That is true but when it finishes the steel stock appears if you just wait. Of course it has been a while for me and things may have changed. I used to do this and start several going, put iron and coal in, combine, put more iron and coal in, remove sludge from previous step, combine, return sludge and work on two stocks at the same time. The reason for removing the sludge was to prevent the bad combination error message when doing the second combine I even manged to do three combines on the odd occasion. just keep your stacks of sludge separate and you should be ok, but as I said things may have changed.