ROTARY KILN MADE OF A METAL ALLOY

Abstract

The present invention relates to a rotary kiln made of a metal alloy. The alloy is preferably chosen from the group of Alloy 321, Alloy 321H, Alloy 347, Alloy 347H, Alloy 348 and Alloy 348H. An object of the present invention is to provide a rotary kiln that can be operated under the reducing gas conditions to be experienced by the kiln in a pyrolysis process of scrap rubber. The present invention furthermore relates to the use of such a rotary kiln in a process for the pyrolysis of tyres.

Claims

1. A rotary kiln made of a metal alloy, wherein said metal alloy comprises stabilized austenitic stainless steel.

2. The rotary kiln according to claim 1, wherein said alloy comprises C 0.1 max. %; Mn 2.00 max; P 0.045 max; S 0.03 max; Si 0.75 max; Cr 17-19%; Ni 9-13%, said weight percentages are based on the weight of the alloy.

3. The rotary kiln according to claim 2, wherein said alloy further comprises N 0.1 max; Co 0.20 max; Nb 8?content C to 0.1 max; [Nb+Ta] 8?content C to 0.1 max; Ta 0.10 max; Ti 4?[C+N] min to 0.70 max, said weight percentages are based on the weight of the alloy.

4. The rotary kiln according to claim 3, wherein said alloy is chosen from the group of Alloy 321, Alloy 321H, Alloy 347, Alloy 347H, Alloy 348 and Alloy 348H.

5. The rotary kiln according to claim 1, wherein internal parts of said rotary kiln are made of said metal alloy.

6. The rotary kiln according to claim 1, wherein tubing in communication with one or more product outlets of said rotary kiln are made of said metal alloy.

7. The rotary kiln according to claim 1, provided with one or more welds, wherein the chemical composition of the weld metal comprises about 2-3% ferrite in the austenite.

8. A process for pyrolysis of tyres comprising a use of the rotary kiln of claim 1.

9. The process of claim 8, wherein said kiln is rotated horizontally in a natural gas fired furnace and operated at close to atmospheric conditions.

10. The process of claim 9, wherein said kiln is rotated at 0.5-10 rpm, preferably 1-3 rpm.

11. The process of claim 8, wherein said kiln is positioned on an inclination of between 1 and 5?.

12. The process of claim 8, wherein the internal temperature of said rotating kiln increases over its length from 500-700? C.

13. The process of claim 8, wherein said pyrolysis of tires comprises at least a two-stage pyrolysis process, wherein a first pyrolysis stage is carried out in a first rotary kiln and a second pyrolysis stage is carried out in a second rotary kiln, wherein the temperature during the first pyrolysis stage is 500-700? C. and the temperature during the second pyrolysis stage b) is between 550-700? C., the temperature during the second pyrolysis stage being higher than the temperature during the first pyrolysis stage.

14. The process of claim 8, wherein said kiln is operated under reducing gas conditions.

Description

[0022] FIG. 1 shows a schematic diagram of an indirectly heated rotary kiln with optional cooling zone. The scrap rubber is introduced, together with an inert gas, optionally nitrogen, at the inlet of the rotary kiln. The pyrolysed product flows to the outlet of the rotary kiln and the char material thus produced is withdrawn and further processed.

[0023] FIG. 2 is a DeLong constitution diagram that relates the chemical composition of the balance between austenite and ferrite formers.

[0024] The internal atmosphere (see FIG. 1) or gas composition was determined during two tests and is shown in Table 2.

[0025] Following an extensive investigation a steel chosen from the group of Alloy 321, Alloy 321H, Alloy 347, Alloy 347H, Alloy 348 and Alloy 348H, especially designated Alloy 321 austenitic stainless steel was selected for the metal alloy of the kiln. Whilst there was extensive literature on the use of stainless steels operating under oxidizing conditions there was no direct information of performance under the reducing gas conditions to be experienced by the kiln.

