250 g/L Caustic Soda Bauxite Digestion Liquor Composition and Red Mud Settling

In Bayer process liquors designated by a nominal 250 g/L Na₂O-equivalent caustic soda concentration, the solution is a multicomponent electrolyte of sodium hydroxide, sodium aluminate, and water, with dissolved impurities that largely dictate downstream red mud settling. The concentration corresponds to 322.6 g/L NaOH by mass, using the conversion factor 1.291; conversely, a liquor prepared with 250 g/L NaOH contains 193.8 g/L Na₂O. In plants processing gibbsitic bauxite, digestion is conducted in horizontal or vertical autoclaves at 140–150 °C and 0.36–0.48 MPa saturated steam pressure, with residence times of 0.5–2.0 h. For boehmitic bauxite, the same 250 g/L Na₂O liquor is heated to 200–250 °C at 1.55–3.97 MPa, and for diasporic bauxite temperatures reach 250–270 °C at 3.97–5.50 MPa. The digestion reaction consumes free sodium hydroxide to form sodium aluminate, so the initial free caustic of 250 g/L Na₂O falls as alumina dissolves; the final A/C ratio commonly approximates 0.60–0.70, giving 150–175 g/L dissolved Al₂O₃. The same liquor contains 10–30 g/L sodium carbonate from bauxite carbonates and atmospheric CO₂, 1–10 g/L sodium sulfate, 1–15 g/L sodium chloride, 5–25 g/L total organic carbon from humic and fulvic substances, and 0.2–1.5 g/L silica after desilication. These impurities are not passive; they change liquor density, viscosity, scaling tendency, and particle surface chemistry during red mud settling.

Bauxite slurry for digestion is prepared by wet grinding in rod or ball mills to a particle size distribution of 0.075–0.15 mm P80, with spent liquor added before or after milling depending on predesilication requirements. Gibbsite dissolution in milled slurry reaches equilibrium within 15–30 min at 145 °C, whereas boehmite requires 2–6 h and diaspore requires 6–12 h in multi-compartment autoclaves with sequential steam injection. The extracted slurry leaves digestion at 10–25 wt% red mud solids, depending on ore grade and mud factor; typical bauxite mud factor ranges from 0.5 to 2.5 tonnes red mud per tonne alumina produced. The slurry is flashed to atmospheric pressure through a train of 8–12 flash vessels, reducing temperature to 95–105 °C before sand removal and thickener feed. During flashing, silica can precipitate as desilication product sodalite or cancrinite, and dissolved iron can precipitate as fine hematite or goethite particles that later participate in settling.

Composition Boundaries and Analytical Control of 250 g/L Na₂O Liquor

Analytical control of a 250 g/L Na₂O-equivalent caustic soda digestion liquor requires distinction between free caustic, total caustic, total sodium, and dissolved alumina because each term is operationally defined by titration or instrumental methods. Free caustic is determined by precipitating carbonate and aluminate with barium chloride and titrating sodium hydroxide with 0.1 mol/L hydrochloric acid to phenolphthalein endpoint, corresponding to pH 8.3; total caustic includes sodium aluminate and is determined by tartrate-complexed barium chloride precipitation followed by titration. Dissolved alumina is often measured by the difference between total caustic and free caustic after calculation, or by EDTA-zinc titration after dilution with excess EDTA at pH 4–5. Inductively coupled plasma optical emission spectrometry per ISO 11885 gives total sodium, aluminium, silicon, iron, sulfur, and trace elements after appropriate dilution with 1% volume fraction nitric acid. Anions including sulfate and chloride are determined by ion chromatography per ISO 10304-1. Total organic carbon is determined by high-temperature combustion oxidation at 680–950 °C and non-dispersive infrared detection per ISO 8245. Suspended solids after filtration are determined gravimetrically through 0.45 μm mixed cellulose ester membranes. These methods define the liquor composition table below.

At 250 g/L Na₂O, the liquor is not a simple Newtonian fluid; viscosity increases with dissolved Al₂O₃ and organic carbon. Capillary viscometry or rotational rheometry at 25 °C typically gives 3–8 mPa·s for clarified Bayer liquor with A/C 0.60–0.70, and the value can exceed 15 mPa·s when sodium oxalate and high-molecular-weight humates accumulate beyond 25 g/L TOC. The higher viscosity at digestion temperature is partly offset by a reduction in water-like hydrogen bonding, but it nevertheless reduces the hindered settling velocity of red mud particles by increasing the frictional resistance of the continuous phase. The same liquor also exhibits a negative temperature coefficient of solubility for sodium aluminum silicate; cooling from digestion to thickener temperature promotes desilication product precipitation, which influences both liquor composition and mud particle charge. This interplay between chemical equilibria and transport properties is central to red mud separation.

