Hydrochloric acid is supplied as an aqueous solution of hydrogen chloride gas, identified by CAS 7647-01-0 and transported under UN 1789. Commercial liquid grades extend from regenerated pickling acid at 18% to 20% HCl by mass to 20° Baumé muriatic acid at 31.45% HCl and 22° Baumé material at 35.21% HCl. The product is clear to slightly yellow, fuming, and miscible with water; the hydrochloric acid–water system forms an azeotrope at approximately 20.2% HCl at atmospheric pressure. Total acidity and iron content are commonly determined according to ASTM E224-16.

The largest industrial use is steel pickling, where hot-rolled carbon steel is immersed in HCl baths at 65 °C to 85 °C and acid concentrations from 5% to 20% by mass. Spent pickle liquor from this operation contains ferrous chloride, ferric chloride, free HCl, and water. Because the spent liquor is classified as hazardous in many jurisdictions, regeneration by pyrohydrolysis is preferred over neutralisation and deep-well disposal where steel throughput and environmental compliance justify the capital cost.

Pyrohydrolysis: How Spent Pickle Liquor Becomes Fresh HCl Again

In the pickle bath, mill scale is dissolved by two principal reactions: FeO + 2HCl → FeCl₂ + H₂O and Fe₂O₃ + 6HCl → 2FeCl₃ + 3H₂O. The resulting liquor contains ferrous chloride and ferric chloride in proportion to the oxide scale, the free-acid concentration, and the immersion time. Spent liquor is seldom discharged directly to recovery; it is first concentrated by evaporation to reduce the water load on the high-temperature reactor. Total iron concentrations in spent carbon-steel pickle liquors are commonly in the range 100 g/L to 150 g/L as Fe, while free HCl may remain between 20 g/L and 100 g/L depending on line speed and acid drag-out. Published data for specific site configurations is limited.

Pyrohydrolysis converts the dissolved metal chlorides back into HCl and a dry iron oxide by-product. In a spray roaster, the concentrated liquor is atomised into a refractory-lined chamber operating at 500 °C to 800 °C. The dominant reactions are:

2FeCl₂ + 2H₂O + ½O₂ → Fe₂O₃ + 4HCl

2FeCl₃ + 3H₂O → Fe₂O₃ + 6HCl

The hydrogen chloride gas is drawn from the roaster with combustion products and water vapour. It is quenched in a venturi scrubber and then absorbed in water or weak rinse water. The resulting regenerated acid, typically 18% to 20% HCl by mass, is returned to the pickle line after blending with fresh 20° Baumé acid. The iron oxide reports to the roaster bottom and baghouse as a dry powder.

Preconcentration equipment is usually a multi-effect evaporator using graphite or tantalum heat-exchanger surfaces because wet HCl at temperatures above 80 °C is aggressively corrosive to stainless steels. The concentrated liquor is stored in rubber-lined or fibre-reinforced plastic tanks before atomisation. Atomiser selection—rotary disc, single-fluid nozzle, or two-fluid nozzle—determines droplet size distribution and therefore the residual chloride content and particle size of the oxide. A roast gas outlet temperature above the dew point of hydrochloric acid prevents condensation and corrosion in the ductwork; this is maintained by controlling combustion air and auxiliary fuel.

The absorber loop typically includes a recirculation tank, a packed tower or falling-film absorber, and a tail-gas scrubber. Acid mist carryover is controlled by demister pads and scrubber liquor distribution. The weak-acid purge from the absorber may be used as rinse water in the pickle line only if the accumulated metal cation load does not destabilise the pickling bath. Full-scale units are usually tuned by ion balance rather than a single fixed setpoint.

What Limits Acid Recovery Efficiency When Rinse Water Returns to the Pickle Line?

Water balance is the main process conflict. Each litre of rinse water sent to the regeneration plant must be evaporated before the chloride salts reach the roaster. If countercurrent rinsing is not applied, the weak-acid volume may exceed absorber make-up demand, and surplus acid cannot be discharged without violating effluent limits. In practice, acid drag-out from the pickle bath is therefore minimised by wringer rolls, tank immersion geometry, and strip speed control rather than by additional evaporation capacity alone.

Reactor temperature defines a second constraint. Below approximately 500 °C, hydrolysis of ferrous chloride is incomplete and sticky chloride solids may accumulate on the roaster lining. Above approximately 800 °C, fuel consumption rises and the oxide may sinter, reducing its value as ferrite raw material. Feed solids and chloride species also matter: zinc, lead, and alkali chlorides in mixed steel pickling can volatilise and re-condense as sticky salts in the quench and absorber, requiring more frequent washdown and reducing HCl recovery. Hydrolysis conversion is strongly temperature-dependent; at the lower end of the operating window, residence time must be increased to achieve acceptable chloride decomposition.

Acid recovery efficiency is measured across the absorber and the tail-gas scrubber. When absorber temperature, recirculation rate, and weak-acid concentration are controlled, HCl recovery in commercial spray roasters is commonly reported between 95% and 99%. Residual HCl emissions are site-specific and regulated under applicable air-emissions permits. Continuous readings from the tail-gas scrubber are used to trim acid flow to the evaporator and atomising air to the roaster.

The oxide by-product from spray roasting consists mainly of Fe₂O₃ with low moisture and low residual chloride. It is sold as ferrite raw material for soft ferrite production, as pigment feedstock, or as an iron source in non-metallurgical applications. Oxide quality is influenced by the concentration of chromium, nickel, zinc, and lead in the spent pickle liquor; feeds from plain carbon steel lines produce the most consistent material. The residual chloride specification is often set by the buyer, and chloride is controlled by roast temperature and final washing.

Regenerated acid is not limited to steel pickling. After filtration and adjustment to 20° Baumé, it can be used in water treatment for pH adjustment, cation-exchange resin regeneration, and production of calcium chloride. Oilwell acidizing uses hydrochloric acid at 5% to 28% by mass, but that service usually requires virgin acid rather than regenerated pickling acid because of iron and trace-metal constraints.

Materials of Construction for HCl Regeneration Loops

Materials are selected for the wet, hot, and dry zones separately. The dry roaster shell is carbon steel protected by refractory and insulation; the hot gas duct before the quench is often refractory-lined because too rapid cooling below the acid dew point leads to localised corrosion. The quench venturi, absorber, and tail-gas scrubber are constructed from fibre-reinforced plastic or rubber-lined carbon steel with polyvinylidene fluoride or graphite internals. Metallic materials such as tantalum and Hastelloy C-type alloys are used only for specific high-temperature heat-transfer surfaces and mechanical seals.

Carbon steel piping is not acceptable for wet HCl below the dew point. Storage tanks for regenerated 18% hydrochloric acid are lined with rubber or fabricated from high-density polyethylene or FRP. Pumps use magnetic-drive or mechanically sealed designs with wetted parts in fluoropolymers or ceramics. Oxidising agents—bleach, peroxide, or hypochlorite—must be excluded from HCl tanks because they release chlorine gas. Ammonia and amine-based additives are also incompatible, forming dense white fumes of ammonium chloride.

The regenerated acid is sampled at the absorber discharge and at the pickle tank. Total acidity is determined by acid-base titration using ASTM E224-16 or ISO 905:1999. Density is checked by digital density meter according to ASTM D4052-22; iron and trace metals are measured by ICP-OES or atomic absorption. Free chlorine is monitored where oxidising agents have been used upstream. Hydrochloric acid above 25% is classified under CLP (EC) No 1272/2008 as Skin Corr. 1B and STOT SE 3 with hazard statements H314 and H335.