When Fluorapatite Contains More Than 30% CaO, Sulfuric Acid Demand Shifts Nonlinearly

In wet-process phosphoric acid, digestion of fluorapatite with sulfuric acid is the largest single downstream volume driver. The dihydrate route reacts phosphate rock with sulfuric acid at 70–80°C. Rock grades between 28 wt% and 32 wt% P₂O₅ typically consume 2.6–3.2 tonnes of 100 wt% H₂SO₄ per tonne of P₂O₅. The reaction converts fluorapatite to phosphoric acid and gypsum. Free sulfate in the attack liquor is maintained at 2–4 wt% to control gypsum crystal habit. Excess sulfate produces acicular gypsum that blinds tilting-pan filter cloths. Insufficient sulfate forms crusts on unreacted rock and reduces digestion efficiency. The phosphoric acid leaving the filter contains 26–28 wt% P₂O₅. It is evaporated to 52–54 wt% P₂O₅ for ammonium phosphate production. Terminal products include monoammonium phosphate, diammonium phosphate, and triple superphosphate. Fertilizer compliance is covered by EU 2019/1009, which sets contaminant ceilings for cadmium, chromium, mercury, nickel, and lead. Sulfuric acid handling and registration fall under REACH Regulation (EC) No 1907/2006.

Reactor trains use brick-lined carbon steel with rubber interlayers because dilute acid at 70–80°C is too aggressive for unlined carbon steel. Chloride above 0.05 wt% in phosphate rock causes pitting corrosion in stainless filter feed lines. Flash coolers maintain the digestion setpoint and control gypsum precipitation. Fluoride gases are scrubbed in venturi systems. Gypsum slurry is sent to lined storage ponds or plasterboard operations. Startup rock moisture above 8 wt% dilutes the first-stage acid and drops reaction temperature. Operators compensate by adding 93–98 wt% acid upstream of the attack tank. Phosphate rock with high magnesium oxide content consumes additional sulfuric acid to form magnesium sulfate. That raises liquid viscosity and lowers filter rate. Published data for ore below 20 wt% P₂O₅ is limited, and pilot-scale evaluation is required before plant design.

Refinery alkylation units using sulfuric acid as catalyst maintain acid strength in the reactor emulsion at 88–93 wt% H₂SO₄ rather than the 98 wt% fresh acid strength. The acid phase contains acid-soluble oil, water, and dissolved light hydrocarbons. Isobutane-to-olefin ratio is controlled between 7:1 and 12:1 to suppress polymerization and heavy acid-soluble oil formation. Reactor temperature is held at 4–14°C. Higher temperatures increase olefin polymerization and acid consumption. The terminal alkylate is separated by distillation and blended into high-octane gasoline. Research octane number is measured under ASTM D2699. Motor octane number is measured under ASTM D2700. Sulfuric acid alkylate typically runs between 92 and 97 RON, depending on feedstock composition and reactor configuration.

Spent acid withdrawal is scheduled when acid strength approaches 88 wt% or when acid-soluble oil exceeds 6–8 wt%. The spent acid moves to a regeneration plant where acid-soluble oil is thermally decomposed and sulfuric acid is returned at 98 wt%. Diene and mercaptan levels in the olefin feed directly increase acid consumption and red acid formation. Units processing high-sulfur FCC olefins require more frequent spent acid bleeding. Pressure vessels and contactors are constructed to ASME Section VIII. In the United States, process safety management for the acid inventory and associated hydrogen sulfide hazards falls under 29 CFR 1910.119.

Free-Acid and Ferrous Sulfate Feedback in Continuous Strip Pickling Lines

Before cold rolling or continuous galvanizing, hot-rolled steel strip passes through a sulfuric acid pickling bath to remove magnetite and hematite scale. The bath contains 5–15 wt% H₂SO₄ at 60–85°C. An acid inhibitor is metered at 0.05–0.3 vol% to reduce base metal dissolution while descaling continues. As pickling proceeds, ferrous sulfate heptahydrate accumulates and free acid declines. At 60–100 g/L Fe, bath efficiency falls sharply. Ferrous ion slows scale dissolution and can deposit as iron sulfate crystals below 25°C. Operators bleed spent pickle liquor to crystallizers or spray-roast regeneration plants. The terminal strip is rinsed, dried, and either cold rolled, galvanized, or enameled. Surface cleanliness after pickling is verified against ISO 8501-1 rust grade comparisons. Descaling practices for stainless materials are referenced in ASTM A380/A380M-17.

Copper contamination above 50 mg/L in the pickle bath causes cementation of copper onto the steel surface and induces pitting during subsequent rolling. Brass valves and fittings must be excluded from the recirculation loop. Iron sulfate crystallization in overflow lines is a common production bottleneck in continuous push-pickle lines. Heating coils are fabricated from graphite or tantalum because dilute hot sulfuric acid attacks conventional stainless steel. Scale removal rate depends on strip speed, bath agitation, and free-acid titration. Side headers and turbulent flow reduce diffusion-layer stagnation on the strip surface.

