Published on February 14, 2008
Slide1: 507 E20300 Production and Processing of Materials – Metals and Ceramics Lecture 10 References: * Atkins, 2002 * Hayes, 1993 * Evans & De Jonghe, 1991 8.5 Smelting and Converting of Sulfides The smelting of sulfides to produce metal entails two steps: (1) sulfide of the desired metal is separated from most of the gangue by forming two immiscible liquids, and (2) all of the gangue is separated and the desired metal sulfide is converted to metal. 8.5.1 Reverberatory Smelting Reverberatory smelters have been the traditional unit for carrying out the first step: the formation of two liquids, one an oxide slag containing most of the gangue elements and the other a sulfide “matte” containing the valuable metal sulfide along with some gangue sulfides. Slide2: Reverberatory furnace smelting sulfide concentrate e.g. smelting of copper sulfide concentrates (usually contains some iron sulfide as well): The matte would consist largely of sulfides of copper and iron. Some oxidation of copper sulfide takes place Cu2O Reaction “Cu2O (slag) + FeS (matte) = Cu2S (matte) + FeO (slag)” takes place Copper oxide in the slag phase is readily abstracted (as sulfide) into the matte phase by reaction shown above. Slide3: 8.5.2 Converting Let’s keep on using the example of copper sulfide concentrates. The term converting is used to describe the batch treatment of matte to yield first an iron-rich slag that is recycled to the Reverberatory and then an impure molten copper. A converter is a cylindrical vessel (4m in dia. and 10m long) that can rotated about its horizontal axis. Slide4: The first stage in converting is the oxidation of iron sulfide in the matte to an oxide which forms a slag with silica (sand) added to the converter. 2FeS + 3O2 = 2FeO + 2SO2 Note that any cooper sulfide oxidised tends to react with iron sulfide, so that the transfer of copper into the slag is small (see reaction equation shown in the previous slide). The slags are continuously recycled for the recovery of small amount of remaining copper. After nearly all the iron sulfide originally present in the concentrate has been removed as oxide in the slag, the oxidation of the copper sulfide begins: Cu2S + O2 = 2Cu + SO2 Any oxide formed at this stage tends to react according to Cu2S + 2Cu2O = 6Cu + SO2 The final product is impure copper metal with residual iron, precious metals (Au and Ag), and oxygen as the chief impurities. The oxygen can be removed by contacting the molten metal with a reducing agent prior to casting the metal. (Pure copper can be produced from the electrolytic refining process – discussed in later lectures.) Slide6: 8.6 Steelmaking: Preliminary Refining Technologies Impure iron is produced by the reduction of iron oxides with coke in the iron blast furnace. Most of impurities are removed in the operation that we call steelmaking. The resulting iron-carbon alloy, which we call steel, contains C and Mn at the level of a few tenths of a percent, S and P at a few hundredths of a percent, and Si at even lower levels. The Ellingham diagram for oxides make it clear that the Mn and Si impurities in pig iron have a much greater affinity for oxygen (greater –ve free energy of the oxide) than does iron. This affinity is exploited in steelmaking. By contacting the pig iron with oxygen, we can oxidise the C, Mn, Si and S and P with little loss of iron by oxidation – this is known as oxygen steelmaking, which is the most important and common steelmaking method. The oxidised impurities will either escape as gases (e.g. CO) or become constituents of an oxide slag that floats readily to the surface of the metal. This oxide slag can be made to have a sufficiently low melting point and viscosity (that it can be readily poured from the steelmaking vessel) by adding lime to the vessel. 8.6.1 Basic Oxygen Furnace (BOF) BOF is presently the most established oxygen steelmaking technology. A schematic diagram of a BOF is shown in the next slide. Slide7: Approx. 5m in diameter and 7m in height Oxidation is typically completed in 20-25 mins, and the entire operation is over in 35-40 mins. No fuel need be burned; heat generated by the exothermic oxidation reactions (of the impurities). Steel scrap (15-30%) is added as a coolant to limit temperature increase. Final temperature is controlled to stay around 1600C – 1700C. Supersonic jet of oxygen improves overall “agitation” improve the heterogeneous reaction at the metal-oxygen interface improve refining rate. Slide8: 8.6.2 Electric Furnace Steelmaking A significant amount of steel is made in electric furnaces such as the open arc furnace. Arcs are readily visible Batch-wise operation Employ graphite electrodes Can be tilted for pouring Typically produce steel from scrap or direct reduced iron (DRI), although some use combinations of these and pig iron. (DRI – solid porous iron reduced by H2 using a countercurrent-flow kiln design. Containing residual O2.) Slide9: Typically, in producing steel from scrap or DRI, much