barrel process
A process of extracting gold or silver by treating the ore in a revolving barrel or drum, with mercury, chlorine, cyanide solution, or other reagent.
barrel process: rotating drum extraction for precious metals
The barrel process treats crushed ore in a rotating cylindrical drum to dissolve and concentrate gold or silver. The drum, typically 1 to 3 meters in diameter, rotates at 20 to 40 rpm while containing ore, water, and a chemical reagent. This gentle agitation keeps particles in suspension and in contact with the leaching solution, avoiding the settling and dead zones that plague static tanks.
The choice of reagent depends on the ore type and local conditions. Mercury amalgamation in the barrel suits fine placer material and low-grade ore; cyanide solution handles most hard-rock deposits; chlorine works on some gold ores but is less common now. The drum's rotation ensures even exposure of ore particles to fresh reagent, improving extraction rates compared to simple standing pools. Residence time typically runs 2 to 8 hours depending on ore grade and reagent strength.
Practical operation and wear
Barrels are lined with rubber or ceramic to resist corrosion from acidic or alkaline solutions. The drum must rotate smoothly on roller bearings or trunnions, and seals at both ends must stay tight; leaks cost reagent and create safety hazards. Operators add fresh reagent continuously to maintain concentration. The slurry inside becomes progressively more dilute as the metal dissolves, requiring periodic discharge and refilling.
The process works well for finely ground ore (minus 200 mesh or finer) but struggles with coarse particles or sticky clay, which settle and stick to the barrel wall. Overloading reduces rotation efficiency and causes material to pack rather than tumble. Temperature control matters; warmer solutions extract faster, but excessive heat increases mercury vapor losses or causes reagent decomposition.
The barrel process is now less common in large-scale mills, which favor heap leaching or tank circuits with mechanical agitation. It survives in small operations, custom ore treatment, and reworking of old tailings where low capital cost and batch flexibility outweigh slower throughput. Modern variants use multiple smaller barrels in series to improve leaching kinetics without building a massive single drum.