Platinum Pt · 78

A dense, unreactive metal that speeds up chemical reactions without being consumed by them.

なぜ重要なのか

Platinum cleans diesel exhaust, and it is the catalyst on both sides of a hydrogen fuel cell and most electrolysers.

Where it is in the Earth

Platinum belongs to a group of six chemically similar elements — the platinum-group metals, or PGMs — that share an unusual tendency to remain inert under conditions that would corrode or dissolve almost anything else. In the Earth's crust they are extraordinarily rare, and that rarity is not accidental. When the planet was still largely molten, dense metallic elements sank toward the core, carrying most of the world's platinum with them. What remains in the accessible crust is there because of specific geological accidents that concentrated it to workable grades.

The most important of those accidents is the solidification of large bodies of magma — molten rock — inside the Earth. As a magma body cools slowly over millions of years, different minerals crystallise out in sequence. In certain magmas derived from the mantle (the layer beneath the crust), a process called magmatic differentiation allows iron- and nickel-rich sulfide liquids to separate and sink, scavenging platinum and its sister metals as they go. Where those sulfide-rich layers were preserved, the result is a layered mafic intrusion — mafic meaning a rock rich in magnesium and iron. The Bushveld Complex in South Africa is the largest such structure known, and it hosts the Merensky Reef and the UG2 chromite layer, which together account for the majority of the world's known platinum endowment. Zimbabwe's Great Dyke is a narrower but structurally similar feature.

A separate style of deposit forms where magma carrying sulfide droplets cools more rapidly, often near the surface or in conduit-like feeder channels. The Norilsk-Talnakh system in Russia and the Sudbury Basin in Canada represent this type, though Sudbury has the additional distinction of having been reshaped by a large meteorite impact, which redistributed and concentrated the sulfide ore. In all these settings, platinum occurs partly within sulfide minerals such as pentlandite and partly as its own discrete mineral, sperrylite, a platinum arsenide. The grade of these ores is low by the standards of most metals — the metal content is measured in grams per tonne rather than percent — which means that the geological process of concentration, though remarkable, still leaves most of the rock as waste.

Getting it out

Because platinum occurs at such low concentrations, mining it is fundamentally an exercise in moving enormous quantities of rock to recover a small quantity of metal. The method used depends on how the orebody sits in the ground. In the Bushveld Complex, the platinum-bearing reefs are relatively thin, near-horizontal layers that can extend for many kilometres laterally but are only a metre or two thick. Accessing them requires underground mining — sinking shafts and driving tunnels along the reef — because the overburden (the rock above the ore) is too thick for surface excavation to be economical over most of the deposit. The work is labour-intensive and technically demanding, carried out at considerable depth.

Where a reef outcrops at or near the surface, or where a thick, lower-grade zone exists close to the surface as on the Platreef at Mogalakwena, open-pit mining becomes viable. An open pit strips away the overlying rock in benches, exposing the ore below. The ratio of waste rock removed to ore extracted — called the strip ratio — can be high, but the method allows for larger equipment and higher throughput than underground operations. At Mogalakwena this approach makes a lower-grade, thicker body of ore economically workable in a way that conventional narrow-reef underground methods would not.

In Russia, the platinum at Norilsk-Talnakh is recovered underground alongside nickel and copper, which are the primary products of those mines. This means platinum there is inseparable from the economics of nickel and copper production. A mine operator whose revenue depends mostly on nickel will continue operating even when platinum prices fall, and conversely will not expand specifically to produce more platinum. This by-product relationship shapes supply in ways that are quite different from a mine where platinum is the principal product. Wherever platinum is mined, the grade of the ore — that is, the number of grams of platinum contained in each tonne of rock — determines how much material must be dug, crushed, and processed for every kilogram of metal recovered.

What pulls on it

Platinum's dominant use for several decades has been in catalytic converters, devices fitted to vehicle exhaust systems that convert harmful gases — carbon monoxide, unburned hydrocarbons, and nitrogen oxides — into less harmful ones. Platinum is particularly effective in diesel catalytic converters, where operating temperatures and exhaust chemistry differ from petrol engines. As long as large numbers of diesel vehicles remain in service, this demand continues. The concern for producers is that new diesel vehicle sales have been declining in major markets, and the share of battery electric vehicles, which require no exhaust catalyst at all, is growing. Whether this shrinks platinum demand in autocatalysts depends on the pace of fleet turnover, which is slow: vehicles already in service continue to require their catalysts, and the global vehicle fleet is large.

Alongside the established catalyst market, platinum has a structural role in hydrogen technologies. In a proton exchange membrane, or PEM, fuel cell — a device that combines hydrogen and oxygen to generate electricity — platinum is used as a catalyst on both electrodes. In a PEM electrolyser, which splits water into hydrogen and oxygen using electricity, platinum is used on the cathode side. The intensity figures for these applications show a meaningful quantity of platinum per unit of capacity. If hydrogen production and fuel-cell power generation scale substantially, the demand from these uses could become significant relative to current supply. That said, reducing the amount of platinum required per unit of capacity has been an active research direction for years, and actual deployment volumes depend on factors outside the platinum market itself.

Chemical refining and petroleum processing use platinum-containing catalysts to facilitate reactions such as reforming, where lower-value hydrocarbons are converted into higher-value ones. These are continuous industrial processes with long catalyst lifetimes, so demand from this sector is relatively stable and tied to the overall scale of refinery operation. Medical applications, including chemotherapy agents that contain platinum, represent a smaller but consistent slice of demand. A sharp change in overall platinum demand would most plausibly come either from a rapid collapse of the internal combustion engine fleet — faster than historical fleet replacement has occurred — or from a large-scale build-out of PEM electrolysis capacity, both of which would play out over years rather than months.