less oxidation is necessary than in producing steel from pig iron. Consequently, rust on the steel scrap or residual oxygen in the DRI are frequently sufficient to supply all the O2 necessary to oxidize the impurities. If additional O2 is needed, it is commonly introduced by feeding iron ore to the furnace (which can react according to reactions such as the one shown below), although sometimes air or oxygen lances are used. 2Si + Fe3O4 = 2SiO2 + 3Fe Compare to the BOF, the furnace produces little polluting fume or dust, can handle scrap, pig iron or DRI in any proportion, and provides very easy control of the temperature of the finished steel. However, electric furnaces are expensive to build and operate. The open arc furnace has been widely used to produce special steel (e.g. stainless steels), where small batches are produced, usually from cold starting materials. 8.7 Glassmaking Glass production bears a resemblance to pyrometallurgical operations in that high-temperature liquids are handled and ultimately cast into solid form. Therefore we will discuss glassmaking in this sub-section. A glass is a liquid that has been cooled, without crystallization, to the point where its viscosity has become enormous. Slide10: Most (although no all) glasses are formed from Silica (SiO2). The other principal oxides involved in the formation of glasses are Na2O, B2O3, CaO, MgO, Al2O3, and PbO. Many of the commercially available glasses are colored by addition of Fe2O3, CuO or other oxides. Oxides and/or recycled glasses (A) are fed to the hopper above the screw feeder (B). The feeder conveys the oxides/glasses continuously into the main section of the melter (C). The extremely high temperatures in the melter (gas-fired) cause the oxides to become molten liquid glass; lower-melting-point recycled glasses provide a “liquid bed” to aid the melting of the oxides (D). The molten glass flows under a skimmer block (E), into the forehearth (F), where the material continues to form a stable glass. At the end of the melter, the glass flows out (G) into a water quenching roller pair or tank. A removable block is included at the end of the forehearth (H) to stop the flow of glass if desired. Exhaust gases (I) flow out from the furnace up the square flue, to the air sampling equipment. Refining Section Glass Furnace Slide11: Because most glasses have a significant electrical conductivity at high temperatures, it is also possible to heat them electrically. This is achieved by passing alternating current between molybdenum electrodes placed in the furnace. The principle function of the refining section of the furnace (i.e. the forehearth) is to provide a relatively calm region of the melt, where entrapped gas bubbles can escape by rising out of the liquid. Fining agents such as sulfates that release large volumes of large bubbles by thermal decomposition in the refining section are added to the furnace with the raw materials. As the large bubbles rise through the molten glass, they engulf the smaller bubbles, thereby removing them from the melt. The surface scums on the molten glass and floating unmelted particles are prevented from reaching the refining forehearth section by the skimmer block. Part 9: Hydrometallurgy and Electrometallurgy In this part, we examine some alternative techniques that make use of aqueous chemistry to extract metals from ores and concentrates. This hydrometallurgical technology employs much lower temperature (however, may still need to consume considerable quantities of energy). Hydrometallurgical processes typically pose no air pollution difficulties, but steps must usually be taken to avoid water pollution. Slide12: Leaching step is where the desired metal compounds are abstracted into aqueous solution by suitable reagents. Leaching step is imperfect in that undesirable compounds also find their way into solution. Therefore need to be followed by a purification step. Finally, the desired metal is recovered from solution by an electrowinning process (i.e. electrolysis of the solution; a topic within electrometallurgy). E.g. metal is deposited by the electrolytic reduction of the metal ion at a cathode. Slide13: 9.1 Leaching 3 examples of the chemistry taking place in leaching operations - the dissolution of (1) zinc oxide, (2) gold in alkaline cyanide solution, and (3) copper sulfide in water in the presence of oxygen. E.g. gold ores contain only a fraction of an ounce of gold per ton of ore. The cyanide reagent is highly specific to gold; that is, it is inert to the vast majority of much less valuable minerals that make up the bulk of the gold ore. Economic extraction of gold from such lean ores would be impossible by pyrometallurgical techniques. 