Mineral processing

In short

Mineral processing (ore dressing, beneficiation) upgrades a run-of-mine ore into a concentrate fit for smelting by physical and physico-chemical means, without changing the chemical identity of the minerals. It is the bridge between mining and extractive metallurgy: an Indian hematite ore at 58% Fe or a lead-zinc ore at 10% Zn cannot be fed to a furnace or leach circuit economically until the gangue is rejected and the size is controlled. Its physics - comminution energy, settling, flotation chemistry and mass balance - is heavily examined in GATE MT, university Extractive Metallurgy papers and the ESE/IBM/PSU technical rounds. Employers who use it daily include NMDC, SAIL, Tata Steel, JSW, Hindustan Zinc, Hindustan Copper, IREL, Coal India's washeries, Vedanta and equipment makers such as Metso, FLSmidth, thyssenkrupp and TRF.

Explained

What mineral processing does, and the idea of liberation

• Ore = valuable mineral(s) + gangue. Mineral processing separates them; it does not reduce or chemically convert them (that is smelting/hydrometallurgy).
• Objectives: increase grade (upgrading), reject gangue and deleterious elements (Si, Al, P, S, alkalis), produce a controlled size for sinter/pellet/furnace/leach feed, and recover saleable by-products.
• Liberation: the valuable mineral must be freed from the gangue before it can be separated. Grinding is done to the liberation size - in Indian base-metal ores typically 80% passing 45-75 µm.
• Locked particles (middlings) are partly liberated; they force a choice between grade and recovery, or regrinding.
• Grade-recovery relationship: higher concentrate grade almost always means lower recovery. The operating point is economic, not technical.
• Standard sequence: sampling → crushing → screening → grinding → classification → concentration → dewatering → tailings disposal → agglomeration/handling.
• Unit operations are classified as (i) size reduction, (ii) size separation, (iii) concentration, (iv) dewatering, (v) materials handling and ancillary.
• Preconcentration/ore sorting (hand picking, optical/XRT sorters, DMS on coarse sizes) removes gangue early and saves grinding energy.

Comminution: crushing and grinding

• Why it matters: comminution takes 30-70% of a concentrator's energy and is the biggest single power cost; crushing is efficient, grinding is not (only a few per cent of input energy makes new surface).
• Reduction ratio = feed size / product size. Crushers work at 4-7 per stage; grinding mills much higher.
• Crushing (dry, coarse):
  • Jaw crusher - primary, fixed and swinging jaw, compression; handles ROM lumps up to ~1 m; reduction ratio ~6.
  • Gyratory crusher - primary for high tonnage (NMDC, coal), continuous crushing action, higher capacity than a jaw of the same gape.
  • Cone crusher - secondary/tertiary, choke-fed, finer product.
  • Roll crushers / HPGR - high pressure grinding rolls give micro-cracked product and save mill energy; used in iron ore and diamond plants.
  • Impact crusher / hammer mill - impact and attrition, for soft, friable, non-abrasive ore (limestone, coal).
• Grinding (usually wet, fine):
  • Ball mill - cascading/cataracting steel balls; impact plus attrition; grate or overflow discharge.
  • Rod mill - rods grind by line contact, give a narrow size distribution with less fines - good ahead of gravity circuits.
  • AG/SAG mill - large-diameter mill using the ore itself (AG) or ore + ~5-15% balls (SAG); replaces secondary/tertiary crushing and rod milling.
  • Stirred/vertical mills (IsaMill, VTM, bead mills) - fine and ultrafine grinding below ~20 µm.
• Mill variables: mill speed (as % of critical), ball charge (~30-45% of volume), ball size, pulp density (~65-75% solids wet), liner profile, and circulating load (typically 150-350% in a closed circuit).
• Open versus closed circuit: closed circuit with a classifier/cyclone returns coarse material to the mill, prevents overgrinding, raises capacity and sharpens the product size.
• Laws of comminution (energy E to reduce size):
  • Rittinger - energy ∝ new surface created; fits fine grinding.
  • Kick - energy ∝ volume reduction ratio; fits coarse crushing.
  • Bond - the practical middle law, uses the work index Wi (kWh/t to reduce theoretically infinite feed to 80% passing 100 µm); determined by the Bond ball-mill grindability test.
• Typical Bond work indices: limestone ~11, bauxite ~9-10, hematite/magnetite ~10-15, quartzite ~12-14, taconite/BHQ ~14-18 kWh/t (ore-specific - always use the tested value).

