Coal Inspectorate | Alert | No.410 V 1 | 03 June 2022

Use of liquid nitrogen in the mining industry

What happened?

On Tuesday 17th May, a worker received serious cryogenic burns after immersing their hands in a container of liquid nitrogen whilst trying to shrink a brass bush for inserting into an excavator boom arm. Whilst this incident did not occur on a mine site, this type of work may be conducted on mine sites, and in many cases may be done by contractors. Regardless of who conducts the work, the Mine Safety and Health Management System must manage the risks associated with the safe use and handling of liquid nitrogen or similar substances, if it is used at the mine.

How did it happen?

Work Health and Safety Queensland inspectors reported that the worker was not wearing the correct personal protective equipment for the task. Further details related to the incident are not available at this time

Key issues

Liquid Nitrogen

Liquid nitrogen is one of the cryogenic liquids commonly used in the mining industry. As “cryogenic” means related to very low temperature, it is an extremely cold material. Liquid nitrogen has a boiling point of negative – 195.8 degrees centigrade and can expand to a very large volume of gas.

The vapor of liquid nitrogen can rapidly freeze skin tissue and eye fluid, resulting in cold burns, frostbite, and permanent hand and eye damage, even by brief exposure.

Liquid nitrogen expands 695 times in volume when it vaporises and has no warning properties such as odour or colour. Hence, if sufficient liquid nitrogen is vaporised to reduce the oxygen percentage to below 19.5%, there is a risk of oxygen deficiency which may cause unconsciousness. Death may result if oxygen deficiency is extreme. To prevent asphyxiation hazards, handlers must make sure that the work area is well ventilated.

Without adequate venting or pressure-relief devices on the containers, enormous pressures can build upon evaporation. Users must make sure that liquid nitrogen is never contained in a closed system. Use a pressure relief vessel or a venting lid to protect against pressure build-up.

Handling Safety Practices

• Liquid nitrogen should be handled in well-ventilated areas.
• Handle the liquid slowly to minimize boiling and splashing.
• Use tongs to withdraw objects immersed in liquid nitrogen - Boiling and splashing always occur when charging or filling a warm container with liquid nitrogen or when inserting objects into the liquid.
• Use only approved containers. Impact resistant containers that can withstand the extremely low temperatures should be used. Materials such as carbon steel, plastic and rubber become brittle at these temperatures.
• Only store liquid nitrogen in containers with loose fitting lids (Never seal liquid nitrogen in a container). A tightly sealed container will build up pressure as the liquid boils and may explode after a short time.
• Never touch non-insulated vessels containing liquid nitrogen. Flesh will stick to extremely cold materials. Even non-metallic materials are dangerous to touch at low temperatures.
• Never tamper or modify safety devices such as the cylinder valve or regulator of the tank.
• Liquid nitrogen should only be stored in well-ventilated areas (do not store in a confined space).
• Do not store liquid nitrogen for long periods in an uncovered container.
• Cylinders should not be filled to more than 80% of capacity, since expansion of gases during warming may cause excessive pressure build-up.

Recommendations

Site Senior Executives (SSEs) must:

Identify if liquid nitrogen or other cryogenic substances are used or likely to be used on the mine site. Consideration should be given to the use of alternative engineering solutions.

Where liquid nitrogen or other cryogenic substances are to be used on site:

• ensure risk assessed procedures are developed, implemented and communicated. This should include the provision of appropriate tools (tongs, baskets, lifting jigs), suitably rated PPE (gloves, apron, double eye protection) and task specific procedures.
• provide site-specific training in the use of liquid nitrogen or other cryogenic substances. Records of training shall be maintained.

Supervisors must: