| With the advance of industrialization process, the amount of the three wastes is increased and the pollution capacity of the three wastes to the environment has exceeded the environment self-purification ability. Fluorgypsum is a byproduct of the production of hydrofluoric acid using concentrated sulfuric acid and fluorites in fluorite salt plants. For every ton of hydrofluoric acid, there are approximately 4~5 tons of fluorgypsum produced and the present domestic output of fluorgypsum is more than one million tons per year. The waste fluorgypsum occupies considerable land and affects the surrounding crops and water resources. How to change the waste into the treasure is a very important and urgent problem. Meanwhile, the total emission amount of fluoride wastewater has increased dramatically in millions of tons per year. The fluorine pollution has become more and more serious. How to make the discharge wastewater meet the first level criteria specified in Integrated Wastewater Discharge Standard (GB 8978 - 1996) is a very important and urgent task.The waste fluorgypsum was obtained from Hunan Nonferrous Fluorine Chemistry Co., Ltd., and its main chemical composition and mineral structure were elaborately analyzed. The effects of the ratio of gypsum and water, temperature, soluble impurity, pH value, water retaining agent content and powder grinding fineness on the hydration-hardening properties of the waste fluorgypsum were studied. The acid waste residue from the fluoride salt factory was washed to obtain the acidic modified solution. The acidic modified solution could solve the problem of high basicity when the modified fluorgypsum was used as building materials. The modified technology of the waste fluorgypsum was determined by the results of hydration-hardening properties, and the modified fluorgypsum was abtained. Besides, the production process was proposed. The basic performances of fluorgypsum were tested before and after modification, and the scanning electron microscopy (SEM) was employed to analyze the changes in morphology. Then, the application performances of the modified fluorgypsum were tested and the eight-channel micro-calorimeter (TAM Air) was employed to study the application performance of the modified fluorgypsum respectively. The experimental results indicate that the performance of the fluorgypsum is significantly improved. The morphology of the crystal of the fluorgypsum was changed from block and particle into trimetric and short column. This fluorgypsum crystal staggered mutually and improved the strength of hardened body. The modified fluorgypsum as a substitute for natural gypsum as cement retarder can make the setting time, volume stability, and the SO3 content of cement et al. to meet the national standards. The fly ash and fluorgypsum could reduce the heat of the hydration to avoid the cracking accident of concrete. When the modified fluorgypsum was used as high early-strength cement, the 3-days compressive strength was greatly enhanced comparing to the fly ash single-doped cement, which indicates that the fluorgypsum beared the function of enhancing the early stage compressive strength of concrete. When the modified fluorgypsum was used in plastering gypsum, the performance is in line with national standards.The fluoride wastewater of Hunan Nonferrous Fluorine Chemistry Co., Ltd. can not meet the discharge standard. The two stage fluoride removal of milk-PAC was adopted and the optimum process parameters were determined. Besides, the flocculant effect of PAC and aluminum sulfate also were compared. The results show that the concentrations of fluoride ion in wastewater were under 5.6 mg/L when the process parameters of the first stage were designed like that the pH value was 12 with the value of CaActual/CaTheoretical between 2.0 and 2.5 with the stirring time was between 30 and 50 min, the stirring intensity was 150 r/min, the precipitation time was between 90 and 120 min, and the process parameters of the second stage were designed like that the PAC dosage was 500 mg/L, the pH value was between 6 and 8.0, reaching the first level criteria specified in Integrated Wastewater Discharge Standard (GB8978-1996). The performance of PAC is superior to aluminum sulfate, such as less dosage, strong adaptability of pH and temperature changes, etc.The research subject can really achieve the resource utilization of the waste fluorgypsum and benefits both the environmental and the economic. In addition, the improved treatment process of the high fluoride wastewater is simple, and its treatment effect is good. Besides, this technology has low energy consumption and low reagent dosage. |
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