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Preventing Kaolin Slurry Sedimentation: A Guide to Selecting the Right Suspension Aids
For ceramic body applications, Goway ceramic deflocculants (FG-series deflocculants) and Sodium Tripolyphosphate (STPP, FG-1003) offer dispersant-driven sedimentation control as part of broader slurry stabilization.
Key Takeaways
- Sedimentation is a Stokes' Law problem: settling velocity is proportional to the square of particle radius and the density difference between particle and liquid — and inversely proportional to medium viscosity. Reducing effective particle size (via dispersion) or increasing medium viscosity are the primary intervention points.
- Kaolin surface chemistry matters: Goway FG-K90 kaolin carries Al₂O₃ 35.5% and Fe₂O₃ 0.45% (Source: Goway Technical Data Sheet) — its specific surface charge characteristics determine the minimum effective dispersant dosage. Ball clays like FG-B82 (Fe₂O₃ 1%) may require different stabilization strategies due to higher charge heterogeneity.
- STPP is effective but sensitive: STPP (FG-1003: Na₅P₃O₁₀ 94%, pH 8.0–9.0) provides good electrostatic stabilization in clean water conditions but loses efficiency in the presence of Ca²⁺ and Mg²⁺ ions (Source: Goway Technical Data Sheet).
- Polymer-type aids offer additional steric protection: polyacrylate-based suspension aids adsorb onto clay surfaces and create a physical barrier against re-flocculation, particularly useful for long-duration storage or high-solid-content systems. Goway ceramic deflocculants with higher SiO₂ content (such as FG-MK03: SiO₂ 20–22%) may contribute to steric effects in addition to electrostatic repulsion (Source: Goway Technical Data Sheet).
- Thixotropic modifiers are a separate tool: products like xanthan gum and attapulgite prevent sedimentation by creating a weak gel structure at rest — not by dispersing particles. This mechanism is complementary to, not a substitute for, proper deflocculation.
Section 1: Why Kaolin Slurry Settles — The Physics of Sedimentation
Kaolin slurry sedimentation is fundamentally governed by Stokes' Law, which describes the terminal settling velocity of a spherical particle in a viscous medium:
Where: v = settling velocity (m/s) r = particle radius (m) ρp = particle density (kg/m³) — kaolin: approx. 2,600 kg/m³ (industry reference value) ρl = liquid density (kg/m³) — water: approx. 1,000 kg/m³ g = gravitational acceleration (9.81 m/s²) η = dynamic viscosity of the medium (Pa·s)
Note: Kaolin particles are platy, not spherical. Stokes' Law provides an approximation framework; actual settling behavior of platy particles involves shape factors.
The Four Variables You Can Influence
In practice, this means sedimentation control strategies focus on two levers: preventing floc formation (keeping r small via dispersants) and increasing effective viscosity at rest (via thixotropic modifiers). These two approaches are not mutually exclusive and are often combined in production.
Section 2: Kaolin Properties That Affect Sedimentation Risk
Kaolin is not a uniform mineral — different sources and processing grades present significantly different sedimentation challenges. The following parameters from Goway's kaolin products illustrate the key variables that influence slurry stability:
| Product Code | Sub-Category | Whiteness (1200°C) |
Al₂O₃ (%) | Fe₂O₃ (%) | TiO₂ (%) | K₂O (%) | L.O.I (%) | Sedimentation Risk Notes |
|---|---|---|---|---|---|---|---|---|
| FG-K90 | Kaolin Clay | 90.0 | 35.5 | 0.45 | 0.09 | 1.08 | 13.2 | High Al₂O₃ → higher surface charge density; lower Fe₂O₃ → reduced charge heterogeneity; typically exhibits more predictable dispersion response |
| FG-K86 | Kaolin Clay | 86.8 | 33.71 | 0.43 | 0.02 | 3.17 | 11.35 | Higher K₂O (3.17%) may indicate feldspar contamination; mixed mineralogy can increase optimal dispersant dosage vs. pure kaolin systems |
| FG-B88 | Ball Clay | 88.0 | 30.5 | 0.5 | 0.03 | 1.1 | 11.8 | Lower Al₂O₃ relative to kaolin; higher organic content (typical of ball clay) increases LOI and may cause foaming; organic matter complicates ionic dispersant response |
| FG-B82 | Ball Clay | 75.0 | 32.5 | 1.0 | 0.2 | 2.1 | 12.5 | Higher Fe₂O₃ (1.0%) and TiO₂ (0.2%) indicate more complex charge surface; greater coloring oxide content increases surface charge variability and may reduce deflocculation efficiency per unit dispersant |
| Source: All parameters from Goway Technical Data Sheet (v2.1). Sedimentation risk notes are qualitative assessments based on known mineral chemistry relationships; plant-specific behavior requires laboratory verification. | ||||||||
The Specific Surface Area Factor
Particle size — and consequently specific surface area (SSA) — is the dominant variable in sedimentation behavior that is NOT captured by oxide composition alone. Finer kaolin grades carry higher SSA, which simultaneously:
- Slows settling (smaller r per Stokes' Law)
- Requires more dispersant to achieve full surface coverage
- Creates more complex floc structures when under-dispersed (platy particles stack in face-to-face and edge-to-face configurations)
D50 and D90 values are not included in the current v2.1 TDS. For kaolin grades where SSA is a primary design parameter, request the particle size distribution report from the Goway product team alongside the chemical composition data.
