Under Commission Decision 2000/532/EC as amended by Commission Decision 2014/955/EU, the European Waste Catalogue (EWC) entry 06 01 01* designates sulphuric and sulphurous acid from manufacture, formulation, distribution and use as hazardous. Sulfuric Acid Waste and Neutralisation: EWC Codes and Disposal Rules therefore begin with the asterisk and the corresponding hazard properties in Commission Regulation (EU) No 1357/2014. Free sulphuric acid at C ≥ 15 wt% is assigned Skin Corr. 1A H314 and Eye Dam. 1 H318 under Annex VI of Regulation (EC) No 1272/2008, while the range 5 wt% ≤ C < 15 wt% triggers Skin Irrit. 2 H315 and Eye Irrit. 2 H319. pH measurement to ISO 10523:2012 is therefore not a substitute for concentration-based classification. A waste with pH 2.0 and only 3 wt% H₂SO₄ may not be HP8 unless other dissolved acids or metal species drive corrosivity. Directive 2008/98/EC Article 18(1) prohibits mixing hazardous waste with non-hazardous waste or other substances except under permit; deliberate dilution to fall below the 15 wt% specific concentration limit is not a lawful declassification route.

Which EWC Code Applies When Spent Acid Is Recovered Rather Than Discarded?

EWC codes apply only to waste. Spent sulphuric acid regenerated by thermal decomposition in a spent acid regeneration unit at 1,000–1,100 °C may leave the waste regime as a by-product or end-of-waste substance only if the criteria in Directive 2008/98/EC Article 6 are met: the substance has a lawful use, a market or demand exists, and use does not cause overall adverse environmental or human health impacts. If these criteria are not met, the stream remains waste and the appropriate EWC code is 06 01 01*, 11 01 05* or 16 06 06*, depending on origin. A consignment note that omits the asterisk on EWC code 06 01 01* is a classification error under the Waste Framework Directive and may also affect the ADR transport classification.

Neutralisation of free H₂SO₄ with sodium hydroxide follows H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O. The strong acid–strong base reaction releases approximately 114 kJ mol⁻¹ of H₂SO₄ for the two proton transfers; additional dilution heat from 96–98 wt% acid raises the first-stage temperature rapidly. The addition order is fixed: acid is metered into pre-charged water or dilute alkali through a PTFE-lined dip tube; reverse addition of caustic into 96 wt% acid can boil the liquid at the nozzle and create a corrosive aerosol.

In a two-stage continuous stirred-tank reactor train, the first stage is commonly a glass-lined or PTFE-lined vessel of 5–10 m³ working volume, with liquid height equal to tank diameter. A 45° pitched-blade turbine operating at 100–150 rpm provides bulk circulation; an external recirculation loop discharges through a shell-and-tube heat exchanger because the neutralisation heat must be removed at approximately 2.1 MJ kg⁻¹ of incoming 96 wt% H₂SO₄. The heat load becomes significant when spent acid is received at bulk rates above 2 m³ h⁻¹. Stoichiometric demand for a 10 wt% H₂SO₄ stream at 20 °C is approximately 114 L of 50 wt% NaOH per cubic metre of acid, based on 2.17 kmol NaOH m⁻³; for 96 wt% acid, the sodium hydroxide demand rises to roughly 1.9 m³ of 50 wt% NaOH per cubic metre of acid. The first-stage pH is held at 4.0–5.0 to control gypsum supersaturation when lime is used, or at 2.5–3.5 when iron is present as Fe³⁺, allowing Fe(OH)₃ to precipitate before final neutralisation.

Final pH trim in the second stage is set between 8.5 and 9.5 for lime treatment or 7.0–8.5 for sodium hydroxide, depending on sewer permit conditions under Directive 2010/75/EU. A cascade pH controller with inner flow control on the alkali pump is preferable to direct pH-to-valve control because the titration curve is steep near equivalence; a change of 0.2 pH units near neutrality can correspond to a large change in alkali demand per litre. Redundant pH probes are installed in the recirculation line in retractable holders, with automatic cleaning using 5 vol% HCl every 8–12 h to remove gypsum or metal hydroxide films. Probe response lag in high-solids slurry is typically 10–30 s, so integral time must be longer than the reactor turnover time; otherwise the loop oscillates and overshoots pH 9.5.

