Steuler Acid Cooling Crystallization (SACC)

This process is based on the cooling of the sulfuric acid pickling solution. Through the cooling the solubility of the iron sulfate is fallen short of and the iron sulfate crystallizes out as FeSO4x7H2O. The formed iron sulfate can be used, in particular, as raw material or precipitant. This process is only implementable for sulfuric acid. The free acid can be recovered to more than 99%, the discharged acid bound in the byproduct iron sulfate has to be supplied again to the pickling process.

  • Recovery rate for free sulfuric acid of more than 99% entailing reduced fresh acid consumption
  • Saving in operating costs by reduced sludge production in the waste water treatment plant
  • Production of utilizable iron sulfate crystals
  • Saving in operating costs by reduced consumption of neutralizer in the waste water treatment plant
  • Easy plant design and operation of the recovery system

Process Description

A partial stream of the pickling solution, with a temperature ranging from 55 to 98°C, is continuously drained from the pickling bath and supplied to the mixing tank. To reduce the solubility of iron sulfuric acid is already metered before the regeneration. Thereafter the waste acid is cooled in counter flow to the purified acid and supplied to the multistage cooling crystallization. This crystallization consists of three crystallization tanks equipped with dipped heat exchangers. As cooling medium a cooling brine is pumped through the heat exchangers in counter flow to the acid to be cooled. According to the sulfuric acid concentration and the tank temperature, the concentration of dissolved iron(II) decreases from step to step and the crystallization increases. The suspension with the required crystallization temperature ranging between 10 and 15 °C is supplied from the last crystallization stage to a sedimentation for pre-concentrating the crystalline mush. The crystalline mush is fed by pump to a sieve centrifuge, where the crystals are dewatered. The filtrate from the sieve centrifuge and the clear overflow of the purified acid are collected and pumped back to the pickling line as purified acid via the heat exchanger.

The low temperatures required in the cooling brine cycle are generated by a chiller. The heat from this chiller is dissipated via cooling water.