WO2016083062A1

WO2016083062A1 - A method for preparing an extruded carbon block

Info

Publication number: WO2016083062A1
Application number: PCT/EP2015/074962
Authority: WO
Inventors: Siddhansh ARORA; Sharadchandra Govind Bansode; Parthiv Ripudaman Dave; Viddam Jagan Mohan REDDY
Original assignee: Unilever N.V.; Unilever Plc; Conopco, Inc., D/B/A Unilever
Priority date: 2014-11-24
Filing date: 2015-10-28
Publication date: 2016-06-02
Also published as: CN106999819A; CN106999819B; PH12017500775A1

Abstract

The present invention relates to a porous carbon block and a method for preparing the carbon block continuously by an extrusion process. The carbon block is particularly suitable for use in water purification devices. It is an object of the present invention to provide a process for preparing carbon block that is efficient, economical and has good chlorine removal capacity prepared continuously through an extrusion process. The present inventors have found that filtering capacity and life of a carbon block can be markedly improved by extrusion of the carbon block having a specific combination of two binders in an extruder screw having a specific ratio between flight outside diameter of two different sections of the screw.

Description

A method for preparing an extruded carbon block

Field of the invention The present invention relates to a porous carbon block and a method for preparing the carbon block continuously by an extrusion process. The carbon block is particularly suitable for use in water purification devices.

Background of the invention

Water contains many contaminants, which include particulates, chemicals and microorganisms. For making water suitable for consumption, water must be purified by removing the contaminants to a safe level.

There is growing consciousness among people to consume water after filtering or purifying not just in the urban localities but also in remote villages. In villages where online systems are not readily applicable there also exists a constant need for development of cost-effective gravity-fed systems which have a simple manufacturing process and most importantly overcome the inherent problems of gravity fed systems for achieving good flow rate while providing effective removal of particulate contaminants.

Carbon block made of activated carbon of a selected particle size distribution and binder material having selected melt flow characteristics for providing desired filtration of contaminants from water which includes microorganisms like cysts and bacteria while consistently giving desired high flow rates under gravity flow conditions are known. Carbon block are also useful in adsorbing and removing dissolved organics, taste and odor.

Carbon block are generally manufactured by a compression moulding process using activated carbon and a binder. There are few limitations in compression molding process like huge number of mould requirement, long manufacturing cycle time and the process is labor intensive.

WO2005094966A1 (Unilever, 2005) discloses a carbon block filter media for use in gravity fed filters having a powder activated carbon (PAC) with a particle size distribution such that 95 wt% of the particles pass through 50 mesh and not more than 13wt% passes through 200 mesh and a binder material having a melt flow rate (MFR) of less than 5 grams per 10 minute and a process for preparing the carbon block filter media by compaction. The process for the preparation of a carbon block filter medium includes intimately mixing powder activated carbon with the binder material in a mixer, compacting the mix in a vibratory compactor, compacting the mix in a mould of desired shape and size by applying a pressure of not more 20 kg/cm², heating the mould to a selected temperature, cooling the mould and releasing the carbon block from the mould.

US2009/0274893A1 (Filtrex Holdings PTE Ltd., 2009) discloses a method and apparatus for continuous extrusion of activated carbon of tubular shapes used in filtration of water. A mixture of activated carbon and polymeric binder is gravity fed into an extruder barrel through a slide to facilitate uniform feed and prevents bridging. To prevent the separation of the activated carbon and polymeric binder, the polymeric binder is surface charged by a plasma process to attach to the carbon with a weak force. The barrel is heated by induction heating to facilitate a high and constant heat source. To be effectively processed it is essential that the binder is surface-modified or plasma-treated to create the surface charge prior to mixing with carbon powder, without which there will be separation of carbon and binder or there may be bridging in the barrel. The blocks are made using 40mesh (420 microns) to 320mesh (45 microns). The process requires additional steps of surface modification of the binder.

We have now found that filtering capacity and life of a carbon block can be markedly improved by extrusion molding carbon block having a specific combination of two types of binders. It is further preferably seen that the carbon block has improved characteristics when extruded in an extruder screw having a specific ratio between outside diameters of flight of two different sections of the screw. It was also found that a preferred carbon block with the desired filtering capacity and life was obtainable using activated carbon with particle size ranging from 50 to 1400 microns. It was further found that the present process also eliminates the requirement of an additional auger screw for moving the mixture of activated carbon and binder from the feeder to the barrel of the extruder. It is an object of the present invention to provide a carbon block that is efficient, economical and has good chlorine removal capacity.

