Silica Removal in Water: Lime & Magnesium Dosing Guide – STARK

Introduction

In high-silica raw water treatment scenarios, combining lime softening with magnesium-based additives can significantly improve the removal of dissolved silica. This method enhances operational efficiency and is widely used in industrial water treatment systems.

Understanding Lime Softening and Magnesium Additives

Lime softening not only reduces hardness by removing calcium and magnesium ions but also provides favorable alkaline conditions for magnesium oxide (MgO) to hydrolyze. When silica (SiO₃²⁻) content is high, simultaneous dosing of lime and magnesium compounds—such as magnesite or dolomitic lime—is recommended to achieve better silica removal efficiency.

How to Estimate Lime Dosage

The lime dosage can be roughly estimated using a calculation formula based on the alkalinity and hardness of the incoming water. However, due to the complexity of water chemistry, this is considered a preliminary reference. Actual dosing should be adjusted during on-site operation based on water quality data.
 

Magnesium Dosing Principles

For effective silica removal, magnesium oxide dosage can be estimated at 10–15 times the amount required to remove 1 mg/L of SiO₃²⁻. Two common magnesium-based additives are:

• Magnesite (MgCO₃)
• Dolomitic Lime (a mixture of CaCO₃ and MgCO₃)

Dosage Calculation for Magnesite

Required MgO (mg) = (Target SiO₃²⁻ to be removed in mg) × 10–15

Then, considering the MgO content in magnesite:

Magnesite dosage (mg) = Required MgO / (C₁ / 100)

Where C₁ is the MgO content percentage in the magnesite (≥70% for qualified products, ≥75% for Grade 1, ≥80% for Premium Grade).

Dosage Calculation for Dolomitic Lime

Since dolomitic lime typically contains lower MgO (20–30%), it's rarely used alone. If used, the required amount is calculated similarly, but with different constants:

Dolomitic Lime dosage = Required MgO / (C₂ / 100)

Where C₂ is the MgO content in dolomitic lime.

Combined Use of Magnesite and Dolomitic Lime

In many systems, both additives are applied together to balance cost and efficiency. For dolomitic lime, calculate the CaO dosage based on its CaO content. Then, magnesite is supplemented accordingly to meet the MgO requirement.

Practical Considerations

• On-site pilot tests are crucial before finalizing dosing ratios.
• Real-time monitoring is advised to avoid overuse and ensure cost-effective operation.
• The quality of additives (purity levels) directly impacts the accuracy of dosing.

Conclusion

Lime softening, when combined with magnesium-based compounds, offers a robust solution for silica removal in industrial water treatment. Accurate dosage calculation, based on both theoretical models and real-world adjustments, is key to optimizing system performance.

 

Explore Related Products

For advanced silica removal systems or customized water treatment solutions, explore our range of Industrial RO Pretreatment Equipment or Magnesium-Based Dosing Systems.