Solid State Subtractive Metallurgy

Solid State Subtractive Metallurgy

Learn more about the origins of our Solid State Subtractive Metallurgy (S3M) process from a recent publication with MPT International

How a coal-based DRI process inspired the development of a battery recycling technology


Lithium-ion batteries (LIBs) are globally among the most widely used battery types, which traditionally gained importance in the consumer electronics industry. Over the last 5 years, there has been a strong increase in electric vehicles (EVS) powered by LIBs. It is estimated that there were more than 2 million EVs (including hybrid plug-ins) manufactured and sold in 2018, and the growth is expected to continue into the near and longer-term. Ac cording to the IEA’s Global EV Outlook (2019), the number of EVs on roads is globally expected to exceed 145 million vehicles by 20301, with China contributing to over 50% of new EVS manufactured.2

With current battery technology, the usable life of LIBs for EV applications is in the range of 5-7 years, resulting in a large number of spent LIBs after the battery packs in EVs are retired. The wave of retiring batteries represents the amount of batteries requiring appropriate recycling. 3

Recycling is vital for a number of reasons:

  • Environmental: if not properly handled and recycled, spent EV batteries can be toxic and hazardous
  • Economic return: potential for highly profitable recovery of valuable metals (Cobalt, Nickel and Lithium)
  • Social impact: closing the loop with the reuse of battery materials. Image promotion of the EV industry and brand recognition through engagement in battery recycling
  • Supply chain: without recycling, a shortage of supply in several key raw materials could be faced.
  • Regulatory: market players are assigned or delegated with the responsibility of EV battery recycling by government regulations globally
  • Sustainable: closed loop economy enabled by innovative clean technology

There are currently two dominant process routes for full processing/recycling of LIBs: hydrometallurgical and pyrometallurgical processes. The hydrometallurgical process consumes acidic / basic / industrial solvents, which are expensive and require additional processing to treat the toxic wastewater. It is also limited by a low reaction speed and a complicated process route. The pyrometallurgical process involves melting at high temperatures, which translates into high energy consumption (and cost), and high environmental emissions. The resulting extracted metal requires subsequent hydrometallurgical processing to further separate the products.4

The major drawbacks of current LIB recycling technologies can be summarized as a low product value from partial processing (physical separation) and a high operating cost for full processing (physical + chemical separation), which requires chemical conversion and metallurgical separation.

The current high operation cost of the chemical separation process naturally invites interest to develop a process that can avoid both the high-temperature operations involving the melting of metals or oxides, as well as the operating expenses associated with usage of solvent extraction(and to a degree hydrometallurgical altogether).5 Given these unique design requirements, solid state processes involving conversion of elements of interest into desired forms in a solid solid or solid gas reaction regime become ideal candidates to achieve the required chemical separation in the EV LIB recycling exercise.

One of the most notable and well developed solid-state processes used in modern metallurgical operations is the production of Direct Reduced Iron (DRI) for use in steelmaking, which can shed much light on the design concept of a solid state process for EV LIB recycling.

What can we learn from the production of DRI?

DRI is produced from the direct reduction of iron ore (in the form of lumps, pellets, or fines) into iron by a reducing gas or elemental carbon produced from natural gas or coal. Many ores are suitable for direct reduction. Direct reduction processes can be broadly categorized into two categories: gas-based and coal-based. In both cases, the objective of the process is to remove the oxygen contained in various forms of ore, converting it to metallic form in solid-state.

The direct reduction process is comparatively energy efficient as no melting is involved, with the reduction reactions taking place below the melting point of the metal in question (iron, in this case). Today, direct reduction processes have been developed to specifically overcome the difficulties of conventional blast furnaces, the major production route for primary steel production worldwide. The initial capital investment and operating costs of direct reduction plants are generally lower than integrated steel plants and are more suitable in locations where supplies of high-grade coking coal are limited but steel scrap is generally available for recycling and charging at an electric arc furnace.

