The phrase "were then placed" is correct and can be used in written English.
You can use this phrase to describe the sequence of events, for example: "The books were then placed neatly on the shelves."
All three medications were then placed in the refrigerator in an insulated box.
They were then placed in a brain scanner and shown the image again.
Approved submissions were then placed in a New Tips category, along with the originator's name.
When only the bones remained, they were then placed in a small ossuary.
The eyes were then placed under the loupe microscope and dissected.
He and other prisoners were then placed in a structure, 25m long by 2m wide.
DPPC bilayers were then placed over hydrogel scaffold using Langmuir Blodgett technique.
The peanuts were then placed in separate compartments of a miniature warehouse.
Suspensions containing C2C12 myoblast cells were then placed on the surface of these micromachined channels.
Nucleus pulposus cells were then placed onto the in vitro-formed hyaline cartilage.
The beams were then placed under sustained loads for 300 days.
