Hydrochloric Acid Regeneration - Wikipedia, The Free Encyclopedia

Hydrochloric acid regenerationFrom Wikipedia, the free encyclopedia

Hydrochloric acid regeneration or HCl regeneration refers to a chemical process for the reclamation of bound and

unbound HCl from metal chloride solutions such as hydrochloric acid.[1]

Contents

1 Field of application

2 Known processes

2.1 Regeneration

2.2 Recovery of free HCl

2.3 Transformation of FeCl2 to FeCl3

3 Hydrothermal regeneration

3.1 Step1: Oxidation

3.2 Step2: Hydrolysis

3.3 Advantages

3.4 Known implementations

4 Pyrohydrolysis of spent pickle liquor

4.1 Main differences between different implementations of pyrohydrolytic acid regeneration

4.2 Basic process flow diagram of spray roaster hydrochloric acid regeneration plant

4.3 Process description of spray roaster hydrochloric acid regeneration plant

4.3.1 Preconcentration

4.3.2 Roasting

4.3.3 Absorption

4.3.4 Exhaust gas treatment

4.4 Environmental impact

5 Notes

6 External links

Field of application

The commercially most relevant field of application for HCl regeneration processes is the recovery of HCl from waste pickleliquors from carbon steel pickling lines. Other applications include the production of metal oxides such as, but not limited, toAl2O3 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 absorptionof hydrogen chloride in water, a process invented in the 1960s. However tightening environmental standards and stringent airpermit 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 metalsprocessing industry:

Regeneration

Pyrohydrolysis

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Spray Roaster Pyrohydrolysis

Fluidised Bed Pyrohydrolysis

Hydrothermal Regeneration

Electrolytic Fe-precipitation

Recovery of free HCl

Retardation

Dialysis

Ion Exchange

Transformation of FeCl2 to FeCl3

Electrolytic Oxidation

Chemical Oxidation

Hydrothermal regeneration

Hydrothermal Hydrolysis of hydrochloric SPL from carbon steel pickling lines is a hydrometallurgical reaction which takesplace according to the following chemical formulae:

Step1: Oxidation

12 FeCl2 + 3 O2 -> 8 FeCl3 + 2 Fe2O3

Step2: Hydrolysis

2 FeCl3 + 3 H2O -> 6 HCl + Fe2O3

Today hydrothermal hydrolysis, which operates at very low temperatures, consumes only a fraction of the energy otherprocesses demand and produces virtually no emissions, is considered the most effective way to regenerate any given quantityof spent pickle liquor.

Advantages

low energy consumption (approx. 1300 kJ per litre waste acid)

no gaseous emissions

wide operating range (10 to 100% of nominal capacity)

high value by product (>20m3/g BET specific surface;>2 kg/l specific weight;

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 (Ferric Oxide Powder or Pellets)

Purity and commercial value of Iron Oxide By-Product

Cl content

SiO2 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, Cl2, 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 theventuri evaporator (III), where direct mass and heat exchange with the hot roast gas from the roaster (reactor/cyclone) takesplace. The separator (IV) separates the gas and liquid phase of the venturi evaporator product. The liquid phase isre-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


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Preconcentrated waste acid from the preconcentrator (III / IV) is injected into the reactor (I) by means of one or more spraybooms (VIII) bearing one or more injection nozzles each. Injection takes place at reactor top at a pressure between 4 and 10bar. The reactor is directly fired by tangentially mounted burners that create a hot swirl. Temperatures inside the reactor varybetween 700 C (burner level) and 370 C (roast gas exit duct). In the reactor the conversion of droplets of preconcentratedwaste acid into iron oxide powder and hydrogen chloride gas takes place. Hydrogen Chloride leaves the reactor through thetop, 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 isadiabatically 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 increasesof approx. 200 mbar and are feedback-controlled to maintain a relative pressure of -3 mbar between reactor and atmosphereto avoid any overpressure-related leakinge of acid gas. To rinse the impeller and cool the gas as well as to remove remainingtraces of HCl from the roast gas, the exhaust gas fan is commonly supplied with quenching water, which is separated from theexhaust gas stream by means of a mist eliminator (VII) at the pressure side of the fan. In a final scrubber, commonlyconsisting of a combination of wet scrubbers such as venturi scrubbers (IX) and scrubber columns (X), remaining traces ofHCl 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.[2]

Notes

"Hydrochloric acid regeneration" (http://www.komalgroup.com/waste_water_treatment4.html).1.

U.S. Department of Justice (2006). "Notice of Lodging of Consent Decree Under the Clean Air Act" (http://regulations.justia.com

/view/52449/). Justia Regulation Tracker.

2.

External links

Minimizing Fuel Cost during Regeneration of the HCl Lixiviant (by Hatch) (http://www.hatch.ca/technologies

/fluidization_technologies/articles/minimizing_fuel_costs_thermal_hydrochloric_acid.htm)

3D Animation of Spray Roaster Hydrochloric Acid Regeneration Plant (by SMS Siemag Process Technologies)

(http://www.youtube.com/watch?v=TrmCPkYF4Jg)

3D Animation of Fluidized Bed Hydrochloric Acid Regeneration Plant (by SMS Siemag Process Technologies)

(http://www.youtube.com/watch?v=wfz8N_BdzGs)

3D Animation of Hydrothermal Hydrochloric Acid Regeneration Plant (by SMS Siemag Process Technologies)

(http://www.youtube.com/watch?v=NNR-26Okvco)

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Categories: Inorganic reactions Chemical processes

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Hydrochloric Acid Regeneration - Wikipedia, The Free Encyclopedia