Carbonate, sulfate, and chloride concentrations in a 250 g/L Na₂O liquor are not solely dependent on bauxite inputs; they are amplified by evaporation in closed-loop washing and by the use of sodium-based process chemicals. Sodium carbonate above 30 g/L increases the total sodium oxide content to 280–300 g/L Na₂O and raises the boiling point elevation of the liquor, requiring higher digestion temperatures for the same alumina equilibrium. Sodium sulfate above 10 g/L accelerates sodalite formation, consuming caustic and increasing red mud mass. Chloride above 15 g/L promotes stress corrosion cracking in stainless steel flash vessels and thickener internals, so plant-specific corrosion boundaries are set by autoclave and flash vessel vendor bulletins rather than a single global standard.

How Does Red Mud Settling Respond to Flocculant Dose at 250 g/L Na₂O?

Red mud settling in a 250 g/L Na₂O liquor follows hindered settling behaviour because the red mud concentration after flash cooling is typically 20–80 g/L. Initial particle size measured by laser diffraction per ISO 13320:2020 gives a median diameter d50 of 0.2–5 µm, with a specific surface area of 15–60 m²/g by nitrogen adsorption per ISO 9277. The particles consist of undigested hematite, goethite, quartz, anatase, sodium aluminum silicates, and residual clays; their surface charge in high-pH sodium aluminate liquor is strongly negative, with zeta potentials in the range of −20 to −60 mV measured by electrophoretic light scattering. Anionic polyacrylamide flocculants with molecular weights of 18–24 × 10⁶ g/mol and carboxylate substitution of 20–35 mol% are used because they bridge fines into shear-tolerant aggregates despite electrostatic repulsion, relying on calcium ions and aluminium hydroxo species in the liquor as surface charge modifiers. Flocculant is dissolved to 0.05–0.10 wt% in low-hardness water using a low-shear wetting and maturation unit, and injected into the thickener feedwell at a dose of 10–60 g/t dry mud solids.

At the low end of the dose range near 10 g/t, initial settling rates in a 1 L graduated cylinder at 95 °C are commonly 0.5–1.5 m/h, and overflow turbidity remains above 300 NTU because fine particles are not fully bridged. Increasing dose to 30–40 g/t typically raises initial settling rate to 1.5–3.0 m/h and reduces overflow turbidity to 50–150 NTU measured per ISO 7027-1. Above 60 g/t, further addition does not improve clarity; instead, the underflow solids decrease from 400–600 g/L toward 300 g/L because the floc structure entrains interstitial water and the bed becomes compressible. In a 30 m diameter high-rate thickener handling 1,200 m³/h of red mud slurry, this overdose condition is detected by an increase in rake torque beyond 1.0 MN·m and a lowering of underflow density. Batch settling tests with a 1 L cylinder are not fully scalable to a 30 m thickener because wall effects and low compaction pressure differ; pilot-scale continuous thickeners of 0.5–2 m diameter are required to confirm dose.

The relationship between flocculant dose and settling rate is not linear; at low underflow solids, the compression zone dominates, and at high doses the flocs become shear-sensitive. In a continuous thickener, the optimum dose is determined by the minimum total cost of flocculant and underflow pumping, with overflow turbidity constrained by the wash water circuit. For a 250 g/L Na₂O liquor, increasing the dissolved Al₂O₃ from 150 to 175 g/L raises liquor density by approximately 0.02–0.04 g/cm³ and reduces the density difference between red mud particle and liquor, lowering the Stokes settling velocity by up to 10 %. This density effect is amplified by suspended fine particles that increase the effective liquid density in the free-settling zone. Therefore, red mud from boehmitic digestion at 230–250 °C tends to settle more slowly than red mud from gibbsitic digestion at the same caustic concentration, not only because of finer particle size but also because the higher dissolved alumina content reduces the driving force for sedimentation.

