How to demolish a CHPP safely in Australia

A CHPP is one of the more predictable industrial structures to demolish, which is exactly why it rewards a proper method and punishes an improvised one. This is how we approach one. It is written for mine owners, closure teams and the engineers advising them.

What a CHPP actually is

Coal handling and preparation plant. Sometimes called a washplant or a coal prep plant. It takes run of mine coal, sizes it, washes the rock and clay out of it and produces a saleable product.

Structurally they are consistent. A CHPP tends to be close to square on the footprint and four or five storeys tall, a braced steel frame carrying process equipment on every level. That consistency is the single most useful thing about demolishing one, because the method that works on one works on the next.

Around the building sits everything else:

• Raw coal handling: dump station, breaker or sizer, raw coal stockpile, stacker and reclaimer.
• Conveyors and transfer towers: often kilometres of them, on trestles, in galleries, with transfer points and drive houses.
• The plant itself: screens, dense medium cyclones, centrifuges, flotation cells, filters and the magnetite recovery circuit.
• Thickeners, clarifiers and tailings handling.
• Product bins, stockpiles and train load out.
• Workshops, substations, MCC rooms, control room and the reagent store.

The method: split it and pull it

On a structure of this shape, the method that works is to bring the whole thing to ground in a controlled way and then process it at ground level, rather than sending people and machines up through five levels of framing.

Where the site allows it, that means cutting the structure down the middle, then pulling the legs so the two halves split apart and come down. Once it is on the ground the machines can get in, and the whole job becomes shearing and sorting steel on a flat surface instead of working at height inside a frame that is being progressively weakened.

That is the principle. The detail is where the engineering sits:

• An engineer designs the sequence: which members are cut, in what order, and what the structure does as each one goes. A braced frame carrying heavy process equipment does not behave like an empty shed.
• Heavy equipment comes out or gets accounted for first: cyclones, centrifuges and mills are dense, high and often the reason a floor is loaded the way it is.
• Exclusion zones are set from the analysis, not from experience or optimism.
• The pull is done with the right machine at the right angle with the operator well outside the zone, or by remote means where the structure is unpredictable.
• Where the split and pull is not available, because of what is next to it or because the structure will not behave, it becomes top down with high reach machines and shears, which is slower and costs more. Establishing which of those two you are doing is the first commercial question on the job.

Conveyors and transfer towers

The conveyors are usually the larger part of the scope by length and the part most likely to hurt somebody.

Belt under tension is stored energy. Gravity take up towers hold a suspended counterweight that has to be brought down deliberately and never cut free. Inclined conveyors want to run away once the drive and brake are removed. Trestles and galleries are long, light and unstable once you remove a section in the middle, so they come down in sequence from one end, not opportunistically from the middle.

Transfer towers carry the same problem as the plant building in miniature: heavy chutes and equipment at height in a light frame.

Coal dust and residual coal

A shut down CHPP is not a clean one.

• Coal dust is explosive: it accumulates in galleries, on beams, inside chutes, in bins and in the dust extraction system. Any hot work is a permit activity with cleaning, monitoring and a standing watch, and in some areas hot work is simply not the right answer.
• Residual coal self heats: coal left in bins, chutes, stockpile tunnels and on the floor of a plant that has been idle can be smouldering before anyone opens it. Thermal checks before entry are worth doing, and a bin that is warm gets treated as a fire, not as a cleaning job.
• Bins and chutes are confined spaces with hung material above the opening: the rule is the same as on any silo: the material comes out from outside, never by putting a person underneath it. We have written separately about why nobody should be inside a silo and it applies directly.
• Magnetite: the dense medium circuit runs on magnetite slurry. It is not a health hazard in the way the other items are, but it is heavy, it settles hard in the bottom of vessels and sumps, and it is a disposal stream that has to be planned rather than discovered.

The other hazards on an older plant

• Asbestos: on a plant of any age, in gaskets, lagging, seals, switchboards, roof and wall sheeting, and inside equipment. Surveys covering only the buildings miss most of it.
• Lead paint on structural steel, which matters the moment you introduce heat or abrasion.
• Reagents: flocculant, flotation reagents and the fuels that go with them. The reagent store is a small building that can hold the worst chemicals on the site.
• Hydrocarbons: gearboxes, drives, hydraulic power packs and the fuel infrastructure. Drain and account for them before anything is cut.
• Stored energy generally: springs, accumulators, take ups, tensioned belt and anything suspended.

Sequence that works

1. Settle the end state. Whole site clearance, or one module out while the rest keeps running. They are different jobs.
2. Investigate. Hazardous materials, structural drawings, weights, thickest steel, residual product.
3. Isolate and prove it. Power, water, air, fire systems, fuel. Locked, tagged and physically broken.
4. Clean out. Coal, magnetite, reagents, hydrocarbons, from outside the confined spaces.
5. Strip hazardous materials under licence.
6. Remove heavy process equipment where it helps the structural sequence or where it has resale value.
7. Take the conveyors and transfer towers in planned sequence.
8. Bring the plant structure down by the engineered method.
9. Process, sort and recover at ground level.
10. Slabs, footings, below ground services, then contamination assessment and handover to rehabilitation.

The steel is worth real money, and it is worth it later than you think

A CHPP is scrap heavy. That recovery can fund a serious share of the demolition, and on most sites it should be shown to the owner openly, docket by docket.

Two things change the number. Freight, because a long haul to the nearest market thins the credit quickly. And cleanliness, because a scrap merchant will not take steel carrying residue until it has been cleaned, and cleaning it is a cost that sits on the demolition side of the ledger. Both belong in the price conversation at the start. See steel recycling and scrap recovery for how we handle it.

Questions we get asked

Can a CHPP be taken down while the mine is still operating?

Yes, and it is common. It changes the sequence completely, because every isolation has to be proven against plant that is still running and the programme is driven by production rather than by demolition logic.

Do you use explosives?

Rarely, and only where the site, the structure and the surroundings all suit it. Mechanical methods with engineered pulls give more control and far fewer approvals, which is why they are the default.

What is the longest lead item?

Usually not the plant. It is the hazardous material clearance and the waste classification, followed by getting the right machine to a remote site.

How long does one take?

It depends entirely on what is around it, how much of it is contaminated and whether the site is live. Anyone who gives you a duration before they have the weights, the thickest steel and the hazmat position is guessing.

Where to next

Before you go out to tender, it is worth reading what a demolition contractor needs from you before they can price a plant. For the wider scope, see mine demolition, decommissioning and closure, and for the machines involved, high reach excavators and shears.