Chemical Engineering in Daily Life - Question 8 - Standard answers available
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This post was last edited by yjqin1 on 2015-8-20 at 19:14. Today’s rural areas in China are different from those in the 1980s and 1990s after the reform and opening up, and even more different from those in the 1960s and 1970s; they are vastly different from what they were before the liberation. The 1970s were my teenage years; I didn’t have to go to school or do farm work, but we often went hungry. Especially in winter, we only had two meals a day of thin porridge. Aside from the children of the party secretary (the village leader), everyone was pale and thin – there was no such thing as needing to lose weight. But it was fun back then too; for example, one could play with ice monkeys (a type of top) and make firecrackers by oneself. Making firecrackers relies on nitrate, that is, nitrates. At that time, the common fertilizers used in rural areas were only ammonia water, ammonium bicarbonate, superphosphate, etc.; urea and ** were not yet available. So where can we get nitrates? Let me explain it slowly. First, either make a small broom in a special shape by yourself or ask your brothers and uncles to help, then bring along a basket and a winnowing basket. We went to people’s toilets, pigsties, animal shelters, and other such places, carefully cleaning the walls and floors. Sometimes the big fat pigs would grunt in protest, and we had to be extra careful not to get kicked by donkeys or cows. Gather these ‘soils’ and take them home ； Then add well water and stir for some time; boil the supernatant to concentrate it by a factor of about ten, until white solids appear in large quantities. However, these solids must be discarded. Take out the small amount of water that is left over, put it in another container, and place it in the yard at night; the next day, beautiful crystalline formations will be found inside, usually in a radial pattern. Fishing out these crystalline solids yields what is known as “nitre”. With nitrate, charcoal, and sulfur stolen from adults, one can make \"explosive bombs,\" but firecrackers and other similar items still have to be bought from small vendors. We, the children of farmers who asked our classmates on the first day of high school in 1978 what \"hydrogen dioxygen one\" was, are naturally born chemists and chemical engineers. Dear readers, as a new generation that has grown up playing games, can you achieve excellence in gaming? Is it possible to produce the double helix structure of DNA? Is it possible to win a Nobel Prize this way? Enough chatter. May I ask, how did we produce nitrates from dung back in those days?
Standard answer: Few people pay attention to this question; perhaps crystallization is not covered in much detail in the textbook \"Principles of Chemical Engineering.\" After all, there is only one academician related to crystallization, namely Wang Jingkang from the School of Chemical Engineering at Tianjin University. The principle behind the production of nitre in rural areas in the past was as follows: The soil in toilets, pigsties, animal shelters, and similar places contained soluble chloride ions, sulfate ions, nitrate ions, sodium ions, potassium ions, and others. After soaking these soils in well water, chlorides, nitrates, and sulfates dissolve into the water. After filtering the supernatant, it is boiled in a large pot until it reaches a boil; as the water evaporates, the solution becomes concentrated several times or even dozens of times, until it reaches saturation. The solubility of these salts is quite high. However, the saturated solubility of sodium sulfate remains relatively constant above 35 degrees, while that of sodium chloride stays almost unchanged across all temperature ranges. So, first, sodium sulfate and sodium chloride crystallize out at 100-108. The potassium ions and nitrate ions among them are concentrated as water evaporates and sodium chloride and sodium sulfate crystallize. Above room temperature, the solubility of potassium sulfate, sodium nitrate, and potassium nitrate increases significantly as the temperature rises. By cooling this highly concentrated solution from over 100 degrees to zero degrees or even below, potassium nitrate experiences the greatest decrease in solubility due to temperature effects; therefore, it precipitates first at low temperatures. Sodium sulfate, which is greatly affected by temperature, as well as sodium nitrate, which is less affected by temperature, may also precipitate under such conditions. There may be some randomness involved here, depending mainly on what specific soluble components the nitrate consists of. Can’t potassium nitrate and sodium nitrate be used as **? Since the Northern Song Dynasty, modern-style firecrackers came into existence, and that’s how we Chinese have been producing nitrate all this time. We Chinese are smart, right? It depends on whether it’s used in a proper context.