Production of Hydrochloric acid

Hydrochloric acid (HCl) is listed as a Title III Hazardous Air Pollutant.

Hydrochloric acid is a versatile chemical used in a variety of chemical processes,
including hydrometallurgical processing (e. g., production of alumina and/or
titanium dioxide), chlorine dioxide synthesis, hydrogen production, activation of
petroleum wells, and miscellaneous cleaning/etching operations including metal
cleaning (e. g., steel pickling). Also known as muriatic acid, HCl is used by
masons to clean finished brick work, is also a common ingredient in many
reactions, and is the preferred acid for catalyzing organic processes. One example
is a carbohydrate reaction promoted by hydrochloric acid, analogous to those in the
digestive tracts of mammals.
Hydrochloric acid may be manufactured by several different processes, although
over 90 percent of the HCl produced in the U. S. is a byproduct of the chlorination
reaction. Currently, U. S. facilities produce approximately 2.3 million megagrams
(Mg) (2.5 million tons) of HCl annually, a slight decrease from the 2.5 million Mg
(2.8 million tons) produced in 1985. 8.6.2 Process Description1-4 Hydrochloric
acid can be produced by 1 of the 5 following processes:
1. Synthesis from element
H2 + Cl2 → 2HCl
2. Reaction of metallic chlorides, particularly sodium chloride (NaCl), with
sulfuric acid (H2SO4) or a hydrogen sulfate:
NaCl + H2SO4 → NaHSO4 + HCl
NaCl + NaHSO4 → Na2SO4 + HCl
2NaCl + H2SO4 → Na2SO4 + 2HCl
3. As a byproduct of chlorination, e. g., in the production of dichloromethane,
trichloroethylene, perchloroethylene, or vinyl chloride:
C2H4 + Cl2 → C2H4Cl2
C2H4 + Cl2 → C2H3Cl + HCl
4. By thermal decomposition of the hydrated heavy-metal chlorides from spent
pickle liquor in metal treatment:
2FeCl3 + 6H2O → Fe2O3 + 3H2O + 6HCl

5. From incineration of chlorinated organic waste:

C4H6 + Cl2 + 5O2 → 4CO2 + 2H2O + 2HCl
After leaving the chlorination process, the HCl-containing gas stream proceeds to
the Figure
PCC (Product) pyridinium chlorochromate
After leaving the chlorination process, the HCl-containing gas stream proceeds to
the Figure 8.6-1. HCl production from chlorination process. (SCC = Source
Classification Code.) absorption column, where concentrated liquid HCl is
produced by absorption of HCl vapors into a weak solution of hydrochloric acid.
The HCl-free chlorination gases are removed for further processing. The liquid
acid is then either sold or used elsewhere in the plant. The final gas stream is sent
to a scrubber to remove the remaining HCl prior to venting. 8.6.3 Emissions4,5
According to a 1985 emission inventory, over 89 percent of all HCl emitted to the
atmosphere resulted from the combustion of coal. Less than 1 percent of the HCl
emissions came from the direct production of HCl. Emissions from HCl production
result primarily from gas exiting the HCl purification system. The contaminants
are HCl gas, chlorine, and chlorinated organic compounds. Emissions data are only
available for HCl gas. Table 8.6-1 lists estimated emission factors for systems with
and without final scrubbers. Units are expressed in terms of kilograms per
megagram (kg/Mg) and pounds per ton. 8.6-2 EMI
If HCl is produced as a byproduct of another process such as the production of
dichloromethane, trichloroethane, perchloroethylene, or vinyl chloride then the
emission factor and SCC appropriate for that process vent should be used.

Fig. HCl production from chlorination process.

