Decommissioning a BIOX plant: the arsenic problem nobody prices

14 September 2026

BIOX plant decommissioning looks straightforward from the outside. A row of large agitated tanks, some walkways, some pipework, a cooling system and a lot of structural steel.

What is on the inside of those tanks is the reason it is not a normal steel strip, and it is the item that most tenders for BIOX plant decommissioning either miss entirely or bury in a contingency.

What a BIOX plant does

Bio-oxidation applies where gold is refractory, locked up inside sulphide minerals where conventional cyanide leaching cannot reach it. The circuit uses bacteria to oxidise the sulphides and liberate the gold so the downstream leach circuit can recover it.

In Australia those sulphides are frequently pyrite, pyrrhotite and arsenopyrite. Arsenopyrite is the important one, because it is an arsenic bearing mineral and the process works by breaking it down.

A typical circuit has:

Primary and secondary reactors, commonly a bank of large agitated tanks, stainless or lined, each with an agitator, air spargers and cooling.
Cooling towers and heat exchangers: the reaction is exothermic and the bacteria have a narrow temperature range, so cooling is a permanent part of the plant.
Air blowers, usually large, and a lot of ducting.
Nutrient and acid dosing, because the circuit runs acidic and the bacteria need feeding.
Launders and interstage pipework between reactors.
A neutralisation and thickening circuit downstream, usually lime dosed, that conditions the oxidised product before it goes to leach.
Thickeners, filters and the associated pumping.

The arsenic problem in BIOX plant decommissioning

When a bio-oxidation circuit treats arsenopyrite ore, the arsenic does not disappear. It ends up in solution, in precipitates and in scale, and it builds up on every surface the process liquor touches.

In practical terms that means arsenic bearing scale and residue through the reactors, on the agitator shafts and blades, in the launders, in the pipework, in the neutralisation circuit and in the thickeners. It does not stay in one tank. It runs through the circuit.

That has consequences no structural method statement will solve:

Treat every surface as contaminated until proven otherwise: an empty reactor is not a clean reactor.
Dry scale becomes airborne dust the moment you put a shear or a hammer through it. This is the hazard that catches out crews used to normal steel demolition.
An occupational hygienist sets the standard, based on the Safe Work Australia workplace exposure standards for airborne contaminants, and there is air monitoring and biological monitoring through the works, not just a survey at the start.
Decontamination water is a waste stream. The project team has to capture, treat and dispose of it, and on a remote site there may be nowhere nearby that will take it.
The steel is not scrap until it is clean: a merchant will not accept arsenic bearing steel, so the recovery credit is conditional on a cleaning cost that sits on the demolition side.
Waste classification, under the NSW EPA's waste classification guidelines, drives the disposal cost, and the project team needs to establish it early, because the nearest licensed facility that will accept the material may be a very long way away.

None of this makes the job impossible. It makes it a hazardous materials job with a demolition component, and it needs pricing and programming on that basis.

Crane and EWP coordinating an industrial vessel lift during demolition. — Crane and EWP coordination during an industrial vessel lift.

Staged BIOX plant decommissioning site showing the scale of the work zone. — Staged demolition site showing overall work-zone scale.

Multiple excavators demolishing a steel-frame industrial building during a staged site clearance. — High-reach demolition of elevated building elements.

Acid, and what it does next door

The circuit runs acidic. That matters for two reasons.

First, residual acid attacks people and equipment, and acid residue in a vessel is a hazard in its own right during cleaning.

Second, and more seriously, bio-oxidation circuits usually sit on a site that also has a cyanide leach circuit downstream. Acid and cyanide must never meet. Any decommissioning plan that cleans both circuits in the same period needs to control the interface between them deliberately, write the sequence down and prove the isolations.

This is the point where a contractor who has only ever done structural work will get into trouble.

Legacy, which predates the plant

Bio-oxidation came into commercial use in the late 1980s and 1990s. Many of the Australian sites that host a BIOX plant had a much longer history before that, and some of them roasted arsenopyrite concentrate decades earlier. Roasting arsenic bearing concentrate produces arsenic trioxide, and that legacy sits in the ground around the older parts of those sites.

It has nothing to do with the bio-oxidation plant and it will not appear in the plant's records, but it is on the same site and it will turn up during the earthworks. Ask what was on the ground before the current plant went up, and get a contamination assessment that covers the historical footprint, not just the current one. See soil and site remediation.

How we sequence a BIOX plant decommissioning

Establish the process history, not just the current circuit: what ore the circuit treated, what reagents it used, and what stood on the site before.
Sampling and characterisation: scale, sludge, residue and soil. This is a laboratory programme with a real duration, and it sets everything downstream.
Confirm waste classification and the disposal route before anyone mobilises. If there is nowhere to take the material, nothing else matters.
Isolate and air gap the circuit physically, and control the interface with the downstream cyanide circuit.
Decontaminate from outside the vessels: high pressure cleaning with capture, not people inside tanks with scrapers.
Get written clearance to a standard the hygienist sets, item by item.
Remove agitators and internals, the items carrying the heaviest scale and the ones that change the tank loading.
Demolish the tanks, frame, walkways and cooling structures, and keep dust suppression and monitoring running throughout, because clearance is never perfect.
Segregate steel by cleanliness, because clean steel and contaminated steel have very different values and very different destinations.
Slabs, bunds, sumps and below ground, then contamination assessment and handover.

Multiple specialist excavators operating across a large industrial demolition work zone.

What to ask for before you tender it

If you are tendering a BIOX plant decommissioning, the answers that matter most are:

What ore was treated, and what was the arsenic content.
What cleaning has already been done, what it was done with, and what clearance exists.
Whether the reactors have been drained, flushed and air gapped, and who verified it.
Dimensions, weights and materials of construction for the reactors and the internals.
Where the waste can actually go, and whether that facility has confirmed it will take it.
Whether tank cleaning is owner scope or contractor scope.

The full list is in what a demolition contractor needs from you before they can price a plant.

Questions we get asked

Is BIOX plant decommissioning the same as demolishing a normal tank farm?

No. Structurally it is similar. The difference is that every internal surface is contaminated, which changes the method, the protective equipment, the monitoring, the water handling, the waste classification and the scrap value.

Can the reactors be sold rather than scrapped?

Sometimes, and it is worth asking early, because a buyer changes the method completely. A vessel coming out for reuse comes out whole and clean, which is a different job from shearing it in place.

Who sets the clearance standard?

An occupational hygienist, engaged for the project, working to the relevant exposure standards. Not the demolition contractor, and not a verbal agreement on a site walk.