Molybdenum: many cleaner stages, and often the mine as well

Molybdenum flotation looks simple. Molybdenite is naturally hydrophobic, and a few drops of fuel oil will lift it into a rougher froth. But cleaning that froth to a roaster or ferromolybdenum buyer's specification is where the trouble hides. Say a mine owner asks: "Why does my rougher concentrate grade well but the final shipment still attracts penalty charges?" The answer usually sits in three places: overgrinding, copper and lead deportment, and oxidised surfaces.

Contact angle data tell the story. A review of molybdenite flotation (ScienceDirect) reports that natural molybdenite faces are hydrophobic while the edges are hydrophilic. One study on natural crystals obtained a contact angle around 80° for the faces. That anisotropy means a liberated, unoxidised molybdenite flake will attach to a bubble strongly, but any broken edge or attached gangue can drag it back. You'll float molybdenite with a light hydrocarbon collector because the surface already wants air. Rougher recovery is rarely the problem. The problem is cleaning. Because the mineral is so floatable, it doesn't need a strong collector, so selectivity against pyrite and chalcopyrite comes mostly from depressants, pH and staged regrinding. If you pull too hard early, you'll entrain fine gangue and sulphides that no amount of cleaning will shed.

Most porphyry molybdenum comes with copper. You'll face a choice: bulk flotation followed by selective molybdenum flotation after copper and iron sulphide depression, or preferential molybdenum flotation from the start. Bulk flotation then separation is the common route because both minerals float naturally together. The separator chemistry matters more than the collector. Sodium hydrosulphide is the benchmark copper depressant, but it carries HSE concerns and releases toxic H₂S if the pH drops. Polysulphide alternatives offer safer handling, but they change the pH control strategy (patent).

One locked-circuit test on a copper-molybdenum bulk concentrate reported 21.05% Cu and 1.10% Mo in the bulk product, with copper recovery of 87.37% and molybdenum recovery of 77.07%. After selective flotation, the molybdenum concentrate assayed 46.19% Mo and 0.75% Cu at 93.80% molybdenum recovery, while the copper concentrate reported 21.50% Cu and only 0.069% Mo at 99.92% copper recovery (SAIMM). Those numbers show the separation has to land the molybdenum in one product and keep copper almost entirely out of the other. For a deeper look at the copper side, see the copper circuits page.

Oxidation strips the face hydrophobicity that makes molybdenum flotation easy. A fresh basal plane may have a contact angle above 60°, often around 80° (ScienceDirect). Weathering or an oxidised zone can drop that to near zero, especially if clays form. One study reports the molybdenite face completely loses its hydrophobicity, contact angle 0°, with just 50 ppm of clay in fresh water, and close to 80 ppm was required in seawater (PMC). That's a dramatic change. Oxidised molybdenite won't attach to bubbles. It reports to tailings even though the assay still shows molybdenum in the feed. Weathered ore also generates slimes that coat the few remaining hydrophobic faces.

Before you commit to a flowsheet, run bench oxidation tests. Measure the contact angle, or use a simple micro-flotation test on a freshly ground sample and on a sample left to weather for a week. Compare the kinetics. If the oxidised test loses more than a trivial amount of recovery, the flowsheet needs a different collector, a sulphidiser or a pre-aeration step. No pilot plant will fix a problem that bench tests can show for the cost of a few hours.