Case Study: 55 t/h Four-Effect Evaporation and Concentration System for Sodium Silicate/Sodium Hydroxide Wastewater

Project Background

A large-scale manufacturer of silicone and personal care intermediates generates significant quantities of mixed alkaline wastewater containing sodium silicate and sodium hydroxide, with a design treatment capacity of 55 tons per hour. This wastewater presents complex characteristics, including high alkalinity, high viscosity, a tendency to foam, and a marked propensity for silicate scaling. Conventional evaporation equipment frequently suffers from reduced heat transfer efficiency, pipeline clogging, and corrosion-induced perforation. The client's existing single-effect evaporation unit suffered from excessive energy consumption and inconsistent concentrate quality, failing to meet the dual requirements of subsequent resource recovery and regulatory discharge compliance.

Solution

To address the specific challenges of this application, we customized a four-effect evaporation and concentration system—combining falling-film and forced-circulation technologies—with a capacity of 55 tons per hour. Recognizing that sodium silicate viscosity rises sharply with concentration and that sodium hydroxide is highly corrosive to standard stainless steel, we adopted a staged process design: rapid water removal via falling-film evaporation in the initial stages, followed by high-concentration processing via forced-circulation evaporation. The system is equipped with an integrated online alkaline and acid cleaning unit to effectively manage silicate scaling issues.

Core Equipment Configuration

The complete system comprises a four-effect series evaporator assembly, preheaters, surface condensers, a vacuum unit, and an automatic control system. Falling-film evaporators are utilized for the first and second effects to ensure efficient heat transfer during the low-concentration, low-viscosity phase. Forced-circulation evaporators are employed for the third and fourth effects to maintain turbulent flow within the tubes for high-concentration sodium silicate solutions, thereby preventing localized overheating and dry-wall scaling. Critical components in contact with the process fluid—including heating tubes, separators, and circulation piping—are constructed from nickel-based alloys or nickel-lined materials; this offers a service life regarding sodium hydroxide corrosion resistance that is more than three times that of 316L stainless steel. Process Highlights

The four-effect series design maximizes the utilization of latent heat from secondary steam across all stages. Live steam is introduced only into the first effect, while the subsequent three effects utilize the secondary steam generated by the preceding stage as their heat source; secondary steam from the final effect is recovered via a condenser. This design reduces steam consumption to under 0.28 tons per ton of water evaporated, achieving energy savings of over 72% compared to single-effect evaporation. To address the tendency of sodium silicate to foam, each effect's separator is equipped with a high-efficiency wire-mesh demister and an automated antifoam dosing port; this ensures steam purity and prevents compressor or vacuum system damage caused by liquid carryover. The concentration endpoint is monitored via both online density meters and conductivity meters, with automatic adjustment of feed and discharge valves to ensure continuous, stable operation.

Project Outcomes

Since commissioning, the system has consistently met its design processing capacity of 55 tons per hour. It concentrates dilute alkaline liquor (approx. 8% solids) into a high-concentration sodium silicate/sodium hydroxide mixture (over 45% solids), which is either directly reused in the client's downstream production lines or sold as raw material for commercial water glass. The system processes over 400,000 tons of wastewater annually, recovers nearly 70,000 tons of concentrated liquor, and saves over 10 million RMB in annual steam costs. The equipment has operated continuously without unplanned downtime due to corrosion or scaling, and the client fully complies with environmental discharge standards, achieving a harmonious balance between economic and environmental benefits.

If you require an efficient evaporation and concentration solution for high-viscosity or highly corrosive wastewater, please contact our engineering team for a free process assessment.