In softwood kraft pulp bleaching, chlorine dioxide is generated on site from sodium chlorate, sulfuric acid and methanol or hydrogen peroxide in an ERCO R8/SVP-LITE generator; the resulting ClO₂ solution is applied in a D0 delignification stage at 0.18–0.25 active chlorine multiple on oven-dry pulp, pH 2.5–3.5, 55–70 °C and 30–60 min retention. For kraft pulps entering bleaching with kappa number 28–32, D0 reduces kappa number to 6–10 before an oxidative extraction stage using H₂O₂ at 0.3–0.8 wt% on oven-dry pulp, NaOH at 1.2–2.5 wt% and MgSO₄ at 0.05–0.1 wt% as peroxide stabiliser at 75–90 °C and 60–90 min retention. A final D1 ClO₂ charge of 0.5–1.0 wt% active chlorine is applied at pH 3.5–4.2 to achieve kappa number 1.0–2.5 for paper-grade bleached pulp or below 1.0 for dissolving pulp. ISO 302:2015 kappa number and ISO 2470-1:2016 diffuse blue reflectance brightness are the routine release tests. Bleach towers are upflow vessels fabricated from titanium or 316L stainless steel; static mixers are used for ClO₂ addition and gas-phase monitors are required because ClO₂ partial pressure above 300 mm Hg is explosive. The main process failure is peroxide decomposition caused by transition-metal carryover; Fe²⁺ and Mn²⁺ in filtrate above 0.5 mg/L and 0.2 mg/L, respectively, force an upstream Q chelation stage with EDTA or DTPA at 0.2–0.5 kg/ton pulp. The terminal outputs are bleached softwood kraft pulp for fluff pulp, coated paperboard and dissolving pulp for viscose staple fibre.

What limits sodium persulfate microetch uniformity on fine-line substrates?

Inner-layer copper roughening prior to dry-film lamination uses sodium persulfate microetch solutions formulated at 80–120 g/L Na₂S₂O₈, 1–3 vol% H₂SO₄ and 20–40 g/L Cu²⁺ at 28–35 °C. Spray impingement pressure is held at 1.0–2.5 kg/cm² in conveyorised etchers with 316L or titanium nozzle banks; this yields etch depth of 0.5–1.5 µm and copper removal rate of 0.8–1.8 µm/min. The terminal surface topography is evaluated by IPC-TM-650 2.5.17D peel strength after oxide replacement; a peel strength of 0.8 N/mm is the common line acceptance threshold for Class 3 inner-layer adhesion. The limiting variable is cupric ion accumulation. Bath life ends when Cu²⁺ reaches 40–55 g/L because etch rate becomes non-linear and sidewall attack on fine-line geometries increases. Sodium persulfate decomposition accelerates above 50 °C; temperature-control loops holding ±2 °C are required to avoid exothermic decomposition and localised over-etch. Replenishment is controlled by copper-dissolution counts from the rectifier amp-hour total and specific gravity measurement at 1.08–1.15. The terminal product is high-density interconnect and multilayer printed wiring board with copper surface roughness suitable for dry-film adhesion and layer-to-layer registration.

Advanced oxidation of refinery secondary effluent under Fenton chemistry

After secondary clarification, refinery effluent with chemical oxygen demand 120–400 mg/L is treated in a pH-controlled oxidation basin by dosing H₂O₂ 35 wt% at 500–1,500 mg/L and FeSO₄·7H₂O at a molar ratio H₂O₂:Fe²⁺ of 2:1 to 10:1. The reaction pH is maintained at 2.8–3.5 with 93–98 wt% sulfuric acid; redox potential is typically above 450 mV versus Ag/AgCl. Hydraulic retention time of 30–90 min in glass-lined or 316L stirred reactors reduces chemical oxygen demand to 30–80 mg/L. The spent iron is then neutralised with Ca(OH)₂ to pH 8.0–9.0 and flocculated with anionic polymer at 0.5–2 mg/L, producing ferric hydroxide sludge at 0.3–0.7 kg dry solids/m³. Treated effluent is tested according to ISO 6060:1989 for chemical oxygen demand and ISO 7887:2011 for colour. Carbonate and bicarbonate alkalinity above 300 mg/L as CaCO₃ scavenges hydroxyl radicals, so acidification to below pH 3.5 strips alkalinity as CO₂ but increases sulfuric acid consumption. Residual H₂O₂ after the clarifier must be quenched with sodium bisulfite or catalase to below 0.5 mg/L before discharge because peroxide interferes with chlorine disinfection residual measurement and exerts an oxygen demand in receiving water. The terminal output is treated refinery effluent suitable for discharge or reuse in cooling-tower make-up after pH adjustment.