Desilication products formed during digestion and cooling introduce a separate settling surface that is not adequately described by bulk red mud composition alone. In a liquor containing 0.2–1.5 g/L SiO₂, the addition of bauxite-derived quartz and clay minerals under high caustic conditions converts reactive silica to sodium aluminum silicate phases of the sodalite–cancrinite family, with stoichiometry approximately 3(Na₂O·Al₂O₃·2SiO₂)·Na₂X, where X is carbonate, sulfate, chloride, or hydroxide. These precipitates crystallize on existing quartz surfaces and on digester walls, reducing available heat transfer and changing the red mud particle mineralogy. X-ray diffraction of red mud from a 250 g/L Na₂O circuit typically shows hematite, goethite, quartz, anatase, sodalite, and cancrinite with minor calcite, depending on lime addition. Sodalite-bearing red mud settles differently from hematite-rich red mud because the sodalite particles have a lower density and a platy or prismatic morphology, creating more porous flocs and slower compaction. In high-throughput thickeners, the presence of 5–15 wt% sodalite in red mud is associated with a 20–40 % reduction in underflow density at the same flocculant dose, requiring either a higher solids residence time in the compression zone or a reduction in thickener feed rate.

The interaction between desilication product precipitation and organic carbon in red mud settling is particularly relevant at 250 g/L Na₂O because high caustic accelerates the breakdown of bauxite humic substances into sodium oxalate, formate, acetate, and bicarbonate. Sodium oxalate concentrations in such liquors commonly reach 2–5 g/L, and the needle-shaped crystals can co-flocculate with red mud fines, increasing flocculant demand by 10–20 %. The presence of carbonate and sulfate in sodalite further modifies the ionic strength of the liquor, compressing the electrical double layer and reducing the effective radius of fine particles; this can improve coagulation but also increases the risk of fine particle carry-over if shear is excessive. In continuous thickeners, the feedwell design must provide gentle mixing at tip speeds below 0.5 m/s because high shear breaks anionic polyacrylamide flocs and releases fines that cannot be recaptured by reflocculation. This operational boundary is one reason why centrifugal pumps and throttling valves are avoided on flocculant solution lines and on thickened mud transfer lines.

Flocculant Selection and High-Rate Thickener Control

Flocculant selection for a 250 g/L Na₂O red mud thickener is governed by molecular weight, carboxylate substitution, and resistance to alkaline hydrolysis. High-molecular-weight anionic polyacrylamides with 18–24 × 10⁶ g/mol are standard; at liquor pH 13.5–14.0, the amide groups hydrolyse slowly to carboxylate, increasing net negative charge and reducing bridging efficiency after incomplete solution aging. Stock solutions prepared at 0.05–0.10 wt% should be used within 24 h at 25 °C, or within 72 h at 10 °C; extended storage in caustic liquor results in molecular weight reduction and requires re-qualification by intrinsic viscosity measurement. For red mud containing >10 wt% sodalite or >20 wt% goethite, hydroxamated polyacrylamide derivatives are sometimes used at doses of 30–80 g/t because the hydroxamate functional group coordinates iron oxide surfaces more strongly than carboxylate. However, the higher cost and lower water solubility of hydroxamated products restrict their use to high-iron bauxite operations where standard anionic polyacrylamides do not achieve overflow clarity.

High-rate thickener control requires closed-loop adjustment of flocculant dose against overflow turbidity, underflow density, rake torque, and bed level. In a typical 30 m diameter thickener with 1,200 m³/h feed, bed level is measured by a pressure transmitter on the rake arm; the setpoint is maintained at 1.5–2.5 m below the overflow launder. When overflow turbidity exceeds 150 NTU for more than 15 min, the dose is increased by 5 g/t increments until clarity recovers. If underflow solids fall below 400 g/L while overflow clarity is acceptable, the flocculant dose is reduced or the bed level is raised to increase compression residence time. Rake torque above 0.8 MN·m in a 30 m unit indicates excessive bed accumulation or underflow withdrawal failure; the response is to increase underflow pump speed or add dilution water, not to stop raking, because a stationary rake can lead to mud heeling and thermocline disruption. These operating limits are based on published high-rate thickener vendor bulletins and continuous pilot data; values for a specific bauxite and liquor composition must be confirmed by on-site settling tests because published data for this specific configuration is limited.

Thickener operation at 250 g/L Na₂O is also constrained by temperature and boiling point. The feedwell temperature is maintained below 105 °C to avoid flash boiling and flocculant degradation; if the flash train discharges at 110 °C or above, dilution water is injected into the liquor line to cool the slurry before flocculant addition. At temperatures above 100 °C, the hydrolysis rate of polyacrylamide increases, and the polymer solution's intrinsic viscosity can fall by more than 25 % within 30 min, making settling performance batch-to-batch variable. In addition, high caustic liquors require thickener rake drives with enhanced torque density because the dense underflow, at 500–600 g/L solids, exerts yield stresses in the range of 50–150 Pa measured by vane rheometry. Under these conditions, the rake lift and variable-speed drive must be interlocked with the underflow pump so that a sudden viscosity increase does not stall the mechanism.