Will a filter press successfully remove phosphate sludge?

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.

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.

Regards,

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 :-)

Regards,

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.

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?

Regards,

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.