Fire in a deposit of metallic waste

The event occurred at a centre handling metal waste. A fire broke out in a container for aluminium alloy shavings, while handling it with a grapple excavator. The fire spread throughout the entire building.
An operator intervened with a fire extinguisher while the excavator operator, assisted by two others, removed the machine from the building. The 61 employees were evacuated.
A plume of dense smoke, visible for several kilometers, rose from the building. Ten residents were confined within the path of the smoke.

The INITIATING CAUSE was the accidental formation of an explosive atmosphere by metallic waste and its ignition during the handling.
The waste in question was a historical batch of turnings made of the 7075-aluminium alloy. It had been stored for five years in a humid environment with aged cutting oil. These conditions led to oxidation of the turnings, resulting in a concentrated hydrogen cloud above the container.

The IGNTITION SOURCE was probably provided by a spark generated by the collision between one of the grapple's fingers and the container wall
The presence of suspended dust may also have contributed to the formation of an explosive atmosphere. The melting of the translucent roof panels may have contributed to the fire spreading within the building.

The ROOT CAUSE could be identified in an inadequate risk assessment of the waste storing and handling processes.

metallic cotnainer, crane

BACKGROUND INFORMATION ON THE ACTIVITIES
The company was specialised in recycling of ferrous and non-ferrous materials. On the site effected by the fire, metallic batches were sorted, consolidation and packaged for further processing/recycling.
7075 aluminium alloy (AA7075) is an aluminium alloy with zinc as the primary alloying element and composition:
zinc: 6–6.1% ,
magnesium: 2.1–2.5% ,
copper: 1.2–1.6%
[https://en.wikipedia.org/wiki/7075_aluminium_alloy].

Material damage = €2 million
Business interruption losses = €400,000.

Unwanted hydrogen production from oxidation of metallic waste is a well-documented occurrence. It can come to incidents, due to a combination of factors such as high humidity, long storage periods and, above all, storage in confined spaces, what enable the formation of an explosive hydrogen-air mixture.
ARIA source reports that the operator took measures aiming at improving the efficiency of the emergency actions. Nothing is said on possible preventive measures, which could be reduce the probability of formation of the explosive mixture, such as an adequate room ventilation and explosive gas detection.

Following this incident, the operator:
(1) relocated the fire hose reels.
(2) Added a 15-tonne cement stockpile to the site.
(3) Install another emergency exit within the building.