Section 3: How Suspension Aids Work — Three Stabilization Mechanisms
Dispersant molecules (phosphates, polyacrylates, silicates) adsorb onto the kaolin particle surface and introduce negative surface charges. When two particles approach each other, overlapping electrical double layers generate a repulsive force that opposes flocculation.
Key concept: Zeta potential — the electrical potential at the slipping plane of the double layer — serves as a proxy for electrostatic stability. More negative Zeta potential (typically below −30 mV) indicates stronger repulsive force. (Reference: colloidal chemistry literature, DLVO theory)
Limitation: Sensitive to pH and multivalent cations (Ca²⁺, Mg²⁺). Divalent ions compress the double layer, reducing repulsive range — a critical consideration when process water is hard.
Polymer-type suspension aids (polyacrylates, polycarboxylates) anchor onto particle surfaces and extend hydrophilic polymer chains into the solution. When particles approach, overlapping polymer layers create an osmotic pressure increase that pushes particles apart.
Key advantage: Steric stabilization is much less sensitive to ionic strength and pH than pure electrostatic repulsion. This makes polymer-type aids more robust in high-salinity or variable water quality conditions.
Limitation: Higher cost per unit than inorganic electrolytes; may introduce organic load (relevant for body composition control). Adsorption reversibility varies by polymer architecture.
Viscosity modifiers (xanthan gum, attapulgite, CMC) build a weak gel network in the slurry at rest. This network physically suspends particles by providing a yield stress — a minimum force required to initiate flow — preventing particles from settling through the medium under gravity alone.
Key advantage: Highly effective for long-duration static storage (24–72 hours). Does not depend on particle surface chemistry — works regardless of ionic conditions.
Limitation: Must be broken down before processing — requires mechanical agitation or a defined re-dispersion step. If the gel structure is too strong, it can cause pump cavitation or poor flow into the ball mill. Balance between stability and processability is critical.
Section 4: Four Types of Suspension Aids — Comparative Overview
| Type | Primary Mechanism | Key Advantages | Key Limitations | Best Fit Scenario | Indicative Dosage Range |
|---|---|---|---|---|---|
| Inorganic Electrolytes (STPP, SHMP, Sodium Silicate) |
Electrostatic repulsion; some chelation (SHMP) | Low cost; well-documented in ceramic industry; SHMP effective for Ca²⁺/Mg²⁺ sequestration | Sensitive to hard water; STPP hydrolyzes over time; limited steric protection | Soft-water systems; standard kaolin with low coloring oxide content; short-to-medium storage | 0.1–0.5% by dry weight (industry reference; requires plant-specific verification) |
| Ceramic Deflocculants (Polysilicate / mixed ionic type, e.g., Goway FG-series) |
Electrostatic + partial steric (from SiO₂ component) | Broader ionic tolerance vs. pure STPP; formulated for ceramic body pH range; dual-mechanism action from NaO/SiO₂ balance | Specific performance data for sedimentation prevention not published in standard TDS — requires application trial; higher cost than commodity STPP | Ceramic body slurry with moderate water hardness; systems requiring both deflocculation and improved ionic tolerance; formulations with mixed clay types | Refer to Goway product-specific starting dosage guidance; 5-point dosage curve test recommended |
| Synthetic Organic Polymers (Polyacrylate, Polycarboxylate) |
