ARBOK-Selenium (metal case)

Overview

Selenium is the element industry pays to destroy. It is an essential trace nutrient with the narrowest gap in the periodic table between requirement and toxicity, and it therefore carries one of the tightest numerical discharge limits in water law: 12–23 µg/L. Under a single US steam-electric rule, compliance was priced at ~$480 M/yr.

The difficulty is manufactured by the treatment train itself. Selenium leaves the boiler volatile, as SeO₂ in the vapour. Wet desulphurisation dissolves that vapour and oxidises it to selenate, SeO₄²⁻ — an ion close enough to sulphate that no conventional separation distinguishes them, in a liquor where sulphate sits at thousands of mg/L. Precipitation does not discriminate at that ratio; ion exchange saturates with sulphate; RO concentrates the problem. What remains is biological reduction — a living fixed-film reactor with the reliability characteristics of a living thing.

The ARBOK position is not a better selenium removal step. Selenium is taken while it is still volatile, before selenate exists. The separation the industry has spent two decades attempting is never performed, because the ion it targets is never formed.

> Core technology and architecture: see ARBOK-VC (Vacuum Cracking).

Applications

Feedstock streams: coal-power purge water; copper anode-slime and decopperising solutions; mine, phosphate and irrigation drainage; refinery sour water and desalter brine.

Outputs: selenium in the volatile cut; sulphur as a dry mineral sulphate salt (marketable, not waste); returned water; no discharge.

End uses: metallurgy (free-machining steels, copper alloys), glass decolourising, photoreceptors and thin-film PV, animal feed additives, electronics.

Operating Principle

Separation by volatility and density in a single pass, with gas, vapour and liquid phases handled inside one unit; where required, gases are concentrated and separated in the same apparatus. Water is returned to the process and there is no discharge. Selenium is removed as the volatile oxide, upstream of the aqueous oxidising environment that would convert it to selenate. Sulphur is fixed into mineral sulphate salts on the same principle as ARBOK-HS (Hydrogen Sulfide Capture). No membranes, no biological stage, no consumables.

Key Parameters

Discharge limit: 12–23 µg/L depending on stream and jurisdiction.

US steam-electric compliance cost (one rule): ~$480 M/yr.

Selenium price: $27–29/kg world; $33–35 Asia; $22–23 North America.

Supply: no dedicated mine anywhere — essentially all primary output is a by-product of copper anode slimes, alongside tellurium and PGMs.

Stream volume: 200–500 m³/day per power unit = 1–2 standard Arbok systems.

Feed concentration (case basis): 1–5 mg/L Se; 2–5 g/L sulphate.

Incumbent: anaerobic fixed-film biological reduction, O&M $150–400k/yr, carbon-source dependent, nitrate-inhibited, temperature-sensitive, generates Se-bearing biosolids.

Architecture and Components

Standard modular unit, 200 m³/day = 73,000 m³/yr. Single-pass separation train; sulphur fixed as sulphate salt; condensate returned. Shares the platform with ARBOK-HS (Hydrogen Sulfide Capture), ARBOK-SULPHUR, ARBOK-Copper-Waters, ARBOK-Tellurium.

Advantages

Technical: the selectivity problem is not solved but avoided — the sulphate/selenate discrimination never has to be made.

Operational: replaces a living reactor with a physical separation; no biology to upset, no carbon feed, no biosolids.

Regulatory: with no discharge the outfall leaves the regulated set entirely rather than improving a number inside it.

Commercial: sulphur becomes a saleable dry salt and selenium a by-product on a stream that previously produced only cost.

Integrations

FGD water treatment on coal units; copper refinery slime circuits; mine water plants under reclamation bonds; refinery sour-water systems. Connects to ARBOK-HS (Hydrogen Sulfide Capture), ARBOK-SULPHUR, ARBOK-Tellurium, ARBOK-Copper-Waters, ARBOK-VC (Vacuum Cracking).

Deployment & Operation

Stream assay (Se, sulphate, nitrate, chloride, TDS) → configuration → install on the existing purge. Purchase or BOOM (PAY&GO + off-take on the recovered selenium and sulphate).

TRL

Platform TRL 9. Selenium-specific field reference pending; configuration requires site validation.

Market Potential

Selenium is not a volume market — world output is a few thousand tonnes a year, all of it by-product. The market here is compliance, not metal: every coal unit, copper refinery, seleniferous mine and refinery outfall inside a jurisdiction with a µg/L selenium limit is a candidate, and each of them is currently paying to run biology. The metal is the bonus that makes the compliance case profitable rather than merely cheaper.

Typical Project Economics

Basis: one unit, 200 m³/day, 73,000 m³/yr, 1–5 mg/L Se.

• Selenium recovered: 73–365 kg/yr = $2–10k/yr. Trivial. Never lead with it.
• Biological treatment O&M not incurred: $150–400k/yr
• Water returned rather than discharged and replaced: up to $1 M/yr
• Marketable sulphate: $20–50k/yr per unit (150–365 t dry salt at 2–5 g/L sulphate, $120–180/t)
• Penalty and enforcement exposure removed: $0.3–1 M/yr for a facility with an exceedance history (US statutory maximum ~$66k per day of violation — a ceiling, never to be quoted as an expected figure)
• Aggregate: $1–2.5 M/yr per installation.

Risk Factors

Se concentration and speciation vary widely by stream — assay per site. Nitrate and chloride load affect the configuration. Sulphate salt off-take requires a local buyer or the $20–50k line disappears. Water valuation is jurisdiction-specific and carries the largest single line in the economics. Selenium price is thin and by-product-driven; it cannot be hedged. Selenium-specific field reference pending.

Related Technologies

ARBOK-HS (Hydrogen Sulfide Capture) · ARBOK-SULPHUR · ARBOK-Tellurium · ARBOK-Copper-Waters · ARBOK-VC (Vacuum Cracking) · ARBOK-VC (Vacuum Cracking)