Thickeners: how they work, the types and parts, rake torque and the variables to watch, common problems, and maintaining a thickener
Thickeners from the operating and maintenance side: how feedwell, flocculant, settling zones and the rake work together, conventional, high rate, high density and paste thickeners, the parts and what to watch on each, rake torque, bed level, underflow density and overflow clarity, problems from bogged rakes to ratholing, and the maintenance program. With a section drawing of a high rate thickener.
Key takeaways
- A thickener separates a slurry into clear water at the top and a dense underflow at the bottom, using settling, flocculant and a slowly turning rake.
- The feedwell, the flocculant and the bed do most of the work. The rake moves settled solids to the centre and keeps the bed from setting.
- Rake torque, bed level, underflow density and overflow clarity are the variables to watch. Rising torque is the early warning of a heavy or sanded bed.
- The worst thickener events are bogged rakes and solids setting around a stopped rake. Both are prevented by watching torque and managing the bed before a stop.
- The rake and its blades can only be inspected when the tank is drained, so the drive and torque system get the most routine attention and the drain-down is planned as a major event.
A tailings thickener trips on rake torque on a Sunday night. Operations lift the rake and keep the underflow pumps running, but the bed has been building for days behind a faulty bed level reading, and a ring of compacted solids has formed out near the tank wall. It takes a week of dilution, pumping and careful rake work to recover, and the plant runs at reduced rate the whole time because there is nowhere else for the tailings to go.
This guide covers thickeners from the operating and maintenance side: the types, the parts and what each does, the variables to watch, the common problems and how to prevent them, and the maintenance program. It sits beside the guides to slurry pumps, industrial gearbox maintenance and fixed plant maintenance in mining. Thickener design and process control belong to the process engineer and the supplier.
How a thickener works
Slurry enters the feedwell at the centre, where it is slowed, diluted with clear liquor and mixed with flocculant. The flocs that form leave the bottom of the feedwell and settle, first through a zone of hindered settling and then into the compression bed, where the weight of solids above slowly squeezes water out. Clear liquor rises to the overflow launder at the rim.
The rake turns slowly, often once every several minutes, with blades angled to move settled solids toward the central cone. It also opens channels in the bed that let water escape, which raises the underflow density. The drive is built for very high torque at very low speed, and the torque it measures is the best single indication of what is happening in the bed.
| Part | What it does | What to watch |
|---|---|---|
| Feedwell | Slows and dilutes the feed and mixes in flocculant, so particles form flocs before they enter the tank | Wear and build-up inside, damaged baffles |
| Rake and drive | Arms with blades turned slowly by a high torque drive, moving settled solids to the centre cone | Drive gearing and bearing, blade wear, torque measurement |
| Rake lift | Raises the rake out of a heavy bed to avoid overload | Lifting mechanism and its controls |
| Bridge or centre column | Supports the drive and rake, across the tank or on a central pier | Structural condition and access |
| Overflow launder | Collects the clear liquor around the rim | Weirs level so overflow is even, launder clear of solids |
| Underflow cone and pumps | Draws the thickened solids from the centre | Pump and pipe wear, blockage, standby |
| Flocculant system | Makes up, ages and doses polymer flocculant into the feed | Mixers, dosing pumps, dilution water |
| Instruments | Torque, bed level or pressure, underflow density and flow, overflow turbidity | Calibration, since control depends on them |
Thickener types
Most modern mineral thickeners are high rate machines, which use flocculant and a well designed feedwell to handle a given tonnage in a much smaller tank than the older conventional design. High density and paste thickeners go further, with deeper beds and stronger drives, to produce underflow dense enough to recover more water or to stack.
| Type | What it is | Suits |
|---|---|---|
| Conventional | A large diameter, shallow tank relying on settling area | Older plants, low rates, where space is plentiful |
| High rate | Flocculant and an efficient feedwell, so a smaller tank handles the same solids | Most modern mineral plants |
| High density or high compression | A deeper bed and stronger drive for a denser underflow | Tailings, where water recovery matters |
| Paste | A deep cone tank producing underflow with a yield stress that stacks | Paste backfill and stacked tailings |
| Clarifier | Designed for overflow clarity rather than underflow density | Process water and water treatment |
The variables to watch
A thickener changes slowly, which makes it easy to ignore until it is in trouble. The variables below, trended together, show the state of the bed hours or days before a problem. They depend on instruments that are easy to neglect, a bed level probe that has fouled or a density meter out of calibration, so the instruments belong in the maintenance program as much as the drive does.
| Variable | What it shows | Why it matters |
|---|---|---|
| Rake torque | How hard the rake is working against the bed | Rising torque warns of a heavy or sanded bed before the drive trips |
| Bed level or bed mass | How much solids inventory is in the tank | Too high risks torque and overflow solids, too low gives a thin underflow |
| Underflow density | How thick the product is | The main control target, set by underflow pumping rate |
| Overflow clarity | Solids escaping with the water | Points to flocculant dose, feedwell performance or overload |
| Flocculant dose | Grams of polymer per tonne of solids | Too little gives poor settling, too much costs money and can harm downstream |
| Feed rate and solids | The load the thickener has to handle | Sets everything above, and changes with the plant upstream |
Rake torque is the thickener's early warning. Trend it, alarm it, and act on it before the drive does.
