MINERAL PROCESSING KNOWLEDGE · CHROME ORE BENEFICIATION
Chromite breaks down readily; the fine fraction below 20 micrometres is where gravity circuits quietly lose their value.
Technical overview. Actual process selection depends on representative ore samples, mineralogy, testwork and site conditions.
Every gravity plant produces slimes, and every gravity plant loses some of its value to them. In chrome ore beneficiation the loss tends to be larger than in most, because chromite crystallises in a brittle habit and breaks down readily during crushing and grinding. A significant part of the feed can finish below 20 micrometres, where conventional jigs, spirals and shaking tables no longer separate cleanly. Those particles leave with the overflow, are usually booked as ordinary tailings, and never appear as a separate line in the mass balance.
Technical note: This article explains the principles of slime generation and slime control in chromite gravity circuits. It is not an engineering design or a performance guarantee, and it does not substitute for testwork. Desliming cut points, reagent selection and equipment sizing must be established on representative samples from the actual deposit.
1. Where the slimes come from
Chromite is not a tough mineral. Its crystals are brittle, and under the impact and abrasion of a crusher or a mill they fracture into progressively finer fragments rather than rounding into durable grains. The result is a natural tendency towards over-grinding: material that is already liberated continues to be reduced because the mill has no way of knowing it is finished, and a portion of the chromite is converted into particles that gravity equipment cannot hold.
Two operating habits make the problem worse. Grinding circuits that are pushed hard for a fine product size generate more fines than the flowsheet expects, and material returned to the mill because it was never removed at the right size is ground again without benefit. Slimes are therefore a grinding outcome before they are a separation problem.
2. Why a gravity circuit loses them
Gravity separation depends on particles of different density settling at different rates, and that difference collapses as particles get smaller. At fine sizes, turbulence and pulp viscosity dominate a particle’s motion, so a chromite grain and a silicate grain of the same size follow almost the same path through the unit.
There is a second effect that is easier to miss. Slimes raise the viscosity of the pulp as a whole, and a viscous pulp blunts the pulsed flow that a jig needs in order to stratify its bed, so even a modest quantity of fine material can degrade the performance of the whole unit. In practice the plant sees the symptom as a concentrate that gradually becomes dirtier rather than as a tailings that suddenly becomes richer, which is why the loss escapes attention.
3. Three practical controls
Three measures address the problem at different points in the circuit.
- Pre-desliming ahead of gravity separation. Micro hydrocyclone clusters, typically 50 mm and 75 mm units, remove the fraction below about 10 micrometres before the pulp reaches the gravity stage. The purpose is to protect stratification: with the slime load reduced, pulp viscosity falls and the jig or spiral works on the size range it was designed for. The overflow is not automatically waste, however; it carries chromite as well as slime, so it needs its own recovery step and its own sampling point.
- Fine centrifugal recovery on the deslimed stream. Once the very fine fraction has been separated, it can be treated in a high-frequency fine centrifugal concentrator that holds particles conventional gravity units would lose. This converts part of an overflow loss into concentrate, and it is normally the highest-value addition to a slime-troubled circuit.
- Selective flocculation ahead of gravity. Polymer flocculants can agglomerate fine chromite selectively into larger, denser units that gravity equipment can then treat. Selectivity matters far more than dose: the flocculant has to prefer chromite over silicate slime, otherwise the process simply builds larger particles of the wrong material, and the reagent therefore has to be screened on the actual pulp.
4. Designing a circuit that expects slimes
None of the three measures is a remedy bolted on after commissioning. Slime control works when the circuit is designed around the expectation that slimes will be present.
- Control the grind rather than the tailings. Remove finished material early with a vibrating screen or a classification stage, instead of letting it recirculate through the mill.
- Treat the overflow as a stream. Give the deslime overflow a defined destination, a flow meter and a sampling point so the loss is visible in the mass balance.
- Separate the circuits. Coarse, medium and fine material need different equipment and different recovery targets, and blending them hides the band that is failing.
- Watch the water balance and re-test after every change. Dilution changes viscosity, and a cut point that suits one ore type may not suit the next.
Conclusion
Slimes are usually described as a nuisance. In a chromite gravity plant they are a recovery line item, and often a large one. Chromite breaks down easily, the resulting fines settle too slowly to be captured by jigs, spirals and tables, and the loss stays quiet because it leaves as an overflow rather than as a visibly poor concentrate. Pre-desliming, centrifugal recovery of the deslimed fines and selective flocculation each address part of the problem, but only if the circuit is arranged to expect slimes in the first place and to measure the fine streams separately.
How LiCheng Tech can help
LiCheng Tech supplies jig machines, spiral chutes, shaking tables, centrifugal concentrators, and the classification equipment and fine screening used to prepare feed for each stage. We work with operators on slime balance problems, where the visible concentrate looks acceptable but the recovery does not, and we help define desliming cut points, select equipment for the fine band and set up the sampling needed to show what is actually leaving in the overflow.
Explore the Mineral Processing Solutions, the chrome ore beneficiation pages, or send your project information.