Excessive Sludge Buildup in Zinc Phosphating Bath

A. Hi Sagar. The thing is: "sludge making is very fast" compared to what? Sludge generation is always fast but sometimes it's faster than it needs to be. TSN Sankara and Tony Wagstaff offer good advice right from the beginning, and the helpful comments just keep on coming :-)

It's probably difficult for people from other countries and with other native languages to always use the same anglicized name on this site, but please try because it makes it easier for readers to help when we can search the site for your name and see all of your postings and get a better feel for who you are and what your overall situation is :-)

Luck & Regards,

A. Dear Amitabh Kohi,

Phosphating is a conversion coating process in which metal dissolution by the attack of free acid (phosphoric acid) is the basis of coating formation. The ferrous ions formed at the metal/solution interface gets oxidized to ferric ions which in turn combines with the phosphate and gets precipitated as ferric phosphate sludge.

If your bath is producing more sludge, then check the free acid value. If the free acid content is higher than the recommended level, then it will cause the formation of excessive sludge.

I am not sure about the green colouration of the bath solution.

A. Hi
You have given only basic info, but simply there is nothing you can add to reduce the amount of sludge being produced; however you can look into reducing the amount of sludge produced initially.

1 Is your present filtration/sludge removal system efficient, are pipes becoming blocked, are pumps working correctly.
2 Are you maintaining the correct pointage for total acid and accelerator? High accelerator causes higher sludge levels.
3 Are you getting carryover from the cleaners? Any alkali going into the phosphate tank will cause sludge to be produced.

This is just the start, there is no magic additive.

A. Without further information I agree with Tony Wagstaff.
In principle the loss of "green colour" is due to the loss of nickel content of the bath. This will be due to mechanical effects or chemical instability resulting from the wrong TA/FA ratios or the wrong Zinc Phosphate replenisher.
The excessive sludge will be controlled by correct TA/FA ratios, accelerator content and no excessive metal loss, in principle the metal loss should be 50% of the coating weight.
Tank turn-over should be between 4-7 per hour and avoid high temperature above 60 °C.
Good filtration is always important using filter press or paper.

A. Mr. Kohli,

If the green colouration of your bath is due to the nickel salt then I agree with Martin Ings that the loss in green colour is due to the loss in nickel content.

What is the nickel content of your bath?

If you are losing nickel (loss in green colour) then is it also precipitating along with the sludge with ferric phosphate?

Ferric phosphate is white colour. What is the colour of your sludge?

It is surprising to note that your bath is producing heavy sludge for a free acid pointage of just 0.6.

A. Phosphating baths are formulated with a zinc salt, phosphoric acid and an accelerator. The amount of zinc salt and phosphoric acid in a bath are chosen in such a way that it contains soluble zinc phosphate with a small excess of free phosphoric acid (Free Acid). The free acid causes dissolution of the metal and enables a raise in interfacial pH (due to the consumption of acid at the metal-solution interface). When the raise in interfacial pH reaches a particular level (called as the point of incipient precipitation), the soluble primary phosphate gets converted to insoluble tertiary phosphate and deposit on the metal substrate.

The equilibrium between soluble primary phosphate and insoluble tertiary phosphate will be altered by a change in pH and temperature of the bath.

If you operate your phosphating bath at a pH or temperature higher than the recommended level, then the conversion of the soluble primary phosphate and insoluble tertiary phosphate will occur throughout the bath and will get precipitated. The tertiary zinc phosphate is also white in colour.

This is the reason why Martin Ings has suggested you to avoid high temperature above 60 °C. Also as suggested by him, check the metal loss and coating weight. The ratio between the metal loss and coating weight (called as conversion ratio) should be around 0.4-0.6.

Check the free acid, total acid, free acid to total acid ratio, pH and temperature of the bath as to meet the recommended level. Also check the conversion ratio. This will give an idea about the amount of metal loss and the quantity of sludge that will be formed.