If spent acid contains amphoteric zinc or lead, pH 9.0 is not the correct endpoint

Spent pickling acid and battery electrolyte frequently contain dissolved lead, zinc, copper, iron and arsenic. Single-stage neutralisation to pH 7.0 may fail the leachability criterion for a non-hazardous filter cake because amphoteric Zn(OH)₂ and Pb(OH)₂ re-dissolve at elevated pH. Iron(III) hydroxide precipitates most effectively between pH 3.5 and 4.5; copper(II) hydroxide has its minimum solubility between pH 8.5 and 9.5. Zinc hydroxide solubility increases below pH 8.0 and above pH 11.0, while lead hydroxide redissolves above approximately pH 10.5. A staged neutralisation sequence is therefore required when the spent acid carries mixed metals: first raise pH to 3.5–4.5 for iron hydrolysis, separate the iron hydroxide sludge, then raise the clarified liquor to 9.0–9.5 for copper and zinc precipitation. If the flow is raised to pH 11.0 to destroy cyanides or precipitate nickel, the final discharge must be re-acidified to below pH 10.0 to prevent lead and zinc redissolution.

Leach testing of the dewatered sludge under EN 12457-2:2002 is used to determine whether the filter cake is hazardous by HP14. A sludge that passes the leach criteria may still be assigned EWC code 06 05 03 for non-hazardous effluent-treatment sludge; if leachate metals exceed the thresholds, the sludge is 06 05 02* and must be sent to a hazardous waste landfill or thermal treatment. The exact endpoint cannot be inferred from pH alone because sulphate-bound complexes such as lead sulphate and calcium arsenate have different solubility responses.

Why gypsum scaling restricts filter-press throughput in lime-based neutralisation

Calcium hydroxide slurry at 10–20 wt% Ca(OH)₂ neutralises sulphuric acid to gypsum, CaSO₄·2H₂O. Gypsum is less soluble than sodium sulphate, which reduces the dissolved solids load but creates a crystalline deposit on pH probes, overflow weirs and filter-press feed lines. The solubility of gypsum in water is approximately 2.4 g L⁻¹ at 25 °C; below the first-stage pH setpoint of 4.5 calcium hydroxide dissolution is rapid, and the resulting supersaturation can blind filter cloths within one shift. Scale control is mechanical rather than chemical: the first-stage CSTR is fitted with high-turbulence baffles and a bottom-cone discharge to prevent accumulation, and transfer lines are flushed with 5 vol% HCl after every 4–6 h of continuous operation.

Filter-press operation for lime-neutralised sludge uses polypropylene chamber plates with membrane squeeze at 8–15 bar. Cake solids of 35–45 wt% are common for gypsum-dominant sludge; lower solids indicate excess ferric hydroxide or colloidal silica. If the cake does not release cleanly, the filter cloth is conditioned by soaking in 3–5 vol% HCl and scraping with a polypropylene plate. Sodium hydroxide avoids gypsum scaling but the resulting filtrate can exceed 50 g L⁻¹ dissolved solids, which may violate a sewer consent for total dissolved solids or conductivity. The choice between lime and caustic is therefore not only a sludge-management decision but a compliance decision for the wastewater discharge permit.

Off-site transfer of waste sulphuric acid is governed by ADR 2023. Waste sulphuric acid with more than 51 wt% acid is assigned UN1830, Sulphuric Acid, Class 8, Packing Group II; spent acid at 51 wt% or less is UN2796, Battery Fluid, Acid, Class 8, Packing Group II. A tanker carrying 06 01 01* sulphuric acid therefore displays the corresponding UN number and the Class 8 label, and the transport document must include the EWC code and the hazardous property codes HP8 and HP14 where applicable. Under Regulation (EC) No 1013/2006, transboundary movement of 06 01 01* or 16 06 06* waste requires prior written notification and consent before shipment; Annex IA of the Regulation requires the EWC code, composition, and treatment method. Liquid waste sulphuric acid is prohibited from landfill under Directive 1999/31/EC Article 5(3)(a), so disposal routes are limited to neutralisation, regeneration, or thermal decomposition at a permitted hazardous waste facility.

The final neutralised effluent may be discharged only under an environmental permit issued under Directive 2010/75/EU, with continuous pH recording and alarm setpoints at pH 6.0 and pH 9.0; batch discharges require recorded pH within 6.5–9.0 and immediate shutdown of the discharge valve upon breach. After neutralisation, a liquid that still fails pH 6.0–9.0 or contains dissolved metals above permit thresholds remains hazardous under EWC code 06 05 02* if it is an effluent-treatment sludge. A filter cake that passes EN 12457-2:2002 and is not flammable or ecotoxic may be coded 06 05 03. The operator should not use general municipal waste codes for the original acid sludge because the original acid code is retained until the waste has been fully transformed and classified.