Indonesia’s strict mine-water limits are forcing operators to pair low-cost sulfide precipitation with high-affinity ion-exchange polishing. The economics are stark: pennies per m³ for bulk removal, but resin to drive Ni/Co to sub-ppb.
Regulatory targets and raw water
Indonesia is the world’s largest nickel producer and co-produces significant cobalt. Wastewater from laterite/leach processing and ferronickel smelters can carry elevated Ni and Co, often in the 0.1–10 mg/L range or higher. To protect freshwater, PerMenLingNo. 9/2006 limits dissolved Ni to 0.5 mg/L and Co to 0.4 mg/L in mine wastewater (id.scribd.com). Meeting these “polishing” targets often requires advanced treatment beyond conventional neutralization.
Raw ferronickelous process waters (RKEF) may carry Ni at tens of mg/L. Typical raw mine waters can have Ni and Co levels orders of magnitude above the regulatory limits, making effective polishing essential to avoid violations.
Sulfide precipitation chemistry and performance
Sulfide precipitation removes dissolved Ni²⁺ and Co²⁺ by forming insoluble sulfides (NiS, CoS) when sulfide (e.g., Na₂S or H₂S) is added, typically at pH 7–9. The solubility products (Ksp, a measure of how readily a solid dissolves) are extremely low—on the order of 10⁻²⁰ for NiS and 10⁻²⁶ for CoS—so most metal precipitates. Tests on Ni-rich effluents (e.g., 100 mg/L Ni) show 90–95% Ni removal at optimal [S²⁻]/[Ni] ratios (iwaponline.com).
Under carefully controlled pH (e.g., pH≈5 optimized then raised) and stoichiometry, Ni abatement can reach ~98%, producing a sludge of millerite (NiS) with some NiO (iwaponline.com). Cobalt behaves similarly, often quantitatively precipitating as CoS under comparable conditions. In mixed-metal waters, Ni can be less completely removed than more insoluble sulfides (e.g., CuS) due to slower kinetics and partial re-dissolution: NiS tends to re-dissolve under oxygen at pH<10, which is why systems often use closed, de-aerated units or rapid solids removal (nepis.epa.gov).
Sulfide trains, cost and constraints
The method is chemically simple and cheap. Reagent costs are low: precipitating 1 mg Ni (from 1 mg/L feed) requires ~1.3 g Na₂S (priced at ~$0.50–1.00/kg), translating to fractions of a cent per m³ for typical Ni concentrations. Reactions are fast, supporting high throughput. Industrial precedents include processes like SAVMIN® that use sulfide for precious-metal leachates (pmc.ncbi.nlm.nih.gov). Dosing is typically delivered by a metered injection system; in practice, sites pair such metering with a pH control tank and a clarifier, with the dosing handled by an industrial dosing pump.
Limitations are material. Large reagent doses produce voluminous sludges: every mg of Ni removed generates roughly 10–20 mg of Ni-containing solid (NiS/CoS mixed sludge). These sludges are often hazardous and require neutralization or landfill, adding disposal cost. Sulfide alone seldom meets tight standards when influent is high: for a 100 mg/L Ni influent, 94% removal still leaves ~6 mg/L—well above the 0.5 mg/L limit (iwaponline.com). Reviews note that precipitation uses large volumes of chemicals and yields toxic sludge, even though the reagent chemistry is “simple, safe to operate and low-cost” (pmc.ncbi.nlm.nih.gov). Process complexities—precise pH control and the risk of NiS re-oxidation—demand engineered reactors and reliable settling.
Chelating ion‑exchange polishing
Specialized polymeric ion-exchange resins with chelating functional groups (e.g., iminodiacetate, bis‑picolylamine) exhibit very high affinity for Ni²⁺ and Co²⁺. In a pilot on dilute wastewater—flue-gas desulfurization water with ~0.09 mg/L Ni—a chelate resin (Purolite S930) removed Ni essentially to zero: from 89 µg/L to <0.1 µg/L (>99.9% removal), even with abundant competing cations present (pmc.ncbi.nlm.nih.gov). Such resins typically have high capacity—on the order of 100–200 mg Ni per gram of resin (www.mdpi.com)—and serve as an effective polishing step after bulk removal.
Uptake kinetics for Ni and Co are reasonably fast (minutes to hours) at mild pH 2–6 (ru.scribd.com). Selectivity is tunable by resin choice (thiourea-based resins focus on precious metals; iminodiacetate resins target Ni/Co). Spent resins are regenerated by acid stripping, yielding concentrated Ni/Co in small volume, with less solid waste than sulfide sludge. In practice, operators deploy ion-exchange columns—see Ion-Exchange systems—charged with specialized ion-exchange resins to achieve parts‑per‑trillion (ppt) levels if required.
Other adsorption media options
Adsorptive media can remove Ni²⁺ and Co²⁺, though generally at lower efficiency than chelating ion-exchange. Activated carbon and modified polymer beads are widely studied; one screening found a carbon resin with Ni capacity ~144 mg/g versus 90–208 mg/g for several chelating resins (www.mdpi.com). Commercial carbon products, such as activated carbon, can play a role but typically cannot match the polish of chelate resins.
