Boehmite Digestion Kinetics in High-Alumina Bayer Liquors

In the context of Bayer process intensification for diasporic and monohydrate bauxites, the digestion of boehmite (γ-AlOOH) in high-alumina liquors represents a kinetic regime where mass transfer limitations, caustic depletion, and alumina activity coefficients deviate markedly from the ideal behavior assumed in classical shrinking-core models. Industrial practice at refineries processing Weipa, Gove, or Sangaredi ores has shown that when the liquor alumina-to-caustic weight ratio (A/C) exceeds 0.65, the apparent activation energy for boehmite dissolution shifts from a surface-reaction-controlled value of approximately 85–95 kJ·mol⁻¹ to a mixed-control regime with values as low as 40–55 kJ·mol⁻¹, indicative of pore diffusion limitations within the hydrate layer. This transition is accompanied by a measurable increase in the sensitivity of the digestion rate to free caustic concentration, quantified by a reaction order with respect to NaOH activity that rises from 1.2 to 1.8 as the molar ratio Na₂Oₜₒₜ/Al₂O₃ in the liquor falls below 1.4. Tube digester operations at 255–265 °C with residence times of 8–15 minutes experience instability in specific throughput when the boehmite fraction in the feed exceeds 35% of total alumina, requiring adjustment of the blow-off flash sequence to maintain liquor supersaturation below the critical nucleation threshold of 1.08 relative supersaturation ratio, per measurements using in-line thermometric titration apparatus calibrated against lithium metaborate fusion reference samples.

What Operational Parameters Shift the Rate-Determining Step in Boehmite Attack?

The kinetics of boehmite digestion in concentrated sodium aluminate solutions are frequently described by the rate equation −d[AlOOH]/dt = k₀·exp(−Ea/RT)·(C_NaOH − C_NaOH,equil)^n·(S/V)^m, where the pre-exponential factor k₀ and the orders n and m are conditional upon the liquor’s total soda concentration and organic carbon burden. Using a continuously operated 45 m³ horizontal autoclave train with five compartments at the Dazun refinery (Shandong, China), plant trials documented that raising the free caustic concentration from 180 g/L to 235 g/L Na₂Oₖₒ while holding the dissolved alumina at 145 g/L increased the boehmite conversion after 12 minutes from 78% to 94%, but at the cost of elevating the liquor’s equilibrium boehmite solubility such that the driving force for precipitation in the subsequent security filtration step narrowed to 3.2 g/L Al₂O₃, dangerously close to the self-precipitation limit of 1.5–2.0 g/L observed at that temperature. Under these high-liquor-alumina conditions, the rate-determining step transitions from the chemical reaction at the boehmite–liquor interface to the outward diffusion of aluminate ions through a coherent layer of secondary boehmite needles that form epitaxially on the surface of partially leached particles, a phenomenon confirmed by scanning electron micrographs of quenched solids taken from the interstage sampling valves of the digestion unit. The effective diffusivity of Al(OH)₄⁻ species through this layer was estimated at 8.2 × 10⁻¹¹ m²/s at 260 °C, based on fitting the Ginstling–Brounshtein diffusion model to the tail of the extraction curve where conversion exceeded 85%.

Addition of lime during boehmite digestion—typically in the form of finely ground quicklime (CaO) with a particle size d₉₀ < 75 µm and a reactivity measured by the ASTM C110-20 slaking rate—acts to sequester phosphate and vanadate impurities but also catalyses the dissolution of boehmite through formation of calcium hydroaluminates such as tricalcium aluminate hexahydrate (TCA, 3CaO·Al₂O₃·6H₂O). The catalytic effect becomes kinetically significant only at lime dosages above 2.5% of dry bauxite mass, as documented in a series of batch autoclave tests conforming to the ISO 6130:2020 procedure for aluminium ores, where the time required to reach 98% extraction of boehmite from a Boké bauxite blend was reduced from 28 minutes to 19 minutes at a digestion temperature of 250 °C. However, the concurrent increase in caustic consumption due to desilication product precipitation—incrementally 4.2 kg Na₂CO₃ per metric ton of alumina for each additional 0.5 wt% of reactive silica—places a practical upper bound on lime addition that diminishes as the reactive silica content of the bauxite surpasses 3.5%. This trade-off is most acute when processing mixed gibbsite–boehmite ores where the silica mineralogy includes chamosite, which dissolves incongruently and releases iron into the liquor, depressing the activity of free OH⁻ by forming ferrate complexes detectable by an increase in the liquor’s magnetic susceptibility above 4.2 × 10⁻⁶ emu/g.

