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Why that small amount of platinum in a drum of catalyst is worth the price — its pedigree is far more complicated than most people imagine.
1. The Starting Point: Pitifully Little Platinum in the Ore
Platinum group metals rarely form orebodies of their own. The overwhelming majority occur as associated constituents of nickel–copper sulfide ores, with head grades typically of only a few grams per tonne (ppm level). The principal producing region is the Bushveld Complex in South Africa, followed by Norilsk in Russia and the Great Dyke in Zimbabwe.
That associated nature is what determines the supply rigidity discussed in the previous article: the mine's primary products are nickel and copper, and platinum merely comes along for the ride.
2. Step One: Mineral Processing and Concentration
After the ore is crushed and ground to a suitable particle size, flotation is used to concentrate the sulfide minerals. This step does not separate platinum; it merely raises "a few grams per tonne" to "tens to hundreds of grams per tonne" in the concentrate, lightening the load on the downstream pyrometallurgical stages.
3. Step Two: Pyrometallurgical Smelting — Platinum Reports to the Matte
The concentrate is smelted in a flash furnace or an electric furnace. Nickel sulfide and copper sulfide form the matte, and the platinum group metals have one crucial habit — they are chalcophile elements and preferentially report to the matte phase rather than reporting to the slag.
The slag is discarded or returned for reprocessing, while the matte carries the platinum group metals onward.
4. Step Three: Secondary Concentration — Driving Out the Base Metals
The platinum concentration in the matte is still not high enough, so further concentration is required. Two routes are common in industry:
By the end of this step, platinum group metal concentrations have typically been raised to a few percent up to tens of percent, and the material is ready for the hydrometallurgical route.
5. Step Four: Aqua Regia Dissolution — the Platinum Group Enters Solution
The insoluble residue is dissolved in aqua regia (a mixture of concentrated hydrochloric and nitric acids). Most of the platinum and palladium pass into solution; rhodium and iridium dissolve only partly, while ruthenium tends to volatilize and be lost under oxidizing conditions and requires a dedicated recovery step.
This is the entry point to hydrometallurgical refining. The solution now contains several different platinum group metal ions crowded together, and the task ahead is to separate them one by one.
6. Step Five: Platinum's Signature Separation — Ammonium Chloride Precipitation
This is the most classic step in the entire history of platinum metallurgy. Ammonium chloride is added to the platinum-bearing solution, and the platinum precipitates as ammonium hexachloroplatinate, (NH₄)₂PtCl₆ — a bright yellow crystalline powder. Under the same conditions, most of the other platinum group metals remain in solution.
The reaction has excellent selectivity and an immediately visible endpoint; it is still in use today, and it is also platinum's signature identification reaction in the history of chemistry.
Palladium takes a different route: ammonia is added to form a soluble ammine complex, which is then acidified to precipitate palladium(II) diammine dichloride.
7. Step Six: Repeated Refining, Purity Climbing Step by Step
A single precipitation is nowhere near enough. The ammonium hexachloroplatinate goes through a cycle of calcination to platinum sponge → redissolution in aqua regia → precipitation again, usually repeated three to five times, pressing the impurities down level by level.
Modern plants increasingly use solvent extraction (a specific organic extractant selectively picks up platinum) and ion exchange resins to replace or supplement the traditional precipitation route, with the advantages of higher recovery, shorter flowsheets and less waste liquor. But because of its reliability and its visible endpoint, ammonium hexachloroplatinate precipitation is still retained at quite a few stages.
8. Step Seven: Reduction to Metallic Platinum
The refined chloroplatinic acid is reduced with a reducing agent to yield metallic platinum:
Platinum black and platinum sponge are the starting raw materials of the catalyst industry — the platinum black spontaneous ignition experiment mentioned in an earlier article used precisely the product made here.
9. What the Purity Grades Mean
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|
Grade |
Typical Use |
|
99.9% |
General industrial applications |
|
99.95% |
Standard catalyst feedstock |
|
99.99% |
High-end catalysts, electrode materials |
|
99.999% |
Electronics grade, reference materials |
What is that remaining fraction of a percent? Mainly other platinum group metals, base metals (iron, copper, nickel), and non-metallic impurities such as carbon, sulfur and silicon. Article No. 12 deals with this specifically: how much these extra nines actually affect catalyst performance. One spoiler — the effect is bigger than most people assume.
10. Assaying Throughout: Fire Assay and ICP
Precious metals analysis has one "gold standard" procedure — the fire assay. The sample is fused at high temperature with lead and flux; base metals report to the slag phase while precious metals are collected by the lead, which is then removed by cupellation, and the final button is weighed for quantification. It can accurately preconcentrate trace platinum, with accuracy higher than most instrumental methods.
Trace impurities are left to ICP-OES / ICP-MS, which can measure down to the ppb level.
11. Closing the Loop: Mother Liquor and Scrap Must Be Recovered
There is an iron rule running through the whole flowsheet — no platinum-bearing stream may be discharged directly. Precipitation mother liquor, wash water, filtration residues and spent reagents all return to the front end of the process for recovery.
This is both an environmental compliance requirement and, more importantly, the crux of cost. In a poorly managed workshop, platinum losses in the mother liquor alone can eat away a sizeable slice of the year's profit. In our plant the mass balance is done by shift, and every gram of platinum has to reconcile.
12. The Endpoint: From Platinum Sponge to That Drum of Pale Yellow Liquid
Back in the catalyst workshop, the flowsheet is short but critical:
