In cyanidation circuits treating free-milling gold ores, sodium hydroxide is not merely a pH modifier; it controls cyanide speciation, hydrogen cyanide volatility, reaction kinetics, and downstream scale formation. The acid dissociation constant of hydrogen cyanide is 9.21 at 25°C, and below this pH the equilibrium shifts toward volatile HCN, which creates both an occupational exposure hazard and a direct loss of cyanide from the leach solution. A leach circuit maintained at pH 10.5–11.5 converts more than 95% of total cyanide to free cyanide ion at the lower bound and more than 99% at the upper bound, as determined by cyanide speciation procedures aligned with ISO 17690:2015. Where 99% sodium hydroxide pearls are used instead of hydrated lime, addition can be controlled through a recirculating dosing loop with a pH analyser calibrated according to ASTM D1293-18, and the absence of calcium addition reduces the risk of calcite or gypsum scaling on carbon screens, interstage screens, and launder surfaces. The pearls themselves are hygroscopic; at relative humidity exceeding 60%, storage silos require dry air purging to prevent caking and bridging in rotary valve feeders. Dissolution is strongly exothermic with a heat of solution of approximately 44.5 kJ/mol. The resulting alkali solution can be stored in lined steel or high-density polyethylene tanks provided the concentration is held below 50 wt% and the temperature below 60°C to avoid stress cracking of polymer components. In heap leach operations, sodium hydroxide is often selected over lime for pH adjustment of barren solution because it does not contribute to clogging of drip emitters; however, sodium accumulation in recirculated process water must be managed where sodium adsorption ratio limits are imposed by discharge permits. The decision to use 99% pearl rather than 50% liquid caustic is generally driven by freight cost, distance from chlor-alkali production, and the need to avoid freezing in cold climates. The high assay of 99% material, with typical sodium carbonate below 0.5 wt% and chloride below 0.1 wt%, minimizes carbonate introduction that would otherwise consume acid during activated carbon acid washing and increases alkali delivery efficiency. Pyrite oxidation and carbon dioxide absorption in aerated slurries continuously consume alkalinity, so maintaining the cyanide-stable pH window is an ongoing loss-control and neutralisation task rather than a single-dose correction.

Is Caustic Soda Always Preferable to Hydrated Lime in High-Carbonate Gold Pulps?

At the operating pH window of 10.5–11.5, hydrated lime and caustic soda are not interchangeable without consequence. Hydrated lime often has a lower delivered cost per ton of alkalinity, but it introduces calcium into a circuit that may already be saturated with bicarbonate from carbonate gangue minerals such as calcite and dolomite. When lime is used in high-hardness recycled water, calcium carbonate precipitation can blind carbon adsorption screens, reduce interstage screen flow, and generate scale in carbon regeneration kilns. Caustic soda shifts the water chemistry toward sodium carbonate rather than calcium carbonate, which is much more soluble; this is particularly relevant when sulfate concentrations exceed 1,500–2,000 mg/L and calcium sulfate coprecipitation becomes a severe fouling mechanism. The stoichiometric alkalinity equivalence of 1.0 kg of 99% NaOH pearls is approximately 0.93 kg of pure calcium hydroxide on an equivalent hydroxide basis. Practical lime requirements are usually higher because slaking efficiency, grit removal, and slurry line blockages reduce available alkalinity. Caustic is not a universal substitute: in circuits with high sodium background levels, sulfate disposal constraints, or barren bleed restrictions, continuous sodium addition may increase reverse osmosis recovery energy or crystallizer load in zero-liquid-discharge plants. Lime also supplies divalent cations that can improve clarification by co-precipitating silica and fine gangue. Selection of 99% sodium hydroxide pearls is therefore typically justified when scaling, sodium adsorption ratio limits, remote logistics, or elevated carbon dioxide in process water make calcium-based alkalinity operationally expensive. The comparison is not simply an economic decision; it is a mass-balance decision involving calcium, sodium, carbonate, sulfate, total dissolved solids, and slurry viscosity.

In pressurised Zadra and Anglo American Research Laboratories elution circuits, 99% sodium hydroxide pearls are dissolved to prepare eluant at typical concentrations of 1.0–3.0 wt% NaOH, with free sodium cyanide commonly maintained at 0.1–0.5 wt% NaCN. The desorption of Au(CN)₂⁻ from activated carbon is a reversible exchange with hydroxide and cyanide, so a high hydroxide concentration drives gold cyanide into the solution phase. In AARL circuits, carbon is first washed with acid to remove calcium and zinc foulants, then contacted with hot caustic cyanide solution at 110–120°C for 4–8 h under pressure. In Zadra circuits, the same solution type is recirculated through the column at 95–110°C and 200–500 kPa. The acid wash step is directly affected by the carbonate content of the caustic pearls: sodium carbonate consumes acid and releases carbon dioxide gas, reducing acid wash effectiveness. This is why 99% pearls with Na₂CO₃ below 0.5 wt% are specified. Caustic concentration below 0.5 wt% has been associated with slower elution and higher residual gold loading on stripped carbon; concentration above 3.0 wt% can increase carbon fines generation through chemical attack of the carbon surface. Eluant is prepared in jacketed mix tanks with high-shear agitators to dissolve pearls without localized hot spots; the resulting solution is filtered through 50–100 µm cartridge filters to protect elution column distribution nozzles. Sodium hydroxide pearls also reduce calcium load in elution because they do not introduce calcium, which otherwise precipitates as calcium carbonate during acid washing of carbon and interferes with elution column pressure control.

