Classifying – separating – and assorting solids – Electrostatic – Free fall type particle charging
Patent
1993-04-09
1994-12-06
Bollinger, David H.
Classifying, separating, and assorting solids
Electrostatic
Free fall type particle charging
B03C 700
Patent
active
053702352
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a process for the separation of plastic particles of a plastic mixture of plastics of a chemically different type which, however, have an approximately the same density range, for example polyethylene (PE) and polypropylene (PP), by an electrostatic separation process by means of a free-fall separator.
The polyolefins polyethylene (PE) and polypropylene (PP) belong to the mass plastics used most. They, therefore, also represent the predominating portion of the plastics contained in waste. The density of PE lies between 0.92 and 0.97 g/cm.sup.3, and the one of PP between 0.9 and 0.91 g/cm.sup.3.
Many commodity and one-way articles consist of said plastics. An example thereof are the medical one-way syringes. One-way syringes consist of a cylinder made of polypropylene and a plunger made of polyethylene. After the removal of the injection needle they are discarded as waste and heretofore eliminated predominantly by incineration. On the waste sector, the current trend is toward recycling. In many hospitals there are already pilot projects for the collection and recycling of plastic objects. The plastic quantity of a syringe consists in about equal parts of the plastics PE and PP. There is no corresponding demand for such mixtures, so that only low or no profits at all can be achieved therefor. There are even users who demand a credit for taking them.
As opposed to the above, it is possible to achieve for purely sorted recycling materials profits that are oriented in the price of the new materials and that may come up to 60% of the price of the new material. Consequently it is of interest, economically, to be able to separate such mixtures.
According to the state of the art there are the following possibilities for the separation of such plastic mixtures:
1. Sorting by hand
This method is used again extensively in the field of recycling for lack of better methods even though it is very labor-intensive and thus also uneconomical.
2. Sorting according to density, too, represent a not very successful method because PP and PE have similar densities and separation by means of a water-alcohol mixture with a density of 0.91 has not found any practical application. From this follows that a density separation with water is not possible due to the almost identical density range of the two substances.
The separation of plastic mixtures in a free-fall separator is specified in DE-PS 30 35 649; however, the known process is not usable for separating the above-mentioned plastic mixtures because the different plastics PE and PP are not selectively charged in the triboelectric charging of the plastic mixture.
The consequence thereof is that after passing through the free-fall separator, a considerable amount of medium material is collected, i.e., the plastic particles predominantly have a low charge that is not sufficient for a deflection in the field. The charging is frequently totally unselective.
The invention, therefore, is based on the problem of further developing a process of the type specified at the beginning in such a way that a high degree of purity of the respective type of plastic is achieved especially in the separation of PE and PP, and whereby the resulting quantity of medium material is kept as low as possible.
According to the invention, the problem is solved in that prior to the triboelectric charging, the plastic mixture is subjected to a surface treatment. According to one form of execution, the surface treatment comprises a treatment with mineral acid; according to another form of execution, the plastic mixture is contacted with an alkali lye. In this connection, particularly diluted hydrochloric acid can be considered as the mineral acid, whereby the dilution of the hydrochloric acid is selected in such a way that a pH of about 3 is adjusted.
Preferably diluted soda lye is used as the alkali lye, whereby the dilution of the soda lye is adjusted in such a way that a lye with a pH of about 10 to 12 is obtained.
It has been found in tests that through a surface treatment with the r
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Geisler Iring
Hollstein Axel
Kleine-Kleffmann Ulrich
Neitzel Ulrich
Stahl Ingo
Bollinger David H.
Kali und Salz Aktiengesellschaft
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