Ultrapure water by Electrodeionisation

In a semiconductor process, phosphorus, arsenic and stibium are n-type impurities in silicon or germanium, whose atoms can contribute negatively-charged electrons to the conduction band of the semiconductors. Boron and gallium are p-type impurities in silicon or germanium, which can contribute positively-charged holes to the valence band of semiconductors. Both electrons and holes contribute to electric current. So boron is a p-type impurity and much of it will invert the n-type silicon and have an effect on the concentration of carriers.

Reverse osmosis (RO) and Mixed bed (MB) units are usually used to remove silicon and boron from ultrapure water. The average removal rate of silicon by RO can reach 80%. The removal rate of boron by RO usually is 40%.

Normally, through mixed bed unit, silicon in product water can be 0.02 mg/l or less and boron is under 10 µg/l. But mixed bed unit needs chemical regeneration.

Other removal methods such as coagulant, coagulant plus clarifying filtration, flotation and ultrafiltration only remove some silicon and boron from water, and the removal rate is not very high. But for Electro-Deionisation, there is no need for chemicals and it can provide a high quality product steadily and continuously.

Fundamentals of Electro deionisation (EDI):

EDI is mainly composed of ion-exchange resins, ion-selective membranes and two electrodes. There are many cells between the two electrodes. Each cell contains a dilute chamber and a concentrate chamber. The dilute chamber from the concentrate chamber by ion-selective cathode, it is separated by cation-selective membrane and if the dilute chamber is near the anode, it is separated by anion-selective membrane.

When the feed water enters the dilute chamber, the impurities will be absorbed by resins, and then they move onto the surface of the resins under the influence of an electric field. The anions move towards the anode, and the cations move towards cathode (Fig), then they continuously enter into the concentrate chamber, and are brought out of EDI with the concentrate stream. At the same time, water molecules on the surface of resins near the outlet of the dilute streams in the dilute chambers, dissociate into H + and OH- because of the high potential gradients at the localized areas.

H₂O ↔ H⁺ + OH^–

Thus the generated H + and OH- can regenerate resins automatically, which makes the EDI work continuously without chemical regeneration. The pH in anion-resins can reach above 11 and less than or equal to 3 in cation-resins. But the pH of the product water is 7 because the generated H+ and OH- can move onto the surface of resins as other impurities ions quickly move to the concentrate chambers.

Also the amounts of generated OH- can ionize silicon and boron in ultrapure water, which gives EDI a good ability to remove silicon and boron

SiO₂ + OH^– ↔ HSiO₃^–

HSiO₃^– + OH^– ↔ SiO₃^2- + H₂O

H₃BO₃ + OH^– ↔ B(OH)₄^–

The ionised Silicon and Boron molecules get removed by ion selective process.