Gravity and Magnetic Concentration

Introduction

After ore is crushed enough to expose distinct grains, some minerals can be separated by physical differences. Gravity methods exploit density-related settling; magnetic methods exploit differences in attraction to a magnetic field. Neither method creates metal atoms or changes an oxide to free metal. They enrich a feed for later processing.

Core explanation

In a gravity separation, denser particles may settle differently from lighter ones in water or air under carefully controlled size and flow conditions. A simple classroom picture shows heavy valuable grains collecting while lighter gangue washes away. Real performance depends on particle size, shape, fluid flow and how fully grains were liberated. A large low-density particle can behave differently from a small high-density particle, so density alone is not the full design criterion.

Magnetic separation works when one phase responds more strongly to a magnetic field than another. Magnetite, Fe₃O₄, is strongly magnetic and can sometimes be separated from less magnetic gangue after suitable preparation. Hematite, Fe₂O₃, has different magnetic behavior and may need a different technique or field strength; one should not assume all iron-bearing minerals behave exactly like magnetite. Moisture, particle attachment and equipment settings also influence a real separator.

Both methods need liberation. If magnetite grains remain locked inside quartz-rich pieces, the mixed particles may carry too much gangue into magnetic concentrate or lose magnetite to tailings. Additional grinding could help, but it costs energy and may make very fine particles hard to capture. A test of grade and recovery across candidate sizes informs the decision.

For a numerical example, a 500 kg ore contains 100 kg magnetite. A magnetic stage produces 150 kg concentrate with 80 kg magnetite. Concentrate magnetite grade is 80/150 = 53.3%, compared with 20.0% in feed. Magnetite recovery is 80/100 = 80.0%. The 20 kg magnetite outside the concentrate belongs in rejected or other streams. A report that quotes only “53.3% grade” hides the 20% valuable-mineral loss, while a report quoting only “80% recovery” hides the remaining 70 kg gangue in concentrate.

Gravity and magnetic processes can be combined with screening, flotation or other methods. A single ore may include multiple valuable minerals, some magnetic and some not. Separating one phase may leave another in tailings unless the flowsheet accounts for it. “Gangue” is relative to the chosen target; a phase discarded in one product stream might be valuable in another process.

The concentrated mineral is still chemically combined. If magnetite is the feed to ironmaking, its iron remains in an oxide and must later be reduced. A concentrate mass is not an elemental iron mass. To estimate contained iron, multiply magnetite mass by its iron fraction from Fe₃O₄, and then apply later recovery as appropriate.

Wet separation can make measured product mass include water. For fair grade and recovery comparisons, use a consistent dry-mass basis. Assays should specify whether grade means whole mineral phase or elemental iron. These distinctions prevent apparent mass-balance contradictions.

Step-by-step reasoning

1. Identify a density or magnetic contrast between valuable mineral and gangue. 2. Check that particles are liberated at a suitable size. 3. Choose a method and define feed, concentrate and rejected streams. 4. Calculate grade within each stream and recovery from initial valuable content. 5. Keep physical concentration separate from later chemical reduction.

Visual explanation

Draw two parallel branches from crushed ore: a water-flow tray where dense grains settle and a magnetic drum where magnetite grains follow the magnet. In each branch, label concentrate and tailings, then put “same mineral formulas before and after” under the sketch.

Real-world analogy

Sorting a mixed box by weight or by response to a magnet can collect different objects without changing their material. A dense nut might settle while an iron clip follows a magnet. Ore concentration uses the same separation logic at mineral-particle scale.

Real-world example

A plant processing magnetite-bearing rock may use magnetic separators after crushing and grinding. Operators adjust settings to capture magnetite while rejecting enough nonmagnetic rock. They track both iron-bearing mineral recovery and product grade before sending concentrate to ironmaking.

Why?

Why can a method with high concentrate grade still be poor? It may discard a large fraction of valuable mineral. Grade measures purity of the captured stream, whereas recovery measures how much of the original valuable content survived the separation.

Common misconception

“Any iron ore can be pulled out with a simple magnet.” Different iron minerals respond differently, and grain size and gangue attachment matter. Magnetite is a useful example, but the label “iron ore” does not guarantee easy magnetic separation.

Worked example

From 500 kg ore at 20.0% magnetite, a separator produces 150 kg concentrate at 53.3% magnetite. Feed magnetite is 100 kg; concentrate magnetite is approximately 150 × 0.533 = 80.0 kg. Recovery is 80.0%. Concentrate gangue is about 70.0 kg, and about 20.0 kg magnetite was not captured. The approximate decimals reflect the stated grade precision; the separator has enriched the mineral without reducing it to iron metal.

Quick check

1. Does magnetic concentration convert Fe₃O₄ into elemental Fe? Answer: No. It physically enriches magnetite; a later chemical reduction is needed for iron metal.

Exam focus

Name the property exploited and a relevant mineral example. Use consistent dry masses and distinguish concentrate grade from recovery. Avoid claiming all iron ores are equally magnetic or that physical sorting produces elemental metal.

Advanced insight

Magnetic response has degrees rather than a simple yes/no property. High-gradient magnetic separation can capture more weakly magnetic particles than a simple magnet. Gravity separation similarly depends on fluid mechanics and particle size distribution, so pilot tests are needed for process design.

Summary

Gravity and magnetic concentration use physical particle differences after suitable liberation. They can raise ore-mineral grade but may lose some valuable material. Compute grade and recovery separately, and remember the concentrated mineral still needs chemical extraction of its metal.

Practice questions

1. What property is exploited in gravity separation? Answer: Different particle settling behavior related to density, size and flow. 2. What mineral is a common strongly magnetic iron-ore example? Answer: Magnetite, Fe₃O₄. 3. If 80 kg of 100 kg magnetite enters concentrate, what is recovery? Answer: 80.0%. 4. If 80 kg magnetite is in 150 kg concentrate, what is its grade? Answer: About 53.3% magnetite by mass.