  COSINE-200 is the next phase experiment of the ongoing COSINE-100 that aims
to unambiguously verify the annual modulation signals observed by the DAMA
experiment and to reach the world competitive sensitivity on the low-mass dark
matter search. To achieve the physics goal of the COSINE-200, the successful
production of the low-background NaI(Tl) detectors is crucial and it must begin
from the mass production of the ultra-low background NaI powder. A clean
facility for mass-producing the pure-NaI powder has been constructed at the
Center for Underground Physics (CUP) in Korea. Two years of operation
determined efficient parameters of the mass purification and provided a total
of 480 kg of the ultra-pure NaI powder in hand. The potassium concentration in
the produced powders varied from 5.4 to 11 ppb, and the maximum production
capacity of 35 kg per two weeks was achieved. Here, we report our operational
practice with the mass purification of the NaI powder, which includes raw
powder purification, recycling of the mother solution, and recovery of NaI from
the residual melt that remained after crystal growth.
