WO2010149188A1

WO2010149188A1 - Procedure and composition of activated carbon obtained from waste tyres

Info

Publication number: WO2010149188A1
Application number: PCT/EP2009/004630
Authority: WO
Inventors: Pedro Romera Lorca
Original assignee: Desarrollos Tecnicos Mc, S.L.
Priority date: 2009-06-26
Filing date: 2009-06-26
Publication date: 2010-12-29

Abstract

The object of the present invitation is to develop a process and a composition of activated carbon from waste tyres that in a single stage achieves and activated carbon of high quality and excellent mechanical properties at low cost, this process achieving the above mentioned stages by adding natural components to the mixture, in addition the use of waste tyres makes it possible to obtain an active composition of high quality mechanical properties and at a very low cost.

Description

PROCEDURE AND COMPOSITION OF ACTIVATED CARBON OBTAINED

FROM WASTE TYRES

OBJECT OF THE INVENTION

Procedure and composition of activated carbon obtained from waste tyres and natural clay silicates.

TECHNICAL BACKGROUND

Activated carbon, or active carbon, is a porous carbon material. A carbonised material that has been made to react with oxidising gases (such as CO₂ or air), or with water vapour; or treated by adding chemical products such as H₃PO₄, during (or after) a carbonisation process, with a view to increasing its porosity. Activated carbons have a high absorption capacity and are used for purifying liquids and gases. Appropriate control of the carbonisation and activation processes produces a wide range of activated carbons with various pore size distributions.

Practically any organic material with relatively high proportions of carbons can be transformed into activated carbon. Activated carbons can be obtained industrially from wood and forest waste or other types of biomass, peat, lignite, and other mineral carbons, as well as from various polymers. Nevertheless, there are certain limitations. Thus, from a structural point of view activated carbons are very disorderly and isotropic. Therefore, carbonous materials that pass through a fluid or pseudo-fluid state during their carbonisation will not be suitable for preparing activated carbons given that re-solidification of this phase tends to form ordered structures in the resulting carbons. Hence, cokeable carbons are not adequate, unless their cokeable properties can be eliminated through prior oxidation, and similarly thermoplastic residues are not adequate either. The factors that need to be taken into account in order to choose an adequate precursor are: high availability and low cost, low content in mineral material and for the resulting carbon to possess good mechanical properties and absorption capability. Wood, coconut shells and other husks, in addition to mineral carbons and petroleum coke are the most commonly used precursors.

Heat activation

Also known as physical activation, despite the fact that the activation occurs through the (chemical) reaction of the activating agent (an oxidant such as air, water vapour, CO₂, etc.) with the carbon of the material that is being activated. This type of activation comprises several stages. Hence, occasionally certain pre-^*treatments are required such as grinding and sifting to obtain a suitably-sized precursor. If the precursor is a cokeable carbon then an oxidation stage will be necessary in order to eliminate the cokeable properties. On other occasions, the material used is milled until it forms a fine powder, then compacted with a binder in the form of briquettes and then milled again until the required size is obtained. This achieves a better diffusion of the activating agent and therefore an improved porosity in the resulting activated carbon. Another prior stage to activation itself is carbonisation, wherein the precursor is subjected to high temperatures (in the region of 800 ⁰C) with a lack of air, in order to eliminate the volatile substances and leave a carbonous residue which will be subjected to activation. During de-volatilisation, the gas and vapour output from the precursor produces an "incipient" porosity in the carbonised material, which develops further during the activation phase.

Activation itself may be a totally independent process from carbonisation or may be carried out subsequently. It consists of making the activating agent react with the carbon atoms of the carbonised material which is being activated in such a way that a "selective burning" takes place gradually perforating the carbonised material, generating pores and increasing porosity until transforming it into an activated carbon. The activating agents that tend to be used are: Oxygen (rarely on an industrial scale), air, water vapour (most commonly) and CO₂.

Chemical activation

In this type of activation the precursor is made to react with an activating chemical agent. In this case, the activation tends to take place in a single stage at temperatures that may vary between 450 and 900 ⁰C. Nevertheless, in this type of activation a subsequent stage is necessary of washing the activated carbon in order to eliminate the remains of the activating agent. There are numerous compounds that could be used as activating agents, however the most commonly used industrially are zinc chloride (ZnCI₂), phosphoric acid (H₃PO₄) and potassium hydroxide (KOH). Chemical activation through ZnCL₂ was the most frequently used method up until 1970, especially in order to activate wood residues. However, its use has now been seriously restricted due to the environmental problems the use of ZnCI₂ entails. However, in some countries such as China this method continues to be used for production.

