Managing post blast fume (oxides of nitrogen), is a complicated issue for some mining companies, given the exposure it creates for onsite personnel, nearby communities and the environment. However, there are measures that can be taken into account to minimize its production. Understanding how to deal with fume is an inevitable consequence of blasting, and has recently developed into a major issue impacting the “license to operate” for mines in many parts of the world.
Whilst some post blast gases are toxic, not all of them are. In terms of health impacts, the most dangerous gases are nitrogen dioxide (NO2) and nitric oxide (NO). Other gases often produced are nitrous oxide (N2O) and nitrogen pentoxide (NO5). We commonly group these all together as “oxides of nitrogen” or simply NOx as normally it isn’t possible to define which particular gases were produced in a fume event. Incomplete detonation, or detonation without a correct chemical oxygen balance is likely to result in higher levels of post blast fume and higher levels of NOx and so one of the targets of managing blasting is to ensure complete detonation of the bulk explosive product.
Different jurisdictions manage fume in different ways. For example, in Australia there is a specific code of practice provided by AEISG for post blast fume which looks to prevent occurrences and manage those that do occur. The code of practice specifies levels of severity of post blast fume production based on a scale of 0 – 5 where Level 0 means there is no visible post blast fume, all the way through to Level 5 which is the worst possible outcome (deep purple/brown fume cloud containing high levels of harmful NOx). The assessment is meant to be used to help determine the actions that should take place as a result. Remember, depending on the prevailing weather conditions, the post blast fume produced could disperse quickly or could remain concentrated for some time.
Ammonium Nitrate Fuel Oil (ANFO) is a popular bulk explosive often used for blasting as it is a particularly efficient and cost effective bulk product for removing overburden material. However, it is water soluble and therefore is damaged if water is present (this is one of the common causes of post blast fume). On the other hand, emulsions and water gels have more resistance to water damage than ANFO mixtures, so they can cope with varying ground conditions and are often used in wet ground. Similarly, if the ground contains many cracks or those cracks open during drilling, bulk explosive can fill those spaces and the reduced diameter can hinder a complete detonation.
To develop products for areas that are “fume prone” explosives companies aim to supply very stable, oxygen balanced emulsion products. These can take the form of an increased range of specialised emulsions whilst others rely upon a robust standard product. Specific emulsion formulations that are designed to prevent or minimize fume generation are often marketed as specialised products and may have lower water content, higher energy or sensitivity, higher viscosity, different types or amounts of emulsifier and/or have additional additives, compared with standard emulsions. These emulsions can be more costly than standard products but that may be compensated by a lower cost base product. Ultimately, it becomes important for mines to manage which products are used where.
Management of oxides of nitrogen (NOx) in open cut blasting should include but not be limited to:
• Procurement: Select explosives suppliers that have managed post blast fume before and can provide both products and procedures for managing blasts in fume prone areas, based on the service level they are contracted to provide.
• Contracts: When writing your explosives supply contract, ensure that it contains flexibility to deal with post blast fume events, e.g. access to alternative products and services available in the market to mitigate post blast fume, including new technology, so you are not locked in.
• Accountability: Demand your suppliers maintain updated technical data sheets and training programs, not only for their personnel but for the mine’s as well. Ensure there are quality control records for explosives manufacture and calibration records for the bulk explosives delivery vehicles used.
• Storage & Handling: Ensure correct storage of all raw materials including AN prill, emulsion, and any sensitizing chemicals where applicable. Correct storage reduces the risk that raw materials are out of spec when used.
• Design & Implementation: Correct selection of the initiation system is paramount to the performance of the blast. Initiation timing of the blast should consider the factors that are likely to generate post blast fume (e.g. bulk explosive type & energy, burden and spacing, hole depth, relief) and the environmental conditions (e.g. ground conditions, moisture levels, and weather conditions). Correct blast design may be a compromise between production based outcomes and environmental compliance. Risk assessments should be used to ensure the correct balance is maintained
• Training & Education: Be aware of the factors that contribute to post blast fume production, and how these can be managed at site level. Support responsible design practices that seek to mitigate fume whilst maintaining productivity. Remember it is usually more than one factor that contributes to the production of post blast fume.
• Continuous improvement: Evaluate the outcomes of blasting through ongoing measurement and use the data to feed into blast design and planning for the future. Monitor the supplier’s loading practices, the quality of the explosive products, and the QA/QC data. Record & file all post blast assessments including the level of fume produced and cross reference this against the blast parameters and prevailing conditions to build up your knowledge base.
At Moncourt Group we have the expertise and knowledge to support mines to reduce the risk of post blast fume and to manage any fume events. We know the products, suppliers, services, and technology available in the market as well as fully understanding the commercial propositions. Coupling all of this with our independence from any supplier or solution means we can help our clients to lower the likelihood and severity of post blast fume production at the site.