Extraction of the Metals

Transcript

0:00Introduction

Hello everyone! Let me start with a small puzzle. To get iron from its ore, we feed coke, which is carbon, into a huge furnace. So the carbon pulls the oxygen out of the iron oxide, right? Well, mostly not, and the real answer may surprise you. And copper is even stranger: in its last step no carbon is used at all, and copper sulphide itself reduces copper oxide. So how does that work? Today we follow metals from ore to pure metal. These steps come up often in JEE, so let's begin.

0:34Step one: concentrating the ore

So, the first step is concentration, separating the useful mineral from the rest of the ore. The methods include hydraulic washing, magnetic separation, and leaching. Bauxite is leached with N a O H, sodium hydroxide, and silver and gold, written A g and A u, with cyanide. Okay? For sulphide ores, we use froth flotation. The collector, pine oil or xanthates, makes the sulphide particles water repellent, so they attach to air bubbles. The stabiliser, cresols or aniline, keeps the froth stable. Right? Now let me ask you this. The ore has lead sulphide, P b S, and zinc sulphide, Z n S, and we want only the lead sulphide in the froth. Both are sulphides, so how do we hold the zinc sulphide back? With a depressant, yes! N a C N, sodium cyanide, keeps zinc sulphide out of the froth, while lead sulphide is floated. That gives us selectivity. Clear?

1:36Calcination, roasting, and zinc

Next, we heat the ore, and there are two ways. Calcination heats carbonates and hydrated ores in little or no air. For example, Z n C O three, zinc carbonate, gives Z n O, zinc oxide, plus C O two; the arrow means gives. With me? Roasting heats sulphides in air. Two Z n S plus three O two gives two Z n O plus two S O two, sulphur dioxide. Got it? After roasting, we get zinc by reducing zinc oxide with coke: Z n O plus C gives zinc plus carbon monoxide. Then purify it by distillation. Okay?

2:14The Ellingham diagram

Now, what can remove oxygen from an oxide? The Ellingham diagram tells us. It plots delta f G naught of the oxides against T, the temperature. The capital delta with a small f below, and the little circle above, make it the standard Gibbs energy of formation. Right? Here is the rule: any metal whose line lies lower can reduce the oxide of a metal whose line lies above it. Clear? Now look at carbon: two C plus O two gives two C O, carbon monoxide. This line slopes downwards, so tell me, why does that happen? Carbon is a solid, so count only the gas: one mole of oxygen gives two moles of carbon monoxide. So delta S naught, the entropy change, is greater than zero. Then in delta G naught equals delta H naught minus T times delta S naught, delta G gets more negative as T rises. With me? So at high temperature, carbon reduces most oxides.

3:13Iron: the blast furnace

Got it? Now iron, in the blast furnace. Coke burns first: C plus O two gives C O two. Then this carbon dioxide meets more coke, and C O two plus C gives two C O, carbon monoxide. Okay? Now the iron ore, F e two O three, iron three oxide. Remember our first puzzle. What actually takes the oxygen away from it? Carbon monoxide, yes, exactly! F e two O three plus three C O gives two F e plus three C O two. So carbon monoxide is the main reductant of the ore, not the solid coke; only in the hottest zone does some coke reduce F e O directly. That settles our first puzzle. Right? To remove silica, S i O two, we add limestone, which gives C a O, calcium oxide. C a O plus S i O two gives C a S i O three, calcium silicate, which leaves as slag. Clear? The iron from the furnace is pig iron, with about four percent C, carbon; the percent sign means per hundred. Remelt it with scrap iron and coke, and you get cast iron, about three percent. Wrought iron is the purest, and steel is made by oxidising away the excess carbon. With me?

4:27Silver and gold: the cyanide process

Next, let us look at how gold is extracted, by the cyanide process. Four A u, plus eight C N minus, plus two H two O, plus O two, gives four A u C N two minus ions, plus four O H minus. C N minus is cyanide, and O H minus is hydroxide. The square brackets mark one complex ion, and the round brackets hold its two cyanides, so it is gold with two cyanides, with a charge of minus one. Got it? Then zinc pushes the gold out. Two A u C N two minus, plus Z n, gives Z n C N four, two minus, plus two A u, gold metal. Silver is extracted by the same cyanide process. Okay?

5:07Refining: the last step

Finally comes refining. Distillation is used for zinc and mercury, liquation for tin, and electrolysis for copper and zinc. Right? Zone refining is used for silicon, germanium, boron, gallium and indium. The Mond process refines nickel, and van Arkel refines zirconium and titanium. Chromatography is also used. Clear?

5:31Example 1: calcination vs roasting, one example each

Now Example one, an easy one. Distinguish calcination from roasting, with one example each. Try to pick examples other than zinc. Pause the video and try it first. Done? Okay, let's check. Calcination drives off carbon dioxide or water, in limited air. Think of the limestone in the blast furnace: C a C O three, calcium carbonate, gives C a O plus C O two. Okay? Roasting oxidises sulphides in air. Two P b S, lead sulphide, plus three O two gives two P b O, lead oxide, plus two S O two. Right? So, in one line: limited air for carbonates, and excess air for sulphides. Match the ore to the process; students often mix these up.

6:18Example 2: copper from pyrites by self-reduction

Clear? Now Example two, a harder one, and it settles our second puzzle. Write the reactions for extracting copper from copper pyrites, C u F e S two. Pause the video and try it first. Done? Okay, let's check. First we roast the pyrites. Two C u F e S two plus O two gives C u two S, plus two F e S, plus S O two. With me? Next, the iron must go. Two F e S plus three O two gives two F e O plus two S O two. Then, in a reverberatory furnace with silica, F e O plus S i O two gives F e S i O three, which leaves as slag. Got it? What is left is called matte: copper sulphide, C u two S, with some iron sulphide. In the converter, two C u two S plus three O two gives two C u two O plus two S O two. Okay? Now we have copper oxide. To get copper metal, we must add coke, just like in the blast furnace, right? No, no, wait, there is no coke here! Two C u two O plus C u two S gives six C u plus S O two. The copper sulphide itself reduces the copper oxide, and that is called self-reduction. That settles our second puzzle. Got it? The product is blister copper. Why is it called that? As the copper solidifies, sulphur dioxide escapes and leaves blisters on its surface. Clear?

7:43Quick revision: roasting, Ellingham, iron, copper, gold

So, quick revision. First, concentrate the ore, then calcine carbonates in little air, or roast sulphides in air. Second, on the Ellingham diagram, a lower line reduces the oxide above it, and carbon wins at high temperature. And third, in the blast furnace carbon monoxide is the main reductant, copper comes by self-reduction, and gold and silver by cyanide, then zinc. Now go and try the two practice problems on this page. All the best!