| Large amounts of waste dilute acid containing high concentrations of arsenic and lead, cadmium, chromium, copper and other heavy metal contaminants is produced during process of furnace gas purification when producing sulphuric acid by pyrite. Each ton of sulfuric acid will produce about 100 kg of this waste dilute acid. The composition of waste dilute acid is complex and it has great harm to the environment, meanwhile, the waste dilute acid also contains a mass fraction of 15%-25% sulfuric acid which has a large value to be reused. At present, companies producing sulphuric acid by pyrite without fertilizer always use lime and iron salts together to treat this waste dilute acid and then emission wastewater. However, there are limitations about removal efficiency of arsenic and heavy metals and stability to reach emission standards. What’s more, Large amounts of sludge containing arsenic and heavy metals is produced with the use of lime, which can easily result in secondary pollution to environment as hazardous waste. Companies producing sulphuric acid by pyrite with fertilizer always reuse this waste dilute acid to produce phosphate directly in order to achieve the so-called "zero-emission" gole. Actually, it transforms pollutants into the compound fertilizer which poses a serious threat to the soil environment with long-term administration of this kind fertilizer.The paper explored the Na2S-PFS acid treatment process based on summarizing the current treatment technology for acid wastewater with arsenic and heavy metals. By single factor and orthogonal experiments analyzed the optimum conditions and effect of dosage of Na2 S and PFS, pharmaceutical dosing sequence, p H, redox potential on the removal efficiency of arsenic and heavy metals. It analyzed the environmental feasibility of reusing waste dilute acid to wet process of phosphoric acid leach processing before and after treatment. Conclusions are as follows:(1) Single-factor experiments investigated the effect of dosage of Na2 S and PFS,pharmaceutical dosing method, p H, settling time and redox potential on the removal efficiency of arsenic and heavy metals. The content of residual H2SO4 in waste dilute acid was investigated after treatment under conditions of different factors. The results showedthat:a.The dosage of Na2 S has a significant impact on the removal efficiency of arsenic and heavy metals. The removal efficiency of arsenic and heavy metals got an increasing tendency with the increasing dosage of Na2 S until it reached to 1.6g/(L waste dilute acid),when the removal efficiency of Pb, Cd, Cr and Cu reached 97.65%, 97.02%, 93.15%,97.88% stably. The removal efficiency of arsenic obtained maximum 99.91% but the H2SO4 in waste dilute acid got a great loss.b. PFS has the function to reduce the amount of Na2 S, avoid H2SO4 to be lost,improve removal efficiency of arsenic by flocculation, enhance the quality of waste dilute acid after treatment. The removal efficiency of arsenic and heavy metals in waste dilute acid is low and the treatment effect is not ideal using PFS alone. With the use of Na2 S to treat waste dilute acid, controlling the dosage of Na2 S and PFS at 1.2g /(L waste dilute acid) and 0.8g /(L waste dilute acid) respectively, the arsenic and heavy metal elements could be were removed by precipitation and meanwhile the concentration of H2SO4 in waste dilute acid could maintain at 15.4%.c.It is more favorable to improve the removal efficiency of arsenic and heavy metals and maintain H2SO4 in waste dilute acid by the way to add Na2 S firstly and then add PFS into waste dilute acid. It is beneficial for S2- to react chemically with arsenic and heavy metals in waste dilute acid which can improve the utilization of Na2 S and play a catalytic role in treatment of waste dilute acid by adding Na2 S with the method of immersion, in multiple and prolonging.d.The main form of arsenic in waste dilute acid is H3 As O3 when controlling p H at 3and redox potential at +300m V~+800m V. H3 As O3 in acid environment can generate As3+,which can be precipitated effectively by Na2 S. H3 As O4 can generate As O43- which can react with Fe3+ from PFS, so it is able to obtain a good result by using Na2S-PFS process to treat waste dilute acid.(2) Orthogonal experimental results show that the primary and secondary relationship between each factor affecting the removal efficiency of arsenic and heavy metals is as following: Na2 S dosage> PFS dosage> p H values> reacting time> settlingtime. The optimum combination conditions in the laboratory to treat waste dilute acid is as follows: dosage of Na2 S 1.2g/(L waste dilute acid), dosage of PFS 0.8g/(L waste dilute acid), adjusting p H at 3,controlling reaction 60 min,settling 30 min. On this condition,residual concentrations of total arsenic and heavy metal elements are able to meet the emission limit requirements, meanwhile, residual concentration of H2SO4 in treated waste dilute acid still amounted to about 15.4%. Quality conditions of treated waste dilute acid can well meet the requirements of wet phosphoric acid production.(3) Reusing untreated waste dilute acid with arsenic and heavy metals directly can cause heavy metals and arsenic in leaching phosphate being exceeded. Waste dilute acid after treatment by Na2S-PFS process would not introduce arsenic and heavy metals into phosphate, which brings good economic, environmental and social benefits by achieving a valuable component in resource utilization of waste dilute acid under the premise to guarantee the quality of phosphoric. |
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