Steric stabilization (primary); electrosteric (combined) | Strong performance in hard-water and high-ion-strength systems; robust at variable pH; longer effective duration | Higher unit cost; organic load introduced; potential foaming at high dosage; molecular weight selection critical | High-solid-content slurry (>65% w/w); long storage requirements (>24 hours); hard-water or recycled-water systems | 0.05–0.3% by dry weight (typical for ceramic applications; source: general polymer chemistry literature) |
| Natural Biopolymers / Viscosity Modifiers (Xanthan Gum, CMC, Attapulgite) |
Thixotropic gel network formation; viscosity increase | Effective for very long-duration static storage; ionic condition independent; attapulgite is mineral-based (no organic load) | Requires re-dispersion agitation before processing; adds viscosity that may conflict with spray dryer or pump requirements; cost of xanthan gum per unit is relatively high | Long-duration static storage (>48 hours); intermittent production with extended shutdown periods; pipeline transport | Xanthan gum: 0.01–0.05% w/w (typical use in industrial suspensions); CMC: 0.05–0.2% w/w. (Industry reference values; plant verification required) |
| Note: All dosage ranges cited above are from general industrial and ceramic literature references and represent typical industry starting points. Actual optimal dosage for your specific system must be established through laboratory testing. Combining mechanism types (e.g., electrostatic dispersant + low-level thixotropic modifier) is a common industrial approach for optimizing the stability-processability balance. Data not verified against Goway TDS unless explicitly noted. | |||||
Section 5: Goway Dispersant Products — TDS Parameters Relevant to Slurry Stabilization
Goway's ceramic deflocculant and STPP product lines are primarily formulated for ceramic body deflocculation (viscosity reduction). Their sedimentation prevention function is a secondary benefit of their dispersant action — they reduce flocculation tendency by enhancing electrostatic or electrosteric repulsion between kaolin particles. The following TDS data is from the v2.1 product database.
5.1 Ceramic Deflocculants (FG-series)
The NaO:SiO₂ ratio in each product determines the balance between purely ionic (NaO-driven) and partially steric (SiO₂-driven) stabilization mechanisms:
5.2 STPP Products (FG-1003 as Primary Grade)
STPP provides electrostatic sedimentation control through phosphate adsorption onto clay surfaces. Its effectiveness is well-documented in ceramic literature but is subject to hard-water limitations.
| Parameter | Value | Relevance to Sedimentation Control |
|---|---|---|
| Na₅P₃O₁₀ (%) | 94 | High active content → efficient phosphate dosing per unit weight |
| P₂O₅ (%) | 56 | Phosphate availability for clay surface adsorption |
| Fe₂O₃ (%) | 0.015 | Low iron → minimal discoloration risk in high-whiteness kaolin bodies |
| pH (1% solution) | 8.0–9.0 | Mildly alkaline — compatible with typical ceramic body pH range; less aggressive than higher-pH phosphate grades |
| Insoluble Matter (%) | 0.1 | Low insoluble content → clean dispersion without introducing contamination |
| Source: All values from Goway Technical Data Sheet (FG-1003). Hard-water limitation (Ca²⁺/Mg²⁺ sensitivity) is a well-documented characteristic of sodium tripolyphosphate in ceramic literature — see our STPP vs Deflocculant comparison guide for detailed evaluation. | ||
Section 6: Selection Matrix — Matching Suspension Aid Type to Your Conditions
Use the following decision framework to identify the most appropriate suspension aid category for your kaolin slurry system. Final selection and dosage must be confirmed through laboratory testing.
Section 7: Lab Trial Protocol — Sedimentation Column Test
The sedimentation column test is the simplest and most direct method for quantifying kaolin slurry sedimentation behavior. The following protocol is based on standard laboratory practice in slurry stability evaluation.
