I've treated leaching rock in a curing bin.
The procedure I used: put the rock in new salt water ;
a small power head in the bin for circlatltion and aeration;
wait a few days; test PO4 ; if near zero wait and test it again after a week or so;
if still near zero the rock is not leaching;
if it is leaching add lanthnaum chloride to the bin to get the water to near zero( say .15 ppm PO4 or less);
Dose daily until the water maintains a low PO4 reading for a week.
I don't bother with a filter sock in this application,probably wouldn't hurt ; I rinse the loose lanthanum phosphate and lanthanum carbonate off the rock before it goes to the aquarium.
For dead rock( dry rock) you may get even better results in the low range using fresh water since you will get less lanthanum carbonate precitant and more lanthanum phospahte, I think.