Sizing: screening and classification

• Screening separates by geometric size, effective down to about 0.5 mm wet (finer with high-frequency and sieve-bend screens); classification separates by settling behaviour, used below that.
• Screen types: grizzly (coarse scalping), vibrating screen (inclined/horizontal, single or multi-deck), banana screen, trommel (on mill discharge), sieve bend/DSM, high-frequency fine screens, Derrick stack sizers in Indian iron-ore plants.
• Screen terms: aperture, near-size (0.75-1.5 × aperture, the hardest to screen), blinding/pegging, probability of passage, capacity versus efficiency trade-off.
• Screening efficiency = fraction of the undersize present in the feed that actually reports to the undersize product.
• Classifiers: mechanical (rake/spiral classifier), hydraulic (hindered settling, elutriators), hydrocyclone (no moving parts, high capacity, fine cut).
• Hydrocyclone: feed enters tangentially under pressure (~70-170 kPa); coarse/dense particles leave as underflow (spigot), fines as overflow (vortex finder). The cut size d₅₀ falls with larger spigot/smaller vortex finder, lower pulp density and higher pressure.
• Free versus hindered settling: in free settling (dilute) particles fall independently; in hindered settling (dense pulp) the effective fluid density rises, which favours separation by density rather than size - the basis of hydraulic classifiers and jig beds.
• Classification is not perfect: dense fine particles misreport to the underflow (why heavy minerals are overground) and light coarse particles to the overflow.

Gravity, magnetic and electrostatic concentration

• Gravity concentration exploits density difference in a moving fluid. Feasibility is judged by the concentration criterion: > 2.5 easy at all sizes, 1.75-2.5 workable above ~75 µm, 1.25-1.75 only for coarse sizes, < 1.25 impractical.
  • Jig (Baum/Batac for coal, Harz for ore) - pulsating water column, stratification by hindered settling and consolidation trickling; coarse feed (0.5-50 mm).
  • Shaking table (Wilfley) - riffled deck with differential motion; fine feed (0.05-3 mm); gives multiple products - used for tin, tungsten, chromite, beach sands.
  • Spiral concentrator (Humphreys) - helical trough, film flow plus secondary circulation; the workhorse for IREL's beach sands and Indian iron-ore fines (0.075-3 mm).
  • Heavy media / dense medium separation (DMS) - sink-float in a magnetite or ferrosilicon suspension of chosen SG; dense medium cyclone used widely in Indian coal washeries for coking-coal beneficiation.
  • Centrifugal units - Knelson/Falcon concentrators, Multi-Gravity Separator, Kelsey jig, for fines below 75 µm.
  • Enhanced gravity + classification - Floatex density separator, teeter-bed for iron-ore fines.
  • Sluices, pinched sluices, Reichert cones - high-capacity film separators.
• Magnetic separation by magnetic susceptibility:
  • Ferromagnetic/strongly magnetic: magnetite, pyrrhotite → low-intensity (LIMS) drum separators, wet or dry.
  • Paramagnetic/weakly magnetic: hematite, ilmenite, monazite, garnet, chromite, wolframite → high-intensity (WHIMS/HGMS), induced roll, rare-earth roll/drum separators.
  • Uses: tramp-iron removal (overband magnets), magnetite recovery in DMS circuits, upgrading BHQ/BHJ iron ore, ilmenite-monazite split at IREL, Chhatrapur and Manavalakurichi.
• Electrostatic separation by surface conductivity, on dry, close-sized, heated feed: high-tension roll separators split conducting rutile/ilmenite from non-conducting zircon and monazite. Standard in Indian beach-sand plants.
• Other physical methods: optical/XRT sorting, selective flocculation (tested for Indian iron-ore slimes), heavy liquid tests (bromoform, LST) for laboratory washability.

