Sodium-Ion Battery Black Mass Recovery and Yield

When we commissioned the recovery line for our sodium-ion pilot cells last spring, the first surprise had nothing to do with chemistry. It was the accounting. Our lithium-ion team had spent years tuning black mass recovery against a metal value that pays for the process: cobalt, nickel, lithium, copper. Sodium-ion black mass hands you iron, manganese, aluminium and hard carbon, and the ledger does not balance the same way. Run a sodium-ion black mass recovery route through a lithium-ion spreadsheet and you will conclude the material is worthless, then landfill a recoverable stream. That conclusion is wrong and expensive. What follows is what is inside sodium-ion black mass, how recovery yield should be defined and measured, which routes hold up at pilot scale, and what designers can change upstream to improve the downstream numbers.

What Is Actually Inside Sodium-Ion Black Mass

Black mass is the fine fraction you get after a cell is discharged, dismantled, shredded and screened. Its composition is set by the chemistry you built, so the first step in any recovery project is to stop treating it as one material. A sodium-ion battery stream is not a diluted lithium-ion stream; it is a different periodic table profile.

Cathode Fractions: Layered Oxides vs Prussian White

Layered oxide cathodes such as NaFe0.5Mn0.5O2, NaNi1/3Fe1/3Mn1/3O2 and their doped variants dominate the energy-oriented sodium-ion battery market. They leach readily in mineral acid and return an iron-manganese-nickel liquor that can be re-precipitated as a mixed hydroxide precursor. Prussian white, Na2Fe[Fe(CN)6] and its analogues, behaves differently: it is cheap, it cycles well, and it carries cyanide ligands that change your process safety case entirely. Hard carbon anodes, typically 250 to 320 mAh/g, contribute the largest single mass fraction of the electrode stack.

The Aluminium Collector Advantage

Sodium-ion cells use aluminium foil on both electrodes. There is no copper foil and no lithium metal in the as-built cell, which is a genuine economic gift. Copper is 8 to 12 percent of lithium-ion black mass and it historically paid for shredding and sorting. Aluminium foil is easier to separate by eddy current or alkaline dissolution, but the recovered metal credit per tonne is far smaller. That reframes the business case: sodium-ion recovery is a materials-recirculation play, not a precious-metal play.

Hard Carbon and the Electrolyte Residue

Hard carbon arrives as a disordered, non-graphitic powder that most lithium-ion flowsheets discard as tailings. It is also the fraction with the clearest second-life path, because sodium-ion anode demand is growing faster than hard carbon precursor capacity. The electrolyte residue, usually NaPF6 or a sulfonimide salt in carbonate solvents, is where fluoride risk lives and where mass balance errors are born.

Defining Recovery, Yield, and Purity Before You Measure Anything

Most disputes about black mass recovery numbers are definition disputes, not measurement disputes. Lock the definitions before you quote a number to anyone.

Three Numbers That Get Confused

Recovery is the mass of target output divided by the mass of that same target in the feed, expressed on a dry basis. Yield is recovery measured over a single pass, before any rework of intermediate streams. Purity is the concentration of the target in the output, and it trades directly against recovery: you can always raise purity by rejecting more material and lowering yield. Recycling efficiency, as defined under the EU Battery Regulation, is a mass-based figure for the whole battery excluding energy content, and it is not the same as either recovery or yield. Quote all four, or expect someone to quote them at you.

The Mass Balance Worksheet

Feed mass, moisture content and assay by ICP-OES in triplicate.
Coarse fractions: casing, busbars, tabs, fasteners, by alloy.
Foil fraction: aluminium by eddy current or alkaline leach.
Separator and polymer fraction, by density separation.
Black mass fraction, screened to a defined top size.
Leach liquor assays for Na, Fe, Mn, Ni and any dopants.
Residue mass and hard carbon content after washing and drying.
Moisture in every filter cake, because that is where yield silently disappears.

Where the Losses Hide

In our pilot line, unrecovered mass concentrated in four places. Filter cake moisture at 25 to 35 percent water explained most of the apparent shortfall until we reported on a dry basis. Fines below 20 micrometres carried over in overflow during the first weeks. Dust collection captured 2 to 5 percent of feed as mixed powder. Sampling error reached 3 to 6 percent until we replaced single-grab samples with riffle-split composites.

Selecting a Process Route

Mechanical Pre-Treatment and Density Separation

Discharge cells to a defined low state of charge, then shred under an inert or wetted atmosphere. Two-stage shredding with intermediate screening gives a cleaner foil fraction than single-pass shredding, at the cost of more dust control points. Expect 55 to 65 percent of feed mass as black mass, 12 to 18 percent aluminium, 12 to 20 percent casing, and 3 to 5 percent separator and polymer.

