Technical grade hydrochloric acid (HCl 31%–37%) is specified for steel pickling and synthesis duties where controlled impurity levels, predictable acid activity, and cost per kilogram of available HCl are more important than the sub-mg/kg metal limits of reagent or food-grade material. The as-delivered liquid is a fuming, colorless to pale-yellow aqueous solution with a density from 1.153 g/cm³ to 1.184 g/cm³ at 20°C; total acidity is determined by titration according to ASTM E224-16, and density-concentration conversion uses ISO 905:1976. At these concentrations the solution is above the 20.2% atmospheric azeotrope, so the headspace above an unvented vessel always contains hydrogen chloride. Storage therefore requires vapor-tight tanks vented through a scrubber charged with 5–10 wt% sodium hydroxide, with pH control and caustic recirculation interlocks. Transfer piping and tank linings are commonly rubber-lined carbon steel, fiberglass-reinforced vinyl ester, or unpigmented polypropylene; titanium and unstabilized stainless steels are not considered acceptable continuous-immersion materials for concentrated HCl service. The material is shipped under UN 1789 as a corrosive liquid and carries GHS hazard statements H314 and H335. Table 1 lists representative procurement limits for a technical grade product; exact values are contract-specific and should be verified against the supplier certificate of analysis before tank acceptance.

Table 1: Representative technical-grade HCl 31%–37% procurement limits

Property	Method	Typical range
Total acidity as HCl	ASTM E224-16	31.0–37.0 wt%
Density at 20°C	ISO 905:1976	1.153–1.184 g/cm³
Iron	ASTM E224-16 photometric	≤ 5 mg/kg
Sulfate	ASTM E224-16 /supplier ion chromatography	≤ 50 mg/kg
Free chlorine	Iodometric titration	≤ 10 mg/kg
Residue on ignition	ASTM E224-16	≤ 100 mg/kg

What Are the Corrosion and Storage Constraints for 31%–37% HCl?

Corrosion resistance in this concentration range is strongly temperature dependent. Rubber-lined carbon steel tanks provide acceptable service at ambient temperatures up to approximately 40°C, but the rubber compound must be specified for acid service and tested for cure condition and Shore hardness. For heat exchanger tubing, graphite, tantalum, and Hastelloy C-276 are used because the metal surface remains serviceable under hot chloride conditions; titanium is attacked by warm chloride acid and must be excluded from wetted parts. Gasketed FRP piping with vinyl-ester resin is acceptable up to the design pressure of the transfer pump, normally 0.3–0.5 MPa, while clear PVC is restricted to low-temperature vent lines due to softening. At high ambient temperatures, tank headspace HCl concentration increases, and the scrubber must be sized for vapor–liquid equilibrium at 35–40°C rather than at nominal storage temperature. Unloading pumps are typically vertical canned pumps with double mechanical seals and clean-water flushes; acid mist from the vent is captured by a packed column with a superficial gas velocity below 1.5 m/s to prevent caustic carryover. Hydrogen generation is a safety consideration when concentrated acid contacts carbon steel during maintenance or an accidental line break. Ventilation must be evaluated for hydrogen accumulation above the lower explosive limit; normal tank venting and nitrogen padding after maintenance are specified in plant operating procedures.

On continuous carbon steel strip lines, the as-delivered 31%–37% acid is not injected directly into the pickling bath. The acid is diluted with recovered rinse water to an operating concentration of 18–20 wt% HCl; this window balances scale dissolution rate against base-metal attack and hydrogen embrittlement tendency. The principal scale components are wüstite, magnetite, and hematite, with the following dissolution reactions:

FeO + 2 HCl → FeCl₂ + H₂O Fe₃O₄ + 8 HCl → FeCl₂ + 2 FeCl₃ + 4 H₂O Fe₂O₃ + 6 HCl → 2 FeCl₃ + 3 H₂O

