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EXPLOSION AT A BALL MILL

A ball mill in the United States was wet-milling a mixture of pelletised metal oxides in water in the presence of an organic polymeric dispersing agent, using steel balls as the milling media.  After an extended milling period the hatch to the mill was being opened when an explosion occurred seriously burning the operator who had opened the mill.   After the explosion the air space in the mill was found to contain methane and carbon monoxide.  The presence of these organic species immediately led the mill operators to suspect the polymeric dispensing agent as being the cause of the incident.

On further investigation it was found that a redox reaction can occur on the surface of the steel balls used in the milling process.  This reaction results in oxidation of iron on the surface of the milling media and the liberation of hydrogen from water.

It is believed that the explosion was caused by the accumulation of hydrogen in the mull which resulted in the formation of an explosive hydrogen-air mixture on opening the hatch.  The presence of methane and carbon monoxide supports the theory that a reductive atmosphere was present, resulting in incomplete combustion.

This incident is of particular importance given the common practice of wet milling using a steel milling media.

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