Common problems
Most serious thickener events start with the bed: too much of it, the wrong kind, or material left to set. A bogged rake or a tank full of set solids can take days to recover and sometimes needs the tank drained and dug out. Managing the bed before a planned stop, by drawing it down and keeping the rake turning, prevents most of them.
| Problem | Cause | Response |
|---|---|---|
| High torque and rake bogging | Bed too deep or too dense, coarse sands, islands of solids | Lift the rake, increase underflow, find and break up the island |
| Ratholing | The underflow draws a channel through the bed, so thin material bypasses | Underflow density falls suddenly, lower the draw and rebuild the bed |
| Dirty overflow | Poor flocculation, feedwell short circuiting, overload | Check dose and dilution, feedwell condition, feed rate |
| Rakes stuck after a stop | Solids settle and set around the rakes when the drive stops | Keep the rake turning when possible, lift before restarting, reduce the bed before a planned stop |
| Underflow line blockage | Dense material settles in a stopped line | Flush on stop, avoid dead legs, standby line or pump |
Maintaining a thickener
The parts in the tank, the rake arms, blades and the inside of the feedwell, can only be inspected when the thickener is drained, which is a planned event with a long lead time and somewhere for the process to put the slurry. Everything that can be checked from above gets regular attention instead: the drive, through oil analysis and condition checks, the torque measurement and trip settings, the underflow pumps, the flocculant system and the instruments.
The drive is usually the most critical spare question. A drive failure stops the rake, and a stopped rake in a full bed risks the solids setting, so the spares, the repair plan and the procedure for a long stop are worked out before they are needed.
| Task | What it involves | When |
|---|---|---|
| Drive | Gear and bearing condition, oil analysis, lubrication, seals | Rounds and condition monitoring, internal inspection at shutdowns |
| Torque system | Sensor calibration, alarm and trip settings, rake lift response | On a set interval and after any drive work |
| Rake arms and blades | Wear, bent arms, missing blades, scaling | Inspected when the tank is drained, which is a planned event |
| Feedwell and launders | Wear, build-up, baffles, weir level | Planned stops |
| Underflow pumps and lines | Wear, seals, valve condition, flushing | As for slurry pumps, with standby proven |
| Flocculant system | Mixer, aging tank, dosing pumps, dilution | Rounds, with dose checked against target |
| Instruments | Bed level, density and flow meters, turbidity | Calibration on a set interval |
Common mistakes
- Running on a faulty bed level or density reading without knowing it.
- Torque alarms set so close to the trip that there is no time to act.
- Stopping the rake with a full bed for a planned shutdown.
- Flocculant dose set once and never checked against the feed.
- Underflow lines left full when the pumps stop.
- Draining the thickener without a plan for where the slurry goes.
- No spare or repair plan for the drive.
Common questions
What is a thickener in mineral processing?
A thickener is a large circular tank that separates solids from liquid by gravity settling. Slurry enters through a central feedwell, usually with flocculant added, the solids settle into a bed, and a slowly rotating rake moves them to a central cone where they are pumped out as a dense underflow. Clear water overflows at the rim and is usually recycled to the process.
How does a thickener work?
The feedwell slows and dilutes the feed and mixes in flocculant, which binds fine particles into larger flocs that settle quickly. Below the feedwell, the solids settle through a hindered settling zone into a compression bed, where the weight of material above squeezes out water. The rake turns slowly through the bed, moving solids toward the underflow cone and releasing trapped water.
What is the difference between a thickener and a clarifier?
Both are settling tanks. A thickener is designed and run for a dense underflow, with the solids as the product or the main concern. A clarifier is designed for clear overflow, with the water as the product. A high rate thickener can produce both reasonably clear overflow and a dense underflow, but its design priority is the underflow.
What causes high rake torque in a thickener?
A bed that is too deep or too dense, coarse sands that settle quickly and pack, islands or donuts of solids building on the floor, and material that has set while the rake was stopped. Rising torque is the warning to lift the rake, increase the underflow rate and reduce the bed before the drive trips on overload.
What is flocculant used for in a thickener?
Flocculant is a long chain polymer that binds fine particles into larger flocs, which settle much faster than the individual particles. It lets a smaller thickener handle the same solids and gives clearer overflow. The dose, the dilution and how well it is mixed in the feedwell all matter, and both too little and too much cause problems.
Key terms
Plain-language definitions from our glossary for the concepts this article leans on.
Standards and further reading
Related case studies and tools
- Predictive maintenance and condition monitoring (case study)
Related reading
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