Refining Crude Metals: Methods & Examples

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

For most uses of metals the pure metal is required, but in nature it is typically found with impurities. In this lesson we will learn several methods used to refine metals.

Refining Methods

Imagine with me for a moment that you just bought your first piece of property, and start digging it up to build a house. And then, the ultimate dream comes true - you strike a gold mine! You excitedly grab the rock with bits of gold in it - but now what? How do you get that gold out of the rock? The process of separating the gold (or other metals) from the impurities surrounding it is called refining. There are several different methods that can be used for refining metals including:

  • Distillation
  • Liquation
  • Poling
  • Electrolysis
  • Zone refining
  • Vapor Phase refining

Let's go over these methods so you can better understand what it takes to refine metals.

Distillation

For metals that have a particularly low boiling point, such as mercury and zinc, they can be distilled to remove the impurities. Distillation is the process of vaporizing something, and then allowing it to condense back in a separate location. Mercury and zinc will readily vaporize, while most other metals and impurities won't. So if you heated up a sample of impure mercury, the mercury will vaporize but nothing else around it will. The vapor can be collected, and when it condenses back you are left with pure mercury.

Liquation

Metals that have particularly low melting points can use the liquation method, this includes metals such as lead and tin. Liquation is similar to distillation, except the impure metal is only melted instead of vaporized. When the other impurities have a much higher melting point, they will remain solid and can be removed from the pure liquid metal.

When metal reaches high temperatures, it melts and turns into a red hot molten compound
Melted metal

With this method, the impure metal is typically put into a sloped container and heated to the melting point of the desired metal. Since it is sloped, the liquid metal will run down, leaving behind the solid impurities.

Poling

Poling is a method used to purify metals that have oxidized impurities. It is typically used to purify metals like copper or tin that are in the impure form of copper oxide or tin oxide. This method may seem strange, but in order to do this we take a log of wood that is still green and use it to stir the liquid metal. The hydrocarbons in the green wood can reduce the metal, and the oxygen leaves as CO2 gas.

Electrolysis

Electrolysis uses currents to give the energy needed to help a chemical reaction occur to break apart chemical bonds between metals and impurities. This method was first used in the late 18th century to separate tin and zinc from their salts. This method is also used for purifying aluminum. Aluminum oxide is melted and a current is passed through it. The electric current separates the aluminum from the oxygen. A cathode adds electrons on the aluminum, making pure aluminum, and an anode collects the extra electrons on the oxygen, combining it with carbon to form CO.

Zone Refining

Zone refining works by crystallizing the metal. Since impurities don't crystallize, they will fall out of the matrix. This is often used for semiconductor metals such as silicon and germanium. Using this method, the impure metal is formed into a rod. The rod is placed horizontally, with one end of the rod surrounded by an inert gas and heated. Slowly the molten zone is moved from the top to the bottom of the rod. The metal will crystallize, staying put on the rod, while the impurities will become liquid, running down the rod as the heat moves down. Thus we end up with the impurities at the bottom of the rod, and pure metal at the top.

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