Extraction and Refining of Metals & Reactivity Series

Extraction and Refining of Metals

• The Earth's crust contains metals and metal compounds like gold, iron oxide, and aluminum oxide
• Metals are often mixed with other substances in the Earth
• To obtain useful metals, they must be extracted from the mixed substances
• A metal ore is a rock containing a metal or metal compound in a high enough concentration to make extracting the metal economical
• Major steps in metal extraction:
  • Ore concentration: purifying and concentrating the ore, removing unwanted rocks
  • Reduction to crude metal: reducing metal oxides to pure metals, creating a mixture
  • Refining: purifying the metal and removing impurities to obtain a pure metal

Reactivity Series

• A reactivity series is an empirical, calculated, and structurally analytical progression that arranges metals by their reactivity, from highest to lowest
• Reactivities from highest to lowest are listed in the image
• Reactivity series is used to summarize information about reactions of metals with acids, water, and double displacement reactions
• It displays the extraction method of metals from their ores

Electrolysis

• Highly reactive metals are extracted using electrolysis from their ores due to the formation of very stable compounds that make reduction by carbon unfeasible
• Electrolysis uses a direct electric current to drive a non-spontaneous chemical reaction
• Electrolysis is commercially important for separating elements in ores using an electrolytic cell. Voltage needed for electrolysis is called the decomposition potential
• Electrolysis passes a direct electric current through a molten or dissolved ion-containing substance or electrolyte, creating chemical reactions at the electrodes, and separating materials
• Main components for electrolysis are:
  • Electrolyte: an ion-conducting substance, usually a polymer that contains free ions to carry current through the electrolyte (solid salts cannot conduct current unless molten)
  • DC electrical supply: provides energy to create or discharge ions in the electrolyte.
• The external circuit's current is carried by electrons

Electrolysis: Extraction of Aluminum from Bauxite

• Raw materials
  • Bauxite (Al₂O₃∙2H₂O): an ore containing hydrated aluminum oxide
  • Cryolite (Na₃AlF₆): a sodium aluminum fluoride, used to lower the melting point of bauxite
  • Carbon electrodes
• Melting Point: about 2000°C
• Cryolite is added to lower the melting point to about 900°C
• In electrolysis, the aluminum ions are attracted to the negative cathode and oxygen ions to the positive anode.
• Aluminum ions gain electrons, forming molten aluminum at the cathode. Oxygen loses electrons to form oxygen gas at the anode, reacting with the carbon anode and creating Carbon monooxide/carbon dioxide requiring regular replacement

Reduction with Carbon (Blast Furnace Method): Extraction of Iron

• Raw Materials
  • Iron ore (e.g., haematite, Fe₂O₃)
  • Coke (C)
  • Limestone (CaCO₃)
• Iron is extracted from iron ore in a blast furnace
• Reactions during iron extraction:
  • Coke (carbon) reacts with oxygen from hot air to produce carbon dioxide
  • Limestone decomposes to calcium oxide and carbon dioxide
  • Carbon monoxide reacts with iron oxide ore to remove oxygen and free the iron
• Impurities (silica) react with limestone to form a molten slag
• The molten iron, denser than the slag, sinks to the bottom of the furnace, creating pig iron

Reduction with Carbon: Extraction of Iron - Stages

• Stage 1: Production of carbon dioxide. Coke burns in air to form CO2. The limestone also decomposes into quicklime (CaO) and CO2
• Stage 2: Production of carbon monoxide. The carbon dioxide formed from the limestone reacts with more coke to form carbon monoxide
• Stage 3: Reduction of haematite. The carbon monoxide removes the oxygen from iron oxide (in the ore), releasing pure iron that's molten due to the high temperature. This iron flows down to the base of the furnace as pig iron.
• Stage 4: Removal of impurities. Silica from the ore reacts with calcium oxide to form calcium silicate (slag), which floats on the molten iron. The impurities are removed from the iron.

Refining of Metals

• Most metals obtained via reduction are impure. They need further refining
• Refining is based on the difference in properties between metals and their impurities
• Common refining methods:
  • Liquation: melting the metal; low-melting metals like tin, lead are purified by liquation, where the pure metal melts and flows, leaving impurities behind
  • Distillation: used for volatile metals; heated to generate vapors to be condensed and purified; impurities are left behind
  • Oxidation: removing impurities by oxidation of impurities to form gaseous oxides, which are removed
  • Electrolytic refining: used for purifying impure metals like copper; involves making the impure metal the anode and pure metal as the cathode; current passing through electrolytic cell dissolves anode leaving behind purer metal at the cathode

Extraction of Low Reactive Metals

Metals found at the bottom of the reactivity series are often found in their native forms. They can be extracted from their ores by heating or roasting. Gold, for example, is often found as a native metal and does not usually require chemical separation. Other chemical reactions may be needed to remove contaminants.

Extraction of Gold

• Gold extraction involves breaking down large rock chunks into smaller pieces, grinding to a slurry or powder, thickening the slurry with water to form pulp, and processing it through leaching tanks.
• Cyanide leaching dissolves the gold, a chemical solvent. It's important to combine it with oxygen in the carbon-in-pulp process to allow dissolution.
• Small carbon grains are introduced to the leaching solution; gold adheres to the carbon. A filter mechanically separates the gold from the carbon.
• A hot caustic solution separates the gold from the carbon in a stripping vessel.
• Additional filtration processes are used to remove the remaining carbon and other materials using screens.
• Electrowinning is used to separate the gold using a specialized cell. A current passes through the solution, which causes the gold to collect on the negative terminals.
• Smelting, involves melting the negative terminals in a high temperature furnace which causes the gold to become nearly pure. A flux is added to separate the gold from the terminals.
• Molds transform the molten gold into gold bars called doré bars during final steps of the process.