Study On The Reaction Behavior Of Na₂S₂O₃ With Cu²⁺ In Waste Acid And Bipolar Membrane Electrodialysis Process

A large amount of waste acid will be produced in the process of producing sulfuric acid from smelting flue gas which contains heavy metal elements such as Cu,Fe,As,Zn,Pb.At present,the vulcanization precipitation method is widely used in major factories because of its high removal rate,simple operation and low cost.The commonly used vulcanizing agents are mainly Na₂S and Na₂S₂O₃.Due to the high acidity of the waste acid,Na₂S reacts with waste acid to produce a large amount of hydrogen sulfide gas,which will pose a serious threat to the environment and operators.But,the reaction of Na₂S₂O₃ with waste acid is mild and controllable,which facilitates the implementation of stepwise precipitation and separation of different ions,and maximizes the recycling of resources.However,the reaction pathway of Na₂S₂O₃ with metal ions such as Cu²⁺is not clear enough,which often leads to problems such that the dosage of vulcanizing agent and the reaction end point are not easy to master.There is also the problem of introducing impurity Na⁺to contaminate the vulcanization system.At the same time the study of the influence law of the main factors of reaction is not systematic and perfect.For this reason,the reaction of Na₂S₂O₃ with Cu²⁺is used as the probe reaction in this paper.First,through thermodynamic calculations and kinetic experimental data,the possible reaction pathways of Na₂S₂O₃ sulfidation reaction are inferred,and countermeasures to improve the reaction rate of the sulfidation process are proposed based on this judgment.It provides theoretical and experimental basis for improving the vulcanization efficiency of different metal ions in waste acid and realizing precise step-by-step vulcanization.Secondly,a new process of producing dissolved H₂S₂O₃ by bipolar membrane electrodialysis is proposed.The specific work content and main conclusions are as follows:（1）Through thermodynamic calculations,experimental phenomena and product analysis,the paper proposes four reaction pathways of Na₂S₂O₃ and Cu²⁺.Through design verification experiments and kinetic behavior analysis,it is finally determined that the reaction system includes direct sulfidation of Na₂S₂O₃ and Cu²⁺and the side reaction of the disproportionation of Na₂S₂O₃ and H⁺.The potential-p H relationship diagram of Cu-S-H₂O and the experiment show that S and Cu S will be produced in the vulcanization system.In the actual vulcanization experiment,the system should be controlled at the stable region of Cu S by adding an appropriate amount of vulcanizing agent according to the potential value of the solution system,and that will reduce the amount of S.Experimental investigations show that increasing the reaction temperature,increasing the amount of vulcanizing agent and extending the reaction time can all increase the removal rate of Cu²⁺to varying degrees,and the improved efficiency of the three decreases in turn.When the reaction temperature is 80℃,the number of moles of vulcanizing agent added is when the theoretical reaction mole number is twice,and the reaction time is 120 min,the removal effect of Cu²⁺is the best,and the removal rate reaches 99.45%.（2）After clarifying the reaction pathway of Na₂S₂O₃ and Cu²⁺under acidic conditions,this paper also carried out the main reaction（sulfidation reaction）and side reaction（disproportionation decomposition reaction）kinetic parameters measurement experiments.And the determination experiment is mainly carried out by the differential method.The results show that the reaction order of S₂O₃^2-in the main reaction is 0.988,the reaction order of Cu²⁺is 0.700,the total reaction order is 1.688,the reaction activation energy of the main reaction is 125.61 k J·mol^-1;the reaction order of S₂O₃^2-in the side reaction is 0.56,the reaction order of H⁺is 1.25,and the total reaction order is 1.81,the reaction activation energy of the side reaction is 113.94 k J·mol^-1.The analysis shows that the reaction order of S₂O₃^2-in the main reaction is greater than the reaction order of S₂O₃^2-in the side reaction.It is quantitatively proved from the aspect of kinetics that increasing the input of vulcanizing agent can increase the selectivity of the main reaction,and it means to improve the Cu²⁺removal rate.The reaction activation energy of the main reaction is greater than the reaction activation energy of the side reaction,which also proves that increasing the reaction temperature can increase the removal rate of Cu²⁺.（3）The bipolar membrane electrodialysis process produces dissolved H₂S₂O₃ to treat Cu²⁺in the waste acid of copper smelting flue gas,which can solve the problem of Na₂S₂O₃being used as a vulcanizing agent to introduce Na⁺into the system.After experimental exploration,when the acid chamber concentration is 1%H₂SO₄,the acid chamber flow rate is30 L·h^-1,and the salt chamber concentration is 0.064 mol·L^-1（mass percentage concentration is 1%）,the current efficiency reaches 76.45%and the removal rate of Cu²⁺reaches 4.7%.