Slippage at the drive pulley occurs whenever the frictional forces between the belt and the pulley are less than the forces holding the belt stationary. I assume that you have not experienced slippage once the belt has started to move and the system has "warmed up".
Icing will interfere with the pulley/belt friction ratio and corrective action must be taken to counter the problem.
Your options include:
 The one you described with a "nubbed" metal pulley. The nubs dig into the bottom cover of the belt (similar to studs on tires for driving on ice). However, this also decreases the service life of your belt's bottom cover.
 The use of ceramic lagging. This works the same way as your pulley option.
 Using a snub pulley to increase the wrap on the drive pulley.
 Increasing the take-up tension. Be careful with this one -- over tensioning can cause mistracking problems, increase bearing failure rates, adversely affect the splice, and increase damage from loading.
 Heat the area around the drive to eliminate the possibility of icing. (This would be my choice, based on the information you provided.)
 Heat the drive pulley only.
You should also investigate the rest of the conveyor. If the belt has frozen to the idlers, skirting, and cleaners, your start-up load can be very high. This may require additional action on your part.
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