Making calcium chloride, ferric chloride and PAC: HCl as the chloride source requires a tightly specified acid feedstock because the chloride ion constitutes the functional anion in each of these inorganic salts and coagulants. Technical-grade hydrochloric acid is most often received at 31.45% or 35.0% w/w HCl and is specified under ASTM E1146-21. The acid is not a minor acidulant but a stoichiometric anion donor; therefore excursions in concentration, iron, sulfate, organic matter, or heavy metals propagate directly into product basicity, insoluble residues, and coagulant performance. Production-scale lines typically combine rubber-lined steel reactors, graphite heat exchangers, and CPVC/PVDF piping with wet scrubbers because acid vapour pressure, hydrogen evolution, and chloride stress-corrosion risks define equipment selection more than general chemical compatibility tables suggest.

Calcium Chloride Production via Limestone Dissolution and HCl Stoichiometry

Calcium chloride is obtained by reacting calcium carbonate with hydrochloric acid according to CaCO₃ + 2 HCl → CaCl₂ + CO₂ + H₂O. The theoretical consumption is 0.657 kg HCl gas per 1.000 kg anhydrous CaCl₂, corresponding to 2.09 kg of 31.45% w/w acid per kg of anhydrous CaCl₂. In production, the terminal pH is held between 4.0 and 5.5 to avoid surplus acid carryover while ensuring complete carbonate reaction. The CO₂ release is 0.396 kg per kg anhydrous CaCl₂, and the reactor headspace must be designed for gas disengagement; acid is often introduced below the liquid surface to suppress aerosol emission. Limestone is dry-ground to a median particle size below 100 µm where rapid dissolution is required, which increases dust-handling requirements. Product quality under ASTM D98-15 is controlled primarily by magnesium, iron, and sulfate impurities because these remain in the dried flake or granule. The process is comparatively stable; the main production bottleneck is not reaction chemistry but solids dewatering and drying energy input. Published data for specific batch-time kinetics in mixed acidulation reactors is limited; therefore the acid feed is trimmed by pH cascade control on the recirculation line rather than by a single fixed time.

What Limits Iron Dissolution Efficiency in HCl-Based Ferric Chloride Reactors?

Ferric chloride production from scrap iron or iron oxide uses HCl as the dissolution medium and as the chloride source for the oxidation step. In the scrap-iron route, Fe + 2 HCl → FeCl₂ + H₂, and the subsequent oxidation 2 FeCl₂ + 2 HCl + ½ O₂ → 2 FeCl₃ + H₂O consumes additional acid. On a theoretical basis, the acid demand is 0.674 kg HCl gas per 1.000 kg anhydrous FeCl₃ via the iron oxide route. The dissolution reaction is exothermic and generates hydrogen; therefore the vessels are inerted with nitrogen and fitted with flame arrestors because the lower flammability limit of hydrogen in air is 4.0 vol%. Temperature control is the main process conflict: low temperatures reduce dissolution rate, while sustained operation above 85°C increases HCl vapour pressure and accelerates degradation of rubber-lined steel. Graphite heat exchangers are used because titanium passivity cannot be relied upon in concentrated hot HCl. The oxidation section is often a packed-bed gas-liquid column, and the circulation pumps must tolerate high ferric ion concentrations and abrasive fine carbon or siliceous matter from scrap feed. Commercial liquid ferric chloride for water treatment is commonly 40–42% w/w FeCl₃ with a density of 1.45–1.50 g/cm³ at 25°C; free acid is usually controlled below 0.5% w/w HCl to avoid excessive coagulant pH depression. Potable water grades must meet NSF/ANSI/CAN 60. The principal field failure mode is hydrogen carryover into the vent header when the liquid-to-gas ratio falls below the scrubber design curve; ferric chloride mist also fouls demister pads and caustic scrubbers when the off-gas is not pre-cooled.

