Low Boron Removal

With the increasing demand for electronic grade silicon, and particularly the growth of silicon solar cells, the semiconductor industry is facing the challenge of finding efficient ways to create molten silicon that is free from impurities. The pure silicon contains very few mobile electrons and is, thus, highly resistant to electrical conductivity. However, any small impurity—such as boron or phosphorus—present at the level of even one particle to a billion, can totally change the electrical properties of the silicon and increase its conductivity. This phenomenon is extendedly used to deliberately create a boron impurity inside a Silicon wafer to change its electrical properties—known as boron (or phosphorus) doping—is the basis of most semi-conductor components such as diodes, transistors, microprocessors and silicon photovoltaic cells. These are the most critical processes in the semiconductor industry, and the precondition is receipt of pure molten silicon.

Different methods, such as removal of boron from molten silicon using CaO–SiO2 based slags, or using a steam-added plasma melting method, are implemented to create silicon free from impurities. Regardless of the method selected, all the processes require high purity water, free of boron.

Recently, we have been witness to governments making efforts to protect natural water resources in some countries in favor of agriculture and water preservation, while industries are left to solve their own water shortage. This is exactly what is happening in Taiwan and until recently, the Formosa Petrochemical Corporation (FPCC) enjoyed fresh water from natural sources. However, the government of Taiwan recently reduced Formosa’s water rights from natural sources to support agriculture, requiring the company to seek an independent water supply. This approach is spreading to other countries as well, and high-quality desalinated water is a solution that industries can no longer evade. This conflict of supplying water for agricultural use, and supplying high-purity water for industry, can be resolved. IDE Technologies was awarded the design and supply of a 36.9 million gallons per day SWRO desalination plant, with a primary feature of boron-free water.         

The removal of boron from fresh water is a well-known process and is commonly done with boron-adsorptive resin inside an ion exchanger vessel. However, removing boron from the permeate of a seawater desalination plant to an undetectable level of 0.01 mg/l requires a different approach, and becomes a more critical challenge for a large-scale seawater desalination plant. Although the ability of the polyamide membrane to reject boron is well known, this process requires careful control of the pH to improve the boron rejection on one hand, while over-dosing of sodium hydroxide (NaOH), could increase the tendency for magnesium hydroxide scaling on the membrane. When the design of the plant is also required to consider low energy consumption and minimum chemical consumption, finding the optimal configuration and the optimal membranes becomes a very challenging task.

This article focuses on a seawater desalination process that takes the boron removal issue to the edge, using four desalination stages and a downstream boron ion exchanger that removes the boron to an undetectable level of less than 0.01 mg/l of boron. In next month’s conclusion, we’ll take a closer look at membrane boron removal and boron removal by ion exchanger, as well as control and instrumentation issues. υ

As vice president of engineering at IDE, Jacky Ben Yaish bears overall responsibility for the engineering design of all IDE products, including both thermal and RO desalination, with the thermal and membrane process departments, as well as the detailed engineering, falling under his responsibility. Jacky has been closely involved in the engineering of the company’s leading mega projects around the world, among them the Sorek, Carlsbad, Tianjin, and Jamnagar plants. Jacky brings twenty-three years of experience in desalination to the management table, from all aspects of the project—process, detailed design, and project management. His across the board experience in all stages of project, from design through to commissioning and operation, provides an unparalleled understanding of complex water treatment projects. He has a broad understanding of alliancing and risk/reward compensation processes associated with alliance. Jacky is a graduate of Ben Gurion University with bachelor and master degrees in mechanical engineering. He also has an MBA from Bar Ilan University, with a focus in financing.

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