Method for obtaining a carbon black powder by pyrolyzing scrap rubber, the carbon black thus obtained and the use thereof

Abstract

The present invention relates to a method for recycling scrap rubber comprising the steps of pyrolyzing scrap rubber to obtain a char material and milling the thus obtained char material. The present invention also relates to carbon black powders and carbon black pellets obtained by the method according to the invention. Moreover, the present invention relates to the use of said carbon black powder and to compositions comprising said carbon black powders.

Claims

1. A method for recycling a scrap rubber, which method comprises the following steps: i) pyrolyzing a scrap rubber to obtain a char material; ii) milling the char material obtained in step i) to obtain a carbon black powder; characterized in that the pyrolysis in step i) comprises at least a two-stage pyrolysis process, wherein the said two-stage pyrolysis process comprises: a) a first pyrolysis stage to obtain an intermediate char material and b) a second pyrolysis stage to obtain a char material, characterized in that the first pyrolysis stage a) is carried out in a first rotary kiln and the second pyrolysis stage b) is carried out in a second rotary kiln, wherein the temperature during the first pyrolysis stage a) is between 630-670 C. and the temperature during the second pyrolysis stage b) is between 680-720 C., the temperature during the second pyrolysis stage b) being higher than the temperature during the first pyrolysis stage a), wherein the char material obtained from the second pyrolysis stage has a polyaromatic hydrocarbon (PAH) content less than 0.5 mg/kg.

2. The method according to claim 1, wherein in the at least first pyrolysis stage a) the percentage of volatiles present in said scrap rubber is reduced to an amount of about 5-10 wt. % based on the total weight of the intermediate char material, and wherein the intermediate char material is introduced in the second pyrolysis stage b) in which the percentage volatiles is further reduced to a percentage of less than 2.5 wt. % based on the total weight of the char material.

3. The method according to claim 1, wherein the residence time of each the first pyrolysis stage a) and the second pyrolysis stage b) are between 20-50 minutes, and the residence time of each of the first pyrolysis stage a) and the second pyrolysis stage b) is equal in duration.

4. The method according to claim 1, wherein in the second pyrolysis stage b) the percentage volatiles is reduced to less than 1.0 wt. % based on the total weight of the char material.

5. The method according to claim 1, wherein the milling of step ii) is carried out by jet milling using compressed air or steam.

6. The method according to claim 1, wherein the milling of step ii) is carried out in such a way that the carbon black powder obtained from step ii) has a particle size distribution of D50<10 m and D99<40 m.

7. The method according to claim 1, wherein the temperature in the second pyrolysis stage b) is at least 30 C. higher than in the first pyrolysis stage a).

8. The method according to claim 1, wherein the rotary kiln is a cylindrical vessel, inclined slightly to the horizontal, which is rotated about its axis, wherein the material to be processed is fed into an upper end of the cylinder and gradually moves down towards a lower end as the kiln rotates.

9. The method according to claim 1, wherein the scrap rubber is tyres.

10. The method according to claim 2, wherein in the at least first pyrolysis stage a) the percentage of volatiles present in said scrap rubber is reduced to an amount of about 5-10 wt. % based on the total weight of the intermediate char material, and wherein the intermediate char material is introduced in the second pyrolysis stage b) in which the percentage volatiles is further reduced to a percentage of less than 2.0 wt. %, based on the total weight of the char material.

11. The method according to claim 3, wherein the residence time of each the first pyrolysis stage a) and the second pyrolysis stage b) are between 25-45 minutes.

12. The method according to claim 3, wherein the residence time of each the first pyrolysis stage a) and the second pyrolysis stage b) are between 30-40 minutes.

13. The method according to claim 6, wherein the milling of step ii) is carried out in such a way that the carbon black powder obtained from step ii) has a particle size distribution of D50<5 m and D99<20 m.

14. The method according to claim 6, wherein the milling of step ii) is carried out in such a way that the carbon black powder obtained from step ii) has a particle size distribution of D50<1 m and D99<10 m.

15. The method according to claim 6, wherein the milling of step ii) is carried out in such a way that the carbon black powder obtained from step ii) has a particle size distribution of D50<0.5 m and D99<2 m.

16. The method according to claim 1, wherein the temperature in the second pyrolysis stage b) is at least 50 C. higher than in the first pyrolysis stage a).

17. The method according to claim 1, wherein the temperature in the second pyrolysis stage b) is at least 30 C. higher than the first pyrolysis stage.

18. The method according to claim 1, wherein the temperature in the second pyrolysis stage b) is at least 80 C. higher than in the first pyrolysis stage a).

