The waste liquid treatment in electroless nickel plating includes the treatment of nickel ions, organic acids, phosphorus and phosphorous.
Author: Marisa
I. Nickel ions
In the electroless nickel plating waste liquid, if there is no complexing agent or the amount of complexing agent is small, sodium hydroxide (concentration of 6 mol/L) can be directly used to adjust the pH value.
According to the concentration of nickel ions in the waste liquid, add an appropriate amount of NaOH to precipitate the nickel ions as Ni(OH)2 for removal. For nickel removal from waste liquid with complexing agent, first use CaO to adjust the pH value of the waste liquid to about 8 to remove most of the organic acid complexing agent, then add CaO or NaOH to the waste liquid to adjust the pH value of the waste liquid to 11-12, so that most of the nickel ions and other heavy metal ions in the waste liquid will undergo precipitation reaction, and then add an appropriate amount of polymer flocculant to accelerate the sedimentation of insoluble matter. During the sedimentation process, add a suitable and appropriate amount of oxidant (potassium permanganate, hydrogen peroxide or chlorine, etc.) to remove hypophosphite and phosphite in the waste liquid, which is beneficial to the precipitation of nickel ions and reduces the chemical oxygen demand (COD) of the waste water.
II. Organic acid
In order to improve the quality of the coating, the stability of the plating solution and the deposition rate of metal nickel, various complexing agents are added to the chemical nickel plating solution. If the waste liquid contains complexing agents such as malic acid, tartaric acid and citric acid, CaO can be used to adjust the pH value of the waste liquid so that these organic acids can be removed by generating corresponding calcium salt precipitation. Since the solubility of calcium malate is relatively large, in addition to the calcium salt method, the method of further increasing the pH value should be used to increase the precipitation amount of the waste liquid and promote the removal of malic acid radicals by generating calcium salt precipitation; while the solubility of calcium tartrate is relatively small, at a pH of about 8, 95% of the tartaric acid radicals will be removed; like calcium tartrate, the solubility of calcium citrate is also very small, at a pH of about 8, 98% of the citrate radicals will be removed.
III. Hypophosphite
Nickel plating waste liquid contains a large amount of hypophosphite and its oxidized product phosphite. Due to the high solubility of calcium hypophosphite, the CaO precipitation method cannot effectively remove hypophosphite, but the CaO added when removing nickel ions will increase the pH value of the waste liquid. At this time, if the temperature of the waste liquid is increased, the hypophosphite in the solution can reduce nickel ions and other heavy metal ions, and the hypophosphite is oxidized to phosphite. If there are more hypophosphites in the waste liquid, appropriate oxidants (such as potassium permanganate, hydrogen peroxide, etc.) can be added to remove them. When the pH value of the waste liquid is around 7, the solubility of calcium phosphite will drop sharply. Experiments show that when the pH value is 5.5-7, the removal rate of phosphite in the plating solution is above 95%. For the phosphite that has not been removed, sodium tungstate can be used as a catalyst to oxidize phosphite to phosphate using hydrogen peroxide; or potassium permanganate can be directly used as an oxidant to oxidize excess hypophosphite and phosphite to phosphate. Add CaO to the waste liquid containing phosphate to adjust the pH value of the waste liquid to above 9.5. The solubility of calcium phosphate is small, and the generated precipitate can be easily filtered and removed. At this time, the phosphorus content in the waste liquid can be reduced to 2-7 mg/L, meeting the requirements for wastewater discharge.
IV. Other treatments
If the nickel and most of the organic acids in the chemical nickel plating waste liquid are precipitated and removed, and the COD in the waste liquid does not meet the discharge standard, the waste liquid still needs to be deeply treated. Under the condition that the pH value is greater than 9, chlorine gas is introduced. At this time, Cl2 mainly exists in the form of CLO-, which has a strong oxidizing ability. If a small amount of copper ions is added to the waste liquid as a catalyst, the waste liquid can be deeply treated, and COD can be easily reduced to the standard of (100 mg/L) for direct discharge. At the same time, organic acids can be oxidized to CO2 and water, hypophosphites and phosphites can also be easily oxidized to phosphates, and phosphates can be easily removed by precipitation reaction.