Phosphorus-Containing Wastewater Treatment Process in Printed Circuit Board Production

In recent years, with the continuous development of science and technology, the printed circuit board industry has made great progress. Phosphorus-containing wastewater will be generated in the production process of printed circuit boards. This kind of wastewater is mainly produced in the electroless nickel plating process. Phosphorus-containing wastewater Phosphorus present in the water is harmful to the ecological environment, especially the eutrophication of water bodies that has often occurred in recent years, which has attracted widespread attention.

Phosphorus in phosphorus-containing wastewater mainly exists in the form of hypophosphite and phosphite. The wastewater also contains complexes, stabilizers and brighteners, such as: citric acid, tartaric acid, malic acid, glycolic acid, succinic acid, succinic acid acid, acetic acid, etc.Secondary, phosphite reacts with organic matter in wastewater to generate more complex substances, including inorganic salts, complexes, organic matter, etc., making wastewater difficult to degrade. At present, the treatment methods for phosphorus-containing wastewater from printed circuit boards mainly include chemical precipitation method, ion exchange method, biological treatment method, membrane separation method and Fenton oxidation method. These treatment processes and their advantages and disadvantages are now introduced, and according to the actual situation of our company, the process of using Fenton oxidation to treat phosphorus-containing wastewater is determined. The following describes the process of Fenton oxidation to treat phosphorus-containing wastewater.

Comparison of Treatment Processes for Phosphorus-Containing Wastewater

1.1 Chemical precipitation method

The chemical precipitation method is to add precipitants (mainly calcium salts, iron salts, etc.) to the wastewater. Under a certain pH value, the precipitant reacts with the phosphorus-containing pollutants in the wastewater to form insoluble precipitates for removal.

Parker in the United States found that the use of lime milk treatment, although the resulting precipitate volume is larger, but the treatment effect is better than the treatment method using caustic soda.

The advantages of the chemical precipitation method are that the process is relatively mature and practical, and the operating cost is low, but the chemical precipitation method can only remove the orthophosphate and phosphite in the wastewater, but cannot remove the hypophosphite in the water, and the lime milk reaction produces a large amount of waste residue. It must be handled properly, otherwise it will cause secondary pollution.

1.2 Ion exchange method

The ion exchange method mainly uses anion exchange resin to adsorb anionic substances such as secondary and phosphite in wastewater. In 1955, Spoulding first published a patent, using anion exchange resin to treat phosphorus-containing wastewater, which can reduce the mass concentration of phosphite in the waste liquid from 110.8g/L to 50.4g/L.

The ion exchange method can remove the phosphorus pollutants in the wastewater very well, but other anions in the wastewater are also adsorbed and removed, which causes the resin to be saturated quickly and needs to be regenerated before it can continue to be used. The one-time investment of ion exchange resin is large and the equipment is more complicated. This method requires high cost and is not suitable for large-scale promotion.

1.3 Biological treatment

1.3.1 Principle of biological phosphorus removal

Biological phosphorus removal is the use of microorganisms such as phosphorus accumulating bacteria, which can absorb phosphorus from the external environment in excess and in excess of its physiological needs, and store phosphorus in the bacteria in the form of polymerization to form high phosphorus sludge. , out of the system to achieve the effect of phosphorus removal from wastewater. The basic process is as follows:

(1) Excessive uptake of phosphorus by phosphorus accumulating bacteria.

Under aerobic conditions, phosphorus accumulating bacteria carry out aerobic respiration, continuously oxidize and decompose the organic matter stored in the body, and at the same time continuously absorb organic matter from the external environment. Due to the oxidative decomposition, energy is continuously released, and the energy is ADP ((adenosine diphosphate)) Obtained and combined with H3PO4 to synthesize ATP (adenosine triphosphate).

(2) Phosphorus release by phosphorus accumulating bacteria. Under anaerobic conditions, the ATP in the phosphorus accumulating bacteria is hydrolyzed to release H3PO4 and energy to form ADP, that is, formula (5).

1.3.2 Biological Phosphorus Removal Process

The biological phosphorus removal process generally adopts the anaerobic-aerobic activated sludge method, which has the functions of removing organic matter and phosphorus at the same time. The specific method is to add an anaerobic biological treatment process before the conventional aerobic activated sludge treatment system. The pretreated wastewater enters the anaerobic section together with the return sludge (phosphorus sludge), and then enters the aerobic section. . The returning sludge absorbs a part of the organic matter in the anaerobic section and releases a large amount of phosphorus. After entering the aerobic section, the organic matter in the wastewater is degraded aerobic, and the sludge will absorb a large amount of phosphorus in the wastewater. It is discharged in the form of mud to achieve phosphorus removal. The specific process flow is shown in Figure 1. This method has a simple process, does not need to add chemicals, and has low operating costs, but it has a large site area, a large one-time investment, and a low phosphorus removal efficiency. Generally, the phosphorus removal efficiency when treating urban sewage is only about 75%. , The phosphorus removal efficiency of our OCT Plant’s activated sludge method is only about 20%, mainly because the phosphorus concentration in the wastewater is low, below 5mg/l, and the wastewater biochemical time is short, resulting in low treatment efficiency.

