Articles

Introduction

The Metals Company recently announced an MOU with PAMCO to research processing of polymetallic nodules from the Clarion-Clipperton Zone using their Rotary Kiln Electric Furnace (RKEF) technology.

If succesful this represents a major step forward for processing of polymetallic nodules, so let's take a quick look at RKEF foundries and how they work

Metallurgy of Polymetallic Nodules

Polymetallic nodules are rounded, multi-metallic accretions found on the abyssal plains of the world's oceans. Various projects and companies have sought to mine these nodules from the seabed since the early 1970s, with some degrees of success (see the Kennecott Consortium) because of they contain large amounts of relatively high grade ores. The Clarion-Clipperton Zone has received significant attention, due to the vast and densely packed volume of high-grade ores that are located there.

The primary constituents of polymetallic nodules in the Clarion-Clipperton Zone are:

• Manganese (27-30%)
• Iron (6-8%)
• Copper (1.3%)
• Nickel (1.4%)
• Cobalt (0.24%)
• Other trace elements

This represent a huge untapped reserve of base and precious metals, that has formed over millions of years by the precipitation of these metals from seawater. Whilst they are an attractive prospect for deep-sea mining operations, the technology required to process the raw nodules into usable ores, mattes or alloys is still relatively nascent.

RKEF Foundries: A Brief Overview

Rotary Kiln Electric Furnace (RKEF) foundries have traditionally been employed in the processing of lateritic nickel ores. They combine the efficiency of a rotary kiln in preparing and reducing the ore, with an electric arc furnace for final smelting.

The RKEF process comprises two major stages:

1. Rotary Kiln: Drying, calcination, and pre-reduction of the ore occur in the kiln. This phase eliminates moisture and reduces the ore in preparation for smelting
2. Electric Furnace: Here, the pre-reduced ore is melted to produce a metal-rich alloy. This is often followed by refining to isolate the desired metals

Raw Material Preparation

Typically the raw lateritic nickel ore being processed in an RKEF foundry go through a number of pre-processing stages. Whilst polymetallic nodules will be fundamentally different to lateritic nickel ores, it's nonetheless likely that they'll be subjected to similar pre-processing stages, which typically involve:

1. Drying: Reducing the moisture content of the ore
2. Crushing: Breaking down the ore into smaller, more manageable pieces
3. Blending: Mixing the ore with additives to ensure uniformity

The Rotary Kiln

The rotary kiln itself is a slowly rotating horizontal cylinder in which the ore is rotated and heated. The prepared ore is fed into the higher end of the kiln, and due to the kiln's inclination and slow rotation, the ore moves through the kiln as it rotates. This process results in the following:

1. Drying and Pre-heating: As the ore enters the kiln, it's first dried by the hot gases from the furnace. Following this, the ore is pre-heated to a specific temperature
2. Reduction: As the ore moves further down the kiln, it encounters a reducing atmosphere, typically provided by the combustion of coal or other reductants. In this stage, the iron and a portion of the nickel in the ore are reduced from their oxides to the metallic state
3. Calcination: In the last part of the kiln, the reduced ore undergoes calcination, where moisture and volatile components are removed

Electric Arc Furnace (EAF)

The calcined product from the rotary kiln, is then charged into the electric arc furnace, where the following processes occur:

1. Melting: The intense heat from electric arcs formed between the electrodes and the calcine melt the reduced ore
2. Slag Formation: Impurities form a slag that floats on top of the molten metal
3. Recovery: The molten metal settles at the bottom of the furnace, whilst the slag floats on top. The metal is then tapped from the bottom of the furnace

Application of RKEF in Polymetallic Nodule Processing

Polymetallic nodules contain a host of different metals of interest. This means that any refining method ideally should be able to handle all these different metals - which is likely why RKEF is being explored by TMC - it's a refining method that can handle complex ores can be beneficial. Here's why:

• Flexibility: The RKEF process has been optimized for lateritic ores but can be adjusted for the composition of nodules
• Economies of Scale: Large RKEF foundries can process significant quantities, potentially making deep-sea mining ventures more economically viable
• Refining Capacity: The electric furnace can separate multiple metals, which fits well with the multi-metallic nature of the nodules

Challenges

Whilst it's great news that TMC and PAMCO are working together on commercializing this tech, there are significant hurdles to overcome with the process itself and with compatibility with nodules from the CCZ:

• Direct Application to CCZ Nodules: RKEF foundries can hypothetically handle polymetallic nodules from the CCZ, but practical operations can be different, and significant modifications may be required to make current equipment effective
• Energy Intensive: RKEF foundries require significant energy, potentially offsetting some of the economic benefits
• Slag Production: Managing and mitigating slag, a by-product, can present challenges, especially given the different composition of nodules compared to lateritic ores

Comparison to Other Processing Methods

RKEF appears to be a very promising route to process polymetallic nodules, but other methods could also be used. These would include hydrometallurgical processes, which uses aqueous chemistry to extract metals. Whilst these methods may offer more selective extraction, they often require more steps and can be more environmentally impactful due to the chemicals involved.

Future of RKEF for Processing Polymetallic Nodules

The future looks promising but hinges on:

• Technological Innovations: Adapting PAMCO's RKEF facilities for nodules might is crucial for TMC's operations and the future of DSM
• Sustainable Practices: TMC has made strong statements around adhering to sustainable practises and zero waste, so it will be crucial to adapt current RKEF facilities to meet these goals
• Economic Viability: The economics need to make sense, and whilst PAMCO offers a depth of skill and experience, their facilities at Hachinohe are dwarfed by the IMIP facilities in Indonesia

Conclusion

RKEF foundries offer a potentially efficient, low-cost means of processing polymetallic nodules from the CCZ, without building new processes, plant and equipment. The process is well-understood, developed, and is normally capable of handling a wide variety of ore types and grades.

That said, the process has not been commercially applied to polymetallic nodules, so it's likely that significant work will be required between PAMCO and TMC. Even then, the PAMCO facilities at Hachinohe are limited in size, accessibility and scalability, so they're likely to only be a stepping stone to a larger operation, plant or facility.

This is going to be fun and exciting to watch!

References

1. Halada, K., Shimada, R., & Iizuka, A. (2014). Development of a new type of rotary kiln and its application to the treatment of polymetallic nodules. Minerals Engineering, 56, 53-58.
2. Mudd, G. M., & Jowitt, S. M. (2014). A detailed assessment of global nickel resource trends and endowments. Economic Geology, 109(7), 1813-1841.
3. Hein, J. R., Koschinsky, A., & Mikesell, M. (2013). Deep-ocean ferromanganese crusts and nodules. Treatise on Geochemistry, 11, 273-291.