How to decommission a gold processing plant safely in Australia

A gold processing plant is not a structural demolition job with some chemicals in it. It is a chemical job with some structure around it, and the order you do things in matters more than the machines you bring. 

This is how we approach one. It is written for mine owners, closure managers and the consultants advising them, and it is deliberately blunt about the parts that go wrong.

The gold processing plant decommissioning sequence

• Crushing and milling: crushers, SAG and ball mills, cyclone clusters, mill liners and the heaviest single castings on site.
• Gravity circuit: centrifugal concentrators and intensive leach reactors.
• Leach and CIL tanks: carbon in leach or carbon in pulp. Large agitated tanks with interstage screens, walkways and launders between them.
• Carbon handling, elution and regeneration: acid wash, elution columns, and a regeneration kiln.
• Electrowinning and the gold room: cells, a calcine oven, a smelting furnace, and the highest security area on site.
• Reagent storage and mixing: cyanide, caustic, acid, lime, flocculant, and often a cyanide destruction circuit.
• Thickeners, tailings pumping and the detox circuit.
• Services: substations, MCC rooms, compressed air, process and raw water, laboratory, workshops and offices.

Cyanide is the item that decides the job

The leach and CIL circuit runs on cyanide. It is kept alkaline for a reason. If cyanide solution or residue meets acid, or simply meets the atmosphere in the wrong conditions, it can generate hydrogen cyanide gas. That is the single biggest life safety item on a gold plant decommissioning and everything else arranges itself around it. Cyanide on site should be managed against the International Cyanide Management Code, the industry standard covering the safe use, handling and disposal of cyanide in gold processing.

Practically, that means a few hard rules:

• Nothing gets cut until there is a written clearance for that item: not a verbal reassurance on a site walk. A document, with a standard, a result and a name on it.
• Residue is assumed present until it is measured absent: tank walls, cone bottoms, launders, interstage screens, sumps, dead legs and pump casings.
• No acid anywhere near the circuit until the whole area has been cleared, including acid brought on for other reasons.
• Gas monitoring and rescue arrangements that match the hazard, not the paperwork.
• Wash down water is captured and treated: it does not go to ground and it does not go to the nearest drain.

Most gold plants sit under the cyanide inventory threshold that would make them a Major Hazard Facility, but that is a question to confirm with the operator in writing rather than assume from the size of the plant.

The rest of the contamination stack

Cyanide gets the attention. It is rarely the only thing present.

• Loaded carbon: carbon in the CIL tanks and the columns holds gold, which makes it valuable, and it also holds whatever else it has adsorbed. It is an owner asset and it needs to be recovered and accounted for before the tanks are touched.
• Mercury: many ore bodies carry mercury, and it concentrates in the gold room, in the retort, the calcine oven and the furnace, and in the ducting above them. Assume it is there in any plant that has smelted on site and treat the gold room as a specialist scope.
• Arsenic: where the ore was refractory, arsenic bearing residues turn up through the oxidation circuit, the launders and the thickeners. That is a scope of its own and we have covered it separately in BIOX and bio-oxidation plant decommissioning.
• Asbestos: in gaskets, lagging, seals, switchboards and sheeting on any plant of age. Surveys that cover buildings and stop at the process equipment miss most of it.
• Lead paint on the structural steel, which matters the instant heat or abrasion is introduced.
• Reagent residues in the mixing area, the bunds and the delivery lines.
• Legacy ground contamination: Many Australian gold sites had a previous life, sometimes a roasting or smelting operation decades before the current plant. That legacy is not connected to the current circuit and it will not appear in the current plant's records, but it is still there under the surface. Ask the question about what was on the ground before.

Tanks are never as clean as the paperwork says

The most common cause of trouble on these jobs is a tank that has been emptied but not cleaned, handed over as clean.

Large agitated tanks hold residue on the walls, in the cone, behind baffles and under the agitator. Interstage screens trap solids. Launders between tanks hold product in every low spot. Pumps and valves hold solution in the body. A system that has been drained and flushed can still have a dead leg full of what you were trying to get rid of.

Settle this commercially before anyone prices the job. Either the owner cleans the tanks and provides a clearance, or the contractor prices the cleaning as a defined scope. Leaving it ambiguous is how a job goes wrong, because the cheapest tender will always be the one that assumed somebody else was doing it. Our approach to tank and vessel cleaning and demolition starts from the assumption that they are dirty.

The gold processing plant decommissioning sequence

Gold processing plant decommissioning follows a fixed order, even when the scope changes:

1. Settle the end state and the battery limits: whole plant or one circuit. Slab level up, or foundations out too.
2. Recover the owner's value: loaded carbon, any gold in circuit, and equipment worth resale rather than scrap.
3. Isolate and air gap: power, reagents, process water, compressed air, fire systems. Physically broken and proven, not just locked.
4. Decontaminate and clear: tanks, launders, screens, pumps, dead legs. Wash water captured and treated. A hygienist sets the standard and the clearance is written.
5. Strip hazardous materials: asbestos and lead under licence. The gold room as its own scope because of mercury.
6. Remove process equipment: agitators, screens, pumps, mill internals, and anything that changes the structural loading when it goes.
7. Structural demolition: tanks, frames, walkways and buildings, by engineered sequence with people kept off the steel.
8. Process and recover at ground level: separate the streams, because contaminated steel and clean steel are worth very different amounts.
9. Slabs, bunds, sumps, below ground services.
10. Contamination assessment and handover to rehabilitation.

What gold processing plant decommissioning does to the programme

On a normal industrial demolition the structure sets the duration. On a gold processing plant decommissioning job, the clearance sets it. You cannot shear a tank that has not been cleared, so the critical path runs through the cleaning, the sampling, the laboratory turnaround and the sign off.

Two things shorten it. Starting the hazardous material and residue investigation months before mobilisation. And sequencing so that crews are working on cleared areas while the next area is still being cleared, rather than everyone waiting on one certificate.

One thing lengthens it more than anything else: finding out on site that the records do not match the plant.

Questions we get asked

Can you take out one circuit while the rest of the plant keeps running?

Yes, and it is common on gold sites where one part has gone redundant. It is harder than a full clearance because every isolation must be proven against operating plant alongside it. See live plant decommissioning.

Who should clean the tanks, us or you?

Either, as long as it is written down. It is the least predictable cost on the job. If the owner has the people and the waste handling already in place it is often cheaper their way. If not, it should be a defined and separately priced part of the contract.

Is the steel worth anything?

Yes, but less than the tonnage suggests. A scrap merchant will not accept steel carrying residue until it is clean, and on a remote site the freight to the nearest market can take a large bite out of the credit. Both belong in the conversation at the start.

How early should we bring a demolition contractor in?

Before the closure provision is set, not after. The cost study is built the same way the tender is built, and the questions it raises are far cheaper to answer years out.

Where to next

If you are preparing to go to market for a gold processing plant decommissioning project, start with what a demolition contractor needs from you before they can price a plant. For the broader picture see mine demolition, decommissioning and closure and mine closure and rehabilitation works.