21.1 Thickening, Filtration & Solid–Liquid Separation
Key Takeaways
- Solid–liquid separation recovers process water, concentrates slurry, prepares product and controls tailings or residue using settling, filtration, centrifugation and drying.
- Clarification prioritizes clear overflow, while thickening prioritizes dense underflow; both depend on particle size, density, surface chemistry, feedwell energy and flocculation.
- Flocculants bridge fine particles into faster-settling aggregates, but overdosing can harm clarity, rheology, filtration and cost.
- Filtration rate is governed by pressure, viscosity, area, cake resistance and medium resistance; washing, air blow and cake discharge determine product quality and cycle time.
- Equipment selection balances feed variability, required moisture, water recovery, throughput, footprint, energy, reagent, maintainability, safety and downstream behavior.
After grinding, flotation, leaching or precipitation, valuable product and waste often remain suspended in water. Solid–liquid separation controls water recovery, downstream equipment size, transport, storage and environmental risk. No one machine is universally best: particles, chemistry, required moisture and operating context determine the route.
Free and Hindered Settling
An isolated fine sphere in laminar flow follows Stokes' law, with settling velocity proportional to diameter squared and density contrast. Industrial pulp is concentrated; particles interfere and displace water upward, producing hindered settling. Very fine charged particles may remain stable until coagulation or flocculation changes interactions.
A settling test records interface height versus time. Initial slope estimates zone-settling velocity; compression begins as solids form a network. Test representative feed across ore type, temperature, pH, salinity and reagent conditions.
Coagulation and Flocculation
Coagulants reduce electrostatic repulsion. High-molecular-weight flocculants adsorb and bridge particles into aggregates. Prepare and age polymer correctly, dilute it for distribution, and mix with enough energy for contact but not enough to shear flocs.
Optimize dose with settling rate, overflow clarity, underflow density and rheology—not rate alone. Overdose wastes reagent and can create sticky, high-yield-stress underflow or poor filtration.
Thickener Anatomy
- feed pipe and feedwell dissipate energy and mix flocculant;
- clarification zone allows flocs to settle and clear water to rise;
- settling zone transports aggregates;
- compression zone compacts bed;
- rake moves settled solids to underflow cone; and
- overflow launder collects clarified water.
Control variables include feed rate and solids, flocculant dose, bed level, rake torque, underflow rate/density, overflow turbidity and interface. High torque can indicate excessive bed, poor dilution or underflow restriction. Do not withdraw too fast and collapse the bed or too slowly and bury the rake.
Mass and Water Balance
A thickener receives 100 t/h dry solids at 20% solids by weight. Total feed slurry is $100/0.20=500$ t/h, containing 400 t/h water. Underflow at 50% solids totals $100/0.50=200$ t/h and contains 100 t/h water. With all solids reporting to underflow, overflow water is $400-100=300$ t/h.
This ideal balance ignores overflow solids, evaporation and inventory change. Real sampling must use simultaneous flows and consistent dry basis.
Filtration Fundamentals
Pressure difference drives liquid through a porous cake and filter medium. A simplified Darcy relation is:
where $A$ is area, $\Delta P$ pressure, $\mu$ viscosity, $R_m$ medium resistance and $R_c$ growing cake resistance. As cake thickens, rate falls. Fine compressible cakes can become less permeable when pressure rises, so more pressure does not always proportionally increase throughput.
Equipment
- Vacuum belt/disc/drum filters: continuous, moderate pressure difference, suitable for readily filtered concentrates or tailings.
- Pressure filter press: batch chambers or membranes provide lower moisture and clear filtrate but require cycle and cloth management.
- Pressure leaf filter: clarification/polishing applications.
- Centrifuge: uses centrifugal acceleration, useful where particle and density behavior fit.
- Thermal dryer: removes remaining moisture at high energy and dust-control demand.
A filter cycle includes fill, filtration, optional squeeze, washing, air blow, depressurization, opening and cake discharge. Cycle bottleneck may be mechanical time rather than filtration.
Cake Washing and Moisture
Washing displaces soluble impurities from cake pores. Poor distribution causes channeling. Counter-current washing can improve efficiency. Final moisture depends on pore size, capillary pressure, cake thickness, pressure, air blow and cracks. Report free moisture and dry basis consistently for transport or sale.
Selection Matrix
Ask:
- required overflow/filtrate clarity and recovery;
- required underflow or cake moisture/rheology;
- feed tonnes, percent solids, particle size and clay;
- variability, corrosivity and temperature;
- batch versus continuous downstream process;
- water scarcity, tailings strategy and residue stability;
- power, air, floor area, labor and maintenance; and
- guarding, pressure release, confined access and chemical exposure.
Exam Trap
A thickener does not “filter” through a medium, and a hydrocyclone is not a substitute for high-clarity filtration. Identify the separation mechanism, required product and limiting particle behavior before choosing equipment.
Thickener Capacity Trap
A conventional thickener is often evaluated by solids flux and clarification area, while high-rate designs depend strongly on feedwell and flocculation. Scaling only by tank diameter can fail when ore changes create finer particles or higher yield stress. Pilot or bench tests should represent expected variability, and design should check maximum solids load, hydraulic load, bed inventory, rake torque and underflow pumping together.
A thickener receives 100 t/h dry solids at 20% solids by weight and discharges all solids in underflow at 50% solids. Ignoring inventory change and overflow solids, how much water reports to overflow?