Equipment Required
- Graduated glass cylinders (250 mL or 500 mL), minimum 3 sets for simultaneous comparison
- Precision balance (±0.01 g)
- pH meter and conductivity meter (for water quality characterization)
- Ford Cup No. 4 (4 mm nozzle) or equivalent rotational viscometer
- Timer
- Temperature-controlled environment (test at process temperature, typically 20–25°C)
Step-by-Step Protocol
- Characterize Your Kaolin Slurry Baseline
Prepare slurry at your standard solid content (note: % by weight vs. by volume). Measure: pH, conductivity (as proxy for ionic strength), Ford Cup flow time (4 mm), and visual appearance. Allow to settle without agitation for 1 hour and note any visible sedimentation line. This is your zero-aid baseline. - Prepare Candidate Aid Solutions
Prepare stock solutions of each suspension aid at 5–10× target use concentration (to avoid diluting the slurry significantly during addition). Prepare minimum 3 dosage levels per candidate: low, mid, high (based on supplier-recommended range or, if unavailable, 0.05%, 0.15%, 0.30% by dry weight as starting points). - Run Parallel Sedimentation Columns
Label 3+ graduated cylinders per candidate. Add aid solution to reach target dosage in each cylinder. Bring slurry to uniform dispersion with agitation (e.g., 2 minutes hand stirring or laboratory mixer at constant speed). Fill each cylinder to the same volume. Record time = 0 and allow to stand undisturbed at room temperature. - Record Sedimentation Height Over Time
Record the height of the clear supernatant layer (in mm) at: 30 min, 1 hr, 2 hr, 4 hr, 8 hr, 24 hr (and 48/72 hr if relevant to your storage window). Plot sedimentation height vs. time for each condition. - Evaluate Sediment Character
At end of test period, invert the cylinder gently and observe: does the sediment re-suspend easily (soft, re-dispersible sediment — acceptable) or does it form a hard, compact cake (hard sedimentation — problematic for pump re-start)? Record qualitatively as soft / medium / hard. - Confirm Slurry Processability
Re-agitate the cylinder after the sedimentation test period. Measure Ford Cup flow time again. Confirm the slurry returns to the target viscosity window. An aid that prevents sedimentation but also permanently increases viscosity or prevents re-dispersion is not suitable for most ceramic processes. - Run Dosage Optimization
For the best-performing candidate(s), run a narrower 5-point dosage curve around the optimal range identified in step 3–5. Identify the minimum effective dosage that achieves your stability target — this is your starting dosage recommendation for plant trial.
Section 8: Dosing Framework
There is no universal "correct dosage" for suspension aids — the optimal dosage depends on the specific kaolin grade, solid content, process water quality, and target stability window. The following framework applies to all product types:
At each point, measure: → Ford Cup flow time (4mm nozzle, target range depends on process — typically 30–60 sec for spray dryer feed) → Sedimentation column height at target stability window (e.g., 8 hours) → pH (confirm no adverse pH shift from aid addition)
Optimal dosage = lowest D-point that achieves: (a) Flow time within target range AND (b) Sedimentation height below acceptance threshold
Note: "More is not better." Over-dosing electrolytic dispersants can cause re-flocculation (electrolyte-induced coagulation at very high ionic strength). Polymer aids typically show a plateau effect above the optimal dosage.
Solid Content — Dosage Relationship
As solid content increases, total kaolin surface area per unit volume of slurry increases proportionally. In principle, dispersant dosage (expressed as % of dry kaolin weight) should be maintained roughly constant as solid content changes — but verify this relationship empirically for your specific kaolin-aid combination, as adsorption behavior is not always linear across solid content ranges.
Section 9: Troubleshooting Common Sedimentation Problems
Most likely cause: Under-dosing of dispersant — particles are transiently dispersed by mechanical energy but have insufficient repulsive force to remain stable at rest.
Suggested action: Run 5-point dosage curve. If increasing dosage shows diminishing returns, consider switching from STPP to a ceramic deflocculant with SiO₂ component for enhanced stability. Also measure process water conductivity — high conductivity may indicate hard water that compresses the double layer.
Most likely cause: Over-dosing of a thixotropic modifier (xanthan gum / CMC / attapulgite), or incorrect product type selected (viscosity modifier should be used at low level only as a supplement, not as primary dispersant).
Suggested action: Reduce thixotropic modifier dosage to sub-0.02% range; ensure primary electrostatic/steric dispersant is present at effective level; confirm Ford Cup readings at operating temperature. See our ceramic slurry viscosity reduction guide for viscosity optimization methodology.
Most likely cause: Complete flocculation has occurred (particles have settled and formed strongly bonded aggregates), likely due to severe under-dosing of dispersant OR high-ionic-strength process water overwhelming the dispersant.
Suggested action: Test process water for Ca²⁺/Mg²⁺ concentration. If hard water is confirmed, evaluate SHMP or a ceramic deflocculant with phosphate chelating component. Consider water treatment at the process water inlet. Mechanical pre-treatment (controlled agitation cycle before pump restart) can help re-disperse soft compacted sediments but does not address root cause.
Most likely cause: Variability in kaolin batch composition (common with natural mineral raw materials), process water quality changes (seasonal variation in municipal water hardness), or recycled material contamination in feed.
Suggested action: Implement incoming QC for kaolin (L.O.I, Fe₂O₃, Al₂O₃) to detect batch-to-batch variation. Monitor process water conductivity daily. Maintain a ±10% dosage adjustment buffer around the nominal optimum dosage. Review the STPP vs deflocculant guide for diagnostic criteria to identify when a product switch is warranted.
Most likely cause: Scale-up effects — lab test is conducted with fresh mixing at controlled temperature; production tank may have localized dead zones, temperature gradients, or longer elapsed time between mixing and stability measurement. Dosage calculated as % of dry kaolin may not have been correctly scaled to production batch volume.
