Biological Regeneration Of Iron Ion Solution Removal Of Hydrogen Sulfide Gas

In this paper removing H₂S gas with ferric solution regenerated by biological method was studied in detail. H₂S gas was removed in a bubble column and ferric ion solution was regenerated in an immobilization cell bioreactor of Thiobacillus ferrooxidan. In view of many advantages such as high H₂S removal efficiency, low running cost, operating in normal temperature and ordinary pressure, no discharge of waste, and the sulphur produced in the process can be reused, this process is regarded as one of the most prospective biodesulfurization methods.The bacteria isolated from the acidic wastewater of a sulfide mineral in Yunnan were preliminarily regarded as engineering bacteria which took Thiobacillus ferrooxidan for superior bacteria after analysis and identification, and the optimal growth conditions and growth kinetics were studied respectively.The immobilization cell bioreactor of Thiobacillus ferrooxidan was constructed with zeolite or plastic fiber as filling. The factors such as the form of immobilization cell bioreactor, the air flow rate, the liquid flow rate, the concentration of ferrous ion and pH were studied respectively. The result showed that the oxidation rate of ferrous ion was increased highly by the immobilization cell bioreactor. With plastic fiber as immobilization filling, the oxidation rate of ferrous ion was 0.36g/L/h under the conditions of initial pH of 1.6, ferrous ion conversion of 95%. When with zeolite as opposite flow of air and liquid immobilization filling, the ferrous ion oxidation rate was 0.63g/L/h which is 1.75 times of that with plastic fiber as filling. When with zeolite as parallel flow of air and liquid immobilization filling, the ferrous ion oxidation rate was 1.10g/L/h which is 1.75 times of that with opposite flow of air and liquid immobilization. But immobilization bioreactor with plastic fiber as filling is simple in structure, has low running cost, is operated simply, and can avoid blockage with hanging filling of plastic fiber.H₂S was removed in a bubble column with ferric ion solution regenerated in an immobilization cell bioreactor as absorbent. The factors such as the ferric ion concentration, H₂S gas concentration of the inlet, gas flow rate of the inlet, gas retention time, and the ferric ion solution flow rate that affected H₂S removal efficiency were studied. The desulphurization efficiency of the system for long time running was also studied. When H₂S gas concentration of the inlet was up to 8100 mg/m³, 99.77% desulphurization efficiency was also achieved through long gas retention time and high ferric ion solution flow rate, H₂S gas concentration of the outlet was below 20mg/m³, which is accorded with H₂S gas standard rate of city coal gas, natural gas and waste gas from petroleum refining, and the treatment load of H₂S gas was up to 1031.85g/m³/h. After long time running the desulphurization result showed that when inlet H₂S gas concentration was 4000 mg/m³, desulphurization efficiency decreased slowly with the prolongation of running time. When the running time was 250 hour, desulphurization efficiency decreased from 99.81% to 98.01%, and outlet H₂S gas concentrationincreased from 7.8 mg/m³ to 82.0 mg/m³ . In order to maintain high H₂S removal efficiency, ferrous ion should be added regularly to the solution to keep the stability of ferrous ion concentration, and the additional ferrous ion can keep the oxidation activity of Thiobacillus ferrooxidan.H₂S was removed with ferric ion solution oxidized by Thiobacillus ferrooxidan from pyrite. The chief factors of grinding size and pulp concentration of pyrite were studied. The contrast experiment removing H₂S gas which used ferric ion solution through pyrite oxidation in airlift reactor was studied, and the result showed that the desulphurization efficiency was about the same as that of ferric ion solution oxidized by Thiobacillus ferrooxidan from FeSO₄·7H₂O. This indicated that it was practicable for pyrite to replace FeSO₄·7H₂O for biodesulfurization.The possibility of producing sulphur and hydroxide of iron in the process of H₂S removal was analyzed through E-pH diagram of S-H₂O system and Fe-H₂O system. The solution chemical characteristic of H₂S gas solution and regenerated ferric ion solution was also studied. The result showed that Fe(OH)₃ precipitate could happen while pH value of solution was greater than 2.9. When pH value of solution is less than 2.9, iron in regenerated ferric ion solution is the form of Fe₂(OH)₂⁴⁺ and Fe³⁺, and then Fe(OH)²⁺.The theory analysis shows that there are three actions in the process of H₂S removal with regenerated ferric ion solution, the direct oxidation of ferric ion, the indirect catalysis oxidation of Fe₂(OH)₂⁴⁺, and the direct oxidation of Fe(OH)²⁺.