Gold mill tailings containing free cyanide 50–350 mg/L are treated with hydrogen peroxide and soluble copper catalyst to oxidise cyanide to cyanate. The reaction is run at pH 9.5–10.5 with Cu²⁺ at 10–50 mg/L and a H₂O₂:CN⁻ molar ratio of 1.5:1 to 3:1 for free cyanide; weak acid dissociable cyanide complexes can require up to 5:1 depending on copper loading. Retention time in agitated tailings-treatment reactors is 15–45 min for free cyanide and 45–120 min for weak acid dissociable cyanide. The treated water is measured by ISO 14403-1:2012 for total and free cyanide; free CN⁻ is reduced below 0.2 mg/L and weak acid dissociable cyanide below 0.5 mg/L before discharge to the tailings storage facility. Thiocyanate and cyanate are not completely mineralised by peroxide alone, so total nitrogen in the treated water may remain elevated. Excess H₂O₂ must be maintained below 5 mg/L in the final tailings water to avoid interference with downstream lime neutralisation and arsenic coprecipitation. Equipment is HDPE or rubber-lined carbon steel because alkaline cyanide solutions are corrosive to unprotected steel. The terminal output is cyanide-compliant process water for recycle or controlled discharge.

When hydrogen peroxide is converted over titanium silicalite-1 in propylene epoxidation

At 40–60 °C and 1.5–3.0 MPa, H₂O₂ 35–50 wt% is fed with methanol and propylene into a fixed-bed or slurry reactor containing titanium silicalite-1 catalyst pellets or extrudates. The pressure maintains propylene in the liquid phase; H₂O₂ conversion is typically 99%, propylene oxide selectivity 95–98%, and the main by-product is propylene glycol from epoxide hydrolysis. Methanol is recovered by distillation and recycled. The epoxidation is exothermic; adiabatic temperature rise is controlled by shell-and-tube reactor cooling with chilled water at 10–20 °C. Alkali metal ion concentration in the feed must be below 1 mg/kg because Na⁺ or K⁺ blocks the titanium silicalite-1 micropores and reduces catalyst life. The crude propylene oxide is purified by extractive distillation to 99.95 wt% before polyether polyol synthesis. Downstream polyol hydroxyl number is determined by ASTM D4274-21. The terminal outputs are propylene oxide, propylene glycol and polyether polyols used in rigid polyurethane foam, unsaturated polyester resin and alkoxylates.

When chlorinated solvent source zones containing trichloroethylene and tetrachloroethylene are treated, in-situ chemical oxidation is implemented via injection well networks screened in the saturated zone. Sodium permanganate is delivered at 1–5 g/L as MnO₄⁻ in potable water, with oxidant demand testing before full-scale injection. The stoichiometric demand for complete oxidation of trichloroethylene is 2 mol MnO₄⁻ per 1 mol trichloroethylene; natural soil oxidant demand from reduced iron and organic carbon typically increases injection mass by 2–10 times the stoichiometric requirement. Injection pressure is kept below 0.5 bar above formation pressure to avoid daylighting and uncontrolled vertical migration. Groundwater monitoring uses USEPA SW-846 Method 8260D for volatile organic compounds; permanganate breakthrough is tracked by pink colour in monitoring wells and spectrophotometric absorbance at 525 nm. The terminal output is reduced volatile organic compound mass in groundwater; however, manganese dioxide precipitation from MnO₄⁻ reduction can reduce well yield by 30–70% if post-injection rehabilitation is not scheduled. In aquifer intervals where natural oxidant demand exceeds 10 g/kg, sodium persulfate is substituted because permanganate persistence becomes insufficient for treatment of the target source zone.

Aseptic packaging sterilant concentration and residue compliance

Roll-fed paperboard and blow-moulded polyethylene terephthalate containers are treated with peracetic acid generated by equilibrium reaction of acetic acid, hydrogen peroxide and water; the sterilant is dosed as a 0.1–0.5 wt% peracetic acid solution at 40–60 °C with contact time 15–60 s. The bath is sprayed through stainless steel nozzles onto the packaging surface and then dried with sterile hot air at 70–90 °C to reduce H₂O₂ residues below 0.5 mg/L in the packaged product. The oxidation process kills spores of Alicyclobacillus acidoterrestris and Geobacillus stearothermophilus; log reduction is validated according to ISO 14161:2009 biological indicators. The sterilant is used in compliance with FDA 21 CFR 178.1005, which permits hydrogen peroxide as a food-contact sanitizer, and the packaging material must meet EU Regulation (EC) No 1935/2004 Article 3 migration limits. Mechanical integrity of the container is inspected on-line by pressure decay at 80–120 kPa and high-voltage leak detection. The terminal product is aseptically packaged fruit juice, dairy beverage or UHT soup.