Silica Precipitation
Technical Profile
The objective of lime softening is the removal of hardness and silica using precipitation. Silica must be removed to prevent fouling in downstream equipment. Lime softening is often used to precipitate silica with magnesium. The efficiency of lime softening increases with temperature.
Warm lime softening is atmospherically operated and consists of a flash mixing vessel where incoming hard water is mixed with calcium hydroxide, soda ash and possibly caustic depending on the hardness to alkalinity ratio. Also, coagulant is added along with a recycle of the sludge collected in the downstream clarifier. In a flocculation chamber, flocculant is added and the mixing regime is much gentler. The third piece of equipment is a gravity clarifier in which the formed flocks settle to be sent to sludge dewatering or recycled upstream as grafting material.
Warm Lime Softening does not reflect on the cold lime (Ca(OH)2) or hot lime (CaO) that is added but just on the temperature. Warm lime softening is conducted around 65°C (150°F). The higher the temperature, the lower the outlet hardness and silica concentration that can be achieved. The temperature is limited to 65°C (150°F) mostly because the flocculation polymer does not dissolve well in hotter water. If this issue can be easily addressed nothing stands in the way of applying higher temperatures.
Ferric chloride is often dosed in or upstream of the flash mixing tank where residence time is approximately two minutes to aid in coagulation. Sludge density in the gravity clarifier worsens with increasing iron content, so maintaining ferric chloride dosing as low as possible is positive for sludge dewatering design.
Silica will co-precipitate with magnesium in a stoichiometric ratio of 1:1 Mg vs. SiO2 at elevated pH (> 10). It is not possible to precipitate more silica than magnesium. Magnesium present in the water can be used for silica precipitation, and MgO or MgCl2 may be added to precipitate additional silica if present. MgO is less soluble at lower temperatures, so MgCl2 should be considered in that case. It is best to underdone magnesium since silica values or less important than magnesium on the downstream RO or steam generator. With varying feed quality, it is most important to get rid of all the magnesium.
Temperature also plays a role in the sense that Magnesium silicate is formed slower at lower temperatures and silica precipitates quicker in the presence of other precipitating salts.
Silica concentration should be lowered to around 50 mg/L for feed to an OTSG. Since the OTSG generates less than 100% steam, silica will stay in solution with the unvaporized water. Silica should be lowered as much as practical for feed to a membrane separation system. Silica limits the recovery of permeate.
