Mineral processing Study Guide

📖 Core Concepts Mineral processing – The series of operations that separate valuable minerals from ore, producing a concentrate and a waste stream (tailings). Beneficiation – Improves ore value by removing gangue; the result is a higher‑grade concentrate. Recovery – Fraction (mass or molar) of the valuable mineral that ends up in the concentrate. Unit Operations – Main blocks: Comminution (crushing & grinding), Sizing (screening & classification), Concentration (gravity, magnetic, electro‑static, flotation), Dewatering (water removal). Physical vs Chemical Separation – Physical uses size, density, magnetism, surface chemistry; chemical uses reagents (e.g., flotation collectors, leaching agents). 📌 Must Remember Crushing = dry, compression/impact; Grinding = wet, attrition, highest energy consumer. Concentration Criterion (CC) determines gravity‑separation suitability: | CC range | Minimum particle size | |----------|----------------------| | > 2.5 | > 75 µm | | 1.75–2.5 | > 150 µm | | 1.50–1.75| > 1.7 mm | | 1.25–1.50| > 6.35 mm | | ≤ 1.25 | not suitable | Magnetic force on a particle: $$F = \frac{m}{k}\,H\,\frac{dh}{dx}$$ m = magnetic moment, k = susceptibility, H = field strength, $dh/dx$ = field gradient. Electrostatic separation works best for dry particles 75 µm–250 µm that are uniformly sized & shaped. Flotation reagents – Collectors (make particles hydrophobic), Frothers (stabilize bubbles), Depressants (prevent unwanted minerals), Activators (enhance target mineral flotation). Dense Media Separation (DMS) uses a heavy liquid or suspension (commonly magnetite) to stratify particles by density before grinding. 🔄 Key Processes Comminution Crushing: Run‑of‑mine ore → jaw/gyratory/cone crusher → size reduction (dry). Grinding: Crushed ore → rod/ball mill → fine particles (wet, closed‑circuit with classifier). Sizing Screening: Choose screen material, aperture, vibration frequency → separate by size. Classification: Use hydrocyclones, gas cyclones, trommels, etc., to exploit settling velocity differences. Gravity Concentration Compute CC → select appropriate equipment (shaking table, spiral, jig, Knelson bowl). Feed particles that meet CC‑size window → dense media or centrifugal forces separate heavy from light. Magnetic Separation Determine magnetic susceptibility of target mineral. Choose high‑field or high‑gradient separator; optionally add water for slurry processing. Electrostatic Separation Ensure particles are dry, 75–250 µm, similar shape. Charge particles (corona discharge) → conductive particles lose charge on drum & are thrown outward. Froth Flotation Adjust pH → set surface hydrophilicity. Add collectors → particles become hydrophobic. Add frothers → create stable bubbles. Introduce depressants/activators as needed. Air bubbles rise → hydrophobic particles attach → skim off concentrate. Dewatering Choose method based on particle size: screens (coarse), thickening/settling (mid‑size), filtration or thermal drying (fine). 🔍 Key Comparisons Crushing vs Grinding – Dry compression/impact vs wet attrition; crushing prepares feed for grinding. Gravity vs Magnetic vs Electrostatic – Density‑based vs susceptibility‑based vs conductivity‑based separations. Flotation Cells vs Columns vs Jameson Cell – Cells: robust, moderate grade/recovery; Columns: finer particles, higher grade, lower recovery; Jameson: fine bubbles → higher kinetic energy, better recovery for fine ores. Dense Media Separation vs Shaking Table – DMS uses bulk density medium before grinding; shaking table separates after grinding based on density alone. ⚠️ Common Misunderstandings “Higher CC = better” – CC only predicts suitability within the proper particle‑size window; a high CC on very fine particles (<75 µm) still fails. “Magnetic separation works on any ore” – Only minerals with sufficient magnetic susceptibility respond; non‑magnetic sulfides need flotation. “Flotation can treat any particle size” – Very coarse particles (>5 mm) have poor bubble attachment; fine particles (<10 µm) may be entrained in slime. “Crushing is a wet operation” – Crushing is typically dry; only grinding uses water. “Electrowinning is part of flotation” – Electrowinning follows leaching; it recovers dissolved metal, not a physical separation step. 🧠 Mental Models / Intuition Density → sink vs float – Imagine a bathtub: heavy marbles settle quickly, light plastic floats; the same principle scales to ore particles. Magnetism → “stickiness” – Materials that “stick” to a magnet feel a pull (force ∝ susceptibility × field gradient). Electrostatics → charge‑and‑release – Dry particles are like static‑charged balloons; conductive ones discharge on contact with a grounded surface. Flotation → “bubble‑hugging” – Hydrophobic particles love bubbles like soap bubbles love water; once attached, they ride up. 🚩 Exceptions & Edge Cases CC ≤ 1.25 – Gravity methods ineffective regardless of size. Electrostatic range 75–250 µm – Outside this range, charge distribution is uneven; separation fails. Magnetic separation of fine particles – Requires high‑gradient fields; low‑grade magnets may miss sub‑50 µm particles. Dense media using organic liquids – Viable but less common due to cost/environment; magnetite suspensions dominate. Flotation columns – Preferable for ultra‑fine ores (<20 µm) where cells lose efficiency. 📍 When to Use Which High specific gravity & coarse size → Gravity concentration (CC check first). Magnetic susceptibility measurable → Magnetic separator (high‑field for weakly magnetic, high‑gradient for fine). Dry, uniform 75–250 µm conductive particles → Electrostatic separator. Sulfide or oxide ores with surface chemistry contrast → Froth flotation (choose cells vs columns based on particle fineness). Very fine, hydrophilic particles → Leaching followed by electrowinning. Need rapid, rock‑by‑rock sorting → Automated optical/conductivity/ magnetic ore sorter. Excess water in pulp → Dewatering screen (coarse) → Thickener → Filter (fine) → Dryer (if moisture < 5 %). 👀 Patterns to Recognize “Heavy → bottom, Light → top” in gravity tables, spirals, or DMS. Magnetic pull on a roller → conductive particles fly outward (electrodynamic separator). Bubble‑laden froth on top of cell → hydrophobic concentrate (flotation). Fine, dry, conductive particles clustering on a charged plate → electrostatic separation. Sudden drop in recovery when particle size < 75 µm → gravity method reaching its limit. Presence of cyanide depressant → targeting sulfide minerals while suppressing others. 🗂️ Exam Traps Distractor: “A CC of 1.2 can be used if the particles are very coarse.” – Wrong: CC ≤ 1.25 is unsuitable at any size. Distractor: “Magnetic separation can replace flotation for all sulfide ores.” – Wrong: Only magnetic sulfides (e.g., magnetite) respond; most sulfides need flotation. Distractor: “Electrostatic separators work on wet slurries.” – Wrong: Particles must be dry; water short‑circuits charge. Distractor: “Higher crusher output automatically means higher recovery.” – Wrong: Over‑crushing creates ultra‑fine particles that may escape gravity or magnetic separation, reducing recovery. Distractor: “All collectors are surfactants that make particles hydrophobic.” – Wrong: Some collectors (e.g., xanthates) are specific to certain mineral chemistries; not all surfactants work universally. Distractor: “Dewatering and drying are interchangeable.” – Wrong: Dewatering removes bulk water (mechanical); drying removes bound moisture (thermal). --- Use this guide for quick review before the exam – focus on definitions, key formulas, decision rules, and the common pitfalls that often appear on test items.