Hydrometallurgical Leaching of Layered Oxides

Sulfuric acid with hydrogen peroxide as reductant at 60 to 80 degrees Celsius, pH 1.5 to 2.0, and 1.2 to 1.5 times stoichiometric acid demand will take iron, manganese and nickel into solution at better than 98 percent in one to two hours. Sodium stays in solution and is easy to wash out. Recover the mixed metals by hydroxide precipitation at pH 4.0 to 4.5 to produce a precursor-grade Fe-Mn-Ni hydroxide at 97 to 99 percent purity, which is the highest-value output in the whole flowsheet.

Alkaline Routes for Prussian White and Aluminium

Sodium hydroxide pre-leach dissolves aluminium foil and produces sodium aluminate, removing the foil before acid is added. For Prussian white black mass, an alkaline route recovers sodium ferrocyanide with the cyanide ligand intact and out of the gas phase. It is slower and more reagent-hungry than acid leaching, and it is what we recommend unless the facility has a cyanide scrubber and continuous HCN monitoring.

Pyrometallurgy: When It Still Makes Sense

Smelting destroys hard carbon, burns the electrolyte and produces an alloy that still needs hydrometallurgical refining. With low metal value in sodium-ion black mass, energy intensity usually kills the economics unless a smelter is already running and the alternative is disposal. Where one is available and the carbon can serve as a reductant, a hybrid route can beat pure hydrometallurgy on total cost.

Hard Carbon Recovery: The Fraction Nobody Models

Hard carbon is 25 to 35 percent of sodium-ion black mass by mass and it decides whether a line is marginal or profitable. Acid washing removes residual sodium, iron and manganese, but it also oxidises the surface and shifts the voltage profile. In our trials, re-milled recovered hard carbon retained 75 to 85 percent of its original reversible capacity: good enough for cost-sensitive applications, not good enough for premium cells.

Ash, Moisture, and the Second-Life Test

Set acceptance limits before you sell recovered carbon: ash below 2 percent, moisture below 0.5 percent, d50 between 8 and 12 micrometres, and a first-cycle efficiency within 4 points of virgin material. Run a 50-cycle coin cell at 0.2C as a gate, not a curiosity. If a buyer will not accept your lot, the reason is almost always ash or particle size, not capacity.

Hazards Specific to Sodium-Ion Black Mass

Hydrogen Cyanide Risk from Ferrocyanide

Acidifying Prussian white black mass below pH 4 can liberate hydrogen cyanide from the ferrocyanide lattice. This is not a theoretical concern; it is the single most important process safety difference between sodium-ion and lithium-ion recycling. Segment the feed by cathode chemistry at the gate, keep Prussian white out of acid leach, install fixed HCN detection with alarm interlocks, and verify scrubber performance before the first tonne is processed.

Fluoride, HF, and Sodium Hexafluorophosphate

NaPF6 hydrolyses in the presence of moisture to release hydrogen fluoride, and moisture is unavoidable in shredded material. Ventilation, alkaline scrubbing and fluoride-resistant materials of construction are all mandatory. We specify pH monitoring on off-gas scrubber liquor and replace wetted elastomers on a fluoride-exposure schedule rather than a calendar.

Residual Sodium Metal and Thermal Events

Hard carbon anodes hold sodium in an intercalated rather than plated form, and our accelerated rate calorimetry work shows self-heating onset temperatures typically 40 to 80 degrees Celsius higher than equivalent lithium-ion cells. That is a real advantage, not a licence to skip procedures. Discharge to a defined low state of charge, verify open-circuit voltage on every module at intake, and quarantine any cell or module above your voltage limit until it has been independently assessed.

Pilot Line Data: What a Small Sodium-Ion Recovery Line Recovers

Our two-tonne-per-day pilot line, processing mixed layered-oxide sodium-ion packs with no Prussian white content, produced the following steady-state numbers after four months of tuning, all reported on a dry basis.

Overall recovery efficiency: 62 to 71 percent of battery mass, excluding energy content.
Aluminium recovery: 94 to 97 percent as a clean foil fraction.
Iron, manganese and nickel leach recovery: 98 to 99 percent.
Mixed hydroxide purity: 97 to 99 percent, suitable for precursor re-synthesis.
Hard carbon residue recovery: 78 to 86 percent of the carbon present in feed.
Sodium recovery as a washable salt stream: 65 to 80 percent, dependent on wash ratio.
Reagent cost: 180 to 320 US dollars per tonne of black mass.
Process water: 3 to 5 cubic metres per tonne, at 85 percent recirculation.

The revenue side is what surprises lithium-ion engineers. Mixed hydroxide precursor and recovered hard carbon together carry most of the value, with aluminium a modest but reliable credit and sodium salts effectively neutral. That mix makes accuracy in the carbon fraction more important than accuracy in any metal fraction, which is the opposite of the lithium-ion priority.