Temperature profiles along the digestion train reveal a counterintuitive sensitivity to the inlet slurry’s solids concentration when boehmite is the reactive phase. In a single-stream facility with a nominal throughput of 7,200 kg dry bauxite per hour, the heat transfer coefficient in the shell-and-tube preheaters dropped from 1,350 W/m²·K to 890 W/m²·K as the non-Newtonian slurry viscosity exceeded 0.35 Pa·s at a shear rate of 100 s⁻¹, a condition triggered by the high yield stress of red mud suspended in liquors with A/C > 0.64. The resulting lag in heating rate—averaging 3.8 °C/min across the 150–220 °C interval instead of the design rate of 5.2 °C/min—extended the effective preheating time by 4.5 minutes and caused a fraction of the boehmite to undergo in situ conversion to α-alumina monohydrate (diaspore) nuclei in local hot spots on the tube wall where the wall surface temperature exceeded the bulk fluid temperature by more than 18 °C. These diaspore seed crystals, once formed at a wall temperature of 270 °C or above, act as passive sinks that reduce the overall extraction by 1.5–2.2% absolute because diaspore does not dissolve appreciably in the available residence time.

Calcium Hydroaluminate Precipitation and Its Retarding Effect on Gibbsite Co-Precipitation Kinetics

In high-alumina circuits where the molar Na₂O/Al₂O₃ ratio in the pregnant liquor leaving digestion is deliberately maintained at or below 1.38 to maximize steam economy in evaporation, the metastable zone width for gibbsite precipitation contracts sharply, and any intentional or unintentional seeding of the liquor with calcium hydroaluminate solids from upstream lime addition can trigger premature nucleation. Measurements using focused beam reflectance microscopy (FBRM) probes installed in the overflow of the primary thickener at the Alunorte refinery (Barcarena, Brazil) indicated that when the liquor alumina concentration reached 168 g/L at 105 °C, the chord count of particles in the 5–20 µm range doubled within 22 minutes of exposure to fine TCA particles, corresponding to a secondary nucleation rate of 1.4 × 10⁹ particles per liter per minute. This level of nuclei generation shifted the particle size distribution of the precipitated gibbsite such that the D(50) fell from 78 µm to 44 µm after a 4-hour residence time in the agglomeration stage of a standard European-type precipitation circuit, violating the minimum median particle size of 65 µm required by the calciner feeding specification to avoid excessive dusting in the electrostatic precipitators. The observed kinetic retarding effect on the boehmite dissolution rate arises because the precipitation of fine gibbsite on the boehmite particle surface creates a mixed-product layer through which aluminate ions must diffuse, the overall diffusion coefficient decreasing by a factor of 3.8 compared to a pure boehmite residue layer.

