Zinc Phosphate Sludge

Question: We run a zinc phosphate spray coating line where mostly mild steel is treated.

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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:

  1. Increased TA/FA ratio to 20.
  2. Run an alkaline cleaner with no free alkalinity.
  3. Installed sludge filters to continuously remove the sludge.
  4. Performed AA analysis on the sludge to determine Fe:Zn ratios; Fe=7% and Zn=2%.
  5. Adjusted operating temperature to 55-60C.
  6. Adjusted operating pressure to 1.5 bar.
  7. 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. L.V.


Just like the punch line of the old sheep joke, “They do, they always do,” zinc phosphates do build up sludge on the bottom of chemical solution tanks. They do, they always do. 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 late energy crisis and the resultant increase in the price of fuel, many pretreatment systems were and are still being 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.

On the other hand, this 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.

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