It is another object of the present invention to provide a process for preparing carbon block that is efficient, economical and has good chlorine removal capacity continuously through an extrusion process. It is a further object of the present invention to provide a process for preparing a carbon block by extrusion process by providing activated carbon having particle size ranging from 50 to 1400 microns.

It is yet another object of the present invention to ameliorate atleast some of the disadvantages in the prior art documents.

Summary of the invention

According to a first aspect of the invention disclosed is a method of extruding a porous carbon block in a screw extruder having a barrel with an inlet at one end and an extrusion die at other end having the steps of:

(i) mixing activated carbon with binder to form a mixture;

(ii) feeding the mixture into the inlet of the barrel comprising a screw having a feed section adjacent the inlet and a heating section proximal the extrusion die, the screw being mounted rotatably along the interior of the barrel;

(iii) rotating the screw to mix and convey the mixture from the feed section of the screw having a first flight outside diameter to the heating section of the screw having a second flight outside diameter;

(iv) heating the mixture during conveyance through the heating section

comprising a metering section adjacent the extrusion die and a compression section intermediate the feed section and metering section;

(v) cooling the mixture during extrusion through the die.

wherein the binder comprises a mixture of low density polyethylene and a polymer with melt flow index less than 5 grams per 10 minute in a ratio in the range 70:30 to 30:70.

An extrusion moulded porous carbon block comprising:

(i) activated carbon

(ii) a binder comprising a mixture of low density polyethylene and a polymer with melt flow index less than 5 grams per 10 minute in a ratio in the range 30:70 to 70:30. According to a third aspect of the invention disclosed is a porous carbon block according to the second aspect obtainable by a method according to the first aspect.

Brief description of the drawing The invention will now be further described with reference to the drawings.

Figure 1 is a longitudinal cross section of the barrel according to the present invention comprising a single screw extruder.

Figure 2 is a side view of a screw of the screw extruder of Figure 1 . Detailed description of the drawing

Referring to Figure 1 , there is shown a longitudinal cross section of a single screw extruder which has a cylindrical barrel (1 ) in which a solid or cored screw (2) runs along the entire length of the barrel and is rotatably mounted. The barrel has an inlet (3) at one end and an extrusion die (8) at the other end. The screw (2) has one or more raised ridges referred to as flight (4); the flight(s) is helically disposed on a surface of the screw. Surface of the screw above which the flight(s) are raised is referred to as the screw shaft (5). Feed having a mixture of activated carbon and binder according to the present invention is poured preferably from a hopper (6) into the inlet (3) which is in communication with the screw (2).

Hopper (6) preferably has a cylindrical top section and a truncated conical section at the bottom. Preferably the angle of the side wall of the hopper to horizontal referred to as cone angle is larger than the angle of internal friction. The cone angle is the included angle measured from the horizontal plane of the barrel to the conically shaped side of the hopper. To improve the flow of the material the cone angle in the present invention was preferably atleast 30°, more preferably atleast 35°, still more preferably atleast 40° and most preferably at least 45°. To improve the flow of the material the cone angle in the present invention is preferably less than 80°, more preferably less than 70°, more preferably less than 60°, and most preferably less than 50°. The hopper slant height was preferably 0 to 300 millimetres.

The feed from the inlet (3) is mixed, melted and conveyed along the length of the screw before extrusion through the extrusion die (8). Heating means (9) is mounted around the outer surface of the barrel (1 ) for melting the feed in operation. The outer surface of the extrusion die is surrounded by a jacket (10) containing cooling liquid for cooling the molten feed extruded through the die. Referring to Figure 2, the screw (2) according to a preferred embodiment has a feed section (A) adjacent the inlet (3, shown in Figure 1 ) and a heating section (B and C) proximal the extrusion die. The heating section has a metering section (C) adjacent the extrusion die (8, as shown in Figure 1 ) and a

compression section (B) intermediate the feed section (A) and a metering section (C). The compression section (B) and the metering section (C) together form the heating section. The feed section (A) has a constant flight height (D) between the screw shaft (5) and the top of the flight, the metering section (C) has a progressively increasing flight height (E), and the compression section (B) has a constant flight height (F) which is much reduced in height as compared to the feed section (A) and the metering section (C).

