One, chemical precipitation
Chemical precipitation is to make the wastewater in the dissolved state of the heavy metals into insoluble heavy metal compounds, including neutralization and precipitation and sulfide precipitation method.
1, neutralization and precipitation method
In the heavy metal-containing wastewater, add alkali neutralization reaction, so that the heavy metal generation of water-insoluble hydroxide precipitate form to be separated. Neutralization and precipitation method is simple to operate, is commonly used in the treatment of wastewater. Practice has proved that in the operation of the need to pay attention to the following points:
(1) neutralization and precipitation, wastewater, if the pH value is high, need to be neutralized and treated before discharge;
(2) wastewater is often a variety of heavy metals **** exist in the wastewater when the wastewater contains Zn, Pb, Sn, Al and other amphoteric metals, the pH value of the high, may be a tendency to re-dissolve, and therefore strict control of the pH, the implementation of sectional precipitation;
(3) the wastewater is often a heavy metal **** exist in the wastewater. Implementation of segmented precipitation;
(3) some anions in the wastewater, such as: halogens, cyanogen, humic substances, etc., may form complexes with heavy metals, so to be in the neutralization before the need for pretreatment;
(4) some particles are small, not easy to precipitate, it is necessary to join the flocculant to assist in the generation of flocculant precipitation.
2, sulfide precipitation method
Add sulfide precipitant to make the heavy metal ions in wastewater to generate sulfide precipitation from the wastewater removal method.
Compared with the neutralization and precipitation method, sulfide precipitation method has the advantage that: the solubility of heavy metal sulfide is lower than the solubility of its hydroxide, the optimal pH value of the reaction is between 7-9, and the treated wastewater does not need to be neutralized. The disadvantages of the sulfide precipitation method are: sulfide precipitates have small particles and are easy to form colloids; the sulfide precipitant itself remains in the water and generates hydrogen sulfide gas when it encounters acid, resulting in secondary pollution. In order to prevent secondary pollution, in the wastewater to be treated selectively add sulfide ions and another heavy metal ions (the heavy metal sulfide ion equilibrium concentration of sulfide ions than the need to remove the heavy metal pollutant sulfide equilibrium concentration is high). Since the sulfide of the heavy metal added is more easily dissolved than the sulfide of the heavy metal in the wastewater, the original heavy metal ions in the wastewater are separated before the heavy metal ions added, and at the same time the generation of hydrogen sulfide and the problem of the residual sulfide ions can be effectively avoided.
Two, redox treatment
1, chemical reduction method
Cr in electroplating wastewater is mainly in the form of Cr6+ ions exist, so the wastewater to the addition of reductants will be Cr6+ reduced to slightly toxic Cr3+, add lime or NaOH to produce Cr(OH)3 precipitation separation and removal. Chemical reduction method of electroplating wastewater treatment is one of the earliest application of the treatment technology, has a wide range of applications in China, the principle of its treatment is simple, easy to grasp the operation, can withstand the impact of large amounts of water and high concentration of wastewater. According to the different reductants, it can be divided into FeSO4 method, NaHSO3 method, iron chip method, SO2 method and so on.
Application of chemical reduction method to deal with Cr-containing wastewater, alkalization is generally used lime, but more waste residue; with NaOH or Na2CO3, there is less sludge, but the cost of pharmaceuticals is high, the treatment cost is large, which is the shortcomings of the chemical reduction method.
2, ferrite method
Ferrite technology is based on the principle of the production of ferrite developed. In Cr-containing wastewater by adding excess FeSO4, so that Cr6 + reduced to Cr3 +, Fe2 + oxidized to Fe3 +, adjust the pH value to 8 or so, so that Fe ions and Cr ions produce hydroxide precipitation. Pass into the air stirring and add hydroxide continuous reaction, the formation of chromium ferrite. Its typical processes are intermittent and continuous. The sludge formed by the ferrite method has high chemical stability and is easy for solid-liquid separation and dewatering. In addition to treating Cr wastewater, the ferrite method is especially suitable for electroplating mixed wastewater containing more heavy metal ions. China's application of the ferrite method has been decades of history, the treated wastewater can meet the emission standards, more in the domestic electroplating industry.
Ferrite method has the advantages of simple equipment, less investment, easy operation, no secondary pollution. But in the formation of ferrite process needs to be heated (about 70oC), high energy consumption, high salinity after treatment, and has the disadvantage of not being able to deal with wastewater containing Hg and complexes.
3, electrolysis
Electrolysis treatment of Cr-containing wastewater in China has been more than twenty years of history, with high removal rate, no secondary pollution, the precipitated heavy metals can be recycled and utilized and other advantages. There are about 30 kinds of metal ions in wastewater solution that can be electrodeposited. Electrolysis is a relatively mature treatment technology that can reduce the amount of sludge generated and can recover metals such as Cu, Ag, Cd, etc., and has been applied to the treatment of wastewater. However, the cost of electrolysis is relatively high, generally after concentration and then electrolysis economic benefits.
In recent years, the electrolysis method has developed rapidly, and in-depth study of iron filings internal electrolysis, the use of iron filings internal electrolysis principle of the development of dynamic wastewater treatment devices for heavy metal ions have a very good removal effect.
