DTW Systems has created a solution that economically removes silica from the discharge of CMP streams.
In a standard two-step reaction, the stream pH is lowered and maintained as a magnesium source is added at a specific stoichiometric ratio. Upon satisfying the retention time requirements, the pH of the stream is increased forming a series of insoluble silica molecules.
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Mg2SiO4 |
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3MgO-4SiO2-H2O |
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Mg(OH)2 |
The calcium addition is not desired, however, it is difficult to obtain a magnesium chloride source that does not contain a small amount of calcium. As shown on the test data, as the pH of the second reaction is increased, the residual magnesium and calcium, which contributes to the hardness of water, is reduced. This is equivalent to cold lime softening techniques.
The resultant stream is filtered with a microfiltration system. The filtrate is then suitable for recycle to a reverse osmosis unit.