Froth flotation

• Principle: a physico-chemical surface process. Reagents make the target mineral surface hydrophobic; it attaches to air bubbles in an agitated pulp and is floated off as a froth concentrate, while hydrophilic gangue stays in the pulp (tailing).
• Three-phase requirement: particle-bubble collision, attachment (contact angle > 0, measured by the Young equation) and transport in a stable but not over-persistent froth.
• Reagents:
  • Collectors - make the mineral water-repellent. Anionic xanthates and dithiophosphates for sulphides; fatty acids/oleates for oxides and non-metallics; amines for silica (reverse flotation of iron ore and in potash).
  • Frothers - pine oil, cresylic acid, MIBC, polyglycols; lower surface tension, give fine stable bubbles.
  • Activators - CuSO₄ activates sphalerite and pyrrhotite for xanthate.
  • Depressants - NaCN and ZnSO₄ depress sphalerite/pyrite in a selective Pb-Zn circuit; Na₂S, dichromate (depresses galena), starch/dextrin depresses hematite in reverse flotation; lime depresses pyrite.
  • pH regulators (modifiers) - lime, soda ash, H₂SO₄; pH is the master variable (galena floats over a wide range, pyrite is depressed at high pH).
  • Dispersants/flocculants - sodium silicate, polyacrylamides.
• Flowsheet terms: rougher (bulk float), scavenger (recovers from rougher tails), cleaner/recleaner (upgrades rougher concentrate), conditioning tank, differential/selective flotation versus bulk flotation.
• Machines: mechanical (Denver, Wemco, Outotec) cells with impeller-stator plus air; column cells (counter-current, wash water, good for cleaning fines); flash flotation; Jameson cell.
• Variables: particle size (best ~10-150 µm; coarse particles detach, ultrafines collide poorly), pulp density, air rate, froth depth, reagent dosage and addition point, water chemistry, temperature, residence time.
• Indian practice: Hindustan Zinc's Rampura Agucha, Rajpura Dariba and Zawar lead-zinc concentrators; Hindustan Copper's Malanjkhand and Khetri copper circuits; graphite, fluorspar, phosphate and reverse-flotation iron-ore work (NML, CSIR-IMMT, JNARDDC).
• Limitations: reagent cost and effluent load, sensitivity to clay/slimes, poor performance on ultrafines, needs fine grinding.

Dewatering, tailings, agglomeration and water

• Dewatering sequence: sedimentation (thickener, with flocculant; underflow 50-65% solids) → filtration (drum, disc, horizontal belt, ceramic disc, pressure/plate-and-frame; cake 8-12% moisture) → thermal drying where needed.
• Thickener sizing uses the Coe-Clevenger/Talmage method on batch settling-curve data; unit area from the settling rate of the critical concentration.
• Centrifuges and screen-bowl units dewater fine coal; vacuum discs dewater iron-ore and base-metal concentrates.
• Tailings: conventional slurry dam, thickened/paste tailings, filtered dry-stack (increasingly adopted after global dam failures), backfill into stopes. Design, freeboard, seepage, decant and seismic stability are regulated engineering - tailings dam failure is the industry's largest single risk.
• Water: modern Indian concentrators run high recirculation and often zero liquid discharge; make-up water, dissolved ions and residual reagents affect flotation.
• Agglomeration of fines (the link to iron making): sintering of -10 mm fines, pelletisation of -45 µm concentrate with bentonite/organic binder, briquetting - needed because fines cannot be charged directly to a blast furnace.
• Dust and environment: bag filters, wet scrubbers, water sprays, enclosed conveyors; dust is both a loss and an occupational-health issue (silicosis).

