When the overflow from a thickener suddenly becomes turbid, it’s a problem that causes headaches for many mineral processing plants. You may have already tried adding flocculants, but the results have been inconclusive. The real cause may lie in details you haven’t noticed.
The root cause of turbid overflow in a thickener is an unstable solid-liquid separation process, which involves four key factors: feed properties, chemical efficacy, operating parameters, and equipment condition. Simply adding more chemicals often addresses the symptoms rather than the root cause; a systematic diagnosis is required for a complete solution.
When the overflow from a thickener becomes turbid, many operators’ first instinct is to add more flocculant. This intuition may lead you astray. Next, we will break down the four most common root causes and teach you how to quickly identify the true source of the problem.
1. Most Common Cause: Excessive Fine Silt
Excessive fines dramatically increase surface area, consuming disproportionate flocculant. Simply adding more reagent creates numerous small, loose flocs that worsen cloudiness. The key is verifying fines content through particle size analysis.
Many processing plants have observed that when the thickener suddenly becomes turbid, the overflow from the hydrocyclone becomes finer.
These two phenomena are often caused by the same underlying issue. This is because an increase in ultrafine particles typically makes thickening significantly more difficult. When the concentration of particles smaller than 10 μm in the slurry rises substantially (Note: For most tailings thickening systems, changes in the -10 μm content serve as a key reference indicator), the settling velocity of the particles drops sharply. The finer the particle size, the larger the specific surface area, and the stronger the surface charge, making natural settling more difficult. This is particularly true for the following ores:
kaolinitic ores, weathered and oxidized ores, severely silty copper ores, lateritic iron ores, sericite-bearing ores, and montmorillonite-bearing ores.
These mineral compositions can lead to increased dispersion of fine silt and reduced settling performance; even with the addition of flocculants, rapid settling may not be achieved.
It is common on-site for grinding fineness to increase, temporarily improving flotation indicators, only for the thickener to subsequently become cloudy. The issue is not that the thickener has malfunctioned, but rather that the properties of the slurry entering the thickener have changed. Therefore, when the overflow becomes turbid, the first step should be to examine the particle size analysis; do not immediately adjust the chemical dosage.
Three-dimensional analysis for fines problems
(1) Source Identification
- Check grinding parameters (ball size distribution, ore hardness)
- Verify hydrocyclone classification efficiency
- Identify potential contamination from clay-rich ores
(2)Process Solutions
| Situation | Solution | Expected Outcome |
| Temporary fines increase | Install pre-settling tank | Reduce 30-50% fines load |
| Persistent fines issue | Adjust grinding classification | ≥40% fines reduction |
| Extreme fines condition | Switch to centrifugal thickener | Overflow SS<300mg/L |
(3) Reagent Optimization
For high fines content:
- Use ultra-high molecular weight flocculants (>18 million)
- Add dispersants as settling aids
- Implement staged dosing
2. Flocculant Degradation: More Common Than Underdosing
Many people believe that turbidity persists because there isn’t enough chemical. In fact, numerous cases show the opposite: the chemical has lost its effectiveness.
(1) Incorrect chemical concentration
When the polyacrylamide (PAM) concentration is too high, the chemical cannot fully disperse, forming “fish-eye clumps,” which significantly reduces its effective utilization rate.
Common concentrations used in most processing plants: 0.05%–0.15% (Note: There are significant differences among chemical manufacturers and preparation methods). Excessively high concentrations may result in insufficient dissolution and dispersion or localized overdose, thereby affecting flocculation performance.
(2) Insufficient maturation time
PAM requires thorough dissolution.
In many operations, the chemical is prepared in the morning and added directly ten minutes later. In reality, the polymer chains have not fully extended, resulting in significantly reduced flocculation effectiveness.
General recommendation: A certain maturation time is typically required; specifics should be determined based on the chemical product specifications and on-site testing. Some high-molecular-weight products require more than 60 minutes.
(3) Incorrect dosing point
In many thickening tanks, the chemical is injected directly into the central cylinder.
While this may seem convenient, it results in insufficient mixing time. Flocs enter the settling zone before they have fully formed, causing large amounts of fine sludge to pass directly into the overflow.
Many sites have found after retrofitting that simply adjusting the dosing location can reduce suspended solids in the overflow by more than 30%.
3. Feed Fluctuations: The Real Killer for Many Thickeners
Besides the influence of fine mud, fluctuations in feed are also an important factor affecting the stable operation of thickening.
Case studies show that ±5% feed density variation can cause 10x overflow turbidity spikes.
Thickeners are continuous settlers with strict capacity limits. When instantaneous feed exceeds 120% design rate, the settling zone collapses. Install online density meters and flow controllers to maintain ±3% variation.
Feed stabilization strategies
(1) Buffer Systems
- Add mixing tanks (> 30 min retention)
- Use variable-speed pumps
- Install auto-dilution systems
(2) Operational Adjustments
- Maintain sump level at 50-70%
- Avoid simultaneous equipment startups
- Make gradual valve adjustments during shift changes
(3) Automation Upgrades
Recommended instruments:
- Electromagnetic flow meters (0.5% accuracy)
- Nuclear density gauges (±0.5% error)
- Ultrasonic mud bed detectors
4. Overlooked Equipment Issues
A copper mine case: 10% rake speed reduction due to gearbox wear caused overflow SS to jump from 200mg/L to 2000mg/L.
Thickener mechanical condition directly impacts performance. Key checks: rake torque (30-70% normal), blade clearance (<10 mm), center well wear (<20% wall loss), and overflow launder levelness (±2mm tolerance).
Critical maintenance points
(1) Mechanical Systems
- Monthly motor current measurements
- Regular bearing lubrication
- Wire rope wear inspection
(2) Structural Components
- Center well liner replacement schedule
- Overflow launder acid cleaning
- Underflow cone anti-clogging design
(3) Control Systems
- Proper torque limit settings
- Automatic lift function testing
- Emergency stop system drills
How to Quickly Identify the Cause of Turbidity in the Thickener Overflow at the Site?
When turbidity occurs, follow the troubleshooting sequence below to help quickly narrow down the potential causes on-site.
Step 1: Measure the suspended solids concentration in the overflow to determine the severity of the issue.
Step 2: Check the particle size distribution of the feed, focusing on changes in the -10μm fraction.
Step 3: Check the flocculant: Verify if the type has changed; ensure the concentration is normal; confirm adequate maturation; and verify that the dosing point is appropriate.
Step 4: Check the feed rate and feed concentration to identify the source of fluctuations.
Step 5: Inspect the equipment condition—including the central cylinder, rake frame, overflow weir, and flocculant piping.
Conclusion
Turbid overflow from a thickener indicates a systemic issue that requires a comprehensive solution addressing ore characteristics, reagent management, process control, and mechanical maintenance. We recommend establishing a “prevention-monitoring-adjustment” cycle through particle analysis, reagent testing, flow stabilization, and regular maintenance. Remember, when problems arise, a systematic diagnosis is far more effective than empirical remedial measures.