Slurry coal and water and oil with superior stability.

High-shear rotor-stator technology for producing stable coal-water and coal-oil slurries

Coal-water slurries (CWS) and coal-oil mixtures (COM) are high-solids suspensions used as alternative fuels, feedstock for gasification, pipeline transport media, and industrial process fuels. Achieving a stable, pumpable slurry with high coal loading, controlled particle-size distribution, and minimal settling over storage and transport periods remains a significant technical challenge. Conventional agitation systems frequently produce incomplete dispersion, rapid sedimentation, and phase separation, resulting in unreliable handling and inconsistent combustion or process performance.

Kady International high-speed rotor-stator dispersion mills deliver the intensive shear, impact, and cavitation forces required to produce coal-water and coal-oil slurries with superior long-term stability. By combining rapid particle-size reduction with thorough wetting and homogenization in a single intensive mixing zone, Kady mills routinely achieve higher solids loadings and more stable rheological properties than traditional propeller or low-shear mixers.

Technical Challenges in Coal Slurry Preparation

Coal is an inherently heterogeneous solid containing varying proportions of fixed carbon, volatile matter, ash, and mineral matter. When pulverized and suspended in water or oil, several interrelated problems arise:

• Wide particle-size distribution – Coarse particles settle rapidly under gravity (Stokes’ law), while ultra-fine fractions contribute disproportionately to viscosity.
• Hydrophobic surface character – Many coals resist spontaneous wetting, leading to incomplete dispersion and the formation of weakly bound agglomerates.
• High solids loading requirements – Economically attractive slurries typically target 60–70 % (or higher) coal by weight. At these concentrations, inter-particle interactions cause steep viscosity increases and non-Newtonian flow behavior.
• Sedimentation and hard-packing – Without adequate particle-size control and steric or electrostatic stabilization, settled solids can form dense, difficult-to-re-suspend beds.
• Rheological instability – Improper dispersion produces thixotropic or dilatant behavior that complicates pumping, atomization, and storage.

Low-shear mixers generate bulk flow but lack the localized energy density needed to break agglomerates, reduce particle size efficiently, and fully wet hydrophobic surfaces. The result is a slurry that may appear homogeneous immediately after mixing yet separates within hours or days.

How Kady Rotor-Stator Technology Produces Superior Stability

A Kady mill employs a high-speed rotor rotating inside a close-clearance stator. Tip speeds typically range from 5,000 to more than 10,000 feet per minute, creating an extremely high-energy dissipation zone. When coal particles and liquid (water, oil, or mixed media) enter this zone, multiple simultaneous mechanisms improve slurry quality:

• High-intensity impact and attrition – Coal particles collide repeatedly with rotor and stator surfaces, rapidly reducing oversized fractions and eliminating weak agglomerates.
• Extreme hydraulic shear – Material is forced through narrow gaps at very high velocity gradients, dispersing fine particles and promoting intimate contact between solid surfaces and the liquid phase.
• Cavitation and micro-turbulence – Localized pressure fluctuations generate and collapse vapor cavities, producing micro-jets that further disrupt particle clusters and enhance surface wetting.
• Continuous high-energy recirculation – The intense flow field surrounding the head repeatedly draws material through the dispersion zone until a uniform particle-size distribution and thorough wetting are achieved.

The combination of size reduction and surface activation produces a slurry in which inter-particle forces are more effectively managed. When appropriate dispersants or stabilizers are present, the resulting suspension exhibits markedly improved resistance to sedimentation and hard-packing.

Key Performance Advantages

• Higher achievable solids loading – Efficient particle-size reduction and wetting allow higher coal concentrations while maintaining pumpable viscosity.
• Narrower particle-size distribution – Controlled attrition reduces the fraction of both coarse settlable particles and excess ultra-fines that drive viscosity.
• Superior long-term stability – Thorough dispersion minimizes the formation of loosely bound agglomerates that act as nuclei for sedimentation.
• Improved rheology – More uniform suspensions typically display lower yield stress and more predictable shear-thinning behavior, facilitating pumping and atomization.
• Reduced additive demand – Better surface wetting and dispersion often allow lower dosages of costly dispersants and stabilizers.
• Faster processing cycles – Batch times measured in minutes rather than hours translate directly into higher throughput and lower energy consumption per ton of slurry.
• One-step capability – In many formulations the same mill performs both primary size reduction and final homogenization, eliminating intermediate grinding or multi-stage mixing steps.