Metallurgical accounting, testing and Indian regulation

• Sampling governs everything: increments must be representative; sample reduction by riffle, rotary splitter or coning and quartering; Gy's theory relates minimum sample mass to top size and heterogeneity. Bad sampling cannot be fixed downstream.
• Characterisation: sieve analysis (IS 460 series test sieves, equivalent to ISO 3310; Tyler/ASTM mesh), sub-sieve sizing (cyclosizer, laser diffraction), density, specific gravity, Bond/Hardgrove indices, chemical assay, mineralogy and modal analysis (XRD, optical microscopy, QEMSCAN/SEM-EDS), washability (float-and-sink curves for coal, giving yield versus ash at a chosen cut density; Mayer and M-curves).
• Metallurgical balance: two-product and three-product formulae from assays; plant data are reconciled by least-squares mass balancing; report grade, recovery, yield, ratio of concentration and enrichment ratio together - never recovery alone.
• Control instrumentation: on-stream XRF analysers, nuclear/magnetic density gauges, mass flow meters, particle-size monitors, mill power and bearing-pressure control, expert/model-predictive control on grinding and flotation circuits.
• Indian regulation and practice:
  • MMDR Act, 1957 (as amended, including the 2015, 2021 and 2023 amendments) with auction-based mineral concessions, the National Mineral Policy, 2019, and the Mines and Minerals (Development and Regulation) Amendment route for critical minerals.
  • Mineral Conservation and Development Rules, 2017 (IBM) require approved mining plans, beneficiation studies and sub-grade/low-grade ore conservation, and define threshold values below which ore need not be mined but must be stacked for later beneficiation.
  • Indian Bureau of Mines is the regulator for conservation and scientific mining; it also runs ore-dressing and pilot-plant test facilities (Nagpur, Ajmer, Bengaluru).
  • Safety in mines and mineral-dressing plants falls under the Occupational Safety, Health and Working Conditions Code, 2020, in force from 21 November 2025, which subsumed the earlier mines/factories legislation; DGMS remains the enforcing authority for mines. Wages, hours and social security now sit under the other three Labour Codes.
  • Environment: EIA/Environmental Clearance under the Environment (Protection) Act, 1986 notifications, Water and Air Acts, consent to operate from the State Pollution Control Board, CPCB norms for dust and effluent; coal washing requirements and ash limits are set by MoEFCC notification and have been revised more than once - check the current notification before quoting a limit.
  • GST on most mineral concentrates and plant equipment falls in the 5%/18% slabs under the structure effective 22 September 2025; the exact rate is item-specific.

Key points

• Mineral processing - physical/physico-chemical upgrading of ore; no change in mineral chemistry.
• Liberation - freeing the valuable mineral from gangue; sets the grind size and hence the energy bill.
• Grade versus recovery - inversely related; the chosen point is economic.
• Reduction ratio - feed size ÷ product size; ~4-7 per crushing stage.
• Bond's law - the practical comminution energy law, through the work index Wi (kWh/t).
• Rittinger fits fine grinding (surface), Kick fits coarse crushing (volume).
• Critical speed - mill speed at which a ball centrifuges on the shell; mills run at 65-80% of it.
• Closed-circuit grinding - classifier returns coarse material, prevents overgrinding, raises throughput.
• Screening is by size, classification by settling velocity; the changeover is near 0.5 mm.
• Near-size particles (0.75-1.5 × aperture) control screening efficiency.
• Hydrocyclone d₅₀ - the size reporting 50:50 to both products; the cut-size definition for classifiers.
• Concentration criterion - (ρ_heavy − ρ_fluid)/(ρ_light − ρ_fluid); above 2.5 gravity separation is easy.
• Jig, table, spiral, DMS - gravity devices, coarse to fine in that order of fineness.
• LIMS for magnetite, WHIMS/HGMS for hematite and ilmenite - magnetic separation by susceptibility.
• High-tension roll separator - splits conducting from non-conducting dry beach-sand minerals.
• Flotation reagents - collector (hydrophobicity), frother (bubbles), activator, depressant, pH modifier.
• Rougher-scavenger-cleaner - the standard flotation circuit skeleton.
• Thickener then filter - the standard dewatering train; dry-stack tailings is the safer modern option.

