Industrial Surface Chemistry

Zinc Phosphate: Questions and Answers

Zinc Phosphate Sludge

Q: We run a zinc phosphate spray coating line where mostly mild steel is treated. Since the startup of the line, we have experienced problems with excessive sludge buildup. We have tried the following to resolve the matter:

  •  Increased TA/FA ratio to 20.
  •  Run an alkaline cleaner with no free alkalinity.
  •  Installed sludge filters to continuously remove the sludge.
  •  Performed AA analysis on the sludge to determine Fe:Zn ratios; Fe=7% and Zn=2%.
  •  Adjusted operating temperature to 55-60C.
  •  Adjusted operating pressure to 1.5 bar.
  •  Dosed phosphate and accelerator with a dosing pump to maintain concentration levels.

Could you please advise on further alternatives? We have considered changing to iron phosphate, but some of the final product is sent to the coast.

A: Zinc phosphates do build up sludge on the bottom of chemical solution tanks. The amount of sludge is related to the quantity of products processed and the amount of chemical solution in the system, which is somewhat dependent on the size of the chemical solution tank.

Owing to the increase in the price of fuel, many pretreatment systems were and still are built with small processing chemical tanks. This is because the smaller the tank, the lower the volume of processing chemicals that must be heated. This has reduced energy consumption; however, it has increased maintenance costs because the smaller the tank, the greater the sludge buildup.

Another alternative for you is to install larger chemical solution tanks in the phosphate stage.

(Follow-up to zinc sludge question).

I would like to present you with another cause for excessive zinc sludging and the solution.

I have found that continual, excessive sludging in phosphate coating tanks is caused by the reaction of the low pH phosphate coating liquid with calcium in the water. An analysis should show that the primary ingredient in sludge is calcium phosphate (Ca3PO4). Fortunately, calcium phosphate sludge is a non-hazardous solid waste and is easily disposed.

A temporary solution is to create a maintenance cycle where the phosphate coating liquid is periodically removed from the tank, the sludge chipped out and the phosphate coating liquid returned to the tank.

A permanent solution is to change water sources or to install water-softening equipment that will remove the calcium from the make-up water. Even though calcium is the most likely cause of the problem, before anything is done, an analysis must be made of the sludge to be absolutely certain that it is indeed a build-up of calcium phosphate. 

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