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Fluidised bed process

A further possibility of spent pickle acid regeneration by a pyrohydrolysis process is given by the fluidised bed process originally developed by Lurgi-Keramchemie and increased its importance in recent years. The spent pickling acid is fed via pumps into the separation system to be concentrated in a venturi afterwards.

A part of this solution now is passed into the reactor to be converted inside the fluidised bed into HCl and iron oxide (~95% Fe₂O₃ hematite, ~5% Fe₃O₄ magnetite). Within the fluidised bed which is composed of iron oxide granules the decomposition takes place at around 850°C by evaporation of water and finally by conversion of the iron salt into HCl and iron oxide.

The fluidised bed is maintained by a stream of air which also allows the intense mixture of the gas and the solids inside the reactor. The reactor can be heated by oil or gas. The generated iron oxide grows continuously along the granules of the fluidised bed to become heavier and finally sink to the bottom. The final product is free of dust and iron oxide fines.

To maintain equilibrium the bigger grains are taken from the reactor continuously and fresh solution is added.

After leaving the cyclone the gases are being cooled down by means of the venturi, whilst the energy is being converted to preconcentrate the incoming pickling solution. The process gases now being cooled down to around 100°C are passed through an adiabatic column to react with incoming rinse water to azeotropic (~18%) HCl.

The combustion gases are further being washed in a final aerosol scrubbing step consisting of an agglomerator and venturi with pneumatic atomizing nozzles as effective system to minimize dust and HCl in the off-gas. In the last scrubber the rest dust and HCl is removed to maintain lowest possible emission values. The vent system is thus placed that the part with high HCl concentration is kept under negative pressure in order to avoid leakages of HCl gases or oxide dust from the plant into the environment.

All process parameters are controlled automatically in order to maintain constant process condition and thus constant acid concentration and oxide quality.

The advantages of the fluidised bed process are given in a much lower content of chlorine within the oxide granules due to longer reaction times of more than 10 hours and higher temperatures. A very pure oxide results, free of dust, easy to handle in a clean way. Furthermore the closed system for the acid concentration and feed to the reactor (almost without pressure compared with the sprayroaster where up to 10 bars are applied) represents a siginificant advantage in terms of operation and safety. On the other hand energy costs are around 20-25% higher in comparison to spray roasting technology.