The present invention concerns the production of lead granules for the dry filling of tube electrodes for storage batteries.
Tube electrodes for lead storage batteries are known and have been described, for example, in U.S. Pat. No. 2,747,077. They are constructed from a grid of lead or a lead alloy, which grid contains a so-called top frame, from which extends a number of conducting bars or rods, generally of the same lead composition as the grid. Each rod is surrounded by active material. The active material of fully charged positive electrodes consists of lead dioxide. In the prior production of the electrodes, the tube sheaths are set on the grid which is then turned upsidedown on the top frame and thereafter the active material, which is provided in the form of so-called lead powder, is charged into the sheaths. The lead powder used in filling the sheaths is lead powder of varying degrees of oxidation and generally from about 65 up to 100% oxidation. The filling proceeded as follows: the electrode with the sheaths set about the rods was set up in a cassette which at the top is provided with a small-sized funnel. Thereupon, the cassette was vibrated and the lead power which was to be conveyed to the electrode was filled down in the funnel and through the funnel into the space between the sheath and the rod. Finally, the electrode was closed with a bottom strip which covers all the tube apertures.
The dusting of the lead powder which arises in connection with the production of tube electrodes according to the above process is very inconvenient and causes great problems. It is therefore very important that dusting be avoided or be reduced or a minimum. In order to reduce or eliminate dusting, various filling methods which utilize wet mass mixtures have been tried. For example, a mass of the lead dust and water and/or other additions in small amounts may be prepared. This mass is then conveyed to an extruder or some similar feeding device and is pressed into the tube sheaths. Such a procedure is described, for example, in Swedish patent application No. 7,408,864. While this wet process reduces dusting, it produces other problems such as the formation of an irregular filling of the electrodes and increased amounts of lead in the waste water.
Polytetrafluoroethylene (PTFE) is also used for dust control purposes not connected with the production of electrodes. In such a process, up to two percent in weight of PTFE are added to the powder characterized by excessive dusting. PTFE is present either as a dry powder or in suspension. The mixture is then heated to a temperature preferably just under 100.degree. C. and is processed. Thereby the PTFE changes from a powder to fibers. This process and the dust-free, free-flowing powder product thereof are described in U.S. Pat. Nos. 3,838,064 and 3,838m092.
In connection with the production of lead accumulator electrodes in the dry powder process described above, various additions to the lead have been attempted for different purposes including suppression of dusting. The use of PTFE in a powder filling process has been described in U.S. Pat. No. 4,071,946.