Chemical Purification Facility
When the center started, the purification group was formed to explore purification techniques to produce radiopure materials such as NaI powder for COSINE and MoO3, CaCO3, and LiCO3 powders for AMoRE.
NaI crystals for COSINE were previously grown from highly pure NaI powder by the Bridgman-Stockbarger technique. During the growing process, contaminations from the ambient environment are highly suppressed. In order to grow a NaI crystal with ultralow radioactivities from 40K, 210Pb, 228Tl, 226Ra, etc., it is necessary that the initial NaI powder is radiopure. We studied several purification techniques to obtain radiopure NaI powder. Among the several techniques, we found that the recrystallization method is more effective than others and can be applied for bulk purification of NaI powder.
Based on our 1 kg scale powder purification study, we designed and built a system to purify ~70 kg of powder, requiring clean and tight connections between each unit, and easy maintenance and control of the entire system. This system consists of the following units,
- Feed tank:
NaI powder is dissolved into DI water inside this tank, and NaI solution is moved to the mixing tank.
- Mixing tank:
Recrystallization for the solution is applied in this tank by heating to 130 °C and then cooling down to room temperature. After this process, there are NaI crystals and solution.
- Filter unit:
NaI crystals and solution are separated by the filer, and solution is moved to one of the receiver tanks.
- Two receiver tanks:
Collect liquid wastes from mixing tank, filter, unit and conical dryer.
- Conical dryer:
NaI crystals from the filter unit are dried inside the dryer and we collect the final product, purified NaI power.
- Control panel:
It controls all adjustable system parameters such as temperature, pressure, rotation speed of the dryer, engagement of the stirrer, etc.
Figure I-31 and figure I-32 show each unit and the purification system.
For AMoRE, The Mo-based crystals, CaMoO4, Li2MoO4, Na2Mo2O7, etc., are required to have radioactivity levels below ~10 μBq/kg for 228Th and 226Ra. In order to achieve such a goal, we developed sublimation equipment for MoO4 powder purification as shown in Figure I-33. This equipment has several features as follows,
- It has a quartz tube about 1.5 m long; the vacuum is maintained in the tube with 10 mTorr during operation.
- Cylindrical crucible for powder loading and collecting purified powder.
- It has three heating zones; the temperature of each zone can be raised up to 1000 °C.
- The heating part can be tilted up to 60 °
In purification with the sublimation method, the powder is loaded in the bottom part of the crucible and the crucible is placed inside the quartz tube. By heating to about 710 °C, sublimated MoO3 moves to the upper part of the crucible.
Powder that has purified by sublimation contains MoxOy (x, y=1,2,3…) forms, which cannot be used to grow a crystal. Before converting MoxOy into MoO3, the MoxOy powder must be dissolved with aqua-ammonia, producing an aqueous solution containing ammonium molybdate (NH4)2Mo4O4. By blowing hot air inside the annealing furnace (see Figure I-34), this solution is heated to about 680 °C, producing MoO3 powder.