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What is the Kleingarn Curve and how can it help my process?

The Kleingarn Curve was developed by J-P Kleingarn with VDF Germany. Their paper Pickling in Hydrochloric Acid was presented at the Intergalva conference in 1988 with results that have changed the way the galvanizing industry treats spent hydrochloric acid. 

When steel is pickled in a 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. At this point, the acid is considered spent. Previously, the options for dealing with the spent acid included on-site neutralization, off-site disposal, decanting the acid for further use, or some other off-site beneficial re-use. 

Essentially, the Kleingarn Curve is the isotherm solubility curve between hydrochloric acid concentration and iron concentration in solution at 20C (68F). The chart depicts an optimum pickling line showing the optimum iron concentration for each hydrochloric acid concentration. It also shows the saturation concentrations where pickling is no longer possible. The information in the Kleingarn Curve now allows galvanizers to control their acid solution with the addition of either water or acid to reduce the iron concentration.

Figure 1: The Kleingarn Curve

What Does it Mean?

The Kleingarn Curve, Figure 1, shows the optimum iron concentration for every hydrochloric acid concentration for steel pickling. Ideally, the two concentrations would intersect on the red line which would produce optimum pickling times. The further the point is away from the red line, the longer the pickling time will be. Pickling cannot take place if the point is above the blue saturation line. If the point is below the green line, pickling may still take place, but at a greatly reduced rate.


Figure 2: Nomogram for HCI g/I

The Kleingarn Curves concentrations are reported in units of grams per liter. However, most galvanizers in North America generally report their hydrochloric acid and iron concentrations in percentage by weight. In order to convert percentages by weight of hydrochloric acid and iron to grams per liter, the following steps should be taken:

  1. Use the chart in Table 1 to convert HCl percentage to grams/liter.
  2. Using the nomogram in Figure 2, draw a line from the acid concentration on the right, to the solutions specific gravity on the left. The iron concentration will be given by the point where the line crosses the middle axis.

Table 1



In his paper, Kleingarn pointed out some curious consequences of the solubility curve that may produce different results than expected when regenerating spent acid. For example, lets 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 approximate solution would be presented in Table 2.  If these values are plotted on the Kleingarn Curve, we see that the solution would have become saturated and no pickling could take place. So the addition of fresh acid, in this case, has made the pickling solution useless. If on the other hand, only fresh water were added to the solution, the composition would be presented in Table 3.  With this composition, the pickling solution would become usable, albeit with less than optimum pickling times.

Table 2

 Volume m3%HCI% Fe g/L
Spent Acid33413.2
Fresh Acid12320
Regenerated Acid4513.18.9

Table 3

 Volume m3% HCI% Fe g/L
Spent Acid33413.2
Regenerated Acid452.89.3

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