19. The method according to claim 1, wherein the char material has a polyaromatic hydrocarbon (PAH) content less than 0.25 mg/kg.

20. The method according to claim 1, wherein the char material has a polyaromatic hydrocarbon (PAH) content less than 0.01 mg/kg.

Description

DRAWING

(1) FIG. 1 shows a process flow diagram for the process according to the invention.

DESCRIPTION OF THE DRAWING

(2) FIG. 1 shows a process flow diagram that explains an embodiment of the present invention. This flow diagram is not limiting to the present invention but merely for illustrative purposes.

(3) Granulated feedstock tyres (scrap rubber) are blended from two feedstock hopper (1) in a feedstock blender (2). The resultant blended feedstock is added to a first rotary kiln (3) in which the at least first pyrolysis stage a) takes place to obtain an intermediate char material. The intermediate char material is added to a second kiln (polisher kiln, 4) to obtain a char material according to the present invention.

(4) The volatiles released in the first and at least second pyrolysis stage are collected in receiving lines (5) and optionally used for steam generation or electricity generation. The condensable volatiles (viz. oils) collected from the at least first pyrolysis stage are condensed in a condenser (7).

(5) Subsequently, the obtained char material is fed into a char cooler (8), which char material is then de-aggregated in a break mill (9). A magnetic separator (10) is used to remove any remaining steel components (resulting from steel reinforcement of the feedstock tyres) prior to feeding the char material into a jet milling apparatus (11). The product from the milling step is known as a carbon black powder and is subsequently pelletized in a pelletizer (12). The pelletized carbon black powder is then fed through a fluid bed (13) to yield the final product carbon black powder (14).

(6) The invention shall now be exemplified by a number of non-limiting examples.

EXAMPLES

(7) The following examples show several process steps of the present invention.

(8) Pyrolysis

(9) This example shows a rotary kiln operating in a two-stage pyrolysis mode.

(10) The scrap rubber obtained from tyres were added to the pyrolysis apparatus in the form of a granulate wherein 100% of the particles have a length of less than 30 mm, a width less than 25 mm and a height less than 30 mm, and 95% of the particles have a length of less than 25 mm, have a width of less than 25 mm and a height of less than 25 mm. The composition of the scrap rubber obtained from the feedstock tyres having either a low silica content (A), an average silica content (B) or a high silica content (C) is shown in Table 1 below. All the numbers are In percentage by weight, based on the total weight of the scrap rubber.

(11) TABLE-US-00001 TABLE 1 The composition of typical feedstock tyres is: A (low B (average C (high silica) silica) silica) Residual material 4.9 9.8 15.4 (Silica) (1.4) (5.5) (11.2) Volatiles 64.4 62.2 61.0 Theoretical yield 30.7 28.0 23.6 carbon black
Table 2 below discloses the conditions that were used for the pyrolysis of several Examples according to the invention (Examples 1-7) and not-according to the invention (comparative Example). The Examples 1 and 2 where carried out in two parts, 1A and 1B, and 2A and 2B, respectively. 1A and 2A relate to the first stage pyrolysis process whereas 1B and 2B relate to the second stage pyrolysis process. This was done in order to determine the percentage of volatiles in the intermediate char material obtained after the first pyrolysis stage (1A and 2A).

(12) TABLE-US-00002 TABLE 2 Conditions used for the first and second stage of the pyrolysis step and the volatile content of the product obtained therefrom. First pyrolysis stage Second pyrolysis Stage Char charring polishing material Feedstock Residence Temperature/ Residence Volatiles Example Scrap rubber Temperature/ C. time/min C. time/min (wt. %) 1A B 500-600 30 Not done Not done 29.96 1B Intermediate char 575 30 3.1 from 1A 2A A 550-650 20-30 2.37 2B Intermediate char 650-750 5-10 1.33 from 2A 3 A 650 30 650 30 2.3 4 B 650 30 650 30 2.2 5 C 650 30 650 30 1.9 6 A 550-650 20-30 650-750 5-10 0.9 7 B 550-650 20-30 650-750 5-10 2.4 Comp. 1 A 550-650 20-30 Not done Not done 2.9

Examples 1-7 and Comparative Example 1

(13) An electrically heated rotary kiln was setup using an expanded 238 mm (9.38) OD cylinder tube with an integral internal flight cartridge, no cooling zone, a sealed feed hopper assembly with two slide gates to minimize air infiltration, two heaters that were installed in series with respect to each other to preheat the nitrogen gas before entering the feed breeching and cylinder. A two-stage condenser system after discharge breeching was installed to collect the condensable oil that was produced during the pyrolysis. A gas totalizer with a bypass arrangement was installed in the vent line downstream of the condenser to take periodic measurements of the off-gas flow rate. The kiln was set up for concurrent flow. A nitrogen purge was used to maintain an inert atmosphere in the interior of the kiln during the pyrolysis; the product bin containers were also purged with nitrogen.

