Seawater Desalination Should Be Combined with Salt Chemical Industry   

Guoxi Cai  Editor in CCR

Water desalination refers to any of several processes that remove some salt and other minerals from seawater. Water is desalinated in order to convert salt water to fresh water so it is suitable for human consumption or irrigation. Sometimes the process produces table salt as a by-product. Most of the modern interest in desalination is focused on developing cost-effective ways of providing fresh water for human use in regions where the availability of fresh uses water is, or is becoming limited.
   Many countries started to build seawater desalination since few decades ago, especially in the Middle East region, and the world's largest desalination plant is the Jebel Ali Desalination Plant (Phase 2) in the United Arab Emirates, which is capable of producing 300 million cubic meters of water per year. As one of the countries lacking fresh water resources, China also seized the opportunity to development seawater desalination industry and currently a few 10000-ton projects are in operation or under construction, involving plants in Tangshan Caofeidian Industrial Zone, Dalian Hongyan River Nuclear Power Station and Zhejiang Datang Wushan Power Station.

Problems: high cost and concentrated seawater
Although there are numerous plants in China, the fresh water obtained by seawater desalination is still not a large part of China's fresh water supply because of its high cost. Producers have to spend RMB5 - 7 to obtain one cubic meter of fresh water from seawater, while the usual production cost of tap water is RMB 3 per cubic meter and the water from China's project that diverts water from South China to North China has lower cost. In this circumstance, the daily output of China seawater-desalination fresh water is less than 1% of the global total.
    The other problem is concentrated seawater from seawater desalination. Concentrated seawater performs as by-product in the process of seawater desalination and most factories directly discharge it into ocean because they have no use for it. The chemicals in concentrated seawater that originates from pretreatment of seawater desalination cause severe environmental pollution. On the other hand, concentrated seawater contains a mass of inorganic salts such as NaCl and bromine which are regarded as important raw materials in salt chemical industry and discharging them can be considered a waste.

Solution: go with salt chemical
In fact, the two problems above are fitted into one because it is reasonable to utilize the inorganic salts in concentrated seawater for salt chemical production which would significantly offset the production cost of fresh water by seawater desalination. Fortunately, some Chinese producers have already started to recycle the concentrated seawater, and the following examples might serve to demonstrate what is possible:
    Qingdao Soda Ash Industrial Co Ltd, a large scale-comprehensive chemical enterprise in China started operating a 20 000 t/d seawater desalination process on June 22, 2010. Approximately 55% of the concentrated seawater from the process of desalination is delivered to the company's soda ash plants as raw material for soda ash production, saving 60 000 tons of industrial salt every year, which basically corresponds to the cost of seawater desalination. Alternatively, the fresh water from seawater desalination could be used as company's operating water and it saves 3 million tons tap water annually.
    Caofeidian Industrial Zone in northern China's Hebei Province linked up with chemical company Tangshan Sanyou Group to construct a combined seawater desalination and chemical plant in the Caofeidian industrial zone, near Tangshan in the northern province of Hebei, where the by-products of the desalination process will be used for the chemical industry. After being completed in October 2011, the plant will be able to deal with 18 million cubic meters of concentrated seawater, extracting 600 000 tons sodium chloride every year.  
    These two projects were designed to convert concentrated seawater into raw materials for soda production. His is a wise strategy, combining seawater desalination with salt chemical industry that is beneficial for both environmental protection and economic interest.

Experts: comprehensive utilization
Experts in seawater desalination say there are still various other routes for utilization of concentrated seawater, not only producing soda ash and NaCl as the two examples above, but also extracting bromine and producing of other salt chemicals including potassium chloride, magnesium chloride, magnesium sulfate and potassium sulfate. Typically, the bromine abstracted from concentrated seawater might dramatically offset the cost of desalination due to bromine's high market price.
    Experts are also enthusiastic about a more complex process of seawater desalination: make nitrate or sulfate from concentrated seawater; extract bromine; use the remaining seawater to produce magnesium hydroxide and magnesium oxide; separate gypsum from the residual; and then obtain sodium chloride for the production of soda ash and chlor-alkali.

Development of China's seawater desalination
Chinese authorities are trying to establish demonstration projects combining seawater desalination with salt chemical industry. They hope to operate seawater desalination plants on an industrial scale, located close to salt works, soda factories and power stations for convenient transportation and utilization of concentrated seawater.