VSEP Industries

Platinum Group Metals

PGM Raffinate Polishing and Recovery with VSEP® Nanofiltration

Advanced Polishing of Platinum Group Metals in Hydrometallurgical Recycling Circuits

Platinum Group Metals (PGMs)—platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), iridium (Ir), and osmium (Os)—are among the most valuable and chemically complex metals processed in modern extractive metallurgy. Because these metals are chemically similar and often occur together, PGM refining and recycling require highly selective, multi‑stage separation technologies.

Today, PGMs are routinely recovered from spent industrial catalysts, electronics, automotive catalyst materials, and process residues. These materials are first converted into PGM‑enriched intermediates, such as smelted alloys, calcined catalyst concentrates, ash, or refinery residues, before entering hydrometallurgical refining circuits.

Oxidative Dissolution of PGMs in Chloride Media

In hydrometallurgical PGM processing, metallic PGMs are converted into soluble ionic species through controlled oxidative dissolution. The metals are oxidized from their zero‑valent metallic state and immediately stabilized by chloride ligands, forming soluble chlorometal complexes in aqueous solution. Hydrochloric acid is used as the primary source of chloride ions because it:

  • Maintains high chloride activity
  • Prevents hydrolysis and oxide precipitation
  • Stabilizes high oxidation states required for dissolution
  • Enables downstream solvent extraction and ion‑exchange chemistry

Each platinum group metal exhibits unique oxidation behavior, which is exploited through staged dissolution:

  • Platinum (Pt) and Palladium (Pd) oxidize and complex readily
  • Rhodium (Rh) and Iridium (Ir) require stronger oxidation and longer exposure
  • Ruthenium (Ru) often passes through high‑valence oxide states
  • Osmium (Os) is typically converted to volatile oxide species and removed early under controlled conditions

This controlled aqueous chemistry sets the foundation for selective downstream recovery.

Selective Solvent Extraction of PGMs

Following oxidative dissolution, PGMs exist in solution as stable ionic chloro‑complexes (e.g., Pt(IV), Pd(II), Rh(III), Ir(III)). These species are selectively removed from solution using multistage solvent extraction (SX) systems. During solvent extraction:

  • The aqueous metal chloride complexes do not transfer intact
  • Instead, PGMs move across the phase boundary via interfacial coordination chemistry
  • An organophilic metal–extractant complex forms in the organic phase

Solvent extraction provides:

  • Very high recovery efficiencies for Pd and Pt
  • Controlled separation of individual PGMs
  • Chemistry‑driven selectivity rather than brute‑force removal

Importantly, solvent extraction is intentionally incomplete. Non‑target PGMs are deliberately left in solution so they can be separated in subsequent stages. This controlled incompleteness is what enables ultra‑high‑purity PGM recovery.

Raffinate Treatment After PGM Solvent Extraction

After multistage solvent extraction, the resulting aqueous raffinate contains:

  • Trace concentrations of previously extracted PGMs (equilibrium losses)
  • Other PGMs not yet targeted in the SX sequence
  • Residual chloride, acidity, and dissolved impurities

Because PGMs are extremely valuable, raffinate solutions are never discharged untreated. Instead, they are recycled, re‑treated, and polished until metal concentrations are reduced to extremely low economic and regulatory limits.

Why Trace PGMs Remain After Solvent Extraction

Residual PGMs in raffinate solutions are expected and unavoidable:

  • Solvent extraction is an equilibrium process, not a one‑way capture
  • Selectivity is preferred over total removal
  • Ultra‑low concentrations are chemically unfavorable for SX
  • Even ppm‑level PGMs represent significant economic value when aggregated

To close the circuit and eliminate metal losses, secondary polishing technologies are required.

Ion Exchange (IX): Primary Trace‑Metal Recovery Step

Ion exchange is widely used for polishing PGM raffinates because it is highly effective at ppm‑to‑ppb metal concentrations. Ion exchange polishing works by:

  • Passing raffinate through resin‑packed columns
  • Selectively adsorbing remaining PGM chloro‑complexes
  • Returning a clean chloride solution for recycle
  • Periodically eluting the resin to generate a small, PGM‑rich concentrate

That concentrate is returned to upstream refining steps for recovery.

