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salt association |
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uses Here, you will find out how salt is used in a variety of industries, from food manufacture, to de-icing. Scroll down for information about water-softening and the general industry. water-softening hard water If you have a tide-mark around the bath, you probably live in a hard water area. In the UK, about 60 per cent of homes have moderate to hard water. These lie mainly in the east and eouth of England. The peaty waters of Scotland, Wales and north-west England tend to be soft. Rain is distilled water, free from impurities. As it passes through the air, it is contaminated by carbon dioxide and other impurities. When it reaches the ground, it picks up other elements, depending on the local geology. In some regions of the country, it passes through chalk or limestone before arriving in reservoirs, rivers and wells. The calcium and magnesium carbonates picked up by the water are not normally harmful, but cause lime-scale to built up on kettles, boilers and water heaters, making them inefficient and more costly to run. Calcium and magnesium react with soap to produce ‘scum’, reducing the effectiveness of washing machines and dishwashers. Scale builds up on toilet bowls and shower heads. Shampoo and bubble baths don’t lather properly and some people with skin problems find hard water makes them worse. softening water Water companies treat water to ensure it is safe for human use, but it is usually impractical and uneconomic to remove hardness. Many businesses use water directly from underground wells or bore holes. Both industrial and domestic water can be softened economically, using an ion-exchange process. The cause of hardness is the presence of calcium and magnesium ions. If those ions are removed, the hardness disappears. In ion-exchange, troublesome ions are exchanged for trouble-free ones. In its original state, an ion exchange polymer or resin carries sodium ions on its surface. This has a strong affinity for larger, more highly charged calcium and magnesium ions. When hard water passes over the resin, the sodium ions are displaced. As sodium salts are more soluble than calcium and magnesium, the symptoms of hardness disappear. Although chemical and other methods can bring some control over hardness, only ion-exchange can completely remove the calcium and magnesium ions which cause it. When the sodium ions are exhausted, the hardness will reoccur. However, the resin is designed to be reusable. By passing a concentrated salt solution (brine) over the resin, the calcium and magnesium are displaced by sodium and the resin reactivated. This process is called regeneration. Regeneration is usually automatic, either when a certain volume of water has been used or at a set frequency, when the water softener is out-of-use. In industry, several resin vessels are used and one is periodically taken ‘off-line’, to avoid interruption to production. In domestic situations, regeneration usually takes place early morning, or a twin tank system is used. Modern domestic machines are designed to be highly salt-efficient. These require special grades of salt to optimise performance. The salt is very pure and larger than normal table salt, to produce brine quickly. Some machines use salt tablets, produced from compressed salt. Softened water is so effective in dishwasher operation that a softener is often built into the machine. The salt used should be a dishwasher grade, rather than table salt. |
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industry Sir Humphrey Davy (1778-1829) first separated salt into its constituent parts of sodium and chlorine in 1807. At the time, no one could think of anything useful to do with them, but subsequently their properties became the bedrock for many manufacturing industries. Today, salt keeps our industries alive. The properties of chlorine and sodium and the principal compounds from them, make it one of the most important of the basic raw materials which industry uses. Chlorine compounds of commercial importance include hydrochloric acid, and sodium hypochlorite. Important sodium compounds include sodium carbonate (soda), sodium sulphate, baking soda, sodium phosphate, and sodium hydroxide. Salt is widely used as a preservative for meats and is employed in some refrigeration processes, in dyeing, and in the manufacture of soap and glass. Because they are transparent to infrared radiation, salt crystals are used for making the prisms and lenses of instruments used in the study of infrared radiation. Chlorine is used in the manufacture of PVC, which is contained in a vast number of products including blood bags and the tiny catheters used to help keep premature babies alive.
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