Extractive metallurgy

Extractive metallurgy

Metal sulfide flotation concentrates can be treated in a number of ways. These include using  pyrometallurgy, or smelting at high temperature, in which process the sulfur in the metal sulfide is oxidized with air or oxygen to sulfur dioxide, and molten metal is produced. The sulfur dioxide needs to be captured for air-quality reasons and is converted into sulfuric acid. Treatment of sulfide mineral flotation concentrates can also be carried out using hydrometallurgy, in which process the metal sulfide is dissolved (leached) into an aqueous solution. Some processes involve both smelting and leaching steps. This is the case, for example, in zinc sulfide processing. The zinc concentrate is first roasted to release sulfur dioxide and form zinc oxide and then the latter compound is dissolved in sulfuric acid and the metal extracted by electrolysis. The recovery of zinc by electrolysis is presented in the Electrowinning Section.


Environmental restrictions on sulfur gas emissions in smelting operations, and the limited market for sulfuric acid, resulted in a search for hydrometallurgical routes from sulfide minerals to metals. In the 1950s, the Sherritt-Gordon ammonia pressure-leaching process for nickel was introduced. In this process, nickel sulfide flotation concentrates are reacted under pressure with oxygen together with ammonia, which helps to facilitate the dissolution by forming soluble nickel-ammonia complex ions. As with flotation, dissolution is a corrosion-type process in which anodic oxidation of the sulfide transfers electrons to the remaining mineral and these electrons are returned to the solution by the cathodic reduction of oxygen. Nickel is recovered from the solution in the Sherritt-Gordon process either by electrolysis or by reduction with hydrogen gas. The latter process, too, is an electrochemical one, and needs the addition of fine nickel particles as seeds. The hydrogen is anodically oxidized to hydrogen ions on the nickel seed and gives electrons to the metal. These electrons are used up in the cathodic reduction of nickel ions to form nickel metal, which deposits on the seed particles. The nickel particles are removed when they reach a suitable size.

The success of the nickel hydrometallurgical process led to research for similar methods to recover other metals. The Sherritt-Gordon pressure leach procedure for the treatment of zinc sulfide concentrates was introduced in 1981; zinc sulfide is oxidized by oxygen in sulfate solutions to form zinc sulfate and sulfur. The zinc is recovered by electrolysis as it is in the roast leach process already described.

Leaching of copper from ores has focused mainly on oxide ore types. Most oxide copper ores are not amenable to present-day flotation techniques and are not rich enough for direct smelting. Thus, the winning of copper from these ores must involve hydrometallurgical techniques. Most oxidized copper minerals dissolve readily in sulfuric acid so that leaching is reasonably straightforward. A concentrated, pure copper sulfate solution suitable for electrowinning is produced from the initial leach solution by selectively transferring the copper ions to an organic phase by a process known as solvent extraction and returning them again to an aqueous phase. Complexing agents are dissolved in an organic solvent such as low vapor pressure kerosene and this phase is intimately dispersed within the aqueous leach solution in a mixer/settler (see Figure 4). The copper selectively complexes with the reagent and is transferred into the organic phase. The copper replaces hydrogen on the complex and hydrogen ions are consequently transferred to the aqueous phase. The two phases are allowed to separate in the settler section and then the aqueous acid solution, termed the raffinate, is recycled to ore leaching. The loaded organic phase is sent to a second mixer/settler where it is reacted with a strong aqueous sulfuric acid solution. Here, the copper is exchanged for hydrogen ions and transferred into the second aqueous phase from which the metal is efficiently recovered by electrolysis. The overall process of dissolution, solvent extraction and electrowinning is known as SX/EW. Copper electrowinning is discussed in the Electrowinning Section.

Mixed oxide and sulfide copper ores are leached following the same process stages as in Figure 4. Also, some sulfidic ores that are too lean to treat by flotation are also leached in heaps or dumps. The recovery of metals from sulfides in this way utilizes bacteria either naturally present or inoculated into the material. The microorganisms aid the oxidation of insoluble metal sulfides to soluble metal sulfates. The commercial application of bacteria to accelerate sulfide leaching in a controlled manner is a relatively new concept. Bacteria did not, of course, wait for man to exploit their activity. The presence of copper in mine drainage reported in early Roman times was, no doubt, a result of bacterial leaching. The treatment of mine waters to recover dissolved copper was applied as early as 1670 at Rio Tinto in Spain and in the 19th century in the USA.