Concentration of ore

What gangue is. Ore as mined is a mixture of the valuable mineral and unwanted earthy, rocky material called gangue (or matrix) — sand, clay, silica and similar impurities with no metal value. Concentration, also called dressing of the ore, is the removal of as much of this gangue as possible before chemical treatment begins.

Why it matters. Two practical reasons drive this step. First, it cuts down the sheer bulk of material that later, costlier steps (roasting, smelting, reduction) must process, saving fuel and reagents. Second, it raises the proportion of actual metal-bearing mineral in what remains, so every subsequent chemical step works on richer material and runs more efficiently.

Step 1 — crushing and pulverisation. Ore lumps as dug out are first broken down by mechanical crushers, then ground into a fine powder. This is a prerequisite, not a concentration method in itself: a fine powder exposes far more surface area, and both concentration methods below only work well once the ore particles are small.

Step 2a — froth flotation (for sulphide ores). Powdered sulphide ore, such as zinc blende ($\text{ZnS}$), is mixed with water and a small quantity of pine oil in a large tank, and air is blown vigorously through the mixture. Sulphide particles are "oil-loving" and stick to the rising air bubbles, forming a metal-rich froth that is skimmed off the surface. The gangue, being "water-loving," is wetted by the water instead, sinks, and is discarded as tailings.

Step 2b — gravity separation (for oxide ores). Powdered oxide ore, such as haematite ($\text{Fe}_2\text{O}_3$), is considerably denser than its sandy gangue. Washing the powder in a strong current of water, or agitating it on an inclined vibrating table, carries the lighter gangue away while the denser ore particles settle or remain behind. The choice between the two methods is not arbitrary — it is decided entirely by which physical property, wettability or density, is the more useful point of difference for that particular ore-gangue pair.

There is no single numerical formula behind concentration of ore — this is descriptive, laboratory-scale physical chemistry — but both methods rest on the same underlying logic: find the one physical property that most clearly separates the wanted mineral from the unwanted gangue, then design a process that pushes particles apart along exactly that property. In froth flotation, the deciding property is surface wettability. Sulphide minerals such as zinc blende have a surface that pine oil clings to readily, while gangue particles are readily wetted by water instead. Once air is bubbled through the mixture, only the oil-coated sulphide particles can attach to bubbles and float; gravity alone would never separate a sulphide ore this cleanly from its gangue, because the two often have similar densities. Wettability, not weight, is what does the sorting. In gravity separation, the deciding property is density. Oxide ores like haematite are markedly denser than the sand and clay around them, so a flowing current of water — which is light enough to sweep away the low-density gangue but not strong enough to move the heavier ore particles — separates the two by settling speed alone. Wettability plays no useful role here, since oxide ores and their gangue do not differ sharply enough in that property to make flotation practical. This is why the two methods are not interchangeable: applying gravity separation to a sulphide ore, or froth flotation to a dense oxide ore, wastes the process on the wrong physical handle and leaves the gangue mixed in. Matching method to property, not memorising which ore "goes with" which method, is the transferable skill.

Question: Zinc blende ($\text{ZnS}$) ore is mined mixed with a large proportion of rocky gangue. Describe, step by step, how it is concentrated, and state the physical property exploited.

Step 1. The mined lumps are first crushed and pulverised into a fine powder, exposing the sulphide particles and increasing surface area for the next stage.

Step 2. The powdered ore is fed into a large flotation tank along with water and a small amount of pine oil, and air is bubbled vigorously through the mixture, whipping it into a froth.

Step 3. Because zinc blende is a sulphide, its particles are oil-loving and attach to the rising air bubbles, forming a stable, metal-rich froth on the surface, while the water-loving gangue particles sink to the bottom as tailings.

Step 4. The froth is skimmed off and carried forward to roasting; the settled gangue is discarded.

Answer: zinc blende is concentrated by froth flotation, exploiting the difference in surface wettability between the oil-loving sulphide and the water-loving gangue — not density, which is why gravity separation would not work here.

Competitive stretch

Contrast this with haematite ($\text{Fe}_2\text{O}_3$), mixed with light, sandy gangue. Since haematite is an oxide and considerably denser than its gangue, the property to exploit is density, not wettability, so gravity separation (a strong water current, or an inclined vibrating table) is used instead: the light gangue washes away while the denser haematite settles or remains behind. Being asked to justify why one method beats the other for a given ore, rather than just naming the method, is the exam-favourite twist on this topic.

• Thinking concentration is a chemical change — it only removes gangue physically; the ore's chemical identity is unaffected, and no new compound forms.
• Swapping which method suits which ore type — froth flotation is for sulphide ores, gravity separation is for dense oxide ores; reversing this pairing is a common exam error.
• Confusing gangue with the ore itself — gangue is the unwanted earthy or rocky impurity, while the ore is the useful mineral being concentrated.
• Skipping or downplaying crushing and pulverisation — fine particle size is what makes either concentration method effective in the first place.

Ask the tutor: Want to work through why a given ore needs froth flotation instead of gravity separation, with more examples? Ask your AI tutor.