... the lower steps that the warmer tunnel air kept free of ice. (Para. 4)
Meaning: the lower steps that the warmer tunnel air kept without ice.
free of (or: from): without (something or someone unwanted); safe from; untroubled or not marked by.
Look at the following two examples:
The old lady is never free of (or: from) pain. 那位老妇人一直在遭受痛苦。
Meals are provided free of charge. 膳食免费供应。