The mud and water separation unit for the tunnel boring shield operates based on a combination of gravitational settling, centrifugal separation, screening, and filtration. The working process consists of five key stages: drilling fluid transportation, coarse separation, medium separation, fine separation, and recirculation.

1. Drilling fluid transportation stage

Formation of drilling fluid:

• During shield tunneling, the cutting disc cuts the soil
• The cut soil mixes with the drilling fluid, forming a highly concentrated suspension
• The drilling fluid contains solid particles of various sizes: soil particles, sand, stones

Drilling fluid transportation:

• The drilling fluid is pumped from the shield's drilling fluid chamber through a pipeline
• The transport pump continuously supplies the drilling fluid to the separation unit
• A certain pressure and flow rate of transportation are maintained

2. Coarse separation stage (first step)

Separation on the vibrating screen:

• The drilling fluid first passes through the vibrating screen (coarse screen)
• The vibrating screen openings are large (usually 10-50 mm)
• The screen vibrates at a high frequency driven by a vibration motor

Separation of large particles:

• Large particles of sand, stones, debris, and waste are retained on the screen surface
• Under vibration, they move along the screen surface to the discharge opening
• Large solid particles are discharged and can be transported for disposal

Preliminary filtration:

• Drilling fluid with particles smaller than the screen openings passes through the screen
• After removal of large particles, the drilling fluid proceeds to the next processing stage
• Protection of subsequent equipment from damage by large particles

3. Medium separation stage (second step)

Separation in the sand separator (hydrocyclone):

• The drilling fluid after the coarse screen enters the sand separator
• The sand separator operates on the hydrocyclone principle
• Usually removes particles larger than 0.074 mm

Principle of centrifugal separation:

• The drilling fluid under certain pressure tangentially enters the top of the hydrocyclone
• Inside the hydrocyclone, a high-speed rotational movement is formed
• A powerful centrifugal force field is created

Sand separation process:

• High-density sand particles are thrown to the wall by centrifugal force
• Sand particles spiral down along the wall to the bottom part
• Discharged through the discharge opening at the bottom

Rising of light drilling fluid:

• Low-density drilling fluid forms an upward flow in the center
• Overflows through the top opening of the hydrocyclone
• Proceeds to the next fine separation stage

4. Fine separation stage (third step)

Separation in the mud separator:

• The drilling fluid after the sand separator enters the mud separator
• The mud separator also operates on the hydrocyclone principle
• But the hydrocyclone diameter is smaller, removing finer particles

Separation of fine particles:

• Can remove fine soil particles sized 0.015-0.074 mm
• Finer centrifugal separation, higher efficiency
• Fine soil particles are discharged through the bottom part

Deep dewatering on the filter press:

• Additional treatment of fine suspension for dewatering
• The moisture content of the sediment can be reduced to a level suitable for direct transport by dump truck
• Effective reduction of solid material volume is achieved

Obtaining clean water:

• After three-stage separation, the solid phase content in the drilling fluid is significantly reduced
• The density and viscosity of the drilling fluid are restored to the required technological parameters
• A clean drilling fluid suitable for recirculation is obtained

5. Recirculation stage

Storage of clean water:

• The purified drilling fluid after separation flows into the clean water reservoir for storage
• The volume of the clean water reservoir is designed according to construction needs
• A certain reserve of drilling fluid is maintained

Property adjustment:

• If necessary, bentonite and other materials are added to clean water
• Density, viscosity, and other parameters of the drilling fluid are adjusted
• Ensuring the drilling fluid meets the requirements of shield tunneling

Return of drilling fluid:

• The prepared drilling fluid is returned by a reverse pump
• Transported through the pipeline back to the shield's drilling fluid chamber
• Continues to be used to support the bottom hole and cut soil

Circulation operation:

• The drilling fluid circulates between the shield and the separation unit
• Continuous separation of solid particles, addition of fresh drilling fluid
• Implementation of continuous use and recirculation of drilling fluid

Processing of excavated soil:

• Separated solids: sand, stones, soil
• After dewatering, loaded onto transport for removal
• Transported to the designated location for disposal or comprehensive utilization