In pad bleaching systems, a continuous
chemical feed to the saturator is usually required.
For wet on dry application, the feed
solution is made up in a stock tank at the same concentration as the
saturator liquor. This solution is then pumped or gravity fed to the
saturator. The feed is controlled by a float valve on the saturator
liquor's surface.
For wet on wet padding a more concentrated
chemical fed liquor will be required, since the maximum volume of liquor
which can be added is governed by the difference in entry and exit mangle
expressions.
Typically the concentrated chemical feed
liquor will be 5-6 times the strength of the saturator liquor. In some systems,
a single feed liquor can be prepared. At high concentrations, particularly
where several chemicals are involved, stability problems may be
encountered e.g., salting out of surfactants or decomposition of hydrogen
peroxide by the unusually high alkali levels. It is more usual to find two
or three tanks supplying the same saturator. For a peroxide bleach, for
example, one tank would supply peroxide, as received 50%, a second tank
would supply caustic soda and sodium silicate, and a third tank would
supply a wetting agent, sequestering agent, etc.
The feed rate of all these tanks will be
controlled to compensate for the removal of chemicals from the saturator
by the fabric.
There are five basic systems available.
- Manual control
- Rotameter flow meters
- Metering pumps
- Proportioning cylinders
- Fully automated units
In all systems a separate water feed is
usually fitted to the saturator with a liquor level controller. |