(14) The tyre pyrolysis process was performed in two stages, with the equipment systems as described above. The first stage being the charring stage where 10 kg of feed material was heated to the point of releasing the volatiles (concurrent operation) and the kiln was rotated at 1-2 rpm; and the second stage being the polishing and cooling stage where the intermediate char material with a small amount of remaining residual volatile matter was removed and the kiln was rotated at 3-4 rpm. As noted in the Table 2 above for some of the Examples and Comparative Example either the first or second pyrolysis stage was omitted.

(15) Prior to conducting the test trials, all of the off-gas vent line system components were weighed and recorded so as to get an accurate mass balance of the material build up for the first stage and the second stage the charring stage of the tyre pyrolysis.

(16) Polyaromatic Hydrocarbon (PAH) Analysis

(17) The char material obtained from the pyrolysis of scrap tyres as described in Examples 6, 7 and Comparative Example 1 was analysed according to DIN 51720-2001 (volatile content), DIN 51719-1197 (ash content) and DIN ISO 11465-1996 (moisture) and Merkbl. 1, LUA-NRW (GC-MSD) (PAH). The material prepared according to the invention (viz. Example 6 and Example 7) was compared to the material prepared according to the prior art (Comparative Example 1), which had only been processed in the first pyrolysis stage. Two additional char materials have been tested, both are non-commercially available products that are denoted as Comparative Example 2 (obtained from Carbon Clean Tech, Germany) and Comparative Example 3 (obtained from Erus d.o.o., Slovenia) These Comparative Examples 2 and 3 are both carbon black powders obtained by prior art methods. The composition of the char materials is given in Table 3 below and the polyaromatic hydrocarbon analysis is given in Table 4 below.

(18) TABLE-US-00003 TABLE 3 Comparison of char material obtained by the invention and prior art. Residual Mois- Carbon material, Volatiles, ture, PAH, Example Black, % % % % mg/kg Total 6 85.0 13.3 0.9 0.8 0.0 100.0 7 75.6 21.4 2.4 0.7 0.0 100.0 Comparative 82.5 13.4 2.9 1.2 71.0 100.0 example 1 Comparative 79.9 13.1 5.6 1.4 60.0 100.0 example 2 Comparative 67.0 17.0 15.2 0.8 15.0 100.0 example 3

(19) TABLE-US-00004 TABLE 4 PAH content for the char material listed in Table 3 Polyaromatic hydrocarbons Units Ex. 6 Ex. 7 Comp. Ex. 1 Comp. Ex. 2 Comp. Ex. 3 Naphthalene mg/kg <0.050 <0.14 39.00 7.0 1.3 Acenaphthylene mg/kg <0.050 <0.14 <0.2 0.43 <0.05 Acenaphthene mg/kg <0.050 <0.14 <0.2 1.4 0.52 Fluorene mg/kg <0.050 <0.14 <0.2 2.0 0.46 Phenanthrene mg/kg <0.050 <0.14 5.3 6.9 1.2 Anthracene mg/kg <0.050 <0.14 1.5 2.5 0.63 Fluoranthene mg/kg <0.050 <0.14 1.3 5.1 1.70 Pyrene mg/kg <0.050 <0.14 2.4 9.8 2.50 Benz[a]anthracene mg/kg <0.050 <0.14 2.3 7.0 1.3 Chrysene mg/kg <0.050 <0.14 2.00 8.0 1.4 Benzo[x]fluoranthene mg/kg <0.050 <0.14 2.1 9.0 2.2 wherein x is b; j; k; or j, k Benzo[a]pyrene mg/kg <0.050 <0.14 2.0 7.5 1.7 Dibenz[a,h]anthracene mg/kg <0.050 <0.14 0.21 0.69 <0.05 Benzo[ghi]perylene mg/kg <0.050 <0.14 0.63 1.2 <0.05 Indeno[1,2,3-cd]pyrene mg/kg <0.050 <0.14 1.4 2.3 <0.05
Milling

(20) The char materials obtained in Examples 6 and 7 were milled in an Lab AFG 100 milling apparatus (from Hosokawa Alpine). The milling apparatus was operated at a temperature of 20 C. and 22 000 rpm using air as the medium at a pressure of 3 bar, the feed was added directly to the miller through 3 nozzles with a diameter of 1.9 mm.

