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Treating the surface of steel with hydrochloric acid (HCl) produces iron chloride (FeCl2) by way of chemical reaction. The property of this FeCl2, to decompose hydrolytically at a high temperature, is utilized in HCl recovery process.
The chemical processes of pickling and regeneration are demonstrated by the following simplified equations:
Pickling: FeO + 2HCl = FeCl2 + H2O
Regeneration: 2FeCl2 + 2H2O + 1/2O2 = Fe2O3 + 4HCl
The complete decomposition of the FeCl2, the recovery of HCl and the separation of the iron in form of useable Fe2O3, are the duties of the recovery process.
The main parts of the plant are:
- Fluidized reactor
- Absorption column
- Final scrubber
- Drop separator
The spent pickle liquor to be treated is introduced via pre-evaporator into the regenerating process and recycled in the pre-evaporator circuit.
In the Venturi of the pre-evaporator, a heat exchange takes place between the hot waste gases of the reactor and the spent pickle liquor, reducing the waste gas temperature to approx. 100°C and evaporation of water in the spent liquor. In this manner, the concentration of Fe2O3 is increased.
In this way an economical utilization of the waste heat is ensured.
Automatic control of the level and concentration of Fe in the pre-evaporator, provide for constant recovery conditions. A partial stream of the pre-concentrated spent pickle liquor is taken from the recirculation's circuit and delivered via lances as a compact jet of liquid - not sprayed and not atomized - into the fluid bed of the reactor.
The FeCl2 introduced into the reactor decomposes and Fe2O3 settles down on the fluid bed by built up granulation to an approximate grain height of 1 mm diameter and is taken out of the reactor in granular form.
Together with the gas stream, are removed from the fluid bed, fine particles of Fe2O3, separated by the cyclone and delivered back to the reactor. The acid recovered in the recovery plant is collected in a storage tank and depending on requirements it will be added to the pickling plant. All process is controlled by PLC.
The advantages of Acid regeneration plant in respect to fluidized bed are: high efficient HCl recovery, high efficient separation of Fe, does not need the initial filtration of spend acid, extremely low HCl and dust emission and producing a dust free oxide. The plants do not show any red stains; neither on the inside nor on the outside.