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How do I calculate acid/water additions to adjust my hydrochloric bath?

The Kleingarn Curve was developed by J-P Kleingarn with VDF Germany and presented in the paper Pickling in Hydrochloric Acid at the Intergalva conference in 1988 with results that have changed the way the galvanizing industry handles ineffective hydrochloric acid solutions. When steel is pickled in a dilute hydrochloric acid solution, the reaction produces soluble ferrous chloride (FeCl2). Once the solution is completely saturated with hydrochloric acid and ferrous chlorides, green ferrous chloride crystals precipitate out of the solution and the pickling reaction can no longer take place and the acid is considered ineffective. Previously, the options for dealing with the ineffective acid included on-site neutralization, off-site disposal, decanting the acid for further use, or other off-site beneficial re-use. Instead, the information from the Kleingarn Curve allows galvanizers to control and adjust pickling solutions by adding water, acid, or both. This results in direct savings to the galvanizer in terms of operating costs.

Essentially, the Kleingarn Curve is the isotherm solubility curve between the hydrochloric acid and iron concentrations in solution at 20C (68F). The Kleingarn Curve depicts an optimum iron concentration for every hydrochloric acid concentration for steel pickling (Figure 1). Ideally, the two concentrations intersect on the green line for optimum pickling rates. The further the point is from the green line, the longer the pickling time. Pickling cannot take place above the saturation line in red. Below the gray line, pickling may still take place, but at a greatly reduced rate. 

Fig 1 Kleingarn Curve General Chart
Figure 1: Kleingarn Curve. Effect of HCl and Iron Concentrations on Pickling Rate

The Kleingarn Curves concentrations are reported in units of grams per liter (g/L). However, galvanizers in North America often report concentrations in percentage by weight. For assistance with unit conversions, the AGA offers curves and figures to assist you.

Practical Implications of the Kleingarn Curve

There are some curious consequences of the curve that may produce different results than expected when regenerating an ineffective acid. For example, let’s assume we have a tank (33 cubic meters) of spent acid with a composition: 4% HCl, 13.2% Fe, and a specific gravity of 1.37 at 20C (68F). One might assume any addition of fresh acid would regenerate the solution and allow more rapid pickling. However, if 12 cubic meters of fresh 32% HCl was added to regenerate the acid, the calculated result after converting acid and iron concentrations to g/L can be plotted on the Kleingarn Curve, where we see the final bath solution results in properties that place it above the iron saturation curve and no pickling can occur (Figure 2). In this case, the addition of fresh acid makes the pickling solution ineffective.

Figure 2 Kleingarn Example 1
Figure 2: Effect of adding 3,170 gals fresh water to adjust 8,737.5 gals HCl pickling bath with initial composition: 4% HCl, 13.2% Fe, and S.G. of 1.37 at 20 C (68 F).

Alternatively, if fresh water were added to the bath instead of fresh acid, the new bath properties can be calculated, converted to g/L, and plotted on the Kleingarn curve (Figure 3).  With this composition, the pickling solution becomes usable, albeit with less than optimum pickling times.

In reality, the adjustments required to bring the pickling bath back toward the optimal pickling curve may involve additions of both acid and water/rinse water. Though the amount of fresh acid may be approximated, a guess-and-check method using small incremental changes to the acid and water amounts is often necessary to determine both the optimal amount of ineffective acid to remove from the bath and the ideal volume of fresh acid and/or water (or rinse water*) to add.

*Note:  Adding rinse water reduces iron concentration at a slower rate than adding freshwater. However, the addition of rinse water is sometimes preferred when trying to stabilize or raise the iron concentration (correcting a high acid content, or makeup of new bath).

Figure 3 Kleingarn Example 2
Figure 3: Effect of adding 3,170 gals fresh water to adjust 8,737.5 gals HCl pickling bath with initial composition: 4% HCl, 13.2% Fe, and S.G. of 1.37 at 20 C (68 F).
General Guidance for Pickle Bath Adjustments
  • If Acid Low & Iron High:  add concentrated acid, then adjust using water/rinse water.
  • If Acid & Iron Low:  add acid and continue pickling until pickle times improve.
  • If Acid & Iron High:  add rinse water and/or fresh water.
  • If Acid High & Iron Low (or new bath):  add rinse water and/or other ineffective acid and continue pickling. Potentially suspend filtration system.
Calculating Acid and Water Amounts for Pickle Bath Adjustments

An AGA Galvanizing Note Pickle Bath Maintenance Calculations is available to assist galvanizers with the necessary conversions and calculations required to adjust iron and acid levels so that desired pickle bath properties can be achieved. This tool speeds up the guess-and-check calculations while providing an instant visual plot of the initial and adjusted baths on the Kleingarn Curve. The calculator also determines the volume of existing pickling acid that must be removed prior to the bath additions and accounts for any adjustments to freeboard height.

The AGA is currently conducting a research program to develop a similar curve for dilute sulfuric pickling solutions. When that research is completed, the guide will be further expanded to include recommendations for sulfuric pickle bath adjustments. This study will also investigate the potential impact of zinc concentration on pickling rates.

Pickle Bath Regeneration Calcuatltor2
AGA Pickle Bath Regeneration Calculator

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