Mufulira underground mine with in situ leaching
Glencore's Mufulira underground mine uses in situ leaching and hydrometallurgical process to produce refined copper from oxide ores. The site also treats sulphide ores by grinding, froth flotation, smelting and electrolytic refining, producing refined copper. Understanding of these industrial processes follows.
Copper bearing rocks are low grade in copper Cu. Most of the rock is waste material, typically referred to as gangue. Depending on the minerals that contain the copper atoms, the rock is processed in different ways in order to extract the copper metal. There are two main copper bearing ores: oxides and sulfides.
Sulfide ores
Copper bearing ores are sulphides with low grade of copper - typically 0.5% Cu in porphyry magmatic orebodies, 8-12% in sedimentary deposits; copper concentrates are produced using fine grinding and flotation; the concentrates are then sent to smelters where sulphur is burnt producing sulphur dioxide SO2 and a copper concentrate grading 97% Cu called matts. This concentrate is then refined to pure copper grading 99.5%+ Cu in an electrolytic process. Copper bearing sulphides are also present in other non ferrous sulphide ores ie. lead, zinc, silver; the ores are concentrated separately producing lead, zinc, silver and copper concentrates when technically and economically feasible. Copper concentrates of these plants are sold to copper smelters.
The method used to separate copper in copper sulfide ores depends on the concentration of the copper. Higher concentrated ores can be separated via smelting; lower concentrated ores are separated via hydro-metallurgical processes. Some supergene sulfide deposits can be leached using a bacterial oxidation heap leach process to oxidize the sulfides to sulfuric acid, which also allows for simultaneous leaching with sulfuric acid to produce a copper sulfate solution. As with oxide ores, solvent extraction and electrowinning technologies are used to recover the copper from the pregnant leach solution. Secondary sulfides formed by supergene secondary enrichment are resistant to sulfuric leaching. When rich enough, native copper ore bodies may be treated to recover the contained copper via a gravity separation circuit. Supergene ores rich in sulfides may be concentrated using froth flotation.
Oxide ores
Copper bearing minerals are also present in oxidised form, from the oxidation of sulphides near the surface during geologic aeons. Processes for recovering copper metal from oxides are different. They rely on hydrometallurgical processes.
Copper oxide ores are not as attractive of an exploration target as the sulfide ores due to their lower grade. However low-grade oxide deposits can be economically extracted because they can be processed at lower cost than the copper sulfide ores. Oxidized ore bodies may be treated several ways:
With hydrometallurgical processes used to treat oxide ores in which by soluble minerals such as copper carbonate minerals are present. These oxide ores are usually leached by sulfuric acid to liberate the copper minerals into a solution of sulfuric acid laden with copper sulfate in solution. The copper sulfate solution (the pregnant leach solution) is then stripped of copper via a solvent extraction and electrowinning (SX-EW) process plant. Alternatively, the copper can be precipitated out of the pregnant solution through a process called cementation, where the copper is contacted with scrap iron. Copper produced through the cementation method is usually less pure than SX_EW copper.
Origin of sulphide orebodies
Massive sulfide orebodies are formed via the flow of heated fluids (usually seawater) through sedimentary and/or igneous rocks due to volcanic activity under the oceans. Fluids drawn down by gravity through sediments or igneous rocks towards the earth's crust encounter rising temperatures. As these fluids are heated to become a hydrothermal fluid, any dissolved sulfates are reduced to sulfide or precipitated as calcium sulfate. As it is heated, the fluid also becomes depleted in magnesium, and this causes a drop in pH. There results a heated acidic fluid that reacts with the solid rocks in which it is contained. Various elements are leached from the rock and dissolved as complexes. This modified hydrothermal fluid rapidly reaches equilibrium with an assemblage of secondary minerals. The hot hydrothermal fluid becomes less dense and flows upwards. As it nears the earth crust's surface, most often a seafloor, it cools. This causes precipitation of minerals such as pyrite, chalcopyrite, bornite, sphalerite and galena that form massive sulfide deposits. Massive sulfide deposits are a major source of many metals including lead, zinc, copper and silver in varying concentrations. The largest volcanogenic massive sulfide deposits are found in greenstone belts in Achaean cratons, such as those in South Africa and Canada.
Sediment-hosted copper orebodies are found in oceans where spreading centers are buried beneath sediments shed from the nearby continents. The process of mineral deposition is similar to that mentioned above for massive sulfide orebodies, but there is an important difference. Instead of the hydrothermal fluids flowing directly into sea water from the oceanic crust, they must first pass through an overlying layer of sediments. When passing through the sediments, the fluid's chemistry is substantially altered. Sediment-hosted massive sulfides have a wider range in mineralogy than volcanic-hosted deposits, reflecting the variation in the composition of sediments. When it comes to their mineral content, sediment-hosted massive sulfide orebodies tend to have higher concentrations of lead, zinc and silver, and relatively smaller quantities of copper and iron than volcanic-hosted deposits. Two of the largest sediment-hosted massive sulfide deposits are the Sullivan in Canada and the Broken Hill in Australia.
Mis en ligne le 08/06/2011 