In addition, the high-voltage pulse electrocoagulation system (HighVoltageElectrocagulationSystem) for today's world of a new generation of electrochemical water treatment equipment, surface treatment, coating wastewater and electroplating mixed wastewater Cr, Zn, Ni, Cu, Cd, CN- and other pollutants in the treatment of significant results. High-pressure pulse electrocoagulation method than the traditional electrolysis current efficiency increased by 20% -30%; electrolysis time shortened by 30% -40%; electricity savings of 30% -40%; less sludge production; removal rate of heavy metals up to 96% 99%.
Three, solvent extraction separation solvent extraction method is commonly used to separate and purify substances. As a result of liquid-liquid contact, can be operated continuously, the separation effect is better. When using this method, to choose a high selectivity of the extractant, wastewater heavy metals generally exist in the form of cations or anions, for example, in acidic conditions, and the extractant complex reaction occurs from the aqueous phase is extracted to the organic phase, and then in the alkaline conditions are counter-extracted to the aqueous phase, so that the solvent regeneration in order to recycle. This requires that care be taken to select the aqueous phase acidity during the extraction operation. Despite the superiority of the extraction method, however, the loss of solvent in the extraction process and the regeneration process of energy consumption, so that this method has certain limitations, the application is greatly restricted.
Four, adsorption method
Adsorption method is the use of adsorbent unique structure to remove heavy metal ions is an effective method. The use of adsorption method to treat electroplating heavy metal wastewater adsorbents are activated carbon, humic acid, sea foam, polysaccharide resin and so on. Activated carbon equipment is simple, widely used in wastewater treatment, but the regeneration efficiency of activated carbon is low, and it is difficult to meet the reuse requirements of the treated water quality, which is generally used for the pretreatment of electroplating wastewater. Humic acid is a relatively cheap adsorbent, humic acid into humic acid resin used to treat Cr-containing, Ni wastewater has been successful experience. There are related studies show that chitosan and its derivatives are good adsorbents of heavy metal ions, chitosan resin crosslinking, can be reused 10 times, the adsorption capacity is not significantly reduced. The use of modified seafoam to treat heavy metal wastewater has a good adsorption capacity for Pb2+, Hg2+, Cd2+, and the heavy metal content in the treated wastewater is significantly lower than the comprehensive sewage discharge standard. Another literature reports montmorillonite is also a good performance of clay mineral adsorbent, aluminum-zirconium column-supported montmorillonite in acidic conditions on the removal rate of Cr6+ reached 99%, the Cr6+ content in the effluent water is lower than the national emission standards, with practical applications before summer. At the same time, you can view more technical documents on China Wastewater Engineering Network.
Fifth, membrane separation method
Membrane separation method is the use of macromolecules have the selectivity to material separation technology, including electrodialysis, reverse osmosis, membrane extraction, more than filtration. Electrodialysis is used to treat electroplating industrial wastewater, the composition of wastewater remains unchanged after treatment, which is favorable for the use of back to the tank. Containing Cu2 +, Ni2 +, Zn2 +, Cr6 + and other metal ions wastewater are suitable for electrodialysis treatment, there are complete sets of equipment. Reverse osmosis has been used in large scale for plating Zn, Ni, Cr rinse water and mixed heavy metal wastewater treatment. Using reverse osmosis to treat electroplating wastewater, the treated water can be reused to realize the closed loop. Membrane extraction technology is a highly efficient, no secondary pollution separation technology, the technology has made great progress in metal extraction.
Six, ion exchange method
Ion exchange treatment method is the use of ion exchangers to separate the harmful substances in wastewater, the application of ion exchangers have ion exchange resins, zeolites, etc., ion exchange resins have gel type and large pore type. The former has selectivity, and the latter is complicated to manufacture, high in cost, and consumes a lot of regeneration agent, so it is greatly restricted in application. Ion exchange is realized by ion exchange between the free-moving ions carried by the exchanger itself and the ions in the solution to be treated. The driving force to promote ion exchange is the concentration difference between the ions and the functional group on the exchange agent on the affinity of the ions, in most cases the ions are first adsorbed, and then exchanged, ion exchanger with adsorption, exchange dual role. The application of this material is more and more, such as bentonite, which is a clay with montmorillonite as the main component, with good water absorption and expansion, large specific surface area, strong adsorption capacity and ion exchange capacity, if improved after its adsorption and ion exchange capacity is stronger. But it is more difficult to regenerate, natural zeolite in the treatment of heavy metal wastewater than bentonite has greater advantages: zeolite is a network structure containing aluminum silicate minerals, its internal porous, large specific surface area, with a unique adsorption and ion exchange capacity. Studies have shown that zeolite from wastewater to remove heavy metal ions in most cases is the mechanism of adsorption and ion exchange dual role, with the increase in flow rate, ion exchange will replace the adsorption role occupies a major position. If the pretreatment of natural zeolite with NaCl can improve the adsorption and ion exchange capacity. Through the adsorption and ion exchange regeneration process, the heavy metal ion concentration in wastewater can be concentrated to increase 30 times. Zeolite removal of copper, in the NaCl regeneration process, the removal rate of 97% or more, can be adsorbed and exchanged many times, regeneration cycle, and the removal rate of copper does not decrease.