雅思真题听力场景精练 第72期:科技类讲座 物理与工业(3)
日期:2013-08-07 16:10

(单词翻译:单击)

Salt is a mineral. Salt enhances the flavor of food, preserves food, helps to regulate and control normal body functions, and acts as a building block for more complex chemicals. Interestingly, salt is the only rock eaten by humans. Salt is produced using three methods: Rock salt mining, solar evaporation, and vacuum evaporation.

In rock salt mining, salt is mined from large underground deposits called beds or domes. It is crushed into manageable chunks and hauled to the surface, where it is screened, bagged, and shipped for further processing into commercial products and food- grade salt.

Solar evaporation is the oldest method of salt production known to man. Seawater is captured in shallow ponds and allowed to evaporate by sun and wind. If you’ve ever flown into San Francisco and looked out the airplane window to see a series of colorful ponds along the edge of San Francisco Bay, those are salt evaporation ponds belonging to Cargill Salt, the world's largest marketer of salt. These ponds have been producing salt for more than a century. Their colors range from pale green to deep pink, caused by micro-algae that change their hue in response to increasing salinity.

Vacuum evaporation is the process of dissolving rock salt or solar salt in water to create a brine solution, then boiling off the water to leave behind pure salt. Cargill dissolves raw solar salt in pure drinking water, removing dust and some trace minerals that cling to the salt crystals. The resulting brine is pumped into a vacuum evaporator—essentially a large vessel from which the air is removed——and it is boiled. Since water boils at a lower temperature in a vacuum, less energy is needed to evaporate the water than if an open-air vessel was used. Once most of the water has boiled away,the remaining salt slurry is dried, filtered, and air-cooled. A series of vibrating screen sort the salt crystals by size for packaging.

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重点单词
  • tracen. 痕迹,踪迹,微量 vt. 追踪,找出根源,描绘 v
  • clingn. 紧抓,紧贴 vi. 粘紧,附着,紧贴
  • commercialadj. 商业的 n. 商业广告
  • evaporatev. 蒸发,失去水分,消失
  • essentiallyadv. 本质上,本来
  • screenn. 屏,幕,银幕,屏风 v. 放映,选拔,掩蔽,遮蔽
  • vacuumn. 真空,空间,真空吸尘器 adj. 真空的,产生(利
  • evaporationn. 蒸发,消失,脱水,干燥
  • manageableadj. 易办的,易管理的,易控制的
  • mineraladj. 矿物的 n. 矿物,矿石