Two people have been killed and 24 injured in explosions in Shenzhen over the past two days.
A blast at Hong Kong 'toy king' Francis Choi Chi-ming's Early Light International Holdings' factory in Longgang district killed one worker. Eight others were admitted to hospital after welding sparks ignited nitrogen in a diesel tank under repair on Wednesday morning.
Six of the injured are in critical condition.
A factory spokesman said maintenance workers had failed to release all the nitrogen, which was used to check for leaks, before welding cracks on the tank.
The underground tank had been in use for six years. A house standing on top of the tank was destroyed in the explosion.
The factory, one of several Mr Choi operates in Shenzhen, employs 17,000 workers.