Description of the Dynamic Drainage Jar
In the process of forming a sheet of paper, a suspension of fiber and other particles is filtered through a screen to deposit a wet sheet, which upon pressing and drying, becomes the paper sheet. The original suspension may contain wood tracheids 3 to 4 mm long, fiber fragments of varied size and shape, and filler particles less than 1 micron in diameter. It follows, therefore, that paper sheet formation is a fractionating process: true fiber is almost entirely retained by the forming web while the finer particles are retained to a greater or lesser degree depending upon a variety of conditions. The suspended solids in the filtrate passing through the screens are almost entirely fines. The use of laboratory sheet molds for retention studies was found to be unsatisfactory because of lack of correlation with machine performance. The need for a laboratory device that would more nearly simulate the paper machine led to the development about 35 years ago of the "dynamic drainage jar."
The Dynamic Drainage Jar
The sample reservoir is a threaded cylinder with a total volume of 1 liter which fits into a base by a screw thread. The screens rest upon a ledge in the bottom compartment. Leakage is prevented by appropriate gaskets and by tightening the screw. We believe the screw design gives longer-lasting and more positive assurance against leakage than other designs. This is especially important when the jar is used as a vacuum water release analyzer. The bottom compartment has an outlet hole which normally carries a stopper with glass and rubber tubing and with a clamp to control the drainage flow.
Above the reservoir cylinder and projecting into it is mounted a stirrer with a three-blade 2-inch propeller. The stirrer is driven by a motor-generator set with control console to provide precisely controlled rpm. The dynamic jar differs from previously used laboratory retention devices in having this feature of precisely controlled, graduated turbulence. It is easily demonstrated that the retention of the fines fraction is highly dependent upon the degree of turbulence. Also, various "retention aids" differ greatly in their response to turbulence. If the retention of a given paper machine is known, then that retention can be matched by adjustment of turbulence in the jar. An alternative reservoir is available with 3 vertical vanes on the inside wall which increases the turbulence at a given rpm. This vaned cylinder is usually necessary to match the low retention of twin wire and tissue machines.
Operation details of the jar will be found in the Information Manual furnished with each instrument and also in TAPPI Method T-261(80).
Applications of the Jar and Typical Data
The original DDJ has two distinct functions:
- As a single screen classifier, to separate a sample of stock into two fractions defined by the hole size of the screen used.
- To determine (under controlled and graduated degrees of turbulence) the relative tendency of the fines fraction of a sample of stock to stay with the fiber (coarse fraction) or to follow the water through the screen.
Multi-screen classifiers have been widely used for pulp testing to determine the distribution of fiber length. In studies around the wet-end of the paper machine, it is much more useful to know the proportion of fines fraction to fiber fraction. The greater convenience and accuracy of the single-screen classifier make it the logical choice for this purpose.
The first point to be considered is how do we define the difference between fines and fiber? Obviously, in paper stock there is a continuous, though not uniform, gradation in particle size. Therefore, any sharp dividing line will be arbitrary.
Over the years, there has developed a consensus that 200 mesh or 76 micron screen hole is the most logical division. However, an investigator may choose another screen size for special or additional information. Unless specified otherwise, we consider that those particles which pass a 76 micron hole size are fines fraction and those which are held back are fiber fraction.