Formulas and examples

Example 1 - Mass balance and recovery (lead-zinc concentrator). Feed 12.0% Zn, concentrate 52.0% Zn, tailing 1.0% Zn; plant feed 3,000 t/day.

• C/F = (12 − 1)/(52 − 1) = 11/51 = 0.2157 → concentrate = 0.2157 × 3,000 = 647 t/day.
• Recovery = 100 × 52 × (12 − 1)/[12 × (52 − 1)] = 100 × 572/612 = 93.5%.
• Ratio of concentration K = 1/0.2157 = 4.64; enrichment ratio = 52/12 = 4.33.
• Check: Zn in concentrate = 647 × 0.52 = 336 t; Zn in feed = 3,000 × 0.12 = 360 t → 336/360 = 93.4% ✔

Example 2 - Bond's law and mill power. Ore Wi = 14 kWh/t, F₈₀ = 10,000 µm, P₈₀ = 150 µm, throughput 200 t/h.

• 1/√150 = 0.08165; 1/√10,000 = 0.01000; difference = 0.07165.
• W = 10 × 14 × 0.07165 = 10.03 kWh/t.
• Mill power at the shell = 10.03 × 200 = 2,006 kW ≈ 2.0 MW; at 95% motor and drive efficiency, installed ≈ 2.1 MW.
• Grinding finer to P₈₀ = 75 µm: 1/√75 = 0.11547 → W = 140 × 0.10547 = 14.77 kWh/t, i.e. 47% more energy for half the product size.

Example 3 - Critical speed and operating speed. Ball mill internal diameter 3.2 m, top ball size 60 mm.

• N_c = 42.3/√(3.2 − 0.06) = 42.3/√3.14 = 42.3/1.772 = 23.9 rpm.
• At 72% of critical: 0.72 × 23.9 = 17.2 rpm.
• Below ~65% the charge slides (attrition only, poor impact); above ~85% it centrifuges and grinding collapses.

Example 4 - Free settling by Stokes' law. Quartz particle 50 µm, ρ_s = 2,650 kg/m³, water ρ_f = 1,000 kg/m³, µ = 0.001 Pa·s.

• v = 9.81 × (50×10⁻⁶)² × (2,650 − 1,000)/(18 × 0.001)
• = 9.81 × 2.5×10⁻⁹ × 1,650/0.018 = 4.047×10⁻⁵/0.018 = 2.25×10⁻³ m/s = 2.25 mm/s.
• A galena particle (ρ_s = 7,500) of the same size settles (7,500 − 1,000)/1,650 = 3.94 times faster - the basis of hindered-settling classification.

Example 5 - Screening efficiency. Feed 100 t/h containing 40% undersize; undersize product 32 t/h assaying 98% true undersize.

• Undersize recovered = 32 × 0.98 = 31.36 t/h; undersize in feed = 40 t/h.
• E = 31.36/40 = 78.4%. The 8.64 t/h of undersize lost to the oversize is the misplaced material.

Where it is used

• Iron ore: NMDC (Bailadila, Donimalai), SAIL, Tata Steel and JSW run crushing-screening-washing plants, hydrocyclones, spirals, Floatex and WHIMS to make sinter and pellet feed from lumps, fines and slimes.
• Base metals: Hindustan Zinc's Rampura Agucha, Dariba and Zawar concentrators (SAG/ball milling plus selective Pb-Zn flotation); Hindustan Copper's Malanjkhand and Khetri copper flotation circuits.
• Beach sands and rare earths: IREL plants at Manavalakurichi, Chavara and OSCOM use spirals, LIMS/WHIMS and high-tension separators to split ilmenite, rutile, zircon, monazite, garnet and sillimanite.
• Coal: Coal India, SAIL and Tata Steel washeries use dense medium cyclones, Batac jigs, spirals and flotation to lower ash in coking coal and to meet notified ash limits for distant power stations.
• Non-metallics and cement: limestone, bauxite (Vedanta, NALCO), chromite (Odisha Mining Corporation, Tata Steel Sukinda), graphite, phosphate, barytes (APMDC).
• Job roles: beneficiation/plant metallurgist, process engineer in a concentrator, comminution and flowsheet design engineer with EPC firms, R&D at CSIR-NML, CSIR-IMMT, JNARDDC and IBM ore-dressing laboratories, equipment application engineering with Metso, FLSmidth and thyssenkrupp.

