxide solution is recommended for the preparation of modified fly ash.