Slurries are mixtures of solid particles suspended in a carrier liquid. They account for approximately 6% of pump applications and represent some of the most demanding and abrasive duties in industry. Incorrect pump selection for slurry service leads to accelerated wear, frequent impeller replacement, blocked passages and unplanned downtime. The key to successful slurry pumping is understanding the particle characteristics, concentration and behaviour before selecting the pump type and materials.
Slurry Classification
- Settling slurries: Coarse particles (above 0.1 mm) that settle rapidly when flow stops — must maintain a minimum velocity to prevent sedimentation (typically 1.5–3.5 m/s depending on particle size and density)
- Non-settling (homogeneous) slurries: Fine particles (below 0.05 mm) that remain evenly distributed at low velocities — behave as a Bingham plastic or pseudoplastic fluid; characterised by yield stress
- Heterogeneous slurries: Mixed particle sizes — partially settling, concentration gradient across the pipe cross-section at normal velocities
- Compound slurries: Contain both coarse settling and fine non-settling fractions simultaneously — the most challenging to pump reliably
Key Parameters
- Solids concentration (Cw): By weight percentage — low 40%; higher concentration increases apparent viscosity and wear rate
- Particle size (d50, d85): Median and 85th-percentile particle diameter — governs settling velocity and passage size requirements
- Particle shape: Angular particles are far more abrasive than rounded ones — shape factor significantly affects wear rate
- Specific gravity of solids (SG): Determines slurry density and settling rate; iron ore (SG 5.2) settles much faster than coal (SG 1.5)
- Abrasivity index: Miller number or SAR (Slurry Abrasion Response) — quantifies how rapidly the slurry erodes pump materials
Wear Mechanisms
- Impact wear: Particles strike the impeller and casing at high velocity — most severe on the leading edge of impeller vanes and discharge volute
- Abrasion: Sliding contact between particles and wetted surfaces — dependent on hardness ratio between particle and pump material
- Erosion-corrosion: Combination of mechanical wear and chemical attack — slurries with pH extremes accelerate metal loss
- Cavitation erosion: Air or vapour bubbles collapse on impeller surface — worsened by undersize pumps operating well below minimum flow
Pump Type Selection
- Low-concentration, fine-particle slurries: Heavy-duty centrifugal slurry pump with open or semi-open impeller and thick-section wear liners
- High-concentration settling slurries: Centrifugal slurry pump at reduced speed (to limit tip velocity and wear rate); rubber lining for fine silica; hard metal for coarse particles
- Very abrasive, high-SG slurries: Peristaltic hose pump or diaphragm pump — fluid contacts only the replaceable tube or membrane, not the pump body
- Fibrous slurries (paper pulp, sewage sludge): Centrifugal with vortex impeller — no vane-to-particle contact; or progressive cavity pump
- Dense pastes: Twin-piston or progressive cavity — positive displacement essential for paste above 70% Cw
Materials of Construction
- High-chrome white iron (27% Cr): Hardness HRC 65–68 — best abrasion resistance for sharp, hard minerals (silica, quartz, magnetite)
- Natural rubber lining: Excellent for fine, rounded, wet quartz slurries at low velocity; not suitable for sharp or large particles or aromatic solvents
- Polyurethane: Good balance of abrasion and impact resistance — used in cyclone feeds and low-corrosivity slurries
- Stainless steel 316 or duplex: For corrosive slurries where pH is below 6 or above 10; lower abrasion resistance than hard metal
- Ceramic (alumina, SiC): Highest hardness — used for highly abrasive, small-volume applications and hydraulic cyclone liners
Applications
- Mining and mineral processing — ore slurry transfer, tailings disposal, concentrate pumping
- Coal preparation plants — dense medium cyclone feed and product handling
- Paper and pulp industry — wood pulp, paper stock and broke handling
- Municipal wastewater — primary sludge, secondary sludge, digested sludge
- Ceramics and building materials — clay slurry, cement grout, tile wash
- Dredging — river, harbour and sand-and-gravel extraction
- Desalination — brine and reject streams with suspended solids