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Term of the Day |
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Additives
Chemicals such as wetting agents, leveling agents, brighteners, and grain refiners. Wetting agents are not electrochemically active. They are adsorbed on the surface of the substrate, and slow down the diffusion of the chemicals in the bath to the surface of the substrate. Brighteners, leveling agents, and most grain refiners, are electrochemically active. Current causes them to change and some of the additive gets incorporated into the deposit, while the rest stays in solution. The portion that stays in solution is called the breakdown product. Carbon treatment is required at some point, to remove the breakdown products, as stress will go out of control otherwise. » More Electroplating Terms |
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DI (Deionized) Water - Part I: System Overview |
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| Home » Videos, Power Points, White Pages & Articles |
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| [ Back to Videos, Power Points, White Pages & Articles ]
September 20, 2004
What is DI water? DI (Deionized) water is water in its purest form, H2O. Pretreatment by Active Carbon Filtration and Reverse Osmosis process removes organic and other uncharged impurities and Deionization removes all ionic impurities, salts, from water through a two-phase ion exchange process. Salts contain both positive (cation) and negative (anion) ions and their total charge is always neutral. Cation and anion resins are used to remove these positive and negative ions respectively that come in with the city water supply (see table #1 below). The cation resins remove the positive ions, and replaces them with Hydrogen, H+, ions. The anion resin remove the negative ions, and replaces them with Hydroxyls, OH-. When the H+ and OH- ions are combined, they form pure water, H2O. The removal process is really an exchange of ions, where the resin beads give off the H+ ions, and absorb the other Cation elements listed below, and the OH- ions are exchanged for the Anion elements seen below. Table #1: Ions Commonly Found in City Water
Cations: Sodium [Na+]
Calcium [Ca2+]
Magnesium [Mg2+]
Potassium [K+]
Iron [Fe3+]
Manganese [Mn2+]
Hydrogen [H+] | Anions: Chlorides [Cl-]
Sulfates [SO42-]
Nitrates [NO32-]
Carbonates [CO32-]
Silicates [SiO2-]
Hydroxyl [OH-] |
The quality of DI water is typically expressed in terms of resistivity. Resistivity is the measure of how difficult it is for a solution to carry an electrical charge. This is a function of how much ionic material is in the water. The more ionic material, the lower the resistivity.
In order to maintain the higher resistivity level, it is important to reduce the amount of cations and anions in the water, prior to the final deionization phase within the system. Using only the final ion exchange canisters, will work sufficiently to make 18 MO/cm2 (“ 18 Meg”) water, but it is expensive, as the mixed bed resin will be exhausted more than 10 times faster than for RO water - the RO process removes more than 90% of the city water impurities. The main purpose of the softener/carbon filtering is to make the water suitable for the RO filtration. It is also important to understand the need for circulation the DI-water in a loop:
· Constant polishing over a mixed bed is required to maintain quality, as the DI-water is picking up impurities from the air and materials it is in contact with.· A flow velocity of 2 m/s (6 ft/sec) minimum prevents micro organic growth, such as algae, ruining the DI-water quality. Once such growth has occurred, it is very difficult to remove.· Constant flow enables a better control of temperature and pressure. · Maintaining flow by continuously de-ionizing city water, and sending theunused portion to the drain would be very expensive. |
Initially, the city water is filtered through an active carbon filter to remove the Chlorine and Organics that normally are found in municipal water supplies. Chlorine is commonly added to city water to kill off bacteria, but can also decay the RO membrane. When the active carbon is close to saturated with removed Chlorine and impurities, it is exchanged for new active carbon, typically on an annual basis.
In the RO process, the water is separated into two parts: The permeate, where most impurities are removed; and the reject (concentrate), carrying the impurities out of the system. A softener filter prior to the RO process prevents water hardness (Calcium and Magnesium) - ions with low solubility - from precipitating and clogging the RO membrane. In the softener filter, cations, including Calcium and Magnesium, are exchanged for Sodium ions. The solubility for Sodium ions is high and can't cause clogging of the RO-membrane. The softener filter is automatically regenerated with Sodium ions from a brine tank.
It is important to note that the ion exchange process is not limitless. Mixed bed canisters contain both positive and negative resin, charged with only so many H+ or OH- ions. When exhausted, the resin needs to be replaced. The life of these ion exchange canisters will be explained in more detail, in an upcoming part of this Tip series. |  |
The composition of impurities in tap water can be very different from one city to another, sometimes varying from season to season and even over time; tap water sources can be changed and pockets of water of different characteristics can show up unexpectedly, affecting the water treatment processes. A tap water with a low rate of impurities (i.e. Total Dissolved Solids, TDS) requires less ion exchange and the mixed bed resin will last longer. It is always best to understand the process of making good DI water first, and then take a pro-active role in making sure that you have a local source of replacement resins available, to keep the system running efficiently. DI water systems will run efficiently if maintained properly. If not, or if ignored, the resistivity will drop from 18.0 to 9.0 in 3 working days, from 9.0 to 3.0 within 5 working days, and to less then 1.0 with one weeks time. In Part II, I will cover City Water specifications in more detail. |
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