Sulphide self-heating behaviours, risks, mitigation and prevention

The self-heating phenomenon

Self-heating is defined as the self-induced temperature rise in materials containing sulphide minerals caused by accumulated heat resulting from internal exothermic reactions. In short, mixtures of sulphide-bearing minerals (in ores, concentrates, waste rock, tailings, paste backfill, etc.) may react with the moisture and air in their environment to generate heat and produce other potentially toxic by-products. These reactions, if gone unchecked, can pose serious risks to an operation.

The self-heating phenomenon can be described by the following three heating stages:

• Stage A (Weathering): Occurs at temperatures below 100°C
• Stage B (Oxidation): Occurs at temperatures between 100°C and approximately 350°C
• Stage C (Roasting): Occurs at temperatures above approximately 350°C

The heating behaviour of a material within these stages is affected by factors such as the material’s mineralogy, oxygen content, moisture content and particle size as well as the temperature and relative humidity of the surrounding environment. As Stage C occurs at temperatures above a material’s ignition point, the primary focus is understanding and controlling a material’s heating in Stage A and Stage B, so that ignition of the material does not occur.

In Stage A, the heating reactions are electrochemical in nature and require a cathodic site and an anodic site in the material to occur. Therefore, a mixture of at least two sulphides is required for heating to occur in Stage A. The exception to this rule is that of pyrrhotite, which can occur in two different forms (hexagonal and monoclinic) and drive Stage A heating reactions on its own. In the series of Stage A reactions, oxygen and water react with the sulphide materials to produce elemental sulphur. This process is exothermic and generates heat. As the material approaches the boiling point of water, the water in the material begins to evaporate and the material dries, slowing down the Stage A reactions. However, the heating and drying of the material allows Stage B reactions to begin to occur.

In Stage B, elemental sulphur is oxidized to form sulphur dioxide (SO₂) gas. Since this reaction requires elemental sulphur to occur, the heating potential of a material in Stage B is generally proportional to its heating potential in Stage A. This reaction is even more exothermic than the reactions in Stage A, and the heating rate of a material in Stage B is generally greater than its heating rate in Stage A until all the elemental sulphur present is oxidized.

Self-heating risks and mitigation

If not dissipated, the temperature of a self-heating material can rise to hundreds of degrees. Self-heating can also result in depleted oxygen levels in confined spaces, emission of toxic gases such as H₂S and SO₂, spontaneous ignition of the material and, in the case of ores or concentrates, can lead to a degradation of product quality. In addition to the possible impact of self-heating events on CAPEX and OPEX costs, there is a risk to both the safety of workers and to the local environment. Failure to recognize and manage these risks can be viewed as a lack of due diligence. Additionally, managing these risks efficiently can have significant CAPEX and OPEX benefits when compared to an inefficient solution.

The movement, storage and handling of materials that are at risk of self-heating can be safely conducted by first understanding the level of risk and then applying appropriate engineering controls. There is no single solution to mitigate all risk situations. Controls can come in the form of operating procedures and appropriate monitoring, removing either moisture or oxygen from the material or environment, adding chemical agents to consume the necessary reaction fuel or appropriately designing engineering facilities to store and handle the material. In some cases, the mining/process route itself can be modified to change the composition of the mineral mixtures. The key is to first understand the potential risk posed by the material and the material handling chain.

How to prevent self-heating in your operation

In order to design a new operation or analyze a current operation to minimize the risk for self-heating at all stages, you first must identify the potential self-heating risks that your material poses. BBA has a dedicated testing laboratory where we conduct material assessment for self-heating (MASH) tests using the proven FR-2 testing equipment developed by Rosenblum, Nesset and Spira.

When you understand the self-heating properties of your material, you can then examine the likelihood of a self-heating event at each stage of the material handling chain and put appropriate controls in place for any stage that is identified as being high-risk. Often, when materials are highly reactive, material handling infrastructure needs to be designed specifically to mitigate risks posed by self heating. Critical areas that require careful design considerations are storage buildings, conveyor transfer chutes and storage bins.

BBA’s team has a wide array of experience in the fields of mine planning, metallurgy, mineral processing and material handling and is capable of developing mitigation solutions for even the most challenging situation, be it a greenfield, brownfield or a retrofit project. We have a large database of tests that have been previously conducted on a variety of materials and can be used for comparative analyses. BBA has the capability and expertise to conduct the detailed design or retrofit of material handling infrastructure, such as bins and chutes, to accommodate a reactive material. Our experienced experts have designed numerous material handling systems for self heating materials. We can also advise you on the design and data analysis for pilot-scale studies.

Because of our broad expertise in self-heating and our unique testing facilities, BBA is in an exceptional position when it comes to helping clients both understand the potential self-heating risk posed by their materials and guiding them to solutions that will minimize the risks of moving, storing and handling these materials.

If you’re interested in any of the services listed below, feel free to contact Patrick Beaulieu (patrick.beaulieu@bba.ca) or Kevan Ford (kevan.ford@bba.ca) for more details:

• Material assessment for self-heating tests and results analysis
• Self-heating risk assessments for existing operations or new projects
• Design and data analysis of self-heating pilot studies at client sites
• Detailed material handling infrastructure design for self-heating
• Process engineering for risk mitigation
• Self-heating awareness training for operations and technical staff
• Self-heating mitigation solution selection
• Fundamental screening studies for chemical additives