Making Glass
Abstract
In this chapter packaging materials are considered from a chemical engineering perspective, that is, processes involving the building blocks of certain raw materials, such as ores, and methods of extraction and exploitation. The use of rigorous extraction in industrial processes and its influence on material quality and waste production including scrappage (slag and clinker) follows. Processes including wood‐pulp manufacture and ore‐smelting are core contributory stages in the fabrication of modern packaging materials. The description of these processes is accompanied by an outline of the manufacture of glass and its use according to its starting materials. The raw materials are used combinatorially in numerous grades and forms of complex and composite‐type materials.
Keywords life cycle; extraction; materials; commodities; smelting; haematite; Kraft paper;
2.1 Introduction
Raw materials united through a combination of the processes indicated in Figure 2.1a following on from refining, purification, or gradation are able to produce a suitable ‘working material’ for packaging use. The working material may require further adaptation, such as polyethylene (PE) lamination to create better gas or moisture impermeability. Needless to say, such chemistry then creates further waste. The testing that takes place aligns the steps of chemical modification to produce both the working material and various types of waste. Testing has to ensure the correct degree of purity (as befitting the final end use) as well as optimal functionality and performance. Smelting of metals, fractionation and cracking of crude oil and natural gas sources for plastics, silicate mining for glass, and wood‐pulp generation for paper are the usual sources of raw materials. These materials are then moulded and shaped into forms suitable for carrying and retaining a commercial product.
Figure 2.1a shows a range of unit operations in the form of an organogram associated with the life cycle of commercial packaging materials. Following adequate screening and refining or purification (particularly when recycled materials are being used), the raw materials are converted into the working material and shaped into the final product. For example, this can be cullet, sand, soda, and lime for glass manufacture. However, as part of the refining process natural waste materials are generated [1]. Some working material may also require chemical modification, such as the embedding of nano‐materials [2], the hydroxypropyl derivatisation of cellulose to create polymer for use in biopolymer film packaging [3], or the corona discharge treatment of polypropylene to form carbonyl, carboxylate or amide groups on the surface [4] that aid printing. Working material in the form of assorted designations of various packaging materials produces stock such as multi‐lamellar laminate films for a range of adaptations. At this point materials can pass on to manufacturing processes for various applications, such as bottled product or vacuum‐packed modified‐atmosphere trays [5]. These formed packaging products then pass on to the distribution chain and, either as a result of this distribution and poor storage or as a result of transit damage and natural loss of materials during manufacture, on to accrued waste. Importantly, at all steps of the process from sourcing materials to finished distributed product, assessment of quality and performance is essential.
Figure 2.1 Packaging materials chemical engineering unit operations. (a) Organogram of unit operations involved in the life cycle of commercial packaging. (b) Raw materials.
Figure 2.1b shows the five different types of raw materials used for packaging materials. These consist of ores and minerals; silicates and sand; wood and wood pulp; plant and animal matter; and, lastly, oil and gas. The first of these, ores and minerals, are used to create pigments, alloys, and pure metals. Silicates and finely ground sand are used to make glass in its various guises. Glass may use some of the pigments taken from ores such as malachite, uranates (luminous glass), cobalt blue, borate, lead, silver, gold, and iron oxides. Wood is used of course to make paper and cardboard but also for direct use in pallets and crates (tertiary packaging). Wood can also be used as a source of cellulose for cellophane and in modern‐day bioplastics, which are made from hydroxypropyl methyl or ethyl celluloses (see Sections 3.4 and 3.4.1). Plant and animal materials may include starch and gums for use in paper and bioplastics, proteins, waxes, exudates such as natural rubber or amber, leather, and natural biodegradables. Finally oil and gas are most routinely used to make polyolefins (plastics), waxes, dyes, and synthetic polymers [6]. Figure 2.1b highlights the inter‐relationships and end products of the principal sources of raw materials and also of the process aids used in making the packaging. The five starting materials are also sources of key functionalising additives, including pigments, silicates (for paper sizing), natural biodegradable materials, dyes, and the polymers that are used across all packaging media. Smelting of ore is a prime example of taking a crude starting commodity in the form of an inorganic mineral ore and creating an entirely different material. When extracting the metal for direct or indirect further use, this can create many derivative product materials; common examples would be bauxite (aluminium) or haematite–magnetite (iron). High temperatures are needed in this energy‐intense and highly polluting fabrication process [7], such as 1560 °C for iron and 660 °C for aluminium (see Table 3.9). Glass‐making (discussed in Section 2.4) takes various adjuvants and sand, cullet, soda, and lime and occasionally other compounds such as boric oxide to create a supercooled highly viscous amorphously structured fluid or ‘glass’. Pigments such as cobalt are used to give the glass a blue hue or colour.
Wood chips, another starting material, are mechanically or chemically degraded to fabricate paper that, after further bleaching processing, produces white paperboard. Plant and animal matter can be used to harvest cellulose and exudates or proteins [8] that can be used in bioplastics and leather. Finally, crude oil, by processes such as cracking and fractional distillation, is used to create polyolefin plastics such as PE. The breakdown products of the oil and gas industries such as aniline are also used to create a range of nitrogenous azo dyes, such as mauveine (aniline purple), which was invented by William Perkin in 1856.
2.2 Building Blocks, Extraction, and Raw Materials
The Earth's crust and therefore the most valuable ores consist of about 12 or so of the 115 possible elements found in the natural environment, where base metals are present at concentrations much less than 1%. In descending order of abundance for the most common elements these are (percentages are approximate values): oxygen (47%), silicon (28%), aluminium (8%), iron (5%), calcium (4%), potassium (3%), sodium (3%), and magnesium (2%). The most abundant rock minerals in the crust are plagioclase (40%; e.g. diabase), feldspar (12%), quartz (12%), pyroxene (11%; e.g. augite), amphibole (5%), mica (5%; e.g. biotite or muscovite), and clay minerals (5%). A mere 8% of the Earth's crust is made from non‐silicates, e.g. carbonate rocks such as limestone, the source of lime used in glass and iron manufacture. Obviously, the Earth's surface or subsurface is not just populated by mineral rocks but also by vegetation and a subterranean source of oil, natural gas, and coal all derived from chemically degraded plant and animal materials. Iron and aluminium ores are among the most common components of the crust, with common iron ores being magnetite, haematite, siderite, goethite, and limonite. More than 95% of iron ores recovered from the crust (mostly magnetite and haematite) are converted to
