Sulfuric Acid Recovery Plants Through Chiller Methods

Pickling time decreases as temperature increases for sulfuric acid. Most pickling tanks are operated between 45 oC and 60 oC with a Sulfuric acid concentration of 15-20%. If that solution is cooled down to around 10-15 oC, most of the ferrous sulfate will come out of the solution as ferrous sulfate heptahydrate crystals.

Solubility of ferrous sulfate in sulfuric acid can be seen in Figure below. Iron concentration will be around 55-65 g/L Fe in pickling tanks. Process can be described as follows;

1. After filtering the acid solution, it is cooled down through the economizer before the crystallization. It is cooled with the solution that is coming out from the crystallization system.

While temperature of the recovered acid is increased through the economizer, temperature of the feed solution which enters to the crystallization system is decreased. Thus, required energy for heating and cooling is minimized.

2. Cooled acid solution is fed to the crystallization system. Further cooling in the crystallizer will give Ferrous Sulfate Heptahydrate crystals (FeSO4.7H2O) because of the supersaturation created. Heat exchanger should be carefully designed in order to prevent crust formation.

3. Since the acid recovery process is a closed loop continuous system, optimum and constant acid/iron concentrations can be maintained in the pickling tanks.

4. The by-product ferrous sulfate heptahydrate (FeSO4.7H2O) which has a commercial value can be used in agricultural sector or waste water treatment plants.

TYPICAL SKETCH FOR SULFURIC ACID RECOVERY PLANT THROUGH CRYSTALLIZER