Replacing Sulfuric Acid with Carbon Dioxide for pH Control

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Replacing Sulfuric Acid with Carbon Dioxide for pH Control

The iron dynamics facility of Steel Dynamics Inc. (SDI) has an 800 gpm cooling water loop that makes direct contact with flue gas from a submerged electric arc furnace for emissions control. The loop consists of a scrubber for contact with the flue gas, a thickener and clarifier to remove solids from the scrubber effluent, and a cooling tower. Due to fluctuations in the ore feedstock, large changes in pH are observed in the cooling water, sufficient to raise the effluent water pH above the limit of 9.

Sulfuric acid was added to reduce pH, but cost was high and promoted the formation of gypsum (CaSO4) scale, in conjunction with concentration of the salts due to evaporation in the cooling tower. Gypsum forms at pH levels above 2 and is only slightly soluble (2-2.5 g/L at 25 degrees C). Gypsum scaling at SDI was so difficult to remove that transfer lines had to be replaced. An additional drawback of sulfuric acid is that slight excesses of acid addition can lead to very low pH values (less than 4), creating acidic process water.

Due to a lower cost and reduced tendency to form scale, carbon dioxide (CO2) addition to the process water was proposed as a replacement for sulfuric acid for pH control. While carbonate scale is even less soluble (0.013 g/L) than gypsum (2-2.5 g/L), carbonate scale formation can be avoided with pH controlled to below 10. Below pH 10, calcium is in the form of calcium bicarbonate (Ca(HCO3)2), which is highly soluble at over 160 g/L (25 degrees C). As an additional advantage, excess CO2 addition leads to a pH value that is close to neutral (6-7) at atmospheric pressure, so there is no risk of overshooting.

Praxair’s proposal
SDI chose Praxair’s CO2-based pH control system. The system features both liquid and gas injection. Four injection points were chosen throughout the cooling water loop: the cold well, thickener, clarifier inlet and clarifier outlet. The various locations required different pH levels. For example, a higher pH (above 11) is maintained in the thickener and clarifier to promote the precipitation of calcium carbonate, which is removed in these unit processes.

Use of CO2 reduced pH control costs by 35 percent. Additional savings have been realized due to reduced maintenance, downtime and solids handling (from dredged scale) associated with sulfate scale formation.

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