Under Indonesia’s September 2026 pricing revision, two limonite feeds reported at 1.2% Ni can share the same nickel-grade correction factor while imposing different HPAL reagent, recovery, and residue burdens. The pricing formula values reported elements; the autoclave responds to mineral phases. Nickel hosted by goethite is released as the iron oxyhydroxide dissolves, and subsequent hematite precipitation can return part of that acid. Reactive magnesium silicates create soluble magnesium sulfate and a more persistent acid load. Aluminum phases and silica-bearing minerals can also change sulfate partitioning, slurry behavior, and residue composition. These relationships apply to limonite-dominant sulfuric-acid HPAL, not automatically to saprolite or untested ore blends.

What Did Indonesia Change in the Nickel-Ore Pricing Formula?

Indonesia’s Ministry of Energy and Mineral Resources issued Decision No. 363.K/MB.01/MEM.B/2026 on September 11, 2026. The decision took effect on September 15 and revised correction factors used in the benchmark pricing of nickel ore.

For low-grade ore containing 1.2% Ni, the correction factor was reduced from 26% to 14%. The cobalt coefficient was also reduced from 30% to 17%, according to Reuters’ report on the revised formula.

This changes the benchmark treatment of reported nickel and cobalt grades. It does not determine:

• which minerals host the nickel or cobalt;
• how much sulfuric acid an ore consumes under HPAL conditions;
• whether dissolved iron subsequently precipitates and regenerates acid;
• how much magnesium remains in solution for downstream neutralization;
• whether silica-bearing phases produce manageable or difficult slurry behavior;
• or how much residue is generated per unit of recovered nickel.

The full benchmark value may still vary with other formula inputs and commercial terms. Sharing a 1.2% Ni assay or correction factor does not mean that two ores have the same economic response inside an HPAL plant.

Why Does the Same Nickel Assay Produce Different Leach Behavior?

A bulk assay reports the amount of an element in the sample. It does not identify the mineral phase containing that element or the fraction accessible under a particular leach condition.

The USGS nickel-cobalt laterite deposit model describes laterite profiles in which nickel may be associated with iron oxyhydroxides in the limonite zone, while deeper and less weathered material contains more magnesium-bearing silicates. Cobalt may be enriched in manganese-oxide phases rather than following nickel uniformly.

These differences matter because HPAL dissolves mineral structures, not isolated assay numbers:

• Goethite-hosted nickel becomes available as the iron oxyhydroxide structure dissolves.
• Nickel in residual or partially weathered silicates may have different dissolution kinetics and acid demand.
• Cobalt associated with manganese oxides can respond differently from cobalt incorporated in iron phases.
• Magnesium in acid-reactive silicates forms soluble magnesium sulfate and contributes to net acid consumption.
• Aluminum and silica phases can affect sulfate chemistry, secondary solids, slurry rheology, and residue properties.

Total Ni, Fe, Mg, Al, Si, Co, and Cr analyses establish elemental inventory. They cannot independently resolve these host-phase differences. Mineralogical evidence and condition-matched leach tests are needed before converting grade into an HPAL cost conclusion.

Why Do Goethite and Magnesium Produce Different Acid-Cost Signatures?

The most important distinction is between acid that remains consumed in soluble products and acid that may be regenerated by high-temperature iron precipitation.

Goethite dissolution can be followed by acid regeneration

A simplified representation of goethite dissolution is:

$$

2\mathrm{FeOOH}+3\mathrm{H_2SO_4}

\rightarrow

\mathrm{Fe_2(SO_4)_3}+4\mathrm{H_2O}

$$

Under suitable HPAL temperature, acidity, and residence-time conditions, dissolved ferric sulfate can hydrolyze and precipitate as hematite:

$$

\mathrm{Fe_2(SO_4)_3}+3\mathrm{H_2O}

\rightarrow

\mathrm{Fe_2O_3}+3\mathrm{H_2SO_4}

$$

Combining these simplified reactions gives:

$$

2\mathrm{FeOOH}\rightarrow\mathrm{Fe_2O_3}+\mathrm{H_2O}

$$

This cancellation does not mean that goethitic ore requires no acid. Acid is still needed to initiate mineral dissolution, release nickel, and maintain the leach environment. Aluminum reactions, incomplete iron precipitation, entrainment, side reactions, and operating losses prevent ideal recovery. The equations explain why total Fe is not necessarily proportional to permanent acid consumption.

A 2025 experimental HPAL study tested laterite ore at 265°C using acid-to-ore ratios of 0.25, 0.35, and 0.45 and slurry solids of 22%, 26%, and 30% by mass. Higher acid addition increased nickel extraction but also increased iron and aluminum dissolution. XRD and SEM observations linked the behavior to the conversion of goethite and gibbsite into hematite and hydronium alunite. Those findings apply to the tested ore and conditions; they do not establish a universal acid dosage for all limonites. See the original Hydrometallurgy study.

Reactive magnesium creates a more persistent acid load

For acid-accounting purposes, reactive magnesium may be represented on an MgO-equivalent basis:

$$

\mathrm{MgO}+\mathrm{H_2SO_4}

\rightarrow

\mathrm{MgSO_4}+\mathrm{H_2O}

$$

“MgO equivalent” is a reporting convention, not proof that free periclase is present. In laterite, magnesium may occur in serpentine, smectite, chlorite, or other silicate phases. Their dissolution rates vary with mineral structure, particle size, temperature, residence time, and acid activity.