In gas-based DRI processes, the charge mix is introduced into a cylindrical refractory-lined shaft furnace, where it descends by gravity flow and is contacted by upward flowing reducing gas (hydrogen and carbon monoxide), which reacts with it to reduce the material prior to producing DRI for discharge.6

In various forms of coal-based DRI processes, the iron-bearing charge materials to the DR reactor consist of a mixture of pellets and/or lump ore, fluxes such as limestone and/or dolomite and high volatile coal or lignite. There are two major temperature zones: the preheating zone and the reduction + metallization zone. The feed material is heated by a burner in the rotary kiln. The burner fuel is combusted with a deficiency of air so that a reducing atmosphere is maintained. In the reduction zone, the combustion of coal provides heat for the endothermic reduction reactions. Axial airflow through inlet tubes spaced along the length of the kiln is adjusted to control the combustion of the CO formed from the reduction zone and volatile matter in the coal. Metallization of the product occurs in the discharge half of the kiln.

The principles of producing DRI from iron ore under reducing conditions in these processes have applicability to other metallurgical products that can utilize a similar process set up to achieve reduction of metal oxides into their valuable constituent components.

Inspiration and challenges

Inspired by the simple yet ingenious technology of DRI, XproEM’s process design incorporates unit operations similar to those used in DRI processes, such as agglomeration, reduction, grinding, magnetic separation, etc. The most important unit operation is the reduction step, which is where key learning from DRI processes need to be translated. There are several key technical challenges to overcome to achieve successful reduction for LIB materials applications:

Achieving a high reaction efficiency depends on the fine-tuned control of several key reaction conditions and process parameters, including temperature, residence time and reagent stoichiometric ratio, etc.

  • For different LIB types, the amount of reducing agents and gas composition need to be adjusted to account for the change in composition of the feed material to achieve optimal efficiency. If the system is operated in a sub-optimal state, the conversion ratio will decrease considerably and negatively impact economics.
  • It is critical to control the fuel combustion at the burner so it can both provide sufficient heat for reaction to occur as well as generate a gas mixture with the required stoichiometric ratio for the conversion to occur readily and properly.
  • It is fundamentally important to optimize the reaction time and temperature together so that a high reaction rate can be achieved as well as high product purity.  

The XproEM Process

XproEM’s patented and proprietary Solid-State Subtractive Metallurgy (S3M) process provides a unique and sustainable solution to tackle the imminent problem of recycling spent EV LIBs by directly recovering cobalt and nickel into their metallic powder forms via a solid-state dominant pyro-metallurgical technology.

The feedstock is agglomerated into the appropriate particle sizing to withstand the heating and avoid excessive dusting against the overboard gas flow in a countercurrent solid/gas reaction setting. These agglomerated pellets of feedstock, solid reductant and binder are then sent to the reduction reactor for heating and reduction. The first section of the reduction reactor, the heating zone, increases the temperature of the incoming pellets to its expected temperature. The atmosphere in this zone is oxidative and incompletely combusted fuel is fully oxidized before leaving the heating zone.

Once the pellets move into the reduction zone, metal oxides react steadily with gaseous reducing agents such as CO and H2 and are reduced to their pure metallic form such as cobalt or nickel. Solid reductant such as coal or char can also produce the desired gaseous reducing agents to react adjacent metal oxide to promote the reaction efficiency. Metallization occurs gradually as the pellets travel through the reduction reactor.

The reduced solid-state mixture is then cooled in inert gas and sent for wet grinding, and the slurry stream is subsequently classified and gravity separated to remove carbon and other lighter materials. Anti-oxidizing agents are added to the washing water to prevent re-oxidation of metallic cobalt and nickel. The nickel and cobalt are then separated from the non-magnetic gangue through magnetic separation circuit using a combination of high and medium intensity magnetic separators. The head streams collected from each stage of the circuit are then sent for cleaning and upgrading into high purity metallic products using a proprietary refining technique. Any re-oxidation that may have occurred during the earlier steps is reduced during the subsequent steps of the process.

