Spiral concentrator slurry density control for chromite recovery

Chromite producers across the Australian outback know a spiral concentrator behaves like a precision instrument when its feed is steady and an unpredictable sieve when it is not. The most influential variable behind that swing is slurry density, the mass of solids carried in every litre of pulp. Get it wrong and chromium units are lost to tailings, middlings go to the wrong launder, or downstream equipment is starved. Operators in the Pilbara, around Kalgoorlie-Boulder, and in the historic chromite fields near Coobina have all wrestled with the same sensitivity.

Pulp density influences separation in two ways at once. It governs the rheology of the flowing film inside the spiral, changing how fast heavy particles migrate toward the inner trough and how easily light gangue is swept outward. It also sets volumetric throughput. Dilute the feed too far and capacity collapses; thicken it too much and chromite settles prematurely. A circuit on target runs quietly; one drifting toward its limits becomes a constant source of metal loss.

This article examines the levers plant teams use to hold slurry density inside a narrow band: ore characterisation, set-point selection, measurement, splitter and wash-water interactions, sampling, and the wider plant context. The aim is a framework metallurgists can adapt from a modular concentrator in regional New South Wales to a flagship plant shipping concentrate through Port Hedland.

How slurry density governs spiral performance

A spiral concentrator separates minerals by exploiting specific-gravity differences under a flowing film along a helical trough. The film carries lighter particles outward while denser chromite grains of specific gravity 4.5 to 4.8 migrate inward, where they are collected through take-off ports. Separation depends on the slurry behaving as a stable, moderately viscous fluid, neither so watery that particles tumble freely nor so thick that they cannot slide.

Below 25 percent solids the pulp acts almost like water and the film becomes too thin to support segregation. Above 45 percent the slurry turns plastic and chromite grains lose mobility. Most operations target a window between 28 and 38 percent solids, chosen after bench and pilot testing. Within that window, shifts of two or three percentage points move the partition curve. A heavier pulp drags more silicate gangue and lowers grade; a lighter one reduces recovery because some chromite fails to migrate in time.

Feed character differences for chromite compared with other heavy minerals

Chromite is challenging because its specific gravity sits close to several common gangue minerals, especially when the ore contains serpentinite or talc. Where iron ore or coal spirals can rely on a wide density gap, a chromite spiral operates across a narrow window, making feed preparation unusually important. Desliming, classification, and trash screening upstream remove fines that would otherwise raise pulp viscosity and consume density headroom without adding recovery.

A feed that is too broad in size amplifies density sensitivity. Coarse chromite settles too quickly in thin pulp, while slimes of serpentine or chlorite hold water and create a viscous carrier that traps fine chromite. Australian operators commit to a tight cyclone cut, typically closing the circuit to give 0.1 to 1 millimetre feed, before slurry density is even considered. The desliming cyclone overflow goes to a separate fines circuit.

Practical indicators that density is drifting outside the intended range:

• Visible carry-over of dark sand into the middling or tailing launder
• A sudden drop in concentrate mass pull when feed tonnage is unchanged
• Frothing or air entrainment at the head of the trough
• Pressure fluctuations on the pump box or sump level swings
• A change in the colour of the underflow from one of the cyclones upstream

When any of these signs appear, the shift team should treat density as a likely suspect before chasing mechanical or electrical causes.

Setting density targets for Australian plant conditions

The water chemistry of Australian mine sites rarely matches bench water used in laboratory tests. Many Pilbara operations draw process water from saline borefields or recycled thickener overflows carrying dissolved salts, which subtly change slurry viscosity and froth behaviour. In the Kalgoorlie region, hypersaline groundwater is common, while in Tasmania fresh dam water with low total dissolved solids produces a more fluid pulp at the same solids content. Both effects shift the optimal density window by three or four percentage points.

Local regulations shape the envelope as well. The Department of Mines, Industry Regulation and Safety in Western Australia, along with the NSW Resources Regulator, sets strict water abstraction limits that push plants toward higher recycling. Higher recycled water fractions carry more fines and reagents, altering slurry rheology and shifting the density target downward. Site altitude and temperature also matter: a plant near Dampier operates warmer, while one in the New South Wales highlands runs cooler through winter and produces a stiffer pulp at the same set point.

Wash water, splitters, and secondary controls

Once bulk density is on target, the operator has two refinement levers: wash water addition along the spiral and the position of the take-off splitters. Wash water dilutes the film at the lower half of the spiral and helps wash residual gangue outward, but it also lowers local film density and can pull chromite out of the inner trough if applied too aggressively. A modest, evenly distributed wash of one to two cubic metres per start is usually sufficient.

Routine actions that keep density, wash water, and splitter settings aligned:

• Recording density, wash flow, and splitter position on every shift handover sheet
• Verifying densitometer calibration against a Marcy cup at least once per shift
• Walking the bank of spirals to listen for irregular flow patterns or blockages
• Adjusting one variable at a time and allowing two cycles before judging the effect
• Keeping the sump screen clean so trash does not bias the density reading
• Flagging any ore source change that might alter feed sizing or mineralogy

These habits compound. A plant that follows them consistently produces a metallurgical balance that reconciles within one or two percent, while a plant relying on memory often finishes the month with unaccounted losses that no audit can explain.

Sampling, online measurement, and reconciliation

Density control depends on measurement the team trusts. Manual Marcy cup or weigh-and-divide sampling remains the reference method for calibrating online instruments, and most Australian plants run at least two cross-checks per shift on different spirals in the bank. Online nucleonic densitometers are common at larger operations but drift with scale build-up and must be cleaned on a schedule that fits the local water chemistry.

Sampling the concentrate, middling, and tailing streams alongside density readings lets the metallurgist build a partition curve. When density moves and grade does not follow the expected pattern, the cause is usually worn splitter lips, blocked launders, or a change in feed mineralogy. Density paired with assays drives real improvement. For operations recovering gold as a by-product, the gravity circuit philosophy transfers from chromite spirals. Teams refining their approach to centrifugal concentration can review practical guidance on Knelson concentrator gold recovery and adapt the same density principles to their site.

Integrating spirals into a chromite recovery circuit

Spirals rarely operate in isolation on a modern chromite project. They typically sit between grinding and classification and a downstream cleaning stage that may include wet shaking tables, magnetic separators, or flotation cells. The density window chosen for the rougher spirals must be compatible with what follows. A concentrate that is too dilute overwhelms downstream pumps and sumps; one that is too thick will not flow at all.

The wider plant context includes the choice of comminution, launder layout, and operating philosophy. A turnkey approach that aligns crushing, grinding, classification, gravity separation, and downstream cleaning around a single set of metallurgical objectives avoids mismatched density targets. When teams plan the whole flowsheet together rather than bolting spirals onto an arbitrary grinding circuit, density control becomes a consequence of good design. See the discussion of copper heap leach design for broader perspective on how solution management and equipment selection interact across unit operations. That philosophy matters on remote sites, allowing a FIFO crew based in Perth to hand a running plant over to a relief team in the Pilbara with confidence that recovery will not suffer during the changeover.

The path to consistent spiral performance on chromite starts with a clear density target, a measurement system the team trusts, and the discipline to keep variables aligned over time. Plant operators across Western Australia and the eastern states who invest in those fundamentals recover more chromium per tonne mined and reduce the noise in their monthly metallurgical balances. To explore how integrated engineering and mineral processing solutions can be tailored to a chromite project of any scale, contact the team at Lozova.org and request a flowsheet review specific to your ore body and site conditions.