Chemical activation through H₃PO₄ has practically replaced ZnCI₂ and the precursors used for this type of activation are mostly, as in the case of ZnCI₂, forest residues (wood, coconut shell, olive stones, etc.). Activation through H₃PO₄ implies the following stages: (i) milling and sorting of the source material, (ii) mixing of the precursor with H₃PO₄ (recycled and cooled),

(iii) heat treatment in an inert environment between 100 and 200 ⁰C, maintaining the temperature for approximately 1 h, followed by a new heat treatment up to 400 - 500 ⁰C, maintaining this temperature for about 1h, (iv) washing, drying and sorting of the activated carbon, and recycling of the H₃PO₄. The most frequently used proportion of H₃PO₄:precursor tends to be 1 :5 (although different proportions give rise to carbons with different properties), and the performance of the activated carbon tends to be around 50%. Chemical activation through KOH was developed in the 70s in order to produce the so-called "superactivated carbons", with specific surfaces in the region of 3000 m2/g. Unlike the other two activating agents, the preferred precursors for activation through KOH are those with a low content in volatile substances and high content in carbon, such as top class mineral carbons, carbonised materials, petroleum coke, etc. In this activation KOH is mixed with the precursor in aqueous suspension or by simple physical mixing, with KOH: precursor proportions of between 2:1 and 4:1. When impregnation takes place in the aqueous medium, the activation is carried out in two consecutive heat treatments in an inert environment. The first at low temperatures but above 200⁰C (which is used only to evaporate the water and disperse the KOH) and the second at between 700 and 900 ⁰C. In the case of physical mixing, the first heat treatment is not necessary.

Types of activated carbons

Activated carbons can be classified on the basis of particle size in powder activated carbon (PAC) and granular activated carbon (GAC). PACs present sizes of less than 100 μm, and typical sizes are between 15 and 25 μm. GACs tend to have an average particle size of between 1 and 5 mm. GACs can be divided into two categories: (i) activated carbon in pieces (or shapeless) and

(ii) moulded activated carbon (with a specific shape, cylinders, disks, etc.). Activated carbons in pieces can be obtained by milling, sifting and sorting carbon briquettes or larger pieces. Moulded carbons can be obtained in the form of pellets or by extrusion of the carbon powder with various types of binders. There are also other forms of carbon absorbers, such as activated carbon fibres, activated carbon sheeting and felts, monolithic structures, carbon membranes, etc.

In principle all carbons are hydrophobic; however, we can reduce this hydrophobic nature by adding polar surface groups. This can be achieved by oxidation with some type of oxidant agent. Oxygenated groups give rise to primary centres of water molecule absorption which in turn will absorb new molecules by forming hydrogen bridges. This increases the hydrophile nature and "wettability" of the carbons. In the case of absorbing inorganic compounds in the aqueous phase, this could be beneficial. However, in the case of the activated carbon being used to absorb compounds in gas state, the fact that the pores are occupied, or even blocked by water molecules can substantially diminish the carbon's absorption capacity. In fact, this combined humidity oxidation/absorption effect of activated carbons is known as the ageing effect and is something to be avoided inasmuch as possible, especially for applications in the gas phase.

Another important aspect of the surface chemistry of activated carbon is its amphoteric nature, which means that on the surface of the carbon acid surface groups and base surface groups coexist. Whether a carbon is overall acid or base will depend on both the concentration of these groups and on their strength as acids or bases. Intuitively, it is possible to deduce that a base-type carbon will be preferable for absorbing acid compounds than an acid-type carbon and viceversa.

At the same time, acid groups tend to liberate protons, especially in base media, whereas base groups tend to capture them when they are in an acid medium. This means that positive or negative charges may appear on the surface of the carbon. In a generic fashion, if the pH of the medium is higher than the zero charge point of the carbon (pH_Zcp. pH where the number of positive charges is equal to the number of negative charges in such a way that the net charge on the surface of the carbon is zero), we will have predominance of the negative charges on the surface of the carbon; on the contrary, if pH < pHzcp we will obtain a positively charged surface. The conditions in which the carbon has a net negative charge will be preferable for absorbing cations and those in which it has a net positive charge will be preferable for absorbing anions. Given that it is not always simple to modify the pH of contaminating effluents it is preferable to optimise the surface chemistry of the activated carbon bearing in mind the abovementioned criteria so that absorption is maximum.

The object of the present invention is to develop a process and an activated carbon composition from waste tyres wherein a single stage produces a high quality activated carbon with good mechanical properties at low cost. This process manages to combine the abovementioned stages into a single stage by adding natural components to the mix. Also the use of waste tyres makes it possible to obtain an activated carbon composition of high quality, with excellent mechanical properties and at a very low cost.

DESCRIPTION OF THE INVENTION

The object of the invention is a process for preparing an activated carbon composition using waste tyres, wherein the activated carbon can be prepared in a single phase, in other words by carrying out carbonisation and activation in a single stage in such a way that the resulting product is low density, having a higher mechanical resistance and a structure that allows a larger absorption surface, this activated carbon can be used in filters for chimney outputs or fluid discharge pipes with a view to preventing the toxic emissions of various contaminants.

The raw material for producing this activated carbon is fundamentally waste tyres, meaning that the produced activated carbon is very low cost, and also presents special mechanical properties, in addition to high surface performances, resistant to breakage and abrasion and easily mouldable, through integration of the properties of natural silicates from clays, in particular bentonite.

The surface available for absorption will depend on the molecule size of the absorbed material and on the diameter of the pore of the absorbing material; if the aim is to absorb large-size molecules it is more interesting to have absorbents with a suitable pore diameter.