A. Dear Sir,

The green color of the bath is because the ferrous ions of the material coated in it. First, the material dissolves in the bath, it causes a raise of pH and that results in the precipitation of a tri-basic phosphate layer of zinc and nickel (and iron) over the surface.

The sludge is produced (I think) because of the low free acid in the bath, if the sludge is white it is probably zinc phosphate or iron (ferric) phosphate. They precipitate if the pH is higher than 3 or so.

If it is an accelerated (hydroxilamine/chlorate if it is an internal accelerant or nitrite if it is external) phosphate bath, the sludge is almost only ferric phosphate, you can avoid it only changing the product or asking for a new formulation.

I hope that you can find a solution to your problem, the buildup of sludge is unavoidable in phosphate baths, but they can be minimized maintaining the parameters fixed during the process.

Best regards,

A. Something is not right with the bath itself. Unless you're using it 24 hours per day you should not generate that much sludge. Sludge in a Zinc phosphate bath comes from three sources - one is the water being used to make up the bath, anything in the water will form sludge when the water evaporates, (hardness ions like Calcium and Magnesium make a hard, whitish scale). Another source is the metallic phosphates that naturally form during the processing of metals, being a conversion coating process there are some by-products that will form sludge. The third can be a byproduct of the accelerator.

OK, there's a fourth... excessive drag-in of dirty rinse water or cleaning materials WILL cause excess chemical usage and sludge.

In short, first get the bath under control, then be sure the water going into it is good, then keep the rinse ahead clean. If drag-in is a problem you'll have to address it.

My money is on process control and water issues. Take care of those two things and the savings will pay for the effort.

A. If the temperature is more than 15 or 20 degrees differential, you form zinc phosphate flakes inside the heat exchanger. The temperature must be modulated. That's what makes the flakes. Use a step down heat exchanger and an automatic johnson valve to control the temperature. It works. Good luck.

A. I have seen zinc phosphate baths work with very little sludging. The tanks were heated with steam coils that were electropolished with quick release couplings to switch coils in and out quickly. The electropolish allows the dried sludge to fall off with one hit. One tip is the tank size should allow you to keep parts 6 inches off the bottom so as not to disturb the sludge. Every week let the tank sit overnight and draw off the liquid -- clean the walls and bottom of the tank and put the solution back in your tank. This keeps the tank broken-in, keeps your ratios in line.
If you idle the tank without running work thru it the ratio changes so it is best to have a basket of steel to run to keep the ratio up if you only have the occasional parts.

Hi, Tahir.

It isn't clear to me in what way the thread isn't answering your questions. Can you please post with a lot more words? For example, it's not clear whether you are asking for clarification of previous answers about how to calculate sludge generation rates, or whether you are asking whether you are supposed to leave a certain amount of sludge unfiltered for best operation. It's also not clear if you are asking for formulation information on how to concoct a home-brew solution (some readers are associated with suppliers, and are not at liberty to share their company's trade secrets), or for confirmation of unexpected titration results.

Please try your very best to explain your own particular situation in detail rather than casting questions in the abstract. When you ask abstract questions, the first thought of many readers is probably "Read a book", and only rarely does anyone respond :-(

Thanks!

Regards,

A. Hello dear,

Sludge content in Phosphate bath depends on following some factors as :

1. Activation tank condition. How old is activation bath? Do you have any overflowing/refreshing mechanism of activation tank in regular production?
2. For activation bath should maintain TA<6 & pH<9
3. Keep Phosphate TA, FA & Toner value at lower side of Specification.
4. Keep Phosphate bath temperature at middle of spec. (<55°C)
5. Filtration pump capacity should be such as it will filtrate 1/3rd of phosphate chemical per hour.
6. last but imp. point that Main chemical & toner of phosphate should not be added at one location of phosphate tank to avoid its faster reaction. You can keep main chemical addition at tank entry & Toner addition at tank exit.

