The electroplating industry can improve the corrosion resistance of metals, but the electroplating process usually produces huge industrial wastewater, and such wastewater often contains a large amount of chromium ions, which will seriously affect the ecological environment. In particular, the hexavalent chromium ions contained in electroplating wastewater are carcinogenic and must be effectively treated to reduce their components before they can be discharged in an appropriate amount. In the treatment of chromium-containing wastewater, the chemical precipitation method is more commonly used, because it can reduce the hexavalent chromium ions in the electroplating wastewater to trivalent chromium ions that are relatively less harmful, and form precipitation after a special process. The harmful chromium ions are separated, which has the effect of reducing pollution.
The relationship between the existence form of chromium element and its ecological effect
Chromium is a commonly used metal substance in the electroplating industry. Its appearance is silver-white because its oxidation rate in the air is very slow and it is insoluble in water. When used as a coating, it often has high hardness, good corrosion resistance and durability. sex, can play a significant protective role.
In nature, chromium element often does not exist alone, and mainly exists in the form of a compound formed by trivalent chromium ion and hexavalent chromium ion, wherein the compound formed by hexavalent chromium ion usually has both water solubility.
Both kinds of chromium ions are toxic, but the toxicity of hexavalent chromium ions is far more than that of trivalent chromium ions, because trivalent chromium ions exist in a completely oxidized state, and other chemical reactions will not easily occur. Stability, so it is difficult to be excessively absorbed by the human body.
Hexavalent chromium ions have strong oxidizing properties and relatively poor stability, and are easily absorbed by the human body through digestion, respiration, skin and other channels, thereby causing damage to the oxidation and reduction behavior of human cells. If humans stay in the environment polluted by hexavalent chromium ions for a long time, it will have adverse effects on health to a large extent, resulting in heavy metal poisoning.
As a process product of the electroplating process, electroplating wastewater not only contains cyanide, but also contains many heavy metal components, which are extremely harmful. According to the types of heavy metals, electroplating wastewater can be divided into chromium-containing wastewater, zinc-containing wastewater, and cadmium-containing wastewater. Faced with the seriousness of the harm of electroplating wastewater, after several years of research, the treatment methods of heavy metal electroplating wastewater can be divided into physical methods, biological methods, and chemical methods.
In particular, the chemical precipitation method in the chemical method has the advantages of simple operation, quick effect, and less harmful sediment. It is favored by the society and is the most commonly used method in the treatment of heavy metal electroplating wastewater.
Technical analysis of chemical precipitation method
Many electroplating plants use chemical precipitation when treating cadmium-containing electroplating wastewater. The principle of the chemical precipitation method is to add a certain amount of reducing agent to make the hexavalent chromium contained in the electroplating wastewater undergo a redox reaction and convert it into trivalent chromium, and then add alkaline substances to adjust the pH of the wastewater, and then make the hydrogen in it. Oxygen ions react with trivalent chromium, eventually resulting in a flocculent precipitate, chromium hydroxide, in the wastewater. Commonly used reducing agents in the chemical precipitation treatment of electroplating wastewater are Na2S2O5, SO2, FeSO4, etc. The chemical precipitation method mainly uses the strong reducibility of the above-mentioned reducing agents in the face of chromium-containing wastewater, so that the hexavalent chromium ions in the wastewater are reduced to trivalent chromium ions, and then converted into precipitates by other reagents. Complete the precipitation of harmful chromium ions and reduce the proportion of chromium in the chromium-containing wastewater.
Nowadays, most power plants will build two reaction tanks when using chemical precipitation to treat chromium-containing wastewater, one of which is used to adjust the pH of the wastewater so that the subsequently added reducing agent can be fully effective; the other The reaction tank is used for precipitation, and hydroxide and flocculant are added to precipitate chromium ions.
