Most hydrometallurgical processes include one or more leaching steps to initially dissolve the metal of interest or to remove unwanted impurities.

The leaching process may employ one of a variety of chemicals such as dilute sulfuric acid, dilute aqueous cyanide, or other reagents. Methods for contacting ore and solution include agitating ground solids and solution (slurry) in open tanks, agitating slurry at a high temperature in a pressure vessel, or percolating solution through crushed ore that has been piled in a heap or vat and collecting the solution that issues from the bottom.

High-temperature and pressure leach processes are performed in autoclaves. Hazen's batch laboratory autoclaves and continuous pilot autoclaves have been operated under oxidizing, acidic, or alkaline conditions (see High Pressure Leaching). In other processes, oxidizing or reducing gases, such as air, oxygen, or sulfur dioxide, are used, taking into account the gas-liquid contact requirements.

Some leaching systems are complex and are difficult to simulate in single-stage laboratory experiments. Such cases require special equipment and procedures to produce accurate data to predict how the system will perform in a large-scale commercial operation. For example, in the recovery of gold by cyanidation using the carbon-in-leach (CIL) process, the ground ore is suspended in a cyanide solution and is fed to a series of tanks. The slurry cascades from one agitated tank to the next, contacting activated carbon in each tank where the gold is adsorbed. Each overflow stream passes through a screened funnel into the subsequent vessel to collect the carbon from the stream. The collected carbon is manually transferred back to the preceding vessel (or other parts of the system) to realistically simulate the countercurrent flow of ore and carbon until the desired gold content is attained.

Leaching processes are also used to chemically remove contaminants from soils or other solids. Examples include removing mercury, organics, and other contaminants. Hazen has experience in a wide range of chemical soil washing projects for clients.

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