Views: 289 Author: Site Editor Publish Time: 2024-06-05 Origin: Site
Anodizing technology is widely used as a standard and leading technology in surface treatment in the electroplating industry.
Usually, to have better surface characteristics and glossiness, most metal components, such as aluminum parts, need to undergo the anodizing process, and a dense metal oxide film with a certain glossiness, such as nickel film, is covered on its surface.
In the anodizing process, the metal to be plated, such as nickel, is usually used as the anode, and the metal component to be plated, such as aluminum parts, is used as the cathode. The metal to be plated at the anode loses electrons and becomes nickel ions by electrochemical method, then covers the metal component to be plated under the action of the electric field, thereby completing the electroplating process of the plated metal component.
Usually, before the anodizing process, the metal component needs to be degreased by acid and alkali, and after anodizing, the surface of the plated metal component needs to be sealed. At present, most electroplating companies use nickel acetate as a sealing agent. In this process, the company will produce a large amount of degreasing wastewater, acid, alkali, and nickel-containing wastewater. These wastewaters contain a strictly controlled type of pollutant nickel, so they must be adequately treated before discharge.
Table of contents（Click to go to where you want to see）
Specific process of anodizing wastewater treatment
7.1 Anodic oxidation oily wastewater treatment
7.2 Anodic oxidation pickling wastewater treatment
7.2.1 Chemical Neutralization
7.2.2 Ion exchange treatment technology
7.2.3 Membrane treatment technology
7.3 Anodic oxidation alkaline washing wastewater treatment
7.4 Anodic oxidation phosphorus wastewater treatment
7.5 Anodic oxidation dyeing wastewater treatment
7.6 Treatment of nickel-containing wastewater from anodizing
8. Anodizing comprehensive wastewater treatment
9. Acid recovery in the process section
11. Overview
Anodizing is an electrochemical oxidation process in which a metal or alloy, such as aluminum and its alloys, forms an oxide film on its surface under the action of an applied current in the corresponding electrolyte and specific process conditions. This oxide film has protective, decorative, and other functional properties, such as improving corrosion resistance, wear resistance, hardness, and insulation performance.
The principle of anodizing is mainly to use electrolysis to make metal or alloy parts act as anodes and to form an oxide film on the surface in electrolytes. The formation of this oxide film not only changes the state and properties of the metal surface but also provides additional protection.
Anodizing technology is widely used in many fields, such as mechanical parts, aircraft and automobile parts, precision instruments, radio equipment, daily necessities, and architectural decoration. It can enhance the product's corrosion resistance, wear resistance, and decorativeness, making it more suitable for harsh environments.
In general, anodizing is a vital metal surface treatment technology that provides additional protection and decorative effects by changing the state and properties of the metal surface.
The main process of anodizing treatment:
Degreasing and oil removal → hot water washing → rust removal → cold water washing → phosphating/passivation → cold water washing → anodizing → cold water washing → powder spraying and coloring → sealing treatment → water washing → drying → finished product.
Due to its unique treatment process, anodizing technology uses a large amount of sulfuric acid, nitric acid, phosphoric acid, and nickel-containing sealing agents in the treatment process, which leads to high acid and phosphate content in the wastewater generated in the entire process section. At the same time, different products will produce a large amount of high-color sewage, and the sealing agent will also make some nickel-containing wastewater.
Currently, the treatment process of anodizing wastewater in the aluminum industry is based on "acid-base neutralization-coagulation precipitation." This process requires low investment, mature technology, and relatively stable operation. However, with the increase in wastewater volume, the effluent treated by this process often has problems such as excessive color, ammonia nitrogen, and total phosphorus, significantly impacting the surrounding environment. At the same time, the amount of sludge generated by the entire process is high, resulting in high overall operating costs.
With increasingly stringent environmental protection requirements, some treatment processes add A/O biochemical processes after neutralization and precipitation to remove excessive ammonia nitrogen and total phosphorus. However, the disadvantage of the A/O biochemical process is that after the neutralization and precipitation of anodizing wastewater, the salt content is high, and the biodegradability is poor. Carbon sources must be added frequently during operation, which is troublesome for later maintenance. Therefore, the treatment process after neutralization should be determined according to the actual water quality of the factory's production wastewater.
