Paste Backfill Plant

Overview

A paste backfill plant turns mine tailings into a high-density paste, mixes it with a small amount of cement, and pumps it back underground to fill the voids left by mining. It serves a double purpose. The filled stopes regain ground support, which lets adjacent ore be mined safely and increases recovery, and a large fraction of the tailings goes back underground instead of onto a surface dam. Paste differs from older hydraulic fill because it carries the full range of fine particles at high solids and does not bleed water, so it stays where it is placed and cures into a competent mass.

The plant is a process line, not a single machine. Tailings are thickened to paste consistency, blended with binder and trim water to a controlled slump, and pushed through a piped network to whichever stope is being filled. Each step has to be held within tight limits: too wet and the paste will not stand or will segregate, too dry and it will not pump, too little binder and the cured fill is too weak. The plant is built to hit a strength and slump specification continuously while running on a feed that varies through the day.

Deep cone thickener

The front of the plant is the Deep Cone Thickener. Unlike a conventional thickener, its Thickener Tank is tall with a steep cone, giving the residence depth needed to consolidate solids all the way to paste density. A Feedwell dilutes and flocculates the feed, and a very high torque Rake Drive turns a Picket-Fence Rake fitted with vertical pickets that open drainage channels through the thick bed so trapped water can escape upward. An array of Bed Pressure Sensor taps reads the density profile up the cone, and the Underflow Valve meters the paste-like underflow out. Some plants add a Disc Filter to pre-dewater part of the stream and blend the cake back when the thickener alone cannot reach the target.

Mixing and binder

Thickener underflow, binder and trim water come together in the Continuous Paste Mixer, an intensive twin-shaft unit. Its Mixer Shaft pair, driven by a high-torque Mixer Drive, shears the stiff material with wear-tipped paddles until it is homogeneous. Binder comes from the Binder Dosing System, where cement and ground slag sit in a Binder Silo and are metered by a Screw Feeder under a Weigh Feeder that controls dose by mass. Binder dose is the lever on cured strength, so it is set per stope: a primary stope that will later have ore mined next to it needs a stronger fill than a secondary one.

Pumping and reticulation

Paste has a yield stress, meaning it will not flow until pushed hard enough, so it is moved by the Paste Pump, a twin-cylinder piston pump like an oversized concrete pump. Two Pump Cylinder units alternate, a S-Tube Valve swinging between them to keep delivery near continuous, all driven by a Hydraulic Power Unit. From the pump the paste enters the Reticulation Network, a network of cased Borehole Casing runs and lined Paste Pipe Spool spools joined by grooved couplings. The vertical drop in a borehole helps drive the paste, but the long horizontal runs underground and the Wear Elbow bends are where the pressure budget and the abrasion both get spent.

Reagents, water and control

A Flocculant Make-Up Plant wets, ages and dilutes dry polymer into the flocculant dosed at the thickener feedwell, since fast settling is what lets the deep cone reach paste density. A Process Water System recovers thickener overflow into a Process Water Tank for reuse and supplies the trim water that sets final slump at the mixer. Everything is coordinated by the Plant Control, where a Density Meter and a Slump Sensor feed closed loops that trim water and binder against the recipe. Holding density, slump and strength together while pumping pressure stays within the pipe rating is the daily balancing act the control system exists to manage.

Variants and the wider circuit

Not every plant looks the same. Where the Deep Cone Thickener alone can reach paste density, the plant is built around the thickener and mixer with no filtration at all. Where the tailings settle poorly or the strength specification is demanding, a Disc Filter or a press is added to make a drier feed, and the line becomes a hybrid of thickening and filtration. The choice between a pumped delivery and a gravity-assisted one depends on the depth and layout of the mine: a deep mine can let the head in the Borehole Casing do much of the work, while a shallow or flat layout leans harder on the Paste Pump. The backfill plant does not stand alone. It takes its feed from the concentrator tailings stream, returns recovered water through the Process Water System to the mill, and delivers paste on a schedule set by the mine's stope sequence, so the plant has to ramp up and down as different stopes are blasted, mucked and made ready to fill. Getting the rheology right matters beyond the plant fence: paste that is too stiff stalls in the line and is hard to clear, while paste that is too wet bleeds water in the stope and can run, so the operators live in a narrow band of slump set by the Continuous Paste Mixer and the trim-water loop.