(21) The milled product was measured for particle size distribution (D50 & D99) using wet laser diffraction on a Malvern Mastersizer S Ver 2.19. The range lens was 300 RF mm, the beam length was 2.40 mm. The analysis mode used was Polydisperse. A mixture the commercially available Morvet+Supragil (used in a ratio of 70:30) was used as the wetting agent and external ultra-sound was applied to prevent aggregation of the particles. The obtained results are shown in Table 5 and Table 6 below. In Table 6 below measurements were carried out on two commercially available furnace blacks denoted as Comparative Example 4 (N550 of Birla Carbon) and Comparative Example 5 (N660 of Birla Carbon).

(22) TABLE-US-00005 TABLE 5 Particle size distribution of milled carbon black powder according to the invention. Milling apparatus Example used D99 D50 BET 2B Vibratory + air jet 5.4 m 1.4 m 90.4 m.sup.2/g (1.3 kWh/kg) 2B Vibratory + air jet 8.2 m 2.3 m 83.9 m.sup.2/g (1 .3 kWh/kg) 2B Vibratory 35.0 m 9.0 m 70.7 m.sup.2/g 6 Vibratory + air jet 4.14 m 0.95 m Not measured 7 Vibratory + air jet 3.95 m 0.80 m Not measured

(23) TABLE-US-00006 TABLE 6 Particle size distribution and BET surface area of scrap rubber derived carbon black powder (according to the invention) and furnace black (prior art). Type of carbon black Particle Size BET Surface Area, Example powder Distribution m.sup.2/g Comparative N550 [D50 < 2.06 m/ 40-49 example 4 (furnace) D99 < 6.2 m] Comparative H660 [D50 < 2.6 m/ 33-30 example 5 (furnace) D99 < 10.9 m] 2 Scrap [D50 < 1.56 m/ 90.4 rubber D99 < 5.4 m] derived
EPDM Rubber Compounding

(24) Rubber compositions were made by mixing scrap rubber derived carbon black powder or commercially available furnace derived carbon with an EPDM rubber and other components as shown in Table 7.

(25) TABLE-US-00007 TABLE 7 Composition of an EPDM rubber comprising scrap rubber derived carbon black. Ingredient PHR Keltan 8340A (EPDM) 100 Carbon black of Ex. 2 OR 110 comparative example 4 OR comparative example 5 Paraffinic oil 70 Zinc oxide 5 Stearic acid 1 Sulphur-80 1.9 ZDEC-80 2.5 MBT-80 0.6 TMTD-80 0.6 Total 291.6
The composition of the commercially available rubber used (Keltan) is further elucidated in Table 8 below.

(26) TABLE-US-00008 TABLE 8 Composition of Keltan 8340A Keltan 8340A Value Ethylidene norbornene, wt. % 5.5 Oil, wt. % 0.0 Ethylene, wt. % 55 ML (1 + 4) 125 C. 80
Measurements on the mechanical properties of the rubber compositions are summarized in Table 9 below. Comparative Example 6 is a rubber composition according to Table 7 wherein a carbon black according to Comparative Example 4 is used. Comparative Example 7 is a rubber composition according to Table 7 wherein a carbon black according to Comparative Example 5 is used. Example 8 is a rubber composition according to Table 7 wherein a carbon black according to Example 2 (after steps 2A and 2B) is used.

(27) TABLE-US-00009 TABLE 9 Mechanical properties of EPDM rubber compositions comprising scrap derived carbon black and furnace derived carbon black. Rubber Tensile DeMattia flex rebound composition strength M100 M300 fatigue resilience Example Carbon black (MPa) #1 (MPa) #2 (MPa) #3 (kilocycles) #4 (Schob) #5 Comp. Ex. 6 Comp. Ex. 4 15.9 3.1 10.4 >20 40 Comp. Ex. 7 Comp. Ex. 5 15.5 2.5 8.8 >20 43 Ex 8 Ex. 2 15.7 1.8 6.7 >20 46 #1: Tensile strength was measured according to ISO 37-2011. #2: M100 was measured according to ISO 37-2005. #3: M300 was measured according to ISO 37-2005. #4: DeMattia flex fatigue was measured according to ASTM D2230-2012. #5: Rebound resilience (Schob) was measured according to ISO4662-2009.

(28) The measurements shown in Table 8 clearly show that the EPDM rubber composition of Example 8 according to the present invention has a much lower M100 and M300 stiffness moduli while maintaining the tensile strength compared to the prior art rubber compositions.

(29) Such combination of properties allows higher loading of the carbon black powders according to the present invention compared to the carbon black powders of prior art without compromising physical properties. This will result in reduced compound cost due to increased dilution of the more expensive polymer.