XproEM Advantages

In comparison to the existing hydrometallurgical and melting pyrometallurgical process routes, the XProEm process provides an environmentally friendly, energy-efficient and cost-effective LIB recycling technology, with the following key advantages:

  • Closed loop recycling: transform LIB waste into high value materials
  • Environmentally friendly: solving the LIB recycling problem without acidic/basic/organic solvents
  • Low cost operation: high profit margin due to low operating costs and capital expenditures

The abovementioned advantages are developed based on the technical leadership in the following key areas: Firstly, the XproEM process is operated without any consumption of acidic/basic/organic solvents, which is fundamentally environmentally friendly, as opposed to replacing acidic solvents by other organic substitutions that are as or more harmful to the environment per many self-claimed “green” technologies currently under development. Secondly, since the conversion step keeps reactants in a solid state, the XproEM process minimizes the overall energy consumption and reduces its carbon footprint. In addition, the consolidated design employed by the XproEM process with multifunctional unit operations offers a more compact and manageable process flowsheet. Lastly, the XproEM process is able to accept feed materials with a wide range of elemental compositions and high levels of impurities – a highly sought feature lacking in currently existing technologies.

Thanks to these outstanding technical features, the economic performance of XproEM process also shows clear advantages compared to other technologies:

  • Significantly lower operating cost due to:
  • Savings on consumption of expensive solvents as well as wastewater treatment
  • Lower usage of energy and utilities
  • Lower maintenance requirements
  • Lower expected capital expenditure due to:
  • Condensed flowsheet and fast reaction dynamics
  • Ability to recycle without costly equipment such as acid leaching/extraction circuits
  • Process units being based on proven technologies already in use at industrial scale
  • Low technology development risk:
  • XproEM technology is derived from traditional base metal processing routes, combined with a unique innovative design.
  • Most process units are based on proven technologies already in use, resulting in a low overall technical risk profile.

A Valuable Opportunity

The XproEM process recovers valuable battery metals (Ni, Co) from spent LIBs, which have considerably higher historical average product prices, on a per tonne basis, compared to steel7 :

A high-level operating cost analysis was developed based on the DRI plant’s operations, and a breakdown of the individual cost items for each processing area for the XproEM process technology flowsheet for conversion, processing and purification steps. Unit consumptions used for each area are based on the results of a mass & energy balance (e.g. for process flows, key reagents, fuel requirements)and in-house factored/benchmarked estimates from similar projects for items such as labour and maintenance.

The operating cost structure above shows the clear applicability of direct reduction technology to battery metals and the anticipated margins for the XProEM process.


Due to the high similarities of chemical and metallurgical properties between Fe in DRI process and Co and Ni in XproEM process, the direct reduction system using appropriate reductants can readily reduce the transition metals back to their metallic forms with high metallization and yield, enabling subsequent processing for refining and upgrading to final products for their respective process route. The development of XProEM technology has built its success on the essence of the design concept from the DRI process to achieve better operation efficiency and economics. In the near future, the XProEM technology can be expected to become an integral part of the solution framework to tackle the imminent problem of EV LIB recycling.


  1. IEA (2019), “Global EV Outlook 2019”, IEA, Paris,
  2. Lithium-ion Battery Costs and Market. Bloomberg New Energy Finance (BNEF) Research (2017).
  3. The future of Battery Production for Electric Vehicles (2018). Boston Consulting Group (BCG).
  4. Zhang, Xihua & Xie, Yongbing & Li, Haitao & Cao,Hongbin. (2013). An overview on the processes and technologies for recycling cathodic active materials from spent lithium-ion batteries. Journal of Material Cycles and Waste Management. 15. 10.1007/s10163-013-0140-y.
  5. European Li-Ion Battery Advanced Manufacturing(ELIBAMA) Batteries End of Life Management: batteries recycling (2014).
  6. Midrex Process Brochure (2018).
  7. InfoMine: Prices and Price Charts (2019).

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