Acid Cooling Crystallizat.

Asid Colling Crystallizition: This process is based on the cooling of the sulfuric acid pickling solution and

is only suitable for sulfuric acid. However the free acid can be recovered to more than 99%.

Through the cooling  the solubility of  the iron sulfate is reduced and the iron sulfate crystallizes out as FeSO4x7H2O. The formed iron sulfate crystals can be used, in particular, as raw material or precipitant,

the discharged acid bound in the byproduct iron sulfate has to be supplied back to the pickling process

again.

 

Process description:
A partial stream  of the pickling solution with a temperature ranging  from 55 °C to 98 °C is continuously drained from the pickling bath and supplied to a mixing tank. To reduce the solubility of iron, sulfuric acid

is already added before the regeneration.Thereafter the waste acid is cooled in counter flow to the purified acid and supplied to the multi-

stage 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 con-

centration 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 from 10 °C to 15 °C is  supplied from the last crystallization stage to 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.

 

The main benefits are:
— The recovery rate for free sulfuric acid of more than 99% reduces the consumption of fresh acid
— Saving in operating costs by reduced sludge production and consumption of neutralizer in the waste water treatment plant
— The iron sulfate crystals can be used as raw material or precipitant
— Easy plant design and operation of the recovery system