Suggested action: Verify that production dosage calculation converts lab optimum (% dry kaolin) correctly to production batch volume. Confirm tank mixing uniformity (no dead zones near tank base). Repeat lab test at production temperature. Run a dedicated small-scale production trial (one tank) before full-line adoption.
FAQ
This is counterintuitive: higher solid content typically increases slurry viscosity, which per Stokes' Law should slow settling. However, if dispersant dosage is not proportionally increased, the fraction of particle surfaces that are adequately covered decreases, reducing net repulsive force per unit of slurry volume. Flocculation becomes more likely, and flocs — which have larger effective diameters — settle faster than individual particles, even in a more viscous medium. The solution is to maintain an appropriate dispersant-to-surface-area ratio rather than a fixed dosage per unit volume.
Sodium Tripolyphosphate (STPP) is widely used as a dispersant in ceramic body slurry and can contribute to sedimentation prevention by reducing particle flocculation. However, STPP is sensitive to multivalent cations such as Ca²⁺ and Mg²⁺. In hard-water or high-impurity systems, phosphate ions may preferentially react with Ca²⁺ rather than adsorbing on clay surfaces, reducing effective coverage. A ceramic deflocculant with broader ionic tolerance may be more appropriate in such conditions. See our STPP vs Deflocculant comparison guide for detailed evaluation criteria.
Required stability duration depends on your specific production cycle. Most tile plants design for a minimum of 8–24 hours of static stability. Some operations with weekend shutdowns may require 48–72 hours of stability without mechanical agitation. Define your required stability window based on your actual production schedule before selecting and dosing the suspension aid. Laboratory sedimentation column tests at the target stability window are recommended for pre-qualification.
Most suspension aids burn off during the firing cycle. For organic polymers (polyacrylate or xanthan gum), residues are typically eliminated well below standard tile firing temperatures (above 900°C). The inorganic components of silicate-based deflocculants (sodium content) may contribute to the Na₂O level of the fired body — generally not a concern at typical dosage levels, but should be verified against your body composition targets if tight oxide control is required. Laboratory trial firings are recommended before full-scale adoption.
Starting dosage depends on the specific product and the kaolin type. A five-point dosage curve test is the recommended approach: start at the lower range of the product's recommended dosage and measure flow time and sedimentation column behavior at five incrementally higher dosage levels. Actual optimal dosage depends on kaolin surface area, solid content, process water quality, and target stability window. Contact Goway for product-specific starting dosage guidance based on your kaolin specification.
Technical Notes & Data Sourcing
- P1 Data (Goway TDS v2.1): All FG-series deflocculant parameters (NaO, SiO₂, P₂O₅, L.O.I) and kaolin product parameters (Al₂O₃, Fe₂O₃, TiO₂, etc.) are from the Goway v2.1 product database sourced from official product data sheets.
- Industry Reference Data: Stokes' Law parameters (kaolin density ≈ 2,600 kg/m³), STPP/SHMP mechanism descriptions, polyacrylate steric stabilization mechanisms, and xanthan gum dosage ranges are based on established colloidal chemistry literature and general ceramic industry practice — these are not Goway-specific performance claims.
- Data Gap — Suspension Aid Performance: Goway's current v2.1 TDS does not include specific sedimentation prevention performance data (e.g., measured sedimentation rate, critical dosage for stability at defined solid content). Mechanism descriptions for FG-series products are qualitative assessments derived from composition data. Contact Goway for application-specific testing.
- Prohibited Claims: This article does not state specific percentage improvements in sedimentation stability for any product without verified supporting data. Dosage ranges for non-Goway product types (xanthan gum, CMC, polyacrylate) are cited as general industry reference values and require plant-specific verification.
- Disclaimer: Final parameters should be verified against the latest batch COA. Laboratory trials are recommended before full-scale application. Actual suspension aid performance depends on kaolin mineralogy, particle size distribution, solid content, process water quality, temperature, and storage conditions.
Get a Kaolin Slurry Stability Solution for Your Specific Conditions
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Providing this information enables Goway to suggest the most relevant product and trial protocol for your system.
Submit for Free Technical ConsultationRelated Resources
- Goway Ceramic Deflocculant Product Portfolio — FG-2017, FG-MK03, FG-N203B, FG-SL01A specifications
- Goway STPP Products — FG-1003 and full-range STPP specifications
- STPP vs Ceramic Deflocculant: A Data-Driven Selection Guide — cost, performance and switching protocol
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