Sulfurization Experiment And Relevant Theory For Waste Acid Treated From Smelting Flue Gas

There are a large amount of low-concentration H₂SO₄ solution containing As,Pb,Zn,Cu and Fe,which are produced during the production of smelting flue to produce acid.The processes of neutralization-sulfurization-neutralization are used in many industries.That is,metal ions is sulfurated sodium synchronously by sodium sulfide as sulfiding agent.However,this process can not make full use of high-value heavy metals by stepwise sulfurization,and new pollutants are formed at the same time.The use of H₂S for the treatment of heavy metals in waste acids has its unique advantages and does not result in the formation of sulfates.However,in the production process of H₂S,there are a lots of problems,including complicated problems,huge equipment,inconvenience for storage and use,and so on.In this paper,we have grasped the key issues in the treatment of acidic wastewater;theoretical analysis of the conditions and control methods for sulfide formation in the system is conducted;dissolved H₂S is produced by bipolar membrane electrodialysis;metal ions is treated by stepwise sulfurization using H₂S as sulfiding agent.The main works and results are summarized as follows:（1）Theoretical calculations show that in the dilute acid solution in which As³⁺and Cu²⁺coexist,for the first sulfurization under the condition of n_Cu.S2-:n⁰_Cu.S2-=1,the removal rate of Cu²⁺in the solution can approach 100%,while that of As³⁺is approximately 0.The removal rate of As³⁺can be close to100%for the second sulfurization under the condition of n_Cu.S2-:n⁰_Cu.S2-=1.In practical applications,the potential value of the solution should be considered in order to calculate the molar ratio of Cu²⁺/Cu⁺,and accurately calculate the amount of sulfiding agent required to completely sulfide the precipitated copper ions.Sulfur may be generated in Cu-S-H₂O acid system,and the pH value of sulfur formation is linear with lg[Cu²⁺].In the actual sulfurization process,the amount and adding manner of the sulfiding agent should be selected and controlled based on the pH value and potential value of solution to avoid the formation of sulfur.（2）The study on the process conditions of sulfurization of metal ion.The effects of factors such as acidity,metal ion concentration,presence of impurity ions,amount of sulfiding agent,and amount of hydrazine hydrate on reducing agent were investigated.First,the acid solution containing a single Cu²⁺/As³⁺is subjected to a sulfidation treatment to provide a basic reference for the simultaneous presence of a plurality of ions.Then,the waste acid solution containing only Cu²⁺and As³⁺metal ions is simultaneously vulcanized.The results show that for simultaneous sulfurization,when 1.2 times of theory Na₂S which is to precipitate Cu²⁺and As³⁺,the removal rate of Cu²⁺could reach 99%,and that of As³⁺could reach more than 90%.Then,we took a simultaneous sulfurization method to treat the industrial waste acid from the Central Plains gold smelter,which regulator potential with hydrazine hydrate.The results show that:in the dilute sulfuric acid solution with concentration of 8%,when 1.2times of theory Na₂S which is to precipitate Cu²⁺and As³⁺,the removal rate of Cu²⁺is more than 98%,and that of As³⁺is more than 90%.And the amount of hydrazine hydrate added should not be less than 0.4 mL·L^-1（industrial hydrazine hydrate of 40%concentration）.Finally,on the process conditions of simultaneous sulfurization of metal ion,for the first sulfurization under the condition of n _Cu.S2-:n⁰_Cu.S2-=0.8,the removal rate of Cu²⁺in the solution can approach 100%,while that of As³⁺is approximately 0.The removal rate of As³⁺can be close to 100%for the second sulfurization under the condition of nCu.S2-:n0Cu.S2-=1.（3）The study on the technological conditions of electrodialysis pilot plant.In this part,the technological conditions of the bipolar membrane electrodialysis pilot plant for generating hydrogen sulfide are studied.Based on the principle of higher current efficiency and higher concentration of production,the best operating voltage for the membrane stack（3 groups of anion exchange membrane,cation exchange membrane and bipolar membrane）should be 7.0V,and the flow rate in the acid chamber is 60 L·h^-1,and the concentration of Na₂S in the salt chamber is 0.5 mol·L^-1（about 4.5%by mass）.Then we treated waste acids for pilot experiments by electrodialysis.When the current is 1.5 times of the theoretical current,Cu,Pb,As,Zn and Fe have the highest removal rates totally.Among them,the removal rate of Cu²⁺and Pb²⁺can reach 100%,and the removal rates of Zn²⁺and Fe³⁺are 50%and 60%,respectively.