A holistic sludge plan built around high-pressure filter presses and second-stage leaching promises >50% volume cuts and ~80% metal recovery — and could flip waste streams into revenue at scale.
Indonesia’s nickel sector — now ≥60% of global supply (MDPI; Reuters) — is moving fast to clean up its wet waste. The legal backdrop is tough and getting tougher, but the economics are starting to look attractive.
The emerging playbook: neutralize and clarify tailings, squeeze them hard with membrane-plate filter presses, then leach the “dry” cake to pull back nickel and cobalt. The goal is simple: meet discharge rules, shrink footprints, and monetize the metals left behind.
Regulatory requirements and incentives
Two pillars set the tone: Law No.32/2009 (Environmental Protection) and Law No.4/2009 (Minerba Law), which impose strict waste management and AMDAL (environmental impact assessment) requirements (Scribd). Government Regulation 78/2010 mandates mine reclamation (Scribd), while MoEF (Ministry of Environment and Forestry) Regulation No.16/2019 (the 2nd amendment of Permen 5/2014) sets numeric discharge quality standards mining wastewater must meet (Scribd).
Permen LHK No.5/2022 (Apr 2022) adds a nature-based layer, requiring mining effluent to be treated via constructed wetlands (engineered reed-bed systems) (peraturan.bpk.go.id). And Government Reg.22/2021 explicitly exempts nickel (ferro-Ni) slag from hazardous-waste status — a key enabler for reuse (Kompas).
Industry guidance now leans hard toward dry solutions: storing HPAL (High Pressure Acid Leach) tailings in wet dams is “environmentally hazardous,” and pressurized filter‑press dry stacking is being “aggressively implemented” globally (dewaterfilterpress.com). The upshot: plans must hit Ni/Co limits and favor dry, resource-recovering flowsheets (Scribd; Kompas).
Tailings and sludge characteristics
HPAL plants dissolve nearly all Ni/Co into solution, leaving tailings high in Fe/Si and acidity. One study found HPAL tailings at ≈38% Fe and <7% S (Kompas). RKEF (Rotary Kiln Electric Furnace) smelters produce a slag with ≈40.7% Fe, ~0.6% Ni, and 0.2% Co (MDPI).
Waste volumes are huge: ~6–16 tonnes of slag are produced per tonne of Ni metal (MDPI). Harita Nickel’s dry‑stack tailings facility (DSTF, Dry Stack Tailings Facility) alone holds ~25×10^6 m³ — about 49×10^6 t — of HPAL tailings (Kompas). Untreated mine effluent readily exceeds heavy‑metal limits and must be neutralized and clarified (NIH/PMC). In practice, HPAL tailings are first neutralized with lime (to raise pH and co‑precipitate metals), then sent to dewatering (Kompas).
High‑performance dewatering and volume reduction
The core objective is to minimize sludge volume. Thickening and mechanical dewatering do the heavy lifting; among options, filter presses produce the driest, “drip‑free, stackable” cakes (McLanahan). Typical filter‑press cakes reach 30–60%+ solids (i.e., 40–70% moisture or less) (Nihao Water), and HPAL tailings trials have hit 22–24% moisture (≈76–78% solids) (dewaterfilterpress.com).
The plan centers on membrane‑plate (batch‑operated) filter presses for final dewatering, with press pressure and timing tuned for target dryness (dewaterfilterpress.com). One case used multiple 6×6′ HP (high‑pressure) membrane presses to treat >800 DMT/h (dry metric tonnes per hour), aiming for ≤24% cake moisture at “Conc.: 35” (dewaterfilterpress.com; dewaterfilterpress.com).
Filter presses can also eliminate coagulants at the press stage (unlike belt presses) and automate solids capture, returning clear filtrate — nearly all water — for reuse (McLanahan). High cake solids translate to a 50–70% reduction in waste‑stream volume. Example: dewatering 1,000 m³ of 35%‑solids slurry to 22% moisture roughly halves the disposal volume. At scale, DSTF dry stacking stores the damp cake in lined basins with a smaller footprint; Harita Nickel neutralizes HPAL tailings, filter‑presses to “dry” cake, then stacks it in secured pits (Kompas).
