Question: In addition to zinc, nickel, copper and chrome, we have a large area in which we phosphate. In addition to classic zinc phosphating, we also have manganese phosphating. In recent months, we have repeatedly encountered problems with the limit values. The situation escalated when an official inspection found that the value in the final inspection was ten times the permitted value. Can you give us any tips on how to get the problem under control permanently?
Answer: As the authorities are now "in the game", it will not only be necessary to solve the problem at a technical level, but also to prove that you are doing everything possible to avoid such incidents in future. In addition to the actual problem, it is therefore important to create trust, as otherwise you may face increased inspections and possibly new requirements. We will take this into account in our considerations below.
Sources of phosphate
Phosphate does not have to come exclusively from phosphating. Phosphates are mainly found in degreasers. In plastic metallization, there are phosphorus-containing reduction solutions and corresponding rinsing solutions, which may contain other phosphorus compounds in addition to phosphinates and phosphonates. It can also be a component of other electrolytes and processes that contain phosphoric acid, for example.
You should fully determine where phosphates occur in the plant and laboratory (if this wastewater also ends up in the wastewater treatment plant and is not disposed of separately) and in what concentration. This also applies to eluates.
Controls and analyses
Experience has shown that wastewater values are mainly checked where they are suspected. Due to the separation of wastewater in separate storage tanks and appropriate wastewater treatment, phosphate values are therefore mainly checked for batches in which the phosphating wastewater has been treated. If the phosphate comes from other sources, it is quite possible that the phosphate values were checked less or not at all and you will therefore miss them.
Also carry out a detailed analysis of the previous data to identify patterns or causes for the exceedances. You should also question your analysis methods. Are the methods up to date and reliable? Are they being carried out properly?
Process optimization
It is generally advisable to avoid as much wastewater as possible instead of detoxifying it. Once you have identified all sources, you should consider where you can avoid or at least reduce it. There will be little room for maneuver when it comes to phosphating; recirculation via appropriate rinsing systems is more likely to come into play here. In degreasing, you should check whether you can use substitutes or even do without them completely.
Phosphates are divided into orthophosphates and polyphosphates in cleaning solutions. Orthophosphates (PO43-) have a good dispersing capacity, i.e. they keep insoluble substances finely dispersed in suspension. Polyphosphates have good lime soap dispersing properties in the slightly alkaline range. Polyphosphates are complexing agents and have a stabilizing effect. For example, the hardness formers in the water are complexed and dirt particles are suspended. It is therefore not so easy to do without phosphates in degreasers, but you should still contact chemical suppliers and ask for phosphate-free alternatives.
You should also optimize the addition of chemicals. Check whether the concentrations and quantities are dosed correctly.
Waste water treatment
Orthophosphate can be reduced to a limit value < 2 mg/L P by precipitation with calcium, aluminum or iron compounds. Precipitation takes place at pH 8.0 - 9.5. Polyphosphates, pyrophosphate or tripolyphosphate cannot be precipitated in this form as they are complexing agents. In order to fall below the limit value, polyphosphates must first be converted into orthophosphate. This is achieved by lowering the pH value to < 2.5. Retention time at least 15-30 minutes. The splitting of the polyphosphates must be carried out before the metal precipitation.
In some cases, complete conversion is already achieved at pH values of just < 3.0; depending on the literature, a pH value of < 2.0 is also specified. The different specifications are usually based on the dosing and stirring technology. The more HCl has to be added and the slower the pH value falls, the closer the pH value can be to 3.0. The faster the pH value falls and the less acid has to be added, the closer the pH value should be to 2.0.
Then precipitate with calcium hydroxide:
H3PO4 + Ca(OH)2 → CaHPO4 + 2 H2O
Solubility = 70 mg/L at pH > 9.5
3 H3PO4 + 5 Ca(OH)2 → Ca5(OH)(PO4)3 + 9 H2O
(calcium hydroxophosphate (hydroxyapatite))
Solubility = 3 mg/L PO43- corresponds to approx.
1 mg/L phosphorus.
The precipitation of the phosphate is also favored by the addition of iron salts. Polyphosphates are precipitated by adding aluminum, iron or magnesium salts.
The addition of metal salts results in an anion exchange. (PO43-) is removed and (Cl-) or (SO42-) is released into the water. Divalent iron can only be used successfully if it is oxidized to trivalent iron in water containing oxygen. This is rarely the case in waste water from electroplating plants.
Another method is UV oxidation. By combining hydrogen peroxide and UV light, the phosphine ions can be oxidized to phosphine ions or further to phosphate ions.
H2PO3- + OH→ PO3H- + H2O
H2PO2- + OH→ PO2H- + H2O
2 PO3H- + H2O→H2PO4- + H2PO3-
PO3H- + OH⇄ H2PO4-
This results in the following overall reaction:
H2O2 + H2PO3-→H2PO4- + H2O
The organic compounds contained in the solution are also partially oxidized by the hydroxyl radicals.
Technical improvements
Consider upgrading or replacing outdated equipment that may no longer be operating efficiently. Implement automation technology for accurate dosing and monitoring of process parameters. This applies equally to production and wastewater plant.
Training and awareness
Train your employees regularly in the specific requirements and techniques of phosphating and wastewater treatment [1, 2]. The same applies to your personnel who carry out the analyses. Promote awareness of the importance of compliance with limit values and the consequences of exceeding them.
External advice
If necessary, bring in external experts or consultants who specialize in phosphating and environmental management or electroplating in general to identify specific problems and develop solutions [3]. A trained outside perspective can often uncover problems that you are now blind to.
Emergency measures
Develop a plan for immediate action if limits are exceeded to quickly control and remedy the situation. Emergency measures are short-term reactions and long-term preventive strategies.
By combining such immediate and preventive measures, you can minimize the impact of limit value exceedances and ensure long-term compliance with legal requirements. It is important that you demonstrate to the authorities in particular that you are prepared for any eventuality in the future. In combination with training and, if necessary, external experts, you will succeed in creating new trust.
LiNKS
[1]Online course Environmental Technology Part 1 - Wastewater Treatment: https://www.galvanotechnik-for-you.de/uebersicht-kurse/umwelttechnik-teil-1-abwasserbehandlung/
[2]Environmental Technology Part 2 - Energy and Recycling Technology: https://www.galvanotechnik-for-you.de/uebersicht-kurse/umwelttechnik-teil-2-energie-und-recyclingtechnik/
[3]Agency Binary Circle: https://www.agencybc.de/
