Translation:The metal was thirty kilograms in weight.
It's technically not an accusative case here, but one of the other functions of the -n ending. You can read about them here.
In this case it's the type of -n that is used when expressing length, quantities, price, distance and measures:
“La ŝtofo estas 2 metrojn longa”, The fabric is 2 metres long;
(2 metrojn describes the measurement of longa)
“La domo kostas multan monon”, The house costs a lot of money;
(multan monon describes the amount of kostas)
“Ili marŝis dudek kilometrojn”, They walked twenty kilometres.
(dudek kilometrojn describes the distance of marŝis)
Ili staris kelkajn metrojn for de mi. = They stood a few meters away from me.
(kelkajn metrojn describes the distance of for)
I challenge you to find a native English speaker who would utter the above sentence. Much better translations would be any of: - The metal weighed thirty kilograms. (acknowledging Makaber00's correct statement about the difference between mass and weight) - The weight of the metal was thirty kilos. - The metal weight was thirty kilos. - There were thirty kilos of metal.