Research On Treatment Of Acidic Wastewater Of The Flue Gas Absorbed By Crystal Membrane Distillation

With the increasing demand of mineral resources , the discharge amount of industrial wastewater in mineral resources exploitation and processing is growing. According to statistics, the mineral wastewater of various types is about 10% of the total discharge of industrial wastewater in China. The acidic mine wastewater is one of the most polluted and dangerous in the mine wastewaters. In American, about 70% of the acid in water body comes from the acidic mine wastewater. Acidic mine wastewater can not be recycled directly; it is discharged into the rivers, lakes and other water bodies nearby mines. These behaviors cause the pH change and so on. Furthermore, it also restrains the growth of bacterium and microbe, hinder the self-purification of water. Acid mine wastewater interact with minerals and then produce some certain salts which will have bad impact on the growth of freshwater biology and plant and even threaten the lives of animals and plants. Polluted by acid in a long term, the natural water quality will be deteriorated gradually and damage the ecological environment seriously. Acidic wastewater of the flue gas absorbed is acidic absorption which absorbed flue gas of SO₂ and SO₃ by water. Its major pollutants are SO₄^2- and Cl-. High acidity and difficulty to treatment are its main characteristics. It is known that the method of removal of SO₄^2- is barium method, calcium method, frozen method and biological method etc. The ways of removal Cl- are ion exchange, adsorption and extraction. Now there is no practical way to recycle and reuse the components and separates SO₄^2-and Cl- at the same time. So, it is necessary to invent a new way which is sustainable development to deal the acidic wastewater of the flue gas absorbed.Membrane distillation is a relatively new separation technology which developed in recent years. Compared to traditional distillation technology and other membrane separation technologies, The excellent characteristics of them are low running pressure,temperature and high energy utility. So it was used widely in desalination of seawater, the preparation of ultrapure water, concentration of juice, concentration, crystallization and separation of volatile substances, and so on. This process has advantages of simple equipment and easy to operate and enlarge and can also obtain high purity liquid solution and make solution supersaturated.JiAn liyuan gold Ltd provided the wastewater. The water came from the treatment of concentrated gold mine with high concentration of S and C. Technics of"DL-chloride and sulfur oxidation pretreatment and 3YL soak-comprehensive recovering multimetals of refractory gold concentrated mine was used in the company. Acidic wastewater comes from the ore roasted process of pretreatment of one stage oxidation roasting sulfur with chloride which try to remove S and C and reuse the valuable metals. Some certain proportion of the solid chloride agent was added to the gold concentrated mine, the roasted atmosphere and temperature were mastered by controlling air volume to remove S and C during the roasting process. Most iron of roasted concentrated mine was oxidized to Fe₃O₄, S entered the smoke in the form of SO3, C entered the smoke in the form of CO₂ and CO. The smoke produced in the roasting process was absorbed by the water in absorption tower after be filtered and then became acidic wastewater of the flue gas absorbed.According to above paragraphs, these characteristics of the wastewater were high acidity and high concentration of SO₄^2-. The neutralization pretreatment was combined with crystal membrane distillation to deal with the wastewater. Pretreatment used calcium carbonate ore, which produced in the gold mine, to neutralize acidic wastewater of the flue gas absorbed. The reasons for the selection of this process were that the massive calcium carbonate ore and about 50℃heat in the production process of roasting ore could be provided by the gold mine. In the process of pretreatment, some important factors, such as the calcium carbonate ore size, the dilution effect of initial water sample, the effect of chloride ion concentrations in the process of neutralizing, Fe²⁺ and surplus sulfate removal were observed. In the process of crystal membrane distillation, we studied the effect of flux and ion rejection ratio under different experimental conditions. We discussed the different experimental conditions and the feasibility of this process. In the process of pretreatment, we came to a conclusion that under the conditions of the ore size 5mm, feed solution dilute multiples 6, aeration time 10 min, saturated calcium carbonate 7mL/L, the effluent could enter the membrane distillation equipment directly.In the process of membrane distillation, the optimum conditions were determined that temperature 50℃, the velocity of flow 18.5mL/min, the pH of feed solution 7 from discussing influenced factors. The CaCl₂ crystal in the concentrated feed solution began to appear after 18 hours running in these conditions and the crystal was preferable, the content of Cl- in effluent was less than 100 mg/L, meeting the Discharge Standard (250 mg /L) completely. The effluent could not only be discharged but also used as backwash water. The CaSO₄ produced in the process of pretreatment could be used as materials in the cement plant directly. The CaCl₂ produced in the process of membrane distillation could be sent to the upper roasting process. All of these saved the cost of productions, reused the resources and eliminated the secondary pollutions.In a word, the technics used in this paper made full use of the heat and mine ore, utilized the minerals comprehensive, reused the water resources in the premise of protecting environment and eliminating secondary pollution in the production process. This process reduced the operating costs greatly and achieved sustainable development at the same time.