Slurry Separation Plant - Understanding and Optimizing

Slurry Tunnel Boring Machine

A Slurry TBM (or Slurry Shield) is specifically engineered for tunneling in unstable, high-permeability soils like sand, gravel, or areas with high groundwater pressure. 

• Face Stability: It uses a pressurized bentonite slurry injected into the cutterhead chamber to counteract earth and water pressure, preventing the tunnel face from collapsing.
• Material Transport: The rotating cutterhead grinds the soil, which mixes with the slurry to form a "muck" suspension. This mixture is then pumped out of the tunnel through a pipeline to the surface.
• Versatility: Modern designs like those from Herrenknecht AG allow these machines to handle heterogeneous ground conditions by adjusting the separation components. 

The Separation Plant

The separation plant (or Slurry Treatment Plant) is the "kidney" of the operation, filtering the returned muck so the bentonite can be reused. 

Core Components & Stages:

• Shale Shakers: The first stage of primary separation, using vibrating screens to remove large stones, gravel, and coarse solids.
• Desanders & Desilters: These use centrifugal force and hydrocyclones to remove finer particles. Desanders typically target sand (down to ~50 microns), while desilters target silt (down to ~20 microns).
• Dewatering Shakers: Positioned under hydrocyclones to further dry the separated solids for easier disposal.
• Decanter Centrifuges: High-speed units used for secondary separation to remove ultra-fine particles (2–5 microns).
• Chemical Dosing Units: Often integrated with centrifuges to remove particles smaller than 1 micron using flocculants. 

Key Benefits

• Cost Savings: Reintroducing cleaned fluid into the circuit significantly reduces the need for new bentonite and water.
• Environmental Impact: It minimizes waste volume and prevents contaminated liquid discharge, which is critical in urban or sensitive regions.
• Efficiency: A well-designed plant prevents "bottlenecks," ensuring the TBM can advance at its maximum rate without waiting for slurry processing. 

In slurry TBM (Tunnel Boring Machine) operations, the slurry is defined by its ability to provide structural support to the tunnel face and transport excavated soil (muck) to the surface. 

1. How Slurry is Defined 

Slurry is a non-Newtonian, thixotropic fluid. Its core definition is based on two primary properties that must be strictly monitored during tunneling: 

• Density: Measured in g/ml or lb/ft³, it must be high enough to counteract groundwater and earth pressure but low enough to remain pumpable. 
• Viscosity: Usually measured with a Marsh Funnel, viscosity defines the slurry's "thickness" and its ability to suspend sand and gravel during transport. 

2. How Much Bentonite is Needed 

The amount of bentonite required depends on the soil conditions (e.g., highly permeable sand requires more bentonite to create a "filter cake" seal). 

• Standard Concentration: Typically, 4% to 6% bentonite by weight is mixed with water for a fresh slurry. 
• By Weight: This equals approximately 40kg to 60kg of bentonite per cubic meter (\(1,000\) liters) of water. 
• Industry Rule: A common starting point is 1 lb of bentonite per 1 gallon of water (an 8:1 ratio) for initial mixing, which is then diluted to the target concentration. 

3. How Much Water is Needed 

Water is the continuous phase of the loop. The volume required is determined by the excavation volume and system losses. 

• Theoretical Volume: For every cubic meter of soil excavated, you need at least one cubic meter of slurry to fill that space.
• Circulation Factor: To account for fluid lost into the surrounding ground or trapped in the discarded "muck," operators maintain 1.5 to 2 times the theoretical bore volume in active circulation at all times.
• Daily Makeup: As the separation plant removes dry solids, a small percentage of water and bentonite must be added daily to replace what was lost to the "filter cake" on the tunnel walls or absorbed by the excavated dirt.  

Checking the pH of the ground and groundwater is critical because bentonite slurry is chemically sensitive. If the ground's pH is significantly different from the slurry's, the entire tunneling operation can be compromised. 

Operators!

The operator checks this for three main reasons:

1. Slurry Stability

• Bentonite performs best in an alkaline environment, typically with a target pH between 8 and 10. 
• If the ground is too acidic (Low pH): The bentonite particles will "flocculate" or clump together. This causes the slurry to lose its ability to suspend sand and rocks, potentially leading to clogged pipes or the TBM getting stuck.
• If the ground is too basic (High pH): Often caused by nearby concrete or cement grout, an extremely high pH (above 11 or 12) can increase fluid loss and thicken the filter cake too much, making it harder for the machine to move. 

2. Filter Cake Formation

• The "filter cake" is the thin, waterproof seal the slurry builds on the tunnel face to prevent collapse.
• Proper pH ensures the bentonite particles stay dispersed and flat, allowing them to overlap like scales on a fish to create a tight seal.
• Incorrect pH leads to a leaky seal, which can cause the tunnel face to collapse or allow groundwater to flood the machine. 

3. Detecting Contamination

A sudden change in pH is often the first warning sign that the TBM has hit something unexpected, such as:

Old construction debris: Like buried concrete piles or grout.

Chemical spills: Organic matter or industrial pollutants in the soil that could degrade the slurry.