| Wastewater type | Production process | Wastewater characteristics | Main pollutants |
| Pickling wastewater | Rust removal, anodizing | The wastewater is highly acidic, with a pH value of 1-2 | Sulfuric acid, nitric acid, ammonia nitrogen |
| Alkaline washing wastewater | Degreasing and oil removal | Wastewater is strongly alkaline | Petroleum, sodium hydroxide, emulsified oil |
| Phosphating wastewater | Phosphating, cleaning | Wastewater is acidic | Phosphate, color |
| Dyeing wastewater | Powder spraying | Wastewater has different chromaticity according to different products | Chroma, COD |
| Nickel-containing wastewater | Sealing treatment and cleaning | The wastewater is acidic and has a certain color | Nickel ions, color |
| Comprehensive wastewater | Drips from each process section and floor cleaning | The wastewater is acidic and contains many types of pollutants | Aluminum ions, COD, chromaticity, ammonia nitrogen |
Characteristics of anodizing wastewater
Anodizing wastewater is waste generated during the metal surface treatment process in the electroplating industry. Its hazards are mainly reflected in the following aspects:
Contains toxic and harmful substances
Anodizing wastewater contains heavy metal ions, such as chromium, copper, zinc, etc. These heavy metals are highly harmful to the environment and the human body. Once they enter the environment or ecosystem, they will not be decomposed and may remain, accumulate, and migrate, causing various hazards.
Affecting the ecological balance
The heavy metals contained in wastewater will accumulate in algae and sediment, be adsorbed by the surface of aquatic organisms such as fish and shellfish, and concentrate through the food chain, thereby causing harm to the entire ecosystem.
Threat to human health
Once heavy metals enter the human body and accumulate to a certain amount, they will cause various poisoning reactions, affect human health, and even endanger life.
Causes water quality deterioration
When nitrogen, phosphorus, and other elements in wastewater enter the water body, they will cause the massive reproduction of diatoms, blue-green algae, green algae, etc., resulting in a decrease in dissolved oxygen in the water and an increase in chemical oxygen demand, which will in turn lead to the deterioration of water quality and affect the survival of aquatic organisms such as fish.
Since the properties of wastewater generated by different processes are different, the wastewater generated by different process sections can be collected separately, and targeted physical and chemical pretreatment can be carried out on different sewage.
Oily wastewater mainly comes from degreasing cleaning, electrophoretic cleaning, and grinding cleaning of workpieces, and the primary pollutants are grease and surfactants.
The acid content in pickling wastewater is high, which can be neutralized with caustic soda flakes, thus reducing the amount of sludge produced by about 40%;
Phosphorus-containing wastewater mainly comes from washing water in production processes such as chemical polishing and surface phosphating, and the primary pollutant is phosphate. Although the acidity of phosphating wastewater is high, neutralization treatment with flake caustic soda alone cannot effectively remove phosphates in the wastewater. It needs to be neutralized with lime, flake caustic soda, or magnesium oxide so that the total phosphorus removal rate can reach more than 98%;
Nickel-containing wastewater mainly comes from the sealing process of anodizing. Since most of them use nickel-containing sealing agents, the primary pollutants are nickel ions and their coordination agents. For nickel-containing wastewater, the pH value can be adjusted to 9 with caustic soda flakes, and the nickel removal rate can be as high as 99%;
Dyeing wastewater has different colors due to different products, and the treatment effect of general decolorizers is very poor. Therefore, special decolorizing powder can be used to remove the color in the wastewater, and the color removal rate can reach more than 95%;
Cleaning wastewater mainly comes from the relatively clean cleaning water in each process, with low concentrations of various pollutants and large water volume. The pre-treated oily wastewater, RO concentrated water of cleaning wastewater, wastewater generated by other methods (such as dyeing, anodizing, neutralization, etc.), workshop floor cleaning water, spray tower circulating water, etc., are collected together as comprehensive wastewater.
The composition of comprehensive wastewater is complex and generally acidic. Alkaline washing wastewater can be neutralized with comprehensive wastewater to achieve the effect of "waste treatment with waste" in the pretreatment process of nickel-containing and acid-base oil-containing wastewater. Neutralized wastewater can be treated further according to the actual water quality. The above wastewaters are mixed after different pretreatments and meet the discharge standards after secondary physical and chemical precipitation or A/O biochemical treatment.
For oily wastewater, the main focus is on the high concentration of COD and suspended solids in the wastewater. Most of the COD comes from oil (including emulsified state). Therefore, demulsifier is first added to demulsify part of the emulsified oil, and the relative density difference is used to separate and remove suspended solids and oil by Air flotation, thereby removing most of the COD. After the wastewater is pretreated, it enters the comprehensive wastewater for deep treatment. Figure 1 shows the oily wastewater treatment process.(Learn more about oily and emulsion wastewater treatment).