4 Membrane separation method

The main principle of the membrane separation method is that under a certain pressure, when the stock solution flows through the membrane surface, many tiny pores densely covered on the membrane surface only allow water and small molecular substances to pass through and become permeate, while the volume of the stock solution is greater than The substances with micropores on the membrane surface are trapped on the liquid inlet side of the membrane and become concentrated liquid, thus realizing the purpose of separating and concentrating the original liquid. Membrane separation method is widely used because of its high separation efficiency, no secondary pollution, and small equipment footprint. Generally, the main equipment used is the reverse osmosis membrane. The reverse osmosis membrane can intercept substances larger than 0.0001 micron, and is the most delicate membrane separation product. It can effectively intercept all dissolved salts and organic substances with a molecular weight greater than 100. The pure water produced by the membrane separation method can be recycled and used.

The main disadvantage of the membrane separation method is that the one-time investment of the reverse osmosis membrane is more expensive, and it needs to be used in conjunction with pretreatment when treating wastewater. Otherwise, the membrane is easy to block, and chemical cleaning is required after the blockage before it can continue to be used, and the treatment efficiency of the membrane is reduced after multiple chemical cleanings. Even lose the selective permeability and can not continue to use.

1.5 Fenton oxidation method

1.5.1 Introduction of Fenton Oxidation

Fenton oxidation is one of the few inorganic chemical reactions named after a person. In 1893, the chemist FentonHJ discovered that the mixed solution of hydrogen peroxide (H2O2) and ferrous ions has strong oxidizing properties, which can oxidize many known organic compounds such as carboxylic acids, alcohols, and esters to inorganic states, oxidation The effect is very significant. However, for more than half a century, this oxidizing agent has not been paid much attention because of its strong oxidizing property. But in the 1970s, Fenton’s reagent found its place in environmental chemistry. Fenton’s reagent with a high ability to remove refractory organic pollutants was used in printing and dyeing wastewater, oil-containing wastewater, phenol-containing wastewater, coking wastewater, and wastewater containing It is widely used in wastewater treatment such as nitrobenzene wastewater and diphenylamine wastewater. When Fenton discovered Fenton’s reagent, it was unclear what oxidant was produced by the reaction of hydrogen peroxide with ferrous ions that had such a strong oxidizing power. More than two decades later, it was hypothesized that hydroxyl radicals might have been generated in the reaction, otherwise, the oxidative properties would not have been so strong. Therefore, a more widely cited chemical reaction equation is used to describe the chemical reaction occurring in Fenton’s reagent, as shown in formula (6).

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It can be seen from the above formula that 1 mol of H2O2 reacts with 1 mol of Fe2+ to generate 1 mol of Fe3+, along with 1 mol of OH and 1 mol of hydroxyl radicals. It is the existence of hydroxyl radicals that makes Fenton’s reagent have strong oxidative power. It is calculated that the oxidation potential of ·OH radical is as high as 2.73V in the solution of pH=4. In nature, the oxidizing power is second only to fluorine gas in solution. Therefore, persistent organic compounds, especially aromatic compounds and some heterocyclic compounds that are difficult to be oxidized by common reagents, are all degraded by non-selective oxidation in front of Fenton’s reagent.

1.5.2 Fenton Oxidation Phosphorus Removal Process

This process draws on the application of Fenton oxidation in other wastewater treatment, and adopts Fenton oxidation to oxidize secondary and phosphite in wastewater to orthophosphate. During the reaction process, Fe2+ is oxidized to Fe3+, Fe33+ and orthophosphate Phosphate radical reaction produces ferric phosphate precipitation, thereby removing phosphorus in wastewater, and the reaction equation is as formula (7) ~ formula (9):

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The Fenton oxidation process can well oxidize secondary and phosphite in the phosphorus-containing wastewater of printed circuit boards to orthophosphate, and form ferric phosphate precipitation when the pH value is greater than 8, thereby achieving the purpose of phosphorus removal. The phosphorus removal rate of this method can reach more than 95% in the phosphorus-containing wastewater of the Circuit OCT Plant.