Lead-Acid Battery Electrolyte Quality Is Defined by Trace Metal Limits

Fresh acid for lead-acid cells is diluted from 93–98 wt% H₂SO₄ with deionized water to a filling gravity of 1.250–1.285 at 25°C. The resulting electrolyte contains 30–38 wt% H₂SO₄. Trace iron, copper, manganese, and chloride catalyze self-discharge at the negative plate. Electrolyte-grade acid is supplied with iron below 50 mg/kg, copper below 5 mg/kg, and chloride below 10 mg/kg in the diluted bath. Dilution water is prepared to ASTM D1193 Type II or equivalent demineralized specifications. Flooded SLI batteries must meet electrolyte density and capacity requirements under IEC 60095-1. Terminal electrolyte supports automotive starter batteries, VRLA cells, and traction batteries.

Filling temperature is kept below 35°C to avoid accelerated grid corrosion. Organic contaminants from uncured plastic tanks or dirty mixing equipment lower hydrogen overpotential and increase water loss. Mixing tanks are constructed from polypropylene, PVDF, or ebonite-lined steel, never unlined carbon steel. Batch-to-batch density checks use a hydrometer or digital density meter calibrated at 25°C. Operators adjust with concentrated acid or ASTM D1193 Type II water. Electrolyte-specific gravity shifts with temperature, and correction tables are applied before final fill.

In the sulfate route for titanium dioxide, concentrated sulfuric acid is consumed both as digestion medium for ilmenite and as hydrolysis control agent in titanyl sulfate solution. Ground ilmenite or primary slag is digested with 85–93 wt% H₂SO₄ at 160–210°C. The acid-to-feed mass ratio is held between 1.4 and 2.0, depending on titanium dioxide content and iron content. The resulting sulfate cake is dissolved in water and clarified. Ferric iron is reduced to ferrous iron with scrap iron to prevent premature hydrolysis. Hydrolysis is conducted at 94–105°C with seed crystals to precipitate hydrous titanium dioxide. The precipitate is filtered, washed, calcined at 800–1000°C, and milled to pigment fineness. Titanium dioxide pigment is classified under ISO 591-1 by rutile content, color, and matter volatile at 105°C.

High calcium and magnesium impurities in the feed consume sulfuric acid to form stable sulfates and increase acid demand beyond the nominal ratio. Iron sulfate heptahydrate is a large co-product stream and must be crystallized, filtered, and either sold or neutralized. Digestion vessels are brick-lined and operate under high gas generation. Incompatibility with unlined stainless steel is severe at 160–210°C because hot concentrated sulfuric acid is highly oxidizing to stainless alloys. Hydrolysis seed ratio and calcination residence time determine whether the pigment develops anatase or rutile structure. Published data for low-grade ilmenite below 45 wt% TiO₂ is limited for this specific reactor configuration.

What Governs Linear Alkylbenzene Sulfonic Acid Color and Sulfone Formation?

Sulfonation of linear alkylbenzene with oleum or concentrated sulfuric acid converts the alkylbenzene to linear alkylbenzene sulfonic acid, the core anionic surfactant intermediate. Batch reactors using 98 wt% H₂SO₄ or oleum with 20–30% free SO₃ operate at 20–50°C with an aging time of 1–4 h. The acid-to-LAB molar ratio is controlled near 1.2:1 for concentrated acid sulfonation. Higher ratios generate sulfones and darken the product. Water formed during sulfonation dilutes the acid and reduces reaction rate. Spent sulfuric acid is separated by gravity and may be recycled or regenerated. The acid form is neutralized with sodium hydroxide to sodium alkylbenzene sulfonate. Anionic active matter is determined by two-phase titration per ISO 2271. Finished surfactant must satisfy biodegradability limits in EU Detergents Regulation (EC) No 648/2004 Annex II, with ultimate aerobic biodegradability above 60% in OECD 301B tests. Free oil in LABSA is maintained below 1.5 wt%, and free sulfuric acid below 2.0 wt% after neutralization.

Feed moisture above 0.5 wt% accelerates dilution of oleum and shifts the SO₃ balance. Low-temperature aging below 20°C produces a high-viscosity product that separates poorly from spent acid. Glass-lined or enameled reactors are specified because hot oleum attacks stainless steel. Terminal LABSA and sodium sulfonate are used in laundry powders, dishwash detergents, and institutional cleaners.

For oxide copper heaps, sulfuric acid-conditioned raffinate dissolves malachite and chrysocolla at pH 1.8–2.2. Irrigation liquor contains 5–15 g/L H₂SO₄. Acid consumption ranges from 2–5 tonnes per tonne of copper cathode, depending on limestone, chlorite, and iron oxide gangue. Pregnant leach solution is clarified and forwarded to solvent extraction. Organic extractant loads copper selectively. Stripped electrolyte is electrowon to LME Grade A copper cathode with 99.99 wt% Cu minimum. Bismuth and lead are controlled below 2.0 mg/kg and 5.0 mg/kg respectively in the cathode. Solvent extraction crud formation rises when soluble silica in the pregnant leach solution exceeds 0.5 g/L. Acid addition is adjusted by continuous pH probes in the raffinate return line. HDPE drip lines and multi-stage centrifugal pumps with silicon carbide mechanical seals are standard. Terminal copper cathode is sold to wire rod mills and brass producers.

Silica dissolution can also decrease heap permeability if colloidal silica precipitates in the ore pores. Operators install automatic acid dosing linked to pH setpoint. Sulfuric acid with high sulfate salt concentration may form gypsum scale in pipes if calcium is present. Published data for high-altitude heap operations with temperature swings above 30°C is limited for this specific control scheme.