The feed section (A) includes 26% of the total length of the screw. It receives the feed through the inlet (3), preferably from a hopper (6). In this section, the outer diameter of the screw shaft (5) is wider than the outer diameter of the screw shaft (5) in all subsequent sections, the outer diameter of the shaft is 48 millimetres. The outer diameter of the flight (first flight outside diameter) is also the longest as compared to any of the subsequent sections. The first flight outside diameter measures 94.5 millimetres. In the feed section the barrel inside diameter measures 97 millimetres. Depth of screw channel which is the vertical distance between the screw shaft and the inner surface of the barrel of the extruder (not shown) of the feed section is larger than the depth of the screw channel in all subsequent sections of the screw (2). The depth of the screw channel in the feed section which measures 24.5 millimetres gives the feed section of the screw a high material delivery rate and causes the particles to be compacted and compressed. The feed section conveys the feed towards the subsequent sections of the screw. In this section, there is no heating of the material and the material is compacted and compressed. The screw in the feed section has a constant screw pitch measuring 86 millimetres and also a substantially constant flight height (4) which measures 23.25 millimetres. Downstream the feed section (A) is a heating section having a compression section (B). The length of the compression section is 27% of the total length of the screw. Preferably the flight height of compression section is shortest and said height progressively increases in the heating section in a direction towards the extrusion die. In the compression section the screw flight height (4) reduces sharply to 7.91 millimetres and the depth of the screw channel also becomes shallower than the previous feed section (A). The depth of the screw channel measures 8.25 millimetres. The barrel inside diameter in the compression measures 62.5 millimetres. In this section the second flight outside diameter decreases to 62 millimetres and the screw pitch decreases to 61 millimeters. The binder begins to melt in this section; the melting initially takes place in an interface between a film of molten binder and a solid bed of packed binder particles. As the melting continues, the solid bed breaks up and small particles of solid binder particles become dispersed in the body of a molten binder. Melting of the binder occurs as a result of the combined effect of heat produced by heater means (9) mounted outside the extruder barrel (1 ) and the shearing forces to which the screw subjects the binder causing friction between the internal wall of the extruder barrel and the binder. Melting softens the binder to such an extent that it flows freely and will assume any shape. The metering section (C) follows the compression section (B) either with or without intervening transition section. Typically, the metering section receives feed which has been fully melted. In this section the molten material is stabilized in temperature, is further mixed, and is delivered under pressure, usually to a final mixing or kneading stage before exiting the barrel. The metering section constitutes 47% of the total length of the screw. The metering section (C) functions to convey and pump the softened binder, as extrudate, out through the downstream end of the extruder which is typically an extrusion die (8) or some other form of restricted orifice. In this section, the shaft outside diameter narrows gradually in a direction towards the extrusion die. Thus, in the metering section channel depth progressively increases in the direction towards the die. The barrel inside diameter in the metering section measures 62.5 millimetres. The second flight outside diameter and the screw pitch in the metering section and the compression section is substantially constant. The flight height in the first half of the length of the metering section measures 13.9 millimetres and the other half of the length of the metering section closer to the extrusion die has a flight height measuring 24 millimetres.

Preferably the ratio of the flight height of the compression section to the flight height of the metering section is 1 :1 .8 to 1 :3. Preferably the flight height in the metering section progressively increases in a direction towards the extrusion die.

Detailed description of the invention

The expression wt% used throughout the specification means percentage by weight.

Single screw extruder:

Screw extruder generally has at least three distinct sections corresponding to the function performed by each section. The functions include solid conveying, melting and melt conveying. Solids conveying occurs in the feed section. The melting and melt conveying occurs in the heating section. The heating section includes a compression section which follows the feed section. The melting occurs mainly in the compression section, although the melting is not necessarily completed at the end of this section. The melt conveying occurs in the metering, although melting can continue in this section. The metering section melts the last particles and mixes to a uniform temperature and composition. Method of extruding a porous carbon block in a screw extruder:

According to a first aspect of the invention disclosed is a method of extruding a porous carbon block in a screw extruder.

