This invention relates to metering and wetting systems for dry materials and is particularly adapted, inter alia, for wetting polyelectrolytes or the like. Polyelectrolytes are polymers which have been found effective in promoting coagulation and are often used in waste water treatment.
Handling of polyelectrolytes is difficult because of their hygroscopic nature and the difficulties involved in properly wetting them and bringing them into solution. Polymers such as polyelectrolytes require from about 5 to about 60 minutes to "age" after they have been properly wetted, to transfer into solution. It might appear that a very simple method of wetting a polyelectrolyte would be to meter the polymer directly into a tank and mix it with a mixer. However, because of its extremely hygroscopic nature, if one added a clump of polyelectrolyte to a bucket of water, the outer portion of this clump would wet instantaneously upon contact with the water, preventing the center core of the clump from being wetted. The wetted outer area then would become a viscous shell and would isolate the still dry, encapsulated material. In fact, it is very difficult for liquid to penetrate the outer wetted shell to wet the dry material encapsulated therein. These unwetted particles or globules are generally known in the trade as "fisheyes." Even mechanical mixers have difficulty in breaking down these lumps, particularly within the period of time required for the aging process or within allocated process time requirements.
Thus, it is essential that every grain of polyelectrolyte be wetted in order for a polymer metering/wetting system to be completely functional.
Another serious handling problem derives from the extremely adhesive nature of improperly wetted polymer. The unwetted or partially wetted polyelectrolyte globules will adhere to practically any surface with which they come in contact. Therefore, when a polyelectrolyte solution which contains unwetted or partially wetted particles is transported, there is a tendency for these particles to agglomerate and adhere to each other and to the inside of the pipes, creating a serious problem. Thus, if a polyelectrolyte solution containing these unwetted particles is permitted to pass through pipelines, pumps, etc., clogging may result very quickly, rendering the system useless. On the other hand, a properly wetted polyelectrolyte solution does not cause any handling problems per se.
My U.S. Pat. No. 4,077,612 is also directed to the problem of wetting polyelectrolytes. The present invention differs from that of the '612 patent in at least two major aspects. First, the present invention does not utilize an air dryer, and second, it does not utilize a dry air/polymer atomizing system. The system of the present invention is less costly to manufacture and is smaller in size than the system disclosed by the '612 patent.
In the system of the '612 patent, the polyelectrolyte is first atomized with dry air and is then conveyed via a pneumatic conveying line to a mixing-wetting chamber where wetting occurs. In that system it is important that the air be substantially moisture-free (col. 4, lines 16-18) because if the atomized polyelectrolyte contains moisture, it will adhere to the surfaces of the system.
The current invention, by contrast, is simpler and more convenient in that the air need not be dry, and therefore atmospheric air may be used. The present invention also abrogates the need for any pneumatic conveying line because of the different kind of wetting chamber used. The atmospheric air used to disperse the polymer simultaneously creates air/water turbulence in the lower part of the wetting chamber. The polymer is not atomized or otherwise dispersed before it enters the wetting chamber. It is simply fed into the inlet of the wetting chamber where it is drawn in by gravity aided by an eductor effect. This is also simpler and more convenient than the procedure disclosed in the '612 patent.