How are MVR (Mechanical Vapor Recompression) and Mother Liquor Dryers Integrated in Industrial Brine Evaporation Systems?

In industrial brine "zero-discharge" processes, the MVR evaporator only undertakes the "main concentration" task, turning 99% of the water into clean condensate for reuse. However, it cannot overcome the final bottleneck of "mother liquor." The mother liquor dryer (also called a mother liquor drying machine, scraper/drum dryer) is designed to completely solve the mother liquor problem. In engineering, both are generally combined in a "three-stage" closed-loop configuration. The specific division of labor and coupling points are as follows:

I.System Positioning and Process Boundaries

1.MVR Stage – "Reduction of Over 90%"

Task: Evaporate 8% membrane concentrate or softened liquid to TDS 20-30%, producing recycled water and centrifuged salt.

Bottleneck: Evaporation intensity drops sharply after boiling point increases and viscosity surges; organic matter and impurities accumulate, requiring periodic discharge of mother liquor to maintain system stability.

2.Mother Liquor Drying Section – “Fully Utilizing the Liquid”

Task: To dry the MVR discharge mother liquor (containing 70–85% water, 25–45% TDS, and highly enriched with COD and heavy metals) in one step to a solid mixed salt with 5–15% water content, achieving “zero liquid discharge.”

Features: Low-temperature (45–90℃), vacuum or atmospheric pressure thin-layer drying; capable of handling extreme materials with viscosity >5000 cP and boiling point elevation >25℃.

II.Typical Coupled Process

1.Preheating, Concentration, and Crystallization

Concentrated brine → MVR forced circulation evaporator (87–95℃) → centrifuge → industrial salt (NaCl, Na₂SO₄, etc.)

The centrifuged mother liquor at 70–80℃ is directly pumped into the mother liquor dryer, eliminating the need for secondary heating and saving 20–30% of heat consumption.

2.Vacuum Low-Temperature Drying: The dryer incorporates a scraper/drum, rotating at 2–10 rpm to form a 0.5–1 mm liquid film. Secondary steam is drawn to 70–90 kPa by a vacuum pump, lowering the boiling point to 45–65℃, preventing the pyrolysis of organic matter.

The condensate, after mixing with the MVR condensate, has a TDS <200 mg/L and can be completely reused.

3.Dry Salt Collection and Tail Gas Treatment: The dry salt has a moisture content ≤15% and is directly bagged for hazardous waste disposal or placed in a rigid landfill. Secondary steam containing trace amounts of dust is treated with a "water film dust removal + demisting + activated carbon" process before being discharged in compliance with standards.

III.Energy and Equipment Coupling Techniques

1.Cascaded Utilization of Heat Source: The 120–130℃ steam from the MVR compressor outlet first heats its own chamber, while the condensate (75–80℃) is then fed into the jacket of the mother liquor dryer, achieving secondary utilization of waste heat and reducing overall energy consumption by 40–60%.

2.Shared Vacuum System: The dryer vacuum pump simultaneously draws in non-condensable gases from the MVR, reducing the MVR vacuum pump load by 15-20% and decreasing compressor shaft power.

3.Synergistic Material and Cleaning: The MVR section uses 2205/2507 duplex steel to resist Cl⁻; the dryer cylinder uses 316L steel with enamel or titanium alloy scrapers, resisting high COD and high-hardness scaling; the cleaning cycle is extended from 7-10 days for a single MVR to 21-30 days, reducing annual maintenance costs by more than 50%.

IV.Application Case Overview:

1.Landfill Leachate Concentrate: DTRO + MVR + mother liquor scraper drying, water production rate >90%, dry salt content ≤10% water, reducing landfill volume by 75%.

2.Lithium Battery Wastewater Sodium Sulfate System: The MVR concentrates sodium sulfate to 25% TDS, and the mother liquor dryer performs vacuum drying at 45℃, saving approximately 3.2 million yuan in hazardous waste disposal costs annually. 3. Coal Chemical Salt Separation Zero Discharge: NF salt separation → MVR (NaCl) + multi-effect (Na₂SO₄) evaporation, the mother liquor from both systems is combined and fed into a drum dryer, ultimately producing 700 t/a of mixed salt, all of which is rigidly landfilled.

V.Summary MVR is responsible for the front-end reduction of "large water volume, low energy consumption," while the mother liquor dryer completes the terminal solidification of "small water volume, high impurities." The two are seamlessly connected through "heat sharing, vacuum sharing, and direct mother liquor delivery," together forming the most economical, stable, and space-saving technical route for zero discharge of concentrated brine.