The screw extruder has a barrel having an inlet at one end and an extrusion die at other end. The process of preparing the carbon block preferably includes mixing activated carbon with binder to form a mixture. The binder includes a mixture of low density polyethylene and a polymer with melt flow index less than 5 grams per 10 minute in a ratio in the range 30:70 to 70:30. The activated carbon and the binders are preferably mixed in a horizontal ribbon blender. To obtain a uniform mixture the mixing in the blender is preferably carried out for 10 minutes at a speed of 20 rotations per minute. The next step includes feeding the mixture into the inlet of the barrel comprising a screw. Preferably the amount of mixture fed into the inlet of the barrel exerts a pressure in the range of 3.4 X 10⁴ N/m² to 1 .59 X 10⁵ N/m². Preferably the mixture exerts a pressure of not more than 1 .4 X 10⁵ N/m² more preferably not more than 1 .25 X 10⁵ N/m² and still more preferably not more than 1 .0 X 10⁵N/m² but preferably the mixture exerts a force not less than 7.5 X 10⁴N/m² more preferably not less than 5.0 X 10⁴ N/m² still preferably not less than 4.5 X 10⁴ N/m² and more preferably not less than 3.0 X 10⁴ N/m². It is preferred that the pressure exerted by the feed is maintained between the desired range which provides carbon block extruded continuously having uniform desired porosity. After feeding the mixture into the inlet of the barrel having a screw including a feed section adjacent the inlet and a heating section proximal the extrusion die, the screw being mounted rotatably along the interior of the barrel, the next step involves rotating the screw to mix and convey the mixture from the feed section of the screw having a first flight outside diameter to the heating section of the screw having a second flight outside diameter. Preferably ratio of the second flight outside diameter to the first flight outside diameter is 1 :1 .2 to 1 :1 .75 more preferably the ratio is 1 :1 .4 to 1 :1 .6 and the optimum ratio is 1 :1 .5. Preferably the ratio of the length of the section of the screw having a first outside flight diameter to the length of the section of the screw having a second outside flight diameter is 1 :1 .5 to 1 :3 more preferably 1 :1 .5 to 1 :2 and most preferably 1 :1 .9. The ratio of the screw pitch of the heating section to the screw pitch of the feed section is in the range of 1 :1 .3 to 1 :1 .5 more preferably the ratio is 1 :1 .4 to 1 :1 .5 and most preferably 1 :1 .4.

The following step includes heating the mixture during conveyance through the heating section of the screw. The heating section of the screw includes a compression section and a metering section. The metering section is adjacent the extrusion die and the compression section is intermediate the feed section and metering section. The heating of the mixture is preferably by means of a heating means mounted on the external surface of the barrel. The heating means is preferably an induction heating coil. Preferably the mixture is heated to a temperature in the range 90°C to 260°C more preferably the mixture is heated to a temperature in the range 100 to 200°C, still more preferably to a temperature in the range 125 to 150°C. Preferably the mixture is heated to a temperature of 100°C in compression section of the heating section of the screw. Subsequent step includes cooling the mixture during extruding through the die. The cooling of the mixture is preferably by means of the cooling liquid which is circulated inside a cooling jacket that surrounds the extrusion die. The mixture extruding through the die is cooled to a temperature in the range of 10°C to 70°C. The extrusion die may have any shape depending on the shape of the carbon block that is desired. Preferably the die is in the form of the cylinder with the shaft of the screw running along the die; this provides a cylindrical hollow carbon block.

The specific dimensions for the screw designed in accordance with the present invention may be determined empirically, calculated using conventional equations, or determined by the use of commercially available computer programs. Porous carbon block

According to a second aspect disclosed is an extrusion molded porous carbon block having activated carbon and a binder comprising a mixture of low density polyethylene and a polymer with melt flow index less than 5 grams per 10 minute in a ratio in the range 30:70 to 70:30.

