Sulfide vs Sulfur Feedstock: Reading the Origin Behind Your Acid Spec

Sulfuric acid (CAS 7664-93-9) enters merchant supply chains from two primary sulfur carriers: elemental sulfur and sulfide-bearing metal concentrates. The certificate of analysis is a feedstock record because the impurity distribution differs between a sulfur furnace and a sulfide roaster offgas system. Total acidity is determined by ISO 910 or ASTM E223, while trace metals, sulfur dioxide, and nonvolatile residue are measured by methods such as EPA Method 6010D, EPA Method 1631, ISO 3423, and ASTM E223.

Sulfur-burned acid originates from elemental sulfur that is melted at 130 °C to 145 °C, filtered through 25 µm to 50 µm leaf filters, and combusted in a sulfur furnace. The cooled gas enters a 4-pass vanadium pentoxide converter with typical inlet temperatures of 410 °C to 440 °C. In double-contact double-absorption plants, acid strength is controlled at 98.5 wt% to 99.2 wt%, with iron below 5 mg/kg and SO₂ below 20 mg/kg in high-purity merchant material. Sulfide-derived acid from copper, zinc, or pyrite smelters passes through hot gas cyclones, quench towers, venturi scrubbers, wet electrostatic precipitators, and two-stage candle mist eliminators. Its final impurity load depends on the wet gas cleaning train and the ore blend being processed.

Elemental sulfur from Claus recovery often carries residual hydrogen sulfide, carbonaceous matter, and amine degradation products. These contaminants are removed by diatomaceous earth filtration and precoat filtration before combustion. Sulfide concentrates such as chalcopyrite, sphalerite, and pyrite, by contrast, release volatile arsenic, selenium, fluoride, chloride, mercury, and cadmium during roasting or smelting. The gas cleaning train is the origin-control unit; a specification reader therefore looks beyond total acidity to the trace constituents that survive drying and absorption.

In phosphate fertilizer acidulation, the origin signal appears in filterability and gypsum morphology. Phosphate rock attack with sulfide-derived acid containing residual fluoride and chloride increases corrosion in the attack tank and changes the crystal aspect ratio of gypsum. Attack tank slurry is maintained at 70 °C to 85 °C with sulfate excess of 2% to 4% over stoichiometric; iron and aluminum in the acid can stabilize gypsum nucleation, increasing fines and raising the specific cake resistance of the horizontal belt filter. Producers using sulfur-burned acid with iron below 10 mg/kg typically observe lower defoamer demand and longer filter cloth service. A change from 10 mg/kg to 80 mg/kg iron in merchant acid can increase wash water consumption by 5% to 15% on a fixed P₂O₅ throughput basis.

Aluminum anodizing lines using 15 wt% to 20 wt% sulfuric acid at 18 °C to 22 °C and current density 1.0 A/dm² to 2.0 A/dm² are sensitive to transition-metal carryover. Sulfide-derived acid containing copper, iron, and zinc at 20 mg/L to 150 mg/L can suppress uniform oxide growth and alter dye uptake when aluminum anodizing at 12 V to 18 V produces porous films with pore diameters below 25 nm. Bath maintenance with sulfur-burned acid containing 1 mg/L to 5 mg/L iron avoids the brown staining and nonuniform adsorption that otherwise require bath replacement or chelate addition.

When wet electrostatic precipitator outlet mist exceeds 5 mg/Nm³, sulfide-derived acid carries the fingerprint

Wet gas cleaning performance is the main variable separating low-impurity smelter acid from acid that fails high-purity applications. If the wet electrostatic precipitator outlet mist load exceeds 5 mg/Nm³ or the secondary voltage drops below 45 kV, arsenic, selenium, and residual dust pass into the drying tower and appear in the final acid. Plants that operate a mercury removal stage using selenium addition and calomel precipitation can hold final mercury below 0.1 mg/kg, provided the sludge blowdown rate and reaction temperature are controlled between 50 °C and 70 °C. Batch-to-batch variation in roaster feed changes gas phase arsenic concentration; a shift from 2 wt% to 6 wt% pyrite in the feed can raise weak acid impinger arsenic by a factor of 3 to 5. For this reason, producer certificates often report arsenic as a separate line item rather than folding it into nonvolatile residue.

