AU2973395A

AU2973395A - Manufacture of carbon compacts/pellets from cellulose based materials

Info

Publication number: AU2973395A
Application number: AU29733/95A
Authority: AU
Inventors: Maxwell Wilkinson
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-08-03
Filing date: 1995-07-27
Publication date: 1996-03-04

Description

MANUFACTURE OF CARBON COMPACTS/PELLETS FROM CELLULOSE BASED MATERIALS

This invention relates to a process for treatment of

carbonaceous materials and the product of that process, in one aspect the invention provides a process for treatment of sawdust to produce a product from which activated carbon may be obtained.

BACKGROUND OF THE INVENTION

Activated carbon is an amorphous form of carbon which is highly adsorbent and is widely used for the removal of unwanted components such as colours, odours and a wide range of contaminants from, for example, foodstuffs

including sugar and edible fats and oils and is also used extensively for water purification and in the brewing industry. Activated carbon is also extensively used in the gold industry in the carbon-in-pulp (CIP) and the carbon- in-leach (CIL) processes for recovery of gold.

In the prior art, activated carbon has been made from

carbon obtained by the destructive distillation of

carbonaceous material such as wood, coal or coconut shells. Activated carbon may be used in powdered, granular,

spherical or pallatised form.

For manufacture of granular activated carbons the most common feedstocks are coconut shells and coal. Coconut shells are crushed, carbonised, crushed again, activated (usually by physical activation with steam) and crushed and sieved into sized products. Production of activated

carbons from coal involves briquetting or pelletising and normally requires the use of a binder. The coal is first crushed, and sometimes carbonised at this stage, then

pulverised. The pulverised coal is then briquetted or pelletised using a binder material, eg bitumen or lignosulphonate. The briquettes are crushed before further treatment. Crushed briquettes and pellets are

processed by sieving, drying, carbonisation, activation, and sieving into the required product forms. Sawdust and other wood wastes have been used for the production of activated carbon.

Activated carbons produced from sawdust are made most commonly by the process of chemical activation. The sawdust is impregnated with an activating agent in the form of a concentrated solution, usually by mixing and kneading. This step is carried out at up to 100°C and results in the degradation of the cellulosic material. The impregnated material is then extruded and pyrolysed in a rotary kiln between 400 and 1000°C in the absence of air. The

pyrolysed product is cooled and washed to remove excess activating agent which may be recycled. On calcination, the impregnated chemicals dehydrate the raw material which produces charring and aromatisation of the carbon skeleton and creates a porous structure. The most widely used activating agents for wood waste are phosphoric acid, zinc chloride and sulphuric acid although a number of other chemicals have been used. The common feature of the activating agents is that they are

dehydrating agents which influence the decomposition of the wood (during pyrolysis) and inhibit the formation of tar. They also enhance the yield of carbon by decreasing the formation of acetic acid, methanol etc. In the case of sawdust the most commonly used activating agents are phosphoric acid and zinc chloride. The process described above produces an activated carbon of the pelletised type consisting of formed uniform

cylindrical shapes. Powdered activated carbon may be produced from sawdust by treating lumps of sawdust mixed with the activating agent and subsequently grinding the activated product.

Activated carbons may also be produced from sawdust by the process of physical activation in which the activation is effected by a gaseous agent (eg steam, carbon dioxide). Steam is most commonly used for physical activation and the process is normally a two stage process. Firstly, the material is carbonised below 800°C in the absence of oxygen to an intermediate product, the pores of which are either too small or too constricted for it to be a useful

adsorbent. The next step, which in some cases takes place at a later stage in the same kiln, is a process of

enlarging the pore structure so that an internal surface is produced which is more accessible. This is achieved by reacting the carbonised product with steam between 800 and 1000°C. At this temperature the rate determining factor is the chemical reaction between the carbon and steam. Thus the reaction takes place at the internal surfaces of the carbon, removing carbon from the pore walls and thereby enlarging them.

Activated carbons produced from sawdust by the physical activation process are ground to the appropriate size to produce powdered activated carbons. Existing technologies for the production of activated carbons have a number of disadvantages.

1. Processes involving chemical activation have the

potential to cause environmental pollution and require careful control of process effluents. The residual quantities of the chemicals used for activation may also contaminate foodstuffs when the carbons are used in these applications. This is clearly undesirable in the light of the growing concern in health related matters.