Activated carbon

Activated carbon particles are preferably selected from one or more of bituminous coal, coconut shell, and wood and petroleum tar. It is preferred that surface area of the activated carbon particles exceeds 500 m²/g, more preferably exceeds 1000 m²/g. It is preferred that size uniformity co-efficient of the activated carbon particles is less than 2, more preferably less than 1 .5. It is preferred that Carbon Tetrachloride (CCU) number of the activated carbon particles exceeds 50 %, more preferably exceeds 60 %. Preferably Iodine number of the activated carbon particles is greater than 800, more preferably greater than 1000. It is preferred that not more than 5 % of the activated carbon particles pass through a sieve of 150 mesh, and not more than 5% particles are retained on a sieve of 12 mesh. It is further preferred that not more than 5 % by weight of the activated carbon particles pass through a sieve of 75 mesh and not more than 5% by weight is retained on a sieve of 30 mesh. It is particularly preferred that less than 1 % activated carbon particles pass through a sieve of 200 mesh. Preferably the particle size distribution of the activated carbon is 50 micrometers to 1400 micrometers.

Binder

The binder includes a mixture of low density polyethylene and a polymer with melt flow index less than 5 grams per 10 minute in a ratio in the range 70:30 to 30:70. It is preferred that the ratio is in the range of 60:30 to 30:60 more preferably in the range 60:40 to 40:60 and most preferably 50:50. The ratio between the low density polyethylene and a polymer with melt flow index less than 5 grams per 10 minute is preferably a weight ratio. Low density polyethylene (LDPE):

Disclosed carbon block has a low density polyethylene binder. Suitable binders include LDPE sold as LUPOLEN™ (from Basel Polyolefins) and LLDPE from Qunos (Australia). Polymer with melt flow index less than 5 grams per 10 minutes:

Disclosed carbon block has a polymer with melt flow index (MFR) less than 5 grams per 10 minute preferably less than 2 grams per 10 minute, and more preferably less than 1 grams per 10 minute. Bulk density of the polymer with melt flow index less than 5grams per 10 minutes is preferably less than or equal to 0.6 g/cm³, more preferably less than or equal to 0.5 g/cm³, and most preferably less than or equal to 0.25 g/cm³. Preferred polymer with melt flow index less than 5grams per 10 minutes is selected from Ultra High Molecular Weight Polyethylene or Ultra High Molecular Weight Polypropylene. Their molecular weight is preferably in the range of 10⁶ Daltons to 10⁹Daltons.

Polymer with melt flow index less than 5 grams per 10 minutes are

commercially available under the trade names HOSTALEN™ from Tycona GMBH, GUR, SUNFINE™ (from Asahi, Japan), HIZEX™ (from Mitsubishi) and from Brasken Corp (Brazil).

The melt-flow rate of the binder is measured using ASTM D 1238 (ISO

(International Organization for Standardization) 1 133) test. This test measures the flow of a molten polymer through an extrusion plastometer under specific temperature and load conditions. The extrusion plastometer consists of a vertical cylinder with a small die of 2 millimetre at the bottom and a removable piston at the top. A charge of material is placed in the cylinder and preheated for several minutes. The piston is placed on top of the molten polymer and its weight forces the polymer through the die and on to a collecting plate. The

temperature for testing the present polymeric binder material could be chosen at 190 °C and the load at 15 kg. The amount of polymer collected after a specific time interval is weighed and normalized to the number of grams that would have been extruded in 10 minutes; melt flow rate is thus expressed in grams per 10 minutes.

It is preferred that the ratio of the activated carbon to the binder is from 2:1 to 10:1 parts by weight, more preferably from 2:1 and 8:1 parts by weight.

According to a third aspect of the invention disclosed is a porous carbon block according to the second aspect obtainable by a method according to the first aspect.

Examples

Example 1 : Method for preparing carbon block in a screw extruder according to the disclosed invention

A porous carbon block was prepared from 10 kg of powdered activated carbon (particle size 250 micrometer to 500 micrometres) mixed with 1 kg of low density polyethylene binder (Grade 16MM440 of 30 MFI at 190°C from Reliance

Industries Limited, India) and 1 kg of ultra high molecular weight polymer binder with 0 Melt flow index at 190°C (Grade Gur 4120 from Ticona) in a horizontal ribbon blender for 10 minutes at a speed of 40 rotations per minute to obtain a mixture.

