How to Control the Treatment of Lime-Softened Water in Ultrapure Water Softening Process?

We know that in chemical softening treatment, various agents can be used depending on the quality of the raw water and the requirements for the quality of the extracted pure water. Quicklime softening is a relatively common method. It involves adding lime slurry to water, which reacts with the hardness components in the water—carbonic compounds—to form insoluble CaCO3 or other insoluble substances, causing them to precipitate and achieve softening.

Lime water can remove the bicarbonate hardness of water, i.e., temporary hardness, thus partially softening the water. The softening effect is greatly affected by the amount of lime used. Too little lime results in less CaCO3 precipitation; too much lime raises the pH too high, causing CaCO3 to easily form colloids and affecting the precipitation effect. The effectiveness of lime softening is tested by changes in phenolphthalein alkalinity and methyl orange alkalinity; that is, it is difficult to achieve 2P-M = 0 with the water's alkalinity and pH. Therefore, in actual production, 2P-M is used as a control indicator, keeping it as close to zero as possible. There are three control methods:

1. Controlling 2P-M within a certain positive range, such as 0-20 mg/L (calculated as CaCO3), ensures more complete softening. This method is generally recommended, with an ideal 2P-M value of 5-20 mg/L (calculated as CaCO3).

2. Controlling 2P-M within a certain negative range, such as 0-20 mg/L (calculated as CaCO3). In this unsaturated state of CaCO3 in the water, scaling is less likely, but the water is corrosive.

3. Controlling 2P-M within a certain positive and negative range, such as ±21 mg/L (calculated as CaCO3). This method is generally not recommended because the effluent is unstable and can easily cause post-precipitation, leading to scaling in filters or desalination devices.

Besides 2P-M, pH is often used as an auxiliary indicator for control. The pH value after softening varies depending on the raw water quality, and manufacturers often set specific targets based on actual conditions, generally controlling the pH value around 10.0.