Process description of spray roaster hydrochloric acid regeneration plant

Process description of spray roaster hydrochloric acid regeneration plant

Preconcentration

The metal chloride solution (in the most common case waste pickle liquor from a carbon steel pickling line) is fed to the venturi evaporator (III), where direct mass and heat exchange with the hot roast gas from the roaster (reactor/cyclone) takes place. The separator (IV) separates the gas and liquid phase of the venturi evaporator product. The liquid phase is re-circulated back to the venturi evaporator to increase mass and heat exchange performance.

• approx. 25 to 30% of the waste acid (H₂O, HCl) are evaporated
• roast gas is cooled down to approx. 92 to 96 °C
• dust particles are removed from the roast gas

Roasting

Preconcentrated waste acid from the preconcentrator (III / IV) is injected into the reactor (I) by means of one or more spray booms (VIII) bearing one or more injection nozzles each. Injection takes place at reactor top at a pressure between 4 and 10 bar. The reactor is directly fired by tangentially mounted burners that create a hot swirl. Temperatures inside the reactor vary between 700 °C (burner level) and 370 °C (roast gas exit duct). In the reactor the conversion of droplets of preconcentrated waste acid into iron oxide powder and hydrogen chloride gas takes place. Hydrogen Chloride leaves the reactor through the top, while iron oxide powder is removed from the reactor bottom by means of mechanical extraction devices. A cyclone (II) in the roast gas duct ensures separation and feed back of larger oxide particles carried by the roast gas.

Absorption

In the absorption column (V) the hydrogen chloride compound of the saturated roast gas leaving the preconcentrator is adiabatically absorbed in water (which in many cases is acid rinse water from a carbon steel pickling line). Regenerated acid (typical strength: 18% wt/wt) is collected at absorption column bottom.

Exhaust gas treatment

The roast gas is conveyed through the system by means of an exhaust gas fan (VI). Fans in plants provide pressure increases of approx. 200 mbar and are feedback-controlled to maintain a relative pressure of -3 mbar between reactor and atmosphere to avoid any overpressure-related leakinge of acid gas. To rinse the impeller and cool the gas as well as to remove remaining traces of HCl from the roast gas, the exhaust gas fan is commonly supplied with quenching water, which is separated from the exhaust gas stream by means of a mist eliminator (VII) at the pressure side of the fan. In a final scrubber, commonly consisting of a combination of wet scrubbers such as venturi scrubbers (IX) and scrubber columns (X), remaining traces of HCl and dust are removed. In some plant, absorption chemicals such as NaOH and Na₂S₂O₃ are used to bind HCl and Cl₂ (which is created under certain circumstances in several, but not all spray roasting reactors).

Environmental impact

Pyrohydrolysis based acid regeneration processes produce a considerable amount of stack emissions containing HCl, particles and chlorine, which has led to numerous violations of the U.S. clean air act in the past.