The mixture was fed into the feed section of a screw extruder through a hopper placed above the feed section of a screw extruder as shown in Figure 1 . The level of the mixture in the hopper was maintained such that it exerts a pressure in a range between 3.4 X 10⁴ N/m² to 1 .59 X 10⁵ N/m² on the inlet of the barrel. The screw extruder had a screw rotatably mounted along the interior of a barrel. The first outside flight diameter of the screw in the feed section was 94.5 millimetres and had a flight height of 23.5 millimetres. Feed section was 324 millimetres long and had a screw pitch of 86 millimetres. Immediately after the mixture was fed into the feed section, heating coil surrounding the barrel in the subsequent heating section was switched on and the temperature was set at 150°C. The second flight outside diameter of the heating section was 62 millimeters. The heating section has a compression section adjacent to the feed section followed by a metering section. When the temperature in the

compression section recorded a temperature of 100°C the screw was rotated at a speed of 10 rotations per minute. The screw conveyed the mixture from the feed section into the following compression section. In the compression section, the outside flight diameter measured 62 millimetres and the flight height was shallower than the feed section and measured 7.91 millimetres. The shallower flight height allowed the mixture to form an agglomerated mass as the temperature in the compression section was raised to 100°C as a combination of the heat generated by friction and the heating coil. From the compression section the agglomerated mass was conveyed by the rotating screw to the adjacent metering section. The second outside flight diameter in the metering section was constant and same as the previous compression section. The flight height in the metering section progressively increased in the direction of extrusion die. The heating section was 873 millimeters in length. The

temperature of the agglomerated mass further increased in the metering section and when the temperature in the heating section reached the set

150°C, cooling water was let into the cooling jacket surrounding the extrusion die at a rate of 3 to 5 liters/minute. From the metering section the agglomerated mass which now assumed the shape of the porous carbon block was extruded through the extrusion die with simultaneous cooling. The temperature of the extruded mass coming out through the die had a temperature of 50 to 65°C. The extruded mass was then cooled to room temperature and cut to a desired length. Example 2: Comparison of Chlorine removal efficiency of preferred and comparative carbon block

Preparation of carbon block: The carbon block according to the present invention and the comparative examples were prepared using activated carbon and binders in amounts as shown on Table 1 . All the examples shown in Table 1 were prepared using a screw extruder as described in Example 1 .

Filtration of water: The carbon block shown in Table 1 was used for filtration of water in a gravity fed filtration device.

Method of measuring Chlorine removal efficiency:

Sample chlorine solution: 50ml_ of the treated water after filtration through the porous carbon block as shown on Table 1 was mixed with 2 drops of 5% acetic acid and 2 drops of Leucicrystal violet indicator.

Standard sample: Standard samples with 50ppb, 100ppb, 200ppb and 500ppb chlorine was prepared from 8% hypochlorite solution. Spectrophotometer measurement: A UV-visible spectrophotometer (Shimadzu UV 1800) was standardized using the standard samples. The chlorine content of the water filtered through the carbon block according to the present invention and through the comparative carbon block was measured and the chlorine content in the treated water was determined using the standard graph. The calculated chlorine content was recorded and is presented in the Table 1 below.

Drop Test: The integrity of the block is determined by a drop test. 5 identical carbon blocks were packed in a corrugated box. The corrugated box having 5 carbon blocks was dropped vertically from a height of 1 .2 metres. Each of the 5 carbon block were inspected for breakage. The carbon blocks were reported to have failed the test if at least one carbon block out of the 5 breaks. When all the 5 carbon blocks are intact the test is passed.

Table 1

The data on Table 1 shows that when the binder in the porous carbon block has a combination of LDPE and UHMPE in a ratio according to the present invention (Ex 1 , Ex 2, Ex 3, Ex 4, Ex 5) the chlorine removal efficacy of the carbon block is improved as compared to a carbon block having only LDPE (Ex a) or only UHMPE (Ex d) or a carbon block where the two are combined outside the disclosed ratios (Ex b, Ex c).

