Chemistry at Work in Industry

1Where does cement come from?

A builder in Kigali mixes grey powder from a 50 kg bag with sand, gravel and water. Within a day the paste has set into concrete as hard as rock. The grey powder is cement, and it starts life as an ordinary white rock: limestone. Changing a rock into something that can hold up a building is industrial chemistry.

2Teacher demonstration: turning limestone into lime

Your teacher holds a small chip of limestone (calcium carbonate) with tongs in the hottest part of a roaring Bunsen flame, just above the tip of the inner blue cone, for about ten minutes (Figure 2.1 A). The chip glows brightly. After it cools, it is placed in an evaporating basin and a few drops of water are added (Figure 2.1 B).

Figure 2.1: Heating limestone strongly, then adding water to the product.

3What happened?

On heating. Calcium carbonate breaks down into calcium oxide (quicklime) and carbon dioxide gas. A breakdown caused by heat is called thermal decomposition:

$$\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_3(\mathrm{s})\to \mathrm{C}\mathrm{a}\mathrm{O}(\mathrm{s})+\mathrm{C}{\mathrm{O}}_2(\mathrm{g})\backslash mathrm\{CaCO\_3(s)\; \backslash rightarrow\; CaO(s)\; +\; CO\_2(g)\}$$

On adding water. The quicklime hisses, gives out so much heat that some water boils off as steam, and the lump swells and crumbles into a white powder, slaked lime (calcium hydroxide):

$$\mathrm{C}\mathrm{a}\mathrm{O}(\mathrm{s})+{\mathrm{H}}_2\mathrm{O}(\mathrm{l})\to \mathrm{C}\mathrm{a}(\mathrm{O}\mathrm{H}{)}_2(\mathrm{s})\backslash mathrm\{CaO(s)\; +\; H\_2O(l)\; \backslash rightarrow\; Ca(OH)\_2(s)\}$$

The letters in brackets are state symbols: (s) solid, (l) liquid, (g) gas, and (aq) dissolved in water. Slaked lime is used to make mortar and whitewash, and farmers spread lime on acidic soils.

4How cement is made

Cement works use the same reaction on a huge scale. Limestone and clay are crushed, ground together and fed into a long, slowly turning steel tube, the rotary kiln. There the mixture is heated to about $1450{}^{\circ }\mathrm{C}1450\backslash ,^\backslash circ\backslash mathrm\{C\}$. The calcium carbonate decomposes, and the calcium oxide combines with the clay to form hard grey lumps called clinker. The clinker is ground to a fine powder with a little gypsum (calcium sulfate), which controls how fast the cement sets (Figure 2.2). In Rwanda, cement is made this way by CIMERWA at Muganza in Rusizi District.

Figure 2.2: How cement is made. Carbon dioxide leaves the kiln as a by-product.

5Other chemical industries

Table 2.1: Some chemical industries

Soap is made by boiling a fat or oil with a strong alkali. The reaction is called saponification:

$$\text{fat or oil}+\text{sodium hydroxide}\to \text{soap}+\text{glycerol}\backslash text\{fat\; or\; oil\}\; +\; \backslash text\{sodium\; hydroxide\}\; \backslash rightarrow\; \backslash text\{soap\}\; +\; \backslash text\{glycerol\}$$

6Fertilisers: reading the bag

A fertiliser bag carries three numbers such as 17-17-17. They give the percentage by mass of the three main plant nutrients, always in the same order: nitrogen (N), phosphorus (as ${\mathrm{P}}_2{\mathrm{O}}_5\backslash mathrm\{P\_2O\_5\}$) and potassium (as ${\mathrm{K}}_2\mathrm{O}\backslash mathrm\{K\_2O\}$). Urea, $\mathrm{C}\mathrm{O}(\mathrm{N}{\mathrm{H}}_2{)}_2\backslash mathrm\{CO(NH\_2)\_2\}$, contains 46% nitrogen.

7Tea: chemistry in a green valley

Tea is one of Rwanda's main exports. At a tea factory, freshly plucked leaves are first withered (some of their water evaporates), then cut, torn and curled so that the juices inside the leaf cells meet the air. Substances in the juice react with oxygen, helped by the leaf's own enzymes, and the green leaf turns copper-brown: this oxidation gives black tea its colour and flavour. Hot air then dries the leaf, which stops the reaction (Figure 2.3).

Figure 2.3: Making black tea. Only some steps make new substances.