Rhodium Breakthrough in Ion Exchange Systems

  1. Palladium (Pd) – highest selectivity
  2. Platinum (Pt) – very high selectivity
  3. Iridium (Ir) – moderate selectivity
  4. Rhodium (Rh) – low to moderate selectivity
  5. Ruthenium (Ru) – low selectivity

Despite excellent performance, ion exchange cannot achieve absolute PGM removal under real‑world operating conditions.

Scavenger Precipitation: The Inevitable Final Safety Net

In advanced hydrometallurgical flowsheets, scavenger precipitation becomes unavoidable—not because it is selective, but because it is decisive. Scavenger precipitation:

  • Collapses aqueous PGM solubility
  • Overcomes equilibrium limitations of SX and IX
  • Forces residual metals from solution regardless of chemical speciation

It is typically used:

  • During upset or off‑spec operation
  • At campaign end for circuit cleanup
  • When impurities degrade SX performance or poison IX resins

While effective, scavenger precipitation produces mixed PGM solids that must be reprocessed upstream.

Enhancing PGM Recovery with VSEP® Nanofiltration

Nanofiltration for Raffinate Polishing

VSEP® (Vibratory Shear Enhanced Processing) nanofiltration systems provide a powerful additional polishing step for PGM chloride solutions after solvent extraction and ion exchange.

Nanofiltration membranes:

  • Reject multivalent metal ions, including PGMs
  • Allow monovalent ions such as chloride to pass
  • Concentrate remaining PGMs into a smaller recycle stream
  • Produce a clean chloride/water permeate suitable for reuse

VSEP uses torsional oscillation to suppress scaling and fouling, enabling stable operation on high‑salinity, metal‑bearing process streams where conventional membrane systems fail.

With VSEP nanofiltration, trace PGMs can be concentrated and routed back to recovery steps—reducing or eliminating the need for scavenger precipitation solely for metal recovery.

VSEP® Nanofiltration Pre-concentrating Before Ion Exchange

In chloride‑based ion‑exchange systems, rhodium typically breaks through 5–20× earlier than palladium and 3–8× earlier than platinum, with resin capacities an order of magnitude lower—making Rh the limiting metal in IX polishing unless upstream concentration or alternative recovery steps are used. VSEP can be used to increase concentrations prior to ion exchange to improve loading capacity.

  • Nanofiltration can pre‑concentrate Rh before IX
  • Higher Rh concentration improves resin loading efficiency
  • NF reduces Rh leakage downstream
  • NF delays or eliminates scavenger precipitation campaigns

Case Study 1: PGM Raffinate Polishing with VSEP® NF

In 2021, New Logic installed a VSEP nanofiltration system to recover PGMs from depleted raffinate solutions following solvent extraction and ion exchange.

Metal Concentration Performance

Source Metal Feed Permeate Concentrate
Pd Raffinate Pd 3 mg/L <1 mg/L 35 mg/L
Pt Raffinate Pt 12 mg/L <1 mg/L 163 mg/L
Rh Raffinate Rh 3 mg/L <1 mg/L 36 mg/L
Mixed PGMs Pt/Pd/Rh 30 mg/L <1 mg/L 108 mg/L

Operating Performance

Stream Avg Flux Recovery
Pt Raffinate 58 GFD 82%
Rh Raffinate 48 GFD 87%
Mixed PGM 32 GFD 84%

GFD = Gallons per square foot of membrane per day

The system continues to operate with high permeate flux and stable recovery rates across multiple raffinate streams.

Case Study 2: PGM Recovery in Catalytic Converter Manufacturing

Automotive catalyst manufacturers use platinum‑based ceramic washcoats, where precious metals dominate production costs. Wash water from coating operations is collected to account for every PGM ion. New Logic has installed 12 VSEP microfiltration systems in continuous operation for one such manufacturer to:

  • Dewater insoluble PGM oxides
  • Restore solids concentration for reuse
  • Maintain closed‑loop metal accountability

The first system was installed in 2003 and remains in continuous operation today, demonstrating the long‑term reliability of VSEP technology in precious‑metal service. Typical VSEP performance observed during long-term plant operation is shown in the process diagram below.

Process diagram of VSEP used in Platinum Group Metals recovery.

Why VSEP Matters in Modern PGM Refining

  • Complements solvent extraction and ion exchange
  • Concentrates trace PGMs for recovery
  • Reduces reliance on scavenger precipitation
  • Maintains closed‑loop chloride circuits
  • Improves overall PGM recovery and economics

Unlike solvent extraction and ion exchange, which rely on coordination chemistry and equilibrium selectivity, nanofiltration provides a physical separation mechanism that is independent of PGM speciation.