Influencing factors of chemical precipitation method
3.1 The amount of reducing agent added
When using chemical precipitation to treat chromium-containing wastewater, the most important procedure is the addition of reducing agent. By analyzing the calculation formula of chemical reaction, it can be concluded that in order to ensure the effect of the reduction reaction, that is, to remove the chromium element to the maximum extent, it must be based on the equilibrium state of the chemical reaction, so the addition amount of the reducing agent should be neither too much nor too much. Less, to strictly abide by the requirements in the reaction calculation formula. Under normal circumstances, the reaction effect is best when the amount of reducing agent added is 20 to 23 times the total amount of chromium ions.
3.2 Control of pH value
In the process of chemical precipitation, the pH value will have a decisive impact on the result of the reaction, so in the entire treatment process, the relevant personnel should make a reasonable control of the pH value. The first step of the chemical precipitation method is to reduce the hexavalent chromium to trivalent. During this process, the pH of the chromium-containing wastewater shows an upward trend, which is mainly acidic. The pH was maintained between 2-3 until the reaction was completely terminated. The second part of the chemical precipitation method is precipitation, in which a certain amount of hydroxide will be added. At this time, the pH value of the wastewater will gradually rise to about 9. The experimenter must maintain an alkaline environment to ensure that the three prices can be Full production of chromium hydroxide precipitate for maximum chromium removal.
3.3 Chromium concentration in wastewater
In the reaction process of the chemical precipitation method, chromium ions are the reacting substances throughout, so the concentration of chromium ions in the wastewater will also affect the treatment results. Especially when the concentration of chromium ions in electroplating wastewater exceeds 220 mg/L, it is easy to cause the removal effect of chromium ions to be far from the ideal effect. Therefore, before using chemical precipitation to treat chromium-containing wastewater, relevant units must determine the concentration of chromium ions in wastewater by scientific testing methods, so that the experimenter can choose the most effective treatment method, and it is also helpful to determine the amount of reducing agent added. ,avoid wasting.
Characteristics of chemical precipitation method
4.1 Advantages of chemical precipitation
Using chemical precipitation to treat chromium-containing electroplating wastewater has the advantages of simple principle, economical benefits, and fast reaction speed. Because the principle of chemical precipitation is a redox reaction, the process of the reaction is relatively simple and easy to understand, making many experimenters competent for this task. In addition, the strong reducing agent used in the chemical sedimentation method is not limited to one, but has a variety of options, so that enterprises can choose a reasonable reducing agent according to the degree of harm of the wastewater when treating wastewater, and the method has a negative impact on the impact of the sediment. The requirements for separation equipment are not high, which also reduces the processing cost of the plant. Finally, the chemical precipitation method can complete the reduction reaction in a short time in the treatment of chromium-containing wastewater, thereby precipitating the chromium ion precipitate, which is beneficial to the factory to save time and cost.
4.2 Disadvantages of chemical precipitation
The chemical precipitation method still has certain problems in practical application. For example, in the control of the addition amount of the reducing agent and the pH value, the operation process is very cumbersome. Once a mistake is made, it will cause an irreversible failure, resulting in a waste of costs. Secondly, the method is to reduce the hexavalent chromium in the waste water to trivalent, and finally precipitate in the form of a precipitate to complete the removal of the toxicity of the chromium element, but the recycling of the precipitate is not realized, and in the subsequent treatment of the precipitate Secondary pollution can easily occur.
To sum up, due to the needs of industrialization, the electroplating industry has been rapidly expanded, which has led to an exponential increase in the discharge of electroplating wastewater, which has caused serious damage to the environment around the electroplating plant. The chemical precipitation method is based on redox reaction, which can effectively treat chromium-containing electroplating wastewater, and the method is simple in principle, low in execution cost, fast in sedimentation, and can complete the precipitation treatment of chromium ions in wastewater in a short time. , so it has been widely used. However, the chemical precipitation method also has certain risks, and it only achieves the removal of chromium ions, but does not achieve recycling. Therefore, relevant departments should increase the research and development of this technology, and strive to achieve the treatment and secondary development of chromium-containing electroplating wastewater through parameter control in the treatment process and analysis of precipitated substances.