How to Control Crystal Size Distribution
In lithium refining and other specialty chemical processes, the ability to control crystal size distribution (CSD) defines both the quality of the product and the efficiency of downstream operations. Uniform, predictable crystals improve filtration, washing, and drying — directly influencing yield, purity, and product handling. Within Whiting Equipment Canada’s technology portfolio and its licensed Swenson crystallization systems, CSD control is one of the most technically demanding aspects of producing battery-grade lithium.
CSD Basics
Crystal size distribution refers to the range and uniformity of crystal sizes produced during crystallization. It directly reflects how supersaturation, mixing, seeding, and residence time interact within the crystallizer.
A narrow CSD — where most crystals fall within a defined size range — helps ensure consistent purity and easier solid-liquid separation, while minimizing fine losses and agglomeration. Broader distributions may indicate poor control of nucleation or growth kinetics, leading to variable particle performance and potential rework.
In lithium systems, CSD is tightly linked to the final morphology of lithium carbonate (Li₂CO₃) or lithium hydroxide monohydrate (LiOH·H₂O) crystals. These products must meet stringent battery-grade specifications, often exceeding 99.5% purity, with defined particle size ranges that balance flowability, reactivity, and surface area during downstream cathode production.
Agitation & Seeding
Agitation provides the hydrodynamic environment that governs crystal suspension, contact frequency, and mass transfer. Insufficient mixing can lead to localized supersaturation spikes, resulting in excessive fines; excessive shear can break crystals or induce secondary nucleation.
Swenson’s crystallizers, such as draft-tube-baffle (DTB) and forced-circulation designs, manage this balance by combining high internal circulation rates with low localized shear zones. The DTB’s central draft tube and baffle configuration promote uniform supersaturation and controlled crystal growth, yielding a narrower size distribution without excessive fines.
Seeding, meanwhile, is the operator’s most direct method for defining CSD. Introducing seed crystals of known size and quantity at controlled supersaturation stabilizes nucleation and creates a foundation for uniform growth. In industrial practice, the seed load (typically 2–10% of the slurry mass) and seed size are adjusted to meet the product specification: fine seeds yield small, uniform crystals, while coarse seeds drive larger particle growth with reduced fines formation.
Cooling and Evaporation Profiles (Supersaturation Control)
Crystallization is driven by supersaturation—the thermodynamic condition where the solution holds more solute than it can stably contain. Controlling the rate at which supersaturation develops determines whether the process favors nucleation (the formation of many small crystals) or growth (the formation of fewer large crystals).
Two main strategies dominate lithium refining systems:
- Cooling Crystallization – Used when solubility decreases sharply with temperature. Gradual cooling (1–2 °C/min) promotes uniform growth, while rapid cooling induces excessive nucleation. Jacketed or multi-zone cooling circuits in Swenson designs provide precise temperature control, allowing for a more effective management of this trade-off.
- Evaporative Crystallization – Used when solubility depends more on solvent removal than temperature. Here, evaporators (forced-circulation or falling-film) regulate the evaporation rate to maintain stable supersaturation levels. High-efficiency heat exchangers and vapor recompression can fine-tune solvent removal to avoid runaway nucleation events.
Effective supersaturation control is achieved by balancing solvent removal, temperature change, and agitation intensity, all of which are coordinated by advanced control systems to maintain operation near the metastable limit—where crystals grow but do not spontaneously nucleate.
Instrumentation
Monitoring and maintaining CSD in real time requires precise analytical tools. Two widely used instruments in crystallization systems are:
- Focused Beam Reflectance Measurement (FBRM): Provides real-time chord length distribution data, which correlates closely with particle size. It enables operators to detect changes in nucleation or growth rates and adjust parameters, such as temperature ramps or agitator speeds, instantly.
- Particle Vision & Measurement (PVM): Captures in-situ microscopic images of crystals, giving direct insight into crystal habit, agglomeration, and breakage. When paired with FBRM, PVM helps confirm whether changes in the signal represent genuine growth or variations in particle shape.
Whiting-integrated control architectures (PLC/SCADA platforms) can accommodate these measurement feeds, enabling automated feedback control in which supersaturation and agitation are adjusted dynamically to maintain CSD within design targets.
QA & Analytics
Quality assurance for CSD encompasses the entire process, from lab validation to full-scale production.
- Bench Testing: Early trials in glassware determine yield, purity, and morphology relationships under varied supersaturation conditions.
- Pilot Testing: Provides quantitative data on washing requirements, centrifuge performance, and scaling tendencies, validating how CSD affects process stability.
- Full-Scale QA: Inline sensors and historical data trends ensure long-term reproducibility. Operators can benchmark real-time CSD curves against pilot baselines to confirm product consistency.
This layered testing strategy not only stabilizes quality but also shortens commissioning timelines for new crystallization trains.
Spec Targets by Industry
Each downstream application sets its own CSD specification envelope:
- Battery-grade lithium carbonate: 100–250 µm average particle size, low fines (<10%), smooth morphology for consistent cathode mixing.
- Industrial lithium hydroxide: Coarser crystals (up to 400 µm) with controlled porosity for reactivity in grease and polymer applications.
- Specialty chemicals: Tailored habits (needle- or plate-like) achieved via solvent or additive control.
Meeting these specifications consistently demands not only robust crystallizer design but also precise supersaturation control, agitation tuning, and real-time analytics — areas where Whiting and Swenson’s combined expertise excels.
FBRM/PVM Calibration and Case Learnings
Routine calibration of FBRM and PVM systems anchors their reliability. Using reference slurries or known particle standards ensures that signal drift does not skew process feedback. In practice, continuous data trending enables operators to predict deviations before they impact purity or yield.
Case studies from lithium and chemical refining operations show that integrating inline particle measurement with temperature-controlled supersaturation profiles significantly reduces off-spec production, improving throughput and product consistency.
Conclusion
Controlling crystal size distribution is both a science and an art — one that combines thermodynamics, hydrodynamics, and data analytics to achieve optimal results. For battery-grade lithium, mastering this control is crucial to competitiveness. Through Whiting Equipment Canada’s engineering expertise and Swenson Technology’s crystallization systems, producers gain the precision to stabilize supersaturation, shape crystal morphology, and deliver consistent, high-purity products that meet global energy-storage standards.
Partner with Whiting Equipment Canada to design or optimize your crystallization train. From pilot testing to full-scale automation, Whiting’s engineering team helps you balance supersaturation, agitation, and analytics to achieve tighter CSD control, higher yields, and faster QA validation — essential advantages for any lithium refinery. Contact us to discuss your requirements.