Waste tyres are first chopped into small pieces of 100 to 120 mm, which are placed in a furnace and subjected to pyrolysis. The furnace must have no oxygen and pyrolysis will be carried out at a temperature of between 800 ad

1000⁰C, where carbonisation takes place, and the obtained result is mixed with a natural clay, in particular bentonite and the mixture is placed in contact with vapour in such a way that a product is achieved with a high absorption capacity and high mechanical resistance.

The thorough mixture of the abovementioned components produces as a result materials that present a significant increase in the specific absorption surface and a high volume of meso and macropores as compared to the values that would correspond to the source products bearing in mind their percentages in the mix.

In the mix any natural silicate from clay or other components can be used, in particular bentonite, enstatite or ά- sepiolite, atapulgite, zeolites, in pure state or forming mixtures; the binder increases its mechanical capabilities.

The process of preparing the composition involves the following stages: chopped pieces of tyre between 100 and 120 mm long are placed in a pyrolytic furnace, the furnace having no presence of oxygen, and then the temperature is increased to between 800 and 1000 ⁰C for a time that will depend on the volume of tyre strips placed in the oven. The resulting product is mixed homogenously with clay silicates, preferably bentonite and a stream of water vapour is made to circulate for approximately 2 hours, the result being moulded or extruded, in order to obtain the required shapes, which may be moulded into granules, tablets, balls, hollow cylinders, plates or parallel channel structures along the longitudinal axis, the composition presenting a parallel channel structure along the longitudinal axis of between 5 and 100 channels per square centimetre of its transversal section and is preferably shaped into hollow cylinders of the "macaroni" type, smooth, striated, straight, curved, with a regular or irregular edge, in any shape and size in the form of hollow cylinders, balls, or similar they may be from approximately 0.4 cm long to approximately 10 cm and the wall thickness may be between 0.03 and 0.5 cm.

The moulded elements are dried at room temperature for 2 hours at minimum and then the product is reinserted in the furnace that gives it a heat treatment in an inert environment at a temperature that ranges between 800 and 1000⁰C until the heat treatment is completed, producing as a result an activated carbon product that can be used for the elimination of impurities on both a domestic and an industrial scale.

In the resulting mixture, the weighted ratio after coming out of the furnace between the activated carbon from tyres and clay silicates can vary between 1 :0 and 1 :1 , although an increase in the aggregate of activated carbon from tyres increases the absorbing capacity to the detriment of the mechanical properties of the obtained pieces, whereas the clay aggregate makes it possible to increase the size of the porous surface and improve as mentioned the mechanical properties of the resulting mix.

The resulting composition presents an activated carbon composition of high quality at low cost and with special mechanical properties for use in filters for absorbing contaminants in various processes where the elimination of impurities is required, in other words chemical plants, distillers, and filters also for domestic use such as water filters and similar.

Claims

1. Procedure for preparing an activated carbon composition from waste tyres characterised in that it comprises the following operative stages: - Cutting the tyres into strips between 100 and 120 mm long.

Placing the strips in a pyrolytic furnace at a temperature between 800 and 1000 ⁰C

Mixing with a natural silicate from clays, preferably bentonite and passing a stream of water vapour during 5 hours - Moulding the mixture and drying at room temperature for 2 hours

Moulded components are kept in the furnace at between 800 and 1000 ⁰C until the heat treatment is completed.

2. Procedure for preparing an activated carbon composition from waste tyres, according to claim 1 characterised in that the weighted ratio of the mixture of carbon from tyres and clays varies from 1 :0 to 1 :1

3. Procedure for preparing an activated carbon composition from waste tyres, according to claim 1 characterised in that the composition consists of granules, tablets, balls, solid cylinders, hollow cylinders or plates.

4. Procedure for preparing an activated carbon composition from waste tyres, according to claim 4, characterised in that the hollow cylinders are selected from the group comprising: smooth cylinders with regular or irregular edges, striated cylinders with regular or irregular edges, straight cylinders with regular or irregular edges, curved cylinders with regular or irregular edges.

5. Procedure for preparing an activated carbon composition from waste tyres, according to claim 5 wherein the hollow cylinders have a length of approximately 0.4 cm to approximately 10 cm and the thickness of the wall can be between 0.03 and 0.5 cm.

6. Procedure for preparing an activated carbon composition from waste tyres according to claim 4, characterised in that said composition presents a structure of parallel channels along the longitudinal axis of between 5 and 100 channels per square centimetre of its transversal section.

7. Activated carbon composition from waste tyres characterised in that it comprises: activated carbon from tyres and natural clay silicates.

8. Activated carbon composition from waste tyres according to claim 8 characterised in that the natural clay silicates are preferably selected from the group comprising: bentonite, enstatite, ά- sepiolite, atapulgite, zeolites or a mixture of the former.

9. Activated carbon composition from waste tyres according to claim 8, characterised in that the proportion of carbon from tyres and clay silicates varies in the weighted proportion from 1 :0 to 1 :1.