Technique Of Producing Na₂Cr₂O₇ From Chemosynthesis Aluminum-foil Industrial Wastewater

The research significance of Cr-contained wastewater treatment not only lies in making the concentration of pollutants in the discharged water below the discharge standard limit but also in recycling the resource. Based on reviewing internal and external relative literature, this paper presents a new recycling and innocuous process to reclaim Cr(VI) and Cr (III) from the industrial wastewater generated by a chemosynthesis aluminum-foil plant in Guangdong province, then produce Na₂Cr₂O₇ with it. The whole process involves five major steps: reducing, chromium-precipitation, oxidation, acidizing and concentration. Series experiments were performed to grope the optimum process parameters for the first three steps. The results indicate that under certain process parameters the total recovery rate could be more than 79% while the final production is Na₂Cr₂O₇. Later, a primary economical analysis was made.Adopting sugar as reducer to reduce the Cr (VI) in sample (which concentration is 27 g/L) to Cr (III). For 100 mL wastewater sample the optimum dosage of reducer was 2. 6 g, the optimum reaction temperature was 97℃, the optimum reaction duration was 8 minutes while the reducing rate was98. 9%.In order to precipitate the reduced Cr(III) and the original existed Cr (III) ( which concentration is 20 g/L) in water sample, MH₄CO₃ and NaOH were put in. The optimum process parameters are as follows: for 100 mL reduced sample, 24 g NH₄HCO₃ and 15 g NaOH were consumed and produced Cr(OH)₃ 8.69 g, while the reaction temperature fluctuated between 60 and 70℃. Totally, 93.9% Cr (III) was got rid of from the sample.Transforming the Cr(III) in Cr(OH)₃ sediment into CrO₄²-with H₂O₂ under the condition of high pH value. The optimum process parameters are as follows: for 8.69g Cr(OH)₃ maintaining the pH value at 13. 5, 29 mL H₂O₂ and 14. 6 g NaOH were consumed. 40 minutes later, the oxidation rate was 92. 1%.Adopting H₂SO₄ to regulate the pH value, when pH = 2, 91. 5% CrO₄²- transformed to Cr₂O₇²-Treating 1000 kg wastewater could produce 93 kg Na₂Cr₂O₇ crystal, the gross profit is about 726 Yuan RMB per ton production. Hence, considerable economic benefits could be achieved.