Hope this will help you to resolve your problem.

Thank you.

Hi Rajesh. We appended your inquiry to a thread which generally addresses your questions.

If you are having a specific phosphating problem I am confident that the readers can help you. Please tell us what items you are phosphating, what metal the substrate is, what type of phosphate you are dealing with, and why you are phosphating (as a break-in coating, as a pretreatment for powder coating, etc.)

Are you using a proprietary phosphating process from a supplier, or attempting to do a home-brew? Thanks.

Regards,

A. Hi Razan. This thread offers some good insights. Biestek and Weber's "Conversion Coatings" .
suggests that heavy sludging is due to incorrect coefficient "f", or presence of Ca and Al. If you explain your own situation in detail you will usually get more & better response than to abstract questions. Thanks.

Regards,

A. Hello Zain!

Getting rid of the sludge is a batch process, so if now you are pumping the sludge two times per week, if you use the same filter/tank for both processes, you may be using it four times per week instead of the old two.

We have a phosphating line for 1,4 MT coils with one phosphating bath, and two times per week for a 7000 liter bath is enough to clean it up.

Hope that information can help you decide how you make this work!

Best regards,

Daniel

A. Hi cousin Ashwin. We appended your inquiry to one of at least a half-dozen threads about phosphating sludge. Please search the site if you don't see the answers you need on this page.

I don't know of contactless heating methods, nor of a practical way to dissolve the sludge, but I believe there are five possible things to do --
1). Minimize the sludge generation through the process control measures described here.
2). Keep the sludge from adhering, and make it easier to clean, by using electropolished type 316SS wherever possible.
3). Whatever heating method you employ, use 'derated' heaters so they will not be as hot.
4). Try to design hopper-style tanks so there is a desired place for the sludge to accumulate.
5). Remove, filter, and dispose of the sludge in the hopper frequently.

Luck & Regards,

A. Hi Ashwin. There is no way to avoid sludge formation, you can only minimize it, as explained on this page, by proper chemical balance and avoiding high temperatures.

I assume PHE means plate heat exchanger? My personal opinion is that an external heat exchanger is not a good way to heat the phosphatizing solution: heat exchangers rely on high fluid flow rates to generate high U factors so that high temperatures and smaller heat exchange areas can be used. This is the opposite of the low temperatures and high surface area desired for heating the phosphate solution.

Oversized (derated) electropolished stainless steel heating coils in the phosphatizing tank are (in my opinion) a better approach.

Regards,

A. I used to be an on site service technician for a treatment company many years ago at an automotive manufacturing facility. The zinc phosphate system at that time generated a lot of sludge. My current employer a coil coater, in the last few years has been using zinc phosphate on HDG as a preferred system where previously it was only used occasionally. Apparently some break-though was made by my former employer which greatly reduced the sludge production.

The system here uses a heat a exchanger that uses thermal fluid. This probably also helps.

The sludge can be cleaned out with HCl but the tanks and piping must be rinsed well after to prevent what they call white nubbing.

A. Hi Josh. Although your process is iron phosphate rather than zinc phosphate, your question is so similar to Ashwin's that we put them on the same page. I personally believe that heating coils within the tank are a better idea than external heat exchangers for phosphating solutions.

Regards,

A. Thanks very much Aaron! I once read a 'Hints from Heloise' newspaper column where a woman wrote that she spilled red wine on her blouse, and Heloise's answer began: "If you can catch it before it starts to dry ...". Seriously? The writer had composed a letter, mailed it, awaited it's selection and syndicated publication in a newspaper. The stain had certainly started to dry :-)

The point being that, while we try to answer people's inquiries in timely fashion if we can, there are (just like with Heloise) far more silent readers than posters. 37 people read this page in the last month, spending an average of 7 min 22 secs on it -- so your posting will be seen and appreciated. There are no "dead" threads on this site -- only threads with recent input and threads without. Thanks again!

Regards,