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Will a filter press successfully remove phosphate sludge?

A discussion started in 2007 but continuing through 2020


Q. Hi my name is Steve Gray, I work for an appliance manufacturer in South Africa. We are trying to reduce the effects of iron phosphate sludge on our metal pre-treatment process.
Currently we require a clean out of our tanks once a month which is not the end of the world but I would really like to solve the sludge problem and not just treat the "symptoms"

I am thinking of using a filter press to continually filter the iron phosphate solution, with a filter clean out as required. The manufacturer of the filter press is confidant that it will work, but I would like to be sure. I would like to ask if filter presses are used with iron phosphate, and what kind of results I should expect? Can I pump the liquid directly out of the treatment tank into the filter or do I need a clarifier? the filter press manufacturer says his equipment will filter out solids in suspension down to 10 micron. I have no idea of the typical size of iron phosphate precipitate (iron-3 phosphate). Please can you help me better understand the treatment of iron phosphate sludge?


Steve Gray
Project Engineer - Durban, KZN, South Africa


A. I doubt that the filtration will cause any problem with the process because the solution is already designed to be pressurized and sprayed out of nozzles anyway. What could be a problem, though, is that filter presses are meant to be run all the way until they are full and the sludge becomes, although still damp, essentially solidified -- like damp wallboard and thicker than toothpaste. The issue with trying to do this is that the press will slow down very dramatically and run at a dribble for many hours as you approach that endpoint. It probably could de done with two small presses running on a staggered schedule, but it would be best if there is a hopper bottom tank from which to draw the sludge rather than requiring that the full volume of the phosphating tank be passed through the filter.

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

To minimize search efforts and to offer multiple viewpoints, we combined previously separate threads onto this page. Please forgive any resultant repetition, failures of chronological order, or what may look like readers disrespecting previous responses -- those other responses may not have been on the page at the time :-)

Can we use a filter press to remove iron phosphate sludge from phosphoric pickle?

November 15, 2011

Q. Hi, my name is Jens, we just recently installed our powder coating line and the pre-treatment (spray-on) system here in TN and so far everything works just fine except our stage 3. It's our pickle stage (Deoxidine 182B Henkel product with 10% phosphoric acid at 140 °F).
The amount of sludge build-up in the tanks and nozzles requires too much time to deal with. We need to clean the tanks (chemically and mechanically) once a month and the risers and nozzles every day.
Based on the amount of sludge we take out every month we believe we generate ~17 lbs/day (at 1.4 M sqft/year). Doesn't sound much but it accumulates over a months pretty quick.
Will a filter press work in a phosphoric acid bath? If 'yes', what do we have to look for?
What kind of filter media would we need?

Jens Polte
employer - Greeneville, Tennessee, USA

Suggest suitable filter cloth to handle fine particles from phosphating bath

March 10, 2014 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hi,

We are facing filter cloth blinding in the plate & frame filter press due to fine particles coming from phosphating bath. The filter press has chamber plates and barrel neck filter cloths with O-ring. The particle analysis of dry phosphate solids revealed that the mean diameter of the particles are 14 micron. We used woven PP filter cloth of air permeability of 70 liters/dm2.min which got blinded within 2 months of operating and subsequent acid washing of cloth has not improved the filtration and solid collection rate. Later used woven PP filter cloth of air permeability of 6 liters/dm2.min which gave good cake formation better than filter cloth of AP 70 but got blinded within 2 months of operation.

I request your help by suggesting suitable filter cloth which can work for at least 6 months.

Thanks & Best Regards

Raj K.S.
- Bangalore, India.

March 2014

A. Hi Raj. This is the third inquiry on this page about how best to employ a filter press on phosphating sludges. Readers might infer, from the presence of the three inquiries on the topic, that a filter press is the right and generally accepted type of equipment for the task, but I feel that this is definitely not so!

Phosphate sludges build continuously during operation and ought to be removed continuously, whereas a filter press is definitely designed for batch operations not continuous operations. Consider: the press is opened and cleaned; then it is closed; then the solution is pumped through it virtually without restriction for a while; then the flow slows down as the filter cake builds; then it gets really really slow for a very long time as we try to build a high solids filter cake (if opened too soon the press and the cake and the whole operation is just a syrupy mess). While the filter press is operating at a trickle for all those hours, sludge is building up in the phosphate tank and is not not being removed.

Consider instead the suitability of a continuous gravity or vacuum filter, where the solution passes through a continuous slowly moving web of disposable newsprint or other moving belt of some sort. I think it is much better to use filters designed for ontinuous operation, rather than filter presses, which are designed for batch-type operations.

But if you will be continuing to use a filter press anyway, I would suggest that you give strong consideration to the use of filter aid on this filter press, so it fills faster because you're not using 'toothpaste' as your filter media, and that you ensure that you have some sort of clarifier, sludge hopper, decant tank or something of the like which allows you to filter a thin sludge rather than the entire contents of the tank. Good luck.


Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

Phosphating desludging filter in automobile paint process

March 17, 2016 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. I want to know the details about filter use for desludging of phosphating chemical, etc. Mesh size in microns, use without filter press. We want to do filter through gravity flow, as well as info about complete filter press equipment for phosphating desludging. Name of supplier, etc.

Ahmed Ali
Employee - Pakistan

March 2016

A. Hi Ahmed. We appended your inquiry to a thread where I've expressed my opinion that a filter press is the wrong device for filtering phosphating processes. You want, in my opinion, some sort of "continuous", "belt" filter which uses newsprint as a disposable filtering surface; either "gravity" or "vacuum" operated. I think if you use those search terms, along with "filter" and "phosphate", that you will be able to very easily locate suppliers. We don't offer names of suppliers in these forum postings for a number of reasons (why?), but suppliers are certainly welcome to place advertisements on this page as well as in any other media rather than trying to present their ads as public service announcements :-)


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Ted Mooney, P.E. RET
Aloha -- an idea worth spreading
finishing.com - Pine Beach, New Jersey

July 25, 2016

A. Yes, iron phosphate sludge can be removed by filtration system. Basically it is in two forms. It is dissolved and suspended; we can easily remove the suspended particles, and dissolved particle can be removed by converting it to suspended form.

vishal suryawanshi
- Pune, India.

September 18, 2016


mohamed aboulfadl
automotive - cairo EGYPT

A. Hi mohamed. The basic problem with filter presses in this application is that they are devices designed for batch operations and a phosphating process generates sludge continuously, not in batches. The press will handle many liters per minute for, say, the first 12 hours, and then will handle only a trickle or a drip for, say, the next 36 hours. What do you plan to do about the accumulating sludge during the 36 hour period?


pic of Ted Mooney
Ted Mooney, P.E. RET
Aloha -- an idea worth spreading
finishing.com - Pine Beach, New Jersey

March 11, 2020

A. Centrifuge separators are a better choice for removal of phosphate sludge. The separator can remove up to 5 micron size particles. Moreover, since it is attached directly to the phosphate tank, it removes the sludge continuously from the tank. There are no filter media used, hence no clogging or cleaning of filters.

Nitin Vaidya
- Pune, Maharashtra, INDIA

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