Hydrochloric acid regeneration or HCl regeneration is a chemical process for the reclamation of bound and unbound
HCl HCL may refer to:
Science and medicine
* Hairy cell leukemia, an uncommon and slowly progressing B cell leukemia
* Harvard Cyclotron Laboratory, from 1961 to 2002, a proton accelerator used for research and development
* Hollow-cathode lamp, a spe ...
from
metal chloride
In chemistry, a transition metal chloride complex is a coordination complex that consists of a transition metal coordinated to one or more chloride ligand. The class of complexes is extensive.
Bonding
Halides are X-type ligands in coordination ...
solutions such as
hydrochloric acid
Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid
Acid strength is the tendency of an acid, symbol ...
.
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 Al
2O
3 and MgO, as well as
rare-earth
The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides (yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous silve ...
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
Roasting is a process of heating a sulfide ore to a high temperature in the presence of air. It is a step in the processing of certain ores. More specifically, roasting is often a metallurgical process involving gas–solid reactions at elevat ...
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 liquor
Pickling is a metal surface treatment used to remove impurities, such as stains, inorganic contaminants, and rust or scale from ferrous metals, copper, precious metals and aluminum alloys. A solution called ''pickle liquor'', which usually contain ...
s have been adopted by the ferrous metals processing industry:
Regeneration
*
Pyrohydrolysis
Roasting is a process of heating a sulfide ore to a high temperature in the presence of air. It is a step in the processing of certain ores. More specifically, roasting is often a metallurgical process involving gas–solid reactions at elevat ...
**
Spray roaster
Spray or spraying commonly refer to:
* Spray (liquid drop)
** Aerosol spray
** Blood spray
** Hair spray
** Nasal spray
** Pepper spray
** PAVA spray
** Road spray or tire spray, road debris kicked up from a vehicle tire
** Sea spray, refers to ...
pyrohydrolysis
**Fluidised bed pyrohydrolysis
*Hydrothermal regeneration
*Electrolytic Fe precipitation
Recovery of free HCl
*Retardation
*Dialysis
*
Ion exchange
Ion exchange is a reversible interchange of one kind of ion present in an insoluble solid with another of like charge present in a solution surrounding the solid with the reaction being used especially for softening or making water demineralised, ...
Transformation of FeCl2 to FeCl3
*
Electrolytic oxidation
An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon di ...
*
Chemical oxidation
Hydrothermal 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 FeCl
2 + 3 O
2 → 8 FeCl
3 + 2 Fe
2O
3
Step 2: hydrolysis
2 FeCl
3 + 3 H
2O → 6 HCl + Fe
2O
3
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 m
3/g BET specific surface; >2 kg/L specific weight; <0.05% water-soluble chlorides)
* theoretically unlimited operating capacity
Known 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 FeCl
2 + 4 H
2O + O
2 = 8 HCl + 2 Fe
2O
3
2 FeCl
3 + 3 H
2O = 6 HCl + Fe
2O
3
The process is an inversion of the chemical descaling (pickling) process.
Main differences between different implementations of pyrohydrolytic acid regeneration
*Furnace Type (
spray roaster
Spray or spraying commonly refer to:
* Spray (liquid drop)
** Aerosol spray
** Blood spray
** Hair spray
** Nasal spray
** Pepper spray
** PAVA spray
** Road spray or tire spray, road debris kicked up from a vehicle tire
** Sea spray, refers to ...
, fluidised bed or combined furnace)
*Physical Properties of Iron Oxide By-Product (
ferric oxide
Iron(III) oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It is one of the three main oxides of iron, the other two being iron(II) oxide (FeO), which is rare; and iron(II,III) oxide (Fe3O4), which also occurs naturally a ...
powder or pellets)
*Purity and commercial value of Iron Oxide By-Product
**Cl content
**SiO
2 content (typically 40 to 1000 ppm)
**other impurities
**specific weight (typically 0.3 to 4 kg per litre)
**specific surface (typically 0.01 to 8 m2/g)
*Energy Consumption (between 600 and 1200 kcal/L)
*Fuel type
*Concentration of regenerated acid (typically approx. 18% wt/wt)
*Purity of regenerated acid (remaining Fe content, Cl content)
*Recovery efficiency (typically 99%)
*Rinse water utilization
*Stack emissions (HCl, Cl
2, Dust, CO, NOx)
*Liquid effluents (composition, amount)
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 (H
2O, 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
Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline (petrol), diesel fuel, fuel oil, biodiesel blends, or coal. According to the type of engine, it is discharged into the atmosphere through an ...
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 leakage 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 Na
2S
2O
3 are used to bind HCl and Cl
2 (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.
Notes
External links
Minimizing Fuel Cost during Regeneration of the HCl Lixiviant (by Hatch)3D Animation of Spray Roaster Hydrochloric Acid Regeneration Plant (by SMS Siemag Process Technologies)3D Animation of Fluidized Bed Hydrochloric Acid Regeneration Plant (by SMS Siemag Process Technologies)3D Animation of Hydrothermal Hydrochloric Acid Regeneration Plant (by SMS Siemag Process Technologies)
{{DEFAULTSORT:Hydrochloric Acid Regeneration
Inorganic reactions
Chemical processes