Table 2

Ex Processability Block integrity Flow rate under Number of

after drop test gravity head of unbound carbon

140 millimetre particles on

(mL/minute) surface of carbon block

Ex a Very good No breakage. Water did not flow Less than 25

through the filter

Ex b Very good No breakage. Water did not flow Less than 25

Edge chipped through the filter

Ex 1 Good No breakage. 30 to 100 Less than 25

Ex 2 Good No breakage. 50 to 100 Less than 25

Ex 3 Good No breakage. 200 to 350 Less than 25

Ex 4 Good No breakage. 200 to 350 Less than 25

Ex 5 Good No breakage. 200 to 350 Less than 25

Ex c feed mixture No breakage. 200 to 350 More than 100 does not move Edge chipped

in extruder.

Ex d feed mixture Carbon block 200 to 350 More than 100

does not move could not be

in extruder. formed.

The data on Table 2 demonstrates that when the binder in the porous carbon block has a combination of LDPE and UHMPE in a ratio according to the present invention (Ex 1 , Ex 2, Ex 3, Ex 4, Ex 5) the flow rate through the carbon block is higher as compared to a carbon block having only LDPE (Ex a) or only UHMPE (Ex d) or a carbon block where the two are combined outside the disclosed ratios (Ex b, Ex c). The carbon block according to the present invention pass the drop test and are easily process-able in a screw extruder.

Claims

1 . A method of extruding a porous carbon block in a screw extruder having a barrel provided with an inlet at one end and an extrusion die at the other end comprising the steps of:

(i) mixing activated carbon with binder to form a mixture;

(iv) heating the mixture during conveyance through the heating

section comprising a metering section adjacent the extrusion die and a compression section intermediate the feed section and metering section;

(v) cooling the mixture during extrusion through the die.

wherein the binder comprises a mixture of low density polyethylene and a polymer with melt flow index less than 5 grams per 10 minute in a ratio in the range 30:70 to 70:30.

2. A method as claimed in claim 1 wherein the ratio of the second flight outside diameter to the first flight outside diameter is 1 :1 .2 to 1 :1 .75.

3. A method as claimed in claim 1 or 2 wherein the ratio of the length of the section of the screw having a first outside flight diameter to the length of the section of the screw having a second outside flight diameter is 1 :1 .5 to 1 :3.

4. A method as claimed in any one of the claims 2 to 3 wherein the ratio of the low density polyethylene and a polymer with melt flow index less than 5 grams per 10 minutes is 50:50.

5. A method as claimed in any one of the claims 2 to 4 wherein the polymer with melt flow index less than 5 grams per 10 minute is selected from ultra high molecular weight polyethylene (UHMWPE) or ultra high molecular weight polypropylene (UHMWPP).

6. A method as claimed in any one of the preceding claims wherein the ratio of the screw pitch of the heating section to the screw pitch of the feed section is in the range 1 :1 .3 to 1 :1 .5.

7. A method as claimed in any one of the preceding claims wherein the flight height of compression section is shortest and said height progressively increases in the heating section in a direction towards the extrusion die.

8. A method as claimed in claim 7 wherein the ratio of the flight height of the compression section to the flight height of the metering section is 1 :1 .8 to 1 :3.

9. A method as claimed in any one of the preceding claims wherein the mixture is heated to a temperature in the range 90°C to 260°C.

10. A method as claimed in any one of the preceding claims wherein

extrusion die is cooled to a temperature in the range 10°C to 70°C.

1 1 . A method as claimed in any one of the preceding claims wherein the mixture exerts a pressure of 3.4 X 10⁴ N/m² to 1 .59 X 10⁵ N/m² at the inlet of the barrel.

12. An extrusion moulded porous carbon block comprising:

(i) activated carbon

(ii) a binder comprising a mixture of low density polyethylene and a polymer with melt flow index less than 5 grams per 10 minute in a ratio in the range 30:70 to 70:30.

13. An extrusion moulded porous carbon block as claimed in claim 13

wherein the ratio of low density polyethylene to a polymer with melt flow index less than 5 grams per 10 minute is 50:50.

14. An extrusion moulded porous carbon block as claimed in claim 13

wherein the polymer with melt flow index less than 5 grams per 10 minute is selected from ultra high molecular weight polyethylene

(UHMWPE) or ultra high molecular weight polypropylene (UHMWPP).

15. A porous carbon block as claimed in claim 12 wherein the porous carbon block is obtainable by a method as claimed in any one of the claims 1 to 1 1 .