2. Processes using a binder do so at a considerable cost penalty. 3. Many commercially available activated carbons are not sufficiently hard. This may lead to increased costs in operation caused by losses due to attrition of the carbons. More importantly, such carbons are

frequently not amenable to regeneration. Spent carbon is wasted and must be replaced, thus adding

considerably to the operating costs in that

application, and creating a disposal problem.

4. Activated carbons produced from naturally occurring carbonaceous materials have their pore size

distributions determined to a large extent by the pore structure inherent in the original feedstock. This limits the range of application of these activated carbons.

THE PRESENT INVENTION In essence the process involves the following steps:

1. Firstly the sawdust is heated in the absence of air to dry it and to render it friable for subsequent grinding. This heating or calcination may be carried out in the temperature range 150-300°, preferably 200- 270°C.

2. In the second step the sawdust is cooled in the

absence of air, preferably to just above ambient temperature. 3. The third step involves grinding the calcined sawdust to a fine powder, having a preferred size range between 30 mesh BSS (500 μm) to 200 mesh BSS (75 μm).

4. In the fourth stage of the process the fine calcined sawdust powder is heated in the absence of air to render it formable, for example to a temperature in the range 280-375°C, preferably 300-375°C. The powder should be heated evenly, and this may conveniently be achieved by stirring. 5. In the fifth step the hot material is compacted in the absence of air, for example in a briquetting or extrusion press at a pressure up to 150 MPa. The discharge mechanism of the extrusion press may be of two forms: 1) fitted with a number of small nozzles to produce small or medium diameter pellets;

2) fitted with one large nozzle to produce a single large compact (for subsequent reduction to a granular product). 6. The sixth step in the process involves dusting the compacts and/or pellets with carbon powder if

necessary to prevent them sticking together.

7. The compacts and/or pellets are cured by allowing them to cool in the atmosphere, preferably for about five minutes.

They are then stored at ambient temperature. These are saleable products per se, but may be further treated as follows.

In the case of compacts, these may be crushed and sieved to produce granular material. This may then be activated by the physical activation process described above and then crushed and sieved to produce the desired product size ranges. Alternatively, it may be advantageous to tumble the crushed activated granular material, for example in a rotating drum, before sieving, to polish the granules and thereby reduce the level of attrition loss in subsequent use.

Pellets may be activated by the physical activation process and then sieved to remove any fine material. In the accompanying drawing. Fig. 1 is a diagrammatic representation of the preferred process of the invention as disclosed in the specific non-limiting Examples 1, 2 and 3 illustrating preferred embodiments of the process of the invention which will now be described.

Example 1

The process is applicable to sawdust from a wide variety of species. The data which follows derives from a sample of mixed eucalypt species from East Gippsland, Victoria, which was actually used in this example.

The chemical analysis of this feedstock was as follows:

db = dry basis

The following Table 1 provides data relating to the input and product of each step.

The pellets and compacts produced by this technology are ideal for further processing to high quality activated carbons - using existing technology.

The products having the genesis presented in the table produce activated carbons of high quality. The properties are determined by the carbonisation and activation technique but fall within the range indicated below:

BET Surface area 1300 m²/g

Carbon Tetrachloride Index 60-70

Hethylene Blue index greater than 100

Molasses Decolourisation greater than 200

Ball Pan Hardness Test (ASTM D3802-79) greater than 98

Example 2

The data which follows derives from a sample of mixed eucalypt species from East Gippland, Victoria which was actually used in this example.

The chemical analysis of this feedstock was as follows:

The following Table 2 provides data relating to the input and product of each step.

Samples of the product of Example 2 were activated in the following manner.

A suite of Samples 2A was heated to 850°C, contacted with an excess of steam equivalent to at least twice the stoichiometric steam requirement (based on burnoff) for a period of 3 hours. The yield of activated carbon averaged 33.7 db % of the mass before activation. These activated carbons had the following properties, which are tabulated for comparison with the properties of a commercially available carbon Sutcliffe Speakman 207A, and are suitable for a variety of uses, including use as a vapour phase carbon.

A further suite of samples 2B was activated under similar conditions to those of Sample 2A. The activated carbon was crushed, tumbled and then sieved to produce a granular product in the size range 2.36 mm - 0.85 mm. The average yield in this case was 33.1 db %.