Base steel attack, Fe + 2 HCl → FeCl₂ + H₂, is suppressed by a commercial acid inhibitor metered at 0.1–0.5 vol%. Weight-loss coupon monitoring under ASTM G31-72(2017) shows that inhibited acid reduces carbon steel corrosion by more than 90% relative to uninhibited acid at the same temperature; the exact factor depends on inhibitor chemistry and bath age. Bath temperature is maintained at 70–85°C, and immersion time is controlled between 15–40 s by line speed and tank length. Heating is supplied by graphite plate heat exchangers, and bath agitation is produced by eductor nozzles rather than air sparging to limit acid mist. Acid concentration and iron concentration are measured every 20 min with automatic titration; when iron reaches 150 g/L, the bath is partially dumped or sent to regeneration.

Continuous Push-Pull Pickling Acid Concentration and Temperature Window

Push-pull strip lines are distinguished from batch acid tanks by countercurrent acid flow and continuous renewal. Fresh diluted acid enters the final immersion cell and overflows to the entry cell, where scale loading and iron concentration are highest. The entry cell may operate at 10–12 wt% free HCl with high Fe²⁺, and the exit cell at 18–20 wt% free HCl with low iron. The acid-temperature window is narrow in high-speed operation because the reaction rate increases sharply as temperature rises, but excessive temperature above 85–90°C raises vapor pressure and risks local boiling in entry cells with high chloride activity. Table 2 summarizes a representative carbon steel line envelope.

Table 2: Continuous carbon steel pickling bath operating envelope using diluted technical acid

Bath parameter	Operating range	Measurement /Control
HCl concentration	18–20 wt%	ASTM E224-16 titration
Temperature	70–85°C	Inline RTD, graphite heat exchanger
Immersion time	15–40 s	Strip speed and bath length ratio
Acid inhibitor dose	0.1–0.5 vol%	Metering pump, supplier-defined titration
Spent free acid	20–40 g/L	Automatic acid titrator
Spent iron	120–150 g/L	ICP-OES

Production-line records show that insufficient rinsing after the final bath can drag chloride at 3–8 L/t of strip, causing rust bloom within hours under humid conditions. The trailing rinse cascade is therefore operated with conductivity control and fresh demineralized water makeup. Acid consumption per tonne of hot-rolled scale typically ranges from 12–25 kg of 31%–37% HCl, depending on coil entry scale thickness and line speed. These values are not universal design limits but are tracked on shift logs to detect pump wear and bath contamination.

Spent pickle liquor from carbon steel lines is a high-density chloride solution rather than a simple waste acid. Before regeneration or neutralization, it typically contains 120–150 g/L iron, expressed as dissolved Fe²⁺, and 20–40 g/L free HCl. The liquor is essentially ferrous chloride solution with excess acid; its density often exceeds 1.35 g/cm³. Direct discharge is prohibited in most jurisdictions because of low pH and high dissolved iron. The material is transferred to a holding tank with a solids trap, because residual scale particles and inhibitor decomposition products can accumulate in pump suction. In plants that do not operate an acid regeneration unit, neutralization with lime or caustic produces an iron hydroxide sludge; that sludge requires filter pressing and is classified as a listed or characteristic waste depending on the generator. In plants with an acid regeneration unit, the spent liquor is pumped to a spray roaster or fluidized-bed roaster after filtration to remove particles larger than 200 µm.