Operational countermeasures against this mechanism include the deliberate elevation of the digestion blow-off temperature by 4–6 °C to re-dissolve any newly precipitated gibbsite before it can serve as a nuclei population, a strategy validated by mass balance analysis across the flash vessels at the San Ciprian refinery (Spain). Pulp quench experiments, in which samples of digested slurry were drawn through a rapid-cooling calorimeter at a controlled rate of 30 °C/s, revealed that the crystallization of gibbsite on boehmite surfaces begins at a temperature of 138 °C when the liquor A/C ratio is 0.68, but this onset temperature drops to 121 °C when the A/C ratio increases to 0.72, giving operators a narrow window of less than 8 °C in which to adjust flash vessel pressure without triggering massive nucleation. Maintaining the flash train pressure at a constant 0.85 MPa(g) rather than following the temperature-compensation curve used for low-alumina operations has been found to retard the incidence of gibbsite scaling on the tube side of the shell-and-tube heat recovery exchangers, extending run lengths between descaling outages from 42 days to 61 days.

Where the bauxite feed contains both boehmite and kaolinite in a ratio exceeding 4:1, the digestion silica chemistry diverges from the well-documented sodalite formation pathway and instead produces cancrinite-type zeolites that incorporate calcium and sulfate. These cancrinite phases, specifically the sulfate-rich vishnevite variety with the approximate formula Na₆Ca₂[(AlSiO₄)₆](SO₄,CO₃)₂·2H₂O, precipitate preferentially on the surface of boehmite particles when the sulfate ion concentration in the liquor exceeds 12 g/L as SO₄²⁻. The deposition rate, derived from periodic weight-gain measurements on polished boehmite tiles suspended in an operating digester at 255 °C, was constant at 0.18 g/m²·h under normal operation but accelerated to 0.53 g/m²·h when the sulfate concentration spiked to 18 g/L following the use of sulfur-containing flocculants in the residue washing circuit. The consequent passivation layer reduced the effective boehmite dissolution rate by a factor of 2.7, as determined by back-calculation of the Sherwood number from the observed extraction loss across the digestion train.

When Pregnant Liquor Contains Over 5 g/L Organic Carbon

The accumulation of organic carbon in high-alumina Bayer liquors—predominantly as sodium oxalate, sodium succinate, and humate degradation products—exerts a dual effect on boehmite digestion kinetics. First, the adsorption of mid-molecular-weight humic species (MW 500–2000 Da) onto the boehmite surface reduces the number of available hydroxyl attachment sites, as evidenced by a 22% decrease in the reactive surface area measured by BET nitrogen adsorption after the crystals were equilibrated with a liquor containing 7.5 g/L organic carbon, compared to the same crystals in a synthetic, organic-free solution of identical caustic and alumina concentrations. Second, the formation of stable aluminate-organic complexes increases the true concentration of soluble alumina that is not available for precipitation but is still detected in the analytical determination of total alumina by the standard EDTA titrimetric method of AS 2331.4.1-2010, causing an overestimation of the driving force for dissolution by up to 6%. This analytical artifact, when uncorrected, leads to target extraction ratios that are unattainable, as observed in a plant trial where the measured liquor A/C was 0.660 by titration but the effective reactive A/C was only 0.621 after complexation correction, resulting in a 3.2% shortfall in expected alumina recovery from boehmite.

Kinetic data from a double-jacketed, stirred batch autoclave equipped with a zirconium sampling probe (manufactured by Parr Instrument Company, Moline, IL) and operated at 260 °C with a stirrer speed of 1,200 rpm—sufficient to eliminate external mass transfer resistance as confirmed by a series of tests at 800–1,600 rpm with identical dissolution profiles—showed that the apparent rate constant for boehmite digestion drops from 0.021 min⁻¹ in organic-free liquor to 0.014 min⁻¹ at 6 g/L organic carbon, and to 0.009 min⁻¹ at 10 g/L. The effect is partially reversible: addition of activated carbon or an oxalate crystallization stage that reduces the organic load to below 3 g/L restores the rate constant to 0.018 min⁻¹, but the hysteresis in surface conditioning takes approximately 5–8 hours of continuous operation to fully equilibrate, as indicated by the slow recovery of the slurries’ zeta potential from −32 mV (fully organic-loaded) to −48 mV (clean surface). In a refinery with a total liquor charge of 18,000 m³, this equilibration delay translates to a production loss of roughly 450 tonnes of alumina per organic excursion event.

