Study On Catalytic Oxidation Of The Spent Caustic Solution Of Sodium Sulfide

Industrial sulfur-containing spent caustic mainly comes from the production process of ethylene and oil refining, where the content of sodium sulfide and COD is very high and thus need to be treated before disposal to the environment. Traditionally, this waste effluent is treated by air oxidation at high temperature and pressure, the process is time- and energy-consuming with low efficiency, and thus not suitable for large scale treatment. In this paper, we will study some new and efficient methods, i.e. electrolysis and catalytic oxidation method, so as to reduce the operating temperature and pressure of the treatment. This study is of potential applicability and great importance.This study mainly includes the following two aspects:(1) The traditional treatment technology for sodium sulfide often requires high temperature and high pressure, leading to a high processing cost and potential safty risk. In this study, electrochemical oxidation method was used with sodium chloride as sacrificing electrolyte and ruthenium plated titanium nets as electrode. The effects of some influencing factors on the sulfur removal rate are studied experimentally, including electrode materials, voltage applied, electrode plates interval, processing time, initial concentration of sulfur ion, liquid turbulent degree, electrolyte concentration, and catalyst added. The results show that the present method is applicable for the treatment of the solution with sulfur concentration below 1000ppm level, and the sulfur removal performance deteriorate when the S-content is higher due to the deposition of some semi-solid S-containing intermediates on the surface of electrodes. The optimum conditions are found as follows:ruthenium coated titanium electrode, the applied voltage 3V, the plate spacing 2cm. The sulfur removal ability increases with the decrease of the plate spacing, the removal rate of the sulfide ion is above 98% after 3h treatment for a 1000ppm Na2S solution with the electrode spacing of 1cm. Besides, the iron did not show catalytic effect on the electric oxidation of sulfur.(2)Catalytic oxidation has advantages of high efficiency and less pollution, by which the S2- can be oxided and transformed into S2O32 and SO42-.by the air under the action of catalyst. To investigate the influence of catalyst on the desulfurization reaction, the performance of some catalysts was compared and found that manganese, iron and phosphotungstate have little catalytic effect, while the active carbon loaded N-methylimidazolium phosphotungstate shows the best performance. Meanwhile the dissolution loss of the catalyst can be reduced greatly due to the low solubility of the c salt with a large organic cation. The effects of reaction temperature, the type and amount of catalyst, initial concentration of the waste, and air flow rate have been investigated. It is found that higher temperature, catalyst content and aie flow rate favor the oxidation process of sulfur ions as expected. Under the following conditions the desulphurization ratio reaches to 76.6%, namely initial concentration of S2- 10000ppm, air flow rate 48.9L/h, reaction at 80℃for 5 hours. The ionic specicies formed and their variation of concentration in the oxidation process was identified and analysed using ionic spectroscopy. The result indicates that three principal ions were formed, i.e. SO42-,S2O32-and SO32-. On this basis, the sequential oxidation mechanism was proposed, in which sulfide ion was firstly oxidized into sulfur element S and sulfite SO32-as well as their condensate form S2O32-, then further oxidized slowly to the final form SO42- and the oxidation of S2O32- is deemed as the controlling step.