[0026] The present inventors found that the kiln manufacturing capability is excellent when using an alloy chosen from the group of Alloy 321, Alloy 321H, Alloy 347, Alloy 347H, Alloy 348 and Alloy 348H, especially Alloy 321. In addition, the present inventors also found that the weldability of an alloy chosen from the group of Alloy 321, Alloy 321H, Alloy 347, Alloy 347H, Alloy 348 and Alloy 348H, especially Alloy 321 is excellent with standard welding precautions as well. With regard to the resistance to the kiln operating stresses including fatigue stresses the present inventors found that an alloy chosen from the group of Alloy 321, Alloy 321H, Alloy 347, Alloy 347H, Alloy 348 and Alloy 348H, especially Alloy 321 fulfils the required mechanical strength requirements for the normal operation of the kiln. Furthermore, the resistance to a reducing gas atmosphere for up to 8000 hours continuous operation between 500 and 700? C. provided an acceptable result for an alloy chosen from the group of Alloy 321, Alloy 321H, Alloy 347, Alloy 347H, Alloy 348 and Alloy 348H, especially Alloy 321. On the overall balance of properties an alloy chosen from the group of Alloy 321, Alloy 321H, Alloy 347, Alloy 347H, Alloy 348 and Alloy 348H, especially Alloy 321 is derived as the most effective alloy towards reducing the probability of catastrophic failure (by corrosion and other mechanisms) to a minimum. The next parts of the present description refer especially to Alloy 321 but the alloys chosen from the group of Alloy 321, Alloy 321H, Alloy 347, Alloy 347H, Alloy 348 and Alloy 348H should be understood as including.

[0027] The resistance to chromium carbide precipitation and intergranular corrosion is acceptable, i.e. excellent resistance through using a stabilized stainless steel to avoid chromium carbide precipitation. The resistance to carburisation attack can be qualified as an excellent resistance consistent with the low CO/CO.sub.2 ratio of the gaseous atmosphere. Sufficient CO gas at the metal interface is the requirement for carburisation. The high temperature 475 C embrittlement is associated with ferritic stainless steels and not with austenitic stainless steels, including Alloy 321. The Sigma high temperature embrittlement can form from ferrite present with the austenite. Sigma phase is a hard intermetallic compound which is brittle. Extended exposure of ferrite in the temperature range 500-900 C may lead to its formation but more particularly in the temperature range 750-900 C which is above the maximum kiln operating temperature of 700 C. Only a trace of ferrite is probable (1-2 wt. %) in Alloy 321 but the effect of any retained ferrite from which sigma may form has to be balanced against the beneficial effect of such retained ferrite in combating any susceptibility to weld solidification cracking. The presence of small amounts of ferrite in austenitic solidification reduces crack susceptibility. Sulphidation, corrosion and scaling will occur by reaction of kiln sulphide gases with the nickel in the steel. The extent could be largely prevented by using a ferritic stainless steel that has no nickel content. Ferritic stainless steels however are prone to 475 C embrittlement and sigma formation, etc. 475 C embrittlement is due to the formation of an extremely finely dispersed chromium-rich variation in the ferritic crystal structure. Alloy 321 limits sulphur gas nickel corrosion by having one of the lower nickel contents of the austenitic high temperature stainless steels and a reasonably high chromium content that combat such attack. Regarding hydrogenation one can say that all stainless steels have proved to be resistant to hydrogenisation in most high temperature applications.

[0028] In a preferred embodiment of the present invention the Alloy 321 stainless steel is a stabilized austenitic stainless steel 18% Cr-9% Ni similar to Type 304 but with a titanium addition of at least 5? the carbon plus nitrogen content. The titanium addition prevents chromium carbide and chromium nitride precipitation during welding and also at 427-816 C whilst in service due to the preferable formation of TiC and TiN rather than carbon reacting with chromium. These stabilisers also improve the elevated temperature properties of the alloy including the creep strength. The steel is supplied to the manufacturer in the annealed condition with a typical Brinell Hardness of 175. It is available in sheet, plate and bar.

