| The chromium in tannery sludge was high concentration and toxicity, which could easily promote environmental pollution and threaten houman health. The recycle of chromium-bearing sludge was useful for reducing the pollution and realizing the reuse of resources. This paper developed a technology process, described as "acid leaching-ion exchange method/chemical precipitation method-H2O2oxidation-product", to recover and recycle chromium from chromium-bearing sludge. The chemical characterization showed that sludges contained20.01%(dry base) chromium,2.02%(dry base) calcium and1.87%(dry base) magnesium. The main results are as follows:(1) Leaching studies of tannery sludge were carried out in sulfuric acid, nitric acid and hydrochloric acid media aiming to reach the maximum leaching yield of chromium by orthogonal test. The effect of various factors (the type of acid, solid-liquid ratio, the concentration of acid, leaching time, temperature and stirring rate) were studied. The results indicated that the maximum leaching yield was obtained by HC1, which was followed by H2SO4and HNO3, thus, HC1was the most suitable acid for leaching chromium. The optimum solid-liquid ratio was10g/L, the concentration of HC1was4.5mol/L, the leaching temperature was50℃, and the leaching time was1.5h. The stirring rate affected rarely on leaching yield of chromium. After secondary leaching, the leaching yield of chromium had become91.02%.(2) The present study used732strongly acidic cationic exchange resin to adsorb Cr3+in acid leaching liquor, which investigated the effects of the resin’s amount, pH, contact time, temperature in the adsorption process. The results showed that732resin had a high adsorption rate for Cr3+adsorption. The optimum condition:the amount of resin was0.7g, pH2, the contact time60min and the temperature30℃, leading the adsorption rate of Cr3+to96.1%. The data for the uptake of Cr3+by resin had been processed in accordance with linear for of Langmuir isotherm and Freundlich equation. The analytical results demonstrated that Langmuir isotherm equation gave a better fit with experimental data. Pseudo second order model had better fitting correlation between732resin and Cr3+in the analytical procedure of kinetic data. According to the analysis, the rate of the sorption process was mainly limited to the particle diffusion. The sorption process is spontaneous and endothermic.The metal impurities Ca2+and Mg2+in acid leaching liquor would also be adsorbed during the reaction process. Ca2+and Mg2+had little effect on the Cr3+adsorption efficiency in the initial stage with high concentration of Cr3+. On the contrary, when Cr3+was at low concentration, Ca2+and Mg2+could compete effectively with Cr3+for bonding sites.HCl/H2SO4was used to regenerate the saturated resin. The contact time and concentration of HCl/H2SO4were investigated to reach the maximum desorption percent. The results showed that, after secondary8%HCI/H2SO4regeneration, the desorption rate of Cr3+was95.46%/97.22%. Thus, the products obtained were chromium sulfate solution (concentration of Cr3+, Ca2+and Mg2+was485.094mg/L,7.174mg/L and15.015mg/L, respectively) and chromium trichloride solution (concentration of Cr3+, Ca2+and Mg2+was478.6322mg/L,19.6734mg/L, and15.465mg/L, respectively). In order to improve the utilization rate of resin, the polluted resins were regenerated efficiently. The regenerated resin had high Cr3+adsorption rate of92%. (3) The process "NaOH precipitation-H2O2oxidation-H2SO4acidification-product of Na2Cr2O7" was applied to produce sodium dichromate. NaHSO3was used to reduce Cr6+to Cr3+. When the quality ratio of NaHSO3to Cr6+was5:1, Cr6+was fully reduced to Cr3+. By adding NaOH to adjust the pH of acid leaching liquid to9.0,99%Cr3+was precipitated into Cr(OH)3. H2O2was used to oxidize Cr3+. When pH was10, temperature was60℃, mole ratio of H2O2and Cr3+was2.5:1, and H2O2was added to oxidate Cr3+to Cr6+after2h, the conversion rate reached98.01%. Then, by adding H2SO4to adjust the pH to3, the available product was Na2CrO4with the purity of96%.(4)"Acid leaching-ion exchange method/chemical precipitation method-H202oxidation-product" process was applied to recover Cr from chromium-bearing tannery sludge. The process was easy to operate and low cost. The obtained products as chromium sulfate solution and chromium trichloride solution could be used in various fields, such as trivalent chromium plating. The obtained product as sodium dichromate was raw material of chromium salt, which could be used in smelting metal chromium and chromium carbide. What’s more, sodium dichromate could be reused in leather industry by producing basic chromium sulfate. Thus, this study realized the recycling of tannery sludge in various fields, and limited the generation of a large amount of waste.All above showed that "acid leaching-ion exchange method/chemical precipitation method-H2O2oxidation-product" process was feasible, which was benifit for economy and environment. |
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