As a common surface treatment technology, nickel plating is widely used in electronics, automobile, machinery and other industries.
The wastewater containing Ni2 + is harmful to human health and ecological environment. The common treatment methods include chemical precipitation, vacuum evaporation recovery, electrodialysis, reverse osmosis and ion exchange resin adsorption.
- Although the cost of chemical precipitation is low, the solid waste produced by it needs to be treated twice.
- High energy consumption of vacuum evaporation method.
- The electrodialysis and reverse osmosis methods need large equipment investment and energy consumption, and the membrane is easy to be polluted.
There are advantages and disadvantages in the existing nickel containing wastewater treatment processes.
Ion exchange technology is the perfect upgrade of existing nickel containing wastewater treatment process. Because of the advantages of good effluent quality, recyclable useful substances, it is widely used in the treatment of nickel plating wastewater with low concentration and large waste water volume.
Advantages of using ion exchange method to treat nickel plating wastewater
- High efficiency nickel removal can reach the standard: remove heavy metal nickel ions and meet the national emission index requirements.
- Resource value: recovery of valuable nickel from wastewater.
- Recycling: improve the recycling rate of water and save water resources.
- Energy saving and environmental protection: reduce environmental pollution.
With the increasing awareness of the value of nickel plating wastewater treatment resources, ion exchange technology as an effective method of electroplating wastewater advanced treatment has been gradually valued.
Principle
Ion exchange resin is an insoluble polymer with three-dimensional structure. Its functional group can exchange with ions in water.
The Ni2 + ions in nickel plating wastewater were adsorbed by cation exchange resin.
The resin used can be generally weak acid cation resin. When weak acid cation resin is used for exchange, the resin is usually converted to Na type. When the wastewater containing Ni2 + flows through Na weak acid cation resin layer, the following exchange reaction occurs:
2R－COONa＋Ni2＋→（R－COO）2Ni＋2Na＋Ni2 + in the water is adsorbed on the resin, and Na + on the resin enters the water.
When all resin layers reach equilibrium with Ni2 + exchange, regenerate with HCl or H2SO4 of certain concentration, and the following reactions occur:
(R－COO) 2Ni＋H2SO4→2R－COOH＋NiSO4
At this time, the resin is H-type and needs NaOH to change to Na type. The reaction is as follows:
R－COOH＋NaOH→RCOONa＋H2O
In this way, the resin can be put into operation again and enter the next cycle.
After treatment, the waste water can be reused in the cleaning tank, and the nickel sulfate obtained by elution can be reused in the plating tank after purification.
Process plan demonstration
Selection of resin
At present, there are many resins which can treat wastewater containing nickel, and their properties and characteristics are different, so choosing the right resin is a main problem in the process.
There are many resins that can be used to treat nickel containing wastewater, such as weak acid cation exchange resin (chelating resin), and strong acid cation resin can also absorb nickel ion, but this resin is easily affected by the salt and calcium and magnesium in nickel containing wastewater.
The weak acid cation resin (chelating resin) with high exchange capacity, fast exchange speed, easy regeneration, high mechanical strength and small expansion is often used in the wastewater containing nickel.
Pretreatment of resin
Nickel removal chelating resin is sodium type after activation treatment in the factory. Before use, it can be used only by washing with water to about pH 9.
In recent years, as the mobile bed chromium plating wastewater treatment, the ion exchange treatment of nickel plating wastewater was developed to mobile bed nickel plating wastewater treatment.
Its functions are more and more complete, and its land occupation is smaller and smaller.
In order to prevent the equipment from affecting the exchange of wastewater after the discharge and regeneration of saturated resin, there is a spare resin tank on the device.
The equipment has complete functions and convenient operation, including water pump, flowmeter, filter, air pump, resin regeneration system and power control part.
Wastewater treatment process
Wastewater exchange
During operation, the nickel containing wastewater is pumped into the filter by the water pump from the waste water pool.
The wastewater comes out of the filter and flows back into the exchange column through the flowmeter.
The water coming out of the top of the exchange column is the water that has been removed with Ni2 + ions (the downstream water or the upstream water can be selected according to the specific design process requirements). The reaction is as follows:
2R－COONa＋Ni2＋→（R－COO）2Ni＋2Na＋Wastewater treatment process
Under the same concentration of various cations in water, the exchange order of weak acid chelating resin is as follows:
Cu2＋＞Pb2＋＞Ni2＋＞Co2＋＞Cd2＋＞Fe3＋＞Mn2＋＞Mg2＋＞Ga2＋＞＞Na＋- Regeneration of resin
During regeneration, due to the high shrinkage and expansion rate of the resin, the volume of the resin will be reduced by 30-40% after adsorption of saturated Ni2 +. When the resin is regenerated into Na + type, it will return to the original volume.
During the regeneration of the resin, the H2SO4 or HCl solution (3% – 5%) twice of the volume of the regenerated resin is used for countercurrent regeneration, and the direct regeneration reaction is as follows:
(R－COO) 2Ni＋2H＋→2RCOOH＋Ni2＋
After the resin is completely regenerated, wash it with water, and then use 2 times of the volume of regenerated resin 4% – 5% NaOH solution to flow through the resin, and turn the resin into sodium type (after turning into sodium type, Ni2 + is easy to absorb and exchange, and the exchange capacity is larger).
After the transformation, the resin volume will increase by more than 30%. At this time, use soft water (or pure water) to fully wash the resin (about twice the resin volume).
Thus, the whole process of wastewater treatment and resin regeneration is completed. Operation mode
For resin operation and regeneration, whether it is downstream or upstream. Generally, the operation mode can be confirmed according to the actual process.
With the continuous emergence of new large pore type ion exchange resin and ion exchange continuous process, ion exchange technology shows more and more advantages than other methods in the aspects of advanced treatment of nickel plating wastewater and recovery of high price nickel salts.
In order to improve the recycling efficiency of water and meet the increasingly strict discharge standards, it is expected that the ion exchange technology will be combined with the microcomputer control technology, so that the design of equipment will move towards finalization and automation, and create a new pattern in the field of wastewater treatment.
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