Fenton oxidation treatment of phosphorus-containing wastewater test

2.1 Instruments and Reagents

Mixer, pH meter, total phosphorus measuring instruments and reagents, 20% ferrous sulfate solution, 27.5% hydrogen peroxide.

2.2 Wastewater quality

The waste water is taken from the washing tank waste water after nickel plating in the chemical gold production line, and the water quality is shown in Table 1.

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2.3 Experimental method

Take 100L of wastewater, adjust the pH to 3~4, add a certain amount of ferrous sulfate and hydrogen peroxide solution, stir and react for a period of time, adjust the pH to 8~9, add a flocculant for flocculation and precipitation, take the supernatant Total phosphorus was measured in liquid.

2.4 Results and Discussion

2.4.1 Analysis of the effect of the ratio of ferrous sulfate and hydrogen peroxide on the removal of total phosphorus

(1) 10L of 20% ferrous sulfate was added, and 0.5L, 0.75L, 1L, 1.25L, and 1.5L of hydrogen peroxide were added to test to determine the total phosphorus removal effect. The results are shown in Figure 2.

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It can be seen from Figure 2 that with the continuous increase of the amount of hydrogen peroxide added, the total phosphorus concentration in the effluent continues to decrease. When 1 L of hydrogen peroxide is added, the total phosphorus removal effect is the best. Continuing to increase the additional amount, the total phosphorus concentration in the effluent does not change significantly. Taking into account comprehensively, the amount of hydrogen peroxide added is 1L.

(2) Add 1 L of hydrogen peroxide, and add 1 L to 10 L of 20% ferrous sulfate solution respectively for testing, and determine the removal effect of total phosphorus. The results are shown in Figure 3.

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It can be seen from Figure 3 that with the continuous increase of the addition amount of ferrous sulfate, the total phosphorus concentration in the effluent continues to decrease. When the addition amount of ferrous sulfate continues to increase, the total phosphorus concentration in the effluent does not change significantly. In comprehensive consideration, the amount of ferrous sulfate added is 4L.

From the analysis of the above two results, when the volume ratio of ferrous sulfate and hydrogen peroxide is 4:1, the removal effect of total phosphorus is the best.

2.4.2 Analysis of the influence of reaction time on wastewater treatment effect

Add 4L of ferrous sulfate and 1L of hydrogen peroxide to 100L of wastewater, and analyze the total phosphorus concentration of the supernatant under the conditions of reaction for 0.5h to 3h respectively. The results are shown in Figure 4.

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It can be seen from Figure 4 that the total phosphorus concentration in the effluent decreases as the reaction time continues to prolong. When the reaction time continues to extend after 2 hours, the change in the total phosphorus concentration in the effluent is not obvious. Taking into account comprehensively, the reaction time was taken as 2h.

2.4.3 Analysis of the effect of precipitation pH value on wastewater treatment effect

After the reaction was completed, the pH values were adjusted to 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, and 10.0, respectively, and the total phosphorus concentration in the supernatant was measured. The results are shown in Figure 5.

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It can be seen from Figure 5 that when pH < 8.5, with the increase of pH, the total phosphorus concentration in the effluent decreases continuously; when pH > 8.5, the total phosphorus concentration in the effluent increases continuously with the increase of pH. Taking into account comprehensively, the pH value of precipitation should be between 8 and 8.5.

2.5 Test conclusion

Phosphorus-containing wastewater is treated by Fenton oxidation method. Ferrous sulfate and hydrogen peroxide are added in a volume ratio of 4:1. The addition amount is 40L of 20% ferrous sulfate solution and 10L of 27.5% hydrogen peroxide per cubic wastewater. The reaction time is 2h, and the pH is adjusted after the reaction is completed. When the temperature reaches 8~8.5, flocculant is added for flocculation and precipitation. At this time, the total phosphorus concentration of the effluent is the lowest, and the treatment effect is the best.

Conclusion

The phosphorus-containing wastewater of printed circuit boards is complex in composition and cannot be removed by simple chemical precipitation. It must undergo special treatment to convert the secondary and phosphite in the wastewater into orthophosphate before adding a precipitant. remove.

There are many methods for phosphorus removal. Wastewater of different scales and needs needs to be treated according to the actual situation. For Shennan Circuit’s phosphorus-containing wastewater, the output is about 10-15 tons per day, and the emission value is usually lower than 0.310×10-6. After comparison, the method of precipitation after Fenton oxidation is the best.

With the continuous improvement of wastewater treatment requirements, it is necessary to continue to study the treatment process of phosphorus-containing wastewater, and finding a process method with better treatment effect and lower cost is the direction of future efforts.

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