Читать книгу Packaging Technology and Engineering - Dipak Kumar Sarker - Страница 35

Metals are manufactured from mineral ores. When these are crushed into fine particles and combined with various smelting agents (e.g. limestone flux) at appropriate temperatures, this allows the release of liquid metal (Figure 2.3). The melting point (T_m) of the iron in iron ores (haematite, magnetite) is in the region of 1540–1560 °C [7]. Limestone (CaCO₃) is converted to lime (CaO), which reacts with silicate in the ore to create iron‐contaminated calcium silicate waste (slag or clinker). The ore undergoes three reduction reactions in the furnace and the iron material reacts with carbon monoxide (CO):

Top of the furnace at lower temperature (<1000 °C), Fe(III) to Fe(III/II):

(2.1)

Higher temperature, Fe(III/II) to Fe(II):

(2.2)

Hottest zone, Fe(II) to Fe(0):

(2.3)

It is noteworthy that, at each step of the process to form pure metal, carbon dioxide, a so‐called ‘greenhouse gas’ and one of several causative agents of the ‘global warming’ phenomenon, is generated, as indicated by the ‘given off’ arrow (↑) in the illustrated chemical process. In the above chemical equations brackets show the oxidation state, which for iron means Fe(II) is changed to Fe(III) by the liberation of a negatively charged electron (e), thus Fe²⁺ → Fe³⁺ + e. An oxidation state of zero for a metallic element, Fe(0), indicates a pure elemental substance (pure iron in this case).

Iron ore is first heated in a blast furnace at 2300 °C along with limestone varieties such as dolomite (CaCO₃) and coke (a source of carbon monoxide) and fuel to produce crude, high‐carbon, liquid iron that is cast into the ‘pig iron’ ingots (25 kg). Ingots can then be rolled into sheets or rods and moulded according to requirements in a hot‐press process. Further refinement of iron ingots takes place in a Bessemer (converter) furnace to produce mild steel or Linz–Donawitz‐steel‐making (basic oxygen furnace) at about 1700 °C or with a graphite electrode (EAF) at 1800 °C. The advantage of the EAF is that scrap steel can be used in the process. Aluminium ores are converted via the Bayer process from bauxite (30–60% alumina) with iron, silica, and contaminated sodium hydroxides of aluminium (Na[Al(OH)₄]) to aluminium oxide or alumina (Al₂O₃) at 180 °C under pressure. The alumina alone has a T_m of greater than 2072 °C, but by being dissolved in a lower T_m aluminium compound (cryolite) mixed with excess fluorine in the form of fluorspar (CaF₂) a lower temperature phase transition is permitted. Purified alumina is smelted by the Hall–Héroult process via an electric furnace at 940–980 °C (Figure 2.3). This process is based on electrolysis, where the resultant Al₂O₃ is traditionally dissolved in molten cryolite (Na₃AlF₆). In modern times, because of the scarcity of the natural cryolite mineral a synthetic version (fluorite) is used, with a pure compound T_m of 1012 °C but this is reduced to approximately 960 °C because of the dissolved alumina and added aluminium trifluoride (AlF₃) and by virtue of an ionic fluid being formed that is an electrically conductive medium. The electrolytic decomposition uses a consumable graphite electrode immersed in the sample with a number of concurrent reactions taking place, allowing aluminium oxide hydrolysis with associated emissions and high energy consumption [9]. The basic process in highly simplified form is:

(2.4)

Figure 2.3 Making metal, glass, and paper packaging raw materials, where all processes end with inspection and testing.

This process involves the generation of several electrons at the cathode and their absorption at the anode. Importantly, carbon dioxide is produced in the presence of oxygen and carbon monoxide (Eq. 2.4). These are gases of environmental significance along with the noxious fluorine waste compounds (e.g. hydrogen fluoride and chlorofluorocarbons [CFCs]), with a need for safety and that on mass production leave a weighty carbon footprint [10]. These compounds contribute to global warming (CFCs, along with methane and carbon dioxide), as the perhaps most infamous of the ‘greenhouse’ gases, topped only by the relatively rare coolant gas sulfur hexafluoride (SF₆). The graphitic cathode (negative electrode) and positive electrode (anode) are made of elemental carbon. As part of the process liquid aluminium metal forms at the cathodic electrode and sinks to the bottom of the tank, where it is siphoned off, because of its higher density even at this high temperature. Liquid aluminium is cast into ingots of approximately 20 kg, which is double the ingot size of metals such as copper.