When Acid Pressure Oxidation Liquors Require Neutralization Before Cyanidation

In refractory gold ore circuits employing pressure oxidation, autoclave discharge is typically a hot acidic slurry with pH 0.8–1.5 and temperatures from 100°C to 220°C, containing dissolved iron, arsenic, sulfate, and residual free sulfuric acid. Neutralization is generally staged: the first stage uses limestone to raise pH to 3.0–4.0 because limestone is inexpensive and gypsum generation is tolerated; the second stage raises pH to 10.5–11.0 before cyanidation, using milk of lime or sodium hydroxide. Where 99% sodium hydroxide pearls are used as the final trim reagent, the slurry is cooled and pearls are added through a ring-main dosing system to a series of agitated neutralization tanks. The main advantage is elimination of calcium addition in the second stage, which reduces gypsum supersaturation and decreases scale formation in downstream thickeners, slurry pumps, and carbon-in-leach interstage screens. In high-sulfate pressure oxidation slurries already containing more than 2,000 mg/L sulfate, lime addition during final neutralization can produce a gypsum bulk that raises slurry density and increases oxygen mass transfer resistance. Caustic addition avoids this but increases sodium sulfate concentration, and final liquor may require treatment for sulfate disposal. Each 1.0 g/L of free sulfuric acid consumes approximately 0.82 g/L of sodium hydroxide on a stoichiometric basis, which is used to size metering pumps and dissolution skids. The neutralization reaction is rapid and exothermic; addition must be paced to avoid local boiling and splashing in open-top tanks. pH probes in this service require frequent maintenance because of silica and jarosite scaling, and corrections are made using grab samples and laboratory pH measurement according to ASTM D1293-18. The caustic dose required is determined by residual free acid and dissolved metals, but the buffering contribution of ferric iron and arsenate species makes the titration curve nonlinear. Published data for specific pressure oxidation slurry configurations are limited, but general engineering practice is to provide at least 30 minutes of retention time in the final neutralization tank and to design caustic addition piping for 2.5–3.0 m/s slurry velocity to prevent settling.

Carbon Regeneration Off-Gas Scrubbing with Caustic Soda

Thermal regeneration of activated carbon at kiln temperatures of 650–800°C liberates hydrogen cyanide, ammonia, sulfur dioxide, and fine carbon particulate. A wet packed-bed scrubber using a recirculating sodium hydroxide solution at pH 11.0–12.0 captures hydrogen cyanide and acid gases before stack discharge. The caustic solution is replenished from 99% pearls through a day tank and metering pump, and scrubber blowdown is routed to the cyanide destruction circuit. This application is typically not the main driver for caustic pearl selection unless the site already uses sodium hydroxide for elution and leach pH control.

Alkaline destruction of weak acid dissociable cyanide in tailings slurry is controlled by caustic soda addition to maintain the pH window required by the selected oxidation process. In the sulfur dioxide/air process, copper sulfate is added as a catalyst and pH is held at 9.0–9.5 using sodium hydroxide, while hydrogen peroxide destruction commonly operates at 9.5–10.5. Alkaline chlorination, practiced less often, requires pH 10.5–11.5 to avoid cyanogen chloride formation. Measurement of weak acid dissociable cyanide is performed by distillation and colorimetry according to ISO 17690:2015 or segmented flow injection methods under ASTM D7511-09; these values determine whether discharge meets International Cyanide Management Code limits and local permit concentrations. Sodium hydroxide pearls are used in this duty because they allow precise pH trim without adding calcium, which can cause pipe scaling in the detoxification reactor and tailings pipeline. The dosing rate is set by an online pH controller and verified by laboratory measurements at a frequency of not less than once per shift. A caustic solution of 5–10 wt% NaOH is typically injected into the tailings stream upstream of a static mixer, with mixer residence time designed for 30–60 s before the reaction tank. The operational boundary is that sodium hydroxide does not destroy cyanide alone; it maintains the alkaline environment that prevents hydrogen cyanide volatilisation and enables oxidation. Undertreatment or pH probe failure can produce hydrogen cyanide gas at the tailings surface, while over-addition raises sodium levels in final tailings pore water and may increase salinity management requirements.