Designing Cells and Packs for Higher Recovery Yield

Binders, Tapes, and Delamination

Water-soluble binders such as CMC, SBR and PAA release from foil with a warm water soak and make electrode separation far easier than solvent-borne PVDF, which requires NMP and a solvent recovery system. Every strip of double-sided tape you add to a module becomes a contamination source in the shredder and a manual step in dismantling. If we are honest about which design decisions raised our recovery yield most, binder selection and tape elimination sit near the top of the list.

Casing and Pack Architecture

Standardise on one casing alloy per product family so mechanical separation can run on a single eddy-current setting. Laser-welded busbars separate faster than soldered joints, and bolted module interconnects are faster still. Design for a single break point that isolates the module from the pack without cutting a busbar, and record its location on the outside of the enclosure.

Traceability and the Battery Passport

Persistent identification is now a design requirement rather than a nicety. A QR code carrying the unique product identifier, cathode family, electrolyte salt type and state-of-health endpoint turns a mixed shredder feed into a segmented, chemistry-known stream. That single piece of traceability is worth more to a recycler than any incremental improvement in shredding throughput, because it decides whether a batch goes to acid leach or to an alkaline cyanide-preserving route.

Documentation, Transport, and Standards

End-of-life cells and modules remain dangerous goods. UN 38.3 evidence governs the cell design, and damaged, defective or recalled units follow a stricter transport regime than new production under IATA and ADR rules. Discharging to a low state of charge reduces transit risk for modules with degraded mechanical integrity. There is still no dedicated IEC safety standard for room-temperature sodium-ion cells, so qualification programmes commonly adapt the IEC 62133-2 methodology while declaring that adaptation explicitly. Every black mass lot should travel with a mass balance report, an assay certificate from an accredited laboratory, a chain-of-custody record and a chemical safety declaration that names the cathode family.

Buying and Selling Black Mass: Quality Gates

Incoming Inspection Checklist

Moisture content, target below 2 percent, measured on a composite sample.
Aluminium and copper content, target below 1 percent each.
Cathode family identification, with Prussian white flagged separately.
Total cyanide screening and acidification leachate test.
Screen analysis to confirm top size and fines distribution.
Assay by ICP-OES with an independent check sample.
Voltage verification record for every module received.
Reject criteria and renegotiation triggers written into the purchase contract.

The market for sodium-ion black mass is young enough that specifications are still being written, which means the party with the better measurement discipline sets the terms. Invest in sampling and assay first. Process optimisation is wasted effort on a feed stream you cannot characterise.

If you are specifying a sodium-ion battery for a fleet, an off-grid installation or a backup application and you want the end-of-life path to be clean, ask your supplier for the cathode family, the binder system and the pack break point in writing. Those three answers determine what happens to the material in ten years, and they cost nothing to provide today.

Frequently Asked Questions

What is sodium-ion battery black mass recovery?

It is the process of shredding, separating and chemically treating end-of-life sodium-ion cells to recover aluminium foil, iron-manganese-nickel cathode metals and hard carbon anode material. Unlike lithium-ion recovery, the target output is precursor-grade material and reusable carbon rather than high-value cobalt, nickel and lithium salts.

How much of a sodium-ion battery mass can actually be recovered?

On our pilot line we achieve a recovery efficiency of 62 to 71 percent of battery mass on a dry basis, with aluminium recovery above 94 percent and cathode metal leaching above 98 percent. The figure depends heavily on whether filter cake moisture and dust losses are reported correctly, so insist on a dry-basis mass balance.

Is sodium-ion black mass worth processing if it has no cobalt or lithium?

Usually yes, provided you recover hard carbon and aluminium as well as the cathode metals. The value sits in the mixed hydroxide precursor, the recovered carbon and the avoided disposal cost. A flowsheet that only chases metals will look uneconomic, because the metal value per tonne is genuinely lower than lithium-ion chemistry.

Can Prussian white black mass be acid leached safely?

Not without a dedicated cyanide control strategy. Acidifying ferrocyanide-bearing material below pH 4 can release hydrogen cyanide gas. Either route Prussian white feed to alkaline dissolution that preserves the ferrocyanide ligand, or process it in a facility with fixed HCN monitoring, interlocked ventilation and verified scrubbing.

What documentation should accompany a sodium-ion black mass shipment?

A dry-basis mass balance, an accredited assay certificate, a chain-of-custody record, UN 38.3 evidence for the cells it came from, transport classification for damaged or defective units, and a chemical safety declaration naming the cathode family and electrolyte salt type.

How can pack designers make sodium-ion batteries easier to recover?

Use water-soluble binders, eliminate double-sided tape, standardise on one casing alloy per family, prefer laser-welded or bolted interconnects over soldered ones, provide one accessible module break point, and label the enclosure with a QR code carrying the cathode family and electrolyte type. Each of these raises measured recovery yield at the recycling plant.