The drying tower itself is an origin-sensitive barrier. Acid feeding a chlorine drying tower is maintained at 96 wt% to 98 wt% and at 25 °C to 35 °C to maximize water absorption without excessive mist carryover. Sulfide-derived acid with high nonvolatile solids accelerates plugging of the tower’s ceramic packing and reduces the operating cycle before acid replacement. Plant operators monitor pressure drop across the drying tower; an increase from 150 mm WC to 250 mm WC at constant chlorine flow signals salt or sulfate deposition.

What limits alkylation acid consumption when switching between sulfur-burned and smelter acid?

Stratco-type sulfuric acid alkylation units operate with a circulating acid strength of 90 wt% to 92 wt%, reactor outlet temperatures of 7 °C to 13 °C, and continuous spent acid withdrawal below 88 wt% acid equivalent. Fresh acid must be 98.5 wt% or higher, with water below 0.5 wt% and SO₂ below 10 mg/kg to avoid acid dilution and sulfonation. Sulfide-derived acid containing 20 mg/kg to 50 mg/kg SO₂ and 20 mg/kg to 80 mg/kg iron can raise acid consumption by promoting side reactions and acid-soluble oil formation. Sulfur-burned acid from filtered elemental sulfur feed typically meets these limits without post-treatment, while smelter acid may require aeration, peroxide addition, or high-efficiency demisting before charge pump injection.

Battery-grade dilution and trace metal rejection at the 10 mg/L threshold

Lead-acid battery acid is diluted from concentrated acid to a specific gravity of 1.215 to 1.285 at 20 °C; iron, manganese, chloride, and organic matter are critical. A sulfur-burned acid source with nonvolatile residue below 25 mg/kg and iron below 2 mg/kg can be diluted directly after resistivity-adjusted water addition. Sulfide-derived acid with chloride above 5 mg/kg or iron above 20 mg/kg can shorten float life by accelerating grid corrosion and oxygen recombination inefficiencies. Battery manufacturers frequently test diluted acid for nitrate, chloride, and heavy metals under ASTM D1125 for conductivity and EPA Method 9056A for anion content.

Lithium extraction from spodumene uses concentrated sulfuric acid in sulfation roasting at 250 °C to 300 °C, followed by water leaching and impurity precipitation. Iron and arsenic in sulfide-derived acid report to the sulfated calcine and can later precipitate as arsenates or hydroxides when the leach liquor is neutralized above pH 4.5. Battery-grade lithium carbonate producers set acid feed limits of iron below 10 mg/kg, arsenic below 0.5 mg/kg, and chloride below 10 mg/kg to reduce purification reagent demand and avoid arsenic-laden filter solids. Sulfur-burned acid with a narrow impurity envelope reduces the number of downstream carbonate precipitation stages and increases lithium recovery in the final filtered product.

Copper leach solvent extraction and organic phase crud build-up

Heap leach operations using raffinate makeup acid observe crud formation in solvent extraction mixers when smelter acid contains dissolved silica, humic matter, or colloidal iron. The crud layer in a primary mixer-settler with organic continuous dispersion at 1,200 rpm impeller tip speed can exceed 150 mm thickness, reducing copper transfer efficiency and increasing organic entrainment into the aqueous phase. Sulfide-derived acid that has not passed through post-dilution filtration can introduce 20 mg/L to 80 mg/L suspended solids into the raffinate, whereas sulfur-burned acid typically adds below 10 mg/L. Operators respond by raising the organic-to-aqueous ratio from 1.2:1 to 1.8:1 and specifying acid nonvolatile residue below 100 mg/kg.

Merchant acid terminals blending sulfur-burned and smelter acid can shift trace impurity levels nonlinearly because suspended iron hydroxides and selenium species may precipitate or redissolve depending on final acid strength and temperature. Storage tanks constructed of carbon steel are protected by a passive iron sulfate film only when acid concentration remains above 93 wt% and temperature below 40 °C; dilution with water below 93 wt% at local mixing points can accelerate wall loss. Terminal operators therefore sample after blend tanks have recirculated for 2 h to 4 h and before truck loading slips are released.

Where supply-chain verification is required, sulfur isotope analysis can separate sulfur-burned acid derived from sedimentary elemental sulfur from sulfide-derived acid sourced from magmatic or sedimentary ore bodies. The δ34S signature is preserved through roasters, converters, and absorbers because sulfur isotope fractionation is small relative to source variability. Procurement audits use this signature when origin labelling is disputed or when a batch must be traced to a specific smelter campaign. The test does not replace the certificate of analysis but adds a source-level check to the impurity line items.