Enrichment of Ores
Understanding the critical first step in metal extraction
Introduction
Before metals can be extracted from their ores, the ores must first be processed to increase the concentration of the desired metal. This process is called ore enrichment or ore concentration or ore dressing.
The purpose of enrichment is to remove the unwanted materials (called gangue) from the ore, thereby increasing the percentage of the metal in the ore. This process is essential because most ores contain only a small percentage of the desired metal, making direct extraction economically unfeasible.
Key Terminology
- Ore: A naturally occurring solid material from which a metal can be profitably extracted
- Gangue: The unwanted rocky or earthy impurities in the ore
- Concentration: The process of removing gangue from the ore
- Enrichment ratio: The ratio of the percentage of metal in the concentrated ore to that in the original ore
Why Is Ore Enrichment Necessary?
Most ores contain only a small percentage of the desired metal, with the rest being impurities. Direct extraction would be:
- Economically wasteful due to high energy consumption
- Technically challenging due to the presence of impurities
- Environmentally unsustainable due to excessive waste generation
For example, iron might be present as only 20-30% in its ore, copper as 1-2%, and aluminum as 20-30%. Enrichment increases these percentages significantly.
Benefits of Ore Enrichment
- Reduces the bulk of ore to be handled
- Reduces transportation costs
- Saves energy in subsequent processes
- Eliminates harmful impurities
- Increases the efficiency of extraction
- Makes low-grade ores economically viable
Methods of Ore Enrichment
The choice of enrichment method depends on the physical and chemical properties of the ore, particularly the differences in properties between the valuable mineral and the gangue. Here are the major methods:
1. Handpicking
This is the simplest method of ore concentration, where the desired minerals are separated from the gangue by hand, based on differences in color, luster, or texture.
Process:
The ore is spread on a conveyor belt or a sorting table, and workers manually pick out either the valuable mineral particles or the gangue material.
Advantages:
- Simple and low cost
- No expensive equipment needed
- No chemicals required
Limitations:
- Labor-intensive and time-consuming
- Only suitable for large-sized ore particles
- Not suitable for complex ores
Example
Handpicking is used for separating large lumps of coal from stone and shale in coal mining. It’s also used for separating magnesite (white) from the gangue (dark-colored) in magnesite ores.
Exam Tip
Remember that handpicking is rarely used as the sole method of concentration for commercial operations due to its limitations but may be used as a preliminary step.
2. Gravity Separation
This method is based on the difference in densities between the ore mineral and the gangue. Heavier minerals can be separated from lighter ones using gravity.
Process:
The ore is washed with running water or agitated in water. The heavier mineral particles settle at the bottom while the lighter gangue particles are washed away.
Common Techniques:
- Hydraulic washing: Utilizes flowing water to separate particles
- Jigging: Pulsating water motion causes separation based on density
- Wilfley table: Sloped table with rifts that separate minerals as they flow across
- Spiral concentrators: Helical channels where heavier particles move to the inner edge
Applications
| Ore | Mineral Separated |
|---|---|
| Haematite (iron ore) | Fe₂O₃ |
| Cassiterite (tin ore) | SnO₂ |
| Galena (lead ore) | PbS |
| Gold ore | Native gold |
Exam Tip
Remember that gravity separation works best when there is a significant density difference (at least 1.5 times) between the valuable mineral and the gangue.
3. Froth Flotation
This is the most widely used method for ore concentration, particularly for sulfide ores. It exploits differences in the surface properties (hydrophobicity) of minerals.
Process:
- The finely crushed ore is mixed with water to form a slurry
- A collector (e.g., pine oil, fatty acids) is added that selectively coats the desired mineral particles, making them water-repellent (hydrophobic)
- A frother (e.g., cresylic acid) is added to stabilize air bubbles
- Air is blown through the mixture, creating bubbles
- The hydrophobic particles attach to the air bubbles and float to the top as a froth
- The froth is skimmed off to recover the concentrated ore
Reagents Used:
- Collectors: Create hydrophobic surfaces (xanthates, dithiophosphates)
- Frothers: Stabilize the bubbles (pine oil, cresylic acid)
- Modifiers: Adjust pH and help selectivity (lime, sodium carbonate)
- Activators: Enhance collector adsorption (copper sulfate)
- Depressants: Prevent floating of unwanted minerals (sodium cyanide, starch)
Applications
| Ore Type | Examples |
|---|---|
| Sulfide ores | Copper (chalcopyrite), Lead (galena), Zinc (sphalerite) |
| Non-sulfide ores | Hematite, fluorite, phosphate |
Example Reaction:
For copper ore (chalcopyrite):
CuFeS₂ + reagents → CuFeS₂ (concentrate) + gangue
Exam Tip
Know that froth flotation can achieve high degrees of separation even when the physical properties (like density) of the valuable mineral and gangue are similar.