[0029] The typical chemical analysis (wt. %) is: C 0.08 max. %; Cr 17-19%; Ni 9-12%; Mn 2%; N 0.10%; Si 0.75%; P 0.045%; S and P 0.030%, Titanium 5?% (Carbon plus Nitrogen) up to 0.70

[0030] For fusion welding of Alloy 321 the welding should be preferably carried out using alloy 347 (niobium stabilized) filler metal because direct fusion welding of a titanium filler metal results in some loss of titanium in the arc. Nb preferably captures the carbon as NbC thereby preventing chromium carbide precipitation.

[0031] The welding of the kiln tube, more specifically the technical aspects of the chemical composition of the weld metal and the associated possible effects of weld performance in service, are in some embodiments important. The present inventors found that the chemical analysis of the filler metal is a factor in controlling the ferrite content of the austenite. Residual ferrite at a percentage over approx. 3% has several possible detrimental effects. The present inventors found that it is susceptible to significantly converting to sigma phase after long periods at the kiln operating temperature. This results in room temperature embrittlement and can create problems associated with any needed subsequent maintenance involving any plastic deformation. In addition, there would also be some loss of corrosion resistance at operating temperature. Furthermore, some residual ferrite (i.e. <3%) has the advantage of inhibiting hot weld cracking. On basis of these aspects, a compromise at about 2-3% ferrite in the austenite is preferred. In the enclosed FIG. 2, i.e. a DeLong diagram that relates the chemical composition of the balance between the austenite and ferrite formers, by plotting from the actual chemical analysis of the weld metal used according to the recommended formulae the ferrite content can be predicted.

[0032] The present inventors considered other high temperature stainless steels and stainless alloys categorized as follows: (A) Stainless Steels (Iron greater than 50%, Alloy content less than 50%), Iron plus chromium, Iron plus chromium plus nickel, and (B) Stainless Alloys (Alloy content greater then 50%, Iron less than 50%), Iron plus nickel plus chromium; Iron less than 50% plus nickel plus chromium, Nickel plus chromium plus iron; Nickel based alloys with nickel greater than 50%, Nickel plus chromium; Nickel based alloys with nickel greater than 50%. It was found that many of these alloys operate successfully under oxidizing conditions of service. Under oxidizing conditions oxygen maintains a passive surface film of chromium oxide which reduces the rate of oxidation and attack by sulphide gases. These advantages are not sustained under reducing conditions as the passive oxide film is unstable and lost.

[0033] A ferritic chromium steel (Ferrotherm 4742 X10crALSi18, composition: C: 0.12 m, Si: 0.7-1.4, Cr: 17-19, Al: 0.7-1.2) with aluminium addition has an outstanding resistance to sulphur gas attack (sulphidation) due to the absence of nickel but it has many other disadvantages associated with being a ferritic stainless steel. Low strength at elevated temperatures; 475 C embrittlement between 370-538 C; Suffers sigma embrittlement 500-700 C; Poor weldability due to being prone to excessive grain growth in the heat affected zones of fusion weldments. Inadequate resistance to carburization attack.

[0034] Another ferritic stainless steel, i.e. Alloy 446 (composition: C: 0.15, Cr: 27, N: 0.25), has many similar problems making them unsuitable for the kiln metal alloy.

[0035] A technical disadvantage of Incoloy Alloy 800HT (composition: Ni: 30-35, Cr: 19-23, Fe: 39.5 min, C: 0.06-0.10, Al: 0.25-0.60, Ti: 0.25-0.60) is that the titanium content could sometimes fail to combine with all the carbon making it susceptible to intergranular corrosive gas attack. For example the chemical analysis 0.10% C 0.25% Ti that falls within the element specification range.

[0036] The present inventors found that other alloys such as 253MA; 310; 330; 600; 601; 602 were considered unsatisfactory in a reducing atmosphere to sulphur gas attack.