Figure 1 the oily wastewater treatment process
Enterprises need to use chemical neutralization reactions to treat acidic wastewater. Standard treatment methods include dosing neutralization, filtration neutralization, and recycling. This most basic chemical reaction removes the acidic substances in the wastewater, thereby purifying the wastewater.
When the acid concentration in the acidic wastewater is high, and the composition is relatively simple, it can be recycled. For example, the acid in the acidic sewage can be directly recycled, and the ferrous sulfate in the acidic wastewater can also be recycled.
In addition, acidic and alkaline wastewater can be chemically reacted in the neutralization tank. This treatment method can save enterprises the cost of sewage treatment, and the operation process is relatively simple. Therefore, chemical neutralization has been widely used in acidic industrial wastewater treatment.
The advantage of the acidic wastewater treatment method of dosing neutralization is that it is not affected by impurities and the concentration of acidic wastewater. Commonly used neutralizing agents include alkaline substances such as caustic soda, boiler ash, and lime. When the acid concentration of acidic wastewater is low, it can be treated by filtration and neutralization. The operation process is simple. It mainly allows the acidic sewage to pass through the filter layer directly. The commonly used filter materials are alkaline substances such as limestone and marble.
Currently, aluminum profile factories mainly use acid-base neutralization to treat acidic wastewater. After the acidic wastewater is neutralized, a coagulant is added to the neutralization tank to purify the water quality.
The operation process is as follows: first, the acidic and alkaline wastewater are chemically reacted in the neutralization tank. After the neutralization reaction is completed, the coagulant PAC is added. Purified water quality can meet the standards for cleaning water. The advantage of using acid-base neutralization to treat acidic wastewater is that the operation process is simple. The investment cost is low, and the treated wastewater can be used to a certain extent.
Therefore, acid-base neutralization to treat wastewater is widely used in the acidic wastewater treatment industry.
Enterprises can use ion exchange technology to separate acidic substances chemically when treating acidic industrial wastewater.
The advantage of ion exchange technology in treating acidic wastewater is that it can separate organic compounds with similar structures and inorganic ions in acidic wastewater.
The key to treating acidic industrial wastewater with ion exchange technology is the selection of ion exchange agents. Depending on the composition of the wastewater, the choice of ion exchange resin types will also be different.
Ion exchange resins can be divided into two categories: inorganic ion exchange resins and organic ion exchange resins. Organic ion exchange resins include weak base cationic resins and chelating resins.
The reaction mechanism of ion resins is that the ions in the ion resin dissociate in the solution and then undergo ion exchange reactions with the ions in the industrial wastewater.
The so-called membrane treatment technology refers to fluid separation operation technology.
The membrane divides the fluid into two parts that do not flow into each other. At the same time, the membrane has selective permeability and is selective for substances flowing through the membrane.
With the rapid development of membrane synthesis technology, the application of membrane treatment technology has become more extensive. The industrial field has used membrane separation technologies such as ultrafiltration, nanofiltration, and reverse osmosis.
The use of membrane technology to treat acidic industrial wastewater can effectively purify the wastewater and recycle the usable substances.
Alkali washing wastewater: From the analysis of the water quality of alkali washing wastewater, it is known that the main excessive factors in the wastewater are pH value, COD, BOD5, suspended solids, Pb, and Cu. Because the alkali washing wastewater has high pH, high COD, contains a large amount of emulsified oil, has complex water quality, and a small amount of water (5 t/month), the sulfuric acid neutralization coagulation-oil separation precipitation-evaporation crystallization process is adopted. The condensed water from evaporation is mixed with the cleaning wastewater for post-treatment.
For phosphorus-containing wastewater, the primary pollutant is phosphate. Chemical precipitation methods such as iron salts, aluminum salts, and calcium salts are usually used to remove higher concentrations of phosphate. Considering the high acidity of this wastewater, slaked lime is used as a phosphorus removal agent to control the pH at 10~10.5. Taking advantage of the fact that the solubility of calcium phosphate decreases with the increase of pH value, the phosphate can be treated to 0.5~1 mg/L. After clarification with the aid of flocculant PAM, carbonate is used to remove the residual calcium salt in the effluent before entering the reclaimed water reuse system, which is beneficial to slow down the subsequent RO device fouling. Figure 2 shows the phosphorus-containing wastewater treatment process.
Fig.2 High-phosphorus Wastewater treatment process
The anodizing electrolytic coloring of aluminum profiles generally requires a series of processes such as degreasing, alkaline etching, neutralization, anodizing, electrolytic coloring and sealing.
In general, there are two main types of wastewater and waste liquid in anodizing electrolytic coloring.