Interview questions

• Q: Why must an ore be ground before concentration? A: Because the valuable mineral must be physically liberated from the gangue; an unliberated composite particle behaves as an average of both and cannot be separated cleanly.
• Q: Why can you not simply grind very fine to get full liberation? A: Energy cost rises steeply (Bond's law), ultrafines separate poorly in flotation and gravity devices, and dewatering becomes difficult.
• Q: What does the Bond work index represent? A: The energy in kWh/t needed to reduce a theoretically infinite feed to 80% passing 100 µm; it is a measured ore hardness index used to size mills.
• Q: What is critical speed and why do mills run below it? A: The speed at which the outermost ball is held against the shell by centrifugal force; below it the charge cascades and cataracts, which is what grinds - mills run at 65-80%.
• Q: Difference between screening and classification? A: Screening separates by geometric size through an aperture; classification separates by settling velocity in a fluid, so density and shape also matter - it is used below about 0.5 mm.
• Q: What is d₅₀ of a hydrocyclone? A: The particle size that has equal probability of reporting to overflow and underflow - the practical cut size, obtained from the partition (Tromp) curve.
• Q: How do you judge whether gravity concentration will work? A: By the concentration criterion (ρ_h − ρ_f)/(ρ_l − ρ_f); above about 2.5 separation is easy even in fine sizes, below 1.25 it is impractical.
• Q: What do collector, frother and depressant do in flotation? A: The collector adsorbs on the target mineral and makes it hydrophobic, the frother makes fine stable bubbles and froth, and the depressant keeps a specific mineral wetted so it stays in the pulp.
• Q: How are galena and sphalerite separated in one circuit? A: Galena is floated first with xanthate while sphalerite is depressed with ZnSO₄/NaCN at high pH; sphalerite is then activated with CuSO₄ and floated from the lead tailing.
• Q: Why do hematite plants use WHIMS while magnetite plants use LIMS? A: Magnetite is ferrimagnetic and responds to a low field; hematite is only weakly paramagnetic and needs a high-gradient, high-intensity field.
• Q: Why is iron-ore concentrate pelletised? A: Fine concentrate cannot be charged to a blast furnace - it would choke gas flow - so it is agglomerated into pellets of controlled size and strength.
• Q: Recovery is 95% but concentrate grade has dropped. What do you check first? A: Grind size and liberation, cleaner stage performance, froth depth and wash water, reagent overdose and entrainment of fine gangue - and confirm with a mass balance that the assays reconcile.

Common mistakes

• Using mm instead of µm in Bond's equation, or forgetting the factor 10 - the answer comes out wrong by orders of magnitude.
• Taking (D) instead of (D − d) in the critical-speed formula, or leaving diameters in millimetres.
• Quoting recovery without grade and yield; a 99% recovery at a near-feed grade is no concentration at all.
• Confusing enrichment ratio (c/f) with ratio of concentration (F/C).
• Applying Stokes' law to coarse particles where the flow is turbulent - Newton's law applies above Re ≈ 1000.
• Treating flotation as a density process; it depends on surface chemistry, so pH and reagents matter more than specific gravity.
• Calling grinding a "chemical" step, or claiming beneficiation changes the mineral - only the mineral proportions and size change.
• Ignoring sampling: a flowsheet decision based on a non-representative sample is wrong no matter how good the test work is.

Related topics

Iron making: the blast furnace · Non-ferrous extraction: aluminium, copper and zinc · Metallurgical thermodynamics and kinetics · Hydrometallurgy and leaching · Agglomeration: sintering and pelletisation · Materials characterisation (XRD, SEM and optical microscopy)