4. Magnetic Separation
This method exploits the difference in magnetic properties between the ore and the gangue. It’s particularly effective for separating ferromagnetic minerals from non-magnetic gangue.
Process:
The crushed ore is passed over a conveyor belt with magnetic rollers or through a magnetic field. The magnetic particles are attracted to the magnet and separated from the non-magnetic gangue.
Types of Magnetic Separators:
- Drum separators: Ore passes over a rotating drum containing magnets
- Cross-belt separators: Magnetic particles are lifted from the main belt onto a cross belt
- High-intensity separators: Used for weakly magnetic materials
- Wet magnetic separators: Operate with ore suspended in water
Applications
| Ore | Magnetic Properties |
|---|---|
| Magnetite (Fe₃O₄) | Strongly magnetic |
| Hematite (Fe₂O₃) | Weakly magnetic |
| Pyrolusite (MnO₂) | Weakly magnetic |
| Wolframite (iron/manganese tungstate) | Weakly magnetic |
Exam Tip
Remember that some minerals become magnetic only after heating (roasting). This is called “induced magnetism” and is used for concentration of some ores like siderite (FeCO₃).
5. Electrostatic Separation
This method is based on the difference in electrical conductivity between the ore mineral and the gangue. Conducting materials follow a different path in an electrostatic field compared to non-conducting materials.
Process:
The dry, crushed ore is fed onto a charged rotating drum. Conducting particles lose their charge to the drum and fall off, while non-conducting particles remain charged and are attracted to the drum for a longer period, falling at a different position.
Advantages:
- Effective for fine-grained materials
- Low operating cost
- Dry process (no water required)
- No chemical reagents needed
Limitations:
- Materials must be completely dry
- Limited to coarser particles (typically >75 μm)
- Environment must have controlled humidity
Applications
Used for separating:
- Titanium minerals (rutile, ilmenite) from zircon and monazite
- Tin ore (cassiterite) from gangue
- Coal from ash-forming minerals
- Graphite from siliceous gangue
Exam Tip
Electrostatic separation is particularly important in the processing of beach sands for titanium minerals and in some applications where both magnetic and non-magnetic conductors are present.
6. Leaching
Leaching is a chemical enrichment method where the ore is treated with a solvent that dissolves the desired metal compound while leaving the gangue undissolved.
Process:
- The ore is crushed to increase surface area
- The ore is treated with a suitable solvent (lixiviant)
- The metal dissolves to form a solution (pregnant solution)
- The solution is separated from the undissolved gangue
- The metal is recovered from the solution by precipitation or other methods
Types of Leaching:
- Heap leaching: Ore is piled in heaps and the solvent is sprayed or percolated through it
- Tank leaching: Ore is placed in tanks with the solvent and agitated
- In-situ leaching: Solvent is pumped into underground ore deposits and the solution is pumped back up
- Pressure leaching: Uses elevated pressure and temperature to increase extraction rates
Applications
| Metal | Leaching Agent | Reaction |
|---|---|---|
| Gold | Cyanide solution | 4Au + 8NaCN + O₂ + 2H₂O → 4Na[Au(CN)₂] + 4NaOH |
| Aluminum | NaOH solution | Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O |
| Copper | Dilute H₂SO₄ | CuO + H₂SO₄ → CuSO₄ + H₂O |
| Uranium | Acidic or alkaline solutions | UO₂ + 3H₂SO₄ → UO₂(SO₄)₃⁴⁻ + 4H⁺ |
Exam Tip
For exams, focus on the chemistry of leaching reactions, especially for gold cyanidation and Bayer’s process for aluminum, as these are commonly tested.
7. Roasting and Calcination
These are thermal treatment methods used to convert ores to a more suitable form for further processing. They’re not strictly concentration methods but are often part of the enrichment process.
Roasting:
Heating the ore in the presence of air or oxygen to convert sulfides to oxides while giving off sulfur dioxide gas.
Example: 2ZnS + 3O₂ → 2ZnO + 2SO₂
Calcination:
Heating the ore in limited or no air to decompose carbonates, hydroxides, or hydrates.
Example: CaCO₃ → CaO + CO₂
Purposes:
- Remove volatile impurities (arsenic, antimony)
- Convert sulfides to oxides for easier processing
- Remove water and carbon dioxide from hydrated and carbonate ores
- Make the ore more porous
- Induce magnetic properties for subsequent magnetic separation
Applications
| Process | Ore Examples |
|---|---|
| Roasting | Zinc blende (ZnS), Copper pyrite (CuFeS₂), Lead sulfide (PbS) |
| Calcination | Limestone (CaCO₃), Dolomite (CaMg(CO₃)₂), Bauxite (Al₂O₃·nH₂O) |
Exam Tip
Remember the key difference: roasting involves oxidation (usually of sulfides) while calcination involves decomposition without oxidation (usually of carbonates).