(30) SBR Rubber Compounding

(31) Rubber compositions were made by mixing scrap rubber derived carbon black powder or commercially available furnace derived carbon with an SBR rubber and other components as shown in Table 10. Rubber compositions were made according to ASTM D3191-2010.

(32) TABLE-US-00010 TABLE 10 SBR rubber compositions Ingredient PHR SBR 1500 100 Carbon black of Ex. 2 50 OR Comp. Ex. 4 OR Comp. Ex. 5 Zinc oxide 3 Sulfur - 80 1.75 TBBS 1 Stearic acid 1
The composition of the commercially available rubber used (SBR 1500) is further elucidated in Table 11 below.

(33) TABLE-US-00011 TABLE 11 The specification of SBR 1500 SBR 1500 Value Polymerization cold, emulsion ML (1 + 4) 100 C. 52 Bound styrene, wt. % 23.5
Measurements on the mechanical properties of the rubber compositions are summarized in Table 12 below. Comparative Example 8 is a rubber composition according to Table 9 wherein a carbon black according to Comparative Example 4 is used. Comparative Example 9 is a rubber composition according to Table 9 wherein a carbon black according to Comparative Example 5 is used. Example 9 is a rubber composition according to Table 9 wherein a carbon black according to Example 2 (after steps 2A and 2B) is used.

(34) TABLE-US-00012 TABLE 12 Mechanical properties of SBR rubber compositions comprising scrap derived carbon black and furnace derived carbon black Rubber tensile DeMattia flex rebound composition Carbon strength M300 elongation fatigue resilience Example black (MPa) #1 M100 (MPa) #2 (MPa) #3 (%) #4 (kilocycles) #5 (Schob) #6 Comp. Ex 8 Comp. 25.2 3.5 17.2 450 5 52 Ex. 4 Comp. Ex 9 Comp. 21.6 2.5 12.4 500 20 55 Ex. 5 Ex. 9 Ex. 2 25.1 2.1 10.3 535 20 55 #1: Tensile strength was measured according to ISO 37-2011. #2: M100 was measured according to ISO 37-2005. #3: M300 was measured according to ISO 37-2005. #4: Elongation was measured according to ISO 37-2005. #5: DeMattia flex fatigue was measured according to ASTM D2230-2012. #6: Rebound resilience (Schob) was measured according to ISO4662-2009.

(35) Similar to the EPDM rubber compositions reported above, the SBR rubber composition of Example 9 according to the present invention has a much lower M100 and M300 stiffness moduli and a much higher elongation compared to compositions according to the Comparative Examples, while maintaining the tensile strength.

(36) The composition of Example 9 has a particularly superior combination of mechanical properties with respect to the comparative compositions. Comparative example 8 has a high tensile strength but the elongation is low and the flex fatigue is especially low. Comparative example 9 has a reasonable elongation and flex fatigue, but the tensile strength is low. The composition of Example 9 has good properties in all of these respects.

(37) Such combination of properties allows higher loading of the carbon black powders according to the present invention compared to the carbon black powders of prior art without compromising physical properties.

(38) Activity of ZnO in Scrap Rubber Derived Carbon Black Powder

(39) In order to assess the activity of zinc oxide (ZnO) several tests were carried out. Zinc Oxide has an effect on the crosslink density. Preferably the lowest amount of ZnO that will still give maximum crosslink density is used. In other word, it is preferred to keep the amount of ZnO as low as possible. Therefore, there is sought a carbon black that provides maximum crosslink density at a low amount of ZnO.

(40) Rubber compositions comprising SBR 1500 (ASTM D3191-2010), a carbon black powder (either of Example 2 or of Comparative Example 4) and various amounts of zinc oxide were made and vulcanized. From the results thereof the amount of ZnO at which maximum crosslinking density was obtained was determined.

(41) A rubber composition comprising carbon black according to Comparative Example 4 showed a maximum crosslink-density at 3 wt. % added ZnO.

(42) The composition comprising the carbon black powder of Example 2 showed a maximum crosslink-density at 1.5 wt. % added ZnO. An increase of the added ZnO to 3 wt. % did not give a further increase of crosslink-density.

(43) From these experiments it can be deduced that less ZnO is required when the carbon black according to the present invention is used compared to the prior art. The present inventions believe, without wishing to be bound by any theory, that the carbon black according to the present invention already comprises a certain amount of ZnO so that the addition of extra ZnO during rubber compounding can be reduced significantly, which is a benefit of the present invention.

(44) The above experiments clearly show that one or more objects of the present invention are obtained by the embodiments cited above and in the appended claims.