[0037] In Table 1 some properties of the alloys as discussed above are displayed. Qualitative comparative requirement properties of the alloys discussed based on an assessment scale of 1-10. Relates to the estimated performance of Alloy 321: Ferritic stainless steels in general and other Austenitic/Stainless Alloys in general. Performance comparison as a kiln metal operating in a reducing atmosphere as per Table 2 for continuous operation up 8000 hours between 500 and 700 C.

TABLE-US-00001 Rating 1-4 Poor; 5-8 Intermediate; 9-10 Good. OTHER FERRITIC AUSTENITIC PROPERTY ALLOY STAINLESS AND STAINLESS REQUIREMENT 321 STEELS ALLOYS General mechanical 8 5 8 strength: (Tensile, Fatigue, Creep, Impact Carburization and 9 2 9 Intergranular corrosion resistance Sulphidation resistance: 8 10 5 Sulphur dioxide and hydrogen sulphide 475 C. embrittlement 10 2 10 Hydrogenation: 7 7 7 Hydrogen embrittlement Weldability problems 7 3 7 Chromium carbide 9 1 7 precipitation instability Halogen resistance: 5 5 5 Chlorine and bromine gases. Relative initial and 10 8 3 manufacturing costs TOTALS 73 43 61

[0038] On the basis of the above assessments the present inventors found that a kiln made of Alloy 321 provided the best overall cost effective result considering the potential technical requirements of the kiln.

[0039] Other categories of stainless steel that were unsuitable as high temperature materials were as follows: i) Precipitation hardening stainless steels: the steels are solution heat treated followed by ageing at a temperature around 450 C. They are thus not classified as high temperature materials because if used above the ageing temperature they would overage and lose significant strength. Hence their unsuitability for use as a kiln material operating in the temperature range 500 to 700 C, and ii) Duplex stainless steels: these steels typically have a microstructure of 50% ferrite and 50% austenite. They are not classified as high temperature materials because the ferrite can convert to brittle sigma phase after soaking at temperatures in the range 600-820 C. These steels were developed to combat a wet corrosion problem designated stress corrosion cracking and iii) Martensitic stainless steels: these steels are typically quenched and then tempered in the range 200-650 C. Exceeding the tempering temperature would result in a drastic loss in strength. Hence their unsuitability for high temperature operations.

[0040] The gas composition (see Table 2 below) was determined during two tests from two different pyrolysis trials. Although not analysed, traces of chlorine (Cl) and bromine (Br) will be present in the gas. This is known from the composition of the feedstock and from the analysis of the condensable oil fraction.

TABLE-US-00002 TABLE 2 gas composition from pyrolysis trials Measurement 1 Measurement 2 Component Hydrogen (H.sub.2) 27.14 mol % 23.98 mol % Methane 41.14 mol % 36.54 mol % Ethane 4.02 mol % 7.15 mol % Ethene 12.56 mol % 9.80 mol % Propane 0.13 mol % 1.89 mol % Propene 2.73 mol % 7.67 mol % i-butane 0.03 mol % 0.72 mol % n-butane 0.06 mol % 0.64 mol % i-pentane 0.12 mol % 0.03 mol % n-pentane 0.03 mol % 1.21 mol % i-hexane 0.03 mol % 0.21 mol % n-hexane 0.00 mol % 0.30 mol % i-heptane 0.01 mol % 0.12 mol % n-heptane 0.00 mol % 0.02 mol % octane 0.00 mol % 0.01 mol % Benzene 2.88 mol % 1.76 mol % Toluene 1.31 mol % 1.86 mol % Xylene 0.23 mol % 1.26 mol % CO 3.71 mol % 1.03 mol % CO.sub.2 2.42 mol % 1.48 mol % Nitrogen 1.20 mol % 1.20 mol % Oxygen 0.00 mol % 0.00 mol % H.sub.2S 0.023 mol % 0.021 mol % Mercaptane 0.003 mol % 0.003 mol % Other >C8 0.22 mol % 1.10 mol % Total 100.00 mol % 100.00 mol % Sulphur Compounds H.sub.2S 232 ppm 212 ppm Mercaptane 26 ppm 30 ppm