One is the washing wastewater that is used to wash the liquid attached to the surface of the aluminum profile after each process. The other is the treatment liquid due to the aging of the liquid and the accumulation of impurities.
Acidic wastewater and alkaline wastewater are neutralized and pH is controlled within the range of 7 to 8. The cations such as Al3+, Sn2+, Mg2+, Ni2+, etc. generate hydroxide precipitation:
2NaAlO2+H2SO4+H2O→Na2SO4+2Al(OH)3↓
NiSO4+2NaOH→Ni(OH)2↓+Na2SO4
MgSO4+2NaOH→Mg(OH)2↓+Na2SO4
SnSO4+2NaOH→Sn(OH)2↓+Na2SO4
Fe2(SO4)3+6NaOH→3Na2SO4+2Fe(OH)3↓
Al2(SO4)3+6NaOH→3Na2SO4+2Al(OH)3↓
After neutralization, the precipitates generated by the reaction are: Al(OH)3, Fe(OH)3, Sn(OH)2, Mg(OH)2 and Ni(OH)2, and the solution contains Na2SO4, among which Al(OH)3 is the main component in the sludge.
The process flow of wastewater treatment generated by anodizing and electrolytic coloring of aluminum profiles is shown in Figure 3.
Figure 3, Process flow of wastewater treatment from anodizing electrolytic coloring
Treatment of nickel-containing wastewater from anodizing
Nickel ions are the primary pollutant in nickel-containing wastewater generated by sealing. Due to environmental protection policies and environmental impact assessment requirements, nickel, and other pollutants must not be discharged. Therefore, for nickel-containing wastewater, most nickel ions are removed by chemical precipitation and then treated by disk filtration, ultrafiltration, and multi-stage reverse osmosis system. The produced water is reused in the sealing process. The final concentrated water of the multi-stage reverse osmosis system enters the MVR evaporator for evaporation and crystallization, and the crystallized salt is outsourced for disposal. Figure 4 shows the nickel-containing wastewater treatment process.
Figure 4, Nickel-containing wastewater treatment process
Anodizing comprehensive wastewater treatment
After the pre-treated wastewater is regulated in the regulating tank, most of the organic matter is removed by coagulation and flocculation. After passing through the filter and UF filtration system, the effluent is divided into two parts, of which about 70% of the wastewater can be reused after being treated by precision filters and two-stage reverse osmosis systems.
The remaining 30% of the wastewater is discharged after coagulation, flocculation, Dissolved Air Flotation, and sand filtration.
Comprehensive wastewater treatment process
Since a large amount of acid is used in the process, the subsequent wastewater neutralization treatment cost is high, the sludge production is large, and the total phosphorus removal cost is high. Therefore, many projects have applied acid recovery technology to directly concentrate and recycle the acid of a particular concentration produced in the process section. The acid recovery rate reaches more than 40%, which not only reduces the acid concentration in the wastewater but also can use the concentrated extracted acid to generate new value.
To achieve sustainable development, some newly built anodizing treatment plants will reuse the anodizing wastewater generated. The specific process is to first neutralize and pre-treat the wastewater according to the situation and then reuse part of the effluent in the workshop after secondary filtration. The other part of the effluent can meet the requirements of reclaimed water after filtering through the ultrafiltration and RO membrane group.
The pollutant composition in wastewater from surface treatment industries such as anodizing is complex and challenging to treat. This article classifies wastewater according to the main pollutant composition, conducts targeted pretreatment of the primary pollutants in various types of sewage, and then collects them into comprehensive wastewater for deep treatment, which reduces the difficulty of wastewater treatment and achieves sound treatment effects. The process has high feasibility.
The nickel-containing wastewater is reused by pretreatment + multi-stage reverse osmosis. On the premise of meeting the water quality of the reused water and zero nickel discharge, a higher recovery rate is achieved, the amount of concentrated water is reduced, and the investment and operating costs of the evaporator equipment are reduced.
Most of the relatively clean production cleaning water is reasonably used as the raw water for the reuse of reclaimed water, to reuse reclaimed water, thereby ensuring that the reclaimed water reuse system can achieve stable operation under a lower load and meet the overall reuse rate requirements.This article comprehensively introduces the anodizing process and the sources of wastewater, the characteristics and hazards of anodizing wastewater, and the treatment process of anodizing wastewater. It also introduces in detail the classification of wastewater according to the main pollutant components, and the targeted pretreatment of the main pollutants in various types of wastewater, and then collects them into comprehensive wastewater for deep treatment, which reduces the difficulty of wastewater treatment and achieves good treatment results. The process has high feasibility.
