Physical Processes in metallurgy

Metals generally occur in combined states in the form of ores and minerals as oxides, for example, cassiterite (SnO2), cuprite (Cu2O), chromite (Feo Cr2O3), hematite (Fe2O3), pyrolusite (MnO2), rutile (TiO2), wolframite [Fe(Mn)WO4]; sulfides, for example, chalcopyrite (CuFeS2), cinnabar (HgS), galena (PbS), molybdenite (MoS2), pentlandite [(NiFe)9S8], sphalerite (ZnS), stibnite (Sb2S3); silicates, for example, beryl (3BeO Al2O3 6SiO2), zircon [Zr(Hf )SiO4]; titanate, for example, ilmenite (FeO TiO2); carbonates, for example, azurite [2CuCO3 Cu(OH)2], dolomite (MgCO3 CaCO3), malachite [CuCO3 Cu(OH)2], magnesite (MgCO3); phosphate, for example, monazite [Th3(PO4)4]; vanadate, for example, carnotite (K2O 2UO3 V2O5) and so on. A few precious metals like gold, silver, and platinum are found in the native or uncombined form because they are least reactive. As naturally occurring ores and minerals are associated with gangue such as silica, alumina etc., the first step in the extraction of metals is the removal of gangue from the ore containing the metal value by mineral beneficiation methods incorporating comminution, preliminary thermal treatment and concentration by magnetic separation, heavy media separation, jigging, tabling, and flotation. The choice of the method depends upon the nature of the gangue and its distribution in the ore and the degree of concentration of the metal value required, which depends on the extraction technology to be adopted. The extraction methods incorporate various steps to obtain the metal from the concentrate, ore or some mixture, or from chemically purified minerals; occasionally, the mineral may be first converted to a more amenable form. The mineral beneficiation step lies between mining and extraction.

The extraction processes are classified into three main groups, namely:

1. Pyrometallurgical methods including smelting, converting, and fire refining are carried out at elevated or high temperatures. A step called roasting or calcination may also be incorporated in the flow sheet in the treatment of sulfide or carbonate minerals.

2. Hydrometallurgical methods incorporate leaching of metal values from the ores/minerals into aqueous solution. The resultant solution is purified before precipitation of the metal by pH and pO2 control, gaseous reduction, or cementation. Roasting or calcination also forms an important step in the treatment of sulfide and carbonate ores. In the production of rare metals like uranium, thorium, zirconium and so on, the leach liquor may be purified by fractional crystallization, ion exchange, and/or solvent extraction techniques.

3. Electrometallurgical methods use electrical energy to decompose the pure mineral that is present in aqueous solutions or in a mixture of fused salts. If the metal is extracted from the electrolyte using an insoluble anode the method is called electrowinning. On the other hand if the impure metal (in the form of the anode) is refined using a suitable electrolyte, the method is known as electrorefining.

The choice of the technique mainly depends on the cost of the metal produced, which is related to the type of ore, its availability, cost of fuel, rate of production, and the desired purity of the metal. The fuel or energy input in the process flow sheet may be in the form of coal, oil, natural gas, or electricity. Being an electrically based process electrothermic smelting is an expensive method. This process can only be adopted if cheap hydroelectric power is available. Highly reactive metals like aluminum and magnesium can be produced in relatively pure states by fused salt electrolysis. Electrowinning is often employed as a final refining technique in hydrometallurgical extraction. Hydrometallurgy seems to be a better technique for the extraction of metals from lean and complex ore although it is slower than pyrometallurgical methods. Major quantities of metals are obtained by the pyrometallurgical route as compared to the hydrometallurgical route because kinetics of the process is much faster at elevated temperatures. This is evident from the discussion in the following chapters on matte smelting, slag, reduction smelting, steelmaking, refining, and halides, which deal with the pyrometallurgical methods of extraction. Separate chapters have been included on hydrometallurgy and electrometallurgy. In addition to the well-established tonnage scale production of the ferrous and six common nonferrous metals (aluminum, copper, lead, nickel, tin, zinc), in recent years many other metals, namely, beryllium, uranium, thorium, plutonium, titanium, zirconium, hafnium, vanadium, columbium, tantalum, chromium, tungsten, molybdenum and rare earths have gained prominence in nuclear power generation, electronics, aerospace engineering, and aeronautics due to their special combination of nuclear, chemical, and physicochemical properties. Many of these metals are categorized as rare despite their more abundant occurrence in nature compared to copper, zinc, or nickel.

This is due to the diversified problems associated with their extraction and conversion to usable form.
Production of some of these metals in highly pure form on tonnage scale has been possible recently by efficient improvement of the conventional extraction methods as well as through the development of novel unit processes.

