Inside a Smelter Acid Plant: How Mercury and Selenium Are Controlled

Sulfuric acid produced from non-ferrous smelter off-gases carries an impurity load not present in sulfur-burning acid plants. Roaster, flash-smelting, and converting gases contain sulfur dioxide at 8–14 vol% after dilution and dust removal, along with volatile arsenic, lead, halides, mercury, and selenium. The distribution of mercury and selenium is concentrate-dependent. Published data from copper, zinc, and lead smelter acid plants indicate mercury concentrations entering gas cleaning can range from 3 mg/Nm³ to more than 40 mg/Nm³, while selenium may vary from 5 mg/Nm³ to above 150 mg/Nm³ during high-selenium concentrate campaigns. Without dedicated control, mercury passes into the drying and absorption circuits and contaminates product acid, while selenium dioxide scrubbed into weak acid is reduced to red elemental selenium that fouls tower packing, mist eliminators, and acid coolers.

Smelter gas first passes a hot electrostatic precipitator at 320–380°C to remove metallurgical particulate. The gas then enters a quench tower using recirculated weak acid to cool adiabatically to 65–75°C. In this wet gas cleaning stage, selenium dioxide begins to dissolve as selenious acid, while sulfur dioxide present in the gas drives reduction to elemental selenium. The weak acid circuit is therefore more than a dust scrubber; it is the first selenium control point. A controlled bleed from the quench circuit is maintained at 5–20 wt% H₂SO₄ to limit selenium accumulation. Downstream wet electrostatic precipitators capture acid mist containing dust, chloride compounds, and selenium particles. The removal split across these stages is heavily contaminant-specific. A high-selenium copper smelter acid plant may remove more than 90% of selenium across the quench and wet electrostatic precipitator, while elemental mercury removal across the same stages can remain below 20% because metallic mercury is only sparingly soluble in weak acid.

Why Does Elemental Mercury Survive Wet Gas Cleaning?

Elemental mercury has low aqueous solubility and low mass transfer into weak acid solutions. Wet scrubbers remove oxidized mercury species such as mercuric chloride, but the mercury exiting a smelter after cooling is predominantly Hg⁰. Simple weak-acid scrubbing therefore leaves a large fraction of mercury in the gas phase. The Boliden-Norzink process addresses this by circulating a solution containing mercuric chloride in a packed tower. The scrubbing reaction converts elemental mercury to insoluble calomel:

Hg⁰(g) + HgCl₂(aq) → Hg₂Cl₂(s)

The calomel slurry is withdrawn from the tower sump and sent to thickening and filtration. Mercuric chloride is regenerated by controlled chlorine addition before the solution returns to the tower. Typical design conditions include gas inlet temperatures of 40–60°C, weak acid concentrations of 5–10 wt% H₂SO₄, and chloride concentrations between 1 g/L and 5 g/L. Vendor design data commonly specify residual gas-phase mercury below 0.05 mg/Nm³ and removal efficiency greater than 99% when the chloride concentration and oxidant dosing are maintained. Operation above 60°C reduces Hg⁰ absorption, while excess sulfur dioxide in the wash liquid can compete for oxidant and lower removal efficiency.

Selenium dioxide entering the quench circuit hydrolyzes to selenious acid. Sulfur dioxide dissolved in the weak acid then reduces selenious acid to elemental selenium according to the stoichiometry:

H₂SeO₃ + 2 SO₂ + H₂O → Se(s) + 2 H₂SO₄

If this reaction proceeds without solids management, red amorphous selenium deposits on quench packing, wet electrostatic precipitator plates, and demister surfaces. In plants processing high-selenium copper concentrates, inadequate weak acid purge has produced visible red sludge on wet electrostatic precipitator internals and increased mist carryover. To control the reaction, smelter acid plants operate a selenium reduction reactor with a residence time of 30–90 min at 50–70°C. These conditions encourage agglomeration of red selenium into filterable particles. Filtration is typically performed with plate-and-frame presses or candle filters, frequently using a precoat of diatomaceous earth or perlite. Filtrate is recirculated to gas cleaning, while the selenium-containing filter cake is purged as a hazardous residue. Published data for specific weak acid selenium loadings in smelter acid plants are limited because the load varies with concentrate blend and smelting furnace conditions.

When Selenium Dioxide Reaches the Drying Tower

Selenium that escapes wet gas cleaning enters the drying tower with circulating 93–98 wt% sulfuric acid. The acid absorbs residual sulfur dioxide and selenium dioxide. Dissolved sulfur dioxide then reduces selenium dioxide to red elemental selenium inside the drying acid circuit. This is the same reaction that occurs in weak acid, but the consequences are amplified because the drying tower feeds the main acid system. Red selenium suspended in drying acid increases turbidity, deposits on acid coolers, and plugs mist eliminators. In the final absorption circuit, residual selenium can create red turbidity in product acid. Some metal extraction and battery electrolyte applications impose selenium limits below 10 mg/kg, but published specification data for all industrial acid grades is limited. The practical operating rule is that selenium must be removed before the drying tower. Final acid filtration or product settling is not an effective substitute for upstream selenium reduction and filtration.

Selenium Reduction, Filtration, and Mercury Scrubber Interface

Sequencing of the selenium and mercury removal stages determines solids handling and waste volume. If selenium reduction and filtration are placed upstream of the mercury scrubber, the mercury scrubber receives gas with a lower selenium aerosol load. If selenium particulate passes into the mercury scrubber, calomel and selenium solids mix in the same slurry. The mixed solids are more difficult to separate, and the mercury-containing waste volume may increase. A high-efficiency mist eliminator between the selenium and mercury stages is therefore specified to remove droplets above 3 µm at a separation efficiency of at least 99%. The mercury scrubber itself operates as a packed tower with bed heights commonly between 6 m and 10 m, superficial gas velocities of 1.5–2.5 m/s, and liquid-to-gas ratios from 2 L/m³ to 5 L/m³. These parameters are adjusted for inlet mercury concentration and allowable residual mercury to the drying tower.

Mercury and selenium in commercial sulfuric acid are not always covered by commodity acid specifications. Total acidity may be determined according to ASTM E223-16 or ISO 910:1977, but trace mercury and selenium limits usually appear in user purchase specifications or environmental permits. Laboratories determining trace elements in 96 wt% acid commonly dilute samples to 1–5 wt% before analysis because direct matrix effects in concentrated acid restrict detection limits. Gas-phase mercury monitoring upstream and downstream of the mercury scrubber is often performed by sorbent trap methods such as US EPA Method 30B. Where continuous emission monitoring is required, EN 14181 defines quality assurance procedures for automated measuring systems. Final product testing therefore serves as confirmation of gas cleaning performance, not as a control method. A smelter acid plant cannot adjust mercury or selenium in finished acid by blending or conventional filtration without risking acid quality and downstream fouling.