Novel materials—metal‑organic frameworks, manganese oxides (δ‑MnO₂)—show promising lab‑scale Ni/Co adsorption (often via surface complexation) but are not yet commercialized for mine effluents. Locally, “green” adsorbents like rice‑husk biochar activated with citric acid have been proposed; some report >95% Cu²⁺ removal and hundreds of mg/g capacity for Pb, suggesting potential applicability to Ni (www.ecoeet.com). Practical use of such biosorbents is limited by regeneration, handling (e.g., silica clogging), and durability in mining waters. By contrast, ion‑exchange resins are mature for metal recovery (e.g., Dowex M4195 for Ni separation in battery recycling).
Capital and O&M comparisons
Sulfide precipitation requires little specialized capital—an injection system, pH control tank and clarifier—with very low reagent cost. Na₂S is ~$0.50–1.00/kg; precipitating 1 kg Ni needs ~1.3 kg Na₂S, so chemical cost is ~$0.65–1 per kg‑Ni removed. For typical Ni ~10 mg/L, that’s ~$0.01–0.05 per m³ of treated water (www.mdpi.com; iwaponline.com).
Ion‑exchange demands higher capital: a few resin vessels/columns and roughly 10–20 L of resin for every few cubic meters of throughput. Chelating resins run about $18–70 per liter (www.mdpi.com); 1 m³ of resin (nearly 800 kg) costs ~$18,000–$70,000 up front. One liter can bind many tens of grams of Ni before regeneration, which requires acid/alkali and yields a spent regenerant brine that must be managed.
Performance, polishing and hybrids
Where sulfide excels is bulk removal: dropping Ni from 100→~10 mg/L (≈90% removal) at pennies per m³. But attaining 0.5 mg/L Ni by precipitation alone can require multi‑stage dosing or excess sulfide, creating more sludge. Ion‑exchange is costlier but yields effluent far below discharge limits; one pilot showed Ni “almost totally removed” to <0.1 µg/L (pmc.ncbi.nlm.nih.gov). The pragmatic strategy is hybrid: sulfide or hydroxide precipitation for bulk removal, then ion‑exchange or adsorption polishing to reach regulatory targets (pmc.ncbi.nlm.nih.gov; iwaponline.com).
In numerical terms, precipitation might cost ~$0.05–0.10 per m³ (mainly reagents) to remove >90% Ni/Co, while ion‑exchange O&M (regeneration chemicals, electricity) might add ~$0.20–0.50 per m³. Precipitation alone likely cannot achieve the <0.5 mg/L targets from typical mine water, whereas ion‑exchange can easily do so—with reported levels <0.001 mg/L (pmc.ncbi.nlm.nih.gov).
Environmental and safety considerations
The toxicity and volume of precipitate disposal is a major hidden cost. Toxic sludge (NiS, CoS) must be dewatered and often stabilized or landfilled, with associated fees (pmc.ncbi.nlm.nih.gov). Ion‑exchange’s waste stream—the regenerant acid—is smaller in volume (tens of liters per cycle) but concentrated in Ni/Co; it can be neutralized or processed for metal recovery, potentially offsetting costs. On‑site handling of H₂S or NaHS, if used, also poses safety concerns for sulfide methods.
Throughput and footprint matter: resin systems are compact (no large settling ponds) and operate continuously, but they require careful monitoring for exhaustion.
Data‑backed takeaways
A pilot‑scale comparison showed a H⁺‑form iminodiacetate resin (Purolite S930) drove Ni to virtually undetectable levels, while a simple sulfide dose in a batch test left residual Ni on the order of mg/L (iwaponline.com; pmc.ncbi.nlm.nih.gov). Resin costs are an order of magnitude higher than sulfide chemicals (tens of USD per liter vs cents per kg) (www.mdpi.com), but they yield water quality that easily meets SNI/National Class C standards.
Ultimately, the choice depends on required effluent quality and economics. In Indonesia’s case, the tight 0.5/0.4 mg/L Ni/Co limits almost mandate a polishing step (e.g., ion exchange or advanced adsorbents) after primary treatment (id.scribd.com).
Sources and references
Peer‑reviewed studies and industry reports underpin these findings. Performance data draw from waste‑treatment research (Jerroumi S. et al., Water Quality Research Journal 55(4):345–356, 2020—Ni sulfide precipitation experiments—iwaponline.com; iwaponline.com) and review articles (Matebese F. et al., Heliyon, 2024—mine water tech review—pmc.ncbi.nlm.nih.gov).
Czupryński P. et al., RSC Advances 12:5145–5156 (2022)—pilot ion‑exchange removal of Ni/Hg/Cr—pmc.ncbi.nlm.nih.gov. Z. Zainol & M.J. Nicol, Hydrometallurgy 96:283–287 (2009)—chelating IEX for Ni/Co; ammonium vs H⁺ forms—ru.scribd.com. Wołowicz A. & Z. Hubicki, Processes 9(2):285 (2021)—resin screening; costs and capacities—www.mdpi.com; www.mdpi.com.
Indonesian MoE Permen No. 09/2006—wastewater standards for nickel mining—id.scribd.com. Woolsey S.E. et al., Mine Water Environ. (EPA Report, 1986)—sulfide precipitation kinetics: NiS dissolution—nepis.epa.gov. Additional context on SAVMIN® and precipitation trade‑offs appears in Heliyon (2024) (pmc.ncbi.nlm.nih.gov).
