Pachuca tank

A Pachuca tank is a tall, narrow cylindrical vessel used to mix and aerate pulverized ore (pulp) suspended in water during gold and silver extraction. The tank works by forcing compressed air up through a central pipe, which creates a rising column of bubbles that churns the pulp and keeps heavy solids in suspension while lighter material settles. The design is simple, which is why it has remained standard in metallurgical plants for over a century.

The tank consists of a steel shell, typically 20 to 40 feet tall and 8 to 15 feet in diameter, with a central air lift pipe running from bottom to top. Compressed air, usually at 20 to 40 pounds per square inch, enters at the base of this pipe and expands as it rises, carrying pulp upward. When the mixture reaches the top, the air bubbles escape and the pulp spills over the sides and falls back down, creating a continuous circulation. This convection-style mixing is gentler than mechanical agitation and avoids the corrosion and wear problems that come with moving blades or paddles in caustic leach solutions.

Pachuca tanks excel at handling the cyanide leach process, in which finely ground ore is mixed with a sodium cyanide solution to dissolve gold and silver. The constant gentle stirring keeps ore particles in contact with the solution without settling on the tank bottom, where they would be lost or cause dead zones where no leaching occurs. The same technology works for thickening (allowing fine solids to settle slowly while maintaining suspension), clarification, and various other metallurgical operations where mixing and settling must happen together.

Variants and practical limits

A deep tank design favors leaching time because the pulp spends longer in the solution before being lifted. A shallower tank favors faster turning and easier maintenance access. The air rate is the key control: too low and the pulp settles; too high and the whole mixture turns to foam and slurry spills over the rim, wasting ore and solution. Operators typically run Pachuca tanks in series, passing the leached pulp from one to the next to maximize contact time and gold recovery.

The tank takes its name from Pachuca, Mexico, where the design was developed and refined during silver mining operations in the 19th century. Because compressed air is its only moving part, maintenance costs stay low and downtime is rare. The chief wear point is the central air pipe, which can corrode or accumulate scale from hard water or chemical buildup. Replacement air pipes and routine cleaning of the inside are standard operations that keep the tank productive for decades.