Acid-mine-drainage neutralization: staged lime precipitation

Acidic, metal-laden mine water (dissolved ferric iron, aluminium, nickel and zinc over a sulfate background) is cleaned in a two-stage lime-neutralization circuit — the classic hydrometallurgy / water-treatment precipitation train. Stage 1 doses to pH 4.5, where ferric iron (Fe(OH)3, pKsp 38.6) and aluminium (Al(OH)3, pKsp 33.0) drop out as a sludge while the base metals stay dissolved; stage 2 raises the liquor to pH 9.5, where nickel (Ni(OH)2) and zinc (Zn(OH)2) precipitate, leaving a clarified effluent. Both stages are the new precipitator unit op wrapping the codebase's CRC-cited metal-hydroxide solubility-product chemistry (thermo/electrolytes.hydroxide_precipitation) — dose to a pH setpoint, read off what drops out — the same selective-precipitation workflow a real neutralization plant runs. Honesty notes: equilibrium Ksp precipitation at a specified pH (no self-consistent pH-from-hydrolysis), a dilute-liquor solution-volume estimate from the water flow, B-dot activity coefficients; the lime reagent and the gypsum (CaSO4) a real lime circuit also throws are not tracked as feed streams (the pH is a dosed circuit condition, matching how the plant is actually operated). Screening-grade — a process-design starting point, not a certified sludge-volume/settling deliverable.