HCl - Direct Synthesis or Burner Process
The large scale production of hydrochloric acid, as a desired primary finished
product, is almost always integrated in a large scale chlor-alkali facility. In the
chlor-alkali industry, salt solution is electrolyzed producing chlorine (Cl2), sodium
hydroxide, and hydrogen (H2). The pure chlorine gas can be re-combined with the
hydrogen gas, forming hydrogen chloride gas.
Cl2 + H2→ 2 HCl
The reaction takes place in what is commonly referred to as an acid burner. The
resulting hydrogen chloride gas is absorbed in demineralized water, resulting in
hydrochloric acid. The product resulting from this process is often called burner
grade HCl.
Organic Synthesis of HCl
The largest volume production of hydrochloric acid is a by-product of the
formation of chlorinated and fluorinated organic compounds, chloroacetic acid,
and PVC. These production processes very often also consume large quantities of
HCl. In the chemical reactions, hydrogen atoms are replaced by chlorine atoms.
The released hydrogen atom recombines with the spare atom from the chlorine
molecule, forming hydrogen chloride. Fluorination is a subsequent chlorine-
replacement reaction, producing again hydrogen chloride.
R-H + Cl2→ R-Cl + HCl
R-Cl + HF → R-F + HCl
The resulting hydrogen chloride gas is either reused directly, or absorbed in water,
resulting in hydrochloric acid of various grades. The HCl resulting from these
processes is most commonly referred to as co-product or by-product acid
Application of HCl
Pickling of steel
One of the most important applications of hydrochloric acid is in
the pickling (Note: 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
contains acid, is used to remove the surface impurities. It is commonly used to
descale or clean steel in various steelmaking processes)

of steel, to remove rust or iron oxide scale from iron or steel before subsequent
processing, such as, rolling, galvanizing, (the process of applying a
protective zinc coating to steel or iron, to prevent rusting.)and other techniques.
Technical quality HCl at typically 18% concentration is the most commonly used
pickling agent for the pickling of carbon steel grades.

Fe3O4 + Fe + 8HCl ------ 4 FeCl2+ 4H2O

The spent acid has long been reused as iron(II) chloride (also known as ferrous
chloride) solutions, but high heavy-metal levels in the pickling liquor have
decreased this practice.
The steel pickling industry has developed hydrochloric acid
regeneration processes, such as the spray roaster or the fluidized bed HCl
regeneration process, which allow the recovery of HCl from spent pickling liquor.
The most common regeneration process is the pyrohydrolysis process, applying the
following formula

4 FeCl2+ 4H2O + O2 ---------------------- 8HCl + 2Fe2O3

By recuperation of the spent acid, a closed acid loop is established. [7] The iron(III)
oxide by-product of the regeneration process is valuable, used in a variety of
secondary industries.

Production of inorganic compounds

Similar to its use for pickling, hydrochloric acid is used to dissolve many metals,
metal oxides and metal carbonates. The conversion are often depicted in simplified
equations:

Zn + 2 HCl → ZnCl2 + H2
NiO + 2 HCl → NiCl2 + H2O
CaCO3 + 2 HCl → CaCl2 + CO2 + H2O

These processes are used to produce metal chlorides for analysis or further
production

pH control and neutralization

Hydrochloric acid can be used to regulate the acidity (pH) of solutions.

OH- + HCl -------H2O + Cl-

In industry demanding purity (food, pharmaceutical, drinking water), high-quality

hydrochloric acid is used to control the pH of process water streams. In less-
demanding industry, technical quality hydrochloric acid suffices
for neutralizing waste streams and swimming pool pH control.

Regeneration of ion exchangers

High-quality hydrochloric acid is used in the regeneration of ion exchange

resins. Cation exchange is widely used to remove ions such as Na+ and
Ca2+ from aqueous solutions, producing demineralized water. The acid is used to
rinse the cations from the resins. Na+ is replaced with H+ and Ca2+ with 2 H+.

Ion exchangers and demineralized water are used in all chemical industries,
drinking water production, and many food industries.

Laboratory use

Of the six common strong mineral acids in chemistry, hydrochloric acid is the
monoprotic acid least likely to undergo an interfering oxidation-reduction reaction.
It is one of the least hazardous strong acids to handle; despite its acidity, it contains
the non-reactive and non-toxic chloride ion. Intermediate-strength hydrochloric
acid solutions are quite stable upon storage, maintaining their concentrations over
time. These attributes, plus the fact that it is available as a pure reagent, make
hydrochloric acid an excellent acidifying reagent. It is also inexpensive.

Hydrochloric acid is the preferred acid in titration for determining the amount
of bases. Strong acid titrants give more precise results due to a more distinct
endpoint. Azeotropic, or "constant-boiling", hydrochloric acid (roughly 20.2%)
can be used as a primary standard in quantitative analysis, although its exact
concentration depends on the atmospheric pressure when it is prepared