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Are Heated Trucks Required for Sulfuric Acid Disposal?

According to recent surveys of the galvanizing industry, approximately 40% of galvanizers utilize sulfuric acid for pickling operations. Of the 40% of galvanizers running sulfuric, 25% do not have an acid recycling unit on site. In such cases where sulfuric acid must be sent for disposal, transportation of liquid sulfuric may or may not require additional logistics beyond general hazardous waste protocol.

Sulfuric pickling acids of 7-12% concentration are typically considered “spent” when the iron content reaches above 6%, but may vary according to the operating parameters. According to the crystallization charts presented in Figure 1, a typical sulfuric pickling acid containing 6% dissolved iron can form ferrous sulfate heptahydrate (iron sulfate crystals) when the acid temperature falls below approximately 45-55 F (7-13 C). As a result, some spent sulfuric pickling solutions may be susceptible to the formation of crystals when they are no longer heated in colder climates and/or winter months depending on the acid properties. These crystals cannot be easily re-dissolved back into the acid (even if re-heated).

Dr Galv April 20 Pic
Figure 1: Solubility of Iron in Sulfuric Acid. As a result of iron concentration (%weight/volume) ferrous sulfate heptahydrate forms above the acid % curve on the left side, while ferrous sulfate monohydrate forms above the acid % curve on the right side. (Image Source: Cullivan, Bryan M., Iron Solubility in Sulfuric Acid, Beta Control Systems, Inc.)

If ferrous sulfate heptahydrate is formed en route to a waste handler when the receiver is expecting liquid sulfuric only, the shipment will not be acceptable to the receiver. Upon delivery, any hazardous shipment must match the hazardous material profile or manifest upon delivery, for example:  all liquid, all solid, or a liquid/solid mixture. 

For solutions susceptible to crystallization, it is possible to:  form the ferrous sulfate crystals by cooling in a disposable container, separate the acid from the crystals, re-use or dispose of the acid, and dispose of the crystals separately.

Other measures to mitigate against the formation of ferrous sulfate heptahydrate crystals during transport involve:

  • Avoid excessive dissolved iron concentrations (greater than 7%) unless an acid recovery unit is utilized. This is especially applicable when outdoor temperatures average below 75 F (24 C).
  • When attempting to extend the use of a pickling tank with high iron content, refer to the chart in Figure 1 to avoid increasing acid strength to a value that causes ferrous sulfate to form at a temperature above the expected shipping temperature. 

Example: According to Figure 1, sulfuric acid typically maintained at 7% and with iron levels at 6%, ferrous sulfate heptahydrate will form below approximately 45 F. If the anticipated shipping temperature is approximately 60 F, Figure 1 tells us sulfuric acid concentrations should not be raised above 15% free-acid (light-green curve) when iron levels are near 6%, nor above 10% free-acid (purple curve) when iron levels are near 7%.

  • Ship using a heated truck. Temperature should be maintained above the temperature ferrous sulfate heptahydrate forms. This value can be obtained from Figure 1 using the evaluated acid properties (i.e. iron content and acid strength). 

Example:  For a sulfuric acid of 10% strength (purple curve) and containing 7% iron, Figure 1 states crystallization occurs at temperatures below 60 F. During winter months in a cold climate where outdoor temperatures are near or below freezing, a heated truck would need to be capable of maintaining a transport temperature above the 60 F threshold.

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