The properties of this product make it suitable for use in water treatment. These properties are shown in the following Table, which also includes for comparison the properties of commercially available water treatment carbons Calgon "FILTRASORB 400" and Pica "PICABIOL"

A sample 2C was activated under similar conditions to samples 2A an d 2B for a period of 2 hours 50 minutes.

The activated carbon was crushed, tumbled and then sieved to produce a granular product in the size range 2.36 mm - 0.85 mm. The yield in this case was 39.1 db %.

The properties of this product make it suitable for use in gold recovery. These properties are shown in the following table which also includes for comparison the properties of a commercially available gold recovery carbon: Pica

"PICAGOLD G210 AS".

Example 3

The data which follows derives from a sample of Pinus

Radiata sawdust from Western Victoria which was actually used in this example.

The chemical analysis of this feedstock was as follows:

The following Table 3 provides data relating to the input and product of each step.

The products having the genesis presented in TABLE 3 produce activated carbons of high quality. The properties are determined by the carbonisation and activation

technique.

Sample 3A was activated under similar conditions to the samples in Example 2, to produce a product having the following properties.

Although we do not wish to be limited by any postulated or hypothetical mechanism for the demonstrated benefits of the invention, we believe that the key to our binderiess compaction technology is the controlled utilisation of the binder materials (tars) naturally occurring in the sawdust.

One of the ways in which our process is differentiated from the prior art is by partially devolatilising the finely ground sawdust until it becomes plastic, thus enabling it to be compacted without the aid of chemical additives or added binder materials. We have further found that by adjustment of the conditions employed when the compaction is carried out, that is during the softening stage of the carbonisation phase, we can vary the pore size distribution of the product to suit specific applications.

The process of the present invention has many advantages, including the following:- 1. It utilizes a waste product as feedstock which

otherwise presents a disposal problem. 2. The process and the product are environmentally acceptable.

3. The process does not use a binder and this results in reduced processing costs. 4. The activated carbons produced by this process are hard and regenerable.

5. in the case of the shaped pelletised product the

uniform shape results in low pressure drops in

applications requiring packed beds. 6. The granular and pelletised activated carbons have a high total adsorption. The pelletised carbon also has a high kinetic adsorption.

7. The process has the capability of modifying the pore size distribution of the product to suit particular applications. Hence it is possible to produce a broad spectrum of product types for both liquid and gaseous applications.

It will be clearly understood that the invention in its general aspects is not limited to the specific details referred to hereinabove.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A process for treatment of carbonaceous materials which comprises the following steps:

(a) heating a carbonaceous material in the absence of air to dry it and render it friable;

(b) grinding the product of step (a) to a fine powder;

(c) heating the product of step (b) in the absence of air to render it formable;

(d) compacting the product of step (c) while still hot; (e) cooling the product of step (d).

2. A process according to claim 1 wherein the product of step (a) is cooled in the absence of air before grinding in step (b).

3. A process according to claim 1 or claim 2 wherein the carbonaceous material is sawdust.

4. A process according to any one of the preceding claims wherein the heating in step (a) is carried out at a

temperature in the range 150-300°C.

5. A process according to any one of the preceding claims wherein the heating in step (a) is carried out at a

temperature in the range 200-270°C.

6. A process according to claim 2 wherein the product of step (a) is cooled to just above ambient temperature before grinding in step (b).

7. A process according to any one of the preceding claims, wherein in step (b) the product is ground to a size in the range 30 mesh BSS (500 μm) to 200 mesh BSS (75 μm).

8. A process according to any one of the preceding claims, wherein in step (c) the product of step (b) is heated to a temperature in the range 280-375°C.

9. A process according to any one of the preceding claims, wherein in step (c) the product of step (b) is heated to a temperature in the range 300-375°C.

10. A process according to any one of the preceding claims, wherein in step (d) the product of step (c) is compacted in a briquetting or extrusion press to produce pellets or compacts .

11. A process according to claim 10 wherein the product of step (c) is compacted at a pressure up to 150MPa.

12. A process wherein steps (a) to (d) as defined in any one of the preceding claims are carried out to produce compacts or pellets, and the compacts or pellets are dusted with carbon powder if necessary to prevent them sticking together.

13. A process according to any one of the preceding claims, wherein in step (e) the product of step (d) is allowed to cool in the atmosphere for a period of about 5 minutes.

14. A process for the production of activated carbon, which comprises activating the product of the process as defined in any one of the preceding claims.

15. The product of a process as defined in any one of the preceding claims.