Tearing apart the sulphuric acid chokepoint in Chilean copper.

Sulphuric acid.

Not copper. Not ore grades. Not Chinese property starts. The thing that might actually constrain the world's largest copper producer this year is an industrial chemical most people haven't thought about since high school chemistry. And almost nobody in the financial press is talking about it.

Bloomberg and the sell-side are still running the same copper script they've used all year: tariff front-loading, COMEX stockpiling, whether the surplus is 160 thousand tonnes or 330 thousand. They're arguing about how much metal is sitting in warehouses while the supply chain that actually extracts the metal is quietly seizing up at the input level. It's like debating how full the reservoir is while someone shuts off the pump.

A fifth of Chile's copper output uses sulphuric acid to leach oxide ore. Chile doesn't produce enough of its own. It imports over a million tonnes a year from China alone. And China just turned the tap off.

Beijing announced in early April that it would halt exports of sulphuric acid from copper and zinc smelting starting in May. The reason is entirely domestic: spring planting season, fertiliser supply, food security. Sulphuric acid is a core input for phosphate fertilisers, and China isn't going to let its farmers go short so Chilean mines can keep running. This isn't politics. It's triage. And it landed on an already fractured supply chain.

Story off. Numbers on.

China produces over 40% of the world's sulphuric acid. In the first five months of 2025, Chinese exports hit 1.8 million tonnes — more than double the same period the prior year. This wasn't a marginal supplier. It was the supplier. And the export ban starting May 2026 removes that volume from the global market in one stroke.

Chile took nearly a third of those exports. In March 2025, China shipped 151,268 tonnes of acid to Chile. In February 2026, that had already dropped to 31,870 tonnes. In April? Zero. Not reduced. Zero. For the first time since July 2023, no Chinese acid left port for Chile.

The upstream problem compounds it. Sulphur — the primary feedstock for making sulphuric acid — is heavily concentrated in the Middle East, which accounts for roughly half of seaborne sulphur trade. Sulphur prices are up approximately 70% since the Strait disruptions began. So even if Chilean miners find alternative acid suppliers, the input cost to make that acid has already spiked.

Chilean buyers covered most of their acid needs for the first half of 2026. But the second half is largely uncovered, according to Acuity Commodities. They'll have to go back to the market soon — into a market where the spot price has doubled and the largest single source has shut down.

Meanwhile, copper itself trades near $5.92/lb on COMEX. LME three-month copper sits around $13,030/mt, down 1.4% on the day. Global visible inventories — LME, SHFE, and COMEX combined — topped 1.5 million tonnes earlier this year, the highest in over two decades. The headline number screams surplus. The input chain is screaming something very different.

Here's the part nobody at CESCO Week last week wanted to say out loud, even though the entire conference was — by multiple accounts — more focused on acid than on copper itself.

The replacement options aren't real. Not on this timeline. Acid is dangerous to transport. Shipping costs and safety constraints make this a regional market, not a global commodity you reroute with a phone call. Canada produces some. Japan supplies about 11% of Chile's imports. Peru covers 24%. But none of them have the spare capacity to absorb a million-tonne hole left by China. Building new acid production takes years, not quarters. CRU's Peter Harrisson put it plainly: the loss of Chinese trade cannot simply be replaced with other origins.

Speculative net longs on COMEX copper have surged from 40,200 contracts in early April to 59,200 as of last Tuesday. That's a 47% jump in three weeks. But here's the thing — the money flowing in is chasing the price rebound from $5.40 in March toward $6.12 last week. It's momentum money. CTA trend-following. Very little of it, from what I can see, is priced around the acid supply chain. The specs are long copper because the chart turned up. The chart turned up partly because Chinese smelters just produced a record 1.33 million tonnes of refined copper in March — using acid they already had in stock.

The irony is beautiful in a dark sort of way. Record Chinese refined output is the headline that drew the longs in. But that record output consumed acid inventory that now can't be replenished via exports — because the same government that ran the smelters at full tilt is now banning the acid from leaving. The smelters ate the buffer. The buffer is gone.

I've been wrong about input-chain squeezes before — they take longer to transmit into price than you think, and sometimes producers find workarounds that aren't obvious from the outside. But the math here is hard to work around. You can't leach oxide copper without acid. You can't conjure acid without sulphur. And both are being choked at the same time, from different directions, for different reasons. That's not a narrative. That's a mechanical constraint.

The sequencing here is slower than a classic short squeeze, and that's what makes it harder to trade and easier to miss. This isn't a one-week event. It's a three-to-six-month grind where the constraint at the mine head gradually translates into lower-than-expected refined output, then into tighter physical availability, then — finally — into a spot price that reflects reality instead of warehouse tonnage.

The twist is that the current speculative longs might be right on direction but completely wrong on catalyst. They're long because the chart looks good and the AI-demand narrative has legs. They're not long because Codelco's leaching operations at Chuquicamata and Radomiro Tomic are about to get rationed on a critical input. If the acid squeeze forces output downgrades in Q3, the longs win — but the market structure around them will be violently different from what they expected. Fast-money longs in a grinding physical squeeze tend to get shaken out by the volatility before the thesis pays.

Meanwhile, there's an interesting second-order effect. Sulphuric acid is also essential for Indonesian HPAL nickel projects and for silver — a huge share of global silver is produced as a copper mining byproduct. Constrain the acid, constrain the copper, constrain the silver. At a time when silver is already under pressure from rate-hike fears, the last thing it needs is a supply-side hit from an input most silver analysts have never heard of.

Where does capital go? Not into Chile-heavy copper ETFs that own everything from Codelco-exposed names to fully hedged diversified miners. The edge — if there is one — is in copper producers running concentrate-and-smelt operations with captive or contracted acid, outside the leaching chain entirely. Think Congolese concentrate producers shipping to Asian smelters. Think mid-cap names with sulphide deposits that don't need a drop of imported acid. They're the ones whose output doesn't have a May deadline hanging over it.

The warehouse inventory number says surplus. The acid market says constraint. One trader at CESCO put it best: the copper price is completely unhinged from the fundamentals. I'd agree — but I'd add that it depends on which fundamentals you're looking at. If you're reading warehouse reports, it's overpriced. If you're reading loading dock manifests in Mejillones, it might not be priced high enough. In my experience, the loading docks win eventually. It just takes longer than the charts suggest.