Flow control for gold

Gold leaching plants use valves and pumps for numerous flow control and isolation services. Valve and pump performance affects the whole leaching process, so it is vital to select the right type of valve or pump for the right application.

Requirements for tightness, abrasion and erosion resistance, and pressure range, are among the key variables when choosing the optimal valves or pumps. Reliability should also be considered, since malfunctions may lead to safety risks and added process downtime.

Valves for thickening

Thickeners increase the content of solids and thus the gold concentration of the slurry. Unwanted gangue is separated in the thickener and processed in the tailings stream. Process water, on the other hand, is recycled back for upstream processing.

Valve installations in thickeners are required for overflow as well as underflow service. On the overflow side, butterfly valves provide effective flow control. Knife gate valves are also used. For the thickener underflow, pinch and knife gate valves are the most common solutions.

Valves for pressure oxidation

If the gold ore is refractory by nature, it may require pre-treatment processing before leaching.

One pre-treatment option is pressure oxidation (POX). Due to the conditions that valves are subjected to in the POX process, the highest possible resistance against corrosion, erosion, and deterioration caused by solids is required.

To maintain the sealing abilities of slurry feed valves, the right material, seat and coating choices play an important role. Valmet offers the most competitive valve solutions for the POX process.

Valves for leaching tanks

Regardless of the leaching method (CIL, CIP or CIC), valves must provide reliable control for the liquid, gaseous and air flows.

The presence of cyanide creates challenges for process control and makes the correct valve material selection essential. When performance requirements are a high priority, process ball valves are a good option for the leaching tanks.

With the right material, coating and seat selection, ball valves provide a long service life and precise flow control capabilities.

The gold leaching process starts with comminution. Run-of-mine ore is first crushed and ground to a fine particle size. Water is then added to the process to create a mud-like slurry that is pumped into a thickener where gold-bearing particles sink to the bottom and are passed to downstream processing.

Typically, the next processing phase is a CIL, CIP or CIC circuit. All these methods use cyanide and activated carbon to extract the gold from the slurry. The applied technology varies from one plant to another, but CIL is the most commonly used method.

In the CIL circuit, cyanide and oxygen are added to the slurry, resulting in gold being dissolved from the ore. Activated carbon is then fed into the process from the opposite direction, against the slurry stream, and as it moves through the CIL tanks, dissolved gold adsorbs onto the activated carbon. This loaded carbon then continues into an elution column, where the adsorption process is reversed.

Gold is stripped off the carbon into a pregnant electrolyte and fed into electrowinning cells for plating onto stainless steel wool cathodes. A water spray then removes the gold sludge from the loaded cathodes for filtering and drying. After dewatering, the gold sludge is mixed with fluxes and heated in a furnace to produce doré bars that are transported to a gold refinery for processing into 99.9% pure gold.

In the POX process, the ore is crushed and mixed with water to create aqueous slurry that is then heated and fed into an autoclave.

Oxygen is added to the vessel to react with the slurry. The POX process uses high temperatures (approximately 200 °C/400 °F) and high pressures (10-35 bar/150-500 psi) to facilitate the leaching process.

After the autoclave processing step, the slurry is returned to atmospheric condition by taking it through two or more flashing letdown stages.

Once at atmospheric condition, the slurry is washed and separated, at which point the metal can be recovered from the liquid portion in the downstream processing circuits.