In high-alumina systems, the interaction between organic matter and boehmite dissolution becomes markedly non-linear as the liquor A/C ratio approaches 0.70. Electrochemical impedance spectroscopy (EIS) on boehmite pellet electrodes immersed in Bayer liquor at 100 °C (to avoid autoclave-induced noise) revealed that the charge-transfer resistance R_ct increases from 420 Ω·cm² at organic carbon 2 g/L to 1,150 Ω·cm² at 8 g/L, but the increase is not monotonic; a local minimum in R_ct of 370 Ω·cm² occurs at 3.5 g/L due to a competing chelation effect that removes passivating silica from the surface. This minimum corresponds to a liquor composition window where oxalate-driven silica precipitation creates a transiently cleaner surface, exploited in some refineries by maintaining the organic carbon level within a tight band of 3.0–4.0 g/L through controlled causticisation with solid ladle lime at a dosage of 1.2 kg CaO per kg of organic carbon.

Beyond the mechanistic influence on dissolution, high organic loads alter the rheology of the digestion slurry in a way that exacerbates the kinetic consequences. Capillary viscometer measurements on settled red mud slurries from a boehmite-bearing bauxite (Trombetas, Brazil) processed in a liquor with 9 g/L organic carbon showed a yield stress of 28 Pa versus 12 Pa for the same solids loading in organic-free liquor. The higher yield stress impairs axial dispersion in the digester, effectively creating stagnant zones estimated to occupy 14% of the autoclave volume based on residence time distribution studies with lithium nitrate tracer, which further reduces the effective mean residence time available for boehmite dissolution by 2.1 minutes in a nominal 15-minute cycle. The corresponding decline in extraction is 1.8–2.9% absolute, depending on the particle size of the boehmite fraction, with the coarser fractions ( +150 µm) being disproportionately affected because they settle into the low-shear regions near the digester walls.

Thermal Stability Limits in High-Strength Caustic-Aluminate Solutions

When boehmite is digested at temperatures above 255 °C in sodium aluminate liquors with an A/C above 0.64, the solution becomes thermodynamically susceptible to an autoclave-induced decomposition pathway that generates sodium hydrogarnet (Na₆[AlSiO₄]₆· (NaOH)₂·2H₂O) as a dense scale on the digester walls and heater tubes. This scaling reaction consumes caustic soda and reduces the free OH⁻ concentration in the bulk liquor, thereby lowering the thermodynamic driving force for boehmite dissolution. At the Queensland Alumina Limited (QAL) refinery, monitoring of the shell-and-tube preheater pressure drops showed that when the liquor temperature reached 262 °C and the A/C exceeded 0.66, the scaling rate accelerated to 0.8 mm/h on the tube inner surfaces, as measured by ultrasonic thickness gauging during scheduled downtime. The deposit composition, characterized by X-ray diffraction and Rietveld refinement, was 73% basic sodalite, 18% cancrinite, and 9% calcium hydroaluminosilicate, with a thermal conductivity of only 0.42 W/m·K compared to 16.2 W/m·K for the carbon steel tube wall (ASTM A106 Grade B). This low conductivity forces an increase in the external firing rate to maintain the necessary pulp temperature, elevating the tube wall temperature further and creating a positive feedback loop that ends in a forced shutdown when the pressure drop exceeds the available pumping head, typically within 38–45 days of continuous operation under these conditions.

