Translation:The metal was thirty kilograms in weight.
Not for nothing, but technically Kilograms are a measure of MASS, not WEIGHT. In common use, this sentence is correct, but scientifically... What would the Esperanto be for "The metal's Mass was 30 kilograms"?
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)