Wastewater from the metalworking industry contains high concentrations of heavy metals that harm the environment and pose a threat to public health and biology. Therefore, wastewater needs to be pretreated before it is discharged.
Traditional treatments include chemical methods, heavy metal precipitation, flocculation, sedimentation, and emissions, which are very time consuming. Precipitation of heavy metals reduces the concentration of metals contained. The key is to precipitate the solubility of the metal compound.
That is, if the metal is capable of forming an insoluble compound, the compound can be removed by purification and filtration.
The two heavy metal precipitation methods currently employed are hydroxides and vulcanization processes. Both of these methods involve the reaction of a metal cation with OH- or S2- to form a corresponding insoluble metal hydroxide or sulfide. The precipitation process of heavy metals such as hydroxides is pH sensitive. Therefore, the optimum pH must be determined before wastewater treatment. Most heavy metal hydroxides precipitate at pH 8.5-9.5. Outside this range, the metal hydroxide will re-dissolve or decompose.
Heavy metal hydroxides precipitated as colloidal charged particles require the addition of a coagulant to reduce the charge and increase the size of the precipitated particles. In order to promote sedimentation, a flocculating agent is used to flocculate the solidified particles. The metal floes /mud settle in the purifier or sedimentation tank. If the supernatant reaches the regulatory discharge standard, it can be discharged, and the sludge is filtered by a filter press to form semi-solid waste.
Each step of this treatment needs to be carried out in a separate tank. The entire process requires multiple adjustments of the pH and addition of acid, coagulant, lime or alkali and high molecular flocculant. In addition, this process produces a large amount of hazardous sludge/waste containing high concentrations of heavy metals that require further processing.
Sludge disposal
If the pH drops below the optimum range, the precipitated heavy metals will re-dissolve. The decomposition of municipal solid waste and humic substances produces organic acids (reducing the pH), which in turn increases the solid solubility of heavy metals, and sulfides and cyanides react with these organic acids to form toxic hydrogen sulfide and hydrogen cyanide. gas. Often, waste disposal in landfills or landfills has the least impact on groundwater. However, where these conditions are not met, a collection system is required to collect the leachate and remove heavy metals by sedimentation or other means.
Chemical treatment is used to reduce the exudation rate of heavy metal sludge. Sludge containing heavy metals is treated with chemicals to make them stable or poorly soluble. Most landfills use this approach. However, these treatments increase the cost of high wastewater treatment and typically account for 60% to 80% of waste disposal costs.
2. Stabilization of non-leaching matrix
The best technology for removing heavy metals and other harmful contaminants from wastewater is to keep the non-leaching matrix stable so that once the waste is placed in a landfill, these heavy metals and other harmful contaminants are not hazardous to the environment. This is usually handled by reacting waste with chemicals.
The latest treatment is to use chemicals to remove contaminants from wastewater. This agent has a strong affinity for heavy metals and is less restricted by pH than metal hydroxide precipitation . The metal reacts with the agent at a content of up to 10,000 mg/L or higher. High levels of metal are not limited by pH or oil, surfactants, chelating agents, complexing agents, suspensions, and hardness. This process requires the addition of a chemical to the agitated wastewater of the treatment tank, which forms a dense floating block in the treatment tank, and then rapidly precipitates and dehydrates, which can be carried out on existing equipment. The resulting waste does not require post-treatment. The chemical components in the treatment agent stabilize the pollutants, especially the heavy metals, in the waste, which reduces the solid solubility of the metal. Therefore, the waste passes the toxicity leaching test to meet the requirements of non-hazardous materials.
3. Remove heavy metals by metal removal technology
Metal removal technology (MRT) is a cost-effective technique for removing heavy metals from industrial wastewater. It is called MRT-100 and is a filter tank containing two parts: granular activated carbon (GAC) and methylbenzotriazole (MeBT). When metal-containing wastewater flows through the cell, MeBT combines with metal ions and adsorbs on the GAC to remove contaminants from the water. It is unaffected by pH and does not require adjustment of chemical additives to reduce chemical and labor costs. However, MeBT has a high selectivity for heavy metals and transition metals, so it does not bind non-target metals. This further extends its useful life, reduces frequent replacement of filter media, and ultimately reduces user costs. Once the carbon is saturated, the metal can be removed by a solvent and then reused. In addition, the MRT-100 is simple and efficient.
Summary With the country's emphasis on environmental protection, the metal processing industry must strictly follow the existing regulations and national standards on wastewater discharge, and actively transform into green production and environmentally friendly, in preparation for the long-term development of enterprises.
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