MINERAL PROCESSING KNOWLEDGE · CHROME ORE BENEFICIATION
A size-by-size view of chromite gravity circuits, from coarse jig feed through spiral roughing and shaking-table cleaning to centrifugal recovery of fines.
Technical overview. Actual process selection depends on representative ore samples, mineralogy, testwork and site conditions.
Chromite, FeCr2O4, has a specific gravity of roughly 4.0–4.8 g/cm3; the silicate gangue it is normally associated with, olivine and serpentine, sits far lower at about 2.6–3.0 g/cm3. That contrast is why gravity concentration is the core separation method in chrome ore beneficiation. The difficulty is not the principle but the size range: one comminution circuit delivers material from tens of millimetres down to a few micrometres, and each band responds differently to the same flowing pulp. A chromite gravity circuit is therefore a set of circuits arranged by size.
Technical note: This article explains the principles of size-based gravity separation in chrome ore beneficiation. It is not an engineering design, an equipment quotation or a performance guarantee. Flowsheet selection, equipment sizing and recovery estimates depend on representative sampling, mineralogical and particle-size analysis, suitable testwork and site-specific engineering review.
1. Why density decides the circuit
Density separation is a race between particles of different mass. A chromite grain heavier than its gangue neighbours settles faster, and under the right flow conditions that difference accumulates into layers that can be split. A gravity circuit exists to create those conditions: a pulsed water column, a spiral trough, an inclined shaking deck, or a centrifugal field. Because the race is easy to win with large particles and hard to win with small ones, the same principle needs different machines at different sizes.
Three things should be measured for each band before a circuit is arranged:
- Cr2O3 grade and Cr:Fe ratio in every size fraction, so the plant knows which bands carry value and which carry dilution.
- The liberation size of chromite, where grains become free rather than locked with silicate gangue.
- The share of feed that falls below the working limit of conventional gravity equipment, which sets the fine recovery duty.
2. A circuit map by size band
Chromite feed is normally handled in three working bands, each with its own equipment family.
| Size band | Size range | Primary equipment | Circuit duty |
|---|---|---|---|
| Coarse | 15 mm – 2 mm | Movable-screen jig or hydraulic jig | Density stratification under pulsed water; the underflow is drawn off as a lumpy high-grade chromite concentrate |
| Medium | 2 mm – 0.074 mm | Spiral concentrator, then a shaking table | High-capacity roughing and tailings discard, followed by precise cleaning of the rough concentrate |
| Fine | Below 0.074 mm | Centrifugal concentrator | Enhanced settling and enrichment of fine chromite in an elevated gravity field |
Treat this map as a default rather than a flowsheet. Where primary crushing already produces a fine feed, the spiral and table route may carry most of the tonnage; where the ore is coarse-grained and well liberated, the jig concentrate can justify its own handling as a lump product while the finer bands are treated separately.
3. Coarse and medium fractions: jigs, spirals and tables
The 15 mm to 2 mm band is jig territory, and either a movable-screen jig or a hydraulic jig will do the work. Pulsed water lifts and settles the bed repeatedly, particles stratify by density, and the bottom layer is drawn off as concentrate. Because the particles are large and already partly liberated, this stage can make a high-grade lumpy product before any further size reduction. Crushing an already liberated coarse particle is expensive and counter-productive: it adds grinding power, creates fines a jig cannot hold, and turns a premium lump product into a slime problem.
Between 2 mm and 0.074 mm, spirals do the heavy lifting. A spiral chute has no moving parts in the separation zone, needs no powered drive to establish its flow field, and moves large tonnages through a small footprint, so it suits roughing and tailings discard. Its rough concentrate is deliberately modest in grade and is cleaned on a shaking table, where an asymmetric reciprocating stroke and cross-flow wash water split the bed into visible bands. Spirals make volume; tables make grade.
4. Fine fraction: centrifugal recovery below 0.074 mm
Below 0.074 mm, conventional gravity equipment starts to fail. The settling difference between chromite and gangue becomes small relative to turbulence and pulp viscosity, clean layers no longer form, and the unit reports value to its overflow. The practical answer is to replace the gravity field itself: a centrifugal concentrator subjects the pulp to an acceleration of tens of times gravity, so fine chromite settles against the bowl wall far faster than in an open trough and is enriched into a small, high-grade product.
The trade-off is that a centrifugal unit is a concentrator, not a substitute for the coarser stages. It responds poorly to fluctuating pulp density, oversize contamination and unstable feed grade, and its recovery depends on how the feed was prepared. A well-run fine circuit is fed a screened, classified and deslimed stream, and that preparation usually matters more than the choice of machine.
Conclusion
A chromite gravity circuit is an arrangement of machines matched to size, not a single process step. Coarse feed is settled in a jig machine, medium feed is roughed in spirals and cleaned on tables, and fine feed is driven into a centrifugal field because ordinary settling no longer separates it. Each band needs its own recovery target and its own measurement, otherwise a plant can report a healthy overall recovery while losing most of its value in one size fraction.
How LiCheng Tech can help
LiCheng Tech supplies jig machines, spiral chutes, shaking tables, centrifugal concentrators, and the classification and screening equipment that prepares feed for each band. We work with operators to compare size-by-size circuit arrangements, define the testwork each band needs, and specify equipment against measured feed conditions rather than assumed ones.
Explore the Mineral Processing Solutions, the chrome ore beneficiation pages, or send your project information.