When reactive magnesium dissolves and remains as soluble magnesium sulfate, the acid is not regenerated through the same hematite-forming cycle. The dissolved magnesium can also increase the neutralization duty required before nickel and cobalt recovery.

The established review of pressure acid leaching of nickel laterites identifies magnesium-bearing gangue and ore mineralogy as major influences on acid consumption. Because this source synthesizes multiple studies rather than reporting one universal feed, it supports the mechanism and direction of effect—not a fixed acid penalty per percentage point of Mg.

The valid interpretation is therefore conditional:

Higher total Mg indicates potential acid and neutralization burden only when a significant fraction occurs in phases reactive under the plant’s HPAL conditions.

Bulk Mg alone cannot establish that fraction. Acid-neutralization-capacity measurements and representative HPAL tests are more direct evidence.

How Do Aluminum and Silica Alter the HPAL Response?

Aluminum affects sulfate partitioning and secondary solids

Al-bearing phases such as gibbsite can dissolve readily in hot sulfuric acid. Dissolved aluminum may remain in solution or precipitate into sulfate-bearing phases, depending on temperature, acidity, sulfate activity, and solution composition.

In the cited 2025 HPAL study, hydronium alunite was among the observed secondary phases. That result demonstrates a specific reaction pathway under the study conditions. It does not prove that every aluminum-bearing limonite forms the same precipitate or consumes the same amount of acid.

For cost interpretation, total Al is therefore incomplete. Relevant evidence includes:

• the Al-bearing mineral phases present;
• their dissolution under the intended temperature and acid conditions;
• the secondary phases produced;
• and the resulting neutralization and residue behavior.

Silica is a rheology and residue test trigger, not a standalone cost predictor

Silica may occur as relatively resistant quartz, reactive amorphous material, or silicate minerals that alter during leaching. Total SiO₂ cannot distinguish among these forms.

An original study of a siliceous goethitic nickel laterite found that particle size and temperature affected both acid leaching and rheological behavior. The work demonstrates that mineral breakdown and slurry flow behavior can be coupled in this particular ore system. See the Minerals Engineering study.

Its evidence boundary is important. The study does not directly quantify commercial HPAL throughput, filtration cost, or residue-settling performance. Those outcomes also depend on solids concentration, shear history, particle-size distribution, precipitation products, equipment design, and liquor chemistry.

The defensible conclusion is narrower:

Total silica should trigger mineralogical and slurry-behavior testing; it should not be converted directly into an HPAL cost adjustment without phase-specific and process-specific evidence.

How Does Limonite Chemistry Enter the HPAL Cost Chain?

The causal chain is:

$$

\text{Mineral host}

\rightarrow

\text{dissolution and precipitation reactions}

\rightarrow

\text{acid and slurry response}

\rightarrow

\text{recovery, neutralization, and residue burden}

$$

A pricing formula generally begins near the elemental-assay end of this chain. HPAL performance depends on the earlier mineralogical and reaction steps.

What Evidence Can Distinguish Equal-Grade Limonite Feeds?

Comparisons are credible only when sampling, preparation, analytical method, moisture basis, and reporting basis are aligned. ChemicalCell’s discussion of matrix-specific interpretation of moisture, trace-metal, and anion results explains this broader analytical principle for downstream battery raw materials. That page addresses measurement comparability; the present page remains focused on laterite mineralogy and HPAL response.

What Can—and Cannot—Be Inferred From the Pricing Revision?

Three levels of conclusion should remain separate.

Established fact

Indonesia changed the correction factors used in its nickel-ore benchmark formula, including the factor for 1.2% Ni material and the cobalt coefficient.

Evidence-supported mechanism

Laterite mineralogy controls how Fe, Mg, Al, Si, Ni, and Co react during sulfuric-acid pressure leaching. Goethite dissolution and hematite precipitation can partly recycle acid, while reactive magnesium generally creates a more persistent soluble-sulfate burden.

Conditional process inference

If the revised formula compresses the benchmark-price differences between ores that have similar reported grades but different mineralogy, chemical and mineralogical variability may represent a larger share of the difference in HPAL processing burden.

That inference is technically plausible, but the regulation and published laboratory studies do not establish the profitability of any Indonesian HPAL operation. A plant-level conclusion still requires representative ore variability, acid and neutralizing-agent prices, recovery, residue handling, utilities, equipment constraints, and commercial terms.

Defensible Conclusion

Indonesia’s revised pricing formula changes how reported nickel and cobalt grades enter the ore benchmark. It does not make 1.2% Ni limonites chemically interchangeable.

The strongest evidence supports a phase-specific interpretation: goethite-hosted nickel is released through iron-oxyhydroxide dissolution, hematite precipitation can return part of the associated acid under suitable HPAL conditions, and reactive magnesium silicates create a more persistent acid and neutralization load. Aluminum and silica influence secondary solids and slurry behavior, but bulk Al or SiO₂ values cannot quantify those effects without mineralogical and process testing.

The defensible boundary is equally important. Assay plus mineralogy can identify likely cost drivers. Condition-matched HPAL and slurry tests are still required to determine their magnitude for a particular ore blend and flowsheet.