Factors Influencing the Choice of Enrichment Method
-
Physical and chemical properties of the ore:
- Density difference (for gravity separation)
- Magnetic properties (for magnetic separation)
- Surface properties (for froth flotation)
- Electrical conductivity (for electrostatic separation)
- Chemical reactivity (for leaching)
-
Particle size and liberation characteristics:
- Coarse particles: handpicking, jigging
- Medium particles: gravity methods, magnetic separation
- Fine particles: froth flotation, leaching
-
Grade of the ore:
- High-grade ores may require less intensive concentration
- Low-grade ores may require multiple concentration steps
-
Environmental considerations:
- Water usage and contamination
- Air pollution (especially in roasting)
- Waste disposal requirements
- Chemical usage and toxicity
-
Economic factors:
- Capital investment required
- Operating costs
- Energy requirements
- Recovery percentage achievable
-
Downstream processing requirements:
- Compatibility with subsequent extraction methods
- Required purity for the extraction process
Numerical Example Problems
Problem 1: Calculating Enrichment Ratio
Question: An iron ore contains 25% Fe₂O₃ before concentration. After concentration, the ore contains 65% Fe₂O₃. What is the enrichment ratio?
Solution:
Enrichment ratio = Percentage of metal in concentrated ore / Percentage of metal in original ore
Enrichment ratio = 65% / 25% = 2.6
Therefore, the enrichment ratio is 2.6, meaning the concentration process has increased the Fe₂O₃ content 2.6 times.
Problem 2: Calculating Recovery Percentage
Question: A copper ore weighing 1000 kg contains 2% copper. After froth flotation, 150 kg of concentrate containing 12% copper is obtained. Calculate the recovery percentage of copper.
Solution:
Amount of copper in original ore = 1000 kg × 0.02 = 20 kg
Amount of copper in concentrate = 150 kg × 0.12 = 18 kg
Recovery percentage = (Amount of copper in concentrate / Amount of copper in original ore) × 100%
Recovery percentage = (18 kg / 20 kg) × 100% = 90%
Therefore, 90% of the copper present in the original ore was recovered in the concentrate.
Summary: Key Points to Remember
- Ore enrichment is the process of increasing the percentage of desired metal in the ore by removing impurities (gangue).
- The choice of enrichment method depends on the physical and chemical properties of the ore and gangue.
- Handpicking is the simplest method but is labor-intensive and suitable only for large ore particles with visible differences.
- Gravity separation exploits density differences and is used for ores like haematite, cassiterite, and gold.
- Froth flotation is based on surface properties and is widely used for sulfide ores of copper, lead, and zinc.
- Magnetic separation works for magnetic minerals like magnetite and is sometimes used after roasting to induce magnetism.
- Electrostatic separation separates conducting from non-conducting minerals and is used for titanium ores and beach sands.
- Leaching dissolves the valuable metal using chemical solutions and is important for gold, copper, and uranium ores.
- Roasting converts sulfides to oxides in the presence of oxygen, while calcination decomposes carbonates or hydrates through heating.
- Most commercial operations use a combination of enrichment methods to achieve optimal results.
- Environmental considerations are increasingly important in selecting enrichment methods.
- The enrichment ratio is a measure of the effectiveness of concentration, calculated as the ratio of metal percentage after and before concentration.
- Recovery percentage measures the proportion of the valuable metal that is successfully transferred to the concentrate.
- Ore enrichment is an essential step that makes the subsequent extraction process more economical and efficient.
Exam Focus: Important Questions
Conceptual Questions
- Explain why ore enrichment is necessary before extracting metals from their ores.
- Describe the principle and process of froth flotation with a labeled diagram.
- Compare and contrast roasting and calcination with examples.
- Explain the working of magnetic separation with examples of ores for which it is suitable.
- Describe the leaching process for gold extraction using cyanide solution.
- What factors determine the choice of an ore enrichment method?
- Explain how electrostatic separation works and give examples of its applications.
Numerical Problems
- Calculate the enrichment ratio when an ore containing 30% metal is concentrated to 75%.
- If 500 kg of an ore containing 4% copper yields 100 kg of concentrate, what should be the percentage of copper in the concentrate for 90% recovery?
- A copper ore contains 2.5% copper. After enrichment, 200 kg of concentrate containing 20% copper is obtained from 2000 kg of ore. Calculate:
- The amount of copper in the original ore
- The amount of copper in the concentrate
- The recovery percentage
- The enrichment ratio
