How Gold Refineries Operate
A gold refinery receives unrefined feed — mine doré, recycled jewellery and industrial scrap — assays every lot to establish its exact precious metal content, refines the metal to investment-grade purity (995.0 to 999.9 fineness), casts it into certified bars, and settles with each supplier based on the assayed content of their delivery. Refineries are the quality gate of the world gold market: their accreditation, assay integrity and sourcing controls determine which metal can enter the legitimate market at all.
The Refinery's Workflow
- Receiving. Incoming lots arrive under chain-of-custody seals, are checked against documents and weighed on calibrated scales — the supplier or their representative may witness.
- Melt and assay. Each lot is melted to homogeneity and sampled, with purity established by fire assay and supporting instrumental methods — the process described in How Does a Gold Refinery Assay Work? The assay determines what the supplier is paid.
- Refining. The metal is purified in stages (see below) to the target fineness.
- Casting and certification. Refined gold is cast into bars — kilobars, Good Delivery bars, minted products — stamped with the refinery's mark, fineness, weight and serial number, and recorded on bar lists.
- Settlement. Suppliers are paid on assayed fine content per their agreements — see the settlement process in detail.
The Refining Processes
Miller process (chlorination)
Chlorine gas is bubbled through molten metal; base metals and silver form chlorides that separate from the gold. The Miller process is fast and economical, taking gold to roughly 99.5% purity — sufficient for 995.0 Good Delivery feedstock but short of 999.9.
Wohlwill process (electrolysis)
Miller-refined gold is cast into anodes and electrolytically dissolved and redeposited as ultra-pure gold at the cathode, achieving 999.9 fineness. Slower and more capital-intensive, it produces the four-nines gold required for kilobars and high-specification uses.
Aqua regia and solvent routes
Chemical dissolution and selective precipitation routes are used by many refineries, particularly for complex feeds and for recovering by-metals such as silver and platinum-group metals, which are credited or returned under the supplier agreement.
Accreditation: Why It Matters Commercially
The market distinguishes sharply between accredited and non-accredited refineries. LBMA Good Delivery accreditation — and hub equivalents such as DMCC accreditation in Dubai — certify a refinery's assaying competence, bar quality and responsible sourcing controls, with ongoing audits. Bars from accredited refineries trade globally on the strength of their stamp; bars from unknown refineries face re-assay and discounts. For a seller, delivering to an accredited refinery means the assay will be credible, the settlement bankable and the output marketable.
Sourcing Controls: The Refinery as Gatekeeper
Because refining is where untraceable feed becomes standardised bars, refineries carry the heaviest due diligence duties in the supply chain — OECD-aligned programmes, supplier KYC, origin verification and annual independent audits (see OECD Gold Supply Chain Guidance). A refinery accepts reputational and regulatory liability with every lot; this is why new suppliers face searching onboarding, and why metal without provable origin finds no reputable refinery willing to touch it.
Example
A 30 kg doré lot arrives in Dubai at an accredited refinery. Day 1: receipt, weighing, melt, sampling. Day 3: fire assay confirms 90.1% gold, 7.2% silver. The supplier is settled on 27.03 kg fine gold at the contractual benchmark price less charges, with a silver credit per the agreement. The metal joins the refining stream — Miller chlorination, then Wohlwill electrolysis — and within days exists as 999.9 kilobars bearing the refinery's stamp, sold into regional demand. The supplier's doré has become bankable, market-grade gold.
Key Takeaways
- Refineries assay every incoming lot and settle suppliers on assayed fine content — the assay is the commercial core of the relationship.
- The Miller (chlorine) process refines to ~99.5%; the Wohlwill (electrolytic) process achieves 999.9 'four nines' purity.
- Accreditation (LBMA Good Delivery, DMCC) certifies assay competence, bar quality and sourcing controls — and makes a refinery's bars globally tradeable.
- Refineries are the supply chain's compliance gatekeepers, carrying OECD-aligned due diligence duties and annual audits.
- By-metals such as silver are assayed and credited under the supplier agreement, not lost.
Frequently Asked Questions
How long does refining take?
Receipt, melt and assay typically take days; refining and casting add days more depending on process route and schedule. Settlement timing is contractual and usually runs from final assay, not from refining completion.
What purity do refineries produce?
Standard outputs are 995.0 (Good Delivery minimum for gold) and 999.9 (kilobars and minted products). The Wohlwill process or equivalent chemical routes achieve four-nines purity.
Do refineries buy the gold or refine it for a fee?
Both models exist. Refineries may purchase lots outright (settling on assay) or provide tolling/refining services where the supplier retains ownership and pays refining charges, receiving back refined metal or its value.
What happens to the silver in doré?
It is assayed alongside the gold and credited or returned under the agreement, less applicable charges. For high-silver doré, the silver credit is a material part of settlement.
Why do refineries reject some material?
Primarily for sourcing and compliance reasons — unverifiable origin, sanctions exposure, inconsistent documentation — and occasionally for hazardous contaminants (such as mercury) requiring special handling.