The type of dry chemical feeder that you should use will depend largely on the degree of accuracy and control that your particular process requires.
In general for the greatest feed rate accuracy a Loss-In-Weight feeder (LIW) is recommended. A LIW feeder incorporates a hopper that can be on load cells or level control, which is refilled by a rotary valve from above. The product is then fed to the process make up tank from the LIW hopper by a feed screw. This arrangement can have a feed accuracy of +/- 0.5% once calibrated.
For process requirements that are not as stringent, a simple volumetric screw feeder can be used which has a feed rate accuracy of approximately 2.5% to 5%.