Pirobloc designs and manufactures elution heaters, both with direct heating (direct eluate heaters) and indirect. The elution heaters are boilers specifically designed to supply heat in gold and silver separation processes. The main separation processes of these minerals are:





Elution heaters are an essential element in the main methods of gold and silver separation, such as cyanidation, amalgamation or carbon-in-pulp, with the latter being the most common.

Here at Pirobloc, we have two systems that are used to heat the eluent solution, which is then used for the separation of gold and silver:

  • Direct eluate heaters
  • Indirect heating elution boilers (thermal oil)

The chemical composition of the eluent solution varies depending on the method used. However, it always needs to be heated to the necessary temperature, together with the gold or silver, in an elution boiler, so that the ore can be precipitated when mixed with the absorbing agent.


This is a more economical solution, as its design is simpler and it does not require much maintenance. For direct heating systems, we have a 3-pass stainless steel horizontal helical coil design, which is optimized to reduce the incidence of caustic precipitation.

Our direct elution boilers guarantee the highest quality standards, and they are customised to meet the specific needs of each mining application.

The cylindrical design of the boilers includes two helical coils, which are manufactured according to ASME standards. They are programmed to use radiant and convective heating from the combustion of diesel or gas in the burner, which is installed at the front of the boiler, to raise the temperature of the circulating fluid.

High-density rock wool insulation between the two shells allows low temperatures to be obtained in the outer shell (between 30°C and 40°C), minimizing thermal losses through the structure and avoiding burns caused by unintentional contact with the boiler.

Stainless steel helical coil direct elution boilers provide our customers with the following benefits:

  • Lower acquisition cost compared to indirect heating boilers.
  • Simple design with low maintenance requirements.
  • Automatic boiler operation in most applications, due to the design of the coil tube, allowing for safe operation at much higher pressures than a tank would be able to handle.
  • Absence of corrosion and stability of working temperature.
  • Stainless steel coil construction, with a long service life.
  • Direct elution heating is produced by a modified water boiler, with a gas or oil burner.

As we can see, the main advantage of direct heating over indirect heating in thermal fluid equipment is the cost of acquisition, as it is a simpler system that requires less equipment and components, because it does not need a heat exchanger, a pump or a control circuit.

Direct heating is probably best suited to small operations or short-life process plants.

The obvious disadvantage of direct heating is the fact that it is difficult to control corrosion and scale formation inside the heater, which necessitates regular replacement of the boiler shell. Depending on the water quality and the typical shell service life, this period can range from 9 months to 3 years.

The replacement of the shells (including downtime during the replacement operation) must be taken into account when estimating the long-term costs of the installation. If the shells are large, or if they will require several replacements throughout the service life of the mine, it may be better to move to an indirect heating system.

It is important to create a cost estimate that is as accurate as possible, as direct elution heating can be an economical elution method if the boiler is made from the right material (stainless steel vs. carbon steel), and if proper maintenance is carried out.


An indirect heating system consists of a thermal oil boiler and a plate heat exchanger, or a tube heat exchanger and shell. This is a more sophisticated installation, but it also offers greater efficiency, productivity and durability.

The thermal oil heater is the main component of an indirect thermal fluid heating system, in which the oil is heated to the desired temperature, and it then circulates within a closed circuit to provide energy for the elution process.

Other elements of this system are the heat recovery exchangers, the heat input exchangers, the expansion and deaerator vessels and the circulation pumps, together with the interconnecting piping and valves.

Heat recovery, energy consumption and fuel savings are important aspects of the elution process.

Here at Pirobloc, we offer great versatility in terms of our indirect heating solutions, so that we can adapt to the needs of each customer:

  • We have thermal fluid boilers with horizontal and vertical configurations. There is no functional difference between a horizontal and a vertical thermal oil boiler. The choice of one model or the other depends on the space and height available in the plant, as well as the ease of maintenance.
  • Thermal oil heaters run on diesel/gas/heavy combustion or electric heat.
  • Indirect elution circuits are delivered with all the auxiliary elements that are essential for their maintenance: pumps, expansion and deaeration systems, high temperature valves and shell and tube heat exchangers, or plate and frame exchangers for primary users/recovery and tertiary users in heating circuits.
  • The boilers can be delivered mounted on skids. This solution is becoming more and more common, as it often facilitates assembly at the customer’s site, reducing technical obstacles and assembly costs.
  • The thermal oil system can be constructed inside a container, complete with all its components, piping, instrumentation, electrical panels and lighting. If more than one container is used, they are simply unloaded into position at the customer’s facility and interconnected to make a complete elution heating system that is ready for operation.
  • We manufacture and supply all the spare parts that are required in order to guarantee the correct operation of our systems.
  • Maintenance of the elution heating system.

An indirect heating system that uses a thermal oil boiler and a heat exchanger has numerous benefits:

  • It operates without reduced pressure, and without water-generated steam. This avoids the typical problems associated with steam heating systems, such as leaks, corrosion, water treatment, etc.
  • It is a low-maintenance solution.
  • Significant fuel savings, in comparison to steam heating systems. A thermal fluid system consists of a closed circuit with minimal energy leakage. At least 90% of the fuel is used in the heating process.
  • Highly efficient, due to the uniformity in temperature throughout the entire production process.
  • Automatic boiler operation and indirect monitoring.
  • This is a solution that enables constant production, which is critical in an industry that often operates 24 hours a day, every day of the year.
  • The boilers are head-wearing, easy to operate, and durable, with a service life of up to 20 years or more.


There are three methods of gold and silver separation that require elution boilers to provide the proper temperature for the chemical compound that will cause the gold and silver to separate from the rest of the ore.

  1. Cyanidation is an ore separation process that involves the use of chemicals to separate gold from its contaminants. As part of this process, the ground ore is placed in a tank that contains a weak cyanide solution. Zinc is then added to the tank, causing a chemical reaction in which the end result is the precipitation and separation of ores. Subsequently, the gold precipitate is separated from the cyanide solution in a filter press.
  2. Amalgamation is a similar method, using the same process, but with different chemicals. First, a solution transports the ground ore over plates that are coated with mercury. The mercury attracts the gold, forming an alloy called amalgam. The amalgam is then heated, causing the mercury to boil off as a gas and leaving the gold behind. The mercury is collected, recycled and used again in the same process.
  3. The carbon-in-pulp method also uses cyanide, but it uses carbon to precipitate the gold instead of zinc. The first step is to mix the ground ore with water to form a slurry. Cyanide is then added to dissolve the gold, and subsequently carbon is added to bind with the gold. Once the carbon particles are removed from the slurry, they are placed in a hot, caustic (corrosive) carbon solution, which separates the gold from the carbon.

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