Process flow and equipment selection
The integrated plan combines (a) precipitation of dissolved metals (lime or sulfide dosing to capture residual Ni/Co), (b) thickening (polymers and clarifiers) to concentrate solids, and (c) downstream dewatering (filter presses, or centrifuge with press polishing) to a stackable cake. The equipment blend depends on feed: filter presses excel with fine, clay‑rich slurries; coarse slurries may benefit from upstream centrifugation.
Upstream pH control and flocculation are non‑negotiable. Accurate chemical addition is typically delivered with a dosing pump. Thickener performance can be stabilized in a clarifier, using targeted flocculants; if required, supplemental coagulants improve aggregation. To match batch press cycles, surge capacity and controls fall under wastewater ancillaries for smooth pump‑thicken‑press automation (McLanahan).
On‑site storage and monitoring
Post‑dewatering, the cake is dry‑stacked on‑site or sent off to licensed disposal, as regulations dictate. In Indonesia, deep‑sea tailings discharge (DSTP) for nickel is banned, so all concentrated sludge remains on land (Kompas). Dry stacks reduce dam risk and enable residual water recovery through controlled drainage and evaporation, supported by containment bunds and leachate recirculation. Periodic environmental monitoring — such as quarterly filings to MoEF, as reported at Harita — is required (Kompas).
Metal recovery and revenue potential
Even low‑grade residues add up. Ni furnace slag (RKEF) can carry ~0.6% Ni and 0.2% Co (MDPI), and lab trials show ~79–80% recovery of Ni and Co using citric+ascorbic acid leaching (MDPI). Laterite tailings around ~0.25% Ni and ~0.09% Co have delivered ~79–80% Ni and ~50–58% Co extraction via bioacids or Acidithiobacillus‑generated sulphuric acid (ResearchGate; ResearchGate).
Put simply: if a mine discharges 10,000 t/day of sludge at 0.25% Ni, an 80% recovery flow could yield 20 t Ni/day. At late‑2024/2025 prices (nickel roughly US$14,000–$15,000/t; cobalt ~$21,500/t), each percentage point of Ni in 1 tonne of sludge corresponds to ~$140–150 in metal value (Reuters; Reuters). A burden of ~0.2% Ni and 0.05% Co in sludge yields about ~$37 per tonne if 80% of both metals are recovered. Scaled over tens of millions of tonnes, theoretical recoveries can reach on the order of ($1–2) ×10^9 of Ni/Co value (even if extraction cost‑on yield is half the value).
Practically, the plan recommends a separate hydrometallurgical rinse on the dewatered cake: acid leaching (sulphuric or organic) followed by solvent extraction or precipitation. With very low grades, bioleaching (e.g., Acidithiobacillus thiooxidans cultures) can provide low‑cost acid, as shown in the ~79% Ni recovery from ~0.25% Ni tailings (ResearchGate). Recovery not only adds revenue but also shrinks the residual hazardous fraction.
There are simpler reuses too: PT Harita Nickel (Obi Island) mixes Ni slag with cement to cast bricks, using ~2.8 kg slag per brick. Over 1 million bricks have been produced and sold at IDR 3,000/brick (Kompas; Kompas). This is viable now that Reg.22/2021 classifies slag as non‑hazardous (Kompas). HPAL tailings are also being studied for topsoil (after removing Ni) or as raw material for steel/LFP batteries; the high Fe (~38%) and residual Cr suggest metallurgy feed potential (Kompas).
Expected outcomes and example metrics
Combined, the measures target water reuse >95%, sludge volume reduction >50%, and metal recovery efficiencies ~80% for Ni/Co. For illustration, treating 100 m³/h of tailings slurry at 35% solids could recover ~70 m³/h of water for reuse and produce a dry cake at ~75% solids (moisture ~25%) (dewaterfilterpress.com; McLanahan). A filter press operating 24/7 can meet such throughput with multiple presses in parallel.
With ~80% Ni recovery, even a modest 0.2% Ni in sludge yields ~0.16% Ni in the output solution — adding up to substantial annual nickel tonnes. In short, proven dewatering (filter presses, membrane filters) plus evolving leach chemistries (acid, adsorption, biorecovery) can transform nickel‑mine sludge from a liability into a potential asset (MDPI; ResearchGate).
