Activated carbon filter

Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From hydrocarbon reactant

Reexamination Certificate

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Details

C524S495000, C526S352000, C210S500100, C210S502100, C210S348000

Reexamination Certificate

active

06770736

ABSTRACT:

The present invention relates to a novel polymer as binder for producing activated carbon filters.
Purification filters for purifying liquids or gases are known. In one group of such filters, a loose bed of grains of an adsorbent material such as activated carbon is located between two porous walls which confine the grains, see DE-A-32 04 022. Since there is no cohesion within the granular material, the flow and filter use period are difficult to control. In the flow of the filtrate and as a result of fracturing, relocation of various particle size fractions or washing out or local blockages caused by fines can occur. However, in particular, the adsorbency of the granular material resulting from internal porosity of the grains is effective only to a small extent since most of the filtrate flows past the filter material through the interstices between the grains and essentially contacts only the surface of the grains. Filter units of this type, also known as filter cartridges or filter candles, have to be replaced after a short period of use or the granular material has to be replaced.
Another filter employed for purifying tap water and containing an adsorptive shaped body as filter unit is known from EP-A-0 253 132. The shaped body is configured as a self-supporting sintered body and comprises finely divided adsorbent and thermoplastic binder. During production of the sintered body by thermoplastic sintering, a considerable part of the particle surfaces and micropores of the adsorbent is conglutinated or blocked by adhering and penetrating thermoplastic material; the corresponding internal surface area of the grains is thus lost for adsorption. As a result, the adsorption capacity of the overall filter unit is impaired.
EP-A-0 554 223 discloses a process for producing a filter unit. The high viscosity of the binder results in a lower strength of the filter unit obtained. inhomogeneous distribution. The inhomogeneous distribution in the mixture leads to a fragile product. In addition, the surface area of the activated carbon is decreased by the binder. The consequence is a drop in the filter capacity.
U.S. Pat. No. 4,753,728 discloses an activated carbon filter block which contains activated carbon particles which are bound by a polymer to give a filter block. The polymer has a melt flow index of less than about 1 gram per 10 minutes determined by STM D 1238 at 190° C. and 15 kilogram load. The polymer becomes tacky at elevated temperature. The polymer is not sufficiently fluid to substantially moisten the activated carbon particles. A commercially available polymer GUR® 212 is disclosed which has a melt flow index of less than 0.1 gram per 10 minutes determined by STM D 1238 at 190° C. and 15 kilogram load, and is an ultrahigh molecular weight polyethylene.
It is an object of the present invention to provide a binder for producing activated carbon filters which have a high strength and low binder content and in which the activated carbon is not coated.
The object of the present invention is achieved by a polyethylene homopolymer and/or copolymer having
a melt flow index (MFI 190/15) of from 1.2 g/10 min to 10 g/10 min,
a polydispersity M
w
/M
n
of from 3 to 30,
a bulk density of from 0.05 g/cm
3
to 0.5 g/cm
3
and a particle size from 5 &mgr;m to 300 &mgr;m.
The binder of the invention has a very good biocompatibility. The binder has a high viscosity, low tendency to coat the activated carbon, high porosity and low sensitivity for the process. The binder has a low bulk density and high surface area, a particular morphology and a particle size from 100 &mgr;m to 140 &mgr;m. The binder displays a particularly good thermal stability.
A filter unit produced using the binder of the invention achieves a high adsorbency of the activated carbon and has a very porous structure.
Possible binders according to the invention are organic, thermoplastic materials. A particularly suitable binder has been found to be a powder of a high or ultrahigh molecular weight thermoplastic, in particular polyethylene, which on heating is capable of binding in a viscous transition phase and forms porous structures with activated carbon in the sintering process.
After sintering, a block of activated carbon having a high adsorbency and good mechanical strength is obtained. A high filtrate flow is made possible by the high porosity.
Purification filters can be built as required using one or more such filter units. Applications are envisaged in the fields of process water treatment, wastewater purification, drinking water treatment, in the beverages, food and chemical industry, but also for purifying gases.
A preferred embodiment of the invention is a polyethylene homopolymer and/or copolymer having a melt flow index (MFI 190/15) of from 1.2 g/10 min to 10 g/10 min, a polydispersity M
w
/M
n
of from 3 to 10, a bulk density of from 0.1 g/cm
3
to 0.5 g/cm
3
and a particle size from 60 &mgr;m to 200 &mgr;m. This binder gives an activated carbon filter having a high strength and a low binder content. The melt flow index is preferably 1.3 g/10 min to 10 g/10 min.
A further preferred embodiment of the invention is a polyethylene homopolymer and/or copolymer having a melt flow index (MFI 190/15) of from 1.4 g/10 min to 5 g/10 min,
a polydispersity M
w
/M
n
of from 4 to 8,
a bulk density of from 0.13 g/cm
3
to 0.3 g/cm
3
and a particle size from 80 &mgr;m to 180 &mgr;m. This binder gives an activated carbon filter having a particularly high strength and a low binder content.
A particularly preferred embodiment of the invention is a polyethylene homopolymer and/or copolymer having
a melt flow index (MFI 190/15) of from 1.4 g/10 min to 3 g/10 min,
a polydispersity M
w
/M
n
of from 4 to 8,
a bulk density of from 0.15 g/cm
3
to 0.28 g/cm
3
and
a particle size from 80 &mgr;m to 160 &mgr;m. This binder gives an activated carbon filter having a very particularly high strength and a low binder content.
A further embodiment of the invention is a filter material comprising polyethylene homopolymer and/or copolymer and activated carbon.
In the case of purification filters based on activated carbon for purifying drinking water, a distinction is made between two main groups. In the first and older variant, the loose activated carbon is placed in a porous container. Here, there is no cohesion within the granular material. As a result, the flow and filter properties are difficult to control. The adsorbency of the granular material is effective to only a small degree. The filtrate can flow past the filter material through the interstices between the grains and thus the active surface area of the activated carbon cannot be fully utilized.
A further disadvantage of these filters is that they very quickly become contaminated by bacteria. The activated carbon grains are an excellent nutrient medium for bacteria. In addition, such filters have a purely adsorptive action. The mechanical filtration properties are poor or nonexistent.
The second possibility is to form the filter as a sintered body. This is achieved by mixing finely divided adsorbent, viz. activated carbon, with a thermoplastic and consolidating this mixture in a mold by thermoplastic sintering. Here, the thermoplastic acts as an adhesive. It blocks a considerable part of the surface of the activated carbon grains and thus greatly reduces the active surface area of the activated carbon. The adsorption properties of these filters are low.
The filter block of the invention is produced by a process which gives strong shaped bodies having high adsorptive properties and very good mechanical filtration properties. This filter is produced by dry mixing very fine activated carbon with a high molecular weight polyethylene. The mixture is introduced into a mold and heated in the absence of air to a temperature which is significantly above the melting range of the binder. At these temperatures, the binder melts slowly. The process steps of dry mixing, introduction into the mold, compaction, heating according to a time program and demolding can also be carried out in mass pr

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