Application of Gore membrane filters in the treatment of polluted acid wastewater

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The application of Gore membrane filters in the treatment of acidic wastewater, according to a report by China Water Network on January 23, 2008: The acidic wastewater generated by the smelting and acid production systems of the First Smelting Plant of Tongling Non-ferrous Metals (Group) Company contains complex impurities; it has high levels of heavy metal ions such as copper, lead, and zinc, as well as elements like arsenic and fluorine. The volume of this wastewater discharged is 1,500 m3/h. To thoroughly treat acidic wastewater, it was decided to use chemical flocculation for precipitation combined with Gore membrane filters for treatment. In December 2000, the polluted acid wastewater treatment system was successfully tested in a single trial and put into normal operation. The system has been operating normally for over 3 years, with all technical parameters meeting the design specifications. 　　Wastey acid wastewater from the smelting system and acid production system is introduced into the equalization tank for mixing ； After mixing, the acidic wastewater is pumped to the neutralization aeration tank. Once a certain liquid level is reached, a calcium hydroxide-containing slurry with a concentration of about 20% is added to the tank in order to adjust the pH value to between 7.8 and 8.0; this causes the fluoride ions, heavy metal ions, and some arsenic compounds in the acidic wastewater to precipitate. An appropriate amount of FeSO4 is then added, depending on the arsenide content in the wastewater. Air at a pressure of around 50 kPa is also introduced into the mixture to facilitate stirring and oxidation, ensuring that the reactions take place thoroughly and promoting the oxidation of Fe(OH)2 into Fe(OH)3 colloids with good flocculating properties, thereby allowing the arsenous acid and arsenates in the wastewater to precipitate ； After the aeration reaction is complete, it is determined whether to fine-tune the pH value to 8.5–9.0 based on the level of heavy metal ions in the acidic wastewater ； The acidified wastewater after the neutralization and aeration reaction undergoes solid-liquid separation using a Gore membrane filter; the clear liquid on the upper side meets the required standards and is returned to the production system for reuse, while the bottom stream is filtered through a plate and frame filter press. The filter residue is stored in an environmentally safe manner, and the filtrate is sent back to the neutralization and aeration tank. 　　2 Gore membrane filter The core of a Gore membrane filter is the Gore membrane element. This filter element consists of an expanded polytetrafluoroethylene composite filter membrane and a supporting frame. The expanded polytetrafluoroethylene filter membrane boasts excellent chemical stability, a smooth surface, porosity, and a low friction coefficient; it provides effective filtration by trapping all tiny particles in the liquid on the surface of the membrane. Due to its low friction coefficient, the filter membrane has excellent non-stick properties, making its surface easy to clean. When the filter cake on the surface of the filter membrane reaches a certain thickness, under the control of the controller, backwashing of the membrane surface can be carried out rapidly and automatically, thereby completely removing the filter cake from the membrane surface. 　　3 Evaluation of investment, operating costs, and benefits a. Short process flow and small land area required. In the domestic non-ferrous metal smelting industry, the traditional treatment processes for acidic wastewater involve long procedures, numerous pieces of equipment, complex piping systems, and difficult operations; moreover, the pipes and equipment are prone to blockage. The chemical flocculation precipitation method combined with the Gore membrane filtration process enables the treatment of acidic wastewater and solid-liquid separation in a single step, **simplifying the process flow and reducing the required space to only about 1/3 of that of traditional processes. At the same time, an appropriate bypass design can effectively prevent blockages in equipment and pipelines. 　　b. Low investment costs, short construction time, and low operating expenses. By **simplifying the process flow and equipment configuration, it reduces both the engineering investment and the construction time. The direct investment for this project is nearly 7.5 million yuan; if traditional methods are used, the direct investment is estimated to be between 13 and 15 million yuan. In practical applications, the treatment cost for polluted acid wastewater is 1.5–2.5 yuan per ton, which is significantly lower than the cost of treatment using traditional processes. 　　c. High degree of automation and a favorable operating environment. When the filter cake on the surface of the filter membrane reaches a certain thickness, under the control of the controller, backwashing of the membrane surface can be carried out rapidly and automatically, enabling continuous filtration. This approach overcomes the drawbacks of traditional processes, which involve mechanical separation of solids and liquids followed by natural sedimentation; only after clarification can the supernatant be discharged to meet regulatory standards. It also eliminates the need for multiple filtering units due to the intermittent operation of conventional filtering equipment, thereby reducing labor intensity and improving the working conditions at the site. 　　d. The filtrate meets the **discharge standards**. The levels of harmful impurities in the acidic wastewater before and after treatment are such that the filtrate from the Gore membrane filter can be discharged in compliance with standards. 　　4 Existing Problems The application of Gore membrane filters in the treatment of acidic wastewater has been successful, demonstrating numerous advantages such as a short process flow, small land area required, low investment costs, short construction time, low operating expenses, high degree of automation, and a favorable operating environment. However, for various reasons, there are still some issues that need to be resolved. 　　a. The treated clear liquid has a high calcium content, and it cannot be used directly without treatment; as a result, it is currently discharged directly, failing to achieve the goal of recycling. 　　b. Since the design did not take into account the presence of certain oily substances in the acidic wastewater, coupled with the high calcium content in the treatment solution, scaling occurs on the surface of the filter membranes; therefore, it is necessary to soak and clean the Gore membranes with hydrochloric acid approximately every two weeks during production.