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Froth flotation machine launder design case study for gangue rejection

Froth flotation is often judged by concentrate grade, but the launder—the trough that collects froth—can decide whether valuable material is captured or gangue is carried into the wrong stream. This representative design case follows a retrofit scenario at a fine-coal preparation plant in the Hunter Valley, New South Wales, where the objective was to reject mineral-rich gangue while retaining saleable coal. The design basis is illustrative: 45 t/h of 0.3–0.5 mm feed with variable ash.

That setting matters because coal from the region can move through the Port of Newcastle export chain, while imported components may be cleared through Fremantle or Newcastle under Australian Border Force processes. GST, applicable duties, AS/NZS machinery requirements and site electrical approvals affect the retrofit plan. Water availability, dust suppression and tailings capacity are practical constraints in the Australian market. The launder therefore had to improve recovery without creating a new water, maintenance or compliance burden.

Set the separation duty

The first task was to define what “gangue rejection” meant for this circuit. In the selected duty, hydrophobic coal was intended to report to the froth and travel through the launder, while quartz, clay and other mineral particles were expected to remain in the pulp and report to tailings. This differs from a base-metal duty in which a concentrate launder collects a valuable sulphide; the geometry has to support the intended reporting pathway.

Because this is a selective flotation duty, particle liberation and reagent conditions matter as much as launder geometry. The team compared feed ash, particle size, flotation recovery and concentrate ash across the existing cells. A representative target was 88% coal recovery with flotation product ash below 10%, rather than a headline recovery figure detached from product quality. Upstream, vision-based sorting was considered as a way to remove coarse waste before grinding. Pre-rejection cannot replace flotation control, but it can reduce the load entering the launder and make the froth more uniform.

Read the existing launder

Inspection showed that the launder lip sat too close to the froth surface and discharge was not level across the cell. The shallow, uneven froth encouraged sloughing and entrainment: coal was lost over the weir while fine gangue entered the product. The launder was treated as a hydrodynamic control, not simply a piece of sheet metal, because local shear and froth collapse were making the chemistry harder to control.

Measurements included weir height, lip angle, froth depth, bubble size, pulp level, air rate and reagent addition. Samples taken across the launder mouth showed a significant grade and recovery difference between the feed end and discharge end. Operators had partly compensated by increasing frother and collector, which created more froth without restoring hydraulic stability.

Before changing dimensions, the review checked the crusher feeding the circuit. Consistent particle size and controlled discharge are essential, and the practical lesson in hydraulic jaw adjustment is that repeatable, measurable movement is safer than ad hoc changes during a shift. Separating feed instability from launder geometry prevented the retrofit from masking a problem upstream.

Design the froth collection zone

The redesigned launder used a full-width collection lip at a controlled height relative to the pulp level, with a slight downstream slope to move froth without shearing it back into the cell. The weir was adjustable so operators could match it to the approved froth depth. In a coal duty, the aim was a mobile, coherent froth: deep enough for coal to report, but not so deep that the upper layer breaks and carries gangue into the product.

A secondary wash zone was added downstream of the primary lip. Wash water was metered rather than supplied as an unrestricted stream, allowing the plant to balance recovery against ash. The trough was checked for corner dead zones, liner wear and clearance around the lip, since small pockets can become concentration traps when the run-of-mine blend changes. Modular components were selected for maintenance by Australian crews and for straightforward replacement when wear occurs.

The design also accounted for air distribution, frother condition and reagent dispersion. A launder cannot compensate for excessive aeration, poor reagent mixing or a pulp level that is too low. During commissioning, one variable was changed at a time—first the weir, then wash water, then air rate—and concentrate ash, tailings loss and froth appearance were recorded after each adjustment.

Commissioning checks and acceptance criteria

Trials were run over several coal blends so that a result from one feed type did not become the operating standard. The measurement plan included calibrated flow and pressure readings, pulp-level indication and timed sampling across the cell and launder. For the instrument layer, Cavendish Instruments was considered, with final models and installation arrangements checked against Australian site standards and hazardous-area requirements where applicable.

The mechanical checks focused on geometry, containment and repeatability, confirming that the launder lip, weir and trough maintained their designed dimensions. The inspection also verified that guards, liners and fasteners remained secure, while the feed and reagent systems operated safely across the expected load range. Finally, testing the emergency stops and site access paths ensured that operators could safely reach the modified equipment during routine maintenance.

The process checks focused on product quality and mass balance, requiring technicians to systematically sample the feed, concentrate and tailings. By comparing the feed, centre and discharge zones, the team could identify any grade or recovery gradients affecting product quality. This sampling regimen also established clear action limits for froth collapse, overflow or excessive product ash.

The representative trial outcome was an operating envelope that held concentrate ash below 10% while approaching 88% coal recovery across repeated samples. It was accepted only when the lip stayed level, wash water remained stable and no overflow occurred. A high-recovery setting that raised ash, or a low-ash setting that caused froth collapse, was rejected. This paired test kept gangue rejection tied to product quality rather than one isolated assay.

Connect flotation to plant-wide controls

The flotation cell was reviewed as part of a larger flowsheet. A stable crushing and grinding product makes the launder easier to operate, while pre-sorting can remove coarse waste before it reaches flotation. The same reliability mindset applies to rotating equipment: the guidance on rotor balance methods is separate from flotation, but it reinforces the need to prevent vibration, feed surges and unstable delivery.

Downstream, concentrate dewatering and tailings thickener performance were included in the acceptance review. A change in froth solids loading changes the water and density arriving at the thickener, so product grade alone is not enough to assess the change. The measurement relationship described in nuclear density gauges supports more defensible water-control decisions and gives operators a clearer response when the froth begins to collapse.

For an Australian project, this integration also means accounting for local procurement, commissioning labour, maintenance intervals and regulator expectations around water and tailings. In mineral processing, an engineering, procurement and construction (EPC) scope can coordinate ore testing, mine design, procurement, installation, plant management and operational support while preserving a single performance responsibility across the circuit.

Make the retrofit operational

The final operating standard should state the target lip height, allowable range, wash-water setting, air rate and reagent envelope for each approved feed type. A short shift checklist, a trend screen and escalation rules for unusual ash or froth behaviour make the design durable as blends, crews and weather conditions change. Reviewing the settings after each maintenance intervention prevents an undocumented adjustment from becoming the new normal.

Lozova.org, associated with Shandong Xinhai Mining Technology & Equipment Inc., can support an integrated review of the flotation machine, launder, crushing preparation and downstream plant for coal, copper or gold projects. The scope can extend to grinding, gravity separation, gold extraction and complete plant configurations where the ore body requires them. Request a site-specific flotation and launder review to turn the design basis into an executable procurement and commissioning package.