Titanium in salt and desalination industry
Salt is an indispensable condiment in life and a vital nutrient for the human body. The main component of salt is sodium chloride, and sodium ion is an essential component for transmitting information in the nervous system, which is related to muscle contraction and heart beating. If the human body is deficient in salt, symptoms such as dizziness, fatigue, loss of appetite, rapid heart rate, weak pulse, and muscle cramps will appear. Table salt is one of the most important necessities.
China has a long history of producing table salt, and there are many kinds of table salt. According to their different sources, there are sea salt, lake salt, well salt and rock salt. The earliest one used in ancient China should be sea salt. It is said that during the Xia dynasty period, the people had already been taught how to make sea salt. The method at that time was to collect the salty soil on the coast and pour seawater on it, and later boil the filtered brine to obtain salt. This kind of salt has five colors: green, yellow, white, black and purple, should be the earliest sea salt in China. After thousands of years of development, the modern salt-making process has become mature, and the salt-making industry also occupies an important position in the domestic economy. At present, there are three main methods for salt production from seawater: distillation, electro dialysis and vacuum salt production.
In the modern salt-making industry, most of the salt-making equipment and pipelines are made of metal. In the salt-making workshop, it is often seen that there are a large number of white crystals on the equipment pipelines, welds and some other parts, which are formed by strong corrosion due to the long-term accumulation of brine in some area. Since the brine and salt slurry required for salt production have a strong corrosive effect on equipment and pipelines, problems such as material leakage are common, which is also the major problem plaguing the salt production industry. The leakage not only causes waste of materials, but also causes pollution to the environment with corrosive brine. The maintenance and replacement of equipment caused by leakage will greatly increase the cost and cause significant economic losses to the business entity.
Pipelines and components made of traditional metals and alloys are often perforated or even completely scrapped in a few months. The copper-nickel alloy heating pipe used in the 90,000t vacuum salt-making workshop of Dengguan Salt Works in Sichuan Province put into operation in June 1984, leaked due to corrosion after only five months operation. A carbon steel heating chamber was temporarily fabricated, then the 3.5mm thick heating tube began to corrode and perforate after only two months. Within 3 months, more than 150 heating pipes were corroded and perforated. The heating tube is replaced 4 times in one year. Since then, the salt workshop has switched to heating tubes made of titanium-molybdenum-nickel alloys. After two years of running tests, the walls of the tubes are still as bright as new without any corrosion. Beyond that, the fouling and clogging problems happens inside the heating pipe that have often occurred in the past also been greatly alleviated. Since the surface of the titanium-molybdenum-nickel alloy is very smooth, it is difficult for dirt to adhere to its surface. So far, titanium and titanium alloys have become important metal materials in the salt production industry. Although their prices are higher than those of stainless steel and aluminum alloys, they have a long service life. The equipment maintenance frequency and replacement frequency are greatly reduced, which not only ensures the production efficiency for the enterprise, but also fundamentally solves the worries of corrosion to the salt-making equipment.
Prospects of desalination
The total reserves of water on the earth are very rich, but seawater resources account for 97.3% and freshwater resources only account for 2.7%. The freshwater resources that humans can easily utilize are mainly from river water, freshwater lakes and shallow groundwater. These freshwater reserves only account for 0.07‰ of the world's total water reserves. The world's truly available freshwater resources are about 9,000 cubic kilometers per year, and their distribution is quite uneven. According to data disclosed by the United Nations, more than 1 billion people on earth live in water-scarce areas. With the increase in population, industrial and domestic water consumption continues to increase, coupled with man-made waste, pollution and over-exploitation, freshwater resources are becoming increasingly scarce, which poses a serious threat to human existence. Under such a premise, people have to turn their attention to the treasure house of the earth's water resources - the ocean.
So far, there have been more than 20 kinds of seawater desalination technologies, among which techniques such as distillation, electro dialysis and reverse osmosis have reached the mass production standard. Among the many desalination methods, the multi-stage flash distillation (MSF) plays a dominant role in the world's seawater desalination production, and accounted for about 56% of the world's total seawater desalination. The largest single-machine MSF distillation unit is installed in Italy, with a daily freshwater production capacity of 36,000 tons. In areas with severe water shortages such as the Middle East, MSF has become the most widely used seawater desalination technology.
The multi-stage flash distillation device is mainly composed of seawater heater, heat recovery condenser, heat output condenser, ventilation condenser and jet compressor. Among them, a large number of heat transfer tubes are used in the heat exchange parts. These heat transfer tubes were originally made of copper alloys, but copper alloys are not corrosion-resistant, and problems such as perforation will soon occur under the continuous erosion of seawater. At present, copper alloy heating pipes have been replaced by titanium heating pipes that are immune to seawater corrosion. Titanium material not only has good corrosion resistance to seawater, but also can greatly reduce the occurrence of biological pollution and blockage in the heat transfer tube, which greatly improves the service life of the heat transfer tube.
Apart from industrial pure titanium, titanium alloys are also ideal materials for heating pipes in seawater desalination plants. Japan has widely used titanium alloys in heat exchangers in seawater desalination, power generation and salt production. In order to minimize material loss and reduce costs, Japan's Mitsubishi, Kawasaki, Hitachi, Mitsui, Kobe Steel and other companies have successively adopted titanium tubes with a thickness of only 0.5-0.7mm, which greatly reduces the consumption of titanium materials.
With the continuous improvement and maturity of seawater desalination technology, the extraction of freshwater from seawater has become an important strategy to solve the shortage of freshwater resources worldwide. In the face of highly corrosive seawater, most traditional metal materials have a difficult time to undertake the task of seawater desalination. As the "natural enemy" of seawater corrosion, titanium and its alloys will definitely play a greater role in the desalination process.
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