Most compounds and elements that occur naturally are in an impure state and are mixed with other substances. Often, we want them in their pure state to make them useful to us or so that we can mix them with other chosen substances to make products that are useful to us. We may also want to separate substances to analyze them or determine the proportion of constituent parts. This means that we need to separate the parts that make up the mixture. To do this, we need the science of separation.
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But what is the science of separation? The science of separation is not one science but usually combines several sciences to produce the required results. To achieve the separation desired, it may be necessary to engage in multidisciplinary science projects to reach the specified goal. The technical definition of separation is dividing a mixture into its constituent parts. To separate a mixture or solution, we will normally use a separation process based on the physical properties of the mixture or solution.
The separation process will divide the mixture or solution into two or more separate parts or mixtures with distinct properties. Sometimes the separate parts may have a high purity level with exceptionally low levels of contamination, but that is not always the case. Sometimes a high purity level may be desirable, for example, in ultrapure water, but sometimes we can accept a significant level of certain types of impurities, for example, dissolved salts in drinking water. Separation science relies upon the physical properties of the mixture's ingredients to separate the constituents. Size, shape, mass, density, solubility, volatility, chemical affinity, or charge are among the many properties that could be used to partition the mixture, and separation science will determine how differences can be used to separate the components.
The list of separation processes is long and includes some very familiar processes, such as filtration and sieving, but also some that are more advanced and less well-known, such as electromagnetic separation (mass spectrometry) and photon-enhanced separation (laser isotope separation). An internet search will reveal at least 20 different types of separation processes.
Sometimes different sciences will be required to select a separation process. Separation of living cells from a liquid, for example, may suggest a centrifuge as a good option using the difference in density between the cells and the liquid, and applying a centrifugal force should separate the two. However, if the cells are ruptured and killed by high gravitational forces, then either the speed of the centrifuge may need to be reduced or a different process selected. Therefore biology and physics would be amongst the sciences required to separate the cells from the liquid.
Applications of Separation Science and Processes
We all use products that have undergone separation every day, as most water supplies and virtually all sewage treatment systems use separation to create water quality to meet specific legal criteria. Water and sewage treatment plants may use sedimentation, filtration, flotation, ion exchange, membrane processes, and centrifugation.
Many everyday items use separation in their manufacture. For example, the dyeing of cloth to make our clothes often use colored products, which need to be removed from the wash water effluent produced while rinsing the cloth before being discharged to the sewer. This may require one or more separation processes such as coagulation, flocculation, settlement, flotation or filtration, or a combination of these.
Anyone who has drunk gin or whisky is using a product where the alcohol is separated from the majority of the water by distillation.
If you have ever put table salt on your food, you are using a product separated from water by crystallization.
Separation science is selecting the right separation process for your needs and operating the process to achieve optimum separation. For example, with a simple size-based separation like filtration, how do you stop your filter from getting completely blocked by filtered products? You may also have to consider whether you want the filtered liquid or the filter cake. Which part of the mixture is the product you wish to recover? Additionally, the element you wish to recover may end up in another mixture and may require secondary separation or treatment to produce the desired end product.
Sometimes a separation process may require several steps. An example would be the production of ultra-pure water for kidney dialysis. The scientist will set the criteria that the water must achieve regarding dissolved salts, dissolved solids, suspended solids, organic compounds, and pathogens. This will normally follow national or organizational specifications.
The scientist will then need to identify what unwanted substances are in or could be in the water supplied to the dialysis center. If supplied by a utility company, this may have already gone through various separations such as filtration, sedimentation, and disinfection (this may not be a separation process) to produce potable quality water. If the water contains hardness, a water-softening process may be required, which involves ion-exchange separation to separate the calcium and magnesium ions from the water supply. This will usually be followed by fine filtration removing particles down to 5 microns in diameter before being passed into a membrane filtration process such as reverse osmosis, which will act as a molecular sieve removing up to 98% of dissolved solids.
The water may then pass into a second membrane process called EDI (electro-deionization), which uses electrodes and membranes to purify the water further. EDI helps to eliminate microbes and endotoxins from the water. Most water supplies would cause very rapid fouling of an EDI unit, so there is extensive pre-treatment before the water enters the EDI unit.
Each of the processes outlined above requires specific separation science knowledge for such a scheme to be successful.
Industries and Services using Separation science
Separation science is used in chemical, pharmaceutical, and mining industries, biotechnology, water, and wastewater treatment, material recovery for recycling, and nuclear re-processing. It is used in food and beverage production, medicine, and chemical and biological analysis. Separation science is also used in chemical, biochemical, biological, and medical research.
Separation techniques such as mass spectrometry, chromatography, electrophoresis, and other separation processes can be used on very small-scale applications such as instrumentation and identifying pollutants or contaminants in our environment and manufactured products.
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- Classifying Separation Techniques, updated June 29 2021 Professor David Harvey, De Pauw University chem.libretexts.org/.../7.06%3A_Classifying_Separation_Techniques
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