The thermal boundary for this runaway scaling behavior is sharp: a plot of scaling index (mass gain per unit time per unit area) against temperature and A/C ratio, derived from a series of 28 plant trials, shows that at temperatures below 258 °C, the scaling rate remains below 0.05 mm/h regardless of A/C, but at 263 °C and A/C = 0.68, the rate jumps to 0.95 mm/h. This step change corresponds to the nucleation threshold of basic sodalite, which is pH-dependent and occurs when the effective OH⁻ concentration, corrected for the activity coefficient of NaOH in the mixed electrolyte (γ± below 0.42 at 260 °C), creates a local supersaturation index of 1.3 relative to the solubility product of basic sodalite. Such conditions are frequently encountered in the last compartments of high-temperature tube digesters processing bauxites with silica contents above 4.5%. Mitigation strategies include the injection of spent caustic liquor at a slightly lower A/C ratio (0.58–0.60) into the final digestion stage to quench the temperature to 252 °C and reduce the local A/C below the critical value, a practice that entails a small (0.3–0.5%) loss in overall digestion efficiency but pays for itself in extended run length and reduced descaling costs.

In high-alumina circuits, the risk of exceeding the thermal scaling threshold is compounded when the bauxite contains thermally unstable minerals such as goethite (α-FeOOH) that transform to hematite (α-Fe₂O₃) with a concurrent release of structurally bound water. The dehydroxylation of goethite at 230–260 °C—endothermic at 274 kJ/kg—exerts a cooling effect on the mineral particle surface that masks the true bulk liquor temperature. Thermocouple measurements in a pilot-scale digester ( 250 L volume, high-nickel alloy C-276) recorded surface-to-bulk temperature gradients of up to 14 °C for a 12 mm diameter goethite-rich particle, meaning that the bulk liquor must be heated to 274 °C to ensure the particle interior reaches 260 °C for adequate boehmite digestion. This operational conflict forces the refiner into a regime where the bulk temperature exceeds the scaling threshold, even though the effective reaction temperature at the boehmite–liquor interface remains within the safe window, resulting in gradual scaling of the preheater surfaces but acceptable extraction. At the Alcoa Pinjarra refinery, managing this trade-off involves a feed blend strategy that limits the goethite concentration in the bauxite feed to less than 8% by mass, thereby capping the required bulk temperature overshoot at 10 °C and keeping the autoclave wall temperature below 267 °C for the majority of the campaign.

The presence of fine organic-rich mud particles (originating from bauxite clay contaminants) further destabilizes the thermal window by adsorbing onto the digestion vessel’s internal surfaces and providing nucleation sites for the basic sodalite scale. Scanning electron microscopy of scales extracted from a tube digester after a 90-day run revealed a layered structure in which carbon-rich ( 8–12% C) layers alternating with sodalite-dominated bands, indicating that the scale growth proceeds by episodic spalling and re-nucleation rather than a uniform deposition. The spalling events, correlated with thermal cycling of the vessel during start-up and shutdown, release scale fragments that travel downstream and block the interstage screens, causing a differential pressure rise across the screens from 35 kPa to 155 kPa within 2 hours of a spallation event. This screen blinding forces a reduction in slurry flow rate of 15–20% to prevent mechanical damage to the screen assembly (typically a wedge-wire design with 0.5 mm slot openings), which in turn reduces the production rate and increases the effective residence time, partially offsetting the kinetic detriment of the lower temperature, but at the cost of higher scaling rate—a genuinely perplexing process control dilemma.

Re-tuning the digestion parameters for boehmite in high-alumina liquors therefore requires a delicate balance, usually guided by a real-time scaling index computed from a weighted sum of temperature, caustic concentration, silica concentration, and the specific surface area of the circulating mud measured by an on-line laser diffraction particle sizer. At the Yarwun refinery (Gladstone, Australia), the use of such an index—displayed on the distributed control system as a dimensionless number between 0 and 1 with an action threshold of 0.82—enabled operators to anticipate scaling episodes and reduce the digestion temperature by 3–5 °C preemptively, reducing the annual scaling rate from 0.42 mm/month to 0.18 mm/month without a statistically significant loss in boehmite extraction (maintaining 95.1% ±0.4% over a 12-month monitoring period). The economic benefit of this control loop, exclusive of reduced descaling labour, was estimated at AUD 2.8 million per annum based on avoided high-pressure descaling water consumption and production loss.