Extraction of metals from ores

1. Extraction of metals from ores

2. Pyrometallurgy • Pyrometallurgy uses high temperatures to transformmetals and their ores. These transformations may produce pure metals, or intermediate minerals or alloys, suitable as feed for other refining or commercial applications. • Some pyrometallurgical processes are autogenous, and so the energy required to heat the minerals comes from the exothermic reaction of the minerals in the process and no further energy is required.

3. Pyrometallurgy • Roasting • Solid-gas reactions are used to chemically transform minerals. • As an example, roasting is used to remove sulfur from sulfide ores. The ore is heated in an oxidizing atmosphere such as air. Sulfides are oxidized by the oxygen in the air, with the sulfur liberated as sulfur dioxide gas, leaving an oxidized mineral. ex. 2ZnS + 3O2 --> 2ZnO + 2SO2 • The sulfur dioxide produced in the process is a significant air pollutant. For example, that from smelting around Queenstown, Tasmania killed off all the vegetation making the place look like another planet.

4. Pyrometallurgy • Smelting • The metal oxides can then be smelted by heating with coke or charcoal (forms of carbon), a reducing agent that liberates the oxygen as carbon dioxide leaving a refined mineral. Concern about the production of carbon dioxide is only a recent worry, following the identification of the greenhouse effect. • Carbonate ores are also smelted with charcoal, but are sometimes need to be calcined first. • Other materials may need to be added as flux, aiding the melting of the oxide ores and assisting in the formation of a slag, as the flux reacts with impurities, such as silicon compounds. • Smelting usually takes place at a temperature above the melting point of the metal, but processes vary considerably according to the ore involved and other matters.

5. Electrometallurgy • The branch of process metallurgy dealing with the use of electricity for smelting or refining of metals. The electrochemical effect of an electric current brings about the reduction of metallic compounds, and thereby the extraction of metals from their ores (electrowinning) or the purification of the metals (electrorefining).

6. Hydrometallurgy • Hydrometallurgy involves the use of aqueous chemistry to purify metals or mineral concentrates. Typically hydrometallurgy consists of several specific processes. • Leaching • Leaching uses an aqueous solution containing a lixiviant which is added to the ore. This is usually an acid. The oxidation potential and pH of the solution is often manipulated in order to promote the leaching or dissolving of an ore component into the aqueous phase. For refractory ore and minerals, leaching may require high-temperature and high-pressure treatment.

7. Beverly Uranium Deposit • In-situ leaching • In-situ leaching utilised the same processes as normal heap leaching and leaching. However, in this case, no ore is physically mined. In stead, holes are drilled into the deposit and explosives or hydraulic fracturing used to create open space. Lixiviant is pumped down into the deposit and extracted from other holes and the metals removed. The Beverley uranium deposit is an example of in-situ leaching.

8. Hydrometallurgy • Solvent extraction • Here a mixture of an extractant in a diluent is used to extract a metal from one phase to another. • Ion exchange • Chelating agents, natural zeolites, activated carbon, resins, and liquid organics impregnated with chelating agents are all used to exchange cations or anions with the solution. Selectivity and recovery are a function of the reagents used and the contaminants present.

9. Hydrometallurgy • Precipitation • Precipitation in hydrometallurgy involves the chemical precipitation of either metals and their compounds or of the contaminants from aqueous solutions. Precipitation will proceed when, through reagent addition, evaporation, pH change or temperature manipulation, any given species exceeds its limit of solubility. In order to improve efficiency in downstream processes, seeding to initiate crystallization is often used. • The thermodynamic tendency towards precipitation is shown using Pourbaix diagrams, however these do not show the kinetics or speed at which the salt or metal will be precipitated. Cementation is a type of precipitation.