9.1.1 Leaching Technology (1) Solution mining (see Lecture 1) – in situ leaching as it is sometimes called. This technology has been practiced for the production uranium from sedimentary deposits, using aqueous solutions of ammonium carbonate/bicarbonate with hydrogen peroxide as reagents (lixiviants). Slide14: The reagent solution is continuously injected into the ore body through boreholes a few centimetres in diameter drilled down to it from the surface, with uranium-containing solution pumped out continuously through similar boreholes. (2) Dumping leaching – “watering” rock dumps containing significant amounts of valuable metal. E.g. the dissolution of copper sulfide in water in the presence of oxygen. In reality, the solution sprayed over the dump is acid, the acid usually being self-generated within the dump by the oxidation of other sulfide minerals. (3) Vat leaching – crushed ore is loaded into vats. Leaching solution is pumped continuously from the bottom of one tank to the top of the next. The fresh incoming reagent is to be connected with the ore that has been in the leaching scheme the longest, while the last solid to be contacted with solution is the fresh ore. (Advantages of countercurrent operation.) Vat Leaching Slide15: (4) Agitation leaching – heterogeneous leaching reactions improve by agitation. Operation can be either continuously or batch-wise. Several leaching tanks might be used in series to complete the leaching. Shorter ore residence time because of the rapid reaction of the particles. (5) Pressure vessel leaching – the pressure vessel makes it possible to use gaseous reagents under pressure, thereby increasing the dissolved gaseous reagent in solution and the reaction rate. Ores or concentrates fed to such pressure leaching vessels are usually ground to a small particle size to maximize reaction rate and minimize the danger of particles settling out in the pressure vessel. E.g. the dissolution of copper sulfide in water in the presence of oxygen. Slide16: Leaching in a pressure vessel Faster leaching achievable in more expensive equipment, i.e. higher productivity and shorter residence time. The optimum technology would depend on aspects of economics, technology and science. Slide17: 9.1.2 Leaching Chemistry Eh - a variable indicates whether conditions in solution (i.e. the environment into which we place our minerals) are oxidizing or reducing. pH – indication of hydrogen activity. Eh – indication of electron activity. There are two sloping dashed lines at top and bottom of the fig. representing the stability limits for water. At upper line, oxidation of water proceeds; at lower line, reduction of water proceeds Slide18: Oxidation of water: Reduction of water: E.g. if we want to leach copper sulfide minerals to obtain cupric ion (Cu2+) solutions, we can see that the sulfide minerals Cu2S and CuS can only be leached at a pH value lower than 5 and an Eh exceeding 0.3 volts (higher Eh values if the pH is other than approximately 3). Attempts to leach with alkaline solutions would fail because the (solid) oxides of copper are formed under alkaline oxidizing conditions. Therefore, we can shift leaching conditions to ones that are favourable for the desired reaction by adjusting Eh and pH. Note that areas of stability of the various specimens in a Pourbaix diagram shift as the temperature is changed, providing a third variable that maybe manipulated to achieve the desired leaching effect. The stability of ions in solution can also be manipulated by the concentration of complexing reagents (particularly CN-, NH3 and Cl-). Pourbaix diagrams have now been obtained (by experiment or theoretical calculations or combinations of the two) for a large number of mineral systems and are invaluable in examining the chemistry of potential hydrometallurgical processes. However, they gave no information on the kinetics of leaching reactions and no information on the state of the system if equilibrium is not achieved. Slide19: 9.2 Solution Purification The most common methods of solution purification are chemical precipitation, cementation, ion exchange, and solvent extraction. (1) Chemical Precipitation – precipitation of a valuable metal compound, rather than the impurities, is more common. An example is the processing of seawater to obtain magnesium; the hydroxide of magnesium is precipitated by raising the pH of a solution of magnesium salts (obtained following prior fractional crystallization of sodium chloride). Mg2+ + 2OH- = Mg(OH)2 (2) Cementation – is the reduction of an ion in solution using a metal that has a more negative electrode potential. (Species with more negative electrode potential wants to give electrons, while species with more positive electrode potential wants to accept electrons). E.g. purification of acidic zinc sulfate ZnSO4 solutions (formed by the leaching of zinc oxides with sulfuric acid) with zinc powder. Co2+ + Zn = Zn2+ + Co The zinc powder particles thereby become coated with the impurity metals (e.g. Co, Ni, As etc.) and are replaced periodically. Slide20: (3) Ion exchange – using ion exchange resins to trap either the impurity or valuable metal ion streams from leaching. (4) Solvent extraction – the extraction of metal ions from aqueous solutions when dissolved in an organic medium such as kerosene.