When PAC Basicity Control Moves from Aluminum Hydroxide to Hydrochloric Acid

Polyaluminium chloride is produced by controlled digestion of hydrated alumina or aluminium hydroxide with hydrochloric acid. The general composition is represented as [Al₂(OH)_nCl_6-n]_m. In this chemistry, HCl is not used for complete dissolution; instead the acid input fixes the product basicity, defined as the molar ratio of hydroxylated aluminium species to total aluminium. Industrial potable water grades are commonly specified at 10–17% Al₂O₃ and basicity 45–65%. A lower basicity increases cationic charge density but raises residual chloride and product corrosivity; a higher basicity can produce high-viscosity or gelled solutions, especially after aging at temperatures below 10°C. Published data for low-temperature gel-time kinetics in high-basicity PAC is limited. Hydrochloric acid is preferred over sulfuric acid when the sulfate content must remain below 1.0% w/w in the final liquid, because sulfate shifts aluminium speciation and may precipitate insoluble aluminium sulfate complexes. Digestion is typically run at 80–95°C with high-shear mixing; the exotherm is controlled by metering acid rather than by external cooling alone. Residual insoluble alumina is maintained below 0.5% w/w to protect dosing pumps and filtration systems. The final product is clarified or filtered before storage. For drinking water applications, compliance is assessed under NSF/ANSI/CAN 60; in the EU, EN 883:2004 applies to polyaluminium chloride hydroxide used for water treatment.

Chloride source purity is not a secondary variable. In PAC synthesis, iron can colour the product and manganese or zinc can exceed drinking water limits after dosing. For ferric chloride, by-product HCl streams may contain ferrous chloride that shifts the oxidation balance; batch-to-batch variance above 0.5% w/w Fe is usually sufficient to push the final concentration out of the 40–42% target window. In calcium chloride production, sulfate in the acid reacts with calcium to form gypsum, which deposits on heat-exchanger surfaces; therefore acid sulfate is tightly limited. The acid should be lot-tested for density, total acidity, iron, sulfate, and heavy metals. ASTM E1146-21 gives specification guidance for technical hydrochloric acid, but it does not cover every coagulant-grade impurity concern. A single acid source may be acceptable for calcium chloride and ferric chloride, but PAC production often requires a lower-iron, lower-sulfate acid or additional purification because the aluminium speciation is sensitive to foreign anions.

Rubber-lined steel storage tanks with bromobutyl rubber are used for 31.45% HCl because carbon steel is vulnerable to acid attack and chloride stress-corrosion cracking. Piping at ambient temperature is commonly CPVC; PVDF or graphite is used where temperatures exceed 60°C. Fuming from hydrochloric acid is controlled by vent scrubbers with caustic solution; the scrubber liquid-to-gas ratio must be maintained according to the supplier’s design curve because off-gas composition changes during batch startups and shutdowns. In calcium chloride reactors, acid addition below the liquid level reduces aerosol release; in ferric chloride reactors, hydrogen venting takes precedence over acid fume capture. The two requirements conflict because a single scrubber cannot simultaneously handle an explosive gas mixture and acid mist with optimal efficiency. Production-scale layouts therefore use separate hydrogen and acid vent headers with independent scrubbers and flame arrestors. This separation is not a convenience but a direct consequence of the process safety boundaries.

Water balance controls final product concentration more than acid strength alone. For ferric chloride, excess water must be removed by evaporation under vacuum to reach the 40–42% FeCl₃ specification; falling-film evaporators with graphite tubes are used because ferric chloride solutions are aggressive to stainless steel. For PAC, the water content is set by the aluminium hydroxide slurry concentration; overdilution lowers Al₂O₃ below 10% and may force additional evaporation. In calcium chloride, the final concentration is adjusted in multiple-effect evaporators, and viscosity rises steeply as the product approaches flake-forming concentration. This water-removal step often determines the practical acid concentration selected at the front end: stronger acid reduces water load but increases storage fuming and vapour control demands.

By-product HCl Streams Are Not Automatically Suitable for PAC Synthesis

By-product hydrochloric acid from chlor-alkali or chlorination operations can be used as a chloride source, but the impurity profile must be validated for each downstream coagulant. In PAC production, trace chlorinated organics can stabilize emulsion-like phases and reduce filterability; heavy metals such as chromium or nickel may violate drinking-water chemical limits. In ferric chloride production, some by-product acids already contain ferrous chloride, which changes the oxidation balance. This can be beneficial if the iron content is stable, but batch-to-batch variance above 0.5% w/w Fe can shift the final product out of the 40–42% FeCl₃ specification. In calcium chloride production, organic impurities can cause foaming and odour; activated carbon treatment or distillation may be required before the acid is used. By-product HCl must not be blended into feedstock for potable-water coagulants without lot-specific analytical certification against NSF/ANSI/CAN 60 or EN 883:2004. Published data for specific by-product impurity limits in PAC applications is limited; therefore each acid lot is typically qualified by laboratory-scale digestion before production-scale use.