Metso Insights Blog Mining and metals blog Beginners guide to hydrometallurgical Autoclave processes
Metals refining
Dec 3, 2021

Beginners guide to hydrometallurgical Autoclave processes

The scope of this article is to briefly present the Metso Outotec OKTOP® Autoclave and the equipment required to make the process functional. The typical processes that the autoclave is used for include pressure oxidation (POX), high-pressure acid leaching (HPAL), pressure alkaline leaching (PAL), and pressurized precipitation. Hydrometallurgical pressure processes are used in a variety of different applications to extract different kinds of metals from refractory ore bodies (Figure 1). The process is intended to be continuous with a constant feed.
 Pressure process offering
Figure 1. Pressure process offering

The primary equipment in the process is the autoclave, but the pressure circuit includes many other supporting pieces of equipment. Two of the biggest challenges for the equipment during pressure processes are corrosion and erosion. The slurry that travels through the equipment is typically acidic, meaning it causes wear over time.

The process starts with mining the ore, which is then crushed to a fine material so that it can be mixed with water. The next step is to pump the slurry, consisting of water and fine solids, into the autoclave. The temperature and pressure in the autoclave are much higher than in atmospheric conditions. Exothermic processes create heat and typically require cooling in the form of, for example, a quench water feed or internal cooling coils. Endothermic processes on the other hand require external heating. Preheaters that use ventilation steam from flash vessels can be used to raise the temperature of the slurry feed prior to the autoclave. If needed, heating during operation is usually done with a steam feed.

Inside the autoclave, utility gases and steam are fed through the blowback vessels. The slurry flows through the autoclave and is mixed by the agitator units throughout the process. At the end of the autoclave is the slurry discharge outlet. After the autoclave, the slurry is brought back to atmospheric conditions via one or several flash vessels. When the slurry exits the last flash vessel, the product of the autoclave process can be used for further refining. A generic autoclave process, including the different equipment and how it is connected, is illustrated in figure 2 below.  

A pressure circuit including an autoclave
Figure 2. A pressure circuit including an autoclave


The pressure process includes several different types of equipment, each with a specific function. In the next section we look at the different equipment types and explain how they work in the process.

Feed tank

The feed tank provides a controlled environment to make sure that the process has a homogenous mixed feed to the autoclave. This homogenous mixing is key to ensuring efficient operation and is done with an agitator that continuously mixes the slurry.


Preheaters can be used for endothermic processes or other processes that benefit from preheating. The role of the preheater is to preheat the process slurry before it enters the autoclave. The heating is done by using ventilation steam from a flash vessel, transferring the steam energy to the slurry inside the preheater. During preheating the slurry falls to the bottom of the tank and the excess steam rises to the top.

Feed tank and preheater
Figure 3. Feed tank and preheater


The autoclave is the primary equipment in pressure processes and is the largest single pressurized equipment in the pressure circuit; it is a pressure reactor. This is the part of the process where chemical reactions like leaching and precipitation happen.

The autoclave vessel is divided into compartments consisting of an agitator unit and baffles divided by walls. The agitator mixes the slurry throughout the process. Different types of agitators are available to fulfill the different process requirements of various applications. To improve the efficiency of the agitators there are baffles in every compartment. These improve the mixing efficiency by directing horizontal flow to vertical movements, which prevents vortex formation and enhances axial flow. This leads to reduced mixing time by improving the suspension of solids and dispersion of gas.

Metso Outotec OKTOP® autoclave agitator unit and Mechanical seal
Figure 4. Metso Outotec OKTOP® autoclave agitator unit and Mechanical seal

Typically, a pressure process uses a highly acidic slurry that causes equipment wear. In order to protect the equipment, the inside of the autoclave vessel is made from corrosion-resistant materials or a corrosion-resistant lining is applied. In addition, the vessel can be lined with acid-resistant bricks or PTFE blocks that protect it from thermal and mechanical wear.

The temperature inside the vessel can vary from 100 to 280 degrees Celsius and the pressure can vary from 3 to 70 bar.

Figure 5. Autoclave

Blowback vessel

The blowback vessel protects the feed system from unpredictable backpressure and flows. The steam/utility gases are fed into the autoclave through a blowback vessel to buffer the fluctuating pressure of the autoclave and allow response time in case of backflows. High pressure in the autoclave can push slurry into the feed system, which endangers both the equipment and the overall process. The feed of oxygen to the autoclave is continuous throughout the process. Steam is required for autoclave start-ups and continuous slurry heating in endothermic processes.

Flash vessel

The flash vessel is located after the autoclave and prepares the slurry for further processing. The purpose of the flash vessel is to return the slurry to atmospheric conditions, minimizing solids carryover. This is done in stages if needed using one or several flash vessels. The steam generated in the flash vessel can be used by energy recovery systems.

Autoclave off-gas scrubber

The gas that is generated by the autoclave process needs to be cleaned of harmful components. In hydrometallurgical pressure processes these components can consist of droplets, solid particles, or gaseous components such as hydrogen sulfide. Off-gas cleaning is done with wet scrubbers, and the type of scrubber varies depending on the process. Wet scrubbers use water or scrubbing solutions such as caustics to clean the gas. Autoclave processes typically use an ejector-venturi scrubber to provide high washing efficiency. The scrubber not only ensures that the environmental impact of the pressure process is minimal, but it also improves safety for workers and minimizes losses during the process.

Blowback vessel, flash vessel and ejector-venturi off-gas scrubber
Figure 6. Blowback vessel, flash vessel and ejector-venturi off-gas scrubber

Because all of the equipment in the process with the exception of the feed tank is pressurized, there are strict regulations about equipment design and the materials it is manufactured from. These regulations vary depending on where in the world the equipment will be used. Automation and instrumentation are typically on the system level, which means that the process is highly automated to ensure compliance with regulations.

So why do we need pressure processes that use these types of equipment? Some chemical processes that are used to extract certain products require conditions that are a lot different from atmospheric conditions. Some chemical processes that are used in hydrometallurgy aren’t possible without the high pressure and high temperature provided by an autoclave pressure circuit. Pressure processes are also used for metallurgical processes that, while still possible without the conditions provided by an autoclave, work much faster with one due to the higher pressure and temperature.

Every pressure process is planned and designed specifically for the process it will be used in. This makes every project demanding. When designed and operated correctly, hydrometallurgical autoclave processes are safe for both the operator and the environment.

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