Equipment Configurations for Coal Slurry Production

Kady mills are available in configurations matched to the scale and process requirements of coal-water or coal-oil slurry manufacture:

• Top-entry batch mills – Suitable for standard mixing vessels. The head can be positioned to optimize the draw of floating or partially settled coal.
• Bottom-entry mills – Particularly effective for high-solids systems where dense coal tends to settle. The dispersion head operates directly in the highest-concentration region.
• Continuous-flow mills – Enable staged or single-pass processing for high-volume production lines, integrating easily with upstream crushing or downstream storage and transfer systems.
• Abrasion-resistant construction – Specially heat-treated or hardened rotor-stator components and wear-resistant alloys extend service life when processing abrasive coal and mineral matter.
• Temperature and pressure options – Jacketed vessels, mechanical seals, and vapor-tight designs accommodate elevated temperatures, pressurized systems, or solvent-containing oil media.

Materials of construction, sealing systems, and drive packages are selected according to the specific liquid phase (water, heavy oil, light oil, or mixed media), solids loading, and operating temperature.

Process Considerations and Best Practices

Successful production of stable coal slurries typically involves attention to the following parameters:

• Feed particle size – While Kady mills can accept relatively coarse feed, starting with a controlled top size improves efficiency and reduces wear.
• Order of addition – Sequential addition of liquid, dispersant, and coal, combined with progressive increase in mill speed, often yields the most stable final product.
• Dispersant chemistry – Anionic, non-ionic, or polymeric dispersants can be optimized in conjunction with the high-shear environment to maximize steric or electrostatic stabilization.
• Residence time and tip speed – Laboratory or pilot trials establish the minimum energy input required to reach the target particle-size distribution and stability without generating excessive ultra-fines.
• Rheological monitoring – In-process viscosity and sedimentation tests confirm that the slurry meets pumpability and storage specifications.

Kady International offers laboratory and pilot-scale testing services to evaluate specific coal ranks, liquid media, and target solids loadings before full-scale equipment selection.

Applications of Stable Coal Slurries

Properly prepared coal-water and coal-oil slurries find use in:

• Pipeline transport of coal over long distances
• Direct combustion in boilers and industrial furnaces as a replacement for heavy fuel oil
• Feedstock preparation for coal gasification and liquefaction processes
• Specialty fuel formulations for marine and stationary power applications
• Process suspensions in mineral and chemical industries requiring high-solids coal dispersions

Why Specify a Kady Mill

The intensive, localized energy dissipation of a Kady rotor-stator mill addresses the fundamental physical barriers to stable coal slurry production—inadequate size reduction, incomplete wetting, and residual agglomerates. The result is a suspension with higher solids content, lower sedimentation rates, and more predictable handling characteristics than can be achieved with conventional agitation equipment.

Kady International has designed and manufactured high-speed dispersion mills in Scarborough, Maine since 1949. Our equipment operates in more than forty countries across energy, chemical, and process industries. Whether the requirement is laboratory development, pilot-scale production, or full commercial slurry manufacturing, a properly configured Kady system can deliver coal-water and coal-oil slurries with superior stability.

Need a stable, high-solids coal slurry? Contact Kady International to discuss your coal type, liquid medium, solids target, and production volume. Our engineering team will recommend the optimal mill configuration and process approach.

Kady International
30 Parkway Drive
Scarborough, ME 04074
Phone: 207.883.4141
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KADY® high-speed rotor-stator dispersion mills – mixing, dispersing, blending, emulsifying, and homogenizing solutions for demanding high-solids fluid/solid applications.