 

VSEP® systems are deployed in precious‑metal refineries, catalyst recycling operations, automotive catalyst manufacturing, and specialty metals processing facilities where chloride‑based PGM solutions are routinely processed at scale.

VSEP nanofiltration provides metallurgists and process engineers with a robust, scalable tool for maximizing platinum group metal recovery in advanced hydrometallurgical flowsheets.

Technical Content Authorship Note

This page was authored and technically reviewed by senior process engineers at New Logic Research, Inc., with over 30 years of combined experience in precious‑metal refining, catalyst recycling, hydrometallurgy, industrial membrane systems, and high‑solids wastewater treatment.

These engineers have supported commercial installations in precious‑metal refineries, automotive catalyst manufacturing, and specialty metals processing facilities worldwide.

The flowsheets described represent chloride‑based hydrometallurgical PGM recycling circuits commonly used in catalyst recycling, precious‑metal refining, and specialty metals processing. Specific oxidation states, separation sequences, and polishing requirements vary by feed composition and refinery design.

VSEP nanofiltration does not replace solvent extraction or ion exchange. Instead, it complements these technologies by addressing concentration and recovery limitations inherent to equilibrium‑based separation methods.

Get Started

VSEP systems described here operate continuously in commercial PGM refining and catalyst manufacturing facilities, treating chloride‑based process streams under full production conditions.

If you are evaluating ways to recover trace amounts of platinum group metals and tighten metal accounting—VSEP can help.

As with all industrial systems, site‑specific testing is required to confirm performance under actual operating conditions. New Logic can provide pilot testing showing results for your specific project.

Frequently Asked Questions (FAQ)

What are Platinum Group Metals (PGMs)?

Platinum Group Metals (PGMs) include platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), iridium (Ir), and osmium (Os). These metals are chemically similar, highly valuable, and widely used in catalysts, electronics, chemical processing, and automotive emissions control. Because PGMs often occur together, their recovery and separation require advanced hydrometallurgical processes.

Why are PGMs processed using hydrometallurgy instead of conventional metallurgy?

PGMs are chemically inert and closely related, making solid‑state separation impractical. Hydrometallurgical refining converts PGMs into aqueous ionic complexes, allowing selective separation using solvent extraction, ion exchange, and polishing technologies. This approach enables ultra‑high purity recovery and supports closed‑loop recycling.

Why do trace PGMs remain even after ion exchange polishing?

Ion exchange (IX) is highly effective for ppm‑to‑ppb level recovery, but it is also equilibrium‑limited. Certain PGMs—especially rhodium—have:

  • Low resin selectivity
  • Slow adsorption kinetics
  • Strong hydration and inert complexes

This leads to early breakthrough and residual metal leakage, even in well‑designed IX systems.

Which PGM is the most difficult to remove by ion exchange?

Rhodium (Rh) is typically the most challenging PGM to remove by ion exchange. Compared to palladium and platinum, rhodium:

  • Breaks through IX resin 5–20× earlier
  • Has an order‑of‑magnitude lower resin capacity
  • Exhibits slow adsorption kinetics

This makes rhodium the limiting metal in raffinate polishing circuits.

How does VSEP® nanofiltration improve PGM raffinate polishing?

VSEP nanofiltration provides an additional, physical separation step that:

  • Rejects multivalent PGM ions
  • Allows monovalent chloride ions to pass
  • Concentrates trace PGMs into a smaller recycle stream
  • Produces clean chloride permeate for reuse

By concentrating PGMs upstream, VSEP improves the effectiveness of solvent extraction and ion exchange and reduces reliance on scavenger precipitation.

Can VSEP eliminate scavenger precipitation entirely?

In many cases, yes for metal recovery purposes. VSEP nanofiltration can:

  • Concentrate residual PGMs to economically recoverable levels
  • Return metals to solvent extraction or ion exchange
  • Reduce or eliminate scavenger precipitation used solely for PGM recovery

Scavenger precipitation may still be required for impurity control, but not necessarily for precious‑metal capture.

What industries benefit most from VSEP in PGM recovery?

VSEP systems are used in:

  • Precious‑metal refineries
  • Catalyst recycling and manufacturing
  • Automotive catalyst production
  • Electronics and specialty metals processing
  • Chemical and petrochemical catalyst recovery