Spent Acid Regeneration Demands Thermal Input and Solids Control

In a spray roaster, spent pickle liquor is atomized into a refractory-lined chamber at 800–1000°C. Water evaporates and ferrous chloride oxidizes to Fe₂O₃ and HCl; the HCl gas leaves the top of the roaster with water vapor and combustion products. Hot gas is cooled and absorbed in water to form 18–20 wt% hydrochloric acid, which is returned to the pickling line. The absorber is a packed or tray column with acid recirculation; its concentration recovery is limited by the water balance and absorber temperature, so the recovered acid is not at the original 31%–37% concentration unless a separate concentration step is used. The iron oxide byproduct is collected as a fine dust in cyclones and a downstream scrubber; its particle size and residual chloride content depend on roaster temperature and atomization. Furnace availability is sensitive to feed solids. High suspended solids in the feed cause nozzle plugging and uneven burner flame shape, so feed solids are usually kept below 1 wt%. Ferrous chloride crystallization is another operational boundary: at storage temperatures below 10–15°C, FeCl₂·4H₂O precipitates in tank bottoms and transfer lines. Steam tracing or recirculation is applied to prevent blockages. Regulatory compliance for HCl pickling and regeneration facilities in the United States is governed by 40 CFR Part 63 Subpart CCC, which sets emission standards for HCl and chlorine from affected process vents and requires periodic performance testing of scrubbers.

When Technical HCl Feeds Calcium Chloride and Metal Chloride Synthesis

When limestone is digested by technical grade HCl, the reaction is CaCO₃ + 2 HCl → CaCl₂ + H₂O + CO₂. In a stirred rubber-lined reactor, 31%–37% HCl is added at a controlled rate to crushed limestone. The endpoint is held at pH 4.0–4.5 to avoid excess free acid in the final brine; batch times in a 10 m³ reactor are generally 2–4 h because the exotherm and CO₂ evolution require foaming control. The resulting liquid is clarified by plate-and-frame filtration and then concentrated in a multi-effect evaporator to 35–45 wt% CaCl₂. If the technical acid contains sulfate above 50 mg/kg, calcium sulfate precipitates as gypsum and can scale the evaporator; this is why the specification in Table 1 is enforced at incoming inspection. The same acid can be used to produce ferric chloride by dissolving iron oxide or scrap in HCl; ferric chloride solutions for water treatment are oxidized with chlorine or hydrogen peroxide and stabilized at 30–45 wt% FeCl₃.

For polyaluminium chloride and aluminum chloride synthesis, a parallel acid-digestion route is used. Aluminum hydroxide is reacted with technical HCl at 90–110°C under reflux or pressure. The Al:Cl molar ratio is controlled between 1.5–2.2 to produce a polyaluminium chloride product with a basicity of 40–80%. In this reaction window, viscosity rises sharply as basicity approaches 80%; high-shear mixing and controlled acid addition are required to prevent batch gelling. The final product is cooled and diluted to 10–18 wt% Al₂O₃ equivalent. Because iron and heavy metals in the acid contaminate the coagulant, technical grade HCl with iron below 5 mg/kg is specified when the polyaluminium chloride is intended for drinking-water applications under EN 883 or the applicable national standard. Published data for this specific reactor configuration is limited where non-standardized geometries are used, so pilot-scale titration of the endpoint is necessary.

VCM Oxychlorination Feedstock Purity Limits

In vinyl chloride monomer production, HCl is consumed in the oxychlorination reactor where ethylene, oxygen, and hydrogen chloride react over a CuCl₂ catalyst to form ethylene dichloride: C₂H₄ + 2 HCl + 0.5 O₂ → C₂H₄Cl₂ + H₂O. Commercial reactors operate in the range 220–240°C and 0.3–0.5 MPa, with oxygen or air depending on the plant design. HCl conversion per pass is typically 97–99% in a modern fixed-bed or fluid-bed reactor train. Technical grade 31%–37% HCl is used only after vaporization and drying; water entering the reactor hydrolyzes the catalyst support and reduces selectivity to ethylene dichloride. A glass-lined vaporizer and demister are used before the HCl gas is mixed with ethylene and oxygen. Catalyst degradation is accelerated by iron, sulfate, and organic contaminants in the acid feed; therefore the impurity limits shown in Table 1 are considered minimum purchase specifications. The aqueous HCl byproduct from the quench system is not concentrated back to 31%–37% unless an absorber and stripper are installed; otherwise it is recycled as 20–25 wt% acid. Equipment in this service is specified for wet HCl at high temperature, with graphite or fluoropolymer-lined components instead of metal alloys.