On account of the refractory nature of the minerals and stability of the oxides and carbides of many rare metals direct smelting of the ores with carbon is not feasible for rare metal extraction. The refining methods like fire refining, liquation, distillation and so on are also not applicable. Hence, the flow sheets for rare metal extraction and refining involve many steps, each with the specific objective of successfully removing a particular impurity. On account of the co-occurrence of chemically similar elements, for example, uranium/thorium, columbium/tantalum, zirconium/hafnium and rare earths there are often problems in rare metal extraction. For the separation of such elements, unconventional techniques like ion exchange and solvent extraction have to be incorporated in the process flow sheet for production of high-purity metals. Finally, during the reduction and consolidation stages one has to be extremely careful because rare metals in general, and titanium, zirconium and hafnium in particular, are very sensitive to atmospheric gases that affect their physical, chemical, and mechanical properties.

It would be appropriate to outline here the general steps in the extraction of rare metals:

1. Physical mineral beneficiation: Beach sand, a source of many rare metals like titanium, zirconium, hafnium, and thorium, is processed by exploiting the characteristic differences in the size, shape, density, and electromagnetic and electrostatic behavior of mineral constituents, that is, rutile, ilmenite, zircon, monazite and so on.

2. Selective chemical ore breakdown: In order to bring the metal values to an extractable state, hydrometallurgical unit processes like acid or alkali leaching or pyrometallurgical techniques like fusion with alkalis and alkali double fluorides are employed.

3. Ion exchange: The technique developed long back for purification and deionization of water is currently used extensively for concentration and purification of lean leach liquor and for separation of chemically similar elements.

4. Solvent extraction: An analytical technique once developed for selective transfer of specific metal ions from aqueous solution to an organic phase, has presently come up to the stage of large-scale unit process for purification and separation of a number of rare and nuclear metals.

5. Halogenation: For the production of oxygen-free reactive metals like titanium, uranium, zirconium and so on it has become essential to adopt intermediate routes by converting oxides into chlorides or fluorides prior to reduction.

6. Metallothermic reduction: The traditional “thermite process” has been very successfully employed in rare metal extraction. For example, uranium tetrafluoride is reduced with calcium for tonnage production of uranium metal required in atomic reactors. Similarly, magnesium is used for the production of titanium and zirconium from their respective tetrachlorides.7. Consolidation and vacuum refining: As most of the metals mentioned above are high melting and very corrosive in the molten state they pose problems during melting and consolidation. Special consumable electrode arc melting with super-cooled copper hearths have been developed for the production of titanium and zirconium alloys. Electron beam melting technique has been practiced for melting and refining of columbium and tantalum. The high superheat at temperatures around 3000 C under vacuum helps in removing all impurities including oxygen, nitrogen, and carbon.

7. Ultra-purification: The performance of rare and reactive metals during usage depends on purity. For proper assessment it is important that metals are free from impurities. Similarly, high order of purity is specified for semiconducting elements like silicon and germanium, required in electronic industry. In recent years a number of ultra-purification methods, for example, thermal decomposition, zone refining, and solid-state electrolysis have been developed for large-scale purification of these metals.

A number of textbooks dealing with ironmaking, steelmaking, extraction of non-ferrous metals and principles of extractive metallurgy are available. Each book has some edge over the other in certain aspects of presentation in terms of theory and practice. Some emphasize on technology and some on principles. Thermodynamics and kinetics have been discussed. In this book an attempt has been made to discuss the physical chemistry of different steps, for example, roasting, sulfide smelting/converting, reduction smelting, steelmaking, deoxidation, degassing, refining, leaching, precipitation, cementation involved in the extraction of metals. A chapter on slag which plays an important role in the extraction of metals from sulfide as well as oxide minerals, has been included. Similarly, another chapter highlights the significance of interfacial phenomena in metallurgical operations. The physicochemical aspects of desulfurization, dephosphorization, decarburization, and silicon and manganese reactions in steelmaking have been discussed along with brief accounts on various steelmaking processes highlighting the differences in their chemistry of refining and pretreatment of hot metal. Role of halides, ion exchange, and solvent extraction in metal production and refining have been discussed in different chapters. Methods of construction of predominance area diagrams applicable in selective roasting and leaching have been explained with suitable and appropriate examples in Chapters 2 and 11. At the of end the book, flow sheets demonstrating various steps in the extraction of copper, lead, nickel, zinc, tungsten, beryllium, uranium, thorium, titanium, zirconium, aluminum, and magnesium from their respective ores have been presented.

Relevant worked out examples have been included in each chapter to illustrate principles. While reading the topics on continuous smelting and submerged lance technology in different books and journals one may feel that the chapter on roasting is outdated but one must realize that these developments have been possible only after a sound understanding of the physical chemistry and thermodynamics of all the steps involved in the extraction of metals. Although, currently, almost the entire production of steel comes from top-blown (LD), bottom-blown (OBM) and combined-blown(Hybrid) converters and electric arc furnaces a discussion on the obsolete Bessemer process has been included to highlight the contributions of Henry Bessemer whose invention laid the foundation for the modern steelmaking processes.