Field of application
The commercially most relevant field of application for HCl regeneration processes is the recovery of HCl from waste pickle liquors from carbon-steel pickling lines. Other applications include the production of metal oxides such as, but not limited, to Al2O3 and MgO, as well as rare-earth oxides, by pyrohydrolysis of aqueous metal chloride or rare-earth chloride solutions. A number of different process routes are available. The most widely used is based on pyrohydrolysis and adiabatic absorption of hydrogen chloride in water, a process invented in the 1960s. However tightening environmental standards and stringent air permit policies render it increasingly difficult to establish new pyrohydrolysis-based acid regeneration plants.Known processes
The following processes for the regeneration of HCl from spent pickle liquors have been adopted by the ferrous metals processing industry:Regeneration
* Pyrohydrolysis ** Spray roaster pyrohydrolysis **Fluidised bed pyrohydrolysis *Hydrothermal regeneration *Electrolytic Fe precipitationRecovery of free HCl
*Retardation *Dialysis *Transformation of FeCl2 to FeCl3
* Electrolytic oxidation * Chemical oxidationHydrothermal regeneration
Hydrothermal hydrolysis of hydrochloric SPL from carbon-steel pickling lines is a hydrometallurgical reaction, which takes place according to the following chemical formula:Step 1: oxidation
12 FeCl2 + 3 O2 → 8 FeCl3 + 2 Fe2O3Step 2: hydrolysis
2 FeCl3 + 3 H2O → 6 HCl + Fe2O3 Today hydrothermal hydrolysis, which operates at very low temperatures, consumes only a fraction of the energy other processes demand and produces virtually no emissions, is considered the most effective way to regenerate any given quantity of spent pickle liquor.Advantages
* low energy consumption (about 1300 kJ per litre waste acid) * no gaseous emissions * wide operating range (10 to 100% of nominal capacity) * high-value byproduct (>20 m3/g BET specific surface; >2 kg/L specific weight; <0.05% water-soluble chlorides) * theoretically unlimited operating capacityKnown implementations
Known implementations of the hydrothermal HCl regeneration processes include the PORI process (1974 for J&L Steel, dismantled) and the optimized SMS Demag wet process (2008 for ThyssenKrupp Steel, under construction).Pyrohydrolysis of spent pickle liquor
Pyrohydrolysis of hydrochloric spent pickle liquor from carbon steel pickling lines is a hydrometallurgical reaction which takes place according to the following chemical formulae: 4 FeCl2 + 4 H2O + O2 = 8 HCl + 2 Fe2O3 2 FeCl3 + 3 H2O = 6 HCl + Fe2O3 The process is an inversion of the chemical descaling (pickling) process.Main differences between different implementations of pyrohydrolytic acid regeneration
*Furnace Type ( spray roaster, fluidised bed or combined furnace) *Physical Properties of Iron Oxide By-Product (Basic process flow diagram 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 gasRoasting
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 anEnvironmental 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.Notes
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