Study On Comprehensive Treatment And Disposal Of Wastewater Containing Fluoride And Chromium And Chromium Residue

With the rapid development of industry, environmental pollution caused by industrial wastewater and waste sludge has gradually become widespread and severe, threatening human health and safety. Due to various industry types and different manufacturing technological processes, the pollutant varieties of industrial wastewater are numerous, the water quality fluctuation range is large, the pollutant toxicity is strong and the discharge is huge. Therefore, the industrial wastewater treatment methods differ a lot according to different wastewater types and pollutant varieties and according to different water qualities, the most proper technological process shall be determined to obtain optimal treatment effect. However, the sludge components in the industrial wastewater treatment process are complex and the treatment difficulty is extremely huge. If they are not properly treated, it is easy to lead to secondary pollution of the environment. Therefore, the hazard-free treatment, reduction and reclamation of industrial sludge have been paid wide attention to. Specific to the practical problems of fluoride-containing wastewater, chromium-containing wastewater and chromium-containing waste residue in the treatment process in one industrial park, this paper put forward feasible technical solutions.Specific to the unsatisfactory treatment effect of defluorination using coagulation sedimentation by calcium salt, huge quantity of lime and large sludge quantity, theoretical analysis and laboratory bench-scale study were conducted on defluorination mechanism by calcium salt coagulation sedimentation and technological parameters. The concentration of soluble fluorine ion in aqueous solution was related to the pH value of solution and the concentrations of Ca2+. The concentration of effluent fluoride in fluoride-containing wastewater by calcium salt coagulation sedimentation was higher than the theoretical value. The major reason for this is the limits of lime solubility and the influence of coexistence of anion and cation. Ballasted flocculation process was utilized to treat fluoride-containing wastewater for pilot-scale study. Part of the precipitated sludge was recycled as flocculation carrier, which significantly improved the defluorination effect and the concentration of effluent fluorine ion decreased with the increase of sludge recycle ratio. Ferric chloride coagulant and PAM coagulant aids were jointly used, which could significantly improve the coagulation effect of CaF2particle. The optimal volume of ferric chloride ranged from0.10to0.20mmol/L, the optimal pH in the coagulation process ranged from8.0to8.5, and the optimal volume of PAM ranged from1to3mg/L. Under the optimal technological parameters, operation test was carried out and the mean defluorination and turbidity removal rate by ballasted flocculation process respectively were96.31%and98.00%.Chemical reduction precipitation method is a common treatment process of chromium-containing wastewater. This paper compared and investigated the influence of oxidation-reduction dynamics of Fe（II） and S（IV） on Cr（VI） as well as the influence of pH on oxidation-reduction reaction rate, mainly due to the influence of existing forms of reactants and products and oxidation-reduction potential, Cr（VI） of different pH values and forms and the reductant reaction product and the corresponding product of pH after the reaction. The reaction rate equation of Fe（II） on Cr（VI） is-d[Cr（VI）]/dt=kobs[Cr（VI）][Fe（II）], where apparent rate constantlgkobsis quadratic function of pH, satisfying lgk_obs=6.61-3.38pH+0.43pH2（1.5<pH<7.0）. The reduction reaction of S（IV） on Cr（VI） respectively was the first-order reaction of Cr（VI） and the second-order reaction of S（IV）. The reaction rate equation was as follows,-d[Cr（VI）]/dt=k_obs[Cr(VI_tS（IV）]_t²[H+]_t^0.65. The reaction rate of S（IV） on Cr（VI） is influenced by the pH of reaction system. The correlation between apparent rate constant and pH satisfiedlgk=-0.6466pH+6.1498（1.0<pH<5.0）. The lower the pH value was, the faster the reaction rate was. Under acidic condition （pH<4）, the reduction reaction of S（IV） on Cr（VI） was higher than Fe（II）; while under neutral and alkaline conditions, the reduction reaction of Fe（II） on Cr（VI） was better.Representation was made on the physicochemical property and acid leaching characteristics of chromium-containing waste slag and it showed a strong alkalinity. The major chemical elements included CaO, MgO, Al2O3, Cr2O3, SiO2and Fe2O3, accounting for over90%of the total mass. The contents of total Cr and Cr（VI） in COPR respectively were2.28%,0.80%. The phase composition of chromium-containing slag included Ca2FeAlO5, MgO, CaCO3, CaMg（CO3）2, Mg（OH）2, Ca（OH）2and CaCO3. chromium-containing slag was dangerous waste with leaching toxicity and owned great acid neutralization capacity. After acid neutralization of chromium-containing slag, pH was only relevant to the dosage of H+and the variety of inorganic acid anion had influences on the leaching of total Cr and Cr（VI） in chromium-containing slag. SO42-can replace CrO42-on metallic oxide in amorphous form, which could increase the leaching amount of Cr in chromium-containing slag.This paper compared and investigated the solidification effects of ordinary Portland cement, ordinary Portland cement with slag, ordinary Portland cement with fly ash, Ca（OH）2/slag and Na2SiO3/slag on chromium-containing slag. However, the solidification of Cr（VI） mainly was physical wrapping while lacked chemical fixation mechanism. Properly-adding slag in ordinary Portland cement could significantly improve Cr fixation effect and the optimal ratio of slag to ordinary Portland cement was45%. Adding fly ash in ordinary Portland cement could have adverse effects on the compressive strength and toxicity leaching of the solidification specimen. Under the low requirement of the compressive strength of the solidification specimen, Ca（OH）2/slag was the optimal fixation material of chromium-containing slag, which owned the lowest cost and could achieve better fixation effect under high dosage of chromium-containing slag. In the system of Na2SiO39H2O/slag, the optimal ratio of Na2SiO39H2O fell between15%²5%. When the dosage of chromium-containing slag was less than35%, the compressive strength of the solidification specimen was high and the concentration of toxicity leaching was extremely low. The dosage of chromium-containing slag shall be limited within35%. Fe（II） hydrometallurgical reduction and Ca（OH）2/slag system were used to stabilize and fix chromium-containing slag. After the reduction of Fe（II）, the fixation effect of solidified specimen was improved significantly and the total Cr concentration in the toxicity leaching experiment decreased greatly compared with pure fixation and no Cr（VI） were detected.The transformation of wastewater treatment process in the industrial park was conducted according to the optimized experimental parameters. A more stable and better operational results were obtained compared with the original process. The effluent quality could meet the first grade nationnal discharge standard―integrated wastewater discharge standard‖（GB8978-1996）. The sludge yield significantly reduced with the average sludge production ratio decreased by39.6%. Moreover, the operating costs per ton of wastewater reduced0.71yuan. Finally, a feasible technical treatment solution for residues and sludge in the industrial park was proposed and also a good environmental and economic benefits were gained by immobilized disposition.